Shallow cohesive soil interacts strongly with the atmospheric environment, and its engineering properties deteriorate severely due to the influence of climate factors, leading to geological disasters and ecological environmental problems. In response to the above issues, a combination of bio-glue and rice straw fiber was introduced to improve the mechanical properties of cohesive soil. Through unconfined compressive strength tests and direct shear tests, the changes in the unconfined compressive strength characteristics and shear strength parameters of soil with single admixture and composites were investigated. The effects of bio-glue and fiber contents on the improvement effect of composite treatment were studied, and the effect mechanism of bio-glue and straw fiber composite modified cohesive soil was analyzed based on scanning electron microscope (SEM). The results show that due to the increase in bio-glue content and fiber content, the stress-strain curves of modified soil tend to behave as strain hardening, with a prolonged peak position and reduced post-peak stress decay. Both unconfined compressive strength and shear strength of modified soil are significantly enhanced, which mainly enhances the soil’s mechanical properties by increasing cohesion and has little effect on the internal friction angle. Bio-glue improves soil structure through surface coating, particle bonding, and pore filling, which also improves the reinforcement effect of fiber-soil interface and enhances soil's mechanical properties by combining the network framework formed by fibers.

According to the definition of the barrier effect of underground structures, the concept of equivalent barrier depth was introduced. Analytical solutions for the groundwater head, groundwater retention time distribution, and average retention time in the urban confined aquifer under three boundary conditions of constant flow rate, constant head, and diffuse recharge were derived. Numerical experiments were set up to verify the results, and the results show that the analytical solution is in good agreement with the numerical solution. The results demonstrate that the analytical solution is in strong agreement with the numerical solution. However, the fitting performance deteriorates due to hydrodynamic dispersion when the dimensions of the underground structure are relatively large. Moreover, the analytical solutions of the average retention time agree well with the numerical results in various simulation cases (with a relative error of no more than 5.94%). Even in the three-dimensional flow field, the derived analytical solutions can be used to estimate the average retention time of confined aquifers. Finally, the analytical solution is applied to the confined aquifer flow field before and after the construction of four subway stations in Chengdu Metro Line 2. The calculation results show that compared to the head, the retention time is more sensitive to the parameters of underground structures.

Taking the YD002 borehole in Tongzhou Bay area of Jiangsu Province as the research object, this paper establishes a DFOS (distributed fiber optic sensing) system of land subsidence to monitor the deformation and subsidence of Quaternary sedimentary layer in this area for a long time. Based on the monitoring data, the current situation and trend of land subsidence in this area are studied and analyzed. The results show that the main deformation source in this area is the compression deformation of the soil layer caused by the vertical water release of the confined aquifer group in the upper part of the main pumping layer. The whole stratum shows a compression trend, and the deformation of each compression layer is closely related to the variation law of groundwater level. Compared with the traditional layer-wise mark, the DFOS technology can more finely measure the vertical compression and rebound deformation of soil layer.

In order to study the influence of sisal fiber and interface roughness on the shear mechanical properties of soil-rock interface, concrete modules with rough surface were used as the rock surface similar material and a series of indoor shear tests were carried out. Combined with the SEM test, the mechanism of sisal fiber participating in the strengthening of the interface is further analyzed. The results show that sisal fiber can improve the shear mechanical properties of soil-concrete interface by improving the cohesion. The addition of 0.8% by weight increases the interfacial bond strength of soil-cement interface by 0.8-5 times and the internal friction angle by 0.1-0.2 times. The interface roughness mainly enhances the shear mechanical performance of the interface by improving the bond strength. For an interface roughness of 6.5 mm, the increase of interfacial bond strength ranges from 7 kPa to 15.5 kPa, resulting in an improvement of 0.2-3 times. The enhancement of roughness on the shear mechanical performance of the soil-cement interface exhibits two distinct patterns for the plain soil interface and the fiber-reinforced interface. Regarding the relationship between interfacial bond strength and roughness for the fiber-reinforced interface, the optimal value of roughness appears within a smaller range (0-2.5mm).

In order to study the compressive properties, deformation rules and improvement mechanism of sand improved by the organic polymer composite fiber, through the unconfined compressive strength test and the numerical simulation, the laws of force chain change and microcrack development in the compressive deformation process of improved sand were analyzed, from which the destruction and improvement mechanism of improved sand were summarized. The results show that the organic polymer composite fiber can effectively improve the compressive strength of sand. With the increase of organic polymer content, the unconfined compressive strength of improved sand increases. In addition, with the increase of organic polymer content and strain, the force chain evolution and microcrack development of the improved sand change obviously. The force chain changes from a ring-column structure to an arch structure, and the total number of microcracks and tensile microcracks also increases, indicating that failure changes from single path to multiple paths. The improvement mechanism shows that the organic polymer composite fiber can effectively improve the compressive properties of sand by binding and wrapping the sand grains. When the load is too large, the organic polymer film breaks, the fibers break and slip gradually, and the network structure becomes unstable gradually. In such a condition, the sand is forced to displace and rotate, forming local micro-cracks that eventually develop and extend to form cracks resulting in damages.