Ground Surface Settlement Research Articles

Effect of construction sequence on three-arch tunnel behavior-Numerical investigation

This paper concerns a numerical investigation on the effect of construction sequence on three-arch (3-Arch) tunnel behavior. A three-arch tunnel section adopted in a railway tunnel construction site was considered in this study. A calibrated 3D finite element model was used to conduct a parametric study on a variety of construction scenarios. The results of analyses were examined in terms of tunnel and ground surface settlements, shotcrete lining stresses, loads and stresses developed in center column in relation to the tunnel construction sequence. In particular, the effect of the side tunnel construction sequence on the structural performance of the center structure was fully examined. The results indicated that the load, thus stress, in the center structure can be smaller when excavating two side tunnels from opposite direction than excavating in the same direction. Also revealed was that no face lagging distance between the two side tunnels impose less ground load to the center structure. Fundamental governing mechanism of three-arch tunnel behavior is also discussed based on the results.

Effect of the lateral earth pressure coefficient on settlements during mechanized tunneling

Tunnel excavation leads to a disturbance on the initial stress balance of surrounding soils, which causes convergences around the tunnel and settlements at the ground surface. Considering the effective impact of settlements on the structures at the surface, it is necessary to estimate them, especially in urban areas. In the present study, ground settlements due to the excavation of East-West Line 7 of the Tehran Metro (EWL7) and the Abuzar tunnels are evaluated and the effect of the lateral earth pressure coefficient (K0) on their extension is investigated. The excavation of the tunnels was performed by TBMs (Tunnel Boring Machines). The coefficient of lateral earth pressure (K0) is one of the most important geotechnical parameters for tunnel design and is greatly influenced by the geological characteristics of the surrounding soil mass along the tunnel route. The real (in-situ) settlements of the ground surface were measured experimentally using leveling methods along the studied tunnels and the results were compared with evaluated settlements obtained from both semi-empirical and numerical methods (using the finite difference software FLAC3D). The comparisons permitted to show that the adopted numerical models can effectively be used to predict settlements induced by a tunnel excavation. Then a numerical parametric study was conducted to show the influence of the K0 values on the ground settlements. Numerical investigations also showed that the shapes of settlement trough of the studied tunnels, in a transverse section, are not similar because of their different diameters and depths of the tunnels.

Estimation of Influence Scope of Lateral Displacement of Soft Ground under Vacuum Pressure with PVD

The application of vacuum pressure to a treated area not only induces vertical settlement and inward lateral displacement but also causes the formation of tension cracks near the ground surface. In general, the strain method is applied to calculate the lateral displacement at the boundary of a treated area; however, the influence scope of lateral displacement has not yet been presented. Based on the in situ data of soft clayey soil foundation treated by vacuum consolidation, lateral displacement was estimated in the influence scope in this study. To calculate the influence scope of lateral displacement induced by vacuum pressure, the ratio of the lateral displacement within the influence scope to the ground surface settlement under the centre of the treated area is defined as the maximum value of the lateral displacement (ELD) within the influence scope. This paper proposes a direct relationship between ELD and the distance from the treated area boundary (Lx), considering the length of the prefabricated vertical drain. In addition, the FEA (finite‐element analysis) is used to simulate the process of vacuum preloading to reinforce soft soil foundation. The influence scope simulated is almost close to the calculated value Lx. Accordingly, the safety distance between the boundary of the treated area and the surrounding building can be estimated when the soft soil foundation is consolidated by using a vacuum preloading method.

Roof failure of shallow tunnel based on simplified stochastic medium theory

The failure mechanism of tunnel roof is investigated with upper bound theorem of limit analysis. The stochastic settlement and nonlinear failure criterion are considered in the present analysis. For the collapse of tunnel roof, the surface settlement is estimated by the simplified stochastic medium theory. The failure curve expressions of collapse blocks in homogeneous and in layered soils are derived, and the effects of material parameters on the potential range of failure mechanisms are discussed. The results show that the material parameters of initial cohesion, nonlinear coefficient and unit weight have significant influences on the potential range of collapse block in homogeneous media. The proportion of collapse block increases as the initial cohesion increases, while decreases as the nonlinear coefficient and the unit weight increase. The ground surface settlement increases with the tunnel radius increasing, while the possible collapse proportion decreases with increase of the tunnel radius. In layered stratum, the study is investigated to analyze the effects of material parameters of different layered media on the proportion of possible collapse block.

