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A full-scale field experiment has been conducted in an instrumented saprolite slope in Hong Kong. Soil moisture probes, tensiometers, piezometers, inclinometers, earth pressure cells, and a rain gauge were installed in the slope before and during excavation of a cut. This paper presents the results from soil moisture probes, tensiometers, and piezometers to reveal the surface infiltration process. The soil moisture and matric suction results show that the maximum "wetting front" during the wet season was limited to the top 3 m of soil, and a transient perched water table could develop in the soil during a very heavy rainstorm. A new method based on the variation of volumetric water content in the soil is proposed to analyze rainfall infiltration. The results indicate the following phenomena: (i) rainfall infiltration was around 70% of the total rainfall, (ii) the infiltrated water mainly increased the soil moisture content of the soil at shallow depths, and (iii) the surface vertical infiltration had a very limited effect on the permanent groundwater table at depth. The wetting-front analysis shows that the unsaturated wetting band theory can be used to assess the movement of the wetting front in the unsaturated saprolite soil.Key words: rainfall infiltration, moisture content, matric suction, wetting front, field instrumentation.

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SUMMARYThis paper proposes a two-dimensional finite element model for the analysis of shield tunnels by taking into account the construction process which is divided into four stages. The soil is assumed to behave as an elasto-plastic medium whereas the shield is simulated by beam-joint discontinuous model in which curved beam elements and joint elements are used to model the segments and joints, respectively. As grout is usually injected to fill the gap between the lining and the soil, the property parameters of the grout are chosen in such a way that they can reflect the state of the grout at each stage. Furthermore, the contact condition between the soil and lining will change with the construction stage, and therefore, different stressreleasing coefficients are used to account for the changes. To assess the accuracy that can be attained by the method in solving practical problems, the shield tunnelling in the No. 7 Subway Line Project in Osaka, Japan, is used as a case history for our study. The numerical results are compared with those measured in the field. The results presented in the paper show that the proposed numerical procedure can be used to effectively estimate the deformation, stresses and moments experienced by the surrounding soils and the concrete lining segments. The analysis and method presented in this paper can be considered to be useful for other subway construction projects involving shield tunnelling in soft soils.

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This paper presents the design and construction of a deep excavation for building foundations in saturated soil. This deep excavation was of particular interest because it was located above and beside the Shanghai Metro tunnels. The twin Shanghai Metro tunnels had to be in full operation during the deep excavation. Potential large deformation of the twin tunnels was one of the main concerns during the design and construction for the deep excavation. The paper discusses in detail the criteria and measures for controlling the soil and tunnel deformation. The measures included cast-in-place concrete diaphragm walls with bracing structural members, pumping consolidation, cement–soil mix pile systems, and rational excavation procedures. A simplified theoretical method was proposed to estimate the increment in undrained shear strength in a soft clay layer due to pumping consolidation. Furthermore, conventional finite element methods were used to predict the soil vertical and horizontal displacements induced by the excavation. Using the design and construction methods discussed in the paper, the settlement and horizontal displacement of the tunnels were successfully controlled within 5.0 mm and 9.0 mm, respectively. The curvature of longitudinal deformation curve of the tunnels was less than 1/15 000. The horizontal displacement of the braced diaphragm walls was less than 0.12% of the total excavation depth. Key words: Metro tunnels, saturated soft soil, deep excavation, design, construction, ground improvement, case studies.

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