Abstract
In this article, effect of inter-granular void ratio (eg) on the volume compressibility and undrained shear strength behaviour of a natural silty-sand and base-sand is explored. Natural silty-sand sample was collected from Fatehgarh dam in Kutch region of India and the specimens prepared were subjected to isotropically consolidated undrained compression (CIUC) triaxial tests under two scenarios. In scenario one, silty-sand collected from Fatehgarh dam was used to perform CIUC triaxial tests at in-situ density. However, scenario two was based on CIUC triaxial tests on base-sand, which was extracted from Fatehgarh dam silty-sand soil by removing fines. It was ensured that the two specimens (silty-sand, base-sand) had the same void ratio (e = 0.704) but different inter-granular void ratios (eg-silty-sand = 1.156 and eg-base-sand = 0.704). Volume compressibility during isotropic consolidation phase of specimens was strongly reduced in the base-sand as compared to silty-sand. The undrained shear response of base-sand showed an increase in peak deviatoric stress by a factor of 1.8, 1.6 and 1.7 as compared to silty-sand at an initial effective confining pressure of 100 kPa, 200 kPa and 300 kPa respectively. The angle of friction mobilized at peak deviatoric stress and the work done per unit volume increased with the decrease in the inter-granular void ratio (eg), although the void ratio (e) was same.
Highlights
Stress-strain behaviour and effective stress path response of sandy soils under undrained conditions could be classified in three typical responses as introduced by Castro [1]: strain softening (SS), limited strain softening (LSS) and strain hardening (SH)
At a given p c the presence of smaller silt particles created metastable inter-particle soil structure, which being collapsible in nature, resulted in higher volumetric strains in natural silty-sand specimens
Following conclusions were drawn from the study: 1. At a similar void ratio (e) but different intergranular void ratio, volume compressibility of natural silty-sand specimens was found to be more than 1.5 times as that of base-sand specimens
Summary
Stress-strain behaviour and effective stress path response of sandy soils under undrained conditions could be classified in three typical responses as introduced by Castro [1]: strain softening (SS), limited strain softening (LSS) and strain hardening (SH). Recent studies reported a decrease in liquefaction resistance with the addition of fines till a limiting fines content but with the further addition of fines liquefaction resistance increased This behaviour could not be explained under the CSSM framework due to a number of challenges. This was attributed to the non-uniqueness of the locus of the critical states (critical state line, CSL) in the e-ln(p ) plane; where e and p are the void ratio and mean effective pressure respectively. The unusual “reverse sand behaviour” of silty-sands reported by Lade and Yamamuro [3] and later observed by others could not be explained under the CSSM framework using void-ratio based state parameter Both the compressibility as well as undrained shear strength were significantly affected by the presence of fines. CSL and ICL were determined to be non-unique for sands with different fines content
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