Surface settlement in saturated loess stratum during shield construction: Numerical modeling and sensitivity analysis

In the process of constructing double-track subway tunnels in shallow buried loess areas, the interaction of double-track tunnels is significantly influenced by the net distance and the cross-section size, which is challenging to control the surface settlement. Therefore, the surface settlement prediction is essential while constructing double-track subway tunnels in shallow buried loess areas. The paper analyzed the surface settlement law of shallow buried double-track tunnels in loess areas through theoretical research and numerical simulation. The research results show that with the decrease of the net distance, the surface settlement superimposed curve was double V shape -W shape - single V shape. When the superimposed curve is double V shape and W shape, the Peck formula was used to calculate the surface settlement curve of the single-track tunnel, then superimposed to obtain the final surface settlement curve. When the superimposed surface settlement curve was V shape, based on the Peck formula, the formula for predicting the surface settlement suitable for symmetry and asymmetry was established. The net distance ratio and the area ratio were defined, and considering the tunnel’s interaction, the value and position of the maximum were corrected. Then numerical tests were carried out 16 times with different net distance ratios and area ratios, to determine the parameters of increments and position offsets of the maximum regarding the net distance ratio and the area ratio. Finally, two engineering were conducted for verifying the rationality and applicability exhibited by the prediction formula. The prediction formula served for predicting the surface settlement of double-track subway tunnels in shallow buried loess areas. Which can reduce construction risks and assure the safety of buildings above the ground.

The thickness of aquifer is relatively large, the aquifer is not completely isolated by the diaphragm wall, and the water flow inside and outside the foundation pit flows at the bottom of the diaphragm wall. This article reports indoor experiments and theoretical methods which are used to study the deformation mechanism and law caused by dewatering in foundation pit under the condition of incomplete isolation of the inner and outer aquifers of the foundation pit based on typical geological conditions in South China. Our findings are as follows. (1) Through the experimental analysis of the mechanism of groundwater seepage around the foundation pit, the effective influence depth of precipitation in the foundation pit is greater than that outside the foundation pit, and the effective influence depth of double well precipitation is greater than that of single well precipitation. It reveals that the precipitation curve outside the foundation pit under the influence of seepage around the foundation pit presents the characteristics, and puts forward the calculation formula of surface settlement change outside the pit caused by partially penetrating well precipitation in the completely decomposed granite stratum. The calculation results of the theoretical formula are in good agreement with the experimental results, which has good accuracy and applicability. (2) The surface settlement outside the pit caused by dewatering in the foundation pit is mainly concentrated in the surrounding seepage area, and the change value of surface settlement outside the pit caused by double well dewatering is 2∼3 times greater than that caused by single well dewatering. When the whole well is vertically reinjected, the maximum surface settlement is 0.009 mm at the side near the diaphragm wall, and 0.005 mm at the side far away from the diaphragm wall. When the whole well is at an inclination of 10°, the maximum surface settlement is 0.002 mm at the side near the diaphragm wall, and 0.008 mm at the side far away from the diaphragm wall. When the whole well is at an inclination of 20°, the maximum surface settlement is −0.005 mm at the side near the diaphragm wall, and 0.011 mm at the side far away from the diaphragm wall. The full well depth recharge with an inclination of 20° has the largest influence on the surface settlement of the foundation pit, and the recharge effect is the best. (3) The lateral displacement of diaphragm wall increases by 24% compared with that without reinjection. The maximum lateral displacement of the diaphragm wall is 0.09% of the precipitation depth when the whole well is tilted at 10° for reinjection, and the lateral displacement of the diaphragm wall increases by 13% compared with that without reinjection. The maximum lateral displacement of the diaphragm wall is 0.082% of the precipitation depth when the whole well is tilted at 20° for reinjection, and the lateral displacement of the diaphragm wall increases by 4.9% compared with that without reinjection. The full well depth recharge with an inclination of 20° has the least impact on the diaphragm wall, and the recharge effect is the best. The above results can be provided as theoretical foundation for the study of the deformation mechanism and law caused by dewatering in foundation pit.

Visualization investigation of the large deformation and instability failure of shield tunnel using transparent soil models

Crushing and caving of roof strata in goaf is an important factor affecting surface subsidence. In order to study the influence law of mining under thick loose layer, overburden size parameters, and initial mechanical parameters of collapse zone on surface subsidence, the law of surface subsidence was analyzed through measured data. On this basis, with the help of FLAC3D numerical simulation software, the surface subsidence was simulated by changing the above factors, and the evolution characteristics of the maximum surface subsidence under the influence of various factors were analyzed. Based on the grey relational analysis, the relational degree between each factor and the maximum surface subsidence value is analyzed, and through multiple nonlinear regression, the prediction model of mining subsidence deformation under thick loose layer considering the influence of seven factors is established. The results show that the mining thickness has the greatest influence on the surface subsidence, and cohesion has the least influence on surface subsidence. The rationality of the subsidence prediction model is verified by an example.

