International Journal of Geomechanics

In this study, axisymmetric numerical modeling of continuous impact pile driving from the ground surface to the final depth of pile installation was successfully conducted using arbitrary Lagrangian-Eulerian (ALE) adaptive mesh method. To verify the generated ground vibrations remarkably better with field data compared to previous numerical models, the pile-soil interaction was significantly improved, and all prevalent simplified assumptions were ignored as well. Then, the variation of PPV, the maximum experienced velocity of a soil particle known as peak particle velocity, in different ground depths was meticulously investigated, and it was concluded that PPV can occur in ground depths rather than the ground surface after a threshold radial distance from the pile centerline. The PPV at radial distances more than 8 m and depth of 1 m was between 5 and 15% greater than the values recorded at the ground surface. Moreover, critical depth of vibration was investigated, and it was observed that penetration of a pile up to more than its half-length guarantees PPV occurrence at all radial distances. Finally, a series of extensive parametric studies were carried out in which hammer impact force, pile diameter, tip angle, and damping ratio of soil were concluded as the factors most affecting the ground vibrations induced by impact pile driving.

In the past, the pseudostatic method was the most common method used to evaluate stability of the slope under seismic loads. Through the method, static forces acting on the center of gravity of a sliding body are used to consider the effects of an earthquake. However, seismic loads have different geomaterials at different times and locations; therefore, it is not reasonable to apply the pseudostatic approach to solve dynamic problems. In this study, a pseudodynamic method was developed to compute the dynamic factor of safety of the seismic slope. A sinusoidal wave was applied to simulate the effects of an earthquake. The effects of physicomechanical behaviors of geomaterials on the seismic stability of slopes were taken into account as were inertia and damping forces. In addition, two classical examples were investigated to verify the robustness and precision of the proposed method. The results from the present method are in good agreement with those from studies published previously.

Instability of filling media in large-scale filled-type karst caves often induce mud inrush in tunnel construction. An innovative method is proposed for the integral sliding stability analysis of the filling media on the basis of the simplified Bishop method. A computational model and six assumptions were put forward to interpret the complex system of the tunnel, karst cave, and filled media. In addition, the stability coefficients of the filling media were obtained by solving equilibrium equations of force and moment. The method can be used for analyzing the influence of the hydraulic effect, shear strength parameters, and boundary constraints on the global stability of filling media in practical cases. The results agree well with numerical simulations. In addition, the assumption that the weakened contact interface was taken to be the critical failure surface was proven reasonable by the numerical simulations. The findings indicate that the analytical method is efficient and suitable for theoretical and engineering studies on integral sliding stability analysis of the filling media in karst caves.

Spherical element-based discontinuous deformation analysis (SDDA) usually uses simple types of contact such as sphere–sphere and sphere–plane contact for detection. However, in practical engineering simulations, there is also found sphere–edge contact in the context of interactions between spheres and fixed boundaries. This incompleteness of contact types in the original SDDA restricts its application to problems involving complex boundary shapes. To fill this gap, this paper presents a sphere–edge contact model to extend the application of SDDA to complex boundary shapes such as edges, corners, and finite and infinite planes. The contact formulation is deduced and the algorithm is implemented in SDDA source code developed by the authors. Four numerical examples are simulated for verification. The results show that the proposed model is correct and effective.

This paper presents a comprehensive framework to understand the uncertainties associated with the water retention characteristic curve (WRCC) of fly ash, which is necessary for studying the unsaturated behavior of the fly ash. The measuring devices, range of measured suction, and water content play important roles in inducing the uncertainties associated with WRCC. To account for these uncertainties, a univariate probabilistic modeling was first adopted. Measured suction and volumetric water content were modeled as univariate random variables, the parameters of which were determined using quantile-quantile plots alongside the estimations of the best-fit probability distribution. To handle a wide range of uncertainties associated with WRCC zones and their measurements, the measured data were partitioned according to (1) saturation, desaturation, and residual zones and (2) the measurement range of four instruments. The bivariate dependencies were incorporated using a copula-based modeling and simulation approach wherein the marginals were chosen from the results of the univariate modeling. Results show that the univariate modeling provided a good firsthand estimate of the uncertainties of WRCCs, and when integrated with bivariate modeling and simulation, can yield representative WRCCs under considerably restricted measurement options. The study demonstrates the usefulness of copula-based probabilistic modeling for determining a realistic WRCC of fly ash under limited measured data availability.

