International Journal of Geomechanics

The Zhala hydropower station plant was under the threat of rockfalls induced from a high slope, especially in rainy seasons. This paper proposes an innovative protection structure consisting of a pervious concrete wall and a sand cushion to protect against rockfall impact as well as discharge surface runoff. Laboratory test results show that the peak impact force of the rockfall decreases with decreasing rockfall mass and impact velocity, together with increasing cushion thickness and rockfall sphericity. The most sensitive factor influencing the peak impact force is rockfall sphericity, followed by impact velocity, rockfall mass, and cushion thickness. Based on the sensitivity of influencing factors, a method of calculating the rockfall peak impact force was established by dimensional analysis. Furthermore, a full-scale test was conducted to investigate the feasibility of this structure; test results indicate that an increased rockfall mass results in increased peak impact force, impact stress, and displacement of the pervious concrete wall. By placing a sand cushion in front of the pervious concrete wall, the peak impact force of rockfall and the maximum displacement of the wall are decreased by 70% and 65%, respectively. Meanwhile, the proposed calculation method has good reliability for evaluating the practical rockfall impact force, compared with four classical calculation methods. After the buffering effect of the sand cushion, the impact stress distribution on the pervious concrete wall can be characterized as a bell shape, with high magnitude at the impact point and low magnitude on the sides.

The use of ultrasonic technology for rock breaking, with its high-frequency and microamplitude impact load, is anticipated to help overcome the challenges of high economic cost and low rate of penetration in mineral resource exploration. A three-dimensional finite-element model of a heterogeneous granite mineral crystal was created using random Voronoi tessellation and cohesive elements. The distribution of the multipolycrystalline structure was achieved in a proportional manner. Then, the differences between conventional cutting and ultrasonic-assisted cutting in rock breaking were compared. The effects of different vibration frequencies (20–35 kHz) and amplitudes (10–50 μm) on crack propagation, crack size, stress distribution, and energy consumption were investigated. The results indicate that higher ultrasonic vibration frequencies significantly improve crack formation, with enhancements ranging from 25.16% to 44.30% compared to conventional cutting. It is important to note that ultrasonic load plays a crucial role in enhancing rock-breaking efficiency by enlarging crack sizes and increasing the number of intergranular tensile cracks instead of solely increasing the total number of cracks. When the vibration frequency is 30 kHz, the crack volume and the proportion of intergranular tensile cracks reach their peak values, while the cutting force decreases to its lowest point, 74.51% lower than conventional cutting. Increasing the vibration frequency is advantageous for reducing mechanical energy consumption. However, once the vibration frequency surpasses 30 kHz, further increasing it does not significantly enhance the ultrasonic effect. Additionally, raising the amplitude can increase the quantity and size of cracks, leading to an effective improvement in the stress situation of the polycrystalline diamond compact cutter.

The physical cementation and grain breakage effects play a vital role in influencing the mechanical properties of hydrate-bearing sediment. In this study, a thermodynamic model for hydrate-bearing sediment considering stress history, grain breakage, and drainage condition is developed based on granular thermodynamic and critical state theory. The concept of breakage ratio is defined and incorporated into the critical state line to capture the grain breakage effect under a large range of confining pressures. To describe the structural effects induced by cementation and stress history, a new cementation degradation term and the structure factor of stress history are incorporating into the elastic potential function. By employing a rigorous parameter calibration approach, the theoretical model is applied and verified under various stress histories, hydrate saturations, and confining pressures, and a comparison is drawn between with/without considering grain breakage effect. The theoretical model, accounting for the grain breakage effect, demonstrates superior prediction accuracy and can reflect the inhibitory effect of hydrate saturation on grain breakage under high stress, as well as the strength and deformation characteristics under diverse drainage conditions and stress histories.

Similar to the deep-buried tunnel, the lining of the cut-and-cover tunnel (CCT) exhibits varying degrees of cracking failure during ultrahigh backfill. To explore the whole progressive failure process of a high-filled cut-and-cover tunnel lining structure from the mesodamage to the macrolocal failure and then to the overall instability during backfill, based on the discrete-element program PFC2D, we simulated the mechanical behavior and fracture characteristics of the CCTs with rectangular, circular, and arch lining section features with the backfill height. Two load-reduction measures—load-reduction material (LRM) and the interaction of LRM and concrete walls—were then further proposed, and the impact of load-reduction measures on the progressive failure of CCT lining was analyzed. The simulation results show that the various lining section features indicate differences in the distribution and value of lining stress and fracture. But the CCT lining with features of the three sections exhibits a progressive failure process from cracking to local collapse and falling and finally to overall instability as the backfill height increases. Furthermore, the LRM and concrete walls can effectively reduce the vertical stress of the lining vault and the horizontal stress of the haunch, respectively, to ensure the stability and integrity of the CCT.

