Three-dimensional Centrifuge Model Research Articles

Effects of the skew angle of new tunneling on an existing tunnel: three-dimensional centrifuge and numerical modeling

Tunneling results in stress relief and arching in the ground. New tunnel excavation beneath an existing tunnel but at different angles can lead to different patterns of stress redistribution. Studies on multitunnel interaction have mainly focused on perpendicular tunnels, and the effects of tunneling skew angle remain poorly understood. Three-dimensional centrifuge modeling and numerical back-analyses were conducted to investigate the effects of twin-tunnel interaction at three skew angles (30°, 60°, and 90°). The effects of new tunnel excavation on an existing tunnel were simulated by controlling tunnel volume and weight losses in-flight. A hypoplastic soil model capable of simulating path-dependent and strain-dependent soil stiffness was adopted for the numerical back-analyses. Distinct load redistribution patterns were identified to explain deformations of the existing tunnel at different skew angles due to the advancement of the new tunnel. For 90° tunneling, hoop force increased at the crown and decreased at the left springline of the existing tunnel. The opposite responses were identified at 30° tunneling. A critical skew angle of 30° tunneling led to the maximum invert settlement and tunnel deformation of the existing tunnel. At 30° tunneling, the induced strain in the tunnel lining was 2.3 times larger than that of tunneling at 90°, exceeding the cracking limit suggested by the American Concrete Institute.

Effects of Large-Scale Twin Tunnel Excavation on an Existing Pile Group: Three-Dimensional Centrifuge Modeling

Effects of Large-Scale Twin Tunnel Excavation on an Existing Pile Group: Three-Dimensional Centrifuge Modeling

3D centrifuge and numerical modelling of lateral responses of a vertical loaded pile group to twin stacked tunnels

Tunnel excavations inevitably induce stress release and soil movement, which may affect the safety and serviceability of nearby existing pile foundations. The effects of tunnel construction on existing piles have been extensively investigated, but little attention was paid to lateral responses of piled foundations to tunnelling. In this study, a series of three-dimensional centrifuge model tests and numerical simulations using an advanced hypoplastic soil model were carried out to investigate the response of an existing vertically loaded 2 × 2 pile group in lateral direction due to advancement of twin stacked tunnels in dry sand. In each test, twin tunnelling was simulated three-dimensionally one after the other, with the first tunnel excavated near the mid-depth of the pile shaft and the second either next to the toe of the pile group (test ST) or below but to one side of the pile group (test SB). In addition, influences of tunnel depths and locations with respect to pile group response are investigated. The most significant lateral movement of the pile cap induced after the twin tunnelling in each test. The induced bending moment and pile deflection can be critical on completion of twin tunnels in test ST. In spite of being vertically loaded pile group, the twin tunnelling caused lateral forces (shearing forces to the pile group) in each test. The calculation charts have revealed that the tunnel depth and location relative to the pile group significantly influence induced pile cap lateral displacement and pile head bending moment.

Three-dimensional centrifuge modeling of the influence of side-by-side twin tunneling on a piled raft

Although the effects of tunnel excavation on nearby existing pile foundations have been extensively studied, previous studies mainly focused on response of single piles and elevated pile groups. In this study, three-dimensional centrifuge model tests were carried out to investigate the effects of side by side twin tunneling on an existing two by two piled raft in dry sand. Upon completion of twin tunnel excavation, the measured cumulative settlement of piled raft due to tunneling near the pile toe is 2.7 times of that due to tunneling near the mid-depth of the pile shaft. For twin tunneling near the pile toe, differences of piled raft settlements due to each tunnel excavation are negligible. In contrast, settlement of piled raft due to the second tunneling is about 55% of that due to the first tunneling when twin tunnels are excavated near the mid-depth of the pile shaft. As an advancement of tunnel excavation near the pile toe, the axial loads carried by the front piles are decreased, and the decreased loads are transferred to the rear piles and the raft. On the contrary, twin tunneling at the mid-depth of the pile shaft causes a reduction in the front piles and raft, and the decreased loads are transferred to the rear piles. After completing tunnel excavation, the axial load carried by the raft is increased by 22.0% for tunneling near the pile toe, while it is decreased by 21.7% for tunneling near the pile shaft.

