The pile spacing mainly controls the behavior of pile groups. The spacing should not be too small so that upheaval of ground surface takes place during driving into dense or incompressible material. On the other hand, if spacing is too large, uneconomic pile cap may result. When driving piles in sand and gravels, it is advisable to start driving at the center of the group and then to work outward, in order to avoid difficulty with “tightening up” of the ground.
In the group the following minimum spacing is recommended.
Types of Pile
Perimeter of the pile
Twice the least width
3/2 the diameter of screw blades
Settlement of Pile Groups in Clay
The settlement of a group of piles in clay cannot be predicted from the results of loading test on a single pile because of time effects, remolding of the soil owing to the pile driving and scale effects are quite different for the single test pile and the pile group. To compute settlement the mode of load transform is to be decided first. The following assumptions have been used.
- An equivalent raft at two third the pile length over the area enclosed by the piles at that depth.
- An equivalent raft at two third the pile length over a large area because of the side friction on the group of piles. A spread of one horizontal to four verticals may be reasonable. Figure 1 shows this situation.
- An equivalent raft at the base of the length over the area enclosed by the piles at that depth. This is applicable to point bearing piles and is shown in Fig.2.
Settlement of Pile Groups in Sand
The settlement of pile groups in granular soils can be calculated in a manner similar to that employed for pile groups in clay. In this case a virtual raft is assumed at the base of the pile. The settlement is calculated using the results of Dutch cone or Standard penetration tests.
Generally the above procedure is not adopted. The settlement is calculated using the results of the load tests on individual piles. Skempton in 1953 has compared the settlements of a numbers of pile groups with a settlement of corresponding individual piles and has proposed the following relationship between the settlement sb of a pile group of width B; and observed settlement ss of a single pile of the same loading intensity.
sb/ss = [(4B + 3)/(B + 4)]2
Care should be taken when the piles are driven into sand and gravels which are underlain by clay, if the stresses transferred to the clay from the pile group may result over-stressing or excessive consolidation. The factor of safety against the bearing capacity failure in the clay can be assumed by assuming a spread of load onto the surface of the clay in the manner as shown in Fig.3 below.
The settlement in the underlying layer can be computed in the normal way by first determining the distribution of stress throughout the clay layer using the Fig.7.18 given below.