The uplift capacity of a pile group, when the vertical piles are arranged in a closely spaced groups may not be equal to the sum of the uplift resistances of the individual piles. This is because, at ultimate load conditions, the block of soil enclosed by the pile group gets lifted. The manner in which the load is transferred from the pile to the soil is quite complex. A simplified way of calculating the uplift capacity of a pile group embedded in cohesionless soil is shown in Fig. 15.33(a). A spread of load of 1 Horiz : 4 Vert from the pile group base to the ground surface may be taken as the volume of the soil to be lifted by the pile group (Tomlinson, 1977).

For simplicity in calculation, the weight of the pile embedded in the ground is assumed to be equal to that of the volume of soil it displaces. If the pile group is partly or fully submerged, the submerged weight of soil below the water table has to be taken.

In the case of cohesive soil, the uplift resistance of the block of soil in undrained shear enclosed by the pile group given in Fig. 15.33(b) has to be considered. The equation for the total uplift capacity Pgu of the group may be expressed by

Figure 15.33 Uplift capacity of a pile group

A factor of safety of 2 may be used in both cases of piles in sand and clay.

The uplift efficiency Egu of a group of piles may be expressed as

The efficiency Egu varies with the method of installation of the piles, length and spacing and the type of soil. 

The available data indicate that Egu increases with the spacing of piles. Meyerhof and
Adams (1968) presented some data on uplift efficiency of groups of two and four model circular footings in clay. The results indicate that the uplift efficiency increases with the spacing of the footings or bases and as the depth of embedment decreases, but decreases as the number of footings or bases in the group increases. 

How far the footings would represent the piles is a debatable point.
For uplift loading on pile groups in sand, there appears to be little data from full scale field tests.

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