Three-dimensional centrifuge modeling of soil nail walls

ABSTRACT Assuming plane strain deformation instead of three-dimensional behaviour of nailed convex trenches in the design process might lead to life and property losses during the operation period. In this research, the behaviour of convex nailed trenches was investigated. Various physical models with three-dimensional and plane strain conditions were tested using a geotechnical centrifuge. The failure acceleration was measured as well as the walls deformations. Results showed that convex nailed trenches had less stability than long nailed trenches. It was also revealed that the connection quality of the trench sides at their intersection had a significant effect on increasing stability. Finally, a method was proposed to increase the stability of the convex trench in which the two sides of the trench are sewed together by a series of diagonal nails. Using this method to reinforce the model resulted in the maximum stability and minimum deformation of the convex trench.

Settlement and Load Transfer Mechanism of Pipeline Due to Twin Stacked Tunneling with Different Construction Sequences

The tunnel-pipeline-soil interaction is attracting more and more wide range of scholars and was treated as a plane strain problem, with little attention given to the responses of existing pipeline to twin-tunneling process with different construction sequences. Moreover, there are rare reports available on the influences of tunnel construction sequences on an existing pipeline. In this study, a three-dimensional centrifuge model test and numerical simulations using an advanced hypoplasticity constitutive model were conducted to investigate the influences of twin stacked tunneling with different construction sequences on an existing pipeline. The results shows that the maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the upper tunnel was excavated first were 25%, 18% and 11% larger, respectively, than those when the lower tunnel was excavated first. The maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the lower tunnel was excavated first were slightly smaller than those when the twin tunnels were excavated simultaneously. After the twin-tunneling process, the sagging regions were located within ± 1.5DT (tunnel diameter) in these three conditions. The maximum pipeline bending strain within the hogging regions was approximately half of that within the sagging regions. Because of shielding effects from the upper tunnel, the region where vertical stress of the soil reduced was transferred to both sides of the tunnel due to the second tunneling process when the upper tunnel was excavated first. As a result, the extended sagging regions due to the second tunneling process when the upper tunnel was excavated first were two times than that when the lower tunnel was excavated first.

Responses of pipeline to side-by-side twin tunnelling at different depths: 3D centrifuge tests and numerical modelling

The tunnel-pipeline-soil interaction has gotten momentum recently, but most of the previous studies were focused on the responses of pipeline to single tunnelling and simplified the interaction as a plane strain problem. Up to date, there is still a lack of research on the responses of a pipeline to twin tunnelling at different depths. This study presents a series of three-dimensional centrifuge model tests which were performed to investigate the effects of side-by-side twin tunnelling at varying depths on an existing buried pipeline in dry sand. In the centrifuge model tests, both the volume loss and the weight loss were simulated in-flight using a novel technique. To gain better understanding of the tunnel-pipeline interaction, the centrifuge tests were back-analysed numerically based on an advanced hypoplastic model, which accounts for strain- and path-dependent soil stiffness at small strains. Based on the physical and numerical investigations, it was revealed that the distribution of pipeline maximum shear force is greatly affected by the second tunnelling. The shifted distance of the pipeline maximum shear force induced by the second tunnelling in the case of cover-to-diameter ratio (C/D) of 2 was 80% larger than that in the case of C/D of 4. The major influence zone on the pipeline induced by tunnelling is within 1.2D (tunnel diameter) ahead of and after the tunnel face. When the pipeline is located within the major influence zone due to tunnelling, there was a sharp increase in the relative pipeline-soil stiffness and the additional bending strain in the pipeline. The tests results were compared with the results from a dimensionless chart and an analytic solution in the literature. Although these two approaches can reasonably predict the bending strain in the pipeline due to the first tunnelling, they fail to accurately capture the incremental bending strain solely caused by the second tunnelling. This implies that one should be cautious in using the superposition principle to predict the bending strain in a pipeline subjected to twin tunnelling. Because the deviatoric strain of the soil around the pipeline was increased by the second tunnelling, leading to further degradation of the soil stiffness around the pipeline.

