Shallow Tunneling Method and Control Measure for Ground Surface Settlement

In order to ensure that the tunnel deformation and surface settlement are controlled within the allowable range during the construction process,the design unit has compiled technical measures and monitoring schemes for ground settlement control of this project.Based on the example of a shallow tunneling project on Subway line 8,this paper analyzes and discusses the shallow tunneling method in detail and puts forward corresponding technical measures for ground settlement control.

The Simulation of Grinding Wheels and Ground Surface Roughness Based on Virtual Reality Technology

The paper describes the feasibility and method of the application of virtual reality technology to grinding process, and introduces the modeling method of object entity in the environment of virtual reality. The simulation process of grinding wheels and ground surface roughness is discussed, and the computation program system of numerical simulation is compiled with Visual C++ programming language. At the same time, the three-dimensional simulation models of grinding wheels and ground surface roughness are made with OpenGL tool. The choice of grinding wheels, the forecast of ground surface quality and some simulation results can be realized by interactively inputting grinding parameters. The paper applies virtual reality technology to grinding process,makes the model of virtual grinding wheel and simulates the grinding process. The roughness of ground surface is showed in three-dimensional images, and therefore the grinding technology is studied. Computer simulation can not only be used as a shortcut to analyze and research the grinding process, but also increase the research scope and content. The virtual reality technology used in the paper is an advanced visualized simulation with interaction. The surface roughness Ra on simulated ground workpiece can be calculated by the arithmetic average of contour warp absolute value in sampling length of simulated ground workpiece. The parameters of virtual wheel and simulated grinding process can be changed by interaction input, so the simulated results in the desired grinding condition are gained. The effect of each parameter to ground surface can be analyzed by comparing the grinding results in different condition.

Tunnel flexibility effect on the ground surface acceleration response

Flexibility of underground structures relative to the surrounding medium, referred to as the flexibility ratio, is an important factor that influences their dynamic interaction. This study investigates the flexibility effect of a box-shaped subway tunnel, resting directly on bedrock, on the ground surface acceleration response using a numerical model verified against dynamic centrifuge test results. A comparison of the ground surface acceleration response for tunnel models with different flexibility ratios revealed that the tunnels with different flexibility ratios influence the acceleration response at the ground surface in different ways. Tunnels with lower flexibility ratios have higher acceleration responses at short periods, whereas tunnels with higher flexibility ratios have higher acceleration responses at longer periods. The effect of the flexibility ratio on ground surface acceleration is more prominent in the high range of frequencies. Furthermore, as the flexibility ratio of the tunnel system increases, the acceleration response moves away from the free field response and shifts towards the longer periods. Therefore, the flexibility ratio of the underground tunnels influences the peak ground acceleration （PGA） at the ground surface, and may need to be considered in the seismic zonation of urban areas.

Analysis of ground surface settlement in anisotropic clays using extreme gradient boosting and random forest regression models

Excessive ground surface settlement induced by pit excavation(i.e.braced excavation) can potentially result in damage to the nearby buildings and facilities.In this paper,extensive finite element analyses have been carried out to evaluate the effects of various structural,soil and geometric properties on the maximum ground surface settlement induced by braced excavation in anisotropic clays.The anisotropic soil properties considered include the plane strain shear strength ratio(i.e.the ratio of the passive undrained shear strength to the active one) and the unloading shear modulus ratio.Other parameters considered include the support system stiffness,the excavation width to excavation depth ratio,and the wall penetration depth to excavation depth ratio.Subsequently,the maximum ground surface settlement of a total of 1479 hypothetical cases were analyzed by various machine learning algorithms including the ensemble learning methods(extreme gradient boosting(XGBoost) and random forest regression(RFR)algorithms).The prediction models developed by the XGBoost and RFR are compared with that of two conventional regression methods,and the predictive accuracy of these models are assessed.This study aims to highlight the technical feasibility and applicability of advanced ensemble learning methods in geotechnical engineering practice.

