Tianbin Li

Damage to mountain tunnels related to the Wenchuan earthquake and some suggestions for aseismic tunnel construction

Many tunnels along the Dujiangyan to Wenchuan highway, located near the epicenter of the 2008 Wenchuan earthquake in China, were damaged severely. The characteristics of the tunnel failures were analyzed and categorized as avalanches and landsliding near the tunnels, cracking of the tunnel portals, collapse of the liner and surrounding rock, cracking and dislocation of the liner, uplift and cracking of the ground, deformation and cracking of the preliminary bracing. The main geological factors influencing the tunnel damage are secondary fractures of earthquake faults, sudden change in soil and rock type, weak rocks and the variable geo-stresses in the host material. The tunnel portals and their slopes, unless fully integrated into the tunnel structures and sufficiently reinforced, are likely to suffer significant distress as a consequence of seismic events. The main mitigation measures proposed are the use of reinforced concrete in the secondary lining in the area of fault zones and injection grouting to reduce the differences where there are sudden changes in the character of the host material.RésuméDe nombreux tunnels le long de l’autoroute reliant Dujiangyan à Wenchuan, située près de l’épicentre du séisme de Wenchuan de 2008, ont été gravement endommagés. Les caractéristiques des ruptures de tunnels ont été analysées et classifiées ainsi que les avalanches de roches et les glissements près des tunnels, les fissurations des têtes de tunnels, les ruptures des revêtements et de la roche encaissante, les fissurations et dislocations du revêtement, les soulèvements et fissurations du terrain, les déformations et fissurations de confortements préliminaires. Les principaux facteurs géologiques responsables des dommages aux tunnels sont des fractures secondaires associées aux failles actives, des changements brutaux des types de sols ou de roches, des roches tendres et des variations d’états de contrainte dans le massif rocheux. Les têtes de tunnel et les pentes avoisinantes, à moins qu’elles ne soient complètement intégrées dans la structure du tunnel et suffisamment renforcées, sont susceptibles d’être fortement affectées par l’événement sismique. Les principales mesures d’atténuation de ces effets consistent en l’utilisation de béton armé dans le revêtement secondaire dans le secteur de zone faillée et l’injection de ciment pour réduire les différences là où se présentent des changements brutaux des caractéristiques du massif rocheux.

Temperature effect of rock burst for hard rock in deep-buried tunnel

Much research has been conducted on the influence of rock burst mechanisms and temperature on the mechanical properties of hard rock while research on the effect of temperature on rock bursts is scarce. Therefore, this paper focuses on Rock Burst Proneness Index tests and acoustic emission (AE) tests under the action of high temperature. It was found that the Rock Burst Proneness Index and the AE energy will rise as the temperature rises. It means that the degree of rock burst is increasing instead of decreasing with rising temperature. The research results revealed the temperature effect of rock burst in long deep tunnels under a certain thermal stress condition, which is helpful for explaining the rock burst disaster in tunnels at high ground temperature.

Damage characteristics and influence factors of mountain tunnels under strong earthquakes

A total of 81 mountain tunnels that were damaged in 10 strong earthquakes are studied. They are classified into six typical damage characteristics: lining cracks, shear failure of lining, tunnel collapse caused by slope failure, portal cracking, leaking, and deformation of sidewall/invert damage. Further study and discussion are carried out on influencing factors for mountain tunnels, including seismic parameters, structural information, and rock conditions. Suggestions are also made regarding seismic resistance and reduction.

Progressive modelling of the gravity-induced landslide using the local dynamic strength reduction method

The failure of slope is a progressive process, and the whole sliding surface is caused by the gradual softening of soil strength of the potential sliding surface. From this viewpoint, a local dynamic strength reduction method is proposed to capture the progressive failure of slope. This method can calculate the warning deformation of landslide in this study. Only strength parameters of the yielded zone of landslide will be reduced by using the method. Through continuous local reduction of the strength parameters of the yielded zone, the potential sliding surface developed gradually and evolved to breakthrough finally. The result shows that the proposed method can simulate the progressive failure of slope truly. The yielded zone and deformation of landslide obtained by the method are smaller than those of overall strength reduction method. The warning deformation of landslide can be obtained by using the local dynamic strength reduction method which is based on the softening characteristics of the sliding surface.

Brittle mechanical characteristics of hard rock exposed to moisture

Rich groundwater content can produce a complex geological environment for underground tunnels. Therefore, the brittle mechanical characteristics of hard rock exposed by tunnelling in a moist environment are of great significance. Uniaxial tests and acoustic emission (AE) analysis of sandstone with respect to different moisture conditions are described in this paper. Moisture-controlled trends of the stress–strain relationship, mechanical parameter variation, maximum instantaneous AE energy, cumulative AE energy and macro crack behaviour were analysed in detail. In addition, X-ray diffraction and scanning electron microscopy were performed to analyse the fracture surfaces of rock at the microscale. Sandstone particles were found to be more likely to slip past each other at high moisture contents, resulting in increased plastic deformation and dissipation of internal energy. Increasing moisture reduces the brittleness of hard rock and the probability of rock burst. The results reveal the mechanical mechanisms cause moisture to affect brittle hard rock and can contribute to the improvement of the design and construction of deep tunnels.

