Soil Nailing - Where When and Why

SOIL NAILING: WHERE, WHEN AND WHY A PRACTICAL GUIDE By

Thomas J. Tuozzolo, P.E. Vice President, Moretrench

Presented at the 20 th Central Pennsylvania Geotechnical Conference Hershey, PA, 2003

SOIL NAILING: WHERE, WHEN AND WHY – A PRACTICAL GUIDE

1

Thomas J. Tuozzolo, P.E., M.ASCE

ABSTRACT

Since its development in Europe in the early 1970s, soil nailing has become a widely accepted method of providing temporary and permanent earth support, underpinning and slope stabilization on many civil projects in the United States. In the early years, soil nailing was typically performed only on projects where specialty geotechnical contractors offered it as an alternate to other, conventional systems. More recently, soil nailing has been specified as the system of choice due to its overall acceptance and effectiveness. However, although the theoretical engineering aspects of soil nailing may be well understood, there is a far lesser degree of understanding, even within the geotechnical community, as to the site conditions – where, when and why – under which soil nailing should, and should not, be used. The purpose of this  paper, therefore, is to offer experienced-based guidelines to owners, engineers, designers and general contractors trying to decide if soil nailing is the right system for their project. Typical soil nail details, procedures, design, monitoring and testing considerations, and case studies are  presented as a tool to aid in making those decisions. INTRODUCTION

Soil nailing is a method of providing temporary earth support and retention during excavation for new construction. The technique is also used for construction of permanent retaining walls, slope stabilization, underpinning, and protection of existing cuts (Tuozzolo, 1997). The soil nailing concept was developed in Europe for the permanent and temporary stabilization of natural slopes, for renovation of old retaining walls, and for repair of earth walls that had  prematurely deteriorated. The Th e first recorded application was in France in 1972. Soil nailing was also used as temporary shoring for basement excavations and as permanent and temporary earth support for excavations associated with railroads and tunnels tunnels (Chassie, 1993). 1

 Vice President, Moretrench American Corporation, 100 Stickle Avenue, Rockaway, NJ 07866. Tel: 973.627.2100. [email protected]

1

The first recorded use of soil nailing in the United States was in the mid 1970s as a method of  providing temporary earth support during the construction of a new hospital (Chassie, 1993). The major use of soil nailing in the United States to date has been for temporary excavation support and earth retention during construction of buildings and other structures in urban areas. FUNDAMANTAL CONCEPTS

Soil nailing is an economical, top-down construction technique that increases the overall shear strength of unsupported soils in situ through the installation of closely spaced reinforcing bars (nails) into the soil/rock. Typically, a structural concrete facing (shotcrete) is sprayed against the excavated earth face to connect the nails and reduce deterioration and sloughing. A common misconception is that this structural facing is the major element of the soil nail system. In fact, the nails do the work. Soil nails are passive elements and, unlike tieback anchors, they are typically not mechanically  pre-tensioned after installation. Rather, they become forced in tension when the soil they are supporting deforms laterally as the depth of the excavation increases. Without soil movement, the nails remain in a passive state. This is a fundamental difference between soil nails and tieback anchors, and one that is commonly not examined closely enough or considered during determination of proper use and monitoring. The movements required to mobilize the nail forces are very small and generally correspond to the movements that occurs in a braced system (Chassie, 1993). In some cases, specifications require that the soil nails be tensioned and locked off, as a tieback would. This is not correct. Some specifications even require production soil nails to have a free, or unbonded, length. This is also not appropriate. In some cases, a small pre-tensioned load may be applied to limit the amount of deflection of the soil that is required to mobilize the nail. However, a major posttensioning effort defeats the purpose and effectiveness of the nail. CONSTRUCTION METHODS

Soil nailing is a specialty geotechnical technique. Owners, specifiers and general contractors should ensure that the work is designed and constructed only by firms with the requisite experience.

2

Typical Construction Sequencing and Components •

 Excavate in Lifts

The execution of soil nailing consists of making a 1.2 to 1.8 m (4 to 6 ft) vertical cut extending for a horizontal length to be stabilized and shotcreted the same day. The vertical cut may need to be reduced or other measures may have to be considered if the face stability of the soils is of concern. Typically, the cut is made to 0.3 to 0.8 m (1 to 2.5 ft)  below the elevation of the soil nails so that a suitable bench can be established for the installation of the nails at the proper angle. Usually, mass excavation is performed on the project with relatively large earth moving equipment. Smaller equipment may have to be utilized to provide the necessary face trimming prior to installation of the soil nail wall. It is important that excavation of lifts is  performed in a controlled manner so that over-excavation of the face, and sloughing of the soils, does not occur. After the lift has been excavated to the proper elevation, a level work area, or bench, is constructed in front of the wall to accommodate the drilling equipment. Depending on the type of equipment used, this bench can be as small as 3 m (10 ft) or as large as 12 m (40 ft), although 7.5 m (25 ft) is usual. •

