Surface Settlement Analysis Induced by Shield Tunneling Construction in the Loess Region

The influence and prediction of shield tunneling construction on surface settlement (SS) and adjacent buildings is a hot topic in underground space engineering. In this work, several analytical methods are utilized to estimate the maximum surface settlement (MSS) and conduct a parametric sensitivity analysis based on Xi’an Metro line 2. The results show that there are mainly nine factors influencing the SS induced by shield tunneling construction in loess strata. The disturbance degree of the surrounding soil during the shield advancing stage has the largest influence on the SS, followed by the seepage of the shield lining segments or falling water levels, which lead to the overlying soil consolidation. After this is the grouting filling effect at the shield tail, followed by the reinforcement effect of the tunnel foundation and the track. The smallest influencing factors on the SS are the shield overexcavation and improper shield attitudes during the construction period. The sensitivity analysis results of the above influencing factors may offer a scientific guidance for the control of shield tunneling construction.

Construction Sequence Optimization and Settlement Control Countermeasures of Metro Tunnels Underpassing Expressway

The Chongqing metro line 6 underpass expressway around the city is taken as an engineering background, and the optimal excavation sequence and corresponding control countermeasures for the triangular-distributed three-line metro tunnel underpass expressway are studied. The influences of excavation sequence on the tunnel surrounding rock deformation, surrounding rock stress, supporting structure stress, plastic zone, and surface settlement are analyzed by using MIDAS/GTS NX finite element software. The numerical simulation results showed that Case 1 is the optimal excavation sequence of the metro tunnel. However, the surface settlement under the optimal excavation sequence exceeds the limit value of 30 mm, which cannot guarantee the safety of expressway traffic. On this basis, the control measure for strengthening the three-line tunnels with advanced small pipe grouting and reinforcing the middle tunnel with concrete-filled steel tube piles are proposed. Moreover, the excavation process of the metro tunnel with and without reinforcement schemes is numerically simulated. The results show that the reinforcement scheme can effectively control the surface settlement value within the limited value (16.47 mm), which is close to the maximum surface settlement of 18.31 mm after the metro tunnel excavation is completed, indicating that the proposed reinforcement scheme is beneficial to ensure the safety of metro tunnel construction and the driving safety of the expressway.

The effect of the additional load on the ground support on the settlement of the surrounding ground

Based on the existing engineering examples, this paper uses numerical simulation combined with the actual monitoring values on site to study the effect of the additional load on the support and the settlement of the surrounding ground, and the following conclusions are drawn: (1) When the enclosure structure is good, the settlement curve generally assumes a “spoon shape”. As the distance from the foundation pit increases, the surface settlement curve first increases and then decreases. The distance between the location of the maximum surface settlement and the foundation pit is generally half of the maximum excavation depth of the foundation pit. (2) The existence of additional load accelerates the rate of change of surface settlement, making the soil settlement from the excavation of the first layer of soil as a whole smaller than the unacted additional load to the excavation to the bottom layer as a whole larger than the unapplied load. (3) There will be a certain gap between the numerical simulation and the actual monitoring value. This gap will become larger and larger as the excavation of the foundation pit continues, but the law of change between the two is the same.

The Role of Expanded Polystyrene and Geocell in Enhancing the Behavior of Buried HDPE Pipes under Trench Loading Using Numerical Analyses

In recent years, much research has focused on the use of various materials for relieving and strengthening soil, e.g., steel reinforcing ribs, geosynthetics, geocell, waste tires, and expanded polystyrene (EPS). EPS is being used increasingly in geo-infrastructure, being a super-light material, to replace part of the soil and decrease the ground pressure on buried structures. This paper presents experimental and numerical analyses of the effectiveness of expanded polystyrene and geocell reinforcement for ameliorating the behavior of unpressurized buried pipes exposed to surface loading. A 3-D finite element method (FEM) model of soil, geofoam, geocell, and piping was generated in ABAQUS, and the model was verified by experimental analyses conducted at a laboratory. The results show that reinforcing the soil cover with geocell and geofoam has a substantial impact, decreasing the maximum surface settlement by around 29% and maximum pipe crown displacement by up to 39.5%. In addition, the EPS block density can reduce the maximum pipe crown displacement substantially.

Ground response to tunnelling incorporating soil reinforcement system

The forepole umbrella system (FUS) uses steel pipes installed from within a tunnel to provide a canopy above the tunnel heading that both increases stability and reduces tunnelling-induced ground movements. Although the system is known to be beneficial and has been used in a number of projects, there is little information on how key parameters including length and forepole stiffness combine to produce effective support. To investigate this, centrifuge tests incorporating the three-dimensional (3D) geometry of a tunnel heading in clay and the model FUS have been undertaken. The tunnel heading was supported by a pressurized rubber bag lining with excavation being simulated by a reduction in air support pressure. Image analysis was used to obtain subsurface ground movements and a newly developed 3D imaging system was used to measure the soil surface deformations accurately. The performance of the FUS and the influences of key FUS parameters were quantified via the settlement reduction factor. The results showed that the FUS, arranged in various settings, reduced the maximum surface settlement by 35%–75%. The effects of the FUS parameters on the reinforcing effectiveness are dependent on the ratio of cover depth to tunnel diameter. An optimum design arrangement of the FUS is suggested.