Soil arching is a phenomenon describing stress redistribution due to relative movement between adjoining portions. It commonly exists when soil interacts with structure elements, such as tunnels, retaining walls, buried structures, and piles in pile-supported embankments. A yielding or ultimate state of soil (i.e., the shear stress in soil is equal to its shear strength) is mostly assumed in analytical models for soil arching. In reality, the level of shear stress and the shape of the slip surface in the soil depend on the magnitude of relative movement of the soil. In this study, a custom-made trapdoor test box was used to investigate the progressive development of soil arching with the trapdoor displacement. The dimensions of the test box was 1.76 m long, 0.46 m wide, and 1.48 m high, whereas the trapdoor had dimensions of 0.36 m wide and 0.46 m long. Earth pressures were installed on the top and sides of the trapdoor to monitor the development of soil arching with the displacement of the trapdoor. Furthermore, data from five references available in the literature were adopted to evaluate the progressive development of soil arching with the displacement for different geotechnical applications. Terzaghi’s method was used to calculate the two-dimensional soil arching ratios (i.e., the measured earth pressure divided by the overburden pressure) in all cases in this study using three different values of lateral earth-pressure coefficient (K), and the results were compared with the measured results. Terzaghi’s method calculated the soil-arching ratios reasonably well when the trapdoor displacement was equal to or larger than 10% of the trapdoor width. To facilitate the application of progressive development of soil arching with displacement in engineering practice, a simplified ground reaction curve (GRC) was proposed as three straight lines to represent the progressive development of two-dimensional soil arching with relative displacement. The methods for determining four required parameters were provided, and the proposed method was verified using three trapdoor test results from both this study and the literature.

Classically, lateral earth-pressure, bearing-capacity, and slope-stability problems are considered separately, although these problems are all related to the ultimate limit state of a system. The authors see that these problems are governed by similar ultimate requirements and should be equivalent in terms of the failure mechanism, and these problems can be considered as a different view of the ultimate limit state of the system. It is interesting to note that, although there are many applications of the method of characteristics in various geotechnical problems, there is none that is devoted to the unification of these geotechnical problems. Bearing-capacity, slip-line, and slope-stability problems have been treated as individual topics in all previous works. In the present study, based on the method of characteristics, the authors determined the bearing capacity factors (N_c, N_q, and N_γ) for a shallow foundation and the lateral earth-pressure coefficients (K_ac, K_aq, and K_pγ) for a retaining wall. Through the method of characteristics, K_ac, K_aq, and K_pγ were demonstrated to be related to N_c, N_q, and N_γ, so these two problems can be unified, and each problem can be viewed as the inverse of the other problem. The authors also applied the extremum principle to demonstrate the equivalence between the lateral active-pressure, bearing-capacity, and slope-stability problems. Overall, these three problems can be considered as equivalent in the basic principles, and the classification of a problem is simply a matter of convenience.

Borehole stability during horizontal directional drilling is one of the most important issues affecting success of pipe installations using this technique under roads, railways, and other obstructions. Two failure mechanisms, shear failure (blow-out) and tensile fracture (hydrofracture), have previously been identified. Both failure mechanisms can be seen in saturated clay based on total stress analysis, and shear failure dominates in dry sand based on effective stress analysis. Numerical modeling was used to investigate these failure mechanisms, and parametric studies were implemented. It was found that changes of elastic moduli and dilation angles have a minor impact on the numerical results when linear elastic analysis up to the initiation of yielding is undertaken. It was also found that the numerical results at the crown (and invert) can deviate from the theoretical results if yielding occurs at the shoulder (and haunch) of horizontal boreholes. Linear regression analysis was used to approximate these deviations. It was concluded that the ratios of intercepts of linear regression lines and vertical stresses are related to the undrained shear strength of clay and the critical friction angle of sand. However, there are no obvious relationships between the slopes of the linear regression lines and other factors. The ranges for coefficients in the modified solution are given.