 Install Nail

Typically, the next step is to install the soil nails.  It is important to note that the shotcrete can be applied prior to installing the nails if there is a concern with the standup time of the soil and the possibility of sloughing of the soil. In the United States, common practice

is to use rotary and rotary/percussion drill rigs for the installation of the nail, although it is quite common to utilize augers in cohesive soils. In Europe, some contractors utilize machines to drive the soil nails into the ground (launched nails). Soil nails are typically installed on a 1.2 to 1.8 m (4 to 6 ft) horizontal and vertical grid pattern, with the drill hole inclination being 10 to 20 degrees from horizontal. The holes are either cased or uncased, depending on the type of soil and are, on average, 100 to 200 mm (4 to 8 in) in diameter. Once the hole is drilled, the nail is installed and the hole is tremie-grouted with either type I, II, or III Portland cement and water. The water/cement ratio is normally on the order of 0.45 to 0.50. In the United States, it is common to use No. 25 to No. 36 (No. 8 to No 11),

3

Grade 400 or 500 (Grade 60 or 75) threaded bar as the nail. The bar is centered in the drill hole by means of plastic centralizers spaced every 3 m (10 ft) along the length of the bar, as shown in Figure 1. The length of the bar is based on the design, but a common rule of thumb is that the length of the first level of nails is typically 0.6 to 1 times the overall cut height. Soil-to-grout bond values are used in the design to determine the final length of the nail and the nail spacing.

1 ft = 0.3048 m 1 in = 25.4 mm

Figure 1: Typical Temporary Soil Nail Cross-section •

Place Reinforcing & Drainage

Once the excavation is made and the soil nails are installed, reinforcing material, typically a welded wire mesh, may need to be placed along the face of the excavation to reinforce the concrete facing. It is not uncommon, however, to use reinforcing bars for the length of the wall in lieu of the wire mesh. As an alternate to placing the reinforcing prior to shotcreting, the reinforcing material can actually be added to the ready-mix concrete at the  plant. This is known in the industry as fiber-reinforced concrete. Some common types of reinforcing material are steel or synthetic fibers. When the bearing plate and nut are installed and the nail becomes loaded as the soil deforms, this load is transmitted to the shotcrete facing. Because of this, it is important to ensure that the facing can handle the punching shear induced from the nail load. It may be necessary to strengthen this area directly behind the bearing plate with reinforcing. A typical punching shear detail is shown in Figure 2.

4

1 in = 25.4 mm

Figure 2: Typical Punching Shear Detail If drainage behind the soil nail wall is a concern, a prefabricated geotextile drain mat, typically 0.6 m (24 in) wide, can be applied in strips to the excavated earth face of the wall between the nails prior to applying the shotcrete facing (Figure 3). In most cases, drainage material is not installed for temporary soil nail walls unless drainage is a major concern. For permanent walls, the drainage material is extended from the top of the excavation to the bottom of the excavation and tied into a drainage collection system at the bottom of the wall. If perched water is encountered during construction, weep holes and small horizontal relief drainage pipes can be installed.

1 ft = 0.3048 m

Figure 3: Soil Nail Drainage •

Shotcreting & Installing Bearing Plates

Once the reinforcing and any required drainage medium are placed, the next step is to apply the shotcrete facing. For most temporary shotcrete walls, this is accomplished by applying a 75 to 100 mm (3 to 4 in) thick layer of 21 MPa (3000 psi) concrete. In a temporary application, shotcreting is merely a method of tying the system together,

5

 providing a cover for the soil, and retaining the soil between the nails. The shotcrete can, however, become more critical if the nail spacing increases. Immediately after the shotcrete is applied, the soil nail bearing plates are installed on the fresh shotcrete facing and their nuts are hand-tightened. It is very important to make sure that the bearing plates and nuts are installed as soon as possible, but definitely before the next cut is made since this is what provides the lock off of the nail as the soil deforms. •

 Repeat Steps to Final Subgrade

After the shotcrete has been installed and the soil nails have cured for three days, the  process is repeated in lifts until the predetermined subgrade elevation is reached. Typical temporary and permanent soil nail details are presented in Figures 4 and 5.

1 in = 25.4 mm

Figure 4: Typical Temporary Soil Nail Detail

1 in = 25.4 mm Gr. 60 = Gr. 400 SI Gr. 75 = GR. 500 SI

Figure 5: Typical Permanent Soil Nail Detail

6

Permanent Facing

Although soil nailing began as a method of providing temporary earth support, it has evolved as an economical method of providing a permanent, finished wall. Since soil nailing is a very flexible system in that there is no need to make 90° corners or straight walls, it is very easy to  provide a soil nail wall with curves and bends. If the final product calls for such geometry, more than likely the permanent facing could be shotcrete (Figure 6). Once the temporary shotcrete wall is constructed, a permanent facing typically involves adding another 100 to 150 mm (4 to 6 in) of shotcrete to the existing 75 to 100 mm (3 to 4 in) of shotcrete. The permanent facing will  be reinforced and a screed face finish can be provided. In lieu of using shotcrete as the  permanent facing, there are several other choices. A one-sided concrete wall can be poured against, and attached to, the soil nail wall, or a variety of concrete block or precast panel systems can be mechanically attached to provide a wall that will blend into any retaining wall system (Figure 7).

