TESP12206R0-Transmission Structure Foundations

TRANSMISSION ENGINEERING STANDARD

TES-P-122.06, Rev. 0

TABLE OF CONTENTS

1.0

SCOPE

2.0

APPLICABLE CODES AND STANDARDS

3.0

DESIGN CRITERIA 2.1 2.2

4.0

SUBSURFACE SOIL INVESTIGATION 4.1 4.2 4.3 4.4

5.0

Latticed Towers Single Shaft Structures Framed Structures Externally Guyed Structures

LOADING CRITERIA 6.1 6.2 6.3

7.0

Scope of Investigation Soil Conditions in Saudi Arabia Standard Soil Classification Special Soil Type

PERFORMANCE CRITERIA 5.1 5.2 5.3 5.4

6.0

General Requirements Foundation Requirements

Loads Overload Capacity Factors Seismic Load

CONCRETE FOUNDATION 7.1 7.2 7.3 7.4

Materials Details of Design Details of Reinforcement Types of Foundation

TESP12206R0/JQA

Date of Approval: September 8, 2008 G.

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8.0

FOUNDATION CLASSIFICATIONS AND THEIR APPLICATIONS 8.1 8.2 8.3 8.4 8.5 8.6 8.7

9.0

Procedures for Drilled Pier Foundations Procedures for Driven Pile Foundations Procedures for Anchor Foundations

LOAD TESTS 10.1 10.2 10.3

11.0

Direct Embedment Foundations Driven Pile Foundations (PI) Drilled Pier Foundations Anchor Foundations (AN) Pad & Chimney Foundation (PA) Auger Foundation (AU) Grillage Foundation (GR)

DESIGN PROCEDURES 9.1 9.2 9.3

10.0

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General Instrumentation Scope of Test Program

REFERENCES

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1.0

TES-P-122.06, Rev. 0

SCOPE This section of the Standard establishes the specific parameters, guidelines and procedures for the design of overhead line (OHL) transmission structure foundations, and covers the load encountered as well as foundation performance criteria. Geotechnical considerations, load testing and construction methods associated with each foundation design are discussed.

2.0

APPLICABLE CODES AND STANDARDS Unless specified otherwise in this section, design, materials, manufacturer, workmanship and testing of all foundation works shall comply with an approved standard. All overhead line towers provided under this specification shall conform to the applicable codes and standards of: The latest edition or revision of these approved standards shall apply. 2.1

SSA No. 2

Steel Bars for Reinforcement of Concrete

2.2

SSA No. 142

Cement Tests

2.3

SSA No. 143

Portland Cement -Ordinary & Rigid Hardening

2.4

SSA No. 378

Concrete Aggregate

2.5

SSA No. 690

Test Method of Steel Bar for Reinforcement of Concrete

2.6

ACI 318M

Building Code Requirement for Reinforced Concrete

2.7

ASTM A36

Specification for Structural Steel Standard

2.8

ASTM A121

Specification for Zinc-coated (galvanised) Steel Barbed Wire.

2.9

ASTM A123

Specification for Zinc (hot-galvanised) Coatings on products fabricated from rolled, pressed and forged steel shapes, plates, bars and strips.

2.10

ASTM A143

Safeguarding against embrittlement of hot-galvanised structural steel products and procedure for detecting embrittlement.

2.11

ASTM A153

Specification for Zinc Coating (hot-dip) on Iron and Steel Hardware.

2.12

ASTM A239

Test Method for Locating the Thinnest Spot in a Zinc (Galvanised) coating on Iron or Steel Articles by the Preece Test (Copper Sulphate Dip).

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2.13

ASTM A242

Specification For High-Strength Low-Alloy Structural Steel.

2.14

ASTM A307

Specification For Carbon Steel Externally Threaded Standard Fasteners

2.15

ASTM A325

Specification For High-Strength Bolts For Structural Steel Joints.

2.16

ASTM A370

Methods And Definitions For Mechanical Testing Of Steel Products.

2.17

ASTM A384

Safeguarding Against Warpage And Distortion During Hot-Dip Galvanising Of Steel Assemblies.

2.18

ASTM A385

Recommended Practice For Providing High Quality Zinc Coatings (Hot-Dip).

2.19

ASTM A394

Specification For Galvanised Steel Transmission Tower Bolts.

2.20

ASTM A441

Specification for High-strength Low-alloy Structural ManganeseVanadium Steel.

2.21

ASTM A572

Specification for High-strength Low-allby Columbium-Vanadium Steel of Structural Quality.

2.22

ASTM A588

Specification for High-strength Low-alloy Structural Steel with 50 ksi (345 MPa) Minimum Yield Point. To a 4-inch (100mm) thickness.

2.23

ASTM A615

Specification for Deformed Plain Billet-Steel Bars for Concrete Reinforcement

2.24

ASTM D698

Moisture-Density Relation of Soil

2.25

ASTM A751

Methods, Practice and Definitions for Chemical Analysis of Steel Products.

2.26

ASTM D1143

Testing Piles under Axial Compressive Load

2.27

ASTM D1586

Standard Penetration Test

2.28

ASTM D1194

Test for Bearing Capacity of Soil for Static Load on Spread Fottings

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2.29

ASTM D2049

Test for Relative Density of Conhesionless Soils

2.30

ASTM D3689

Rock Pullout Test

2.31

ASTM D4253

Test Method for Maximum Index Density and Unit Weight of Soil using a Vibratory Table

2.32

ASTM D4254

Test Method for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density

2.33

IEC 826

Loading Strength of Overhead Transmission Lines

2.34

IEC 652

Loading Tests of Overhead Line Towers

2.35

Uniform Building Code (UBC)

2.36

American Society of Civil Engineers (ASCE)

2.37

Manuals and Reports on Engineering Practice -Number 52, "Guide for Design of Steel Transmission Towers".

2.38

Column Research Council (CRC)

2.39

Guide to Design Criteria for Metal Compression Members, Column Research Council.

2.40

American Institute of Steel Construction (AISC)

2.41

Specification for the Design, Fabrication and Erection of Structural Steel for Buildings (AISC Steel Specification).

2.42

Code of Standard Practice for Steel Buildings and Bridges (AISC Standard Practice).

2.43

Specification for Structural Joints using ASTM A 325 & A 490 Bolts Structural Steel Detailing.

2.44

Manual of Steel Construction.

2.45

American Welding Society (AWS)

2.46

D1.1 -Structural Welding Code. International Standards Organization

2.47

ISO Standard ISO 630 Structural Steels

2.48

ISO Recommendation ISO/R657 Hot-rolled Steel Sections.

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TES-P-122.06, Rev. 0

2.49

American National Standards Institute "Safety Rules for the Installation and Maintenance of Electric Supply and Communication Lines" (National Electrical Safety Code).

2.50

ICAO

International Civil Aviation Organisation

2.51

FAA

Federal Aviation Authority

2.52

CAA

Civil Aviation Authority

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3.0

TES-P-122.06, Rev. 0

DESIGN CRITERIA 3.1

General Requirements A transmission line is comprised of a system of interconnected elements, each individually designed. Every decision made for the system shall consider total installed cost, of which foundations are a major consideration. For example, wire tensions are sometimes increased to minimize the number and height, or both, of the structure. Similarly, when developing structure configurations, a wider base may be considered in order to reduce foundation loads, thereby the foundation cost. This must be evaluated against the added cost of widening of the structure. When designing a transmission line, the engineer has to develop a minimum number of standard foundation designs usable at a majority of the sites. Foundations may be standardized by limiting the number to one or two designs for each standard structure type, covering a selected range of subsurface conditions. Verifications of subsurface conditions at each structure site can be made during the construction excavation. In addition, construction excavation may reveal locations that require special foundations due to actual subsurface conditions outside the limits of the preselected range. The benefits of standardization shall be weighed against the additional cost of redesigning the foundation when unusual subsurface conditions are encountered during construction. The amount of standardization will vary with the drilled shaft foundations. A drilled shaft foundation can be varied to suit the actual soil conditions by providing different depths and diameters, or both. Usually, the only change to fabricated material for drilled shaft foundations is the length of rebar which can be readily accomplished with small additional costs. Foundation design shall correspond to the most modern methods and techniques of design and construction. The design of reinforced concrete member shall be in accordance with ACI 318 standard. The tower foundation shall be designed independently for each of the four leg to resist the following loads without excessive rotation or displacement: -Compression -Uplift -Tilting -Horizontal shear forces -Bending Moment

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TES-P-122.06, Rev. 0

All foundation shall make adequate provision for horizontal shear forces in the region of the ground line. As far as practicable, for any standard tower type, the foundation stubs and foundation designs themselves shall be identical for standard towers and extended towers and leg extensions. For all OHL tower foundations A to H, (single circuit & double circuit) tower foundations, no difference shall be made for the design of tensile and compression footings. However, for other tower foundations types C, D, E & F of 380 KV double circuit OHL, the design of the foundations for compression legs may differ in an approved manner from these for the tension legs. Allowable soil bearing capacity values for foundations design work are stated in Table1. Additional values shall be recommended by the geotechnical investigation agency to produce an economical set of foundation designs. The selection of additional or alternative soil bearing capacity values shall be subject to SEC approval. Calculation of resistance to uplift shall be related to the frustum angle. The effective weight of the footing and soil over it shall be included in the calculation of the resistance to uplift. The following basic criteria shall satisfy the foundations for transmission lines:

3.2

3.1.1

They shall be stable, and have an adequate factor of safety or level of reliability against failure.

