Comparing BS 8004 and Eurocode 7 guidelines for single and group piles

CIVIL ENGINEERING DEPARTMENT

Comparing BS 8004 and Eurocode 7 guidelines for single and group piles

By: Lubrun Veeresh

Date: January 2016

Draft Report 01. 2016 UOM, Reduit © Copyright reserved

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1.0 Introduction The British Standard, BS 8004: 1986: Code of Practice for Foundations was formerly used for the design of piles. However, this has been superseded by Eurocode 7: Geotechnical design – Part 1: General Rules.

BS 8004 provides an in depth guideline from the type of piles to the determination of the ultimate bearing capacity. Nevertheless, there are conflicts arising between the BS 8110 and BS 8004 where pile design has to be conducted in conjunction with the code of practice for concrete. When pile foundations are designed using Eurocodes, the specific guidelines to be followed are dependent on the type of pile foundation being constructed. Eurocode 7 does not provide specific guidelines on the method of pile design. Eurocode 7 has to be read in conjunction with BS EN 1993-1: 2005 Eurocode 3, Design of Steel Structures, and Part 5: Pilling, for steel pile and BS EN 1995-1: 2004 Eurocode 5: Design of Timber Structures for wooden piles. National Annex documents provide alternative procedures to Eurocode 7, relevant to particular countries so as to make the design conform to the local practice. 1.1 Aim The aim of this assignment is to differentiate between BS 8004 and Eurocode 7 for a single and a pile group. 1.2 Objectives The objectives of the assignment are: 1. Look up significant differences between the two codes of practice for single piles and group piles. 2. Support the differences with relevant clauses. 3. Outline the practical relevance of the differences.


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2.0 Single piles A single pile can be defined as a long, slender structural member used to transmit loads applied at its top to the ground at lower levels. 2.1 Basis of design The design of piles using Eurocode 7 consists of checking that the ground surrounding the piles can resist compression, tension, and transverse actions at the ultimate limit state. Eurocode 7 proposes three design approaches namely: 1. “Material factoring approach” at load side and a “resistance factoring approach” at resistance side. Reliability is ensured by applying partial factors in two separate checks: for actions and to material properties or pile resistances. The design are checked for both of two separate sets of partial factors. 2. “Load and resistance factoring approach”. It ensures reliability by applying factors to actions and resistance. The design is checked for one set of partial factors. 3. Material factoring approach, at load as well as at resistance side. It applies factors to actions and material properties at the same time. The design is checked for one set of factors. BS 8004 adopts the permissible stress method in which the dead load and the most unfavourable combination of imposed loads are assumed to be applied to the ground. The foundation is deemed safe provided that the permissible stress on the soil has not been exceeded. 2.2 Classification of Piles BS 8004: 1986 defines 3 types of piles which are: 1. Large displacement piles These are solid section or hollow section closed end piles driven or jacked into the ground displacing the soil. 2. Small displacement piles These are small cross sectional area piles driven or jacked in the ground. 3. Replacement piles These are formed by the excavation of soil by boring methods and then casting concrete in the unlined or lined hole.


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Eurocode 7 does not classify piles. However, section 7 provides design guidelines for all types of load bearing piles. 2.3 Actions and factor of safety In Eurocode 7, the loading subjected to a foundation are defined as actions. The actions can be either structural with loadings from the superstructure, or geotechnical: caused by ground movements. The loading behavior are designed to cover ‘permanent unfavourable’, ‘permanent favourable’ and ‘variable’ situations as per clause 2.4.2. The design situations of piles are derived from section 2 under clause 2.2 considering the suitability of the soil strata. Partial factors on action subjected to pile foundations are specified in Annex A: Tables A.1 of Eurocode 7. BS 8004 requires that for single piles, the working load should be less than ultimate bearing capacity including a factor of safety. The factor of safety should be based on the type of soil and the accuracy of the method to obtain the ultimate bearing capacity. Conservative value for the factor of safety lies between 2 and 3. 2.4 Pile load test Eurocode 7, clause 7.4.1 (2), considers 2 types of pile load tests namely: trial pile load test and working pile load test. Trial pile are used for testing prior to completing the design while working load test are piles installed within the actual foundation. Clause 7.5.2.1 (1)P states that the tests should allow the determination of creeping behaviour, deformations and rebound behaviour of piles.

2.4.1 Static loading test Eurocode 7 specifies that the design of a pile should be based on the static load tests or calculation methods conforming to these tests. The methodology of conducting static pile load test is outlined in clause 7.5.2 of Eurocode 7. It is a common practice to take static pile loading tests to failure or to a point where the failure load can be extrapolated. Clause 7.6.1.1(3) of Eurocode 7 permits Rcm, the ultimate limit state resistance, to be the load applied to the pile head which causes a settlement of 10% of the pile diameter if the failure load cannot be found.


