In recent years, regional ground rebound phenomenon caused by the rising of groundwater level has been observed in an urban area in Japan such as Tokyo. This phenomenon might affect the underground structures constructed in those areas. In this study a centrifuge model test was carried out to investigate the fundamental mechanism of tunnel deformation and ground rebound during water rising process. In the centrifuge model test, two model tunnels were installed in different depths of clay ground in order to investigate the influence of ground depth on tunnel deformation. It is found that the relationship between ground surface displacement and elapsed time obtained from centrifuge model test shows similar tendency to the field measurement data. It is confirmed that deformation patterns of the two tunnels were different in water rising process. This is considered to be due to the difference in deformation and pore water pressure in different layers of clay ground. Detail discussions on deformation mechanism of the two tunnels and pore water pressure distribution in clay are presented in this paper.

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For the convenience of shoppers and users, there is an increasing demand for construction of basements in close proximity to existing tunnels. To ensure the safety and serviceability of existing tunnels, attention has been paid to the basement-tunnel interaction. However, most of previous numerical studies simply assumed the complex interaction as a plane strain problem and often they have overlooked effects of stress path and strain dependency on soil stiffness. Based on a dimensional analysis of the basement-tunnel interaction, three-dimensional numerical parametric study is conducted to explore the influence of aspect ratio (i.e., excavation length (L) along longitudinal tunnel direction / excavation width (B) along transverse tunnel direction) on tunnel responses due to basement excavation. Excavation length (L) varying from 2-10 H_e (i.e., final excavation depth) while excavation width (B) changing from 1-6 H_e are considered. Centrifuge test results are used to calibrate and verify soil model and soil parameters adopted. Because of larger inward wall movement and stress relief in a longer excavation, induced heave and transverse tensile strain in the tunnel increase with an increase in aspect ratio. When the aspect ratio and normalised excavation width (B/H_e) are larger than 2, induced tunnel heave at basement centre can exceed the allowable movement limit (i.e., 15 mm). Moreover, the transverse tensile strain of tunnel is larger than the cracking strain limit of unreinforced concrete (i.e., 150 µε) when the aspect ratio is larger than 1.3. This implies that basements with a smaller aspect ratio impose less adverse effects on existing tunnel.

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Numerical evaluation of the effect of blast induced vibration on adjacent structures requires determination of the damping ratio. Selection of the dampring ratio for the rock mass is very difficult because it is heavily influenced by the joint layout. We propose a simple yet robust method to estimate the damping ratio of rock mass that fits the target attenuation curve. The damping ratio can be selected as function of the shear wave velocity of the rock mass and the attenuation parameter of the target attenuation relationship .

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A large-scale field test was carried out to investigate the backfill consolidation behavior of soil-bentonite slurry trench cutoff walls. The results of lateral earth pressure, pore pressure and deformation monitoring show that the weight of SB backfill is initially taken by pore water and sidewall friction, and then is transferred to backfill skeleton due to consolidation. The findings from this large-scale test help understand the stress states of backfill and assess the performance of soil-bentonite slurry trench cutoff walls.

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The behavior of combined soil nail-anchor walls with various heights is analyzed and a wall design procedure is introduced such that maximum allowable deformation and minimum acceptable factor of safety are achieved. Charts of nail lengths are first determined for various cases such that the required factor of safety is obtained. These are similar to the charts published by the FHWA for the design of soil nail walls. These charts are then extended through the determination of the deformation corresponding to each case. Further extension is then made by including soil nail walls with various combinations of anchors. Verification of the results is provided by comparing deformations of actual instrumented combined walls with the results obtained from the current study. Design charts are then constructed for selecting various combinations of soil nail and anchor lengths such that stability and deformation requirements are satisfied. These charts may be used to select a combination for which project cost is minimized.

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In this paper, it aims to examine impacts from three-dimensional effect on the wall deflection induced by a deep excavation in Kaohsiung city, Taiwan. The commercial software PLAXIS 3D was used as a numerical tool for 3D finite element analyses in this study. First, a benchmark analysis was performed to simulate a case history of deep excavation in thick layers of sand (Case A) to verify the performance of 3D numerical analysis model in predicting the wall displacements. It is aware that there is a little limitation in prediction of the wall movements by using a constitutive soil model having single elastic modulus. Next, a series of parametric studies that uses the same input parameters as the benchmark analysis was conducted to model the excavation of Case A with various values of excavation length (L) and width (B). From these parametric studies, plane strain ratio (PSR), which is the ratio of the maximum wall deflection of a certain section to the maximum wall deflection of the section under the plane strain condition, was determined with various values of distance from evaluated section to the excavation corner (d), length (L) and width (B) of excavation. A relationship between PSR, d and ratio of B/L was thus interpreted. It is summarized that PSR is smaller than in sand rather than in clay for B/L more than 1.0, but it is larger than in sand rather than in clay for B/L less than 0.5. Further verification may have to be delivered later in order to explore the reason for the difference.

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For deep and/or large excavation of soil with a high ground water level, three-dimensional seepage flow through soil within a cofferdam is a problem. Three-dimensionally concentrated flow lowers the stability against seepage failure more than in the two-dimensional condition. To clarify the three-dimensional seepage failure mechanism, experiments were conducted under three-dimensional flow conditions for various cases, and analyses of FEM seepage flow and stability against seepage failure of soil were carried out. Theoretical critical hydraulic head differences based on the Prismatic failure concept are defined as H_PF. From experiments, the hydraulic head differences at an abrupt change of discharge H_d, at the onset of soil deformation H_y, and at failure H_f are considered. The self-stabilizing effects and the method for estimating H_f using the value H_PF are discussed.

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