Determination of the geomechanical parameters and in situ stress of petroleum reservoir rocks by hydraulic fracturing is critical in performing fracturing operations and reservoir simulation in petroleum engineering. Many uncertainties are associated with the determination of petroleum geomechanical parameters and in situ stress because of the complexity and nonlinearity of hydraulic fracturing and the variation of rock properties. Inverse analysis is commonly used to determine petroleum geomechanical parameters, but conventional analysis does not allow the uncertainty to be considered. This study proposes a probabilistic inverse analysis method that integrates numerical simulation, Bayesian theory and a multi-output support vector machine (MSVM) to determine in situ stress, geomechanical parameters, and related uncertainties. Furthermore, this study tried to establish the relationships between those parameters and numerically simulated borehole pressure. The proposed method was verified by a numerical example in which the uncertainties in the values of the Young’s modulus, Poisson’s ratio, and in situ stress were modeled as random variables. Compared with the traditional inverse analysis methodology, the proposed method was found to improve the accuracy of maximum in situ stress to 45% and 5% with and without considering the poroelastic effects, respectively. Using the parameters determined by probabilistic inverse analysis, the simulated borehole pressure agreed closely with the measured pressure, indicating that the MSVM model reproduced the relationship between the geomechanical parameters and borehole pressure reasonably well. An analysis of the effect of the uncertainty of borehole pressure showed that estimates of the geomechanical parameters are greatly improved by decreasing the error in monitoring borehole pressures.

The most striking feature of liquid nitrogen (LN₂) fracturing is to drastically reduce the temperature around the rock when LN₂ is injected into the reservoir. The strong thermal gradient can significantly induce the internal thermal stress of the rock, which results in severe damage. In this study, the permeability, ultrasonic wave, and triaxial compression test were performed to investigate the influence of LN₂ treatment on pore structure and mechanical properties of stratified coal. In the pore structure tests, the increasing range of permeability of LN₂-treated samples is 100.3–149.6%, and the P-wave velocity decreases by 3.1–4.6%. In the triaxial compression tests, the compressive strength and elastic modulus of LN₂-treated samples decrease by 11–39.6% and 18.2–32.1%, respectively. The experimental results indicate that LN₂ treatment increases the connectivity of coal pores, improving its flow conductivity and permeability, which can effectively enhance the fracturing effect. Meanwhile, it also reduces the ability of coal to resist deformation and rupture, which contributes to the reduction of initiation pressure in the reservoir simulation. In addition, the analysis further suggests that the super low temperature thermal stress can greatly promote the generation of secondary cracks and the formation of complex fracture networks inside coal. The study provides an important reference for tight reservoir simulation of cryogenic fracturing.

This paper presents an experimental investigation of the reinforcement effects of two vacuum-preloading methods by large-scale laboratory model tests: traditional vacuum preloading and multistaged vacuum preloading. The fill samples obtained from Tianjin, China were subjected to tests and the judge method, and formation time of clogging of these two vacuum-preloading methods was determined. The results show that clogging problems during vacuum preloading have a prodigious influence on ground treatment effect. Different vacuum-preloading methods have different clog-forming processes. The formation of clogging outside of prefabricated vertical drains (PVDs) can be determined from the amount of water discharge, rate of water discharge, amount of settlement, and pore-water pressure. The model test indicated that the multistage vacuum-preloading method is more effective than the traditional vacuum-preloading method under the tested conditions in the coastal area of Tianjin. In the scaled model, it was observed that a clogging mud layer began forming at approximately 4,500–5,500 min and was fully formed between 35,000–40,000 min when using the traditional vacuum-preloading method. With the multistaged preloading method, the forming time and full formed time changed to 1,000–3,000 and 15,000 min. The observations on the average degree of consolidation demonstrate that the multiloading method is beneficial for foundation reinforcement as well. These test and discussion results are useful for overcoming the challenges of clogging that occurs during vacuum preloading in coastal areas.

Stability analysis of potential slip surfaces is one of the core steps of slope safety evaluation. In this study, a novel model was developed that considers the critical unstable condition that represents the limit equilibrium and displacement constraints along a prescribed slip surface. The stress distribution and the safety factor of the prescribed slip surface in the critical state can be obtained by directly solving the nonlinear equations formed by the model. By incorporating the finite-element stress analysis, the proposed model does not require interslice force assumptions as do the traditional limit-equilibrium methods and can consider the influence of stress perturbation from outside of the slip body on its stability. The numerical algorithm is also detailed in the article. In addition, an analysis of three examples is carried out to validate the effectiveness of the proposed model and to demonstrate its feature of fast convergence.

