Pile Group: Efficiency and Settlement | Pile Foundations | Soil Engineering

In this article we will discuss about the efficiency and settlement of pile group in soils.

Efficiency of Pile Group:

It may often be required to use more than one pile below a column, depending on the column load and the load capacity of a single pile. Usually, driven piles should be provided in groups and a single pile should not be used. This is because the pile may move laterally during the driving operation, resulting in the eccentricity of the load, causing additional bending stresses in the pile, decreasing both the structural capacity and the soil resistance capacity.

The use of a pile group instead of a single pile, though not warranted from structural consideration, will ensure in minimizing or eliminating the eccentricity of loads caused by the lateral displacement of piles during the driving operation. This is not the case with bored piles, where there is better control over pile alignment, and the number of bored piles used will be based on design requirements, depending on the loads from the superstructure and the load capacity of the individual pile.

The load capacity of a pile group may not be equal to the sum of the load capacity of the individual piles in the group due to the following reasons:

1. When piles are used in a group at close spacing, the pressure bulbs of individual piles may overlap, causing more stress in the overlapping soil zones. This causes a reduction in the load capacity.

2. When piles are driven in loose- to medium-dense cohesionless soils, it may cause densification of the soil, increasing the load capacity more than the estimated load capacity.

The efficiency of the pile group is defined as –

where Q_ug is the ultimate load capacity of the pile group, Q_u is the ultimate load capacity of the individual pile, and N is the number of piles in the pile group.

Spacing of Piles:

For end-bearing piles founded on a very hard stratum, deriving their load capacity mainly from end-bearing resist­ance, the spacing of piles is governed by the competency of the end-bearing stratum. The minimum spacing of end-bearing piles is 2.5 d, where d is the diameter of the pile shaft. For piles resting on rock, the minimum spacing of piles is 2 d, where d is the diameter of the pile shaft.

For friction piles, the pile spacing should ensure that the pressure bulbs of individual piles do not overlap, which otherwise causes a reduction in the pile capacity. The minimum spacing of friction piles is 3 d, where d is the diameter of the pile shaft. In the case, of non-circular piles, such as square or rectangular piles, the diameter of the circle circumscribing the pile is used to determine the minimum pile spacing.

The spacing of piles should not be so close as to cause direct contact between two adjacent piles in a group at any level, arising due to the tolerance allowed in alignment for installation of piles.

Load Capacity of Pile Group in Sand and Gravel:

When piles are driven in loose- to medium-dense cohesionless soils, the soil around the pile is compacted, increas­ing the group efficiency. For better results, it is desirable to start the driving of piles at the center of the site and work outward. For dense sands, the driving of piles may reduce group efficiency due to dilatancy. In the case of bored piles, there is limited densification of soil around the pile. Group efficiency, therefore, depends on pile spacing, and may be less than 100% if piles are spaced closer than 3 d, where d is the diameter of the pile.

The efficiency of the pile group may be obtained from the converse Lebarre equation for friction piles –

where m is the number of rows of piles, n is the number of piles in each row, B is the diameter of the pile, and S is the spacing of piles.

The load capacity of a pile group is obtained from –

Q_ug = ƞ_gNQ_u …(20.47)

In the case of end-bearing piles, driven in dense sand at a spacing more than 3 d, where d is the diameter of the pile shaft, group efficiency is generally much more than 100%. The load capacity of the pile group is taken as –

Q_ug = NQ_u …(20.48)

For bored piles in sand and gravel, group efficiency is about 2/3 to 3/4.

In the case of piles deriving their support mainly from friction and connected by a pile cap, the group may be visualized to transmit load to the soil as if from a column of soil enclosed by the piles. The ultimate capacity of the group may be computed following this concept, taking into account the frictional capacity along the perimeter of the column of soil as above and the end-bearing of the said column.

Pile Groups in Clay:

In the case of pile groups in clay, the load capacity of the pile group is computed keeping in view two considerations:

1. Piles act individually without overlap of their corresponding pressure bulbs –

Q_ug1 = NQ_u = N(cN_cA_p + αc̅A_s) …(20.49)

2. Piles act as a block of soil enclosed by the piles as shown in Fig. 20.22. The size of the block in any direction is given by –

B_g =[(n – 1)S] + d …(20.50)

where n is the number of piles in the row along the direction, S is the c/c spacing of piles in the row along the direction, and D is the diameter of the piles –

Q_Ug2 = cN_cA_pg + αc̅A_sg …(20.51)

whereA_pg, the cross-sectional area of the pile group, is B_g x B_g for a square pile group, and A_sg, the surface area of the pile group, is 4 B_g x L.

The load capacity of the pile group is taken as the minimum of Q_ug1 and Q_ug2. Where the spacing between the piles is less than optimum, the efficiency of the pile group is usually less than 1. For piles spaced at or more than the optimum spacing, the efficiency of the pile group may approach unity. In the case of driven piles in loose- to medium-dense sand, the efficiency of a pile group may be even greater than 1 due to densification of the sand between the piles during the driving operation.

Block failure occurs generally when the spacing of piles is less than about three times the pile diameter. In this case, the soil, bound by a perimeter of the pile group over the embedded length, acts as a single unit or block.

Individual pile failure occurs when the piles are spaced at about eight times the diameter. In this case, the efficiency of the pile group is equal to 1 or 100%. For piles in sand, the efficiency of the pile group is more than 100% when the piles are at a close spacing and become equal to 100% when the pile spacing is about 5-6 times the diameter. Pile group efficiency of 1 is assumed for pile groups in sand.

For a pile group in stiff clay or compact sand, the sequence of installation of piles shall normally be from the center to the periphery of the group or from one side to the other. However, in the case of very soft soils, the driving may have to proceed from outside to inside so that the soil is restrained from flowing out during the driving operation.

Settlement of a Pile Group:

The settlement of a pile or pile group in clay can be computed from the principles of consolidation. Settlement of a pile group is more than the settlement of a single pile, even when the load is the same. This is because the pressure bulb of the pile group is deeper than that of individual piles, causing the compression of a larger volume of soil by the pile group.

For pile groups in sand, the settlement is computed as follows:

where S is the ratio of pile spacing to pile diameter and r is the number of rows in a pile group.