Travis M. Gerber

Increased Lateral Abutment Resistance from Gravel Backfills of Limited Width

Lateral pile cap tests were performed on a pile cap with three backfills to evaluate the static and dynamic behavior. One backfill consisted of loose silty sand while the other two consisted of 0.91- and 1.82-m-wide dense gravel zones between the pile cap and the loose silty sand. The 0.91- and 1.82-m-wide dense gravel zones increased the lateral resistance by 75 to 150% and 150 to 225%, respectively, relative to the loose silty sand backfill. Despite being thin relative to the overall shear length, the 0.92- and 1.82-m-wide gravel zones increase lateral resistance to approximately 54 and 78%, respectively, of the resistance that would be provided by a backfill entirely composed of dense gravel. The dynamic stiffness for the pile cap with the gravel zones decreased about 10% after 15 cycles of loading, while the damping ratio remained relatively constant with cycling. Dynamic stiffness increased by about 10 to 40% at higher deflections, while the damping ratio decreased from an initial value of about 0.30 to around 0.26 at higher deflections.

Cyclic P-Y Curves for a Pile in Cohesive Soil

A lateral load test was performed on a full-scale steel pipe pile installed in a soil profile consisting of cohesive, fine-grained soils with occasional sand layers. The pile was loaded in nine increments from 0 to 89 mm of displacement, with 15 loading cycles applied for each increment. P-y curves representing static monotonic and cyclic loading conditions were derived from strain gauge data using a moving cubic polynomial technique. The results of push-over analyses using the derived p-y curves as input agreed well with the measured load-displacement response. It was observed that the cyclic p-y curves exhibited a sharp concave-up shape which contrasted with the broader concave-down shape of the static p-y curves. The cyclic p-y curves also exhibited lower peak resistances relative to the static p-y curves. The initial portion of the cyclic p-y curves is relatively flat with near-zero resistance which represents gapping of the soil. The displacement at which the soil reaction component of the cyclic p-y curves begins to dramatically increase is a function of the maximum displacement during previous loading cycles. It was also observed that the effects of cyclic loading were more pronounced in shallower soils.

Behavior of a Nine-Pile Group With and Without a Pile Cap

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.