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2.4.2 Dynamic loading test Dynamic loading tests, defined under Clause 7.5.3. of Eurocode 7, may be used to estimate resistances to axial compression loads ‘provided that there has been an adequate ground investigation and that the method has been calibrated against static loading tests on the same type of pile and of similar length and crosssection, and in comparable soil conditions’. BS 8004 recommends two types of pile tests to determine the ultimate bearing capacity which are the constant rate of penetration, CRP, and maintained load, ML, test. For CRP, BS 8004 prescribes penetration rates of 0.75 mm /minute for friction piles in clay soils and 1.55 mm/minute for piles end bearing in a granular soil. For the ML test, the pile is driven at a constant rate for which the force applied at the top is measured. 2.5 Downdrag (negative skin friction) While piles are subjected to loads from the superstructure, they can also have actions from the soil due to movement of the ground such as consolidation. This effect is known as downdrag and puts further downward force on the pile. The negative friction force causing consolidation in sensitive clays may be estimated as the cohesion of the remoulded clay multiplied by the surface area of the pile shaft. Hence, BS 8004 requires that end bearing pile will cause some consolidation of the surrounding soil and that the skin friction should be estimated from the material properties. Eurocode 7 recommendation to design piles subjected to negative skin effect is to treat the resulting axial force as a permanent unfavourable action. This is given in clause 7.3.2 as actions due to ground displacement for which 2 design approaches have been adopted: pile-soil interaction analysis and an upperbound force exerted on the pile shaft. 2.6 Axially Loaded piles Single piles driven normally and completely in the ground are less prone to buckling. If the pile extends above the ground after it has finished, then that part above the ground is to be designed as a column in conjunction with BS 8110. The effective length of the pile depends on the lateral loading given under clause 7.3.3.4 and degree of fixity provided by the ground. Eurocode 7 does not given particular details to the effective length of piles. However, the same can be found in Eurocode 2 and 3 for concrete and steel piles respectively.


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2.7 Raking piles Raking piles are provided when the ground on which a structure is erected cannot resist horizontal forces. During the design of the raking pile, it is assumed to be an axially loaded pile. However, du to horizontal forces, the top of the raking pile does not move axially and hence causing bending in the pile. Alternatively, BS 8004 proposes the use of vertically bored cast in place concrete pile to resist horizontal forces. Eurocode 7 does not specify any criteria or guideline for raking piles. 2.8 Settlement of single piles The ultimate resistance of the pile obtained from pile load test is divided by a factor of safety to obtain the design working load pile. From the pile test, piles up to 600 mm have a settlement less than 10 mm for a factor of safety of 2.5 and above. As the pile diameter increases above 600mm, settlement increases. This requires a separate analysis for the shaft friction and the base load. Eurocode 7 under clause 7.6.4.1 states that where piles are bearing on medium-dense to dense soils the safety requirements for ultimate limit state design are normally sufficient to prevent a serviceability limit state in the supported structure. 2.9 Tension pile A single pile foundation subjected to tension, Eurocode 7 requires that the design tensile action Ftd acting on the pile to be less than or equal to the design tensile resistance Rtd of the pile foundation. Ftd ≤ Rtd Eurocode 7 allows the ultimate tensile resistance to be found by pile loading tests and more than one test should be carried out and in the case of a large number of pile at least 2% should be tested. Clause 7.3.3.7 of BS 8004 requires that tension pile should be designed to transmit full range of possible loading to surrounding ground. Pulling tests are recommended in cases where the capacity of the tension piles are unknown. The axis of the pile should be co-linear with the direction of the loading.


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3.0 Piles in groups BS 8004 recommendations for pile groups are given in clause 7.3.4. Eurocode 7 does not have any specific section dedicated to the design of pile groups. However, particular criteria for design are listed in sub sections of clause 7.6. 3.1 Installation As per clause 7.3.4.1 under BS 8004 section 7, pile groups installation should avoid heaving and lateral displacement. For stiff soils, it is proposed to start driving piles at center of the group first and then moving outwards. Eurocode 7 does not provide any specific requirement for the installation of pile groups. However, emphasis is laid upon the way piles are installed and monitored under clause 7.9. EN 1997-1 requires that a detailed plan for the pile installation be prepared prior to pilling works. The integrity of the piles has to be checked when required using sonic and vibration tests and coring methods. 3.2 Spacing of piles The spacing of piles prescribed by BS 8004:1986 in clause 7.3.4.2 is mainly governed by the method of installation and the bearing capacity of the pile group. The convention generally adopted are as follows. Table 1: Spacing of piles in a group

Type of pile Friction pile End bearing

Spacing Center to center distance > perimeter of pile For circular spacing = 3 × diameter Spacing > width of piles