The Sensitivity of Ground Surface Temperature Prediction to Soil Thermal Properties Using the Simple Biosphere Model(SiB2)

Using the Simple Biosphere Model （SiB2）, soil thermal properties （STP） were examined in a Tibetan prairie during the monsoon period to investigate ground surface temperature prediction. We improved the SiB2 model by incorporating a revised force-restore method （FRM） to take the vertical heterogeneity of soil thermal diffusivity （k） into account. The results indicate that （1） the revised FRM alleviates daytime overestimation and nighttime underestimation in modeled ground surface temperature （Tg）, and （2） its role in little rainfall events is significant because the vertical gradient of k increases with increasing surface evaporation. Since the original formula of thermal conductivity （A） in the SiB2 greatly underestimates soil thermal conductivity, we compared five Mgorithms of A involving soil moisture to investigate the cause of overestimation during the day and underestimation at night on the basis of the revised FRM. The results show that （1） the five algorithms significantly improve Tg prediction, especially in daytime, and （2） taking one of these five algorithms as an example, the simulated Tg values in the daytime are closer to the field measurements than those in the nighttime. The differences between modeled Tg and field measurements are mostly within the margin of error of -4-2 K during 3 August to 4 September 1998.

Effect of geofoam as cover material in cut and cover tunnels on the seismic response of ground surface

In recent years,special attention has been given to the effects of underground spaces and structures on the seismic response of adjacent ground.Nevertheless,to the best of the authors′knowledge,no method has yet been considered to reduce these effects in technical literature.The present study aims to investigate the role of geofoam as the cover material in cut and cover tunnels on the seismic response of the ground surface.For this purpose,a numerical study was performed using FLAC 2D,a finite difference software,and verified against previous studies.The effects of parameters such as the geofoam type,thickness of the geofoam blocks and interfaces between the blocks,on the ground surface amplification pattern were investigated.Parametric studies demonstrate that the geofoam has a strong potential to attenuate the seismic horizontal movements of the ground surface.As the thickness of the geofoam blocks is decreased,its attenuation ability is increased.It was also determined that the interface between the geofoam blocks plays a key role in the attenuation of the aboveground seismic responses.Based on the results,it was concluded that geofoam is a proper material to attenuate seismic amplifications at the ground surface,induced by underground tunnels.

Ground surface response induced by shallow buried explosions

Based on the spherical cavity expansion theory in the elastic half space,the ground surface movement characteristics of shallowly buried explosions are analyzed.The results show that the induced seismic wave is a longitudinal wave in the near zone and a Rayleigh wave in the far zone.The maximum displacement(velocity) of the longitudinal wave and the Rayleigh wave are inversely proportional to the scaled distance,and can be described by exponential function with exponents equal to 1.4 and 0.5,respectively.The vibration frequencies of the waves have almost no change.The vibration frequency of the longitudinal wave approximates the natural vibration frequency of the cavity in the broken area,and the vibration frequency of the Rayleigh wave is about half that of the longitudinal wave.On the same reduced buried depth and reduced distance,the particle displacement is directly proportional to the product of the boundary loading and cavity radius,and is inversely proportional to the transversal wave velocity.Meanwhile,the particle velocity is directly proportional to the boundary loading and inversely proportional to the wave velocity ratio.In the far zone,the buried depth of the explosive only has a slight effect on the longitudinal wave,but has a larger effect on the Rayleigh wave.

Terrestrial water storage variation in Hebei plain area of China,based on ground surface gravimetry

Variation of terrestrial water storage in the Hebei plain area from March 2010 to June 2014 was studied using ground gravimetry combined with vertical displacement data from the Global Navigation Satellite System.Results show that observed gravity variation in this area increased continuously,basically reflecting a trend toward land subsidence.With the effect of this subsidence removed,a dominantnegative change in gravity variation was evident,reflecting an average rate of decrease in terrestrial water in this area of 0.10±0.053 m/y,and this is equivalent to a volume of 81.5±43.2×108 m^(3)and is consistent with the spatial distribution of groundwater change from measured hydrologic data.These results can be an essential reference and supplement for the study of terrestrial water variation in the Hebei plain area,and indicate that ground surface gravimetry can be used as an important mean for studying changes in terrestrial water.

Recent changes in ground surface thermal regimes in the context of air temperature warming over the Heihe River Basin, China

Changes in ground surface thermal regimes play a vital role in surface and subsurface hydrology, ecosystem diversity and productivity, and global thermal, water and carbon budgets as well as climate change. Estimating spring, summer, autumn and winter air temperatures and mean annual air temperature（MAAT） from 1960 through 2008 over the Heihe River Basin reveals a statistically significant trend of 0.31 °C/decade, 0.28 °C/decade, 0.37 °C/decade, 0.50 °C/decade, and 0.37 °C /decade, respectively. The averaged time series of mean annual ground surface temperature（MAGST） and maximum annual ground surface temperature（MaxAGST） for 1972–2006 over the basin indicates a statistically significant trend of 0.58 °C/decade and 1.27 °C/decade, respectively. The minimum annual ground surface temperature（MinAGST） in the same period remains unchanged as a whole. Estimating surface freezing/thawing index as well as the ratio of freezing index to thawing index（RFT） in the period between 1959 and 2006 over the basin indicates a statistically significant trend of-42.5 °C-day/decade, 85.4 °C-day/decade and-0.018/decade, respectively.

