Shear Strength of Unsaturated Soils with Different Plasticity

Authors

Alaa El-Hosani Refai Kassab, Ph.D. student, Azza Hassan Moubark, Waleed Hamdy Elkamash, Associate Prof, Kamal Mohamed Hafez Ismail, Professor
Department of Civil Engineering- Faculty of Engineering- Suez Canal University – Ismailia – Egypt.

Abstract

A series of shear box tests were conducted on four different types of soils in this study. The first set , (clay1) is the host clay, Middle Delta Nile clay “MDNC” which was obtained from Mid Delta Nile zone, Qalyubiyya Governorate, Egypt. The second set, (clay2) consists of the host clay MDNC but mixed with 40% sand. The third set of tests (clay3) was carried out on natural low plastic clay samples brought from El-Tal El-Kbeer area in Ismailia Governorate. The fourth set (clayey sand -SC) consists of 25% of the host clay MDNC mixed with 75% sand. The purpose is to predict the unsaturated shear strength for various soil types. The results illustrate that for clay1, clay2, and clay3, the relationships between the unsaturated shear strength and matric suction are approximated to two linear failure envelopes intersected at matric suction equals to plastic limit. Each segment has an angle of internal friction due to matric suction  b ranging from 1.80 o to 2.90o for the first zone and  b ranging from 0.28 o to 0.20o for the second zone. Meanwhile, for SC soil it appears to have one linear failure envelope for all saturation levels with angle of internal friction due to matric suction;  b=1.19 o.. Design- Expert® software applied the least square method (LSM) to fit a mathematical model for the experimental data. The Response Surface Methodology (RSM) of the program is used to predict the required model. The paper proposed two general equations to calculate the shear strength of unsaturated clays and one equation for granular soils, which are presented at the end of the paper with their coefficients.