Figure 6: Permanent Shotcrete or CIP Wall Facing Detail

1 in = 25.4 mm

Figure 7: Segmental Block Facing Detail

7

Corrosion Protection

For temporary systems, single-corrosion protection for the soil nails is normally adequate. The nail is simply a bare bar, with sufficient grout cover achieved by providing centralization of the nail in the drill hole. For permanent soil nail systems, corrosion protection has to be examined more closely. In non-aggressive soils, the nail should be provided with an epoxy or galvanized coating as per standard manufacturer specifications. In more aggressive soils, fully encapsulated nails are recommended.

Typical tieback guidelines are normally followed to determine the

appropriate corrosion protection.

WHERE, WHEN AND WHY

Soil nailing is a very versatile geotechnical construction technique with many advantages over other types of temporary and permanent retaining walls. Soil nailing can be used on numerous sites to provide an economical and fast method of temporary earth support, underpinning and stabilization. However, there are many factors to be addressed when determining if soil nailing is the right system for a project, and it is very important to understand the dos and don’ts of soil nailing before it is recommended. When evaluating where soil nailing is appropriate, the following guidelines can be used: The Right Underground Conditions

As with any geotechnical technique, the underground conditions are normally the paramount consideration. A good geotechnical exploration and laboratory testing program is essential. As a minimum, this information should include soil borings, groundwater data, sieve analysis, and  plasticity testing. Whenever possible, test pits excavated with a backhoe are recommended to evaluate the stand up time of the soils prior to production work. Underground Conditions that are Conducive

In general, soil nailing can be used in any soil where a vertical cut can remain stable for at least 24 hours. These cuts should be on the order of 1.2 to 1.8 m (4 to 6 ft), which is standard practice for soil nail lifts. Cuts less than this reduce the economical advantage of soil nailing. Normally soils that have some binder in them are considered the most favorable, such as:

8

1.  Natural cohesive materials, such as silts and low plasticity clays not prone to creep. 2. Glacial till. 3. Cemented sand with little gravel. 4. Find to medium sand with silt to act as a binder. 5. Weathered rock. 6. Residual soils. Underground Conditions that are Not Conducive

The soil nailing system is not recommended if an y of the following conditions are present: 1. Urban fills and loose, natural fill material. 2. Soft, cohesive soils that will not provide a high pullout resistance. 3. Highly plastic clays, since they are susceptible to excessive creep. 4. Loose, granular soils with N-values lower than 10 to 15. 5. Soils without any apparent cohesion and with high gravel content. 6. Expansive clays. 7. The presence of groundwater. Groundwater presents a problem with maintaining face stability during the cut between lifts, and must be properly controlled in the construction of a soil nail wall. Most Effective Applications

Although soil nailing is a very fast and economical earth retention system, the most cost advantages are realized in situations where other, more conventional, techniques have limitations. Soil nailing offers the most advantages during the following applications:  Difficult Soil Conditions : In some situations, the underground conditions may not be conducive

to driving soldier piles for a conventional, tied-back, soldier pile and lagging wall. The presence of cobbles and boulders or the presence of rock above subgrade may require the soldier piles to  be installed with large, costly drilling equipment. In general, this will greatly increase the overall unit price, thus reducing the cost effectiveness of the system. However, since soil nails are typically short, small-diameter, drilled-in elements, the cost of installation generally does not significantly increase under these conditions.

9

On-line Systems: On some projects, due to space limitations, the foundation wall for the

 proposed structure needs to be within millimeters of an adjacent structure or property line. Since most conventional earth retention systems have a general width of 0.3 to 0.6 m (12 to 24 in), installation would be impossible under these circumstances. Other options, such as concrete underpinning pits can be very costly, particularly in difficult, dense soils.

In the above

situations, soil nailing can provide large savings. Since temporary soil nail systems are normally 75 to 100 mm (3 to 4 in) thick, they will not encroach into the new structure as other systems would, and since soil nail walls can be designed to withstand surcharge loads, they can be installed adjacent to existing structures.  Low Headroom Conditions/Tight Access: On some projects, earth retention may be required in

low headroom conditions or within tight access. One such situation would be the widening of an existing highway, where the new portion of the highway must be constructed while an existing, overhead bridge remains active and the overhead bridge girders remain in place. With the development of compact, low-headroom drilling equipment the soil nailing system can be more easily installed under these conditions whereas previously a much more expensive system, such as driving and splicing soldier piles or sheet piling might have been the only option. Permanent Walls: While soil nailing has become a highly effective system for retaining cuts

during construction of new structures, its use for the economic installation of permanent retaining walls is still evolving.