Asymmetric Rock Pressure on Shallow Tunnel in Strata with Inclined Ground Surface

By building a tunnel model with a semi-circular crown, the asymmetric rock pressure applied to the shallow tunnel in strata with inclined ground surface is analyzed. Formulae, which not only include the parameters .related to both tunnel structure and surrounding rock mass, but the overburden depth, are developed. The computation for four tunnel models show that the method presented is feasible and convenient. Furthermore, the influence of the overburden depth on the rock pressure is elaborated, and the criterion to identify the deep or shallow tunnels is formulated as well.

Mechanical responses of multi-layered ground due to shallow tunneling with arbitrary ground surface load

An analytical model based on complex variable theory is proposed to investigate ground responses due to shallow tunneling in multi-layered ground with an arbitrary ground surface load.The ground layers are assumed to be linear-elastic with full-stick contact between them.To solve the proposed multi-boundary problem,a series of analytic functions is introduced to accurately express the stresses and displacements contributed by different boundaries.Based on the principle of linear-elastic superposition,the multi-boundary problem is converted into a superposition of multiple single-boundary problems.The conformal mappings of different boundaries are independent of each other,which allows the stress and displacement fields to be obtained by the sum of components from each boundary.The analytical results are validated based on numerical and in situ monitoring results.The present model is superior to the classical model for analyzing ground responses of shallow tunneling in multi-layered ground;thus,it can be used with assurance to estimate the ground movement and surface building safety of shallow tunnel constructions beneath surface buildings.Moreover,the solution for the ground stress distribution can be used to estimate the safety of a single-layer composite ground.

Changes in the ground surface temperature in permafrost regions along the Qinghaie-Tibet engineering corridor from 1900 to 2014:A modified assessment of CMIP6

Numerous studies were published in the last two decades to evaluate and project the permafrost changes in its thermal state,mainly based on the soil temperature datasets from the Coupled Model Intercomparison Project(CMIP),and discuss the impacts of permafrost changes on regional hydrological,ecological and climatic systems and even carbon cycles.However,limited monitored soil temperature data are available to validate the CMIP outputs,resulting in the over-projection of future permafrost changes in CMIP3 and CMIP5.Moreover,future permafrost changes in CMIP6,particularly over the QinghaieTibet Plateau(QTP),where permafrost covers more than 40%of its territory,are still unknown.To address this gap,we evaluated and calibrated the monthly ground surface temperature(GST;5 cm below the ground surface),which was often used as the upper boundary to simulate and project permafrost changes derived from 19 CMIP6 Earth System Models(ESMs)against in situ measurements over the QTP.We generated the monthly GST series from 1900 to 2014 for five sites based on the linear calibration models and validated them through the three other sites using the same calibration methods.Results showed that all of the ESMs could capture the dynamics of in situ GST with high correlations(r>0.90).However,large errors were detected with a broad range of centred root-mean-square errors(1.14-4.98℃).The Top 5 model ensembles(MME5)performed better than most individual ESMs and averaged multi-model ensembles(MME19).The calibrated GST performed better than the GST obtained from MME5.Both annual and seasonal GSTs exhibited warming trends with an average annual rate of 0.04℃ per decade in the annual GST.The average seasonal warming rate was highest in winter and spring and lowest in summer.This reconstructed GST data series could be used to simulate the long-term permafrost temperature over the QTP.

Predictions of ground surface settlement for shield tunnels in sandy cobble stratum based on stochastic medium theory and empirical formulas

This paper focuses on the prediction of ground surface settlement induced by shield tunnelling in sandy cobble stratum.Based on the stochastic medium theory,an analytical solution to predict the surface settlement is developed considering the difference between soil and tunnel volume loss.Then,the effects of tunnel geometries,influence angle and volume loss on the characteristics of surface settlement are discussed.Through back analysis,a total of 103 groups of field monitoring data of surface settlement induced by shield tunnelling in sandy cobble stratum are examined to investigate the statistical characteristics of the maximum settlement,settlement trough width and volume loss.An empirical prediction is presented based on the results of back analysis.Finally,the analytical solution and empirical expression are validated by the comparisons with the results of model tests and field monitoring.Results show that the soil at ground surface has an overall dilative response for most of the shield tunnelling in sandy cobble stratum.In addition,the developed analytical solution is applicable and reasonable for surface settlement prediction.Meanwhile,the proposed empirical formula also shows good per-formance in some cases,providing an approach or a reference for engineering designers to preliminarily evaluate the surface settlement.