ТЕОРЕТИЧЕСКАЯ МЕТОДИКА ОПРЕДЕЛЕНИЯ МУЛЬДЫ СДВИЖЕНИЙ ДНЕВНОЙ ПОВЕРХНОСТИ ДЛЯ ГОРИЗОНТАЛЬНЫХ И НАКЛОННЫХ ВЫРАБОТОК ПО РЕЗУЛЬТАТАМ ГЕОЛОГИЧЕСКИХ ИЗЫСКАНИЙ

ТЕОРЕТИЧЕСКАЯ МЕТОДИКА ОПРЕДЕЛЕНИЯ МУЛЬДЫ СДВИЖЕНИЙ ДНЕВНОЙ ПОВЕРХНОСТИ ДЛЯ ГОРИЗОНТАЛЬНЫХ И НАКЛОННЫХ ВЫРАБОТОК ПО РЕЗУЛЬТАТАМ ГЕОЛОГИЧЕСКИХ ИЗЫСКАНИЙ

Long-term stability analysis of goaf area in longwall mining using minimum potential energy theory

Estimation of the height of caved and fractured zones above a longwall panel along with the stability conditions of the goaf area are very crucial to determine the abutment stresses, ground subsidence, and face support as well as designing the surrounding gates and intervening pillars. In this work, the height of caving-fracturing zone above the mined panel is considered as the height of destressed zone (HDZ). The long-term estimation of this height plays a key role in the accurate determination of maximum ground surface subsidence and the amount of transferred loads towards the neighbouring solid sections. This paper presents a new stability analysis model of caved material system in the goaf area. For this aim, a theoretical energy-based model of HDZ determination in long-term condition is developed. Then the stability condition of the caved material system is investigated using the principle of minimum potential energy. On the basis of the actual data gathered from the literature, the unstable time period of the caved material system is also calculated. Moreover, the effects of time- and temperature-related parameters and constant coefficients as well as their inherent relations with HDZ are evaluated. Furthermore, sensitivity analysis shows that the two temperature-related constants material constant and time are the most effective variables in HDZ, and the slope of material hardening is the least effective one. The estimated HDZ and the stability time of the caved materials can be successfully applied to determine the induced stress and the maximum surface subsidence, respectively, due to longwall mining.

A fully implicit and consistent finite element framework for modeling reservoir compaction with large deformation and nonlinear flow model. Part I: theory and formulation

Reservoir depletion results in rock failure, wellbore instability, hydrocarbon production loss, oil sand production, and ground surface subsidence. Specifically, the compaction of carbonate reservoirs with soft rocks often induces large plastic deformation due to rock pore collapse. On the other hand, following the compaction of reservoirs and failure of rock formations, the porosity and permeability of formations will, in general, decrease. These bring a challenge for reservoir simulations because of high nonlinearity of coupled geomechanics and fluid flow fields. In this work, we present a fully implicit, fully coupled, and fully consistent finite element formulation for coupled geomechanics and fluid flow problems with finite deformation and nonlinear flow models. The Pelessone smooth cap plasticity model, an important material model to capture rock compaction behavior and a challenging material model for implicit numerical formulations, is incorporated in the proposed formulation. Furthermore, a stress-dependent permeability model is taken into account in the formulation. A co-rotational framework is adopted for finite deformation, and an implicit material integrator for cap plasticity models is consistently derived. Furthermore, the coupled field equations are consistently linearized including nonlinear flow models. The physical theories, nonlinear material and flow models, and numerical formulations are the focus of part I of this work. In part II, we verify the proposed numerical framework and demonstrate the performance of our numerical formulation using several numerical examples including a field reservoir with soft rocks undergoing serious compaction.