3.1.2

The movement shall be within such limits as not to impair the function of the structure.

3.1.3

They shall be economical, or at least cost effective for the particular type of structure.

Foundation Requirements Foundations shall be standardized by minimizing the number of designs for each structure type, covering a selected range of subsurface conditions. If investigations reveal locations that require special foundations outside the limits of the pre-selected range, due to actual subsurface conditions, such foundations shall be designed separately.

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4.0

TES-P-122.06, Rev. 0

SUBSURFACE SOIL INVESTIGATION The cost-effective and technical requirements of assuring foundation design for transmission structures need a thorough knowledge of the subsurface conditions along the right-of-way (ROW). This section is intended to provide guidelines for performing an adequate subsurface investigation for the design of transmission line structure foundations. When designing transmission line structure foundations, the engineer shall take into consideration two factors: a. b.

The ultimate load bearing capacity of the subsurface material. The allowable deformations of the foundation.

Hence, the objectives of a subsurface investigation are to determine the stratigraphy and physical characteristics (particularly the strength and deformation characteristics which are the engineering properties) of the soil or rock underlying a given site. It is necessary to consider the engineering properties of the subsurface materials, construction costs, the construction aspects of a particular foundation type and how they are influenced by such factors as groundwater elevation, safety requirements, Geotechnical Consultant capability and experience, and environmental constraints to determine the most cost-effective foundation. Moreover, recommendation for foundation selection shall be made by Geotechnical Investigation Agency based on the specified foundation types. 4.1

Scope of Investigation Subsurface investigation shall vary depending upon the foundation loads, type of structure and probable foundation types, type of subsurface material, and previous knowledge of the subsurface conditions along the line route. It is necessary to use engineering judgment when considering the scope of the subsurface investigation. Since transmission lines route runs through many kilometres, the route crosses many different kinds of soil strength and characteristics that may vary differently. Of equal significance is the fact that the characteristics of all of the soils encountered may change radically with changes in the season of the year. For the above reasons, subsurface soil investigation and soil testing programmes are imperative, so that a foundation type may be selected which matches the strength required to the strength available at each structure location. This investigation should take place for the entire transmission line route prior to the start of construction so that alternative routings or tower locations may be considered if the required foundations become prohibitively expensive.

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Practically, it is not possible to associate one specific tower type with one specific foundation design, however, it is usually possible to associate one specific structure type with a family of foundation designs, from which one family of design can be selected based on soil consideration and terrain at the tower location. So, it is necessary to classify the soil along the line route as in section 4.3. The geotechnical agency shall investigate the subsurface soil condition providing the information covering the soil mechanics, chemical, geophysical and electrical test results. Approval of the geotechnical agency who shall conduct the test should come from the Company approved list of geothecnical agency. Prior to any design WORK, the contractor shall hire a Geotechnical Consultant, approved by the company, who shall undertake the geotechnical site soil investigations. The geotechnical investigation agency shall visit the site, familiarize and submit a detailed preliminary geotechnical proposal on the following for COMPANY review: a. b. c. d. e. f.

Scope of WORK. Field Investigation method (boring and sampling). Laboratory investigations (soil tests). Foundation types for different soil types, conditions and recommendation. Identification of potential problems such as settlements, de-watering problem, rock excavation, etc. and their solution. Soil electrical and thermal resistivity measurement method and procedure.

Contractor shall not start the investigation and boring until they receive the approval of preliminary geotechnical proposal from company. The soil borehole shall be every structure location (including angle points). Contractor shall provide an A-size drawing in SEC format (drawing number, index, plant number etc.) and showing the location of bore holes. Minimum depth of borehole shall be 2B, where B is the width of the foundation, and in no case shall be less than 8 meters for pad/chimney foundations. For augured drilled soil piers, the borehole depth shall be at least 3 m below the bottom of the pier-anticipated foundation. In case of rock, it shall be 1.0 m under the top of rock. The contractor shall obtain right to entry and access to the required boring location and right to perform the boring and any other tests. He shall take every precaution to protect from damage any existing water sewer, gas and electric conduit lines and other installations. The work shall comply with all the applicable requirements of all codes and regulations at site.

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TES-P-122.06, Rev. 0

The responsibility of the contractor is to provide to SEC all test reports of the results of subsurface soil mechanics, chemical, geophysical and electrical investigations. The report shall clearly indicate: -Saudi Electricity Company (SEC) -Contractor's name and address -Contractor's Project Name and Number -Laboratory name and address -Date of beginning and time -Date of completion and time -Weather conditions -Tower Reference number -Surface elevation -Name of subcontractor (if any), operator, inspector, engineer or geologist. -Signature of authorized Geo-tech Engineer and Contractor Qualified geologist, soil mechanic engineer and more than 10 years experience technicians is required to work in this investigation either planning or field tests. Explosives and blasting of any type shall not be permitted. The subsurface soil characteristics shall be submitted using one or more applicable methods indicated in every project specification along the line route, the measurements shall be done wherever the soil characteristic change is encountered along the line route. The measurements shall be done at every tower location of the OHL. The depth of borehole at each tower location shall be to minimum ten (10) meters below the ground surface and soil data shall be recorded at every one (1) meter depth or wherever the subsurface soil characteristics changes (whichever is less). 4.1.1 General There are three investigatory phases that shall be taken into account namely: a)

Preliminary Investigation

b)

Design Investigation

c)

Construction Verification

The subsurface investigation program shall follow a sequence of preliminary investigation leading to design investigation followed by construction verification. To assist the engineer in judging what scope of subsurface investigation is best suited for a project, each of these three investigatory phases is described in the following sections:

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TES-P-122.06, Rev. 0

4.1.2 Preliminary Investigation The purpose of the preliminary subsurface investigation is to provide sufficient information for the following: a)

Integrating subsurface conditions into the line route selection and structure location process.

b)

Establishing the most appropriate foundation types for cost comparison.

c)

Assisting in the evaluation of the environmental impact of proposed construction; for example, dewatering for excavations.

d)

Verifying that available information from previous experience on or near the right-of-way is applicable to the project under consideration.

e)

Evaluating the terrain for construction purposes.

f)

Evaluating geotechnical problems such as the potential for landslides, erosion, etc.

g)

Establishing a basis for the development of the de-tailed exploration program.

The preliminary subsurface investigation shall consist of collecting existing data relating to subsurface conditions, and making a geotechnical field reconnaissance of the line route. If considered cost effective, preliminary borings should be made to verify and increase confidence in existing data and reconnaissance mapping. a.

Existing Data A considerable amount of data regarding surficial geology, including distribution of surface water, depth of groundwater, depth and physical characteristics of bedrock and type and thickness of soil cover, is available from several resources, such as Geotechnical investigation of existing transmission line rights-of-way.

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b.

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Field Reconnaissance Another useful means of obtaining information during the preliminary subsurface investigation is to perform a field reconnaissance survey of the transmission line route. The reconnaissance shall be performed by a geotechnical engineer or engineering geologist. The purpose of the reconnaissance is to develop a map of the surficial soils showing areas that may offer particular foundation problems such as deep peat or soft organic silt, bedrock outcrops, areas of high groundwater table, and areas of potential slope instability. The soil and rock classifications used in the mapping shall be based on engineering properties, not on geological or agricultural distinctions. By combining the information from the field reconnaissance and existing published information, a preliminary line route map showing basic soil or rock types, inferred depth to bedrock, and elevation of the groundwater table can be developed.

c.

