Shear strength features of soils developed from purple clay rock and containing less than two-millimeter rock fragments

Soil shear strength is an important indicator of engineering design and an essential parameter of soil precision tillage and agricultural machinery and equipment design. Although numerous studies have investigated the characteristics of different soil shear strengths, only a few of these works have paid attention to soils containing considerable quantities of rock fragments. To date, most studies on the effects of rock fragments on the shear strength have paid attention to the role of rock fragments with sizes > 2 mm. The effects of rock fragments < 2 mm in soil are generally ignored. Similar to rock fragments > 2 mm, the presence of rock fragments < 2 mm could also change the mechanical properties of soils. Thus, in the present study we evaluated the potential influence of < 2 mm rock fragments on soil shear strength via an unconsolidated undrained (UU) triaxial compression test. Our results were as follows: (1) A certain quantity of < 2 mm rock fragments presented in purple soils developed from clay rocks; and an appropriate quantity of < 2 mm rock fragments could improve the shear strength of soils. (2) The different PSDs of soils containing < 2 mm rock fragments mainly caused variations in the internal friction angle of soils. (3) The shear strengths of the two mudstone-developed red-brown and gray-brown purple soils was more sensitive to water than that of the shale-developed coarse-dark purple soil. As the soil water content increased from 9% to 23%, the changes in the cohesion, internal friction angle, shear strength, and the maximum principal stress difference were smaller in the coarse dark purple soil than in the two other soils. We therefore concluded that < 2 mm rock fragments in purple soils exerted important effects on soil shear strength. A better understanding of the differences among the shear strength features of purple soils could help improve the design of agricultural machinery and equipment.