Behaviour of anchored pile wall excavations in clays

A database of 35 case histories of anchored pile walls, collected mainly from Chicago, Boston, Texas and Washington, DC, USA as well as Taiwan, has been evaluated to predict the performance of deep excavations in clays. Various factors (including excavation depth, undrained shear strength, wall bending stiffness, factor of safety and pile embedment) affecting the deformation characteristics of excavations supported by anchored pile walls are presented and compared with findings of previous researchers. The normalised maximum wall displacement δHmax/He falls in the range of 0·1–0·7% and the location of δHmax is at a depth of 0·4He to 1·0He. Normalised maximum ground surface settlement δVmax/He is in a range of 0·1–0·4%. The ratio of maximum surface settlement to maximum lateral displacement falls in the range of 0·7–1·4, with an average value of 1·0. Maximum wall displacements are dependent on undrained shear strength and factor of safety and decrease with an increase in these factors. Maximum wall displacements decrease by increasing wall stiffness and most of the cases are bounded by values of EI/He between 35 MNm and 120 MNm. The effect of system stiffness is more pronounced in soft clays whereas deformations in stiff clays are less dependent on system stiffness.

Investigation of the integrated retaining system to limit deformations induced by deep excavation

A series of three-dimensional finite element analyses of deep excavations with the integrated system between buttress walls and diaphragm walls was conducted to investigate the effect of the buttress wall intervals, treatments, locations, height, and thickness on limiting deformations induced by deep excavation. The integrated retaining system was formed by maintaining buttress walls when soil was excavated. The wall deflection control mechanism of the integrated retaining system mainly came from the combined stiffness between the buttress wall and the diaphragm wall. In addition, the ground settlement control mechanism came from the combined stiffness between the buttress wall and the diaphragm wall, and the frictional resistance between the buttress wall and the surrounding soil. For achieving 50% reduction in the wall deflection and the ground surface settlement, the length and intervals of buttress walls that were applied to the integrated retaining system were at least 4 and 8 m, respectively. When the deflection at the diaphragm wall head was well restrained, for example, by the floor slab, the position of the buttress wall head could be located at a depth the diaphragm wall starts to bulge out. In such a case, the performance between the full height and limited height of buttress walls was quite close. Furthermore, a new well-documented excavation project was analyzed to verify the performance of the integrated retaining system. Results showed that the integrated retaining system worked excellently if the joints between buttress walls and diaphragm walls were constructed properly.

Case Study on Deformation Control of Upper-Soft and Lower-Hard Large Span Tunnel Station using Combined Control Technology and Monitoring Demonstration

A large number of shallow buried tunnels are built in the city nowadays and the special strata such as large upper-soft and lower-hard ground often encountered. Deformation control of strata is the focus issue related to the construction safety. Based on Dalian metro Hing Street station with the classical geological condition of upper-soft and lower-hard ground, this paper fully used a combined control method including six different support measures to control the deformation of surrounding rock. 3D finite element model was setup to analyze the construction effect of combined control measures and the monitoring in-site was carried out to verify the deformation control effect of combined control method. It shows that the maximum surface subsidence value is gradually reduced with the support measures gradually increasing. In the case of various supports the maximum sedimentation value is 2.67 cm, which is 42. 1% lower than that of not using control method and the control effect is obvious. In addition, it can be seen that the two-layer initial support and additional large arch foot have the best effect on controlling the ground surface settlement with reduction of 11.7% and 20.2%, respectively. The research results can provide practical experience for the construction of such tunnels, and guide the design and construction of the tunnel in the future.

Face stability analysis of shield-driven tunnels shallowly buried in dry sand using 1-g large-scale model tests

This paper investigates the face stability of shield-driven tunnels shallowly buried in dry sand using 1-g large-scale model tests. A half-circular tunnel model with a rigid front face was designed and tested. The ground movement was mobilized by pulling the tunnel face backwards at different speeds. The support pressure at tunnel face, settlement at ground surface, and internal movement of soil body were measured by load cells, linear variable differential transducers, and a camera, respectively, and the progress of face failure was observed through a transparent lateral wall of model tank. The tests show that, as the tunnel face moves backwards, the support pressure at the tunnel face drops sharply initially, then rebounds slightly, and tends to be stabilized at the end. Similarly, the ground surface settlement shows a three-stage variation pattern. Using the particle image velocimetry technique, the particle movement, shear strain, and vortex location of soil are analyzed. The variation of support pressure and ground surface settlement related to the internal movement of soil particles is discussed. The impact of the tunnel face moving speed on the face stability is discussed. As the tunnel face moves relatively fast, soil failure originates from a height above tunnel invert and an analytical model is developed to analyze such failure.