Since soil nail systems are already designed to withstand

temporary lateral loading, designing the wall to withstand permanent lateral and vertical loading is a rather simple process and although unit costs will increase, the overall wall cost will still be less than the cost of installing a new foundation wall. Soil nailed walls are much more “flexible” than conventional, rigid walls. Thus, they perform  better during a seismic event. These structures can conform to deformation of surrounding ground and can withstand larger total and differential settlements in all directions. Areas of Concern

In the following situations, soil nailing should be ruled out or additional measures should be taken:

10

 Loose Fill & Sandy Soil Conditions : Since soil nailing is a top-down construction technique, it is

essential that the cut face can stand up long enough to install the nails and the shotcrete facing. “Pushing the envelope” and installing soil nailing where this is not achievable is not recommended and can result in face stability failure.  Below the Water Table:. The presence of flowing groundwater will limit the stand up time of the

cut face and can also cause face stability problems. In addition, if the installation is successful there is the added consideration of hydrostatic loading on the soil nail wall and additional drainage requirements. Outside Temperature: As for any concrete application, and as per the ACI code, the shotcrete

should be applied when the ambient temperature is 4.5° C (40° F) and rising. The construction of the soil nail wall below such temperatures is only achievable with an imported heating system and should be a major consideration during evaluation of use of the soil nail system. Temporary heated enclosures and tents can be constructed to keep the temperature above 4.5° C (40° F), but this can dramatically increase the overall con struction cost of the soil nail wall. Utility Interferences and Easements: As for tieback construction, a major consideration when

specifying or choosing a system should be the location and proximity of adjacent underground utilities and adjacent property limits. Existing utilities may cause interference during installation of the nails, and easements may be required if the nails will extend onto adjacent property.  Expansive Clays: As stated previously, clays that have the potential for expansion or are

susceptible to creep should be approached cautiously. An additional monitoring and testing system should be implemented prior to production work to verify the movement of the nails and the wall during construction.  Large Surcharge Loads: Soil nail walls can be designed to withstand significant exterior

surcharge loads (traffic, equipment and building surcharges, and so forth). Nevertheless, it is important that the potential impact of these loads is understood and addressed prior to design finalization, since they can dramatically impact the overall design and the future performance of the soil nail system. Large wall deformations can result in significant settlement, and this should  be considered in the design of the soil nail wall.

11

DESIGN, MONITORING AND TESTING Design

The installation of a soil nail wall results in the construction of a composite, coherent mass similar to that of a mechanically stabilized earth (MSE) wall. Although soil nail walls and MSE walls have inherent differences in both construction and performance, they are similar in the design approach. The reinforced soil mass is separated into two zones, an active zone and a resistant zone. The active zone, or potential sliding wedge, is close to the face, where the lateral shear stresses are mobilized, thus increasing the tension force in the nail and increasing the  pullout force. The resistant zone, or steady zone, is where the nail forces are transferred into the ground. Ground movement during and at the completion of construction is restrained by the frictional bond between the soil and the reinforcing element. Therefore, the most important design consideration is to ensure that the soil nail interaction is effectively mobilized to restrain ground movements and to ensure lateral stability with the required factor of safety for the  particular project (Sakr and Kimmerling, 1995). In the early years, the most common methods used to design soil nail walls were derived from classical slip surface limiting equilibrium design methods of slope stability analysis, modified to incorporate the additional shearing, tensile and pullout resistance provided by the nail reinforcement crossing the failure surface. These methods of analysis evaluate “internal” factors of safety along potential failure surfaces throughout the soil mass.

The failure surface is

assumed to be either bi-linear, parabolic, circular, or a logarithmic spiral. The most common methods used were the German Method (Stocker et al., 1979), the Davis Method (Shen et al., 1981) and the French Method (Schlosser et al., 1983). In the United States, two other methods have been developed and are commonly used for the design of soil nail walls. These are the SNAIL method, developed by the California Department of Transportation (CALTRANS, 1991) and the GoldNail design method developed by Golder Associates of Redmond, Washington. The SNAIL method uses a bi-linear wedge analysis for failure planes existing at the toe of the wall and tri-linear forces for failure planes developing  below and beyond the wall toe. The Golder method can analyze circular failure surfaces. Both methods consider the tensile resistance of the nails crossing the failure surface. These two

12

methods are considered improvements over the other earlier methods in that they consider the limiting pull-out capacity of the nails and take into account the wall facing attached to the soil nails as part of the system. Monitoring

The FHWA’s Manual For Design and Construction Monitoring of Soil Nails (Byrne et al., 1996) estimates that for a typical, vertical soil nail wall, designed with a reasonable factor of safety, the  peak wall displacements at the top of the wall tend to vary from 0.1%H (cut height) or less for weathered rock and very competent and dense soils, to 0.2%H for granular soils, and up to 0.4%H for fine-grained, clay type soils. These displacements are comparable to those experienced by other types of retaining systems such as tieback walls, which generally average maximum lateral movements of 0.2%H (Byrne et al., 1996). As with any retention system, some type of monitoring should be conducted to determine how the system is performing or if additional measures need to be taken. The type of monitoring  program that should be implemented is directly related to the type of system being installed. For temporary systems, monitoring can be performed by simply conducting daily or weekly surveying with a typical transit and level to determine the horizontal and vertical movements that have occurred, and which allows an approximation of the deflection of the system. Typically, this type of device is accurate to 1.6 mm (0.0625 in). When permanent soil nail walls or deeper temporary walls are constructed, it is prudent to install inclinometers in addition to normal survey monitoring to measure lateral deformations. The  probe is inserted into near-vertical, grooved casing of known orientation. Typically, the grooves are set perpendicular and parallel to the object in question. Readings are taken at set intervals while the probe is being extracted. The resultant data is a set of tilt readings at known depths that can be plotted to show the overall lateral deflection of the casing and subsequent surroundings. In addition to monitoring for deflection, there are some circumstances where determination of the actual stress that the nails are experiencing may be useful and important. Such information can be obtained by using vibrating wire strain gages, small instruments used to measure strains in

13

steel structures such as bridges, piles and tunnel linings (Figure 8). Measurements are developed  by plucking the internal tensioned wire at its natural resonant frequency.

