The construction of a slurry shield tunnel produces large amount of excavated soil. Direct discharge of the excavated soil by transportation not only increases the overall construction cost but also raises environmental concerns of pollution. In this study, we explore in-situ recycle of excavated soil in the Nanjing Yangtze River tunnel project as the back-fill grout. A series of laboratory were carried out to determine the synchronous grout parameters, including compression strength and shear strength, initial setting time, fluidity, consistency, and bleeding rate. The mixture ratio of the back-fill grout is also discussed to optimize the grout performance. It is found that: (1) the excavated soil produced from the construction of slurry shield tunnel in this project can be reused as the material of back-fill grout in the sand stratum; (2) Proper selection of binder/sand ratio and fly ash/cement ratio improves the performance of the synchronous grout. The test results demonstrated that the discharged soil can be recycled in-situ as the grouting material in the sand stratum.

This paper presents the corrosion-induced cracking performance of reinforced concrete in a chloride condition. The meso-scale structures of concrete specimens were studied according to the X-ray micro-computed tomography test, including the corrosion propagation, the accumulation and transportation of corrosion products, the defects and cracking behaviors of concrete cover. The experimental results show that the defects could provide a reasonable space for the accommodation of corrosion products, which could significantly increase the corrosion process and postpone the cracking performance. The propagation of the cracking path initiating from the corrosion area of the reinforcement to the concrete cover was also captured based on the experimental images, which can make contribution to the recognition of corrosion-induced cracking behavior of the concrete cover.

In the last decade, the design and construction technologies of subgrade in high-speed railway developed significantly. This paper reviewed corresponding development in five aspects, including mechanical properties of fill materials, dynamic performance of subgrade, foundation treatment, retaining structure and smart construction technologies. It showed that for unbonded granular materials, it was acceptable to use static strength for subgrade design, but for clayey soil it would be more appropriate to base on shakedown theory. The mechanism for lime modified clay has been thoroughly reviewed, and the effect of lime content, curing age, and curing conditions on the behavior of lime-treated clay was discussed. The dynamic response of subgrade, especially the long-term deformation and dynamic stability analysis were important to understanding the behavior of high-speed railway subgrade. The effect of track types, operation speed etc. on the dynamic response of subgrade were reviewed first. Then, the prediction methods, influencing factors, and corresponding issue for long-term deformation of subgrade were presented, following by the methods used for dynamic stability analysis. Three types of foundation treatment methods, including geosynthetic-reinforced pile-supported embankment, pile-raft structure, and pile-plate structure, were reviewed for the corresponding load transmission mechanism, and application scenario. The static and dynamic behavior of four types of retaining structures were presented, including cantilever retaining wall, geosynthetic reinforced soil retaining wall, anchored retaining structure, and retaining wall reinforced by soil nailing. Finally, a series of new technologies correlated to smart construction was introduced, relating to the survey, design, construction, detection and management of subgrade.

An in-situ test to obtain vertical, transversal and longitudinal ground velocities and accelerations caused by high-speed trains was performed alongside the Chinese high-speed railway line between Beijing and Shanghai. The test results indicate that due to excellent track quaity and consolidated infrastructure, whether if the train speed is below or above the Rayleigh wave speed, the dominant frequencies of both the vertical ground velocity and acceleration are both integral multiples of the trainload frequency generated by the centre distance of two neigbouring cars. The frequency-domain distribution of the ground velocity is more sensitive to soil stiffness than is the ground acceleration; this is especially the case for the first dominant frequency. Whether the frequency amplitudes of the vertical ground vibration are higher than the horizontal vibrations depends on the soil stiffness. The greater the soil stiffness, the higher is the frequency-range of the vertical ground vibration compared with that for the horizontal vibration, and the smaller is the difference between the three-dimensional frequency-weighted ground vibration level and the vertical vibration. Thus, effects of horizontal ground vibrations cannot be ignored when the foundation soil is soft.

The deformation of a bridge foundation (i.e. pier) for a ballastless track of a high-speed railway may cause additional irregularities within the track, thereby affecting train operation. By using a unit slab ballastless track bridge system as the research object, this study built a finite element model and a train–track dynamic interaction model. The additional rail deformation caused by the vertical or lateral deformation of a bridge pier was calculated by the finite element model, and then the effects on train operation due to the additional rail deformation were analyzed by the train–track dynamic model. It was found that the lateral deformation of a single pier should be of the most concern for the management and control of a high-speed railway. Specifically, when a pier suffered settlement and lateral deformation concurrently, the evaluation indices of train operation were primarily affected by the magnitude of the lateral deformation, and were only slightly affected by the settlement.

A novel pressure testing method, i.e., a thin-film pressure distribution measurement system, was utilized to investigate the contact area and stress at the ballast bed-soil subgrade interface of conventional railways during cyclic loading in model tests. Factors like the amplitude and frequency of cyclic loading and the existence or nonexistence of the subballast were considered to study their effects on the contact area and average stress over the contact area at the interfaces. In addition, the testing results were compared with those measured by the traditional testing method using earth pressure cells and those calculated by the finite element method (FEM) based on continuum theory. The study shows that, because of the discontinuous characters of the ballast medium, the stress diffused from the ballast bed on the subgrade surface was unevenly distributed but becoming relatively evenly distributed as the subballast was being installed. The subgrade surface stresses measured by the earth pressure cell and calculated by the FEM were very similar, but both were about 50 % lower than the average stress over the contact area measured by the novel thin-film pressure sensor. As the interface stress between the ballast bed and the subgrade is a key factor in subgrade deterioration, new testing methods and numerical calculation methods can consider the discontinuous characters of the ballast appropriately to be necessary for the investigations of the interface contact stress.