Preliminary Borings The development of a surficial map usually is the final step in the preliminary investigation. Occasionally, the information available at this point in the investigation results in the need to interpret the subsurface conditions within a wide range of possibilities. In this case, it may be cost effective to obtain a few preliminary borings in those areas where interpretation is difficult and may affect the foundation design significantly. Preliminary borings are generally used for soil classification purposes only and disturbed samples are thus satisfactory. The most common methods of obtaining disturbed samples are auger borings and using a heavy walled split-barrel sampler which is driven into the soil at selected intervals in the boring. Refer to Company Standard “Geotechnical Investigation” for detailed Guidelines and recommendations. Since ground water affects many elements of foundation design and construction, its elevation shall be established as accurate as possible if it is within the probable construction zone. It is generally determined by measuring to the water level in the borehole after a suitable time lapse. A period of 24 hours is a typical time interval. However, in clays and other soils of low permeability, it may require several days to weeks to determine a meaningful water level. Standpipes or other perforated casings may be used to prevent the borehole from caving in during this period.

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4.1.3 Design Investigation The significance of the design investigation is to provide the design engineer with sufficient subsurface information in selecting types of foundations most suitable at each structure location, determine the size and depth of selected foundation and evaluate potential construction problems. The information required to achieve these goals includes: a) b) c) d) e) h) i) j) k) l) m) n)

Type of structure and allowable foundation movements. Magnitude and duration of structure loading at the ground line. Stratigraphy of the subsurface materials. Elevation of the ground water table. Engineering properties of the subsurface materials. Submerged and saturated weight of soil Modulus of subgrade reaction Coefficient of lateral earth pressure Rock Quality designation (RQD) Cone angle for uplift Standard Penetration Test (SPT) “N” values Dutch Cone Penetration Test “Rp” values

On any transmission line route, these five factors may vary considerably, and the detailed investigation shall provide the required information in a cost-effective manner. Ideally, a detailed subsurface investigation would require a boring at each structure site or foundation Development of the Subsurface Investigation Program shall be done to obtain detailed information of the location of the structure foundations. To promote uniformity in obtaining test data on soil samples obtained in a test boring program, the procedures and relevant standards described in Design Manual NAVFAC DM-7 shall be followed. For all foundations, site soil parameters shall be ascertained from soil borings and from the laboratory soil tests. Soil properties and parameters which are to be determined are listed below for use in various design procedures:

TESP12206R0/JQA

γ

Total unit weight of the soil, kN/m3 (The cohesive strength is taken as one half of the unconfined compressive strength.)

φ

The angle of internal friction of the soil, in degrees

N

Standard Penetration test value


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4.1.4

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Construction Verification The Company shall have representatives in the field during foundation construction to determine if the actual subsurface conditions are similar to those conditions used in the foundation design. If the subsurface conditions used in the foundation design differ from the actual conditions, it may be necessary to enlarge the foundation or change the foundation type.

4.2

Soil Conditions in Saudi Arabia The more significant soil conditions prevalent in Saudi Arabia include: 4.2.1 Areas of Sabkhah Sabkhah as described in TCS-Q-113.02 Earthwork Standard is a material that is firm enough to support roadways when dry but becomes soft and unstable when wet. Sabkhah areas are to be avoided to the extent that such avoidance is practical. Because of the length of transmission lines and considerable number and extent of these areas, the construction of some foundations in this material may be unavoidable. Therefore, it is mandatory at such locations that improvement of soil as stated in TCS-Q-113.02 shall be considered and implemented. 4.2.2 Areas of Aeolian Sand and Marl The presence of Sands introduces the problem of shifting of the overburden at the foundation due to wind action. This problem can be alleviated to some extent by elevating the soil surface at each foundation and stabilizing the elevated surface with crude oil; this practice tends to prevent the depositing of wind-borne sand at the foundation. The more severe situation occurs when, instead of being deposited on the foundation, the, surface sand blows away from the foundation; the same surface stabilization practice tends to alleviate this situation as well. However, because this soil stabilization practice will have a limited life, regular patrols of the line must be made to discover if excessive reveal is causing any foundations to become unsafe. Foundations in these areas should be marked to indicate design reveal and maximum safe reveal. Soil stabilization is discussed in TES-P-122.11, Transmission Line Access roads and Structure Pads. 4.2.3 Rock Areas Where the surface consists of rock outcroppings, the rock is generally a weathered limestone that is drillable to a sufficient depth to install wood poles or tower "rock anchor" type foundations. However, the type of foundations to be adopted shall be the discretion of design engineer.

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4.3

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Standard Soil Classification In addition to reports including the required information outlined in Section 8.1, the soil shall be classified into five types for design purposes. Table-1 shows the main properties of standard soil classification. 4.3.1 Type A Soil Intact or solid bed rock which has not been subjected to fracturing due to severe weathering and which is firmly cemented together. 4.3.2 Type B Soil Shattered rock or medium to dense cohesionless soil with the water table at a depth below the foundation equal to or greater than the width of the foundation. The soil shall have a Standard Penetration Test blow count of 15 or greater. Medium to stiff cohesive soil with a Standard Penetration Test blow count of 10 or greater. The unconfined compression strength shall be greater than 120 kN/m2. 4.3.3 Type C Soil Medium to dense cohensionless soil or medium stiff cohesive soil which may be fissured. The soils may occasionally be below the water table. 4.3.4 Type D Soil Loose or saturated sand or soft clay. Bracing of the excavation will be required. 4.3.5 Type E Soil Areas of Sabkhah or unstable soils where characteristics are subject to dramatic change with the water table. Such soils will require piles to be driven through the soft, unstable zone into a more stable soil as required to develop the required foundation capacity.

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Table-1 Standard Soil Properties

Soil Type SOIL CONDITION

A

B

C D Occasionally Wet Wet Surface Surface Surface

Dry

Location Water Table

Below Below Foundation Foundation

Surface

Unit Weight Soil kN/m3 Ultimate Soil Bearing Pressure kN/m2 (max.) Ultimate Soil Shearing Or Lateral Shear Stress kN/m2 (max.) Plane of Rupture

19.5

15.69

9.81

9.81

9.81

1000

250

100

50

30

100

25

10

5

2

300

300

300

-

-

Average Blow Count Cohesionless Cohesive

Over 50 Over 50

15-50 10-50

8-15 6-10

4-6 4-6

0 0

Dry

E Wet

Soil symbol

Special Soil Symbol 4.4

Special Soil Type In case the subsurface soil investigation encounters a very hard soil or very loose soil properties of which do not fall within Table-l and the total cost of the OHL will be affected and when the line route length in such area exceeds 1000m; SEC shall modify the line route in this area if this modification is economically feasible. Otherwise, the contractor/surveyor shall carry out the required tests in order to provide the special soil properties along with test reports and his recommendations for cement and tower foundations. The special soil shall be given a designation letter as required.

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5.0

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PERFORMANCE CRITERIA In establishing performance criteria, the definition of foundation failure should be thoroughly understood by both the foundation designer and structure designer in order to have a safe and economical foundations design. In essence, failure occurs when pre-established foundation performance criteria are exceeded. For example, performance criteria could be set at a magnitude of displacement that would endanger the stability of the structure at a level that would impair the operational safety of the transmission line. The amount of allowable displacement is dependent on the type of structure. 5.1

Latticed Towers Latticed tower foundation loads consist of vertical tension (uplift) or compression forces and horizontal shear forces. For tangent and small-line angle towers, the vertical loads on a foundation may be either uplift or compression. For terminal and large-line angle towers, the foundations on one side may always be loaded in uplift while the other side may always be loaded in compression. The distributions of horizontal forces between the foundations of a latticed tower vary with the bracing of the structure. Care shall be taken to include both the transverse and longitudinal components in all tower members connected to the foundations. A typical free body diagram is shown in Figure 5.1. When the foundations of a tower displace and the geometric relationship of the tower to its foundations remains the same, any increase in load due to this displacement will have a minimal effect on the tower and its foundation. However, foundation movements that change the geometric relationship between the tower and its foundations will redistribute the loads in the tower members and foundations. This will usually cause greater reactions on the foundation that moves least relative to the tower, which in turn will tend to equalize this differential displacement. At the present time, the effects of differential foundation movements are normally not included in tower design. Several options are available to the engineer considering differential foundation displacements in the tower design, including designing the foundations to satisfy performance criteria that will not cause significant secondary loads on the tower, or designing the tower to with-stand specified differential foundation movements.

5.2

Single Shaft Monopole Structures These structures have one foundation precluding differential foundation movement. Foundation reactions consist of a large, overturning moment and relatively small horizontal, vertical, and torsional loads. Figure 5.2 presents a free-body diagram of the loads.

