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Received Jul 31, 2017; Revised Nov 17, 2017; Accepted Dec 13, 2017
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1. Introduction
Expansive soils, which are regarded as a problem soil, are typically encountered around the world. The total distribution areas of expansive soil are more than one hundred thousand square kilometers in China [1]. These soils have three typical properties (i.e., significant swelling and shrinkage, fissures, and overconsolidation) due to hydrophilic minerals such as montmorillonite. Their strengths are particularly sensitive to changes in the surrounding environment.
The failure of expansive soil slopes, especially surficial failure, is one of the most serious geological disasters that frequently occurs during the construction of highways, railways, and hydraulic engineering projects in expansive soil areas in China [1]. Figure 1 shows a surficial failure of an expansive soil cutting slope in Nanning-Baise expressway in Guangxi Zhuang Autonomous Region, China. Numerous researchers have studied the depth of the surficial failure of expansive soil slopes by field investigations. Liao [1] concluded that the surficial failure depths of most expansive soil slopes were in the range of 1.0 to 3.0 m, which was close to the development depth of fissures [2]. Wang et al. [3] reported that the sliding depth generally ranged from 1.0 to 2.5 m, and the reason for the surficial failure of expansive soil slopes may be a drastic reduction in the strength of slope surface soil. It results from the occurrence of swelling-shrinkage cracking and fissures due to the repeated wet-dry cycles. The Yangtze River Scientific Research Institute and Wuhan University found that the atmosphere influences the depth of the slope at a “critical depth” of approximately 1.5 m by analyzing the measurement results of the suction matrix in expansive soil slopes in the field [4].
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For an engineering project, the shear strength of expansive soils is required to address the stability of expansive soil slopes. A review of the technical literatures revealed that the investigation of shear strength behavior of expansive soils was limited, especially, with respect to the effect of swelling. Dinesh and Chandra [5] reported that the shear strength of the...