Brittle Rock Modeling Approach and its Validation Using Excavation-Induced Micro-Seismicity

With improvements to the bonded-particle model, a custom indicator of crack intensity is introduced to grade rock fractures accurately. Brittle fracturing of rock mass is studied using the bonded-particle model; here, “brittle” refers to the process where more energy is released towards making particles collide and disperse, and hence results in the quick emergence of “chain cracks”. Certain principles concerning how to construct brittle rock are then proposed. Furthermore, a modeling approach for brittle rocks based on the adaptive continuum/discontinuum (AC/DC) method is proposed to aid the construction of large-scale models of tunnel excavations. To connect with actual tunneling conditions, fundamental mechanical properties, the mechanism for brittle fracturing, the joint distribution, and the initial stress field are considered in the modeling approach. Results from micro-seismic monitoring of a tunnel excavation confirmed the suitability of this modeling approach to simulate crack behavior, and results show that simulated cracking exhibit similar trends (evolution, location, and intensity) with micro-seismic cracking.

Weakening effects of the presence of water on the brittleness of hard sandstone

The weakening effects of increasing the water content in rocks have been studied extensively in a wide variety of rock types over the past few decades. High energy release is typically characteristic of rock burst hazards. However, few studies have investigated the mechanism of water involved in rock bursts from the perspective of energy distribution based on the water content. Therefore, in this study we investigated the evolution of microcracks in hard sandstone determined from triaxial tests combined with an acoustic emission analysis for different water contents and confining pressures. Then, the weakening effects of water on the brittleness of the rock were studied from an energy viewpoint. The study shows that the capacity of the storage energy of hard rock decreases due to the dissipation of internal elastic energy in the compression stage in the presence of water. These results revealed the mechanical and energetic mechanisms that caused the weakening effects induced by water on rock burst and can contribute to design and construction improvements in tunnel engineering.

Identifying crack initiation stress threshold in brittle rocks using axial strain stiffness characteristics

As an estimate for the in-situ spalling strength around massive underground excavations to moderately jointed brittle rocks, crack initiation stress marks the initiation of rock micro fracturing. It is crucial to accurately identify crack initiation stress level by proper method. In this study, confined compression tests of sandstone samples are used to examine the validity/applicability of proposed axial strain stiffness method. The results show that by highlighting the minuscule changes in stress-strain curve, the axial strain stiffness curve provided further insight into rock failure process and revealed five stages: (a) irregular fluctuation, (b) nearly horizontal regular fluctuation, (c) irregular fluctuation gradually decreasing to zero, (d) extreme fluctuation, and (e) near zero, which mainly correspond to five stages of stress–strain curve. The ratio of crack-initiation stress to peak strength determined using this approach is 0.44–0.51, similar to the ranges previously reported by other researchers. In this method, the key is to accurately detect the end point of the stage (b), “nearly horizontal regular fluctuation” characterized by a sudden change in axial strain stiffness curve, and the sudden change signifies crack initiation in rock sample. Finally, the research indicates that the axial strain stiffness curve can provide a mean to identify the crack-initiation stress thresholds in brittle rocks.

A Statistical Constitutive Model considering Deterioration for Brittle Rocks under a Coupled Thermal-Mechanical Condition

Due to active actions of groundwater and geothermal, the stability of underground engineering is important during geological structure active area. The damage mechanical theory and statistical mesoscopic strength theory based on Weibull distribution are widely used to discuss constitutive behaviors of rocks. In these theories, a statistical method is used to capture mesoscopic properties of rocks in order to generate a realistic behavior at a macroscopic scale. Based on the above theories, this paper aims at establishing a constitutive relation of brittle rocks under thermal-mechanical coupling conditions. First, a statistical damage constitutive model was established by considering the thermal effects and crack initiation strength. Subsequently, the parameters of the model were determined and expressed according to the characteristics of stress-strain curve. Third, the model was verified by conventional triaxial experiments under thermal-mechanical actions, and the experimental data and theoretical results were compared and analyzed in the case study. Finally, the physical meaning of the parameters and their effects on the model performance were discussed.

The undrained vertical and horizontal bearing capacity of internal skirted foundation in clay

Abstract Three-dimensional (3D) finite element analysis is used to investigate the vertical and horizontal bearing capacity of internal skirted foundations in normally consolidated uniform clay under undrained condition. The vertical and horizontal bearing capacity analysis has taken the effects of embedment ratio, foundation–soil interface roughness and soil strength heterogeneity into account. In this paper, new equations have been proposed for calculating vertical and horizontal bearing capacities based on the results of the finite element analyses. In the proposed equations, the vertical capacity consists of an end-bear resistance and a skin friction resistance, whereas the horizontal capacity consists of a normal resistance, a radial shear resistance and a base shear resistance. By comparing the numerical results, it shows that the proposed equations properly predicted the bearing capacities of the internal skirted foundations in uniform or non-uniform clay under undrained condition.