EVALUATION OF SOIL MODULUS IN HORIZONTAL AND VERTICAL DIRECTIONS FOR COHESIONLESS SOILS

Abstract

The problem. of computing the distribution of the contact pressures below the base of rigid foot-ings and raft foundat!Lons is rather complicated. Such problems can be simplified with the use of values of vertical soil modulus. Design of rigid pavements and grillage foundation also involves the use of vertical ' soil modulus. Large lateral loads act, on pile foundations supporting structures such as bridge abutments, re-taining walls; anchors for bulk heads, piers,fenders, doiphine anchorages, foundation of portal frame, and transmission towers and poles. Such lateral loads are caused by wind, waves, lateral earth pressures, lateral blast forces and earthquakes.. In analysing these problems a knowledge of horizont al soil modulus is quite essential. Technique and test data involved to determine vertical soil modulus is available in the literature but very little informa-tion is available to measure the horizontal soil mo-dulus and vertical soil modulus simultaneously in the field. In the present investigation , rigid plate load test techRique was adopted to evaluate the vertical and horizontal soil moduli at the sane site. Tests were -2- carried out at two sites having cohesionless soils. Tests were conducted on circular plates of diameters, 30 cm, 45 cm 60 cm, 75 cm, 90 cm and 105 cm, and a 30 cm square plate. Plat3s were tested under vertical and horizontal loads to determine vertical and hori-zontal modulus respectively. Three vertical test series were performed at ground level, 54 cm and 190 cm below G.L. whore-'e four horizontal load test sel.ies were performed at various depths with in 2.44 m below G.L. The load was applied by means of hydraulic jack and measured on a proving ring. The reactions in case of vertical tests were taken by gravity loading and re-action loading. In case of horizontal tests the re-action was taken by plate having bigger size than the test plate and abutting it to the vertical opposite face of the pit. Co-efficients of soil modulus were coinJuted both in vertical and horizontal directio -is. Effects of diameter and shape of plate, deflection and depth on these co-efficients have been studied. In framing equations between soil modulus and diameter of plate for various depths a computer programme on I B M 1620 Digit al Computer was run The following conclusions are drawn from the present study:- i) The soil behaves as an elasto-plastic material. ii) The value of co-efficient of horizcutal subgrade reaction is not zero at Ground Surface. iii) The soil moduli in horizontal and vertical direction decreases with the. increase in dia-meter of plate. However there is no signi-ficant change in the value of these co-efficients when the diameter of plate increases above 75cm (30 in). Values of co-efficients obtc.,ined on 75 cm diameter plate can be used in all the problems involved. iv) The soil moduli 5.n both directions vary almost linearely with depth. v) Secant Modulus in both directions decrease with increase in diameter of plate and deflection. vi) The ratio of co-efficient of vertical soil modulus and horizontal soil modulus decreases with the increase in depth. vii) Knowing the value of one co-efficient, diameter of plate and depth, it may be possible to evaluate the other co-efficient by using the equations and curves presented here.