Chapter 32 - Deep Foundations - Index of - Free

FIGURE 32.14Block failure model for pile group in clay.The group block method is also often used to check the bearing capacity of a pile group. Thegroup block is treated as a large deep spread footing foundation and the assumed bottom level ofthe footing is different depending on whether the pile is end bearing or frictional. For end-bearingpiles, the capacity of the group block is examined by assuming the bottom of the footing is at thetip of the piles. For frictional piles, the capacity of the group pile is checked by assuming that thebottom of the footing is located at ⅓ of the total embedded length above the tip. The bearingcapacity of the underlying weaker layers is then estimated by using methods discussed in Chapter 31.The smaller capacity, by using the group efficiency approach, the group block approach, and thegroup block approach with underlying weaker layers, is selected as the capacity of the pile group.32.6.3 Settlement of a Pile GroupThe superimposed stresses from neighboring piles will raise the stress level below the tip of a pilesubstantially, whereas the stress level is much smaller for an individual pile. The raised stress levelhas two effects on the settlement of a pile group. The magnitude of the settlement will be larger fora pile group and the influence zone of a pile group will be much greater. The settlement of a pilegroup will be much larger in the presence of underlying highly compressible layers that would notbe stressed under the loading of an individual pile.The group block method is often used to estimate the settlement of a group. The pile group issimplified to an equivalent massive spread footing foundation except that the bottom of the footingis much deeper. The plane dimensions of the equivalent footing are outlined by the perimeter pilesof the pile group. The method to calculate settlement of spread footings is discussed in Chapter 31.The assumed bottom level of the footing block is different depending on either end bearing orfrictional piles. For end-bearing piles, the bottom of the footing is at the tip of the piles. For frictionalpiles, the bottom of the footing is located at ⅓ of total embedded length above the tip. In manycases, settlement requirement also is an important factor in the design of a pile group.Vesic [79] introduced a method to calculate settlement of a pile group in sand which is expressed asSg= SsBBgs(32.52)whereS g= the settlement of a pile groupS s= the settlement of an individual pile© 2000 by CRC Press LLC