Tunnel driving inevitably induces changes in stress and deformation in the ground, which could cause ultimate and serviceability problems to an adjacent tunnel. The effects of induced stress on an ...existing tunnel and crossing-tunnel interaction are still not fully understood. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the responses of an existing tunnel in sand to the excavation of a new tunnel perpendicularly below it. Three-dimensional tunnel advancement was simulated using a novel technique that considers the effects of both volume and weight losses. This novel technique involves using a “donut” to control volume loss and mimic soil removal in-flight. To improve fundamental understanding of the stress transfer mechanism during the new tunnel advancement, measured results were back-analyzed three-dimensionally using the finite element method. The maximum measured settlement of the existing tunnel induced by the new tunnel constructed underneath was about 0.3% of tunnel diameter, which may be large enough to cause serviceability problems. The observed large settlement of the existing tunnel was caused not only by a sharp reduction in vertical stress at the invert, but also by substantial stress transfer of overburden pressure at the crown. The section of the existing tunnel directly above the new tunnel was compressed vertically because the incremental normal stress on the existing tunnel was larger in the vertical direction than in the horizontal direction. The tensile strain and shear stress induced in the existing tunnel exceeded the cracking tensile strain and allowable shear stress limit given by the American Concrete Institute.
Basement excavation may induce unsymmetrical and highly skewed loadings and (or) stress changes in an existing tunnel, not only in the transverse, but also in the longitudinal direction of the ...tunnel. Although basement–tunnel interaction has attracted intense academic interest recently, it is often simply treated as a plane strain problem. In this study, however, based on a dimensional analysis of the governing parameters, two three-dimensional centrifuge tests were designed and carried out in dry sand to investigate the effects of a basement excavation on an existing tunnel located in two horizontal offsets in relation to the basement. In addition, a preliminary three-dimensional numerical analysis was conducted to back-analyse the centrifuge test and to investigate the effects of the tunnel cover-to-diameter and unloading ratios on the existing tunnel. For the specific conditions simulated and soil type tested, a maximum heave of about 0.07% of the final depth of the basement excavation (H
e
) was induced in the tunnel that ran parallel to and beneath the basement. On the contrary, a maximum settlement of 0.014% H
e
was induced in the tunnel located at the side of the basement. For the former tunnel, the influence zone by the basement excavation on vertical tunnel displacement along the longitudinal direction was 1.2L (basement length). By inspecting the measured strains in the longitudinal direction of the existing tunnel, it was found that the inflection point, where the shear force is at a maximum, was located at 0.8L away from the basement centre. Due to stress relief from the basement excavation, the tunnel located directly beneath the basement was vertically elongated, but the one that lay at the side of the basement was distorted. A preliminary numerical parametric study found that tunnel heave decreased as the cover-to-diameter ratio increased, but at a reduced rate.
Three-dimensional coupled consolidation analysis is conducted to gain insight into the response of a 2×2 floating pile group adjacent to deep excavation in soft clay. By using a validated finite ...element model, the influence of the excavation depth, pile length, pile group location from excavation, the supporting system stiffness, soil state and permeability, and working load are systematically studied. The analysis revealed that the maximum settlement occurs when the pile group is founded at the excavation level and at a distance of 0.75-times the excavation depth, although the induced bending moment is minimum. In contrast to pile group settlement, tilting is maximum when it gets closer to the wall and minimum at a distance of 0.75-times the excavation depth. It is also observed that the influence of system stiffness is more pronounced for the flexible wall and when the pile group is located at the excavation level. Applied working load influences pile group settlement but has relatively minor effects on pile group tilting. Excess negative pore water pressures are generated in the soil elements due to excavation. Pile group experiences progressive long term settlement with the dissipation of excess negative pore water pressure.
A 15.5 m deep multistrutted soft clay excavation of a metro station in Shanghai has been monitored. The deep excavation was supported by a concrete diaphragm wall. The monitoring included wall ...deflections, surface and subsurface ground settlements, total pressures, and pore-water pressures. Relatively small wall deflections and ground settlements were measured as compared with similar case histories worldwide. The small deflections and settlements observed probably occurred because of the use of a short excavation section, application of compaction grouting for improving ground conditions, and the use of active prestress steel struts. No significant “creep” deflection of the diaphragm wall could be identified over a 60 day concrete curing period. Continuous ground settlements accompanied by the dissipation of pore-water pressures were observed. By using Terzaghi’s one-dimensional consolidation theory, the observed settlements were found to be attributed to the primary consolidation rather than creep effects.
•New 3-layer capillary barrier cover system performs well in humid climate.•Flume model test and FEM simulation were conducted.•The lowest clay layer plays an important role.
