Extrem climate engineering geology: Soil engineering properties response to drought climate and measures for disaster mitigation

Abstract

<p indent="0mm">Due to global climate change, extreme climate events occur in increasing frequency and intensity and act on earth surface in various ways, inducing a series of engineering geological disasters, which seriously affect the sustainable economic and social development, and bring many new challenges to current engineering geology research. There is an urgent need to strengthen the basic research on extreme climate engineering and disaster prevention and mitigation. It has important strategic and practical significance for improving China’s defense and decision-making capabilities when major projects are subjected to climate change and extreme climate events, and for improving China’s comprehensive prevention and control capabilities of natural disasters. It is also an important mission and development direction of modern engineering geology. This paper focuses on how the extreme climate affects geological bodies and major projects, and how to induce disasters from the perspective of engineering geology. In addition, this paper systematically summarizes the research advances of drought climate-induced soil engineering property response processes and catastrophic mechanisms, especially the processes and mechanisms of soil evaporation, shrinkage, and desiccation cracking. Generally, the evaporation process of soil water occurs in three fairly distinct stages: constant rate stage, falling rate stage and residual stage. The constant rate stage occurs when the soil is still fully saturated. The moisture transfer is dominated by liquid flow and mainly controlled by capillary force. The change of soil from saturated to unsaturated state results in the evaporation transition from constant rate stage to falling rate stage. In this stage, the moisture transfers in both liquid and vapor forms and the later one gradually dominates the evaporation process. The evaporation reaches residual stage when the pore water is dominated by absorbed water, and the moisture transfers in vapor form only. The volumetric shrinkage soil is mainly attributed to suction-induced pore size reduction. Based on soil shrinkage characteristic curve, three shrinkage stages can be distinguished: normal shrinkage, residual shrinkage and zero shrinkage. Most of the volumetric shrinkage occurs before the air-entry point while the soil is still fully saturated. The shrinkage direction of soil shows obvious anisotropy, which can be quantified by shrinkage geometry factor. At low degree of compaction, the radial shrinkage strain is higher than axial shrinkage strain, and the shrinkage geometry factor is larger than 3, while it is contrary at high degree of compaction. The reversibility of shrinkage deformation depends on soil suction. Shrinkage deformation is reversible when suction is higher than <sc>113 MPa.</sc> Drying-induced soil desiccation cracking process takes place in three typical stages (i.e., primary cracks initiate, sub-cracks initiate and crack network stabilization), and presents evident time-order characteristics. New cracks always start perpendicularly from the existing cracks. Generally, the initiation and propagation of desiccation cracks show evident dynamic characteristics and significantly depend on soil water evaporation rate, stress state and shrinkage potential. The cracks initiate at constant evaporation rate stage where soil is still fully saturated. The cracking water content can be much higher than liquid limit. Cracking is likely to occur if the tensile stress, which is induced by soil suction or shrinkage constraint, reaches the tensile strength of soil. It is believed that soil suction and tensile strength are the two key mechanical parameters that control the cracking behavior. From a perspective of soil structure, cracking is the result of pore shrinkage and microstructure re-arrangement. The mechanical effect and shrinkage potential are proposed as the two prerequisite factors for crack initiation. Quantitative characterization of crack patterns plays a very important role in understanding the cracking dynamic and mechanism. Image processing technique has been proved as a powerful, efficient and high-accurate tool for quantitative description of crack patterns. In the past decades, systematic research methods for soil cracking have been well established, including testing, monitoring, quantifying, data processing etc. These achievements make up for the research blanks of drought climate disasters in the field of engineering geology, and provide scientific basis for guiding engineering geological practice and disaster prevention and mitigation in arid regions. In the future, in addition to strengthening basic research on climate change and atmosphere-geological body interaction, the related technical research should also be strengthened, such as distributed optical fiber sensing technology, microbial geological engineering technology on nature-based solutions, big data and cloud computing technology, artificial intelligence technology, so as to prevent and mitigate disasters in engineering geology by providing advanced theoretical and technical support.