Yongli Xie

Centrifuge modelling of twin-tunnelling induced ground movements in loess strata

A great concern for the safety of large cross-section tunnels, which are being or to be built in the loess strata of China, is attracted. Generally, loess is a multi-phase porous medium and develops complex stress and strain variation while executing a tunnel project. Another problem is that the soil surrounding both tunnel arches is subjected to a complex loading due to the double excavation. To obtain an in-depth knowledge of the mechanism of tunnel deformation induced by the twin-tunnelling, we conducted comprehensive centrifuge tests, which can simulate and reproduce strictly the action process of twin-tunnelling. Through the model tests, the response of twin-tunnelling on loess stratum deformation was obtained. The investigations showed that with the increase of tunnel spacing, the stratum deformation distribution near the vault changes from a single-peak V shape to a double-peak W shape. Additionally, the height of the stratum pressure arch effect increases significantly. The settlement of the preceding tunnel is slightly larger than that of the latter tunnel, and the twin-tunnelling effect gradually decreases with the increase of tunnel spacing. Through comparative analysis of the different combinations of tunnel spacing and tunnel interval, the interaction between two tunnels with different spacing and interval during tunnelling was investigated, further optimizing the reasonable tunnel spacing and construction steps, as well as providing reference for tunnel route selection in the loess strata.

Field Experiment on Soaking Characteristics of Collapsible Loess

In collapsible loess area, migration of soil moisture often causes the temporal discontinuity and spatial nonuniformity of collapsibility, which leads to great damage for infrastructures. Therefore, the research on water infiltration is the key to solving the problem of collapsibility. The aim of this paper is to investigate the spatiotemporal evolution of infiltration characteristics of collapsible loess. A field soaking experiment was conducted on collapsible loess in western China, in which a soaking pool with diameter of 15 m was built. Time-Domain-Reflectometry (TDR) system and soil sampling were employed to measure the water content within the depth of 12 m. Then the saturation isograms were drawn for visualization of the process of infiltration. Also, a pilot tunnel was excavated to investigate how the free face can affect the infiltration behaviors. The experimental results revealed the characteristics of infiltration in both horizontal and vertical directions. Moreover, the response of free face on infiltration behaviors was also found. These findings of research could provide the data for the infiltration laws of unsaturated loess and thereby provide the basis for integrated treatment of collapsible loess.

Study on the Fire Damage Characteristics of the New Qidaoliang Highway Tunnel: Field Investigation with Computational Fluid Dynamics (CFD) Back Analysis

In the New Qidaoliang Tunnel (China), a rear-end collision of two tanker trunks caused a fire. To understand the damage characteristics of the tunnel lining structure, in situ investigation was performed. The results show that the fire in the tunnel induced spallation of tunnel lining concrete covering 856 m3; the length of road surface damage reached 650 m; the sectional area had a maximum 4% increase, and the mechanical and electrical facilities were severely damaged. The maximum area loss happened at the fire spot with maximum observed concrete spallation up to a thickness of 35.4 cm. The strength of vault and side wall concrete near the fire source was significantly reduced. The loss of concrete strength of the side wall near the inner surface of tunnel was larger than that near the surrounding rock. In order to perform back analysis of the effect of thermal load on lining structure, simplified numerical simulation using computational fluid dynamics (CFD) was also performed, repeating the fire scenario. The simulated results showed that from the fire breaking out to the point of becoming steady, the tunnel experienced processes of small-scale warming, swirl around fire, backflow, and longitudinal turbulent flow. The influence range of the tunnel internal temperature on the longitudinal downstream was far greater than on the upstream, while the high temperature upstream and downstream of the transverse fire source mainly centered on the vault or the higher vault waist. The temperature of each part of the tunnel near the fire source had no obvious stratification phenomenon. The temperature of the vault lining upstream and downstream near the fire source was the highest. The numerical simulation is found to be in good agreement with the field observations.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF BLASTING-INDUCED GROUND VIBRATION FROM TUNNEL UNDERCROSSING A VILLAGE

To study the effect of blasting-induced vibration on ground buildings when tunnel undercrossing a village, a blasting vibration trial was carried out in Beizhuang tunnel in Henan Province, China. The safety blasting distance and reasonable explosive charge were pre-estimated according to empirical formula. The attenuation of ground vibration velocity was simulated using a 3D numerical model. Less explosive charge and longer distance to blasting source would result in lower ground vibration. When designed explosive charge was 54 kg, the safe distance was 42.26 m. While the distance between building and blasting source was about 37 m, the maximum explosive charge was 36.24 kg. The numerical results showed that the significantly horizontal affect region of blasting vibration was within 50 m to blasting source. Accordingly, effective vibration control is necessary to avoid disturbing human daily life during tunneling.

Three-Dimensional Numerical Study of the Mechanical Characteristics of a Rectangular Immersed Tube Tunnel

There are many different kinds of cross-section structure forms in the rectangular immersed tube and their stress distributions are complex. In order to confirm the rectangular immersed tube structure features, the finite element software ANSYS is used to analyze the characteristics of the force in the immersed tube structure under different settlement conditions combining specific geological conditions and engineering design requirements. The worst tension parts of the immersed tube are the inner side of the lane floor and the lower side on the outside of the side wall. Chamfer or fillet could be set inside the base lining turning points to reduce stress concentration levels. Combined with tensile stress directions, this paper puts forward feasible suggestions on the structure of the internal reinforcement layout.