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Based on the research background of the damage of high-temperature jointed rock masses, this paper studies the mechanical properties of jointed granite under high-temperature conditions and analyzes the failure law of jointed rock masses with increasing temperature, decreasing temperature and types of joints. The results have important scientific and engineering value. The main research results are as follows: (1) high-temperature conditions change the mechanical properties of granite, especially when the temperature is higher than 400 °C, and the peak stress of granite decreases sharply; the change rate of the mechanical parameters of granite under the condition of water cooling is higher than that under the condition of natural cooling. (2) The uniaxial compressive strength of jointed granite increases with increasing angle α between the joint and the maximum principal plane, and the peak strength of intact granite is higher than that of jointed granite with either conjugate or echelon joints (referred to as conjugate or echelon jointed granite here). (3) The conjugate jointed granite exhibits three failure modes, in which the fractures are all tensile fractures; the echelon jointed granite exhibits three failure modes, most of which include tensile and shear fracturing.

Landslide dams are extremely dangerous because dammed rivers can inundate upstream areas with rising water levels and flood downstream areas after dam breaching. The longevity of landslide dams, which is uncertain, is of great significance for dam failure prevention and mitigation since it determines the time available to take mitigation measures. In this study, the full longevity of landslide dams is divided into three stages (infilling, overflowing and breaching) for better estimation. The influences of dam characteristic parameters (triggers, dam materials and geometric/hydrological parameters) on the full longevity of landslide dams (the period from landslide dam formation to the end of dam failure) as well as on each of the three stages are analysed based on the database. Based on eight dimensionless variables, regression models for estimating the full longevity of landslide dams are developed with a R2 value of 0.781, and regression models for the three-stage longevity (the longevity as the sum of the periods of the three stages) by considering infilling, overflowing and breaching are established with a R2 value of 0.938. It is found that the landslide dam longevity cannot be predicted by one or two influencing factors since it is affected by multiple factors. The relative importance of each control variable is evaluated based on sensitivity analysis: the trigger is the most significant variable in the breaching stage since it affects the size of dam particles, the water content and the inflow rate (e.g. the rainfall trigger results in a larger inflow rate); the lake volume coefficient is more significant in the overflowing stage because it indicates the potential volume of water eroding the dam; and the average annual discharge coefficient is the most important factor in the infilling stage because it controls the time to impound water. The longevity predicted by different models are compared. The models developed in this paper show better accuracy due to the consideration of more parameters based on more cases. In particular, the three-stage longevity regression model shows better accuracy than that of other models because it considers the particular influencing factors for each stage. Three case studies (the “10·10” Baige, Hsiaolin and Tangjiashan landslide dams) are presented to show the application of the regression models developed in this paper. The dam longevity can be predicted more precisely if the timely inflow rate can be estimated by site monitoring or multi-temporal remote sensing images and pre-event digital elevation model (DEM).

Pore‐fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows. Plain Language Summary Debris flows are common geophysical flows consisting of debris grains and muddy water. Debris flows can grow significantly in volume and mobility as they pick up loose sediment from gully bed and banks. The destructive potential of debris flows increases with increasing flow volume and run out. This brings about great challenges for effective early warning of debris flows, design of prevention measures and mapping of hazard zones related to human settlements. It is commonly believed that flow momentum is consumed by carrying static bed sediments. However, flows can gain momentum by overriding wet bed sediments. This can be explained by pore‐pressure generation as debris flows move across wet beds. The increase of measured pore‐fluid pressure is limited for beds with a low water content, but substantial for beds with a higher water content, which strongly affects the erosion rates of bed sediments. Flow velocity and momentum on wet beds are observed to increase significantly but slightly for dryer beds as a result of the pore‐pressure feedback. These findings indicate that the debris composition of the catchment, the water content of bed sediment and the pore‐pressure development should be evaluated when making predictions on debris‐flow hazard. Key Points Propensity for pore pressure evolution of bed sediments during debris‐flow erosion is evaluated by a Deborah number Significant pore pressure and accompanying intense erosion occur for wet bed sediments with undrained behavior Enhanced pore pressure of wet bed sediments reduces flow basal friction, increasing flow mobility and runout

Injecting grout into the gaps between tunnel shield segments and surrounding rocks can reduce ground subsidence and prevent ground water penetration. However, insufficient grouting and grouting defects may cause serious geological disasters. Ground penetrating radar (GPR) is widely used as a nondestructive testing (NDT) method to evaluate grouting quality and determine the existence of defects. This paper provides an overview of GPR applications for grouting defect detection behind tunnel shield segments. State-of-the-art methodologies, field cases, experimental tests and signal processing methods are discussed. The reported field cases and model test results show that GPR can detect grouting defects behind shield tunnel segments by identifying reflected waves. However, some subsequent problems still exist, including the interference of steel bars and small differences in the dielectric constants among media. Recent studies have focused on enhancing the signal-to-noise ratio and imaging methods. Advanced GPR signal processing methods, including full waveform inversion and machine learning methods, are promising for detecting imaging defects. Additionally, we conduct a preliminary experiment to investigate environmental noise, antenna configuration and coupling condition influences. Some promising topics, including multichannel configuration, rapid evaluation methods, elastic wave method scanning equipment for evaluating grout quality and comprehensive NDT methods, are recommended for future studies.

