BEHAVIOUR OF SKIRTED FOUNDATIONS UNDER HORIZONTAL VIBRATIONS

Abstract

When a loose saturated sand deposit is subjected to plusating stresses, positive pore-pressure develop. When the rise in pore-pressure becomes equal to the initial con-fining pressure, the effective confining pressure reduces to zero and the soil looses its strength resulting in sinking of structure. Confinement of the soil beneath the foundation is likely to reduce the settlement. Thus if a rigid skirt is provided around the foundation, it will prevent the soil beneath the foundation to displace laterally resulting in reduction of settlement. The present investigation is carried out to study the behaviour of skirted foundations in horizontal shake table under different vibration characteristics. Tests are carried out in a horizontal shake table with tank size of 105.7 cm x 60.2 cm x 61.5 cm high. A poorly graded locally available sand deposited at two different initial rela-tive densities of 30% and 50% is used in the investigation. The tank is subjected to horizontal accelerations varying from 0.10g to 0,25g at a frequency of vibrations of 1.8333 cps. Wooden shafts of two different diameter 2.54 cm and 7.6 cm with varying depth of foundation from zero to D(i.e. diameter of shaft) have been used. The depth of skirt is varied from zero to D (i.e. equal to diameter of shaft). Test data obtained has been analysed and conclusions based on the study have been drawn. The significant conclusions are:-. (a) The settlement of sand deposit increases with the increase in number of cycles. Sand deposits with higher relative density are less prone to settlements. (b) The settlement of shaft reduces with the increase in its diameter and decreases with increasing depth of foundations. (c) Skirted shafts settle much less at compared to shafts with no skirts. (d) The settlement of shafts increases with increasing magnitude of acceleration. Suggestions for future study have also been made