Studies on Ultimate Bearing Capacity of Shallow Foundation Resting over Geogrid-Reinforced Sand under Centric Inclined and Eccentric Vertical Load

Abstract

Since the original work by Binquet and Lee (1975), a number of studies have been conducted to evaluate the possibility of constructing shallow foundations on reinforced soil to increase their load-bearing capacity and reduce settlement. Results of several laboratory model test studies have been published relating to the improvement of the load-bearing capacity of shallow foundations supported by sand reinforced with various materials such as geogrids, geotextiles, metal strips, metal bars, rope fibers etc. The use of geogrid for soil-reinforcement has increased greatly, primarily because geogrids are dimensionally stable and combine features such as high tensile modulus (low strain at high load), open grid structure, positive shear connection characteristics, lightweight and long service life. The open structure provides enhance soil-reinforcement interaction.
Most of the previous studies relate to using geogrid as reinforcement in the soil under foundations subjected to centric and vertical load to evaluate the ultimate and allowable bearing capacities. Based on the review of the existing literature , it appears that no attention has been paid on the load carrying capacity of shallow foundations resting over geogrid-reinforced soil subjected to inclined and eccentric loadings. The objective of this thesis is to study the two aspects namely (i) the effect of load inclination on the load carrying capacity of shallow strip foundation resting over reinforced sand and (ii) the effect of load eccentricity on the load carrying capacity of rectangular foundations.
In order to arrive at the objective and to quantify certain parameters, two sets of extensive laboratory model tests have been conducted. In the first set the ultimate bearing capacity of shallow strip foundation resting over geogrid-reinforced sand bed subjected to inclined load has been determined. These tests have been conducted on dense and loose sand at relative densities of 69% and 35% respectively. The load inclination with respect to the vertical () has been varied from 0 to 20 with increments of 5, whereas the number of geogrid layers (N) varied from 1 to 4. The embedment ratio (Df /B; Df = depth of foundation, B = width of foundation) has been varied from 0 to 1 with increments of 0.5. Based on the model test results, an empirical non-dimensional reduction factor has been developed to estimate the ultimate inclined load per unit area of the foundation under inclined loading if the ultimate bearing capacity under vertical loading is known.
The second set of laboratory model tests have been conducted to determine the ultimate bearing capacity of a surface rectangular foundation resting over multi-layered geogrid reinforced sand with varying width-to-length (B/L; B, L = width and length of the foundation respectively) ratio subjected to eccentric vertical loading. The load eccentricity ratio e/B (e = eccentricity) has been varied from 0 to 0.15 with increments of 0.05, width-to-length ratio (B/L) has been varied as 0, 0.33, 0.5 and 1. Number of geogrid layers varied as 2, 3 and 4. Based on the model test results, an empirical non-dimensional reduction factor has been developed for each (B/L) ratio. A generalized empirical equation has been developed to estimate the ultimate bearing capacity of rectangular foundations resting on geogrid-reinforced sand subjected to eccentric load.
Numerical modelling based on finite element analysis by using plaxis 3D has been conducted to study the effects of number of geogrid layers, relative density of soil, load inclination, load eccentricity, failure mechanism, displacements, plastic flow on the behavior of inclinedly loaded strip foundation and eccentrically loaded rectangular foundation supported by reinforced sand. A close agreement between the experimental and numerical results is observed.
An equation has been developed for reduction factor obtained by ANN that can be used to estimate the ultimate bearing capacity for two different cases of experiments. The results from ANN analysis have been compared with model test results and appear to be good. Based upon the sensitivity analysis important input parameters is findout.