Comparison of Mohr-Coulomb and Hardening Soil Models Numerical Estimation of Ground Surface Settlement Caused by Tunneling

Comparison of Mohr-Coulomb and Hardening Soil Models Numerical Estimation of Ground Surface Settlement Caused by Tunneling

Analysis on Ground Surface Settlement of Newly Built Tunnel under Existing Expressway

Relying on the Beihuan tunnel project, the numerical calculation of the excavation of the newly built tunnel down-traversing the existing expressway is carried out. The ground settlement deformation law of the expressway under different excavation schemes is analyzed, and the applicability of Peck formula in the calculation of the influence scope of ground settlement is discussed. The results show that: the numerical calculation of surface settlement range corresponds to Peck formula predicted result well.

Behavior of a masonry wall subjected to mining subsidence, as analyzed by experimental designs and response surfaces

In this paper we present the principles of a methodology for studying the impact of mining subsidence on masonry buildings, using numerical experimental designs, statistical analysis and surface responses, where the subsidence is the input variable, joint tensile strength and cohesion are the control variables and the total length of cracks is the output variable. Numerical experiments are carried out using a distinct element model of a masonry wall where masonry blocks are explicitly taken into account. Calculations are also carried out for two types of subsidence according to the position of the building in the convex curvature or the concave curvature zone of the ground surface subsidence. Statistical analysis of the results shows the dominant influence of the joint tensile strength over the joint cohesion and the absence of interaction between these two parameters.

Stability analysis of super-large-section tunnel in loess ground considering water infiltration caused by irrigation

Constructing a super-large-section loess tunnel under the irrigation area is a highly challenging task. One reason is that the strength of the loess decreases when it is soaked, which endangers the safety and stability of tunnels. In this paper, Majiazhuang tunnel is selected as a project case which provides in situ monitoring and measuring data, and Midas GTS NX software is used to build a tunnel structural calculation model under the coupled function of seepage and stress fields. Combined with the change law of physical and mechanical property for unsaturated loess, simulations are performed under different situations, which include the surface infiltration, the integral soaking, and partial soaking caused by concentrated infiltration. The results of in situ monitoring show that the complete structure of super-large-section tunnel buried at 50 m depth below the clayey loess was effected negligibly by irrigation and heavy rainfall, and it is stable during irrigation and heavy rainfall. The results of simulation show that when the integrity of the farmland topsoil is good and the weak structural planes, cracks, and fractures do not exist, the effective depth of irrigation water in Q3 clayed loess is 4 m approximately; when the tunnel soil layer is integral soaked, the moisture content of the Q2 silty clay loess increases from 16.6 to 20.5% and becomes saturated at the moisture content of 24.3%. The crown settlement, ground surface settlement, thickness and strain of the plastic zone significantly rise, and they slowly increase in the early stages while rapidly increase in the later stages. In addition, the rate of increase in the later stages is approximately 9.6 times faster than that in the early stages; when the crown, upper arch, and side wall are partial concentrated soaked, the thickness of plastic deformation zone at the crown has the most significant effect. The thickness of plastic deformation zone at the crown increased more than 100%, at the other two positions both have approximately 40% increasement compared with that without irrigation water. It indicates that the irrigation water infiltrates to the tunnel soil layer, the structural stability of the tunnel is affected by the surface irrigation, and the partial soaking at the upper crown has the greatest effect on the stability of the tunnel structure.

