Prediction of swelling rocks strain in tunneling

Swelling deformations leading to convergence of tunnels may result in significant difficulties during the construction, in particular for long term use of tunnels. By extracting an experimental based explicit analytical solution for formulating swelling strains as a function of time and stress, swelling strains are predicted from the beginning of excavation and during the service life of tunnel. Results obtained from the analytical model show a proper agreement with experimental results. This closed-form solution has been implemented within a numerical program using the finite element method for predicting time-dependent swelling strain around tunnels. Evaluating effects of swelling parameters on time-dependent strains and tunnel shape on swelling behavior around the tunnel according to this analytical solution is considered. The ground-support interaction and consequent swelling effect on the induced forces in tunnel lining is considered too. Effect of delay in lining installation on swelling pressure which acting on the lining and its structural integrity, is also evaluated. A MATLAB code of “SRAP” is prepared and applied to calculate all swelling analysis around tunnels based on analytical solution.     

盾构隧道注浆对既有隧道影响的离心模拟研究

随着地铁网络不断完善,越来越多的新建盾构隧道近距离穿越既有隧道,然而对于盾构隧道近距离穿越既有隧道影响的研究尚不够完善。以上海典型软弱地层为背景,通过离心模型试验,研究了不同注浆率下的盾构上穿越施工对既有隧道以及周围地层的影响。选用排液法在离心场中模拟盾构施工,在不停机状态下成功模拟隧道开挖卸载、地层损失和注浆效应。分析了在不同的注浆率条件下,既有隧道在上穿越施工期和工后长期的位移、周围孔压和纵向应力的变化规律。试验结果表明,新建隧道近距离上穿越既有隧道时,隧道开挖的卸载效应等会导致既有隧道的隆起,但随着注浆率增大,既有隧道的隆起量减小。但过高注浆率对周围土体扰动较大,从而导致工后既有隧道的沉降也越大。     

A logistic regression classifier for long-term probabilistic prediction of rock burst hazard

Rock burst is a complex dynamic process can lead to casualties, to failure and deformation of the supporting structures, and to damage of the equipment on site; hence, its prediction is of great importance in underground construction. We present a novel empirical method to predict rock burst based on the theory of logistic regression classifiers. An extensive database collected from the literature, which includes observations about rock burst occurrence (or not) in underground excavations in projects from all over the world, is used to train and validate the model. The proposed approach allows us to compute new class separation lines (or planes) to estimate the probability of rock burst, using different combinations of five possible input parameters—tunnel depth, H; maximum tangential stress, MTS; elastic energy index, W _et; uniaxial compressive strength of rock, UCS; uniaxial tensile strength of rock, UTS—among which it was found that the preferable model could be developed in H–W _et–UCS space. The proposed model is validated with goodness-of-fit tests and nine-fold cross-validation; results show that its predictive capability compares well with previously proposed empirical methods and confirm that, as expected, the probability of rock burst increases with excavation depth, and that both W _et and UCS have a similarly significant influence on rock burst occurrence. Finally, expressions are proposed for identification of conditions associated with several reference values of rock burst probability, which can be employed in preliminary risk analyses.     

Simplified method for calculating inflow into a deep excavation with consideration of the effects of cutoff walls

Water inflow into deep excavations is a key parameter in the evaluation of environmental impact, and a simplified method is needed to calculate the inflow with adequate accuracy, especially for a deep excavation with a cutoff wall around it. This paper presents a model for calculating inflow based on the method of fragments, assuming two-dimensional sectional flow. The effects of cutoff wall thickness (w), wall penetration, and excavation width on the inflow are well considered in this model. Explicit formulas for the form factor are given. The accuracy and scope of this method are discussed. The method can be applied in both flooding and no flooding cases. Extensive verification shows that, for the flooding case and when one of four conditions is satisfied, the error of the proposed method will be within 10%. These conditions are b/d?≥?0.8 or w/d?≥?0.03 or s/d?≥?0.2 or s₁/d?≥?0.2, for which b is the half width of the excavation, d is the thickness of the soil layer underlying the wall, and s and s₁ are the wall penetration depths on the non-excavation side and excavation side, respectively. For the case of no flooding and when the ratio of the distance from the recharge boundary to the aquifer thickness (a/T) is?≥2, the error does not exceed 15%.     