Variations in the

frequency correspond to variations in the strain, which is converted and displayed on a readout  box. This type of monitoring is costly. Thus, it is typically used only on critical structures and for research and development.

Figure 8: Strain Gage Installation Plan and Section (After FHWA-SA-96-069 Manual for Design and Construction Monitoring of Soil Nails)

Field Testing

It is also important to implement a field-testing program to verify design assumptions. Field testing also verifies that the equipment used on site is providing the assumed specified design  parameters of drill hole diameters, drill lengths, shotcrete thickness, and so forth.  Nail Testing

Testing performed on soil nails includes pre-production testing, verification testing and proof testing. The type of testing can be directly related to the nature of the soil nail wall. For instance, pre-production testing may be required only on a permanent wall to aid in the final design.

Although the type of testing may differ somewhat, there is always some type of

verification required. Nail testing is performed with a center hole hydraulic jack and power pack  pump, similar to that used in tieback testing. As for tieback testing, the load is incrementally applied as the movement of the nail is determined by using a dial gauge capable of measuring to the nearest 0.0254 mm (0.001 in).

14

•

Pre-production testing is performed on non-production or sacrificial nails.

The pre-

 production test helps to determine the ultimate adhesion capacity as it compares to the values used in the design. It is sometimes not possible to achieve ultimate pullout of the nail. •

Verification Testing should be performed on at least one nail per row on all projects.  The

verification test (pullout test) is performed on a shorter, non-production nail. The verification nail should have a short unbonded (free) length so that the nail can elongate during testing and ensure that the load is transferred to the bond zone. The bond zone is purposely made shorter so that better bond values can be achieved and possibly bond failure will occur so that the ultimate values are achieved (see Figure 13). The verification test is performed incrementally, generally to a factor of at least two times the design value. The results achieved are compared to the design, and changes in nail length or spacing can be made if required. •

Proof Testing should be performed on production nails. Typically, five percent of the total

number of production nails should be proof tested. These proof tests are not measuring elongation as a performance test of a tieback would, but merely verifying that the bond is achieved and that creep is not a concern. •

Long Term Creep Testing  can be performed in addition to proof testing when creep is a

concern. Fine-grained soils or plastic soils should be creep-tested. Deflection versus log time results are plotted on a semi-log graph and are compared against the acceptable criteria. Shotcrete Panel Testing

In order to verify the compressive strength of the in-place shotcrete for permanent wall facings, a test panel is prepared from which samples can be extracted, rather than extracted from the wall itself.

ACI 506.2-95 Specification for Shotcrete recommends ASTM C 1140-98 Standard

Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Forms are

constructed from either steel or wood having a minimum width and length of 0.6 m (24 in) and a minimum depth of 90 mm (3.5 in). The panels are prepared using the same reinforcement, equipment, mix design and shooting orientation (slabs, slopes, vertical, overhead) as the actual application. Panels are shotcreted either before or during the production phase of the shotcrete

15

wall. The panel is then cured in the field or in a moist room until ready for testing. Cores are drilled from the panel and tested in compression perpendicular to the surface of the panel. Sawed cubes and beams may also be prepared from the panel and tested either perpendicular or  parallel to the surface. Testing is done at 7 and 28 days, unless project requirements specify otherwise.

CASE STUDIES Robert Wood Johnson Hospital - Cancer Center Addition New Brunswick, New Jersey Underpinning

Construction of a Cancer Center addition to the Robert Wood Johnson Hospital, in New Brunswick, NJ, required excavation to hard rock at up to 7.6 m (25 ft) below existing grade. The 46 m by 61 m (150 ft by 200 ft) excavation site was bounded on two sides by hospital buildings and on the other two sides by an emergency access road and a busy thoroughfare. Earth retention and underpinning to protect the integrity of the structures and roadways was an integral part of the project design. Conventional Approach

A drilled-in soldier beam and lagging system, in conjunction with conventional concrete underpinning, is a common and effective approach to this type of work in the northern New Jersey area. However, evaluation of the subsurface conditions showed a soil profile of 3 m (10 ft) of residual overburden soil with good standup time, below which weathered rock extended to  bedrock. These conditions are ideally suited to soil nailing, where underpinning and excavation support can be achieved in a single operation. Value Engineered Alternate

For this site, the geotechnical contractor calculated that soil nailing would be as effective as conventional support systems, yet less expensive and faster to install. Since the new structure had to be constructed within 100 mm (4 in) of the existing hospital, the soil nail system offered both an economical method of providing the necessary underpinning and an on-line earth retention system into which the new foundation wall could be tied. The geotechnical contractor proposed

16

this value-engineered, alternative design to the excavation contractor and the construction manager for the project. After evaluating the constructability of the system, it was decided that soil nailing would be an innovative and economical solution for the site conditions.  Design and Installation

