Modulus of Subgrade Reaction of Unreinforced and Geogrid-Reinforced Granular Fill Over Soft Clay

Abstract

In the present paper, the modulus of subgrade reaction or subgrade modulus for sand–clay layered soil (granular fill over clay) with or without geogrid reinforcement is determined under vertical loading by a series of model-scale plate load tests (PLTs). Various parameters such as thickness of the sand layer over clay, shape and width of the test plates, number of reinforcement layers, and spacing between two closely spaced plates (to simulate the interference effect of foundations) are included in the study. Five model plates with different aspect ratios (length to width ratio, L/B) are used to investigate the effect of aspect ratio and width of the plate on modulus of subgrade reaction for layered soil. The modulus of subgrade reaction for unreinforced soil increases significantly when the sand thickness increases over clay even if the thickness is more than twice the width of the test plate. However, marginal increment in modulus of subgrade reaction is observed when the sand thickness to plate width ratio is greater than 1.25 for geogrid-reinforced sand–clay layered soil. The maximum improvement in modulus of subgrade reaction due to inclusion of geogrid is observed when the thickness of the granular fill over soft clay is 0.75 times the width of the test plates. For an unreinforced bed, the modulus of subgrade reaction can be reduced up to 16% due to interference effect when the spacing (S) between the plates is less than 1.33 times the width of the plates. For reinforced sand–clay bed, 12% reduction in subgrade modulus can occur due to the interference effect. The subgrade modulus decreases with aspect ratio of the plates for both single and dual plate cases.