Bearing capacity of circular footing on geocell reinforced sand deposit under cyclic loading

Abstract

Sand has the characteristics of low bending and tensile strength. One of the methods to improve the bearing capacity of sand is using geocell, in which the sand is improved through the interaction between the sand and geocell, and through the sand mattress effects as a result of sand filling the pockets of geocell. The aim of this research is to determine the effect of geocell reinforcement on the bearing capacity of circular footing on sand deposit under static and low frequency cyclic loading through the laboratory physical model tests and numerical simulations using ABAQUS 3-D finite element software. The laboratory physical model tests had been carried out using 75 mm diameter (D) circular footing on sand reinforced with geocell, placed at various depth ratio (u/D). The geocell had a 450 mm length, various width (b) and height (h). Homogeneous sand was formed in box models of 620 mm length, 620 mm width and 500 mm height. The relative densities of sand used were 30% and 70%. The ultimate bearing capacity (qu) obtained at the settlement (s) equals to 10%D was used as the basis for calculating the cyclic stress amplitude in the cyclic tests. The frequency of 0.067 Hz and three cyclic stress amplitudes of 0.15qu, 0.25qu and 0.4qu were used. Three patterns of geocell were tested; honeycomb, diamond and chevron. In the numerical simulation, the infill sand was modeled using the Mohr-Coulomb and the geocell was modeled using linear elastic. The optimum u/D was found as 0.1. The settlement ratio (s/D) increased with the number of cycles and reached a sensibly constant maximum value of less than 10% at high number of load cycles. The s/D correlates linearly with the cyclic stress amplitude and relative density. The correlation equations obtained can be used as preliminary design charts. There were good agreements between the results from numerical and experimental models indicating high reliability for prediction of low frequency of cyclically loaded behavior of footing. The static extra safety factor, Fe of between 1.1 to 1.17 was suggested to be used together with the global factor of safety when calculating the safe bearing capacity. Fe depends on relative density and pattern of geocell. The cyclic extra safety factor, Fc is recommended to be used if utilising the settlement obtained from numerical modelling to calculate the expected settlement to be achieved. The range of Fc for unreinforced sand deposits is between 0.8 and 0.9 while it is 0.9 to 0.93 for geocell reinforced sand deposits. The values depend on the pattern of geocell reinforcement, relative density and cyclic stress amplitude. The results revealed that all patterns of geocell increased the bearing capacity of sand under static load and reduced the settlement under cyclic loading, but with more significant improvement in dense sand. The chevron pattern gives the most beneficial effect compared to the honeycomb and diamond pattern of geocell.