Soil pore characteristics, morphology, and soil hydraulic conductivity following land subsidence caused by extraction of deep confined groundwater in Xi'an, China: Quantitative analysis based on X-ray micro-computed tomography

Abstract

The extraction of deep confined groundwater has been widely identified as the main factor leading to land subsidence. However, how soil pore characteristics and morphology may change as a result of land subsidence remains unclear. The objective of this study was to (1) quantify and compare the effects of soil compression due to deep confined aquifer exploitation on soil pore characteristics and morphology and (2) assess the relationships between pore parameters and saturated hydraulic conductivity (Ksat). Three kinds of lithology (clay, silt, and silty clay soils) from two 300-m engineering geological boreholes were investigated at natural saturated state (NSS) and complete drainage state (CDS). Seventeen undisturbed soil samples, 70 and 50-mm in diameter and height, respectively, were collected and subjected to steps of uniformly increasing loads in uniaxial confined compression tests to analyze pore characteristics, morphology, and Ksat. Keeping the samples in a strain-locked state, pore parameters were assessed after each loading step by using X-ray micro-computed tomography (CT; voxel resolution: 3-μm) images. A hundred CT images were acquired and quantified for each treatment in each soil sample. Soil pore characteristics and morphological parameters including number of pores, number of macropores (diameter (∅) > 1000-μm), number of coarse mesopores (∅ 200–1000-μm), number of fine mesopores (∅ < 200-μm), porosity, macroporosity, coarse mesoporosity, fine mesoporosity, largest pore area, average pore diameter, shape factor, structure model index, degree of anisotropy, and the Euler number were analyzed using ImageJ software. All parameters differed between NSS and CDS treatments, especially in silt and silty clay soils below 100-m depth, and the differences seemed to increase with increasing depth. However, parameter variations were not obvious for clay soil. Furthermore, Ksat differed significantly between treatments. The largest pore area, average pore diameter, and regular porosity accounted for 78.2 % of the change in Ksat. Thus, the decrease in proportion of elongated macropores favors land subsidence, particularly in silt and silty clay soils below 100-m depth. Efforts such as optimization of groundwater exploitation depth should be made to alleviate and control land subsidence.