This study investigates the mechanical properties and microstructural evolution of peaty soils under freeze-thaw (NF-T) cycles through unconsolidated undrained (UU) triaxial shear tests and scanning electron microscopy (SEM). The effects of F-T cycles (0-30), confining pressure (100-400 kPa) and fiber content (0%-12%) were systematically evaluated. Results indicate that the ultimate strength and shear strength decrease as the number of freeze-thaw cycles increases, with the most pronounced reduction occurring during the first 5 cycles. Beyond 15 cycles, the rate of decrease diminishes, and the curves tend to flatten. The fiber reinforcement significantly mitigates the strength degradation caused by freeze-thaw cycles. The most significant improvement is observed at a fiber content of 12%. The fibers act as bridging elements that improve soil particle connectivity, thereby strengthening cohesion and mitigating particle displacement and deformation during freeze-thaw processes. SEM analysis reveals that fiber-soil interaction mechanism undergoes a progressive transformation with increasing fiber content, evolving from localized fiber embedding to comprehensive network formation through fiber entanglement. This structural evolution establishes robust inter-aggregate connections that enhance the soil matrix integrity. Following 30 freeze-thaw cycles, no apparently penetrating fissure was formed although the freeze-thaw process damaged the connection between fibers and soil aggregates, demonstrating the effectiveness of fiber reinforcement in mitigating freeze-thaw damage. These findings provide critical insights into the microstructure-mechanical properties relationships of peaty soils, offering practical guidance for foundation treatment in seasonally frozen peat regions and controlling the engineering diseases problems.

Considerable amounts of engineering construction were performed in cold regions where peat soil deposits exist extensively in the Western Sichuan Plateau, China. However, the mechanical properties of frozen peat soil have rarely been reported. In this paper, a series of unconsolidated undrained direct shear tests were carried out on frozen peat soil at varying freezing temperatures and times. The experimental results indicated that the measured cohesion varies from 16 to 31 kPa, and the friction angle varies from 21.2° to 27.8°, with the freezing temperature varying from 0°C to −25°C and the freezing time varying from 6 to 30 h. The growth rate of cohesion is at a maximum when the freezing temperature decreases to −5°C. Both cohesion and friction angle tend to be relatively stable when the freezing temperature decreases to −25°C. Additionally, when the freezing time is greater than 24 h, the growth rates of cohesion and friction angle tend to be stable. The established relations of the shear strength with freezing temperature and freezing time indicate that freezing time contributes more to the growth of cohesion than freezing temperature. This is because the structures of frozen soil have become increasingly dense due to persistent freezing resulting in stronger cementation between ice and soil particles. In contrast, the freezing temperature contributes more to the growth of the friction angle than the freezing time.

This study investigates the deformation and failure mechanisms of gently layered surrounding rock during underground excavation in a hydropower station in Shaanxi Province, China, through base friction physical simulation tests. Results indicate rock mass instability is governed by weak discontinuities (layer planes, joints, and faults). Four failure modes emerge: (1) tensile crack collapse at the tunnel arch induced by stress concentration, (2) shear slip at the sidewalls and shoulders due to exceeding shear strength, (3) bulging deformation of thin-layered rock under compressive stress, and (4) slight uplift at the invert of the main powerhouse due to upward pressure. The physical simulation tests replicate the deformation and failure evolution of the surrounding rock mass during excavation. Initially, the process is characterized by tunnel arch spall and slight slip, progressing to rock mass collapse due to structural plane coalescence. The study reveals the complex mechanical behaviour of failure in gently layered surrounding rock. The tunnel arch exhibits a "tension crack collapse - shear slip - bedding" failure mode, while the sidewall is primarily influenced by shear slip. Findings confirm discontinuities' control on the stability of layered surrounding rock, offering guidance for deformation support and long-term maintenance. These results provide significant theoretical and practical implications for engineering applications.

To investigate the frost damage of Zoige Plateau Peat Permafrost and the stability of engineering foundations in seasonally frozen regions, this study builds upon previous research on the optimal mix ratio of composite stabilized peat soil using mineral powder-steel slag and basalt fibre as primary materials, supplemented with polycarboxylate superplasticizer and carbide slag. Indoor mechanical tests (unconsolidated undrained triaxial shear test, unconfined compressive strength test, and scanning electron microscopy test) were conducted on stabilized soils subjected to varying freezing temperatures and freeze-thaw cycles to explore the mechanical properties and microscopic mechanisms under freeze-thaw action. Based on the experimental dataset, the peak strength of stabilized peat soil was predicted using a particle swarm optimization backpropagation neural network (PSO-BP). The results indicate that the unconfined compressive strength and residual strength ratio of stabilized peat soil are positively correlated with freezing temperature and negatively correlated with the number of freeze-thaw cycles. The stress-strain curve characteristics of stabilized peat soil are generally consistent under different conditions. With increasing cycles, the peak strength and cohesion of the stabilized peat soil first decrease and then stabilize, while the internal friction angle remains relatively unchanged. In the peak strength model constructed using MATLAB, the PSO-BP neural network model exhibited a higher correlation coefficient (R2) and better overfitting performance compared to the BP neural network model, enabling a more accurate strength prediction of stabilized peat soil.

