LIST OF PLATES
7. GROUP EFFECTS 1 GENERAL
Piles installed in a group to form a foundation will, when loaded, give rise to interaction between individual piles as well as between the structure and the piles. The pile-soil-pile interaction arises as a result of overlapping of stress (or strain) fields and could affect both the capacity and the settlement of the piles. The piled foundation as a whole also interacts with the structure by virtue of the difference in stiffness. This foundation-structure interaction affects the distribution of loads in the piles, together with forces and movements experienced by the structure.
The analysis of the behaviour of a pile group is a complex soil-structure interaction problem. The behaviour of a pile group foundation will be influenced by, inter alia :
(a) method of pile installation, e.g. replacement or displacement piles,
(b) dominant mode of load transfer, i.e. shaft resistance or end-bearing,
(c) nature of founding materials,
(d) three-dimensional geometry of the pile group configuration, (e) presence or otherwise of a ground-bearing cap, and
(f) relative stiffness of the structure, the piles and the ground.
Traditionally, the assessment of group effects is based on some 'rules-of-thumb' or semi-empirical rules derived from field observations. Recent advances in analytical studies have enabled more rational design principles to be developed. With improved computing capabilities, general pile groups with a combination of vertical and raking piles subjected to complex loading can be analysed in a fairly rigorous manner and parametric studies can be carried out relatively efficiently and economically.
This Chapter firstly considers the ultimate limit states for a range of design situations for pile groups. Methods of assessing the deformation of single piles and pile groups are then presented. Finally, some design considerations for soil-structure interaction problems are discussed.
7.2 MINIMUM SPACING OF PILES
The minimum spacing between piles in a group should be chosen in relation to the method of pile construction and the mode of load transfer. It is recommended that the following guidelines on minimum pile spacing may be adopted for routine design :
(a) For bored piles which derive their capacities mainly from shaft resistance and for all types of driven piles, minimum
centre-to-centre spacing should be greater than the perimeter of the pile (which should be taken as that of the larger pile where piles of different sizes are used); this spacing should not be less than 1 m as stipulated in the Code of Practice for Foundations (BD, 2004a).
(b) For bored piles which derive their capacities mainly from end-bearing, minimum clear spacing between the surfaces of adjacent piles should be based on practical considerations of positional and verticality tolerances of piles. It is prudent to provide a nominal minimum clear spacing of about 0.5 m between shaft surfaces or edge of bell-outs. For mini-piles socketed into rock, the minimum spacing should be taken as the greater of 0.75 m or twice the pile diameter (BD, 2004a).
The recommended tolerances of installed piles are shown in Table 7.1 (HKG, 1992).
Closer spacing than that given above may be adopted only when it has been justified by detailed analyses of the effect on the settlement and bearing capacity of the pile group.
Particular note should be taken of adjacent piles founded at different levels, in which case the effects of the load transfer and soil deformations arising from the piles at a higher level on those at a lower level need to be examined. The designer should also specify a pile installation sequence within a group that will assure maximum spacing between shafts being installed and those recently concreted.
Table 7.1 – Tolerance of Installed Piles (HKG, 1992)
Tolerance Description
Land Piles Marine Piles Deviation from specified position in plan,
measured at cut-off level 75 mm 150 mm
Deviation from vertical 1 in 75 1 in 25
Deviation of raking piles from specified batter Deviation from specified cut-off level
1 in 25 25 mm The diameter of cast in-place piles shall be at least 97% of the specified diameter
7.3 ULTIMATE CAPACITY OF PILE GROUPS 7.3.1 General
Traditionally, the ultimate load capacity of a pile group is related to the sum of ultimate capacity of individual piles through a group efficiency (or reduction) factor η, defined as follows :
η = ultimate load capacity of a pile group
sum of ultimate load capacities of individual piles in the group [7.1]
A number of empirical formulae have been proposed, generally relating the group efficiency factor to the number and spacing of piles. However, most of these formulae give no more than arbitrary factors in an attempt to limit the potential pile group settlement. A comparison of a range of formulae made by Chellis (1961) shows a considerable variation in the values of η for a given pile group configuration. There is a lack of sound theoretical basis in the rationale and field data in support of the proposed empirical formulae (Fleming &
Thorburn, 1983). The use of these formulae to calculate group efficiency factors is therefore not recommended.
A more rational approach in assessing pile group capacities is to consider the capacity of both the individual piles (with allowance for pile-soil-pile interaction effects) and the capacity of the group as a block or a row and determine which failure mode is more critical.
There must be an adequate factor of safety against the most critical mode of failure.
The degree of pile-soil-pile interaction, which affects pile group capacities, is influenced by the method of pile installation, mechanism of load transfer and nature of the founding materials. The group efficiency factor may be assessed on the basis of observations made in instrumented model and field tests as described below. Generally, group interaction does not need to be considered where the spacing is in excess of about eight pile diameters (CGS, 1992).