However, Eurocode 7 does not give any guideline pertaining to the spacing of piles in a group. 3.3 Effective size of group pile The behaviour of group piles can differ from single pile and it has to be accounted in the design. This behavior is mainly dependent upon the load transfer mechanism through the pile to the soil. Due to variation in soil characteristics, the ultimate bearing capacity and settlement of the pile group differs from that of a similar number of the single pile. Hence BS 8004, clause 7.3.4.3, states that these settlements should be within acceptable limits for the whole foundation. The size of the group affects the depth of soil stressed. Page | 7 Draft Report 01. 2016 UOM, Reduit © Copyright reserved

This is because as the size of the pile increases from a single pile in soil to a pile group, the bulb of pressure also increases. 3.4 Settlements of the group For end bearing piles, the settlement can be calculated using the assumption that the load is applied throughout the area the covered by the pile group at the level of the pile toes. For friction pile, the settlement is found using the assumption that the load is applied on the area larger than the area covered by the pile group at the level of the pile toes. A superposition approach is suggested for the cohesive soil from which an estimation of the settlement of the group is found. The Eurocode 7 recommendation for settlement of pile group is given under clause 7.6.4.2(2) P which states that the assessment of settlement pile group should consider the settlement of individual piles together with the group. However, it does not distinguish whether the settlement analysis assumes that the group is acting as an equivalent large diameter pile or as a block foundation which has been used under clause 7.6.2.1 for the calculation of compressive load. 3.5 Ultimate bearing capacity of the group pile BS 8004 defines the ultimate bearing capacity of a pile as ‘The load at which the resistance of the soil becomes fully mobilized’. BS 8004 states that for end bearing piles on rocks, the ultimate bearing capacity is equal to the nQ where n is the number piles in the group and Q is the ultimate bearing capacity of a single pile. For compacted cohesionless soil, the ultimate bearing capacity of the group can be more than nQ. The ultimate bearing capacity of friction piles in cohesive soils, is predicted to be less than nQ. Hence the ultimate bearing capacity is dependent upon: 1. The spacing of the pile 2. Charactheristics of the soil 3. Number of piles 4. Stiffness of the pile The pile cap involvement in transmitting the load to the ground also affects the ultimate bearing capacity. If the cap is not in contact with the ground, it is assumed not to be taking any load. For pile caps cast on the cohesive soil, it may contribute to the bearing capacity of the pile group. The further distribution of the load on the pile is dependent upon the rigidity of the pile cap.


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Clause 7.6.2.1 of Eurocode 7 requires the compressive resistance of a pile group should include the analysis of both the risk of failure of an individual pile in the group and the failure of the group considered as an equivalent block foundation. In this case the pile group acting as a block may be assumed as a single large diameter pile as stated in the sub clause 4. 3.6 Pile group in tension Two failure mechanisms for tension pile are stated in Eurocode 7 which are pulling the piles out of the ground and uplifting a block of the ground with the piles. Eurocode 7 requires that there is adequate safety against failure for a group of piles uplifting a block of ground satisfying the following equation: Ftd ≤ Wd – (U2d – U1d) + Fd Where, Ftd is the design tensile force acting on the group of piles; Wd is the design weight of the soil block (including the water) and the piles; Fd is the design shear resistance at the boundary of the block of soil; U1d is the design downward force due to the water pressure on the top of the pile foundation; U2d is the design upward force due to the water pressure on the base of the soil block. BS 8004 does not define any failure mechanism for pile groups in tension as Eurocode 7. However, empirical methods has shown that the uplift resistance for a block of soil enclosed by a pile group may be given by: Qu = (2LH_2BH )cu + W Where, Qu is the total uplift resistance of the pile group L is the overall length of the group B is the overall width of the group H is the depth of the block of soil below pile cap level cu is the value of average undisturbed undrained shear strength of the soil surrounding the group W is the value of average undisturbed undrained shear strength of the soil


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4.0 Conclusion BS 8004 is based on permissible stress method while BS 8110 requires limit state design. Hence, the type loading applied to the pile from the superstructure has to be specified. Using the design methods prescribed by Eurocode 7 has the benefit of removing the conflict that exist between BS 8004 and BS 8110. However, Eurocode 7 solely does not provide detailed guidelines on the behaviour of piles and has to be read in conjunction with other Eurocodes as required.


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References Andrew, B. & Andrew, H., 2008, Decoding Eurocode 7. New York: Taylor & Francis Group. Bauduin C. Besix, 2001, Design of Axially Loaded Compression Piles According to Eurocode 7. [Online]. University of Brussels, Belgium. Available at: http://www.besix.com/Besix/media/Besix_Media/pdf/Publications/Design_of_axially_loaded_compressio n.pdf [Accessed on: 24 January 2016]

British Standards Institution, 1986. BS 8004:1986 Code of Practice for Foundations. London: BSI. European Standard, 2004. BS EN 1997-1:2004 Eurocode 7: Geotechnical design – Part 1: General Rules. Brussels: CEN. Tomlinson, M. & Woodward, J., 2008, Pile Design and Construction Practice, 5th Edition. New York: Taylor & Francis Group.


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Comparing BS 8004 and Eurocode 7 guidelines for single and group piles

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