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For single-shaft structures, the foundation movement of concern is the angular rotation of the shaft in the vertical plane and horizontal displacement of the top of the foundation. When these displacements have been determined, the displacement of the conductors can be computed. Under high wind loading, a corresponding deflection of the conductors’ perpendicular to the transmission line can be permitted. Accordingly, a large ground line displacement of the foundation could also be permitted. Due to foundation rotation, the clearance between the conductors and the structure would be decreased only for structures with single string insulators. The mid span ground clearance and the change in line angle would also decrease a negligible amount. In establishing performance criteria for single-shaft structure foundations, consideration shall be given to how much total, as well as permanent, displacement can be permitted. In some cases, large permanent displacements might be aesthetically unacceptable, and re-plumbing of either the structures, their foundations, or both, may be required. In establishing performance criteria, the cost of re-plumbing shall be compared to the cost of a foundation that is more resistant to displacement. For terminal and large-line angle structures, large foundation deflections parallel to the conductor are probably not tolerable. For these structures, the deflection may excessively reduce the conductor to ground clearance, or affect the load capacity of adjacent structures. There are also problems in the stringing and sagging of conductors if the deflections are excessive. This problem is usually resolved by construction methods or use of permanent guys. 5.3

Framed Structures The stability of these structures is dependent in part on one or more of their joints resisting moment. The foundation reactions are dependent upon which joints can resist moment and the relative stiffness of the members. Although these foundation reactions are statically indeterminate, they can be determined by making assumptions that result in a statically determinate structure. In addition, the structure can be analyzed as an indeterminate structure using techniques such as moment distribution or computerized stiffness matrix procedures. Figures 5.3 and 5.4 Shows free-body diagrams of four and two-legged framed structures. If the bases of structures are designed with pins or universal joints, then the moments acting upon the foundations theoretically will be zero. Many different types of two-legged, H-framed structures are in use in transmission lines. This has been particularly true in recent years since visual impact has become of greater concern.

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The H-framed structure is particularly applicable for wood, tubular steel, and concrete poles. The cross arm may be pin-connected to the poles. These structures may be unbraced, braced, or internally guyed as shown in Figure 5.5. As with latticed towers, past practice has usually not included the influence of foundation displacement and translation in H-framed structure design. Significant foundation movement will redistribute the frame and foundation loads. The foundations can be designed to experience movements that will not produce significant secondary stresses, or the structure can be designed to a predetermined maximum allowable displacement and rotation. 5.4

Externally Guyed Structures There are two general types of externally guyed structures. For all types, the guys produce uplift loads on the guy foundations and compression loads on the structure foundation. The guys are generally adjustable in length to permit plumbing of the structure during construction, and to account for creep in the guy and movement of the uplift anchor. 5.4.1 The first type of guyed structure consists of a single shaft as shown in Figure 5.6. This type of structure is often used as a terminal and large-line angle structure, its flexibility allows most of the load to be resisted by tension in the guys and compression in the main shaft. Generally, this type of guyed structure can tolerate significant foundation movement as far as its own structural integrity is concerned; however, like the terminal and large-line angle poles discussed in 5.2, when excessive guy anchor slippage occurs, problems such as conductor-to-ground clearance, security of adjacent structures, and stringing and sagging conductors can develop. 5.4.2 The second type of externally guyed structure is a conventional latticed tower guyed to reduce its leg loads and foundation reactions. This approach is often used to upgrade existing towers.

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The flexibility of the guy combined with the flexibility of the tower is necessary to compute foundation reactions and anchor loads. The maximum amount of anchor slippage can be selected, and the tower and anchors designed accordingly. The initial and final modulus of elasticity of the guys plus the creep of the guys should be considered. The amount of pretension in the guys should be specified. Load testing of the guy anchors is recommended to ensure against excessive slippage. Figure 5.7 shows a typical installation. The leg foundations are required to resist only horizontal shear forces and vertical compression or uplift loads. As in the case of the latticed towers, discussed in 5.1, the load distribution in the component structural elements is sensitive to the foundation performance. Differential displacements of the tower legs will result in load redistribution, and may affect the integrity of the tower.

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6.0

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LOADING CRITERIA 6.1

Loading Saudi Electricity Company (SEC) has established loading cases, as specified in the latest revision of TES-P-122.05, "Transmission Structures", for the design of transmission line systems. Based on this information, the design engineer shall analyze the structural system and appropriate combinations of axial, shear and moment loads acting on every foundation for each loading case.

6.2

Overload Capacity Factors The ultimate foundation reactions obtained from tower analysis shall then be multiplied by foundation overload capacity factors and shall be applied to the design (stability & strength) of foundations. The foundation overload capacity factors are as tabulated below. Table–2-FOUNDATION OVERLOAD CAPACITY FACTORS

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Loads On Foundation

Pile Type Foundations

All Other Type Foundations

Uplift

2.0

1.5

Compression

2.0

1.5

Lateral

2.0

1.5

Overturning

2.0

1.5

Sliding

2.0

1.5


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6.3

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Seismic Load Generally, tower foundations shall survive under moderate earthquake tremors without noticeable distress. Foundations shall be designed with corresponding seismic zone probability of Seismic Design Criteria for R.C structures in Saudi Arabia, King Saud University – College of Engineering and Uniform Building Code or 01-TMSS-01 if the location is other than those in the Table below. TABLE-3

SEISMIC ZONE TABULATION SEISMIC ZONE NUMBER LOCATION (SZN)

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HAFAR AL-BATIN

1

DHAHRAN

1

JEDDAH

2A

KHAMIS MUSHAYT

2B

RIYADH

0

TABOUK

2A


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7.0

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CONCRETE FOUNDATION Foundation shall be designed per the requirement of ACI 318M except as modified in this section. Concrete to be use in the foundation shall meet the requirements of SEC Material Standard 70-TMSS-03. 7.1

Materials 7.1.1 The minimum of 28 day compressive strength of concrete shall be determined from the result of the soil chemical analysis. The degree of exposure of concrete shall be based on the requirements of SEC Material Standard 70-TMSS-03. In the absence of soil chemical analysis, the Design Engineer will specify the degree of exposure of concrete to be adopted from the soil chemical analysis of particular projects in the area. 7.1.2 Reinforcing steel will be deformed bars conforming to SASO SSA 2 with minimum yield strength of 420 MPa as required by design. Deformations shall conform to ASTM A615M. 7.1.3 All steel reinforcement sizes and properties shall conform to SASO Standard.

7.2

Details of Design 7.2.1 Axial forces within the pier, both tension and compression will be considered to extend the full length of the pier. The effect of skin friction in diminishing the axial load will be ignored insofar as reinforcement design is concerned. Skin friction on the pier will be considered in calculating resistance to external loads on the pier. 7.2.2 Shear reinforcement requirements, if any, will be determined from the maximum shear in the foundation due to tower design loads including overload capacity factors. If shear reinforcement is required, it will be provided for full length of the pier. 7.2.3 Pier embedment lengths will be given in 0.30 meter intervals. 7.7.4 Piers for steel monopoles will be located concentrically with the pole shaft. Piers of latticed steel towers will be offset from back-of-angle at top of concrete to allow for stub angle placement.

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7.3

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Details of Reinforcement 7.3.1 Minimum longitudinal reinforcement shall as per ACI 318M requirements. More reinforcement may be required by bending in the pier or as noted below. Not less than 20 mm bar size will be used for longitudinal reinforcement and not larger than 32 mm bar size will be used in any pier. 7.3.2 Minimum tie size will be 12 mm diameter bar. More ties may be required by shear in the pier. Maximum spacing of ties shall not exceed 16 longitudinal bar diameters, 48 tie diameters or pier least dimension, nor more than 50 cm. 7.3.3 Lap splice (compression and tension) lengths have been calculated as 1.7 x development length for longitudinal steel (Class C splice) and 1.7 x development length x 0.8 for 'tie steel (see Chapter 12 of ACI 318M, Code and Commentary). Splice of rebar shall be staggered and not more than 50% at one location. 7.3.4 Vertical bars shall be placed within the ties, and tied to the ties at each intersection. Tie wire shall be a minimum of 1.31 mm2. 7.3.5 Welding is not permitted. 7.3.6 Minimum concrete cover shall be 7.5 cm, measured from the Concrete surface to the outside surface of the ties. (This minimum concrete cover allows for some hole out-of-roundness and/or out-of-plumbness which may occur during construction, and for some flexibility of the reinforcing cage.) 7.3.7 Longitudinal steel shall extend to 7.5 cm below top of concrete, and 10 cm +/2.5 cm above the design bottom of the pier. If, during construction the pier, it is slightly longer than detailed, the rebar cage shall be lifted to within 7.5 cm of the top of the pier.

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8.0

TES-P-122.06, Rev. 0

FOUNDATION CLASSIFICATIONS AND THEIR APPLICATIONS The SEC OHL Survey & Soil Investigation specifies five types of soils: A, B, C, D & E, while OHL Tower foundations shall be classified into: 1.

Pad and Chimney (PA)

2.

Auger (AU)

3.

Anchor (AN)

4.

Pile (PI)

5.