Assessing the vegetation canopy influences on wind flow using wind tunnel experiments with artificial plants

Wind erosion causes serious problems and considerable threat in most regions of the world. Vegetation on the ground has an important role in controlling wind erosion by covering soil surface and absorbing wind momentum. A set of wind tunnel experiments was performed to quantitatively examine the effect of canopy structure on wind movement. Artificial plastic vegetations with different porosity and canopy shape were introduced as the model canopy. Normalized roughness length (Z ₀/H) and shear velocity ratio (R) were analyzed as a function of roughness density (λ). Experiments showed that Z ₀/H increases and R decreases as λ reaches a maximum value, λ _max, while the values of Z ₀/H and R showed little change with λ value beyond as λ _max.     

Orthogonal test and regression analysis of the strain on silty soil in Shanghai under metro loading

Cyclic triaxial test by means of the geotechnical digital system is conducted for the soil near the Guoquan Road Station of Metro Line 10 in Shanghai to analyze the strain characteristics and the variation law of saturated silty soil under subway loading. Orthogonal design method is used to arrange the experiment, considering the following factors: frequency ratio f _R, cyclic stress ratio σ _R, vibration time ratio N _R, and the interaction function among them. Results show that the cyclic stress ratio σ _R, the frequency ratio f _R, the vibration time ratio N _R, and the interaction between the cyclic stress ratio σ _R and the vibration time ratio N _R have a significant effect on the axial strain of the subway tunnel. The effect of the interaction between the cyclic stress ratio σ _R and the vibration time ratio N _R is also significant. From the analysis of variance and regression theory, the nonlinear regression equation of the cumulative plastic strain of silty soil under subway loading is established. Residual analysis proves that the equation is ideal and credible. The results have important value for the design of subway tunnels.     

Effect of Slope on <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">y</Emphasis> Curves for Laterally Loaded Piles in Soft Clay

Pile foundations are often subject to lateral loading due to various forces on a variety of structures like high rise buildings, transmission towers, power stations, offshore structures and highway and railway structures. The present investigation is to study the effect of slopes on p-y curves (where p is the static soil reaction and y is the pile deflection) due to static lateral loading in soft clay (Consistency index I_c = 0.42). A series of laboratory model tests were carried out on the instrumented model pile on sloping ground (slopes of 1V:1H, 1V:1.5H, 1V:2H, 1V:3H and 1V:5H) and with varying embedment length to diameter ratio (L/D) of 20, 25 and 30. From the experimental results, the bending moment curves along the pile shaft are double differentiated to obtain the soil resistance (p) and double integrated to obtain the deflection (y) using curve fitting method. New p-y curves for piles located on crest of soft clay with different sloping ground surface under static lateral loading are developed. Moreover, the effect of sloping angles on proposed p-y curves was studied.     

Numerical Analysis of Sill and Crown Pillar Stability for Multilevel Cut and Fill Stopes in Different Geomining Conditions

A steeply dipping orebody, having decreasing width with depth has been modeled considering horizontal cut and fill method of stoping at four different depth levels. The focus of the study is to identify and understand the behavior of crown and sill pillars in terms of varying stress and geo-mining conditions without reinforcement using finite element method. Analysis of stresses, displacements and extent of yield zones around the excavation is carried out by varying the rock mass conditions such as geological strength index, uniaxial compressive strength (UCS or σ _ci), modulus of elasticity (E), and thickness of crown and sill pillars (T). These analyses have been conducted based on 135 non-linear numerical models considering Drucker–Prager material model in plane strain condition. Results of the study provide valuable insight into the stress concentration factors of the pillars highlighting stress distributions, roof convergence, yield zones and support requirements. Finally, it suggests the optimum thickness of crown and sill pillar with varying thickness of orebody.     

