Settlement of Pile Groups: Understanding the Causes and Estimating the Settlements

Settlement of a group of friction piles can be attributed to three main causes, as identified by Taylor (1948). Firstly, settlement occurs due to compression of the pile and movement of the piles relative to the immediately adjacent soil. This settlement is similar to that observed in a loading test on a single pile when full skin friction is developed. Secondly, settlement occurs due to compression in the soil between the piles. Lastly, settlement occurs due to compression in compressible strata below the tips of the piles. It is important to note that settlements resulting from compression of the soil between piles and compression of the strata below the pile tips are generally of larger magnitude compared to settlements caused by compression of the pile and movement of the pile relative to the soil. However, these settlements may occur slowly in saturated soil due to consolidation and slow dissipation of pore pressure. The disturbance to the soil structure during pile driving can lead to increased settlements after the final loading of a pile foundation. It is well-known that a remolded clay consolidates to a smaller void ratio than that reached under the same load in an undisturbed state. Therefore, structural disturbance during pile driving can result in increased settlements. The magnitude of this settlement increase depends on factors such as the distance the disturbance extends from the pile, the type of soil, the degree of soil disturbance, and the details of the consolidation process subsequent to driving. While definite increases in settlements may not be quantitatively defined, it is possible that in some soils they are much larger than engineers may suspect. Estimates of settlements due to compression in compressible strata below the pile tips can be made using methods based on Terzaghi's theory of consolidation. It is important to note that settlements observed in loading tests on a single friction pile are not representative of the settlements of a pile group. Therefore, such a load test provides information on failure load rather than settlements under actual loading conditions of a friction pile group. The installation of piles alters the deformation and compressibility characteristics of the soil mass in a different way and to a different extent compared to that around and below the tip of a single pile, although this influence extends only to a few pile diameters. As a result, the total settlement of a group of driven or bored piles under the safe design load can generally be estimated roughly as for an equivalent pile foundation, based on the work of Terzaghi and Pock (1967). Several simplifying assumptions are made for this computation. In conclusion, understanding the causes of settlement in pile groups is crucial for accurate estimation of settlements in pile foundations. Settlements can occur due to compression of the pile and movement of the piles relative to the soil, compression in the soil between piles, and compression in compressible strata below the pile tips. The disturbance to the soil structure during pile driving can lead to increased settlements, and the magnitude of this increase depends on various factors. It is important to note that settlements observed in loading tests on a single pile are not representative of pile group settlements. Estimating settlements in pile groups requires considering the altered deformation and compressibility characteristics of the soil mass.