Grillage (GR)

Each of these five foundation types shall be subclassified into several classes: 1.2. 3. 4. ...etc., depending on the applied loads, foundation size, dimensions, reinforcement. etc. Therefore, each tower foundation shall be distinguished by two parts, first part shall be one of the above mentioned types (i.e. PA, AU, AN, PI or GR) while the second part shall be class l or class 2 or class 3, ...etc. (Example AU2 means Auger foundation of class 2). Applicable foundation type in accordance with the type of soils shall generally be: Soil Type A: Anchor (AN). Auger (AU) & Pad & Chimney (PA) Soil Type B: Auger (AU) and Pad & Chimney (PA) Soil Type C: Auger (AU) and Pad & Chimney (PA) Soil Type D: Pad & Chimney (PA) and Pile (PI) Soil Type E: Pad & Chimney (PA). Pile (PI) & Grillage (GR) In general, the foundation shall be of reinforced concrete. Four separate footing foundations shall be offered for each lattice tower. Under all reinforced foundations a blinding layer of concrete of 100 mm has to be laid. All foundation types shall have an average height of 600 mm above ground. In flooded areas the height of foundations above ground shall be minimum (1.2m) and shall be designed to resist the mechanical loads and chemical flood water effect.

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The thickness of the concrete layer above all steel reinforcing parts shall not be less than 75mm. All foundations upper part shall be slightly sloped to prevent accumulation of water. All foundations situated at a distance of less than 30 m from any roadway shall be protected by a crash barrier to ensure the protection of towers against vehicles. The general design of the crash barriers shall be as per TES-P-119.19. Different sizes of each foundation type shall be designed for various types of towers and heights from this standard in order to produce an economical family of foundations for all supports. Special precautions will be required with towers sited in or adjacent to Waddis. The foundations and bases of these towers shall be properly protected to withstand the effects of flash-flooding. The types of foundations usually used for transmission lines of various constructions can be summarized as follows: 8.1

Direct Embedment Foundations Usually wood monopoles and H-frame are directly embedded in the soil in a preaugered hole; the hole is backfilled with compacted sand or crushed rock backfill. Backfill in sandy soils should be compacted to 85% Relative Density in accordance with ASTM D4253 and ASTM D4254; backfill in cohesive soils shall be compacted to 95% Modified Proctor in accordance with ASTM D698. To improve bearing capacity for downward loads, pole bearing plates can be added to produce a greater bearing area. To improve uplift capacity, additional screw anchors can be added to engage a larger volume of resisting soil. To improve lateral load capacity, pole keys may be added to spread the lateral load over a larger area. Direct embedment, coupled with these various improvements, should be suitable in all areas except Sabkhah.

8.2

Driven Pile Foundations Wood structures located in Sabkhah areas will probably require either piling, driven through the soft and unstable zone into soil or soft rock that is more dense and stable, or larger diameter concrete piers extending down to, and anchored into, bedrock. When wood poles are attached to steel piles, care shall be taken in design to avoid removal of wood in locations that cause weakening of the pole. Latticed tower located in areas of Sabkhah, pile foundations or deep piers anchored into bedrock will be required.

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Where low ground bearing capacity and high water table are encountered either concrete pad and chimney with enlarged pad or pile (bored or driven) foundation may be used. Tests will be carried out on test piles installed at specific pile sites to confirm that the design based on the soil data are satisfactory. Piles of material other than concrete require the special experience of the proposed system. In any case, such systems require SEC approval. The minimum bearing capacity of a pile shall be 250 kN. Lateral loads shall be resisted by passive earth pressure. Concrete pile shall contain minimum of 370 kg/m3 cement and a minimum compression strength of 37.5 MPa. The maximum water/cement ratio shall be 0.42. 8.3

Drilled Pier Foundations Usually steel poles are founded on cast-in-place concrete piers, thus disassociating the structure design from the foundation design, except for common loadings. Usually, steel latticed structures also are founded on cast-in-place drilled piers.

8.4

Anchor Foundations (AN) Anchors are long, very slender structural elements. Plate anchors are long rods with flat plates in various levels. They are placed in excavated holes and backfilled with soil. Grouted anchors are long rods which are placed in excavated holes and then grouted to fill the annulus. Anchors often are installed in a group and tied together through a cap to form a foundation. However, many variations are employed for specific situations. Anchor foundation shall be permitted only in a solid, sound non-weathering ledge rock. The holes in rock shall be made in such a manner as to eliminate the possibility of serious cracking of the rock. The dimensions of the hole shall be approved, but the depth of stub actually grouted into the rock shall in no case be less than 1000 mm. The stubs shall be completely galvanised, except that it shall be permissible to cut off, on site, lengths at the bottom ends of the stubes where the upper surface of the rock is at or near ground level. The stubs shall be firmly keyed and grouted into the rock and shall be encased as. for other types of foundation with the exception that the encasing concrete shall extend down to the upper surface of the rock. Where the use of stubes embedded in rock is not economical, other types of anchor foundation may be used if approved by SEC. Field load tests shall be carried out to demonstrate adequacy of used anchor foundation. Rock anchor shall be high strength deformed rods, centered and grouted in holes whose diameters are at least twice the diameter of the rods.

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The uplift capacity of the anchor foundation shall be equated to:

8.5

•

The yield strength of the anchor rod.

•

In sound homogenous rock to the shear resistance of an inverted cone surface of the rock with its apex at the bottom of the anchor contained with an angle of 90° to which is added the weight of the cone of rock thus described.

•

In fissured rock only the weight of a 30° cone of rock shall be considered resisting uplift.

•

The rock to grout bond resistance based on an ultimate rock to grout strength equal to 10% of unconfined compressive strength of the rock or grout whichever is smaller. The minimum grouted length rod shall not be less than 40 times the rod diameter in sound ledge rock but in no case less than 1.5 m, while in fissured rock the minimum length shall be 3 m.

Pad & Chimney Foundation (PA) This type of foundation shall be used wherever possible. The foundation shall be designed with the centreline of the chimney having the same batter as the tower leg and intersecting the footing at its centroid. The pad & chimney crossection shall be of cylindrical, rectangular or square shape. In order to achieve maximum uplift resistance the footing slab shall be preferably engaged into the undisturbed soil by undercutting at the base level, otherwise disturbed soil parameters shall be used. The minimum depth of foundation shall be 1.8m. The minimum size of the chimney shall be 600 mm diameter in circular cross-section OR 0.3 m2 area in rectangular and square cross section with a minimum dimension of 550 mm. The tower stub shall be extended into the main concrete pad and encased in reinforced concrete which shall extend without joints from the pad to a specified height above ground level. All steel work below ground level shall be firmly keyed, grouted and designed to withstand the loads due to the specified condition. Adhesion between the galvanised stubs and pad and chimney foundations shall not be entirely relied upon for transmitting load to the foundations, stubs shall be provided with bolted on cleats which shall comply with the requirements of Technical Data Schedule and shall be capable of supporting 60 percent of the ultimate uplift or compressive load. In the case of foundations employing short stubs eg. rock or caission foundations, the cleats shall be capable of supporting 100 percent of the ultimate uplift or compressive loads.

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Subject to the agreement of SEC in each location, concrete pad and chimney foundations as stated earlier in this clause will generally be employed with undercut excavations. At tower sites where soil conditions will permit a satisfactory undercut without risk of collapse, the base of the excavation walls shall be so undercut. The undercut shall extend to a minimum of 300 mm outside the walls of the excavation which shall be vertical. The outer edge of the undercut shall also be vertical for a minimum of l00 mm and the upper surface sloped to an angle of 50° from the horizontal. All undercutting of excavations for foundations as defined shall be carried out by hand tools and the use of excavators will not be permitted. Care shall be taken to ensure that inclusions of foreign matter do not occur during concreting and the base and sides of the excavation shall be lined with waterproof paper to prevent the migration of cement and fine aggregate. The passive pressure shall be neglected in relation to horizontal forces on pad and chimney foundation for purposes of design. 8.6

Auger Foundation (AU) Where ground conditions permit, the use of auger foundation will be permitted, subject to the satisfactory type test of the proposed auger foundation. The auger foundation shall be undercut type. The foundation design shall be based on uplift frustum principles with the frustum developed from the edge of the undercut. To ensure that the design undercut is properly filled with homogenous concrete, a parallel section of shaft shall be cut for a length of 100 mm below the outer edge of the undercut. The foundation shall be of steel reinforced concrete construction. Lateral loads shall be resisted by lateral earth pressure.

8.7

Grillage Foundation (GR) Grillage foundation may be used for tower Type A, B, C, G & H only. The steel grillage shall consist of angle members suitably connected to the stubs with the flat surfaces bedded on a level layer of sand or fine gravel of not less than 15mm in thickness to ensure that the grillage shall be level and that all loads shall be distributed uniformly on the surrounding earth If the grillage consists of an open grill the spaces between the bars shall not exceed the width of a bar. The depth of excavations shall be carefully taken out to the proper level. Pyramidal grillages shall have their centroid in line with the tower legs. The net area of the bases of steel grillages shall be used for bearing calculation.