Abstract

Due to the inherent random variations in the engineering properties of soils and rocks, considering probabilistic analysis rather than the deterministic one is a safer option for the design of geotechnical structures. However, the probabilistic methodologies often rely on prior knowledge of probability density function (PDF) of the considered random variables (RVs). In this study, a new formulation called the fourth-moment pseudo-normal transformation (FMNT) is employed to perform the probabilistic analysis of a tunnel support system. FMNT requires only the first four statistical moments of the random variables instead of complete information about their PDF to calculate the failure probability (P_f). Further, it is an efficient technique that is demonstrated by performing analytical as well as numerical probabilistic analysis of circular and noncircular tunnels using FLAC 3D. FMNT is used in two different ways to highlight its flexibility: (1) with the first-order reliability method (FORM) called FORM-based FMNT; and (2) with point estimated method (PEM) called PEM-based FMNT. The results of the latter align adequately with the results of Monte Carlo simulation (MCS) and the analysis requires only 21 and 26 simulations for four and five random variables, respectively, instead of 100,000 simulations required by the MCS. The compatibility of this framework in actual field settings is demonstrated through an exercise using the nonnormal random variables.

Practical Applications

Soil and rocks possess inherent variability. This makes it hard for engineers and scientists to predict exactly how they will behave under different situations. Therefore, they use probabilistic methods that deal with this uncertainty. One common method is called Monte Carlo simulation (MCS). It is good at handling uncertainties and giving accurate results and that is why it is commonly used as a benchmark method for comparisons. But to use it, we need to know some information about the input parameters such as their distribution. This information is generally not available when dealing with field data. To solve this, we need a method that gives us results comparable to MCS but is also efficient enough to use in the field. This study presents a new probabilistic technique named fourth-moment pseudo-normal transformation (FMNT) to do this job. It is useful and easy for assessing the safety of geotechnical structures that are demonstrated by conducting a probabilistic analysis of tunnels in the present study. This method will help the field engineers plan and deal with uncertainties in a better way.

Face instability models considering the gradual evolution of the soil arching effect and the characteristics of face instability under different cover-to-diameter ratios are proposed in this paper. The face instability combined-model includes the upper cylinder and lower logarithmic spiral for shallow buried shield tunnels considering the overall soil slipping failure. The two-phase face instability model is proposed for deep buried shield tunnels based on the evolution of the upper soil arching shape and soil local slipping failure. The first-phase combined model is composed of the upper catenary and lower logarithmic spiral. The second-phase combined model consists of the upper cone and lower logarithmic spiral. The earth pressures on the bottom of the upper cylinder, physical catenary, and cone arch are calculated based on the Terzaghi loose earth pressure calculation method and the arch theory, respectively. The carrying capacity at the top surface of the lower logarithmic spiral was calculated according to the slicing method considering the intersection between the logarithmic spiral and tunnel section is in full-section contact. The limit equilibrium equation is established and the calculation model derived by using MATLAB is used to obtain the numerical solution for the limit face supporting pressure of the shield tunnel. The reliability of the theoretical calculation method based on arch theory and the validity of face instability combined model are verified by comparing the calculated values with the measured values of centrifuge model tests and literatures. The influence of cohesion, inner friction angle, and cover-to-diameter ratio on limit face supporting pressure is analyzed. The results provide a basis for determination of limit face supporting pressure at different phases during face instability.

Abstract

The effect of bedding cohesion and loading conditions on the bolting behavioral characteristics of a bedded rock mass was investigated using a self-developed artificial material and loading system. First, physical samples with dimensions of 15 cm × 15 cm × 8 cm containing both the grout and gypsum layers were prepared. The ratio of the thickness of the grout layer to the gypsum layer was 0/100, 25/75, 50/50, 75/25, and 100/0. One grouted bolt was situated in the middle of the sample. Two types of filling materials were chosen as a filling segment around the rock bolt, i.e., gypsum and grout. The tensile strengths of gypsum and grout were 1.4 and 2.4 MPa, respectively. The length and diameter of the bolt were 12 and 2 cm, respectively. The grout dimension was 5 cm × 5 cm × 8 cm. These samples were tested under two different axial loading conditions. A total of 20 distinct tests were conducted concurrently with laboratory experiments utilizing Particle Flow Code in Two Dimensions. The loading rate was 0.05 mm/s. The findings revealed a strong correlation between breakage patterns and loading conditions. Moreover, it was observed that the grout filling significantly impacts the ultimate axial load compared to gypsum filling. When loading the interface between the rock bolt and the filling segment, the final axial load remained consistently similar across all filling types. Notably, the grout filling resulted in a notably higher final axial force. The variation in grout and gypsum bedding layer thicknesses did not demonstrate any discernible effect on the final axial load. Conversely, when loading the interface between the filling and bedding layers, an increase in grout layer thickness led to an augmented final axial load. The grout filling segment also contributed to a heightened final axial force. The results of the numerical simulations exhibited a commendable agreement with the experimental outputs. This research effectively addresses two critical aspects: stability control and support design for the bedded rock mass.