In this study, a series of base-friction tests is carried out to investigate the deformation process and geomechanical behavior of stratified surrounding rock masses with different dip angles during tunnel excavation. Similarity models are constructed to simulate bedding dip angles of 0°, 30°, 60°, and 90°, and the process of deformation and failure for the models is recorded. The results demonstrate that the dip angle strongly affects the geomechanical behavior of tunnels excavated in stratified rock masses. Tensile fracture is the primary geomechanical mode across strata with different dip angles. Bending–toppling and shear slip occur in inclined strata, and bulging occurs in vertical strata. This study highlights the dominant effect of layered structures and the bedding inclination on the mechanisms of deformation and failure of surrounding rock masses. The findings provide a theoretical basis for the design of support measures and the long-term stability evaluation of tunnels under similar geological conditions.

A deep foundation pit for an urban substation in Chengdu, China, excavated in a sandy gravel layer with an excavation depth of 24.6 m, was investigated. Detailed field surveys and field monitoring were carried out to investigate the deformation characteristics of the foundation pit and supporting structure related to excavation. Flac3D software and orthogonal testing were used to analyze optimal pile anchor design parameters by calculating the effects of five influencing factors, namely, pile diameter, pile spacing, anchor cable prestress, anchor angle, and number of rows of anchor cables, on the deformation of the deep foundation pit. Analysis of the optimization effect indicated that although there was a small increase in displacement when adopting the optimized plan, the value of the displacement still satisfied required specifications. The proposed optimization scheme not only ensures the safety and stability of the foundation pit but also reduces construction costs and provides guidance for the design optimization of projects with similar geological conditions.

Considerable engineering infrastructure has been constructed in cold regions with widespread peaty soil deposits, particularly in Western Sichuan Plateau, China. However, the mechanical properties of frozen peaty soils remain poorly documented in the literature. In this paper, a series of unconfined compression tests were carried out on frozen peaty soil at varying freezing temperatures and time. The experimental results indicated that the mechanical behaviours of frozen peaty soil are characteristic of elastic-plastic deformation, greatly affected by freezing temperature. The measured UCS varies from 40 kPa to 1062 kPa when the freezing temperature is between 0 °C and -25 °C, and the freezing time is between 6 h and 30 h. The UCS increases sharply when the freezing temperature decreases to -15 °C from 0 °C, and the rate of increase of UCS slows down, when the freezing temperature is lower than -15 °C. The results of environmental scanning electron microscope demonstrated that the connection strength between ice and soil structure was improved owing to complete freezing, resulting in a significant increase in the cohesion of the ice-soil skeleton and playing a key role in improving the macroscopic mechanical strength. Keywords: organic matter; frozen soil; unconfined compression strength; microstructure

>Layered rock masses represent complex geological formations commonly encountered in the surrounding rock of deep engineering excavations(Hou et al., 2019; Xu et al., 2017; Yang C H et al., 2009; Xian and Tan, 1989). These rock masses are predominantly composed of sedimentary, para-metamorphic, and volcanic rock types, characterized by a set of prominent, primary bedding structural planes(layers) exhibiting relatively consistent orientations and significant spatial continuity.

We investigate a deep foundation pit for urban substation construction in Chengdu, China. The pit was excavated in a sandy gravel layer and has a depth of 24.6 m. Detailed field surveys and monitoring were carried out to investigate the deformation characteristics of the foundation pit and its supporting structure related to excavation. Flac3D software was used to analyze the influence of pile anchor design parameters, including pile diameters, pile spacing, anchor cable prestress, anchor angle, and the number of rows of anchor cables, on deformation of the deep foundation pit. The results showed the pile diameter was the most critical influencing factor for pile deformation. The influences of anchor cable prestress and anchor angle on the horizontal displacement of the pile were inferior to the influence of the pile diameter and the number of anchor cable rows. The results reported in this paper provide guidance for the optimal design of foundation pits in settings with similar geological conditions.