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The design of steel grillages shall be in accordance with allowable unit stresses and design formulation used for tower design. As an allowance for corrosion grillage member thicknesses shall be increased by 2 mm on all surfaces over the design thickness. Minimum bolt diameter shall be 16 mm. The use of steel grillages will not be permitted in highly corrosive soils, below the highest ground water level. Backfilling in sandy soil shall be compacted to 85% relative density, while in conhesive soil shall be compacted to 95%. Filling an over-excavated hole with earth or hard filling will not be permitted and in this event the level shall be made up with a concrete pad not less than 75 mm thick at no extra charge. All steelwork for grillage foundation shall be galvanised and shall be treated with two coats of bitumastic paint (or other approved protective coatings) and carried up the steelwork for 300 mm above ground level.

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9.0

TES-P-122.06, Rev. 0

DESIGN PROCEDURES 9.1

Procedure for Drilled Pier Foundations In general, drilled shaft foundations are applicable to the, three major types of transmission structures, that is, latticed towers, H-structures (framed, pinned, and braced), and single shafts (monopoles). For single-shaft structures, both longitudinal and transverse loads and their resultant overturning moments are resisted by the lateral interaction of the drilled shaft foundation with the materials in which it is embedded. The same is true for transverse loads on pinned H-structures and for longitudinal loads in pinned, framed, and braced Hstructures. However, for framed and braced H-structures, the transverse overturning moments are resisted primarily by axial loads in the drilled shaft foundation. Both transverse and longitudinal loads on latticed towers are resisted primarily by axial loads in the shafts, although the shafts will also be subjected to lateral ground-line shears. Figure 9.1 illustrates the loads applied to the three types of structures and the loads transmitted to their foundations. Drilled concrete shafts are applicable to all three structure types, but are particularly appropriate for single shaft structures where high overturning moments are anticipated. For latticed towers, both straight shaft and belled shafts are commonly used. The drilled shafts can be installed vertically or on a batter that has the same true slope as the leg, as shown in Figure 9.2. Where the shafts are installed with the true leg batter, the shaft shear load is greatly reduced. For H-structures and single-pole structures, the shafts are normally constructed vertically. Direct embedment foundations are applicable to single-pole structures and H-structures. They are not applicable to latticed towers. The uplift capacity of directly embedded foundations is related to the quality of the backfill and the adhesive and frictional forces that can be mobilized at the pole-backfill interface. Significant end-bearing on hollow poles can only be achieved if the pole base is closed with a base plate. Additional bearing capacity can be obtained by installing bearing plates or similar base expanding devices. Precast-prestressed, hollow concrete shafts are applicable where large overturning moments are to be resisted, as in the case of single-pole structures. They may also be used in H-structures and latticed towers. Analysis techniques for drilled shaft foundations subjected to various loading modes are described in Reference 11f.

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9.2

TES-P-122.06, Rev. 0

Procedure for Driven Pile Foundations A pile is a structural element that is used to transmit loads through soft soils to denser underlying soils or rock. The pile is normally installed by top driving with a pile hammer. Piles provide high axial load capacity and relatively low shear or bending moment capacity. Therefore, pile foundations are normally used more often for latticed towers, which have low shear and high axial loads, than for framed structures or single-shaft structures, which have high moment and shear loads. Driven piles will be analyzed as described in Reference 11f.

9.3

Procedure for Anchor Foundations Anchors are used to permanently support guyed structures, as well as to temporarily support other structure types during erection and stringing. The legs of latticed towers can be anchored directly by rock anchors or helix type anchors. The uplift capacity of spread foundations may be increased through the use of anchors as shown in Figure 9.4. Guys and anchors are also extensively used to terminate wire loads on wood structures and to increase wood structure capacity for high transverse loading. At intermediate structure locations, guy and anchors may be utilized to provide additional longitudinal strength. Anchors can be used to increase the load capacity of existing foundations. Anchors will be analyzed as described in Reference 11f.

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10.0

TES-P-122.06, Rev. 0

LOAD TESTS 10.1

General 10.1.1 Purposes of Load Tests Transmission line structure foundations are load-tested for the following reasons: a)

Verification of the foundation design for a specific transmission line.

b)

Verification of the adequacy of a foundation after construction.

c)

Assistance in research investigations.

Load tests conducted as part of a foundation investigation for a particular transmission line help the engineer determine the most cost-effective foundation for support of transmission line structures. These tests are performed after the preliminary subsurface investigation of the right-of-way and prior to the final design of the foundations. Load tests conducted as a check on the adequacy of a foundation after construction verify that the foundation can withstand a particular load. These tests are performed routinely on grouted soil or rock anchors to ensure their capacity. It may be necessary to load test existing foundations if higher loads are proposed, for example, as a result of reconductoring. Load tests may be conducted on transmission structure foundations to improve general knowledge of foundation behavior. Results of these research studies lead to improved transmission structure foundation design methods and, in the long term, help reduce foundation costs. Many load tests have been performed in such a manner that the results are of little value to the engineering profession. For example, the literature contains many examples of load test results that do not include an accurate and complete description of the soil or rock in which the load tests were performed. This section is intended to guide engineers to develop testing programs that provide a sufficient quantity and quality of information to make the tests more useful to the individual engineer and to the engineering profession in general.

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10.1.2 Benefits In general information provided by load tests reduces the uncertainties inherent in the design of foundations, resulting in a more economical and safe design. A load test program shall be justified by a cost/benefit analysis, that is, the expected cost of the load test program shall be weighed against the potential benefits of the information obtained from the load tests. Examples of the benefits of load tests are: a.

When a large number of Foundations Number of foundations is to be constructed, the cost of a load test program may be relatively small when compared to the foundation cost savings that might result from the load test information.

b.

Subsurface Conditions Uncertain soil/rock properties result in uncertainties in determining foundation behavior. One accurate way to determine foundation behavior in a particular soil type is to perform full-scale load tests. Results of load tests performed in one soil type may allow the efficient design of foundations in similar soil types.

c.

Design Methods It may be cost effective to verify the validity of an existing, modified, or new design method. For a particular foundation type, whether it is conventional or unique, there may be several design methods that seem applicable, but result in widely different foundation dimensions. Foundation load test results can lead to the selection of the appropriate method. When a new or modified design method is proposed that would allow a reduction in foundation costs, it is prudent to verify the validity of the method with load tests.

d.

Construction Techniques The construction technique used to build a specific foundation may have a major effect on the behavior of the foundation. For example, the degree of disturbance of the natural soil around a drilled shaft foundation is a significant factor in design. It may not be possible to know in advance to what degree a particular construction technique will disturb certain subsoil. Foundations constructed using several techniques could be tested to determine the actual effect of each construction technique.

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e.

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Structural Design Foundation load tests may be performed to determine if a cost-efficient structural design procedure can be used. It is possible that these tests could be done in conjunction with any of the above.

10.1.3 Types of Load Tests Load tests can be classified on the basis of the type of load applied. Generally, the load types are as follows: a) b) c)

Uplift Compression Lateral Load

d) e)

Overturning Torsional

The design engineer should decide whether to: 1) apply one type of load to the test foundations making it easier to interpret foundation response to loading; 2) apply several types of loads simultaneously simulating actual load conditions, but making interpretation of the foundation response more difficult, or 3) set up the test program employing combinations of the load application scenarios given above. 10.2

Instrumentation The type of instrumentation required will depend on the data to be obtained to meet the needs of the test program. As a minimum, loads applied to the foundation and movements of the foundation shall be measured. The necessity for measuring other parameters such as stresses in the soil and foundation, movements of soil and rock, or both, in the zone of influence of the foundation, and pore water pressures in the soil near the foundation should be evaluated. Selection of the proper instruments to obtain the desired measurements shall be done by a qualified engineer who is fully aware of the advantages and disadvantages of available instruments. A thorough inventory of geotechnical instruments to measure load, deformation, earth pressure, pore pressure, and temperature has been complied by others. Seldom will one manufacturer have all of the instruments best suited to the test program. The fundamental attributes of an instrumentation system have been well documented. A well planned instrumentation system shall consider the following:

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10.2.1 The variables to be measured The measurements made during load tests are, in the order of their importanceloads, displacements, stresses and pore-water pressures. 10.2.2 The Physical Phenomenon employed in the measuring system The technique by which a measurement is made will have an influence on the attributes that follow. 10.2.3 Durability The intrinsic ability of the instrument to survive in its environment - resistance to impact, prolonged submergence, corrosive substances, temperature variations and others. 10.2.4 Sensitivity The smallest significant change in the variable being measured that the instrument will detect. 10.2.5 Response Time The ability of the measuring system to detect rapid changes in the value of the variable being measured. This is very important in dynamic measurements and in pore pressure measurements. 10.2.6 Range The difference between the maximum and minimum quantities that can be measured by a particular instrument without undergoing any alteration. 10.2.7 Reliability The ability of an instrument to retain its specified measuring capabilities with time. 10.2.8 Environmental Calibration In many cases the presence of an instrument alters the behavior of the soil or rock in the vicinity of the instrument. The environmental calibration is the relationship between the real measurement and ideal measurement where the ideal measurement is the value the measured variable would have had if the instrument were not present.