Practical Applications

This study used a unique artificial material and loading system to investigate how bedding cohesion and loading conditions affect bolting behavior in rock mass. The findings showed a significant difference in ultimate axial load when grout was used compared to gypsum filling around the rock bolt, emphasizing the importance of this factor in stability control. The research revealed a strong connection between breakage patterns and the specific loading conditions applied, providing valuable insights into support design in a bedded rock mass. The study also demonstrated a high level of agreement between experimental and numerical simulations, strengthening the credibility of the findings and their relevance to real-world stability control measures.

During tunnel construction, the excavation face frequently encounters heterogenous weathered composite strata that considerably reduce tunnel face stability. To assess the stability of the tunnel face under such conditions, a 3D analysis model is established using the limit analysis approach. The model comprehensively assesses the failure of the entire tunnel face, incorporating the Hoek-Brown criterion to account for the nonlinear failure characteristics of rock. In addition, improvements are made to the shape of the sliding surface in the cover layer to consider the influence of the arching effect. The proposed model is validated by comparisons with existing theoretical methods and numerical simulation results. Further parameter analysis is conducted to investigate the influence of stratum parameters, stratum thickness and tunnel depth. The results indicate that tunnel face instability is more likely to occur when the rock mass quality of the higher layer is greater than that of the bottom layer. The rock mass in the bottom layer has the most remarkable impact on the tunnel face stability compared with the cover layer and higher layer. The stability of the tunnel face initially decreases and then increases with the tunnel depth due to the rock-arching effect. These research results can provide useful references for tunnel face support design in composite strata.

The existing theories or numerical models of thaw consolidation for frozen soil rarely consider the influence of the thermal effect, so it is difficult to analyze the problem of thaw thermoconsolidation for frozen soil under high temperature. Using the piecewise linear approach and finite-difference method, a one-dimensional large strain thaw thermoconsolidation model, called TTCS1, is established for frozen saturated soil under high temperature. The model couples the heat transfer with phase change of frozen soil and the thermoconsolidation deformation, and accounts for the nonlinear changes of soil parameters and large strain during the process of thaw thermoconsolidation under high temperature. When the thermal effect is ignored, TTCS1 shows excellent agreement with the existing large strain thaw consolidation model. When the thermal effect is considered, the numerical solutions of the TTCS1 model are basically consistent with the test values. The influences of the thermal effect, boundary temperature, and boundary conditions on the thaw thermoconsolidation for frozen silty clay are further discussed, and the settlement and settlement rate of frozen soil under high temperature will be significantly underestimated if the thermal effect is not considered.

Abstract

To date, the important problem of Terzaghi’s ground arch model in 1946 has not been solved for loads on circular tunnels. This study comprehensively presented a general analytical solution of Terzaghi’s ground arch model for loads on circular tunnels by the limit equilibrium method, considering the complete boundary conditions of the loosening zone, the effects of water pressure, and the stiffness of the lining and ground. The proposed solution indicates the relationship between the load on circular tunnels and that on trapdoors, and there is a negative correlation between the lateral and vertical earth pressures in the ultimate state. Model tests and field measurements were used to verify the proposed solution. The results indicated that the proposed solution agrees well with the experimental results of circular tunnels in dense sand, loose sand, saturated sand, and saturated clay; the solution is significantly less than the design load from Terzaghi’s formula in good ground conditions. Moreover, the solution demonstrated an interesting rule: the distribution of total loads (including water pressure) on nonrigid circular tunnels is nearly uniform in saturated granular soil. A particular solution was obtained for circular tunnels in saturated granular soil. In addition, the vertical pressure on a circular tunnel in sand increases and the lateral pressure decreases as the groundwater table increases within the tunnel range; the case where the groundwater table is located at the crown is proved to be a critical condition for shield tunnels in sand.

Practical Applications

Previous studies indicate that the load on shield tunnels in good ground conditions would be overestimated by Terzaghi’s formula, which was proposed in 1943 and is widely adopted in tunnel design nowadays. In this study, a general analytical solution for loads on circular tunnels was established based on Terzaghi’s ground arch model, which was improved by Terzaghi in 1946 for arched tunnels but has not been solved. For various overburden depths, the proposed solution agrees well with the experimental results of circular tunnels in dense sand, loose sand, saturated sand, and saturated clay, but Terzaghi’s formula based on trapdoors fails to explain the test results of circular tunnels in sand. The proposed solution is significantly less than the design load from Terzaghi’s formula in good ground conditions. The solution is applicable for both shallow and deep circular tunnels, and it will be very helpful for the economical and rational design of shield tunnels normally in good ground conditions, such as sandy soil, crushed rock, and stiff or hard clayed soil.

