Laboratory studies on the bearing capacity of unsaturated sands

Abstract

In this thesis, extensive experimental studies were undertaken by conducting more than 250 bearing capacity tests on model footings to investigate the bearing capacity of three compacted (coarse-grained, sub-coarse-grained, and fine-grained) sands under both saturated and unsaturated conditions. These tests were conducted in specially designed equipment which includes two test systems for conducting bearing capacity tests using model footings. The designed equipment system had provisions for soil drainage, soil suction profile set for measuring the variation of matric suction with depth of water table, tensiometer holders, vacuum water pumps and other accessories. Two different soil containers (short container with 300 mm in diameter and 300 mm in height and the tall container with 300 mm in diameter and 700 mm in height) were used in the study. The bearing capacity was determined using four different sizes of square model footings (i.e. 20 mm x 20 mm, 25 mm x 25 mm, 37.5 mm x 37.5 mm, and 50 mm x 50 mm). All the equipment was designed by the author and fabricated in the student workshop at University of Ottawa. The experimental results demonstrated that the bearing capacity of unsaturated fine-grained sand increases as the matric suction increases. However, experimental studies on coarse-grained sand show that bearing capacity appears to decrease as the matric suction increases. Some practical reasons for such a behavior are explained in the thesis. Experimental studies have also demonstrated that footing sizes in the range of 20 mm x 20 mm to 150 mm x 150 mm had no significant influence on the bearing capacity of sandy soils. The boundary conditions (i.e., ratio of soil container size to footing size) have some effect on the bearing capacity of sands. A separate experimental program Saturated compacted silt after settlement offers significant resistance to the applied static loads without undergoing creep, however; bearing capacity reduces significantly when it is subjected to dynamic or cyclic loading conditions.