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10.2.9 Accuracy Accuracy can be defined as the tolerance of the instrument, tolerance being the value added to or subtracted from a particular reading such that the resulting computed range of readings bounds the actual value of the variable. 10.2.10 Data Reliability The ability to check for erroneous readings by comparison with a separate instrument installed in a similar position or the ability to recalibrate in-situ and check a reference or zero reading. Generally, the best instruments for field use are those that are of a simple, basic design and, hence, are reliable. When new or innovative instruments are used, it is prudent to have reliable back-up instruments until the new instruments have proven themselves. Elaborate instrumentation programs have often failed to produce useful results because of the use of unsuitable instruments installed and operated by unskilled personnel. Attention to detail in the installation of the instruments is of utmost importance. The process of installation and in-situ calibration shall be reviewed well in advance of installation. Problems during installation should be anticipated and contingency plans developed to cope with the problems. When long term tests are to be performed, stable reference points are usually required for monitoring vertical or horizontal movements, or both. The reference points shall be founded outside the expected zone of influence of the foundation. 10.3

Scope of Test Program 10.3.1 Literature Review The first step toward a successful load test is a review of the literature, including ASTM standards, to determine how tests have been performed in the past. Past load test results may give an indication of expected movements and stresses of foundations under loads similar to those proposed for the test program. When reviewing load test literature, some of the important questions to consider are the following:

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a)

What foundation type was tested and how does it compare to the proposed test foundation?

b)

How was the foundation constructed?


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c)

What type and magnitude of loads were applied and how were the loads measured?

d)

What were the subsurface conditions at the test site?

e)

What parameters were measured and what instruments were used to measure them?

f)

What was the reliability of the instruments?

g)

What were the values of the measured parameters and how do they compare to predicted values?

h)

What were the conclusions of the test program and are they reasonable?

i)

Is there enough information to draw your own conclusions?

10.3.2 Development of Field Testing Program The major elements to consider in developing a field testing program are listed below:

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a)

Foundation Types to be tested: The foundation types to be tested will depend on which foundations are most promising for supporting the proposed design loads in the subsurface conditions at the structure locations. One or several foundation types can be tested. The foundation(s) may be conventional or unique, designed by established, modified, or new techniques.

b)

Location of Test Sites: Selecting proper sites for testing is of extreme importance. The main goal here is to choose site(s) having subsurface conditions representative of those that are expected to be encountered along the proposed transmission line corridor. If subsurface conditions vary considerably on the right-of-way, the engineer should weigh the benefits of conducting tests in each of the subsurface conditions. Access to the site(s) should be as easy as possible, and if more than one test is to be performed at a particular site, adequate space should be available to allow sufficient distance between individual tests to eliminate influence of one test on another.

c)

Number of Test Foundations: The number of foundations to be tested should be determined by a cost/ benefit analysis. The number required is related to the selection of test sites.


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d)

TES-P-122.06, Rev. 0

Additional Geotechnical Investigations: The data obtained from the test program will be of value to the profession only if the subsurface and soil/rock properties are defined thoroughly. The soil / rock properties at each test site should be known with sufficient accuracy to interpret the test results. Commonly, the preliminary subsurface exploration will provide the index properties of the soil or rock, or both, along the right-of-way. In order to permit adequate evaluation of the test results, test site require a thorough geotechnical investigation. In general, undistributed soil sample should be obtained from the immediate test site. Complete soil descriptions based on the Unified Soil Classification system should be made an appropriate index property test performed on all samples. Engineering properties such as soil modulus should be made. Not only will this subsurface information be important to the interpretation of the test results, it will also allow other engineers to assimilate the results with their own experience.

TESP12206R0/JQA

e)

Type of Test to Perform: The types of tests that may be performed are given in section 10.1.3. The test types required should be based on the expected combination of loads to be applied to the transmission line foundations as installed. Generally, much more information is obtained if the foundation can be loaded to failure.

f)

Construction Techniques: The method and materials used to construct test foundation should be the same as those anticipated to be used to construct the production foundations. Some test programs center around the use of various construction techniques to determine the one best suited for the constructing a larger number of foundations. In this case, each technique employed for the test program should be capable of being repeated for the construction of the foundations on the project.

g)

Instrumentation: Deciding on the number and type of instrument to use and the appropriate locations of the instruments is a critical step in the tests program. The engineer should determine what the critical parameters reflecting foundation behavior are and select instruments to measure these parameters. The instruments should provide sufficient information on the actual foundation behavior to allow comparisons to be made with predicted behavior.


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In designing the instrumentation system, it is helpful to anticipate the data that will be obtained and to try to draw conclusions from use of these data. This rehearsal often reveals areas of the foundation that are under-instrumented or over-instrumented. This would lead to a rearranging of the instruments to obtain a better end result. Excavations for construction of the test foundations are helpful in accurately determining the subsurface conditions at the test site. The subsurface conditions revealed during construction operations shall be described in detail. Photographs of the construction operations and subsurface conditions should be taken frequently. Care shall be taken to protect vulnerable instrument parts; electrical wires shall be tied down to prevent them from being pulled accidentally during construction operations. Instruments shall be monitored often during the construction phase. Initial noload readings on instruments shall be taken in the field after sufficient time has elapsed for the instruments to adjust to field moisture and temperature conditions. Electrical instruments shall be protected from moisture. 10.3.3 Test Performance Preferably, the test shall be conducted in good weather; if this is not possible, adequate protection for the-instruments shall be provided. The accuracy of the instrumentation system shall be judged on the day of the test; some instruments perform poorly in inclement weather. Before any loads are applied to the foundation, a set of zero or no load readings shall be taken on all instruments. Electrical readout instruments usually require a warm-up time to obtain stable readings. The loading and unloading schedule shall be established in advance of the test. The number of loading and unloading cycles depends on the requirements of the test program. The loads shall be applied in increments and readings of the instruments taken during each increment. The criteria of proceeding to the next load increment shall be established. This is usually done by plotting, during the test, displacement of the foundation as a function of time for a given load. If, in the opinion of the test engineer, displacements with time become insignificant then the next load is applied. A plot of load versus displacement shall be made as the test progresses to obtain immediate indications of the foundation behavior under load.

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It is important to have good communication between the personnel applying the loads to the foundation, the personnel taking readings of instruments, and the test supervisor. If the instrument readings indicate an unsafe situation, the personnel taking readings must be able to direct the loads to be dropped immediately. Loads shall be applied to the foundation only by order of the test supervisor. This requirement is to ensure safety and to enable instruments to be read on schedule. Loads applied to test foundations for transmission structures can be very large. Therefore, it is absolutely necessary to proceed with caution and provide safety to all. When the performance of a load test requires unusual or difficult timing in applying loads and reading instruments, it is recommended that a mock test be perform to familiarize the test personnel with the required procedures. It is recommended that photographs be taken during the test for documentation purposes. Some test programs will require post-test excavations to inspect the foundation and the mode of failure in the surrounding soil or rock or both. These excavations shall be well-planned so that information critical to the investigation will not be inadvertently destroyed by the excavator. 10.3.4 Analysis and Documentation Analysis of test results can be divided into two parts a)

Those performed while the test is in progress; and

b)

Those performed after completion of the test.

