Shield tunnel is a type of linear underground structure assembled by lining segments, characterized with long joint, weak stiffness, and strict deformation control requirement. The situation of the long-term deformation and defect of the shield tunnel in soft ground in coastal area of China is severe, mainly attributed to the tunneling-induced ground consolidation, frozen cross passage, groundwater pumping, cyclic train load, and nearby construction. Shield tunnel is buried in ground, and the above factors could result in underlying ground settlement, overlying ground loading/unloading, and at-side ground unloading. As a result, the tunnel could suffer from different types of structural deformation and defect. Based upon the aforementioned different reasons, this study investigates the characteristics of the tunnel deformation and defect corresponding to the different types of ground stress change and deformation. It is found that tunneling-induced ground consolidation, frozen cross passage, groundwater pumping, and cyclic train load mainly contribute to the longitudinal differential settlement but negligible transverse convergence, associated with water leakages at circumferential joints. In comparison, surface surcharge and at-side unloading not only cause significant longitudinal differential deformation but also increase transverse lining internal forces, resulting in water leakages at circumferential joints, longitudinal lining concrete cracks and water leakages. Finally, nearby construction could strongly disturb the ground and cause the generation of excess pore-water pressure, making the shield tunnel deformation develops continuously after the nearby construction is completed.

The subgrade structure of high-speed railways is an important foundation for the safe and smooth operation of high-speed trains, and the scientific design of the subgrade structure provides a fundamental guarantee of its durability and technical economy. As, in the development of high-speed railways in China, higher speeds are being pursued, more requirements have been put forward for the dynamic stability of subgrade structures. To address this issue, this article focuses on the control requirements for the long-term stability of subgrade deformation, and various design methods for high-speed railway subgrade structures are presented. Considering the energy dissipation and dynamic stability characteristics of subgrade filling materials, the dynamic performance of coarse-grained soil filling materials in the bottom layer and graded crushed stones in the surface layer are revealed. The methods for determining the values of dynamic parameters such as the dynamic modulus and damping ratio are provided. Based on the dynamic shakedown theory, the stress-strain hysteresis characteristics of fillers and the variation law of dissipated energy are revealed. The correlation between unit volume dissipated energy and shakedown state under cyclic loading conditions is identified. A criterion for determining the critical shakedown state of high-speed railway subgrade structures based on equivalent unit volume dissipated energy is proposed, and a method for determining the design threshold of dynamic stress and dynamic strain is also proposed. The results show that the shakedown design critical values of equivalent unit volume dissipated energy in the bottom and surface layers of the foundation were between 0.0103 similar to 0.0133 kJ/m(3) and 0.0121 similar to 0.0149 kJ/m(3) , respectively. The critical dynamic strain range was 0.8 x 10(-3)similar to 1.3 x 10(-3). On this basis, a high-speed railway subgrade design method based on energy dissipation and dynamic shakedown characteristics was developed. The results can provide theoretical support for the design of high-speed railway subgrade structures with different filling material alternatives and control standards.

Low-concentration of colloidal silica (usually from 5 to 20 wt%) for liquefaction mitigation has been widely studied. But fewer literatures have focused on higher concentration of colloidal silica for cementing sand. For calcareous sand seeped by 40 wt% colloidal silica, cyclic loadings were performed to explore its anti-liquefaction, long-term settlement and self-sensing characteristics. Cyclic triaxial apparatus were to perform anti-liquefaction and long-term settlement tests, and use self -built setup to perform self -sensing tests. The new findings are as follows: (1) for anti-liquefaction, specimens seeped by 40 wt% colloidal silica can't achieve liquefaction, i.e., the excess pore pressure ratios cyclically fluctuates between 1 and negative, which is not discovered by lower concentration of colloidal silica in the previous literatures; (2) for long-term settlement, cumulative strain is small after 20,000 cycles, and the cumulative strain is less than 1 % even the cyclic stress ratio (CSR) is greater than 1.1, indicating that the colloidal-silica-cemented sand has good resistance to deformation; (3) self-sensing characteristics have been discovered, i.e., a stress change of 90 kPa can lead to 59.37 % change in electrical resistivity, compared to Portland-cement-treated sand without change of resistivity under the same stress change. When Portland-cemented materials are added with conductive fillers in the previous literatures, they can exhibit self -sensing characteristics, but the stress sensitivity is still two orders lower than colloidal-silica-cemented sand. For colloidal-silica-cemented sand, based on the fact that the dry silica gel is non-conductive and only the salt solution in the micro porous media is conductive, the self-sensing feature can be attributed to the stress-induced deformation of micro conductive-salt-solution-filled channels (i.e., the deformation of conductive network). The above research indicates that colloidal-silica-seeped sand has a potential for self-sensing subgrades in the marine environments.

It is important to make reasonable prediction about the long -term deformation of high rockfill geostructures. However, the deformation is usually underestimated using the rockfill parameters obtained from laboratory tests due to different size effects, which make it necessary to identify parameters from in-situ monitoring data. This paper proposes a novel hybrid back -analysis method with a modified objective function defined for the time-dependent back -analysis problem. The method consists of two stages. In the first stage, an improved weighted average method is proposed to quickly narrow the search region; while in the second stage, an adaptive response surface method is proposed to iteratively search for the satisfactory solution, with a technique that can adaptively consider the translation, contraction or expansion of the exploration region. The accuracy and computational efficiency of the proposed hybrid back -analysis method is demonstrated by back-analyzing the long -term deformation of two high embankments constructed for airport runways, with the rockfills being modeled by a rheological model considering the influence of stress states on the creep behavior.