The shield construction process of shield tail gap caused by formation damage, the formation damage caused by the settlement is as long as the part of tunnel subsidence analysis, therefore in the finite element software analysis of shield construction settlement value, the shield tail clearance simulation is an important link in the entire model, this article analyzes three common finite element simulation method, shield tail clearance displacement convergence method based on non-uniform distribution and actual matches are higher than layer such as generation method and stiffness discount subtraction. Finally, the displacement convergence method is used to simulate the grouting effect of shield tail on the surface settlement.

The fine numerical simulation is used to study the ground settlement of complex stratum owing to shield construction by ANSYS program. It is shown that the closer the distance between soil layer and the axis of tunnel is, the smaller the disturbance of construction is, the obvious the ground surface settlement is. The value of the maximum settlement at the center of the surface is 7.4mm. The maximum settlement of vault is 14mm. The ground subsidence in cross section distribution is shaped of normal distribution. The closer the distance between soil layer and ground surface is, the smaller the vertical displacement is, the bigger the width of settlement trough of soil layers is. The width of settlement trough is 25m. The volume loss rate of shield tunnel is about 0.32%. The width coefficient of ground settlement trough is 0.56. The tendency of ground settlement decrease to become gentle with the advance of shield construction. The ground settlement keeps constant after tunnel face advancing to 30m.

ABSTRACTRecently, solid waste backfill mining has rapidly developed, finding application in the control of surface subsidence and reduction of accumulated solid wastes. However, with the mining depth, the technique faces a number of other problems, in regard to the source of backfill materials, transportation costs, and control of the surface subsidence in deep multiple coal mining. In this article, an integrated system comprising deep mining, coal-gangue dressing, and underground backfill mining is proposed on the basis field-work. Analysis of the type, construction, and protection standards of the buildings resulted in the elaboration of numerical model aimed at studying the surface subsidence and deformation. Research results demonstrate that backfill mining of the coal seam group leads to a gradual increase in maximum surface subsidence, horizontal movement, inclination, and horizontal deformation, whereas the key index to control is the compression ratio of backfill materials. The results of surface subsidence prediction and measurements in the Tangshan Coal Mine show that the solid waste backfill mining technique can effectively control the surface subsidence, deformation, and safety of buildings. This study is highly instrumental in sustained deep mining and environmental protection.

Taking the shield tunnel project of Guangzhou Metro Line 8 from Tongdewei Station to Shangbu Station as the research background, using the research method of finite element simulation and site monitoring, this paper analyses the influence rules of shield tunneling on ground subsidence under the condition of different hard rock height ratios. The research results show that in the process of crossing different hard rock height ratio composite stratum, as the hard rock height ratio decreases, the value of ground settlement decreases and settlement tank becomes shallow. The surface subsidence in different hard rock height ratio strata is obviously different, and the maximum difference is about 8.6 mm; The influence of the hard rock height ratio on the surface longitudinal settlement is mainly reflected in the position change of the beginning and the end of the settlement. With the increase of the hard rock height ratio, the shield construction reduces the amount of the surface longitudinal settlement and its influence range; Through the research, it is found that the hard rock height ratio in the 0-0.2 and 0.5-1 is the sensitive interval, and the settlement value in these two hard rock height ratio interval varies greatly. It is necessary to pay attention to the uneven settlement of the ground surface caused by shield construction in the sensitive hard rock height ratio interval. The research results of this paper can provide reference value for similar shield construction in upper soft and lower hard composite stratum.

In the 202 section of subway shield Dalian interval in close proximity to existing bridge as the basis, the surface settlement and bridge pier settlement observation station are established. By the 62 day of the crossing construction monitoring, the monitoring data are analyzed in detail. When surface subsidence induced by shield tunnel construction, above the measuring point start speed is faster than settling startup speed measuring points on both sides, about 2-2.5 times, subsidence range higher about 8 to 10 m ahead of time. Away from the pier side first started, and start faster than the side near the pier ,is about 1.2 times , settlement of shield construction are mainly concentrated in the distance before and after the excavation of shield machine 10 m, accounting for 80.72%-81.6% of the total settlement . At highest risk for shield construction area, we should strengthen monitoring and make contingency plans in a timely manner.

Investigation on the 3D Ground Settlement Induced by Shallow Tunneling Considering the Effects of Buildings

The disturbance of ground deformation caused by pipe jacking construction has become one of the important factors affecting the safety of the surrounding buildings and environmental changes. The construction of the sewage pipe network, which is in the Cixian section of the South-to-North Water Diversion Middle Route, is taking as an example. Theoretical prediction and field tests are conducted to research the ground settlement deformation caused by the disturbance of pipe jacking construction. Research indicates that the surface settlement near Hexie Avenue in Ci County meets the engineering safety requirements when the volume loss rate of the local layer is controlled within 6.9%. And the maximum surface settlement within the construction range of the Ci County section of the South-to-North Water Diversion Project was within 2.5-4 mm, which did not break the safety threshold. Suggested values for the formation loss rate and settlement trough width coefficient index are proposed for similar working conditions, which are 3.31% and 0.525 respectively. Relevant research results provide a reference for the study of ground deformation prediction under similar working conditions.