Analysis performed while the test is in progress gives an immediate indication of the behavior of the foundation and allows a better control of the test program. For example, in a static load tests, the time required for sustaining each load increment can be judged by a displacement versus time plot made in the field while the foundation is under a particular load. Usually, the next load increment is applied after a certain time rate of displacement for the foundation has been reached. Applying the next load too soon may cause the load-versus-displacement curve to be erroneous. Plotting measurements in the field can help to point out anomalous readings. These readings can be double-checked to determine if a simple error has occurred or to possibly verify the reading. TESP12206R0/JQA


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If the actual transmission structure is used to apply loads to the test foundation, the engineer shall consider instrumenting the structure to better understand its behavior under actual load conditions. The decision to instrument the structure shall be based on a cost/benefit analysis in the same manner as the foundation test program. Some instrument readings may give an indication of impending failure of a structural member of the test setup. These instruments shall be monitored frequently and the readings analyzed to determine if it is safe to continue the test. It is helpful in analyzing data to put it in a figurative form. For example, a table of lateral displacement values along the length of a caisson tends to be difficult to interpret, whereas a figure showing displacement profiles at each load increment gives a good indication of the lateral displacement behavior of the caisson. Visually depicting the data obtained during the test helps to identify trends in the foundation behavior and allows other engineers to quickly grasp the essential elements of the test. The results of the tests shall be interpreted in a manner which satisfies the requirements of the test program. Some tests will require only a simple determination of whether a foundation moved less than an allowable value under the maximum design load. Others will require analysis to arrive at a new method of designing a particular foundation. The analysis shall consider the actual subsurface conditions at the test site, including additional subsurface information obtained during excavation for the foundation. The behavior of the foundation predicted by widely used analytical methods shall be compared to the actual behavior of the foundation determined on the basis of test results. This comparison shall give an indication of the adequacy of a particular design method for the foundation type and subsurface conditions at the test site. The analysis shall take into account the recent weather history of the test area that is, wet, dry, or frozen ground. When extrapolating the results of load tests to the design of actual foundations on the line, it must be realized that subsurface conditions will not be known at the actual structure sites to the degree of accuracy that they are known at test sites. Also, construction control at structure sites will probably be much less strict than at the test sites. The engineer has the option to add a degree of conservatism in the design of foundations to account for the variability of subsurface conditions and probable variances in construction technique.

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In foundation engineering, the accumulation of experience from full-scale load tests is an extremely important asset. However, unless the experiences gained from individual load tests can be summarized in such a manner that they can be assimilated readily, they lose their value to the engineering profession. One important aspect of making test results readily assimilated is to present complete and accurate subsurface information. The test report shall be presented such that an engineer unfamiliar with the test can easily follow the procedures and the behavior of the foundation and surrounding ground. The technique used to construct and test the foundation should be fully explained.

Figure 5.1 TYPICAL LOADS ACTING ON LATTICED TOWER FOUNDATIONS

HORIZONTAL LOADS (TRANSVERSE AND LONGITUDINAL COMPONENTS) VERTICAL LOADS

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Figure 5.2 TYPICAL LOADS ACTING ON FOUNDATIONS FOR SINGLE SHAFT STRUCTURES

OVERTURNING MOMENTS

HORIZONTAL LOADS

TORSIONAL MOMENTS

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VERTICAL LOADS


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Figure 5.3 TYPICAL LOADS ACTING ON FOUNDATIONS FOR FOUR-LEGGED FRAMED STRUCTURES VERTICAL LOADS HORIZONTAL LOADS MOMENTS

TORSIONAL MOMENTS

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TES-P-122.06, Rev. 0

Figure 5.4 TYPICAL LOADS ACTING ON FOUNDATIONS FOR TWO-LEGGED FRAMED STRUCTURES MOMENTS

VERTICAL LOADS

TORSION

HORIZONTAL LOADS

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Figure 5.5 TYPICAL H-FRAME STRUCTURES

UNBRACED

BRACED

GUYS

INTERNALLY GUYED

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Figure 5.6 SINGLE-SHAFTS, EXTERNALLY GUYED STRUCTURE

GUYS

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Figure 5.7 EXTERNALLY GUYED LATTICED TOWER GUYS

GUYS

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Figure 9.1 LOADS APPLIED TO TRANSMISSION STRUCTURES AND THEIR FOUNDATIONS HORIZONTAL WIND FORCE COMPONENT

DEAD LOAD

DEAD LOAD

UPLIFT ON FOUNDATION

HORIZONTAL WIND FORCE COMPONENT FOUNDATION

DOWNWARD FORCE ON FOUNDATION

VERTICAL DOWNWARD LOAD ON FOUNDATION

SHEAR FORCE ON FOUNDATION a.) Lattice Tower (Four-Legged) Structures

PINNE

MOMENT ACTING ON FOUNDATION

SHEAR FORCE ON FOUNDATION

b.) Single Shaft Structures

FIXED CABLE OR X-BRACE

c.) H - Structures TESP12206R0/JQA


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Figure 9.2 DRILLED CONCRETE SHAFT ORIENTATION

STRAIGHT SHAFT

B

B BELL B

B TOWER LEG LACING A

A

BELL

BATTERED SHAFT

(a) Plan

A

A

LACING TU

LACING TU

TRUE LEG SLOPE TRUE LEG SLOPE

BELL BELL

(b) Section A-A (Battered Shaft)

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(c) Section B-B (Straight Shaft)


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Figure 9.3 TYPICAL PLATE ANCHOR Compacted Backfill

Excavation for Log

Narrow Trench for Rod Narrow Trench to Install Anchor Rod Excavated Trench for Log Installation

SIDE VIEW

TOP VIEW

(a) Log Anchor Rod

Curved Plate

Backfill Driven Rod or Augered Hole for Rod

Augered Hole for Plate

SIDE VIEW

Slotted Hole

PLATE DETAIL (b) Nevercreep Anchor

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Figure 9.4 TYPICAL ANCHORED SPREAD FOUNDATIONS

Grouted Zone Pre-stressed Tendon

PRE-STRESSED ANCHOR FOUNDATION (To resist Overturning Moments of Uplift)

ROCK ANCHOR FOUNDATION (To resist Uplift)

SOIL ANCHOR FOUNDATION (To resist Uplift)

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TES-P-122.06, Rev. 0

Figure 10.1 TEST SETUPS FOR MOMENT AND SHEAR LOADS DYNAMOMETER HYDRAULIC LOAD - DEVICE CABLE

EARTH ANCHO

FOUNDATION

24 in SQUARE 20 TON HOLLOW RAM JACK

1 ½ in DIAMETER PIPE SLEEVE

4 ft 6 in

12 in X 12 in JACK TIMBER STRUT

10 TON JACK

FIN GRADE

6 ft

15 ft

10 ft

15 ft 2 ft DIAMETER REINFORCE CONCRETE PILES

WIRE ROPES

DYNAMOMETER WINCHES

GUY

8 ft X 17 ft CAISSON TESP12206R0/JQA

SCREW ANCHOR


SCREW ANCHOR

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Figure 10.2 TEST SETUPS FOR UPLIFT LOADS NOT LESS THAN 8 FT. CROSS BEAM

HYDRAULIC

STEEL STRAP BEARING PLATES

REACTION BEAMS

TIMBER CRIB MAT OR REACTION PILES TEST PILE

TEST PILE

END VIEW

SIDE VIEW

NOT LESS THAN 8 FT. JACKING COLLAR

BEARING PLATES CROSS BEAMS

BEARING PLATES

HYDRAULIC

TIMBER CRIB MAT OR REACTION PILES TEST PILE TEST PILE

SIDE VIEW

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END VIEW


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Figure 10.3 TEST SETUPS FOR COMPRESSION LOADS TEST BEAM HYDRAULIC JACK RAM TEST PILE

STEEL PLATE DIAL GAGES

TEST PILE

REFERENCE BEAM

ANCHOR PILE

WEIGHTED BOX OR PLATFORM CROSS BEAM TIMBER CRIBBING

STEEL PLATE REFERENCE BEAM

DIAL GAGES TEST PILE

BEARING PLATES

STEEL BEAMS

TEST BEAM HYDRAULIC JACK RAM

TEST PLATE

HYDRAULIC JACK RAM TEST BEAM

DIAL

TEST PLATE

CONCRETE PILE CAP

REFERENCE BEAM

ANCHOR PILES

TEST PILE GROUP

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11.0

TES-P-122.06, Rev. 0

REFERENCES

Title

Author (Manufacturer Agency, etc) Bureau of Reclamation (U.S. Department of the Interior)

Design Standards No. 10

b) Foundation Design M. J. Tomlinson Book and Construction

M. J. Tomlinson

Book

c) Line Manual

Rural Electrification Administration (U.S. Department of Agriculture)

REA Bulletin 62-1

d) Soil Mechanics, Foundations, and Earth

Naval Facilities Engineering Command (Department of the Navy)

Design Manual NAVFC DM-7, March 1971.

e) The Ultimate Resistance of Rigid Piles Against Transversal Forces

J. Brinch Hansen

Danish Geotechnical Institute, Bulletin No. 12

f) IEEE Trial-Use Guide IEEE for transmission Structure Foundation Design g) Foundation Analysis and Design

IEEE

IEEE Std. 691

a) Concrete Footing Design and Details in Transmission Structures

TESP12206R0/JQA

Joseph E. Bowles


Publication

Book

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TESP12206R0-Transmission Structure Foundations

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