Intergranular Water Effects on Shear Behavior of Wet Sand: Phenomenology Based on Direct Shear Tests and Fault Rupture Physical Modeling

Intergranular water in granular soils, such as sands, can produce apparent cohesion in the soil that results in changing the behavior of soil while shearing. Apparent cohesion in wet granular soil arises from surface tension and capillary effects of the small water bridges between adjacent soil grains. This intergranular water exerts an attractive force between the grains and an apparent cohesion is produced. Induced cohesion in wet soil depends on the amount of intergranular water within the pores of soil.
In this investigation, the effect of water content on the sand strength parameters (such as cohesion and internal friction angle) and deformational parameter (such as failure strain) were studied. For this purpose, some series of direct shear tests were conducted in which soil water content and its vertical stress were widely changed. Samples with 0% (dry condition), 5%, 10%, 15% and 24% (saturated soil condition) were examined in nine vertical stress (that categorized into two fields: low and high vertical stress condition). The low normal stress condition contained 0.015, 0.03 and 0.045 kgf/cm2. Vertical stresses of 0.1, 0.25, 0.4, 0.6, 0.85 and 1.25 kgf/cm2 were determined as the high stress condition. The importance of low stress condition is to obtain the exact behavior of soil at low stress condition that corresponds to vertical stresses in small scale physical models.
In this research, the investigation is based on two points of view: 1) from deformational point of view, the most important result is the relationship between soil’s water content and its failure strain. Adding water to sandy soil decreases its failure strain and it has its lowest value at about 5% water. It means that wet soil behaves more brittle than dry one. In this point of view, also a good similarity can be observed between the results of direct shear tests and physical models results (D0/h parameter); 2) From strength point of view, in low vertical stress tests, with an increase in water content to about 5%, internal friction angle increased and beyond this limit decreased. As in this situation, almost a constant cohesion obtained from the data, it can be concluded that by an increase in water content to about 5%, soils shear strength also increases and then decreases (in low stress condition that it corresponds to physical modeling stress level). Besides, discussions were made on the behavior of sand at high stress level and in residual condition.
Above-mentioned results have been employed to interpret the result of some fault rupture physical modeling tests on wet sand. The pure sand was used with various water content (0% (dry), 5%, 10% and 15%). One of the important results that can be achieved in physical modeling tests is the normalized required fault displacement (D0/h) at the bedrock, in which the fault rupture trace reaches the ground surface. Obtained results show that as the water content increased up to a certain value (around 5%), the D0/h parameter decreases at first and beyond this limit it increases a little. Generally, it seems that soil behaves more brittle when it is wet. Besides, it is in its most brittle behavior when the water content of soil is around 5%. The observed trend of D0/h parameter is almost similar to what observed on failure strain of the wet sand that was achieved by direct shear tests.