Rock hardness and abrasivity are among the most crucial properties that can significantly impact the interaction between rocks and mechanical tools in different parts of geoengineering projects. Accurate estimation of these properties is essential for a better understanding and optimization of geoengineering operations. Hence, the main aim of this study was to develop different machine learning (ML) models based on the geochemical measurements for predicting rock hardness and abrasivity. To do this, 159 rock samples were collected from a gold mine, and portable X-ray fluorescence spectrometry (pXRF), Leeb rebound hardness (LRH), and Cerchar abrasivity index (CAI) tests were performed on the collected rock samples. Three different ML algorithms, including random forest regressor (RFR), support vector regression, and gradient boosting regressor, were applied to develop predictive models for LRH and CAI separately. Considering the fact that the geochemical data are of the compositional type, two scenarios were followed: developing predictive models based on the original data obtained from the pXRF and the centered log-ratio (Clr) transformed data, resulting in the development of six predictive models for LRH and CAI. The performance assessment of the developed predictive models showed that RFR models outperformed the other two ML algorithms in predicting LRH and CAI. In addition, the developed models based on the original data demonstrated a better performance in both cases of LRH and CAI than the trained model based on Clr data. The result indicates that integrated pXRF measurements and RFR technique have strong potential to be used for practical and efficient rock materials characterization during exploration and extraction processes.

The response and the safety of geostructures constructed on hilly regions or sloping grounds are receiving increased attention. As the foundation of geostructures resting on slopes is often in the vadose zone, accounting for appropriate properties of partially saturated soil is essential for effective design. Moreover, changes in climatic conditions will adversely affect the load-carrying capacity and the stability of the footing resting on unsaturated slopes. Often, the soil–water retention curve (SWRC) at null stress is adopted but is expected to be affected by various factors, including the stress state of the sample. In the present study, an attempt has been made to demonstrate the effectiveness of using stress-dependent SWRC (SDSWRC) in analyzing a footing resting on a slope exposed to seasonal variations. The SDSWRC along the drying–wetting path determined experimentally was fitted using the standard SWRC model. The key parameters of the SDSWRC, hydraulic conductivity function, and shear strength of unsaturated soils are utilized in numerical modeling. Transient state flux analysis is carried out to mark the effects of SDSWRC in the development of hydraulic response. The stability of an unsaturated slope subjected to a footing load located at a chosen setback distance was assessed under various hydromechanical conditions. The results demonstrate that the seasonal variation, water table position, and net stress significantly influence the stability and the load-carrying capacity of the footing resting on an unsaturated slope.

The thermal-induced pore-water pressure and shear characteristics of lateritic clay under different temperature paths without drainage are studied through a series of temperature-controlled undrained triaxial shear tests with temperatures ranging from 5°C to 50°C. The effects of different temperature paths under undrained conditions on pore-water pressure and shear behavior are discussed. The results indicate that monotonic heating and multistage heating have equivalent effects on the accumulation of pore-water pressure when the initial temperature and target temperature are the same. The initial positive pore-water pressure induced by heating leads to a lower bearing capacity during the early stage of the shearing process. The secant elastic modulus (ε_a = 0.1%) shows a significant decrease with increasing temperature, suggesting that mechanical properties at small strains are influenced by temperature. However, undrained heating has no influence on the shear strength finally. Furthermore, under undrained conditions, temperature variations do not alter the unique relationships among void ratio, deviator stress, and mean effective stress.

The support pressure at a shield tunnel face is significant for the stability of the shield excavation and prediction of ground disturbance. This study presents an optimization measure that relies on the three-dimensional (3D) limit equilibrium method (LEM) to determine the reasonable support pressure of the shield tunnel face. The predetermined support pressure is subsequently incorporated into the finite-element (FE) model, and the ground disturbance that is induced by the shield excavation could be estimated by the FE simulation. The face support pressure that is obtained from the optimization measure aligns remarkably well with those derived from other methodologies, therefore confirming its efficacy. Furthermore, the numerical results demonstrate that the support pressure, as determined by the optimization measure, provides superior control on the ground disturbance. To further estimate the sensitivity of the tunnel buried depth and soil strength in relation to the ground disturbance, orthogonal testing is conducted. The outcomes of this orthogonal testing inform the development of a predictive approach for evaluating ground disturbances that are caused by the shield excavation, which is then validated by comparing it with previous works. The findings of this study serve as a valuable asset in facilitating the rapid prediction of ground disturbance that is induced by shield excavation.