Pilot tests on methods to form working platform on soft clay

Pilot tests on the usage of vacuum preloading combined with short prefabricated vertical drains (PVDs) to form a working platform for future soil improvement work were conducted at a land reclamation site in Tianjin, China. The short PVDs were connected using three methods – the conventional vacuum preloading method, fish-bone connectors and embedded horizontal pipes. To investigate the influence of PVD spacing, short PVDs were installed in square grids with spacings of 0·4 m and 0·6 m. The ground surface settlement, pore water pressures, water content and undrained shear strength of the soil were measured during the specific period of vacuum preloading. The pilot tests indicated that short PVDs connected using embedded vacuum pipes installed at 0·4 m spacing were the most suitable method to form a working platform on the surface of the dredged marine clay. This method yields a substantial saving on construction cost and time while exhibiting similar efficiency to the conventional vacuum preloading method. The undrained shear strength and degree of consolidation of the soil after 60 d of vacuum preloading were 23·6 kPa and 85·1%, respectively, which met the requirements for the working platform.

The mechanism of hysteretic ground settlement caused by shield tunneling in mixed-face conditions

Shield is used more and more widely, such as coal mine roadway, hydropower tunnel, traffic tunnel and so on. But Tunneling with a tunnel boring machine (TBM) may cause inevitable ground subsidence. Though over the world numerous researches have been conducted for surface settlement induced by TBM in soft ground, the researches of surface settlement induced by TBM in a sandy cobble stratum are limited and a comprehensive study of the mechanism of delayed settlement induced by TBM in a sandy cobble stratum is unavailable. A ground stable state or surface settlement can be determined based on real-time monitoring data for surface dynamic subsidence. Subsequently, TBM tunneling parameters can be adjusted to accommodate various geological conditions. A sand-pebble-soil matrix is a typical heterogeneous material. The macro-mechanical performance of this matrix significantly differs from any material. In a general situation with a low water level and minimal disturbance, a stratum can stabilize by itself for a long period of time. Considering the characteristics of the stratum, ground loss can be divided into two phases: immediate settlement, which tends to stabilize, and delayed settlement, which tends to occur in sand-cobble strata, where settlement develops at a much slower rate than in single-medium strata. Monitoring data is not sufficient to guide the construction in the case of delayed settlement. Cobble-soil matrix can be treated as a spatial structural system that is constituted by single granular soil, aggregates of granular soil and pebble grains. Based on Particle Flow Code in 2 Dimensions (PFC2D), the mechanical characteristics of the matrix and the TBM tunneling process were numerically simulated. Movements of the pebble grains were traced and recorded in real time. The model addressed the mechanism of surface collapse from the perspective of mesomechanics. According to the model, a matrix formed self-stabilizing arch that overlies an underground cavity seems gradually wear out with expanding the cavity and eventually penetrating to the ground surface. The law of ground movement and the formation mechanism of ground subsidence in TBM advancement were investigated. The main factors that affect surface subsidence are the speed of advancement, the underground water level and the supporting period. In the numerical analysis, the surface-loss lag was reproduced and the field monitoring data were verified. The findings of this study provide a new method for investigating ground subsidence in similar strata.

Calculation of foundation pit deformation caused by deep excavation considering influence of loading and unloading

A new analytical solution for ground surface settlement induced by deep excavation is proposed based on the elastic half space Melan’s solution, and the analytical model is related to the physical and mechanical properties of soil with the loading and unloading action during excavation process. The change law of earth pressure of the normal consolidation soil after the foundation pit excavation was analyzed, and elastic displacement calculation methods of analytic solution were further established given the influence of excavation and unloading. According to the change of stress state in the excavation process of foundation pit, the planar mechanical analysis model of the foundation excavation problem was established. By combining this model with the physical equations and geometric equations of plane strain problem with consideration of the loading and unloading modulus of soil, constitutive equation of the plane strain problem was also established. The loading and unloading modulus formula was obtained by using the parameter calculation method in Duncan-Chang curve model. The constitutive equation obtained from the model was used to calculate the soil stress state of each point to determine its loading and unloading modulus. Finally, the foundation pit displacement change after excavation was calculated, and thus the soil pressure distribution after retaining structure deformation. The theoretical results calculated by making corresponding programs were applied to engineering practice. By comparing the conventional calculation results with monitoring results, the practicability and feasibility of the calculation model were verified, which should provide a theoretical basis for similar projects.