Evaluation of underground blast-induced ground motions through nearsurface low-velocity geological layers

Surface ground motion produced by underground blasts is significantly influenced by near-surface geological conditions.However,near-surface low-propagation velocity layers were always ignored in past analyses of ground motions due to their thin thickness.With the rising concern about surface ground motions produced by the ascendant scale and frequentness of underground excavation and mining,close attention is gradually paid to ground blast vibrations.Therefore,systemic experiments were conducted and took seven months in an underground mine to clarify the variation of motion from underground rock to surface ground.The attenuation of surface ground peak particle velocities(PPVs)is compared to that in underground rock,and horizontal amplitudes are compared to vertical amplitudes.Differences between bedrock and surface ground vibrations are analyzed to illustrate the site effect of near-surface lower-propagation velocity layers.One-dimensional site response analysis is employed to quantify the influence of different geological profiles on surface ground vibrations.The experimental data and site response analysis allowed the following conclusions:(1)geological site effects mainly produce decreasing dominant frequency(DF)of surface ground vibrations;(2)the site amplification effect of blast vibration needs to be characterized by peak particle displacement(PPD);(3)shear waves(S-waves)begin to dominate and surface Rayleigh waves(R-waves)develop as blast-induced ground vibrations travel upward through rock and lower-velocity layers to the surface.The comparison of response relative displacement to a critical value is best to assess the potential for cracking on surface structures.

A novel ground surface subsidence prediction model for sub-critical mining in the geological condition of a thick alluvium layer

A substantial number of the coal mines in China are in the geological condition of thick alluvium layer. Under these circumstances, it does not make sense to predict ground surface subsidence and other deformations by using conventional prediction models. This paper presents a novel ground surface subsidence prediction model for sub-critical mining in the geological condition of thick alluvium layer. The geological composition and mechanical properties of thick alluvium is regarded as a random medium, as are the uniformly distributed loads on rock mass; however, the overburden of the rock mass in the bending zone is looked upon as a hard stratum controlling the ground surface subsidence. The different subsidence and displacement mechanisms for the rock mass and the thick alluvium layer are respectively considered and described in this model, which indicates satisfactory performances in a practical prediction case.

Shanghai center project excavation induced ground surface movements and deformations

Empirical data on deep urban excavations can provide designers a significant reference basis for assessing potential deformations of the deep excavations and their impact on adjacent structures. The construction of the Shanghai Center involved excavations in excess of 33-m-deep using the top-down method at a site underlain by thick deposits of marine soft clay. A retaining system was achieved by 50-m-deep diaphragm walls with six levels of struts. During construction, a comprehensive instrumentation program lasting 14 months was conducted to monitor the behaviors of this deep circular excavation. The following main items related to ground surface movements and deformations were collected： （1） walls and circumferential soils lateral movements; （2） peripheral soil deflection in layers and ground settlements; and （3） pit basal heave. The results from the field instrumentation showed that deflections of the site were strictly controlled and had no large movements that might lead to damage to the stability of the foundation pit. The field performance of another 21 cylindrical excavations in top-down method were collected to compare with this case through statistical analysis. In addition, numerical analyses were conducted to compare with the observed data. The extensively monitored data are characterized and analyzed in this paper.

An investigation of the influence of ground surface properties and shading on outdoor thermal comfort in a high-altitude residential area

Landscape elements in residential areas can effectively improve the outdoor thermal environment,with different outcomes depending on the climate conditions.This study explores how the ground surface and shading properties affect the outdoor thermal environment in a high-altitude plateau climate where few studies have been conducted.The measurements were conducted during summer and winter in a residential area in Lhasa,Tibet.Without natural shading such as trees,there is a positive correlation between Sky-view factor(SVF)and Physiological equivalent temperature(PET)during winter and a negative correlation during summer.When SVF exceeds 0.65 in summer,it may cause human discomfort.Compared to artificial shading such as a tensioned membrane,deciduous trees are superior at improving human comfort,as they can increase PET by 10.56℃ in winter and decrease it by 9.73℃ in summer.During summer,high-reflection water-permeable bricks can reduce the PET by 1.08℃,and lawns can reduce the mean rachation temperature(Tmrt)by 1.650C;however,the lawns may produce a microclimate with a high air temperature.The results from this paper can be used as a reference for landscape planning and design in residential areas in high-altitude cold-climate regions.