Development of Analytical Models to Estimate Pile Setup in Cohesive Soils Based on FE Numerical Analyses

This paper presents the numerical simulation of pile installation and the subsequent increase in the pile capacity over time (or setup) after installation that was performed using the finite element software Abaqus. In the first part, pile installation and the following load tests were simulated numerically using the volumetric cavity expansion concept. The anisotropic modified Cam-Clay and Dracker–Prager models were adopted in the FE model to describe the behavior of the clayey and sandy soils, respectively. The proposed FE model proposed was successfully validated through simulating two full-scale instrumented driven pile case studies. In the second part, over 100 different actual properties of individual soil layers distracted from literature were used in the finite element analysis to conduct parametric study and to evaluate the effect of different soil properties on the pile setup behavior. The setup factor A was targeted here to describe the pile setup as a function of time after the end of driving. The selected soil properties in this study to evaluate the setup factor A include: soil plasticity index (PI), undrained shear strength (S _u ), vertical coefficient of consolidation (C _v ), sensitivity ratio (S _r ), and over-consolidation ratio (OCR). The predicted setup factor showed direct proportion with the PI and S _r and inverse relation with S _u , C _v and OCR. These soil properties were selected as independent variables, and nonlinear multivariable regression analysis was performed using Gauss–Newton algorithm to develop appropriate regression models for A. Best models were selected among all based on level of errors of prediction, which were validated with additional nineteen different site information available in the literature. The results indicated that the developed model is able to predict the setup behavior for individual cohesive soil layers, especially for values of setup factor greater than 0.10, which is the most expectable case in nature.     

Community vulnerability to hazards: introducing local expert knowledge into the equation

The accuracies of three different evolutionary artificial neural network (ANN) approaches, ANN with genetic algorithm (ANN-GA), ANN with particle swarm optimization (ANN-PSO) and ANN with imperialist competitive algorithm (ANN-ICA), were compared in estimating groundwater levels (GWL) based on precipitation, evaporation and previous GWL data. The input combinations determined using auto-, partial auto- and cross-correlation analyses and tried for each model are: (i) GWL _t?1 and GWL _t?2; (ii) GWL _t?1, GWL _t?2 and P _t ; (iii) GWL _t?1, GWL _t?2 and E _t ; (iv) GWL _t?1, GWL _t?2, P _t and E _t ; (v) GWL _t?1, GWL _t?2 and P _t?1 where GWL _t , P _t and E _t indicate the GWL, precipitation and evaporation at time t, individually. The optimal ANN-GA, ANN-PSO and ANN-ICA models were obtained by trying various control parameters. The best accuracies of the ANN-GA, ANN-PSO and ANN-ICA models were obtained from input combination (i). The mean square error accuracies of the ANN-GA and ANN-ICA models were increased by 165 and 124% using ANN-PSO model. The results indicated that the ANN-PSO model performed better than the other models in modeling monthly groundwater levels.     

Fracture evolution and energy mechanism of deep-buried carbonaceous slate

In order to study the influence of confining pressure and water content on the mechanical properties, fracture evolution and energy damage mechanism of deep-buried carbonaceous slate, uniaxial and triaxial compression tests were carried out under natural and saturated states and acoustic emission monitored. The deep-buried carbonaceous slate samples were obtained at a depth of 1020 m from the Lanjiayan tunnel in Sichuan province, China, where the maximum in situ stress has been measured at 44.2 MPa. The results suggest that water has a significant softening effect on the strength and deformation characteristics of carbonaceous slate, but the effect decreases with an increase in the confining pressure. When both the confining pressure and water content are increased, the acoustic emission events and dissipated energy gradually increase at the pre-peak and post-peak stages. Thus, the AE evolution type seen in the natural state under low confining pressure usually presents as a main shock-type event, and it changes to a foreshock–main shock–after shock event when saturated and at high confining pressures. Based on the S-shaped energy evolution law, the damage evolution process of carbonaceous slate was analyzed. The damage stress thresholds σ _ea and σ _eb were obtained, which can be considered as the thresholds of the rock entering the energy-hardening and energy-softening stages. Finally, a new brittleness energy index BDE is proposed to describe the influence of confining pressure and water content on the damage mechanism of deep-buried carbonaceous slate.     