As an extension of the ...two-layer capillary barrier, a three-layer capillary barrier landfill cover system is proposed for minimizing rainfall infiltration in humid climates. This system consists of a compacted clay layer lying beneath a conventional cover with capillary barrier effects (CCBE), which is in turn composed of a silt layer sitting on top of a gravelly sand layer. To explore the effectiveness of the new system in minimizing rainfall infiltration, a flume model (3.0m×1.0m×1.1m) was designed and set up in this study. This physical model was heavily instrumented to monitor pore water pressure, volumetric water content, surface runoff, infiltration and lateral drainage of each layer, and percolation of the cover system. The cover system was subjected to extreme rainfall followed by evaporation. The experiment was also back-analyzed using a piece of finite element software called CODE_BRIGHT to simulate transient water flows in the test. Based on the results obtained from various instruments, it was found that breakthrough of the two upper layers occurred for a 4-h rainfall event having a 100-year return period. Due to the presence of the newly introduced clay layer, the percolation of the three-layer capillary barrier cover system was insignificant because the clay layer enabled lateral diversion in the gravelly sand layer above. In other words, the gravelly sand layer changed from being a capillary barrier in a convention CCBE cover to being a lateral diversion passage after the breakthrough of the two upper layers. Experimental and back-analysis results confirm that no infiltrated water seeped through the proposed three-layer barrier system. The proposed system thus represents a promising alternative landfill cover system for use in humid climates.
Many of the world reserves of fossil fuels are located at various water depths in fine-grained sediment under the seabed. The fine-grained sediment contains relatively large biogas bubbles, which has ...been posing challenges to the stability of offshore foundations supporting oil and gas platforms. Although fine-grained gassy soil was found to exhibit different undrained shear strengths (c.sub.u) by altering the initial pore pressure, u.sub.i (relevant to water depth), systematic studies concerning the effect of u.sub.i on undrained shear behaviours of the soil are still lacking. This study reports a series of undrained triaxial tests aiming to compare and investigate the responses of reconstituted fine-grained gassy soil with the same consolidation pressure (p'.sub.c), but at a wide range of varying u.sub.i (0-1000 kPa). The shearing-induced excess pore pressure (DELTAu) in the gassy specimens highly depends on u.sub.i. It can be either smaller than that of the saturated specimen with the same p'.sub.c (due to partial dissipation of Au into relatively large bubbles at low u.sub.i) or larger than that of the saturated specimen (related to collapse of relatively small bubbles at high u.sub.i). Consequently, the presence of bubbles had beneficially increased c.sub.u at relatively low u.sub.i (u.sub.i/p'.sub.c < 0.6), and vice versa. The critical stress ratio of the reconstituted fine-grained gassy soil, however, did not appear to be altered by u.sub.i.
An appropriate construction sequence for crossing tunnels can help minimize the adverse impact on the tunnel that is constructed first (considered as the existing tunnel). However, the influence of ...construction sequence on crossing-tunnel interaction is complex. Two pairs of three-dimensional centrifuge tests were carried out to investigate the effects of construction sequence on crossing-tunnel interaction. In the first pair of tests, the new tunnel was excavated beneath the existing tunnel in a reference test, while in the other test the new tunnel advanced above the existing tunnel. To study the effects of cover depth on the construction sequence, the depths of the existing and new tunnels were increased in the second pair of tests. An advanced hypoplasticity constitutive model with small-strain stiffness was adopted to back-analyze the tests. The existing tunnel was found to be vertically compressed when the new tunnel was excavated underneath, but vertically elongated when the new tunnel advanced above. This is because the reduction of stress acting on the existing tunnel in the horizontal direction was larger than in the vertical direction when the new tunnel was constructed beneath. On the other hand, the decrease in vertical stress on the existing tunnel was larger than the horizontal stress reduction when the new tunnel was excavated above. This behavior was observed in both pairs of tests, irrespective of the cover depths of the tunnels. As the cover depths of the existing and new tunnels increased, settlement of the existing tunnel due to the new tunnel construction beneath decreased. This is because with the larger cover depths of the tunnels, the increase in mobilized shear stiffness of the soil dominated the increase in stress relief caused by the tunnel excavation.
Chemical weathering is one of the dominant mechanisms for weathering of granite in subtropical and tropical regions resulting in modifications of its chemical, physical and mechanical properties. ...Thus, the changes in chemical properties may provide insight about the changes in physical and mechanical properties of weathered granite. In this paper three quantitative chemical weathering indices are correlated to the dry density of decomposed granite. A linear relationship is found between each chemical weathering index and dry density for granitic saprolites. Mobiles index (Imob) gives the highest coefficient of correlation among the three chemical weathering indices. Further investigations were conducted to establish the relationships between Imob and other physical and mechanical properties of granitic saprolites. The test results showed that fines content increases with increasing Imob, but peak strength increases with decreasing Imob. The peak states of granitic saprolites are consistent with Rowe's stress–dilatancy relationship. Dilatancy increases with decreasing Imob that contributes to a higher peak strength. Based on some test results presented in this study, it seems that Imob could be a good indicator to quantify the effects of chemical weathering on physical and mechanical properties of granitic saprolites.
•Chemical, physical and mechanical properties of decomposed granite are studied.•Mobiles index is correlated to dry density, fines content and maximum dilatancy.•Rowe's stress–dilatancy can model peak states of granitic saprolites.
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