Behavior of a Nine-Pile Group with and without a Pile Cap

Abstract

The lateral load-displacement behavior of a pile group is a function of the number of piles and their individual stiffnesses, the non-linear response of surrounding soils, and pile spacing. The load-displacement behavior of pile groups also depends upon the degree of pile head restraint provided by the cap and the associated frame action provided by the piles. A full-scale, pile group consisting of nine 610-mm diameter, standard weight pipe piles driven in a generally cohesive soil profile was constructed without a pile cap and then laterally load tested. Later, an above-grade 5.33 m long, 5.18 m wide, 1.12 m high concrete cap was added to the pile group, after which the foundation was reloaded. The pile-to-cap connection had a pile embedment of 75 mm into the cap together with a much longer reinforcing steel cage. Placement of the cap on the pile group increased the load resistance of the foundation by 51% (at a displacement of 25 mm) and decreased displacement by 40% (at a load of 1850 kN); the load increase and displacement decrease would have been greater if the piles in the group with cap had not been previously loaded. The measured stiffness of the pile group with cap was approximately 40 to 50% less than the stiffness of a pile cap with fixed head conditions, but in the range of 15 to 30 mm of displacement, the pile group with cap was still 20 to 50% stiffer than the pile group without cap; hence, the pile group with cap provided some degree of pile-head restraint, but that restraint was less than completely fixed. The pile-to-cap connections used for the cap can be modeled using rotational spring constants in the range of 60,000 to 100,000 kN-m/rad. ABSTRACT: The lateral load-displacement behavior of a pile group is a function of the number of piles and their individual stiffnesses, the non-linear response of surrounding soils, and pile spacing. The load-displacement behavior of pile groups also depends upon the degree of pile head restraint provided by the cap and the associated frame action provided by the piles. A full-scale, pile group consisting of nine 610-mm diameter, standard weight pipe piles driven in a generally cohesive soil profile was constructed without a pile cap and then laterally load tested. Later, an above-grade 5.33 m long, 5.18 m wide, 1.12 m high concrete cap was added to the pile group, after which the foundation was reloaded. The pile-to-cap connection had a pile embedment of 75 mm into the cap together with a much longer reinforcing steel cage. Placement of the cap on the pile group increased the load resistance of the foundation by 51% (at a displacement of 25 mm) and decreased displacement by 40% (at a load of 1850 kN); the load increase and displacement decrease would have been greater if the piles in the group with cap had not been previously loaded. The measured stiffness of the pile group with cap was approximately 40 to 50% less than the stiffness of a pile cap with fixed head conditions, but in the range of 15 to 30 mm of displacement, the pile group with cap was still 20 to 50% stiffer than the pile group without cap; hence, the pile group with cap provided some degree of pile-head restraint, but that restraint was less than completely fixed. The pile-to-cap connections used for the cap can be modeled using rotational spring constants in the range of 60,000 to 100,000 kN-m/rad.