Shear wave velocity as function of cone penetration resistance and grain size for Holocene-age uncemented soils: a new perspective

For feasibility studies and preliminary design estimates, field measurements of shear wave velocity, V _s, may not be economically adequate and empirical correlations between V _s and more available penetration measurements such as cone penetration test, CPT, data turn out to be potentially valuable at least for initial evaluation of the small-strain stiffness of soils. These types of correlations between geophysical (Vs) and geotechnical (N-SPT, q _c-CPT) measurements are also of utmost importance where a great precision in the calculation of the deposit response is required such as in liquefaction evaluation or earthquake ground response analyses. In this study, the stress-normalized shear wave velocity V _s1 (in m/s) is defined as statistical functions of the normalized dimensionless resistance, Q _tn-CPT, and the mean effective diameter, D ₅₀ (in mm), using a data set of different uncemented soils of Holocene age accumulated at various sites in North America, Europe, and Asia. The V _s1–Q _tn data exhibit different trends with respect to grain sizes. For soils with mean grain size (D ₅₀) < 0.2 mm, the V _s1/Q _tn ^0.25 ratio undergoes a significant reduction with the increase in D ₅₀ of the soil. This trend is completely reversed with further increase in D ₅₀ (D ₅₀ > 0.2 mm). These results corroborate earlier results that stressed the use of different CPT-based correlations with different soil types, and those emphasized the need to impose particle-size limits on the validity of the majority of available correlations.     

Determination of ground thermal properties for energy piles by thermal response tests

Thermal properties of ground heat exchanger (GHE) such as effective thermal conductivity and borehole thermal resistance are commonly measured in the field by thermal response tests (TRTs). TRT has been proved to be a consolidated method to determine thermal properties of traditional borehole heat exchangers (BHEs). However, there is still lack of data for adopting TRT on energy piles with often a large diameter and deficiency in validation of TRT results with geological materials. In this study, ground thermal properties for typical configured GHEs of energy piles are investigated. Three TRTs are conducted and the obtained results are analyzed. Effective thermal conductivity, λ_eff, of the ground derived by following the traditional linear source model shows large deviation as compared to the thermal conductivity of the geological materials. In order to determine λ_eff properly, the linear source model is modified and an equivalent radius, r_eq, of energy piles is considered. The λ_eff estimated by the modified model shows a good agreement with thermal conductivity of the in situ geological materials. In addition, there has been no obvious correlation between borehole thermal resistances and thermal efficiency due to heat transport of energy piles that depends not only by borehole thermal resistance but also by the pile’s diameter and ground conditions. The findings drawn from this study indicate that the modified model is reasonable and useful in determining thermal properties of energy piles.     

Mechanized tunneling induced ground movement and its dependency on the tunnel volume loss and soil properties

This study investigates the ground movements due to mechanized tunnel excavation by applying two-dimensional finite element analyses. To assess the contribution of the compressibility and plasticity of the soil on the ground movements, different constitutive models are employed to describe the soil behavior. The influence of volume loss around the tunnel on the surface volume loss is investigated, and a quadratic correlation between them is proposed. Consequently, the empirical Gaussian distribution curve, which is generally used to determine the tunneling induced settlement trough, is improved by applying the proposed quadratic correlation between surface volume loss and tunnel volume loss. Furthermore, the settlement trough width parameter has been derived by a linear function of tunnel volume loss as well. The proposed equations are validated via a case study of centrifuge tests from the literature. The results show that the proposed modification enhances the empirical solution by having better knowledge on the model parameters. Additionally, tunnel overburden and coefficient of lateral earth pressure at rest (K₀) are taken into account to study their influence on the tunneling induced surface settlements. Finally, global sensitivity analysis is applied to evaluate the relative importance of corresponding model parameters in terms of their influence on the ground movements.     