The bearing capacity of strip footings on anisotropic soils was examined by using the method of lower-bound-limit analysis coupled with the finite elements and linear programming technique. The governing equations were first used assuming anisotropic soil shear strength, followed by some modifications to make them applicable for this particular problem. The method was applied to inherently anisotropic soils under a static loading condition, but it can be extended to other problems. The modifications lead to an iterative procedure that solved stability problems assuming shear-strength anisotropy. This iterative procedure is, in fact, a procedure by which the shear-strength parameters are incrementally updated to attain a stationary and converged solution. In this study, the focus was on the bearing capacity of surface footings on anisotropic soils, and eventually, design charts were developed for the problem under study. A few comparisons were also made to show the practical advantages of the results and the verification.

On the basis of a clear understanding of dynamic stress distributions in face slabs of concrete-faced rockfill dams (CFRDs), the cracking failure of a face slab can be avoided using antiseismic design measures. In this study, the distribution of tensile overstresses in face slabs during earthquakes was analyzed while the effects of dam geometries and variable nature of input motions were taken into account. The area of face slab vulnerable to cracking was identified. Furthermore, the installation of a horizontal joint is suggested to mitigate the tensile stress in face slabs when subjected to earthquakes. Three-dimensional (3D) dynamic analyses of CFRDs with various geometries were conducted, and the dynamic slope-direction tensile stresses in face slabs with joints at various positions were compared. The results indicate that peak dynamic slope-direction tensile stress can be significantly decreased by installing the joint where the maximum tensile stress occurs.

The full-height extraction of a 10-m-thick coal seam in one single lift caused severe squeezing deformation of Tailgate 14202 in Majialiang Coal Mine in Shuozhou, China. A combination method of hydraulic fracturing and snake-shaped energy-absorbing rockbolts was proposed to reduce the excessive deformation. Hydraulic fracturing was used to weaken the strength of the overlying strata to improve its caveability. Monitoring of the pressure changes of supports in the longwall working face showed that the supporting working pressure and the periodic weighting interval were reduced. Monitoring also indicated that tailgate deformation was decreased. The application of snake-shaped rockbolts in the field further reduced roadway deformation. Only a small number of rockbolts were found to have been damaged due to the large deformation. Hydraulic fracturing coupled with energy-absorbing rockbolts successfully reduced the large deformation of Tailgate 14202 and safeguarded the basic functions of the tailgate. This paper presents a case study of the improvement of the stability of underground roadways under the effects of the extraction of thick coal seams. The combination method might be helpful in reducing the excessive deformation of underground roadways subjected to large dynamic loads.

Reliability evaluations can be challenging if the limit-state surface (LSS) is implicit and the probability information is incomplete in that only marginal distributions and correlations are given. To address the problem, this study adopted the response-surface method based on an adaptive relevance vector machine (aRVM) to approximate the implicit LSS, and the copula approach was used to reconstruct the joint distributions based on incomplete probability information. The Rosenblatt transformation was used to transform the random variables from the original random space into the independent standard normal space for the first-/second-order reliability method (FORM/SORM) approximations. Five different copulas—the normal, Frank, Clayton, CClayton, and t copulas—were adopted to represent different dependence structures and examine their impacts on the failure probability. Results from the numerical example show that the copula effect was negligible if the shear strength parameters were uncorrelated or fully correlated. However, when the correlation coefficient was 0.6 or 0.8, the probabilistic result corresponding to the commonly used normal copula was 5.6% higher or 3.1% lower if the CClayton or the Frank copula was used to model the dependence structure, respectively.

This article addresses the problem of the stability of a slope within the framework of the yield design theory (YDT), which is a rigorous method that avoids assumptions that could affect the obtained results. The case of a nonvertical slope with an angle $\alpha$ at the toe and $\beta$ at the crest using multirotational rigid-block failure mechanisms is investigated. For fixed geometrical and mechanical parameters, the implementation of the upper-bound kinematic method permitted the definition of an optimization problem with five decision variables characterizing the failure mechanism. A constrained global optimization problem was then formulated and converted into an equivalent unconstrained one to get an upper-bound estimate of the studied slope-stability factor. A well-known particle swarm optimization (PSO) algorithm was then used for both its simplicity and efficiency in unconstrained optimization problems. The previous results for the stability factor have been improved and generalized.