In unobstructed areas of the site, soil nails were installed at 15 degrees below horizontal on a 1.5 m (5 ft) grid pattern. However, a 0.6-m (24-in) diameter sanitary sewer that served the entire hospital complex ran directly beneath existing foundations where the soil nail system had to be constructed on-line and act as the form for the new foundation wall (Figure 9). Compromising this essential utility would effectively shut down all hospital services and lead to very large consequential damages. The geotechnical contractor investigated the precise location and elevation of the sewer and adjusted the angle of the nail installation accordingly. In addition, a very loose sand backfill, most likely used during the construction of the utility, was encountered during the excavation of the upper soil nail lifts. Since this backfill did not exhibit good standup time, the lifts were reduced to 0.6 m to 1m (2 to 3 ft).

Elevations in feet. 1 ft = 0.3048 m

Figure 9: Cross-section Showing Utility and Soil Nail Installation Three to four tiers of soil nails were installed, depending on the depth at which hard rock had  been encountered at the time of the geotechnical investigation. For each tier, a vertical lift of 1.2 to 1.5 m (4 to 5 ft) was excavated, extending for a horizontal length that could be stabilized

17

during the workday. Other than where backfill was encountered, the cut face exhibited good stand up time, allowing the soil nails to be installed first across the entire face of the cut. A geocomposite drainage strip and reinforcing wire mesh was then placed, and a 75-mm (3-in) thick shotcrete facing was applied to bridge the soil nails and complete the structural 2

2

underpinning and retention system. The 1300 m  (14,000 ft ) soil nail system was successfully completed three weeks ahead of schedule, resulting in significant savings. Route 87 & 287 at Meadow Street Tarrytown, New York  Bridge Widening

U pgrading of Route 87/287 from two to four lanes to accommodate increased traffic flow required the installation of a replacement bridge abutment approximately 1 m (3 ft) in front of the toe of the embankment supporting the existing west abutment at the Meadow Street overpass. Since the new abutment foundation would be up to 4.3 m (14 ft) deeper than the existing abutment foundation, an excavation support system would be required to support the embankment soils beneath the existing abutment during this work. The geotechnical contractor was awarded a subcontract by the general contractor for New York State DOT to design and  build the earth retention system. Soil Nail Design and Installation

The embankment soils consisted of silty glacial till with large boulders. Given the presence of these boulders, coupled with the low headroom working conditions beneath the overpass, the geotechnical contractor elected to use soil nailing to support the soils. With the active abutment  bearing directly on soil rather than deep foundations, the soil nail wall was designed to take into account the large vertical surcharge of the existing abutment as well as lateral earth pressures from the embankment behind the abutment. Three levels of soil nails were installed on a 1.4 m H by 1.5 m V (4.5 ft H by 5.0 ft V) grid to support the 4.3-m (14-ft) deep, 61-m (200-ft) long embankment (Figure 10). Given the nature of the embankment fill, the geotechnical contractor chose to first reinforce the cut face with wire mesh and a 75 to 100-mm (3 to 4 in) thick layer of 20 MPa (4000 psi) shotcrete. After the shotcrete had cured, the soil nails were drilled and grouted in place. Specialty drilling techniques

18

were required to penetrate the large boulders during soil nail installation.  Sacrificial test nails were used to verify bond values. The soil nail wall was installed in one mobilization, with no removal of overhead superstructure required.

Elevations in feet. 1 ft = 0.3048 m

Figure 10: Cross-section Showing Installed Soil Nail Wall University of Medicine & Dentistry Dental School Expansion Newark, New Jersey  Earth Retention

Expansion of the Dental School at the University of Medicine & Dentistry of New Jersey (UMDNJ) in Newark, NJ, included construction of a five-story, steel structure with a one-story,  below-grade basement. The site was bounded on one side by the existing Dental School, and on th

the opposite side by 12   Avenue, a major access road for emergency vehicles en route to the adjacent University Hospital. Parallel, underground, electric and telephone duct banks, each 1.5 th

m by 0.6 m (5 ft by 2 ft), ran along 12  Avenue. The telephone bank lay nearest the site, 1.5 m (5 ft) below existing grade and within millimeters of the proposed building. The proposed structure, approximately 85.4 m by 36.6 m (280 ft by 120 ft) in plan, entailed excavations to a depth of 8.2 m (27.0 ft) below grade. In order to facilitate these excavations, a th

temporary earth retention system was required for approximately 80.8 lm (265 lf) along 12

Avenue and for 38.0 lm (125 lf) along the west side of the site. Also, a permanent retention and underpinning system was required for 18.3 lm (60 lf) on the south side of the project, along the

19

existing, 3-story Dental School, where the new building would be constructed on-line and tied in. Preliminary excavation support system concepts included driven steel sheeting and soldier beams th

and lagging along 12  Avenue and the west side, and a permanent, on-line, soil nail wall along th

the existing Dental School. However, the proximity of the duct banks along 12   Avenue  precluded the sheeting and soldier beam options since both systems would encroach into the new  building and would require the structure be moved and redesigned. In addition, the soil borings encountered rock at or just above the proposed subgrade elevation, which could impede the driving of soldier piles or steel sheets and likely nece ssitate a more expensive, drilled-in system.  Engineered Solution

The pre-construction geotechnical investigation indicated that the subsurface conditions consisted of 1.2 m to 1.8 m (4 ft to 6 ft) of fill, underlain by natural silts and silty sands containing varying amounts of gravel, cobbles and boulders. Weathered sandstone bedrock was encountered at or above the subgrade elevation of the proposed structure. After evaluating the soil conditions, space limitations and the schedule, the geotechnical contractor offered a system th

that entailed a temporary soil nail wall along 12   Avenue and the west side of the site, and a combination of drilled-in minipiles and soil nailing along the existing Dental School.

