Study on Suction in Unsaturated Soils with Overlapping Meniscus in Pore Water and Pore Air Phases   Shun-qun Li Department of Civil Engineering, Tianjin Institute of Urban Construction and Tianjin Key Laboratory of Soft Soil Characteristics and Engineering Environment, Tianjin, China e-mail: lishunqun@sina.com Ying-hong Wang Department of Electronic & Information Engineering, Tianjin Institute of Urban Construction, Tianjin, China e-mail: wangyinghong66@sina.com Ling-xia Gao School of Civil and Hydraulic Engineering, Dalian University of Technology and School of Architecture and Civil Engineering, Dalian nationalities University, Dalian, China e-mail: gaolingxia2006@126.com
			ABSTRACT

Based on the investigations on a four ball-shaped particle packing system model with isolated pore water according to thermodynamics and principles for soil mechanics, it is recognized that in additional to the conventional matric suction distributed on contact area of pore water to particle surface, there exists a normal tension force which is distributed along the saturation line on grain surface due to surface tension while the meniscus overlaps on particle surface. The line-distributed tension can be transformed equivalently into an area-distributed tension stress that is defined as tension suction and will pose the similar effect on shear strength and deformation as that of matric suction. According to the actual contact states of meniscus with the surface of soil particles, the unsaturated soil with Continuous pore Water and Continuous pore Air (CWCA) unsaturated soils can be further classified into the overlapping and non-overlapping CWCA unsaturated soil. Therefore unsaturated soils with different water content can be classified as Discontinuous Water & Continuous Air (DWCA) and overlapping CWCA and non-overlapping CWCA and Continuous Water & Discontinuous Air (CWDA) from very low water content to completely saturated state. It is clear that tension suction appears only in DWCA and overlapping CWCA unsaturated soils. Furthermore, the equivalents of matric and tension suctions are put forward to take both intensity and action area of matric suction and tension suction on grain surface into consideration.
The critical water contents of DWCA and overlapping CWCA and overlapping CWCA and non-overlapping CWCA are studied for the four particle packing ideal model illustrated above. In addition to magnitude, effective area of matric suction on particle surface and tension suction are expressed mathematically on the assumption that pore water surface is circle. And variations of matric suction and tension suction and their corresponding equivalents with saturation degree at different contact angle can be determined. Computational results show some important conclusions for the ideal model list as follows. The value of matric suction decreases sharply with saturation degree and obtains a value of zero at a saturation degree of about 30% that is not very big in common conditions. The variation of tension suction with saturation degree has similar tendency with that of matric suction and tension suction vanishes at saturation degree of about 35%-40%. Variation ranges of the two kinds of equivalent suction are smaller than that of the corresponding suctions. And various kinds of suction increase with contact angles and this conclusion displays effect of mineral component on characteristics of unsaturated soils. As a result, both matric suction and tension suction are introduced by instauration and they all contribute to shear strength of unsaturated soils. Therefore the angle of shear strength to matric suction may achieve any value from zero to 90o and this is adverse to the current viewpoint about matric suction in literature.

Keywords: unsaturated soils; matric suction; tension suction; resultant suction; effective area; equivalent suction; overlapping unsaturated soils

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