Green and Sustainable Mining: Underground Coal Mine Fully Mechanized Solid Dense Stowing-Mining Method

China produces and consumes most of coal in the world. This situation is expected to continue within a certain period in the future. Currently, Chinese coal industry is confronted with several serious problems relating to land resource, water resource, environmental, and ecological sustainability. Coal resource exploitation causes the permanent fracture and movement of strata structure, which have caused the fracture and collapse of overlying strata and further led to the subsidence of ground surface as well as the seepage of water in aquifers around the coal seam, which has resulted not only in the loss of land and water resources, but also in serious threats and accidents to underground mining. On the other hand, mining and mineral-processing wastes are one of the world’s long plagued concerns among solid wastes. Coal gangue, as the major waste with a huge amount of discharge, has not only occupied the land, but has also contaminated the ambient land resources and hydrological environment, and further led to ecological system destruction and degradation. What is more, in China there are large amounts of coal—located under railways, buildings, and water bodies—which are unavailable with traditional mining methods. These problems are obviously threaten the concept of green sustainable development. This paper introduces a novel developed solid dense stowing mining method, which is able to significantly reduce or event eliminate the corresponding damages caused by underground mining behavior and realize green and sustainable development. The novelty of this research work is realizing the automation and synchronization of mining and material stowing with an appropriate compaction ratio for adequate support of goaf roof. It can improve the stability of rock strata and the safety and efficiency of underground mining. We also studied and designed a perfect stowing material by using coal gangue and fly ash with appropriate proportions under different particle size gradations. By implementation of the above-mentioned methods in China, the solid dense stowing rate of mined seam areas have reached more than 95% and the overburden strata movements have been reduced to extremely low level which had nearly no damages to above buildings. The solid dense stowing mining method has also realized the reuse and recycling of coal mine solid wastes. Meanwhile, considerable previously unavailable coal resources under buildings, railways, and water bodies have been made available for exploration, which could extend the life of coal mines and increase the sustainability for coal industry and the environment. Ultimately, this method is a reliable way to realize green and sustainable mining. The strata structure protection, the surface subsidence prevention, and coal mine solid waste disposal have been realized at the same time.

The first Italian blast-induced liquefaction test (Mirabello, Emilia-Romagna, Italy): description of the experiment and preliminary results

Soil liquefaction can result in significant settlement and reduction of load-bearing capacity. Moreover, the increase and the accumulation of pore pressure during an earthquake and its post-seismic dissipation can generate permanent deformations and settlements. The quantitative evaluation of post-liquefaction settlements is of extreme importance for engineering purposes, i.e. for earthquake-resistant design of new buildings and safety evaluation of existing ones. Quantifying the extent of these phenomena is, however, rather difficult. Uncertainties arise from the stochastic nature of the earthquake loading, from the simplifications of soil models, and from the difficulty in establishing correlations between the pre-earthquake soil state and the post-seismic deformations. Field scale liquefaction tests, under controlled conditions, are therefore important for a correct quantification of these phenomena. Recent experiences (e.g. New Zealand, United States) show that liquefaction can be induced and monitored with field scale blast tests to study the related effects on soil geotechnical properties. Within this framework this paper introduces the preliminary results obtained from a research project on blast-induced liquefaction at the field scale; tests were performed at a trial site located in Mirabello (Ferrara, Italy), a village strongly affected by liquefaction phenomena during the 2012 Emilia Romagna earthquake. Invasive tests, such as piezocone, seismic dilatometer and down-hole tests, and non-invasive tests were carried out before and after the execution of two blast test sequences to study the variation in physical properties of the soils. Pore pressure transducers, settlement profilometers, accelerometers and an instrumented micropile were installed with the objective of measuring, during and after the detonations, the generation and subsequent dissipation of the pore pressure, the vertical deformations, and the blast-induced ground motions respectively. Variations in load distribution on deep foundations due to soil liquefaction were also evaluated on a test micropile instrumented with a strain gauge chain. Topographical surveys were carried out to measure ground surface settlements. Laboratory tests and trenches also provided increase understanding of the site characteristics.