Influences of loading direction and intermediate principal stress ratio on the initiation of strain localization in cross-anisotropic sand

Since cross-anisotropic sand behaves differently when the loading direction or the stress state changes, the influences of the loading direction and the intermediate principal stress ratio (b = (σ ₂ ? σ ₃)/(σ ₁ ? σ ₃)) on the initiation of strain localization need study. According to the loading angle (angle between the major principal stress direction and the normal of bedding plane), a 3D non-coaxial non-associated elasto-plasticity hardening model was proposed by modifying Lode angle formulation of the Mohr–Coulomb yield function and the stress–dilatancy function. By using bifurcation analysis, the model was used to predict the initiation of strain localization under plane strain and true triaxial conditions. The predictions of the plane strain tests show that the major principal strain at the bifurcation points increases with the loading angle, while the stress ratio decreases with the loading angle. According to the loading angle and the intermediate principal stress ratio, the true triaxial tests were analyzed in three sectors. The stress–strain behavior and the volumetric strain in each sector can be well captured by the proposed model. Strain localization occurs in most b value conditions in all three sectors except for those which are close to triaxial compression condition (b = 0). The difference between the peak shear strength corresponding to the strain localization and the ultimate shear strength corresponding to plastic limit becomes obvious when the b value is near 0.4. The influence of bifurcation on the shear strength becomes weak when the loading direction changes from perpendicular to the bedding plane to parallel. The bifurcation analysis based on the proposed model gives out major principal strain and peak shear strength at the initiation of strain localization; the given results are consistent with experiments.     

Tailings composition effects on shear strength behavior of co-mixed mine waste rock and tailings

The objective of this study was to evaluate the effect of mine tailings composition on shear behavior and shear strength of co-mixed mine waste rock and tailings (WR&T). Crushed gravel was used as a synthetic waste rock and mixed with four types of tailings: (1) fine-grained garnet, (2) coarse-grained garnet, (3) copper, and (4) soda ash. Co-mixed WR&T specimens were prepared to target mixture ratios of mass of waste rock to mass of tailings (R) such that tailings “just filled” interparticle void space of the waste rock (i.e., optimum mixture ratio, R _opt). Triaxial compression tests were conducted on waste rock, tailings, and mixed waste at effective confining stresses (\(\sigma_{\text{c}}^{{\prime }}\)) ranging from 5 to 40 kPa to represent stresses anticipated in final earthen covers for waste containment facilities. Waste rock and co-mixed WR&T specimens were 150 mm in diameter by 300 mm tall, whereas tailings specimens were 38 mm in diameter by 76 mm tall. Shear strength was quantified using effective stress friction angles (?′) from undrained tests: ?′ for waste rock was 37°, ?′ for tailings ranged from 34° to 41°, and ?′ for WR&T mixtures ranged from 38° to 40°. Thus, shear strength of co-mixed WR&T was comparable to waste rock regardless of tailings composition. Shear behavior of WR&T mixtures was a function of R and tailings composition. Tailings influenced shear behavior for R < R _opt and when tailings predominantly were silt. Shear behavior was influenced by waste rock for R ≥ R _opt and when tailings predominantly were sand or included clay particles.     

DEM simulations of sandstone under true triaxial compressive tests

Numerically simulated true triaxial compression tests (σ ₁ ≥ σ ₂ ≥ σ ₃) are conducted in this study to elucidate the failure mechanism of sandstone using 3D discrete element method (DEM), in particular the effect of the intermediate principal stress (σ ₂). Eight series of tests (σ ₃ = 0, 10, 20, 30, 40, 50, 70, and 100 MPa) are conducted. Within each series, σ ₂ is varied from σ ₂ = σ ₃ to σ ₂ = σ ₁ from test to test. For each test, σ ₁ is raised monotonically to failure while keeping σ ₂ and σ ₃ constant. The DEM simulations reveal the effect of σ ₂ on the variations of peak stress, Young’s modulus, failure plane angles, the brittle–ductile transition, and the evolution of failure modes, the effect beyond the well-understood effect of σ ₃. The simulation is in qualitative agreement with the results obtained experimentally. Detailed analyses performed on the particle-scale responses further the understanding of the microscopic mechanisms. The distribution of contact force becomes more homogeneous with the increase of σ ₃, which leads to the resulting damage being more localized rather than diffused. The interaction between contact force distribution and coalescence of cracks determines the processes and patterns of fracturing in the sample scale. σ ₂ is found to affect the microscopic stress distribution as well as structure evolution, and this effect weakens with the increase of σ ₃.     