Three-Dimensional Centrifuge Modeling of Ground and Pipeline Response to Tunnel Excavation

AbstractTunnel excavation inevitably induces stress changes in the surrounding soil leading to significant ground movements and additional stress in nearby pipelines. Most previous studies simplified tunnel–pipe interaction as a plane strain problem and focused on the case in which the pipeline intersects the tunnel perpendicularly. In this study, centrifuge tests were conducted to investigate the three-dimensional response of the ground and pipeline to tunneling, considering different pipeline orientations with respect to the tunneling direction. When the tunnel face was located within ±1.25D (tunnel diameter) away from the monitoring section, the ground settlement and pipe strain induced were more than 95% of the maximum recorded. When the pipeline intersected the tunnel perpendicularly, the maximum pipe strain always occurred above the tunnel centerline. However, when the pipeline intersected the tunnel at an angle of 60°, the location of maximum pipe strain shifted as the tunnel advanced. The maximum ...

Effects of Pillar Depth and Shielding on the Interaction of Crossing Multitunnels

AbstractAny new tunnel excavation may damage adjacent existing tunnels in congested cities. To evaluate the impact of new tunnel construction on nearby existing tunnels, a series of three-dimensional centrifuge model tests were conducted with numerical backanalyses using an advanced hypoplasticity constitutive model. The influences of the pillar depth-to-diameter ratio (P/D) on two-tunnel interaction and the effects of shielding on three-tunnel interaction were investigated. The maximum measured settlement of an existing tunnel caused by a new tunnel excavation at P/D of 0.5 underneath was approximately 50% larger than that when P/D was 2.0. This is attributed to a smaller mobilized shear modulus, resulting from a larger reduction in confining stress of soil acting on the invert of the existing tunnel in the former than in the latter. When the new tunnel was excavated underneath two perpendicularly crossing tunnels, the lower existing tunnel “shielded” the upper one from the influence of tunnel excavation...

Effects of the construction sequence of twin tunnels at different depths on an existing pile

Any tunnelling process inevitably induces changes in stress in the ground and may adversely affect nearby pile foundations. The interaction between tunnelling and an existing pile has been investigated by researchers and a certain amount of fundamental understanding has been gained. However, the effects of different tunnel excavation sequences on an adjacent pile remain to be understood. In this paper, a series of three-dimensional centrifuge model tests and numerical back-analyses were carried out to investigate the effects of construction sequence of twin tunnels on an existing pile in dry sand. Two tunnelling sequences were investigated: (i) a sequence involving tunnelling near the pile toe followed by tunnelling near the mid-depth of the pile shaft (i.e., test TS); (ii) sequence involving tunnelling near the mid-depth of the pile shaft followed by tunnelling near the pile toe (i.e., test ST). The measured cumulative pile settlement was about 33% larger for tunnelling sequence ST than for tunnelling sequence TS. Due to different tunnelling sequences, the apparent losses of pile capacity were 40% and 29% for sequences ST and TS, respectively. Although the computed reductions in normal stress acting on the pile induced by twin tunnelling were almost the same in tests TS and ST, tunnelling near the pile toe induced a larger decrease in the end-bearing and shaft resistances at the lower part of the pile in test ST than in test TS. In contrast to the measured pile head settlements, different tunnelling sequences had a limited effect on measured ground surface settlements and additional bending moments in the pile.

Three-dimensional centrifuge modelling of the effects of twin tunnelling on an existing pile

Tunnelling activity inevitably induces soil stress changes and ground deformation, which may affect nearby existing pile foundations. Although a number of studies have been carried out to investigate the effects of tunnelling on existing piles, the excavation of only one tunnel is often considered. The fundamental interaction between twin tunnel construction and an existing pile foundation has not been thoroughly studied. In this study, a series of three-dimensional centrifuge model tests investigating the effects of twin tunnel construction on an existing single pile in dry sand were conducted. The influence of the depth of each tunnel relative to the pile was investigated by constructing the twin tunnels either close to the mid-depth of the pile shaft or near the pile toe. The pile settlement induced by the excavation of the twin tunnels is found to be closely related to the depth of each tunnel relative to the pile. The measured cumulative pile settlement due to tunnelling near the toe is about 2.2 times of that due to tunnelling near the mid-depth of the pile shaft. Apparent losses of pile capacity of 36% and 20% are identified due to the construction of twin tunnels near the pile toe and at the mid-depth of the pile, respectively. Although there is an increase in the axial force induced in the pile when a tunnel is constructed at the mid-depth of the pile, significant increases in bending moment is not observed in any of the tests.