Mining subsidence, mine solid waste emissions caused by the direct cause of ecological environment deterioration in mining area, effective control and reduce the surface subsidence and reasonable control and utilization of mine wastes, and realize green mining is a fundamental way to reduce the damage of the ecological environment in mining area. To solve this problem, using FLAC3D dimensional numerical simulation method, the above four factors were changed to simulate the surface movement and deformation, and the influence of the above four factors on the maximum surface subsidence was discussed. By using the orthogonal experiment, the importance of the four factors is queued, and the correlation degree ranking of the maximum subsidence value is obtained. The influence degree of the factors on the maximum surface subsidence is: the filling ratio, the thickness of the key stratum, the strength of the filling body and the thickness of the loose layer. In this paper, the content of the three under pressure coal mining area of production practice to provide basic data, can improve the recovery rate of coal resources, reduce the cost of filling, the maximum economic benefits. Has important practical significance to protect the ground structure, reduce the ecological damage, and realize the sustainable development of coal mine.

Excess surface settlement during construction of underground structures such as tunnels is concerned for historic buildings in big cities. A metro line No. 1 located at 20 m deep under the ground surface will be built in Ho Chi Minh City (HCMC) soon, and this metro crosses the City underneath many historic buildings. Reinforcing the soil mass surrounding the tunnel by soil-cement mixing (soilcrete) using jet grouting is considered a feasible solution to reduce surface settlement. However, appropriate characteristics of soilcrete for the HCMC’s geological conditions have not been thoroughly investigated. This study investigated the relationship between the characteristics of soilcrete (young modulus and thickness) and surface settlement at the city opera house, the historic building close to the metro, with a maximum allowable surface settlement of 10 mm.

Based on a metro project in Hangzhou, combined with the investigation of on-site seepage and leakage problems and finite element numerical simulation, the influence of local seepage and leakage in shield tunnels in water-rich silty clay strata on stratum settlement and lining structure deformation was studied. During the simulation process, two working conditions, namely leakage at the joint of the segment and local damage leakage, were, respectively, set up to analyze the distribution of pore water pressure, the development characteristics of stratum settlement and the response of the lining structure. The results show that the pore water pressure near the leakage area is significantly reduced. The pore pressure at the joint of the segment and the local leakage position is reduced by 81.22% and 76.88%, respectively, compared with the hydrostatic pressure at the same burial depth, and the reduction at the bottom of the model is 11.45% and 6.46%, respectively. Under different working conditions, the settlement rates all increased first and then tended to stabilize. The maximum surface settlements were 91 mm and 32 mm, respectively, and the former exceeded the control value. The surface settlement of local leakage is distributed in a concave pattern, and the peak settlement is located directly above the leakage point. The lining structure deforms significantly in both the upper and lower directions, both shifting downward towards the stratum. The maximum displacement and deformation caused by the leakage at the joint of the segment reached 78.26 mm and 24.38 mm, respectively, with obvious over-limits. It is recommended to prioritize the sealing treatment of the leakage area at the joint. The research results can provide theoretical references for the control of water leakage and structural safety evaluation of shield tunnels in water-rich and weak strata.

Currently, there is limited research focusing on the surface deformation caused by shield excavation in strongly weathered rock formations with significant coarse particle content. This study, based on the shield tunnel project of the Pearl River Delta Intercity Pazhou Branch Line, investigates the influence of coarse particle content in strongly weathered rock layers on their mechanical properties, as well as its effect on surface subsidence during tunnel shield excavation. By comparing Peck's formula with random medium theory, the study expands the application of random medium theory in engineering, converting many of Peck's empirical parameters into corresponding parameters within the random medium framework. Additionally, Peck's formula is modified to account for the coarse particle content in strongly weathered rock layers. The results show that as the coarse particle content increases, the force transmission mechanism in the strongly weathered rock shifts from compressive deformation typical of fine-grained soils to point-contact force transmission between coarse particles. Furthermore, as the coarse particle content rises, the strata loss rate tends to decrease gradually. The final settlement curve, calculated using the method that considers changes in coarse particle content, is closer to the measured values. This study's calculation method more accurately reflects the surface deformation behavior caused by shield tunnel construction in strongly weathered rock layers, particularly when coarse particle content is taken into account. It also provides a better understanding of the combined impact of various construction parameters as the coarse particle content changes during the excavation process.