Upscaling of transmissivity,derived from specific capacity: a hydrogeomorphological approach applied to the Doon Valley aquifer system in India

Transmissivity (T) is one of the most important parameters in groundwater studies, and is generally estimated from pumping tests. T can also be deduced from abundantly available specific-capacity (Q/s) data by using analytical and/or empirical approaches, further upscaled by geostatistical methods. A different, remote sensing based, hydrogeomorphological approach is proposed, to upscale T from point- or well-scale to aquifer-scale, and it is applied to the piedmont alluvial aquifer system of Doon Valley in India. In the first step, Q/s and T data-pairs available from aquifer tests were used to establish an empirical, logarithmic relation. Subsequently, satellite imagery along with available data from published and unpublished maps, literature sources and ground surveys were used to divide the aquifer system into major hydrogeomorphological domains that control the groundwater occurrence and flow. Then, the T data derived from Q/s (using the empirical relation) and that available from pumping tests at well-scale were upscaled to aquifer-scale by averaging the T values within each hydrogeomorphological domain. Such a stratification approach is especially useful in areas where availability of only a few data-pairs of known Q/s and T limit the use of geostatistical techniques. A comparative study of the published empirical relations between Q/s and T in various hydrogeologic settings revealed that the relation obtained for Doon Valley aquifer system is close to that found for a similar alluvial aquifer system in Morocco.     

Prediction of strength parameters of sedimentary rocks using artificial neural networks and regression analysis

Accurate laboratory measurement of geo-engineering properties of intact rock including uniaxial compressive strength (UCS) and modulus of elasticity (E) involves high costs and a substantial amount of time. For this reason, it is of great necessity to develop some relationships and models for estimating these parameters in rock engineering. The present study was conducted to forecast UCS and E in the sedimentary rocks using artificial neural networks (ANNs) and multivariable regression analysis (MLR). For this purpose, a total of 196 rock samples from four rock types (i.e., sandstone, conglomerate, limestone, and marl) were cored and subjected to comprehensive laboratory tests. To develop the predictive models, physical properties of studied rocks such as P wave velocity (V_p), dry density (γ_d), porosity, and water absorption (A_b) were considered as model inputs, while UCS and E were the output parameters. We evaluated the performance of MLR and ANN models by calculating correlation coefficient (R), mean absolute error (MAE), and root-mean-square error (RMSE) indices. The comparison of the obtained results revealed that ANN outperforms MLR when predicting the UCS and E.     

Effect of 2-D Random Field Discretization on Failure Probability and Failure Mechanism in Probabilistic Slope Stability

This paper investigates, using the random field theory and Monte Carlo simulation, the effects of random field discretization on failure probability, p _f, and failure mechanism of cohesive soil slope stability. The spatial sizes of the discretized elements in random field Δx, Δy in horizontal and vertical directions, respectively, are assigned a series of combinational values in order to model the discretization accuracy. The p _f of deterministic critical slip surface (DCSS) and that of the slope system both are analyzed. The numerical simulation results have demonstrated that both the ratios of Δy/λ _y (λ _y  = scale of fluctuation in vertical direction) and Δx/λ _x (λ _x  = scale of fluctuation in horizontal direction) contribute in a similar manner to the accuracy of p _f of DCSS. The effect of random field discretization on the p _f can be negligible if both the ratios of Δx/λ _x and Δy/λ _y are no greater than 0.1. The normalized discrepancy tends to increase at a linear rate with Δy/λ _y when Δx/λ _x is larger than 0.1, and vice versa for p _f of DCSS. The random field discretization tends to have more considerable influence on the p _f of DCSS than on that of the slope system. The variation of p _f versus λ _x and λ _y may exhibit opposite trends for the cases where the limit state functions of slope failure are defined on DCSS and on the slope system as well. Finally, the p _f of slope system converges in a more rapid manner to that of DCSS than the failure mechanism does to DCSS as the spatial variability of soil property grows from significant to negligible.