INFLUENCE OF CAPILLARY ZONE ON SETTLEMENT OF FOOTINGS ON SAND

Abstract

The effect of capillary water in the capillary zone of a sand deposit is to increase the in-situ effective vertical stress. This results in an increase in stiffness of the soil. As a consequence, a footing resting in the capillary zone of a sand bed will undergo smaller settlement as compared to a foot-ing resting on a dry/submerged deposit. Though this fact is known, no attempt has so far been made to quantify the effect of capillarity on the settlement of footings on sand. Hence an analytical as well as an experimental investigation has been carried out to study in a qualitative fashion, the effect of capillarity on the settlement of a footing resting on sand. Assuming that the elastic modulus of the soil is proportional to the in-situ mean effective principal stress an analytical solution is presented. The analysis takes into account the increase in effective normal stress due to capill-arity in soils. The results of the analysis are presented in the form of settlement ratio vs width ratio to enable estima-tion of settlement of a footing from the results of a plate load test conducted on a capillary bed of sand. load tests on model footings of 12 cm diameter were conducted on test beds of Solani river sand prepared at a relative density of 82 percent. The tests were conducted for various conditions of test bed, viz., dry bed, submerged bed, capillary bed and wet bed. Tests on capillary bed were conducted for different thickness of capillary zone. The results of load tests are presented in the form of load - settlement curves. The results of analytical and experimental investiga-tion reveal that the capillarity in a sand reduces the _settle-ment of footings substantially. The amount of red Lion is depe~adent on the thickness of capillary zone. If this factor is disregarded in the estimation of the settlement of a footing using plate load test data obtained under capillary bed conditions, it may lead to a serious under estimation of the settlement of a footing which may not be resting on a capillary bed. The error may be of the order of about ¢00 percent or, more in the case of silty sand where the height of capillary' rise can be of the order of l to 3 meters. The ultimate bear-ing capacity of a footing will also increase substantially .under capillary conditions. Results of some 'field load tests on a sandy soil, using 60 cm x 60 cm test plate have also been analysed to examine the possible effect of capillarity on the settlement of the test plate. The analysis revealed that capillarity effect was in fact the reason for smaller settlement observed in the test plate placed at about 1.5 in above water table.