The

minipiles would be drilled through the existing column footings to underpin the building and transfer the loads below the proposed subgrade. Soil nailing would be used to support and retain the soil between the existing columns. th

12  Avenue and West Side th

The soil nail wall along 12  Avenue and on the west side of the site entailed three to five levels of soil nails installed at 15 degrees from the horizontal on a 1.5 m (5 ft) grid pattern. A 75-mm (3-in) thick layer of shotcrete, reinforced with wire mesh, was used to tie the system together and retain the soils between the nails. Construction began with the excavating contractor making an initial, sloped precut to locate the top of the telephone duct bank and remove the existing fill soils which did not exhibit good ‘standup’ time. The precut also allowed the first level of soil nails to be installed at a 15 degree angle below the duct bank, and did not require any redesign or relocation of the proposed structure. Once the first level was installed, and the soil nails and shotcrete had cured, the process was repeated and the remaining lifts were installed until

20

th

subgrade was reached (Figure 11). A view of the soil nail wall along 12  Avenue is presented in Figure 12.

Elevations in feet. 1 ft = 0.3048 m

Figure 11: Cross-section Showing Soil Nail Wall Installation

th

Figure 12: View of Soil Nail Wall along 12  Avenue The wall was designed to withstand the temporary lateral and traffic surcharge loads. The system was also designed to withstand the surcharge loads associated with a large crane scheduled to be  placed on the sidewalk directly behind the soil nail wall, to aid in construction of the new addition. Since the soil nail wall for this part of the project was installed on-line, the new foundation wall was constructed using a one-sided form and poured directly against the soil nail wall. A drainage composite and waterproofing membrane were placed between the soil nail wall 2

2

and the new concrete wall. This 935 m  system (10,000 ft ) allowed the proposed foundation to  be built ahead of schedule and without the need for any design modifications.

21

Soil Nail Instrumentation and Monitoring

In order to evaluate design assumptions and monitor the construction, a comprehensive testing  program was implemented, consisting of field pullout tests, proof testing, surveying, and strain monitoring. Verification Testing: At least one pullout test was performed on ea ch soil nail row (lift) to verify

that adhesion values used in the design program were obtained in the field. The pullout test was  performed incrementally, generally to a factor of at least two times the design value, on a shorter sacrificial nail (Figure 13). In addition, a minimum of five percent of the production nails were  proof tested to 1.33 times the design load to measure the bond and creep values.

1 ft = 0.3048 m 1 kip = 4.45 kN

Figure 13: Pullout Test Detail Survey and Monitoring: Survey monitoring was implemented to measure horizontal and vertical

displacements. The deflection of the wall was generally 0.2% of the cut height, which is within the range of the anticipated deformation for wall construction in these soils (Byrne, et al., 1996). Strain Monitoring:  In addition to the normal monitoring, a strain-monitoring program was

developed and implemented for the temporary soil nail wall to compare the in situ parameters of a multi-tier soil nail earth retention system with that of the original design parameters, information that would be valuable in the future. Strain gage monitoring is discussed more fully in the Design, Monitoring and Testing section of this paper.

22

Two pairs of vibrating wire strain gages were installed on each of three, predetermined soil nails in a vertical plane in order to determine the average strain (Figure 14). The nails selected for instrumentation were located on tiers 2 through 4 of the 5-tier system.

Elevations in feet. 1 ft = 0.3048 m

Figure 14: Instrumentation Layout Strain readings were taken on a weekly basis for a period of two months from initial installation. More frequent readings were taken immediately following nail installation. Readings were also taken prior to and after each new 1.5 m (5 ft) lift. Long term monitoring was not feasible; however, data trends shown in Figure 15 and Table 1 indicate that in situ field stresses were similar to or less than the theoretical stresses calculated using the CALTRANS Design Program (SNAIL). As shown, the nail stress increases as the next cut is made to a new lift. Results in the upper nail are closest to the theoretical design stresses, more than likely because it was monitored longer than the lower nails. Of interest also is how the nail stress spiked in the strain gages installed 1.5 m (5 ft) behind the wall immediately after the crane used to set the steel structure was brought on site and erected approximately 2.5 to 3.5 m (8 to 12 ft) behind the soil nail wall. As previously mentioned, the geotechnical contractor’s design took this loading into account. A summary of the stresses is presented in Table 1.