Effect of Wall Deformation on Dewatering-Induced Ground Surface Settlement

In practice,dewatering for pressure relief is commonly undertaken during ongoing excavation to secure bottom stability against basal upheaval.Simultaneously,through unloading,wall deflection is obviously observed.Noticing that both cause soil deformations,this research is to study the effect of wall deformation on dewateringinduced settlement.A coupled numerical analysis of finite-difference software is employed to model Shanghai soft soils under multi-aquifer-aquitard systems(MAASs)by analyzing the results in association with an empirical approach.Consequently,through gradual force reduction,shear strength at soil-wall interface is significantly diminished.As wall deformation increases instantaneously upon lower loading,wall surface becomes deformedly bending;this condition causes the challenge to workability of shear strength.Moreover,wall deformation caused by unloading affects dewatering-induced settlement substantially.Under smaller loading,large wall deflection is observed;soil plane of failure caused by both sliding and compression occurs along slip curve,with weaker shear-strength soils at rD=0.4 and stronger shear-strength soils between rD=0.4 and rD=0.65,where rD is the distance from the wall that is normalized by the depth measured from ground surface.During dewatering,stronger soils tend to drag weaker soils upward to reduce large differential settlements caused by additional compression.Consequently,settlement becomes larger at rD=0.4 and smaller at rD=0.65.Remarkably,at rD>2.3,both settlement curves that result from numerical analysis and empirical method show overlapping;this indicates that the unloading effect on dewatering-induced settlement at rD>2.3 is insignificant.Furthermore,as wall reaches maximum allowable wall deflection by 67%applied force,additional compression caused by dewatering after loading remains smaller than that under 70%applied force,contributing to smaller dewatering-induced settlement.

Bias Analysis and Correction of Ground Surface Temperature Observations across China

Based on the ground surface temperature(GST)and snow surface temperature(SST)measurements during the period of adjustment from manual to automatic observation systems in China,the influence of observation methods on GST and its relationship with snow cover is analyzed.GST is corrected by SST,and the correction effect is evaluated.The results show that,during the parallel observation period,the winter GSTs from automatic observations are generally higher than those from manual observations,with the automatically observed national daily GST 1.18°C higher.The adjustment has a greater impact on GSTs at 0200,0800,and 2000 Beijing Time(BT)than at 1400 BT,and it has the greatest impact in Northeast and Northwest China,with deviations of 5.24 and 2.09°C,respectively.The GST deviation is closely related to the snow depth and annual snow totals.The average daily GST deviation increases at the rate of 0.66°C per 1-cm increase of snow depth when it is<15 cm,while it tends to be stable at around 10°C for snow depth over 15 cm.The GST deviation at a station is affected by its winter snow totals in Northeast and Northwest China,where the largest deviations are found where snow totals are all above 1000 cm.After the correction with SST,the mean deviation between the automatic and manual observations as well as the false trend can be effectively reduced.Following the correction,the mean deviation of daily GST decreases by 5.8°C,and its trend decreases from 1.87 to 0.65°C decade-1.

Dynamic shear modulus of undisturbed soil under different consolidation ratios and its effects on surface ground motion

The dynamic shear modulus for three types of undisturbed soil under different consolidation ratios is presented by using the resonant column test method. Its effects on surface ground motion is illustrated by calculation. The test results indicate that the power function is a suitable form for describing the relationship between the ratio of the maximum dynamic shear modulus due to anisotropic and isotropic consolidations and the increment of the consolidation ratio. When compared to sand, the increment of the maximum dynamic shear modulus for undisturbed soil due to anisotropic consolidation is much larger. Using a one-dimensional equivalent linearization method, the earthquake influence factor and the characteristic period of the surface acceleration are calculated for two soil layers subjected to several typical earthquake waves. The calculated results show that the difference in nonlinear properties due to different consolidation ratios is generally not very notable, but the degree of its influence on the surface acceleration spectrum is remarkable for the occurrence of strong earthquakes. When compared to isotropic consolidation, the consideration of actual anisotropic consolidation causes the characteristic period to decrease and the earthquake influence factor to increase.