Debris flows can erode mountainsides, cover alluvial fans, and bury people and property by rapid deposition. The deposition characteristics of debris flows are strongly affected by their dynamics and composition, which depend on upstream sediment erosion, but how is still under scientific debate. Here, we conduct a series of experiments to analyze the effects of debris flow grain‐size gradation and eroded bed sediment on deposition characteristics. Debris flows deposit on a gentle runout zone and form coarse‐grained lateral levees and front lobes and a finer‐grained channelized interior due to grain segregation. We show that affected by a high basal pore‐fluid pressure, released mud‐sand‐gravel flows present much flatter deposits than sand‐gravel flows. Runout distance, width and inundated area increase with higher bed water content due to the growths of flow volume and momentum. Inundated area correlates to deposition volume with a power relation for all experiments. Savage number shows the greatest positive correlation with runout and inundated area among all factors, suggesting that potential energy of debris flow is more strongly consumed by grain collision stress than by basal friction stress. Debris flows can deposit as a single nose or multiple fingers depending on the relative magnitude between the friction force at the flow front balanced by downslope gravity and the thrust force of the following channelized flow with a higher speed. Our results facilitate the mapping of debris‐flow impact zones and provide a mechanistic model for predicting deposit shape in debris flows and other geophysical flows like pyroclastic flows.

Affected by earthquakes and heavy rainfall, multiple landslide dams often cluster closely together along river reaches or gullies. Compared with a single landslide dam, the burst flood produced by the cascading failure of multiple landslide dams can be enhanced, seriously threatening life and property downstream. Here, we conduct a series of experiments in a 42 m flume to investigate the failure mechanisms of single and paired dams with fine-grained, well-graded, and coarse-grained debris; analyze the effects of dam geometry and initial water level of a downstream dam on the cascading breach; and quantitatively evaluate the amplification effect of cascading breach discharge. Single dams fail by overtopping along with seepage instability for a fine-grained dam, headcutting for a well-graded dam, and overtopping for a coarse-grained dam, respectively. The type of failure which occurs for a single dam is influenced by the shear strength of the dam material and seepage. However, the downstream dams in cascading tests fail by overtopping irrespective of dam material due to the large outburst floods from the upstream dams. A general flat slope angle is maintained during breaching for the fine-grained and coarse-grained dams, while a step-pool structure is developed for the well-graded dams because the finer grains are easier to wash away than coarse grains. The peak breach discharge for a downstream dam is 1.4–1.9 times the value for an upstream dam in the experimental runs, indicating the amplification effect of breach discharge. The amplification effect has a negative linear correlation with the time interval between the peak breach discharges of the two dams because the overlap of breach processes of upstream and downstream dams is gradually reduced as the time interval increases.

As a crucial solution to the challenge of limited urban underground space development, the assembled shaft offers extensive structure–soil contact surfaces and meantime holds significant potential for shallow geothermal energy exploitation. In this paper, we propose an assembled energy shaft, i.e. a novel geothermal development system, in which the heat exchanger could be easily installed in the shaft concrete with extensive soil–contact area and can have superior protection without extra pre-drilling. This paper establishes a heat transfer model for energy shafts in soft soil areas. By comparing the heat transfer efficiency and additional thermal stress of the energy tunnel in Beijing, the practical feasibility of constructing energy shafts in coastal cities is demonstrated. By proposing the characterization parameters of heat exchange capacity per unit lining surface area and heat exchange per unit length of pipe, it is revealed that thermal interference is minimized when the heat exchange pipe spacing of the energy shaft is 0.25–0.3 m. The heat exchange efficiency is increased when the fluid flow rate is 0.6 m/s ~ 0.9 m/s. According to the deformation characteristics of the lining, the maximum tensile and compressive stresses occur near the inlet of the heat exchange pipe. To minimize stress concentration, it is advisable to position the inlet of the heat exchange pipe at the center of the segment. The research findings confirm the substantial potential of assembled energy shafts in shallow geothermal development and provide valuable insights for the design of such shafts in coastal cities.

A spring-based smoothed particle hydrodynamics (SB-SPH) method was developed to reproduce the progressive failure process and quantify the factor of safety (FOS) of a slope within the intermittently jointed rock mass. In the improved framework, the equations of contact forces between particles are introduced into the Eulerian formulations of SPH, and a unified governing equation for coupled cracking and contact is developed. Moreover, the initial discontinuous domain searching algorithm has been proposed to realize the generation of prefabricated discontinuities. A sliding benchmark test of rectangular blocks on inclines is conducted to verify the validity of the contact behaviour. Jointed rock cells under uniaxial compression with varying dip angles are employed to validate the accuracy and reliability of SB-SPH in cracking and contact behaviour. Additionally, the effects of intermittent joints on the progressive failure process and FOS of slopes characterized by various dip angles and rock bridge lengths are revealed. In comparison to horizontal joints, the FOS of the slope increases by 49.18%, 88.52%, 122.95%, and 173.77% when the joint dip angles are 30°, 45°, 60°, and 90°, respectively. Compared with the situation with persistent bedding planes, when the lengths of the rock bridges are 1 m, 2 m, and 3 m, the FOS of the slope increases by 19.74%, 51.32%, and 78.95%, respectively. The results indicate that as a continuum-discrete method, SB-SPH precisely reproduces the process of crack initiation, propagation, and block contact behaviour, such as frictional sliding at multiple scales.

This paper presents a solution for the time-dependent behaviors of energy piles embedded in transversely isotropic soils, which considers the mechanical and thermal consolidation. By using the transformed differential quadrature method, kernel functions of coupled thermal-hydro-mechanical solution on the soil-energy pile interface are obtained and the boundary integration is conducted. Then, the energy pile is discretized into finite elements. After introducing the displacement coordination and boundary conditions, matrix equations to reflect the interaction between the surrounding soils and energy piles are formulated and solved. Since the consolidation is considered, the solution for energy pile behaviors with time including displacements and thermal stresses are achieved. Computational results are compared with data of existed literatures and field tests to validate the theory in this study. Finally, numerical examples are conducted to discuss the effects of transverse isotropy of soils, consolidation process and the length-diameter ratio of the energy pile.