23

Stress Behavior   All So il Nail s 25.0 Crane set up at top of wall above gages

Excavation of "E" lift 20.0

   )    i   s    k    (   s 15.0   s   e   r    t    S   r   a    B    l    i   a 10.0   n    l    i   o    S

B47-15-U B47-15-L B47-5-U B47-5-L C47-15-U

Excavation of "D" lift

C47-15-L C47-5-U C47-5-L D24-15-U D24-15-L

Excavation of "C" lift

D24-5-U D24-5-L

5.0

0.0 11/1/02

11/15/02

11/29/02

12/13/02

12/27/02

1/10/03

1/24/03

2/7/03

2/21/03

Date

Figure 15: Composite Stress Plot

Table 1: Summary of Stresses Strain Gage Number B47-15-U B47-15-L B47-5-U B47-5-L

Bar Stress (ksi) 16.28 18.99 16.24 17.26

C47-15-U C47-15-L C47-5-U C47-5-L

7.93 7.25 N/A 14.06

D24-15-U D24-15-L D24-5-U D24-5-L

3.63 N/A 6.91 6.52

Average Stress Theoretical Stress (ksi) SNAILZwin output

24

17.64

24.287

16.75

18.854

7.59

20.094

14.06

17.137

3.63

15.901

6.72

14.053

1 ksi = 6.89 MPa

 Existing Dental School (Understanding the Conditions)

During construction of the permanent soil nail wall along the existing Dental School, unanticipated conditions were encountered during excavation of the first 1.5-m (5-ft) lift. A loose, cohesionless fill with poor standup time was encountered directly below the existing first floor slab. To assess the actual conditions, the geotechnical contractor installed a trial test section of shotcrete and test nails to determine if soil nailing would still be viable. The shotcrete did not adhere to the loose fill, causing sloughing of the material and “cave-in” of the test section. Additionally, the test nails did not achieve the bond values that the design assumed. Considering all these conditions, the geotechnical contractor recommended the soil nailing system not be installed and offered a redesign, which consisted of on-line concrete pits, tieback anchors and treated timber lagging to retain the soil between the columns. As originally  proposed, minipiles were installed through the column footing to underpin the column footings and transfer the column loads below the new subgrade elevation. SUMMARY

Soil nailing is an accepted technology, the theoretical aspects of which are well understood and well reported in technical literature. However, research indicates that there are few practical guidelines available that offer a comprehensive, experience-based insight into the construction considerations that should be addressed before a soil nail system design is finalized and implemented. This paper is intended to bridge that void, and is offered as a tool to aid owners, designers, specifiers and general contractors, as well as those within the engineering community, in making the informed decisions that result in appropriate and successful applications of this versatile and economic technology. ACKNOWLEDGEMENTS

The author extends his thanks and appreciation to the following people whose cooperation and input has been invaluable, both on the jobsite and in the preparation of this paper: Steven Lacz of Moretrench Geotec, John J. Peirce, P.E., of Peirce Engineering; and Ken Quazza, P.E. of SESI Consulting Engineers.

25

REFERENCES

Byrne, R.J., Cotton, D., Porterfield, C., Wolschlag, G., and Ueblacker, G. (1996). “Manual for  Design and Construction Monitoring of Soil Nail Walls.” FHWA-SA-96-069, Federal Highway Administration, Washington, DC. Chassie, R.G. (1993). “Soil Nailing Overview” and “Soil Nail Wall Facing Design and Current  Developments,” presented at the Tenth Annual International Bridge Conference, Pittsburgh, PA. Sakr, C. T. and Kimmerling, R. (1995). “Soil Nailing of a Bridge Embankment, Report 2: Design and Field Performance.” Oregon Department of Transportation Experimental Features Project OR 89-07 Schlosser, F. (1983). “Analogies et Differences dans le Compartment et le Calcul des Ouvranges de Soutenement en Terre Armee et par Clouage du Sol.” Proceedings of Annales ITBTP No. 418, Sols et Foundations 184, pp 8-23. Shen, C.K., Herrman, L.R., Ronstad, K.M., Bang, S., Kim, Y.S., and DeNatale, J.S. (1981). “An In Situ Earth Reinforcement Lateral Support System.”  National Science Foundation, Grant No. APR 77-03944, CE Dept., University of California Davis, Report No. 81-03. Stocker, M.F., Korber, G.W., Gassler, G., and Gudehus, G. (1979). “Soil Nailing.” Proceedings of the International Conference on Soil Reinforcement 1, Paris, Vol.2, pp 469-474. Tuozzolo, T.J. (1997). “Stabilizing the Stacks - A hybrid soil nailing system provides permanent underpinning for a historic college library.” Civil Engineering Magazine, December 1997. CALTRANS (1991). “A User’s Manual for the SNAIL Program, Version 2.02.” California Department of Transportation, Division of Technology, Material & Research, Office of Geotechnical Engineering. BIBLIOGRAPHY

Elias, V. and Juran, I. (1991). FHWA RD-89-198, “Soil Nailing for Stabilization of Highway Slopes and Excavations.” Federal Highway Administration, Washington, D. C. Singla, S. (1999). FHWA-IF-99-026, “ Demonstration Project 103: Design & Construction  Monitoring of Soil Nail Walls.”  Project Summary Report. Federal Highway Administration, Washington, D. C.

26