Search

Groundwater levels (GWLs) are closely linked to environmental engineering geological issues in urban areas. With urban pumping control and the increase in groundwater recharge associated with global warming, rising GWLs can lead to hazards, such as seepage and sand liquefaction, which threaten the foundation of engineering infrastructure and adversely affect the lives of urban residents. However, urban engineering geological issues caused by rising groundwater are yet to be comprehensively quantified. In this study, we investigated spatial groundwater variation and the extent of seepage, sand liquefaction, and loess hydro-compaction using empirical equations, statistical analysis, and a numerical groundwater model for the most populous city in northwest China, Xi'an. Our results show that the overall GWL declined significantly before 2002 and gradually increased from 2003. The spatial trend in the GWL was determined to be highly heterogeneous. The maximum potential areas of seepage, sand liquefaction, and loess hydro-compaction accounted for 38%, 8%, and 11% of the total area, respectively. The effects of increasing areal recharge were smaller than the effects of decreasing the pumping rate for the expanding area of sand liquefaction and loess hydro-compaction, based on model projections. Our study provides an evaluation framework and useful insights into the protection of urban groundwater systems. • The spatio-temporal variation in urban groundwater levels showed high heterogeneity. • Spatial patterns of seepage, sand liquefaction, and loess hydro-compaction were investigated. • Urban pumping, more than areal recharge, impacts groundwater-associated hazards. [ABSTRACT FROM AUTHOR]

[Display omitted] • We compared pore features and morphology before and after compression treatment. • Land subsidence transformed elongated macropores to regular and spherical pores. • Silt and silty clay soils > 100-m deep were the main contributors to land subsidence. • Compression decreased saturated hydraulic conductivity. • Saturated hydraulic conductivity was highly related to pore parameters. 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 (K sat). 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 K sat. 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, K sat differed significantly between treatments. The largest pore area, average pore diameter, and regular porosity accounted for 78.2 % of the change in K sat. 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. [ABSTRACT FROM AUTHOR]

Land subsidence is one of the major urban geological hazards, which seriously restricts the development of many cities in the world. As one of the major cities in China, Xi'an has also been experiencing a large area of land subsidence due to excessive exploitation of groundwater. Since the Heihe Water Transfer Project (HWTP) became fully operational in late 2003, the problem of subsidence has been restrained, but other issues, such as ground rebounds, have appeared, and the effect of the underground space utilization on land subsidence remains unsolved. The spatial-temporal pattern of land subsidence and rebound in Xi'an after HWTP and their possible cause have so far not been well understood. In this study, the evolutionary characteristics of land subsidence and rebound in Xi'an city from 2007–2019 was investigated using Small Baseline Subset Interferometric Synthetic Aperture Radar (SBAS-SAR) technology to process the Advanced Land Observing Satellite (ALOS) and Sentinel-1A SAR datasets, and their cause and the correlation with groundwater level changes and the underground space utilization were discussed. We found that the land subsidence rate in the study area slowed from 2007–2019, and the subsidence area shrank and gradually developed into three relatively independent and isolated subsidence areas primarily. Significant local rebound deformation up to 22 mm/y commenced in the groundwater recharge region during 2015–2019. The magnitude of local rebound was dominated by the rise in groundwater level due to HWTP, whereas tectonic faults and ground fissures control the range of subsidence and the uplift area. The influence of building load on surface deformation became increasingly evident and primarily manifested by slowing the subsidence reduction trend. Additionally, land subsidence caused by the disturbances during the subway construction period was stronger than that in the operational stage. Future land subsidence in Xi'an is predicted to be alleviated overall, and the areas of rebound deformation will continue increasing for a limited time. However, uneven settlement range may extend to the Qujiang and Xixian New District due to the rapid urban construction. Our results could provide a scientific basis for land subsidence hazard mitigation, underground space planning, and groundwater management in Xi'an or similar regions where severe ground subsidence was induced by rapid urbanization. [ABSTRACT FROM AUTHOR]