7.3.2 Vertical Pile Groups in Granular Soils under Compression 7.3.2.1 Free-standing driven piles
In granular soils, the compacting efforts of pile driving generally result in densification and consequently the group efficiency factor may be greater than unity. Lambe
& Whitman (1979) warned that for very dense sands, pile driving could cause loosening of the soils due to dilatancy and η could be less than unity in this case. This effect is also reflected in the model tests reported by Valsangkar & Meyerhof (1983) for soils with an angle of shearing resistance, φ', greater than 40°. However, this phenomenon is seldom observed in full-scale loading tests or field monitoring.
Figure 7.1 shows the findings of model tests on instrumented driven piles reported by Vesic (1969). The ultimate shaft capacity of a pile within the pile group was observed to have increased to about three times the capacity of a single pile.
It is generally accepted that, for normal pile spacing, the interaction arising from overlapping of stress fields affects only the shaft capacity and is independent of the type of pile and the nature of the soil. Therefore, it would be more rational to consider group efficiency factors in terms of the shaft resistance component only.
The behaviour of a driven pile may be affected by the residual stresses built up during pile driving. In practice, pile driving in the field could affect the residual stresses of the neighbouring piles to a different extent from that in a model test as a result of scale effects,
which could partially offset the beneficial effects of densification. For design purposes, it is recommended that a group efficiency factor of unity may be taken conservatively for displacement piles.
7.3.2.2 Free-standing bored piles
Construction of bored piles may cause loosening and disturbance of granular soils. In
Notes :
(1) Efficiency denotes the ratio of ultimate load capacity of a pile group to the sum of ultimate load capacities of individual piles in the group. Shaft efficiency denotes the above ratio in terms of shaft resistance only. Base efficiency denotes the ratio in terms of end-bearing resistance only.
(2) Vesic (1969) noted that in view of the range of scatter of individual test results, there was probably no meaning in the apparent trend towards lower base efficiency at large pile spacings.
Figure 7.1 – Results of Model Tests on Groups of Instrumented Driven Piles in Granular Soils (Vesic, 1969)
1 2 3 4 5 6 7
0.5 1.0 1.5 2.0 2.5 3.0
Shaft efficiency
4-pile group 9-pile group
4-pile group
4-pile group
9-pile group Total efficiency with pile cap
Total efficiency Base efficiency
(average of tests)
Pile Spacing/Pile Diameter
Group Efficiency Factor
practice, the design of single piles generally has made allowance for the effects of loosening and the problem is therefore to assess the additional effect of loosening due to pile group installation. This may be affected to a certain extent by the initial stresses in the ground but is principally a question of workmanship and construction techniques and is therefore difficult to quantify.
Meyerhof (1976) suggested that the group efficiency factor could be taken conservatively as 2/3 at customary spacings but no field data were given to substantiate this.
The results of some loading tests on full-scale pile groups were summarised by O'Neill (1983), who showed that the lower-bound group efficiency factor is 0.7. For design purposes, the group efficiency factor may be taken as 0.85 for shaft resistance and 1.0 for end-bearing, assuming average to good workmanship.
If an individual pile has an adequate margin against failure, there would be no risk of a block failure of a pile group supported purely by end-bearing on a granular soil which is not underlain by weaker strata. Where the piles are embedded in granular soils (i.e. shaft and end-bearing resistance), both individual pile failure and block failure mechanisms (Figure 7.2) should be checked. The block failure mechanism should be checked by considering the available shaft resistance and end-bearing resistance of the block or row as appropriate.
Suitable allowance should be made in assessing the equivalent angle of pile/soil interface friction for the portion of failure surface through the relatively undisturbed ground between the piles.
7.3.2.3 Pile groups with ground bearing cap
In the case where there is a ground-bearing cap, the ultimate load capacity of the pile group should be taken as the lesser of the following (Poulos & Davis, 1980) :
(a) Sum of the capacity of the cap (taking the effective area, i.e. areas associated with the piles ignored) and the piles acting individually. For design purposes, the same group efficiency factors as for piles without a cap may be used.
(b) Sum of the capacity of a block containing the piles and the capacity of that portion of cap outside the perimeter of the block.
Care should be exercised in determining the allowable load as the movements required to fully mobilise the cap and pile capacities may not be compatible and appropriate mobilisation factors for each component should be used. In addition, the designer should carefully consider the possibility of partial loss of support to the cap as a result of excavation for utilities and ground settlement.
7.3.3 Vertical Pile Groups in Clays under Compression
The extent of installation effects of both driven and bored piles in clay on pile-soil-pile interaction is generally small compared to that in a granular soil. It should be noted that
the rate of dissipation of excess pore water pressures set up during driving in clays will be slower in a pile group than around single piles. This may need to be taken into account if design loads are expected to be applied prior to the end of the re-consolidation period.
(a) Single Pile Failure (b) Failure of Rows of Piles
(c) Block Failure
Note :
In assessing the ultimate end-bearing capacity of a block failure in granular soils, the effective weight (W') of the soil above the founding level may be allowed for.
Figure 7.2 – Failure Mechanisms of Pile Groups (Fleming et al, 1992)