Experimental investigation of CFFP-soil interaction in sand under cyclic lateral loading

An experimental investigation was conducted on the response of concrete-filled fiber-reinforced polymer piles (CFFPs) in dry sand subjected to lateral cyclic loading. Two types of instrumented model pile tests were conducted. Fifteen tests were conducted with typical load magnitudes and numbers of cycles, and one test was conducted under static loading. The series of tests provided continuous measurements of the bearing characteristics of the model piles and of the responses of the piles and soil because of differences in the pile sizes, cycle frequencies, and numbers of cycles. Significant softening or degradation of the soil near the tested piles was found under large lateral cyclic loads. The rate effect was also determined by changing the cycle frequency. The behavior of the softening and the rate effect were determined by varying the number of cycles and the magnitude of the loads. The tests demonstrate that the maximal bending moment and soil resistance of CFFPs degrade by approximately 15 % and 25 %, respectively, from 50 to 700 cycles at the ultimate loading level. These tests confirm that the maximal bending moment was 35 % to 50 % greater under dynamic loading than under static loading, and the soil degradation was remarkable. The relatively short CFFPs more easily reached the ultimate state than the long CFFPs. The results were used to fit the modulus of subgrade reaction (k), and a new formula to predict k is proposed, which shows good precision.