This study investigated the Shiliushubao landslide's rainfall-induced instability. Using the finite element method and limit equilibrium calculations, coupled transient seepage and stability analyses were carried out to evaluate the dynamic stability of the landslide during numerically simulated rainstorms. A sensitivity analysis of the landslide stability when the landslide was affected by rainfall was also conducted. The results showed that short rainfall durations with low rainfall intensities do not significantly affect landslide stability. However, short rainfall durations with high rainfall intensities have considerable effects on the stability of shallow landslides. Slope stability gradually decreases as rainfall durations increase, but the effect of rainfall duration on stability is much greater for the shallow sliding blocks than it is for the main block. For the same increase in rainfall duration, the safety factors for the shallow blocks decrease more than the safety factor for the main block. Minimum safety factors do not occur immediately during a rainfall but develop during the following days. This behavior demonstrates the hysteresis of rainfall-induced landslide instability.

为实现对峰前及峰后岩石脆性特征的定量刻画,提出了一种基于岩石峰前能量演化及峰后侧向变形特征的岩石脆性评价方法,建立岩石峰前脆性指标Bpre及峰后脆性指标Bpost,将两者之积作为岩石脆性评价指标Bm.为验证脆性评价指标的合理性,选取凉山州某铁矿矿山扩能工程露天采场边坡发育的玢岩、辉长岩及花岗岩为研究对象,开展不同岩石及不同围压条件下的三轴压缩试验;采用已有的脆性评价方法B15～B18、B23与本文所提的岩石脆性评价指标Bm对试样的脆性特征进行验证对比,结果表明:(1)采用脆性指标Bm能够较好地评定不同种类岩石的脆性特征,Bm与岩石脆性呈负相关,即脆性评价指标Bm越大,岩石脆性程度越高;(2)脆性指标Bm较好地验证了围压对岩石脆性特征的影响,随着围压的增大,岩石脆性程度降低.本文所提出的脆性评价方法丰富和完善了岩石室内脆性评价体系,具有一定的理论意义及工程实用价值.

An ancient, large-scale river-damming landslide, located 4 km on the upper stream of the dam site of the Batang Hydropower Station, southeastern Tibetan Plateau, China, was investigated herein. We used an unmanned aerial vehicle (UAV) survey, field investigations, geology and topography maps, borehole surveys, basic laboratory testing, and geological dating to reveal evidence of this river-damming landslide and its cause mechanism. The studied landslide had an estimated volume of 3.5 × 10 7 m 3 , blocked the valley, and formed a dam > 170 m in height. A large quantity of lacustrine sediments was discovered on both banks upstream of the residual landslide dam; this had a maximum thickness of 30 m, mainly comprised of fine sand and silt, with particle sizes of 0.01–0.25 mm, and demonstrated that the landslide blocked the Jinsha River. There was a clear geological structural dependence of the slope failure in the landslide area, dominated by a combination of sliding-toppling failure on cataclinal slopes. The slope instability mode was slip-controlled toppling failure within a thin-layered biotite quartz schist that exhibited inferior mechanical properties. The head scarp developed along a bedding plane that dipped steeply in the same direction as the slope. The upstream and downstream lateral scarps formed along the joint sets J2 and J3, respectively, which intersected with the bedding plane, forming a wedge and facilitating slope instability. Rapid river incision, intense tectonic activity, a high and steep slope, and thin-layered steeply dipping strata were the controlling factors for the occurrence of this landslide, all contributing to slope failure. On the basis of our comprehensive analysis of paleoearthquakes, paleoclimate, and geological dating of the landslide, we conclude that this ancient river-damming landslide event was likely triggered by a strong earthquake related to activity on the Xiongsong‒Suwalong fault in the Late Holocene.

为研究富水砂卵石层地区桩锚支护深基坑变形特性,以成都某变电站深基坑工程为研究对象,基于现场监测结合数值模拟手段,对深基坑及支护结构的变形特征进行研究,揭示深基坑开挖引起的支护桩侧向变形、深层土体侧向变形、周边地表沉降及坑底隆起等变形规律.研究表明,基坑周边地表沉降规律具有显著的时效性与空间性,地表沉降曲线近似呈"凹槽"状分布,最大沉降量位于距坑壁水平距离6.0 m处,主要影响范围为2倍基坑深度;基坑开挖卸荷引起坑底隆起,最大隆起变形量约为基坑深度的0.49％,位于坑底中部.深层土体与桩身侧向位移曲线近似呈悬臂支护的变形特征,变形量随开挖深度的增加而增大,最大变形位于地表.

In this study, an unstable rock slope has been investigated. To evaluate the stability of the pentahedral rock wedge with multi-slip surfaces, an approach based on the limit equilibrium method has been used. The slope has been divided into two parts for the establishment of the model: an upper wedge block and a lower pentahedron block. Namely, a three-dimensional space problem can be resolved into a local equilibrium problem of several sub-blocks. Then, the geometric parameters can be determined, and the mechanical analysis can be carried out. Finally, the minimum safety factor can be calculated by using the optimization method. This proposed approach has been applied to analyze the stability of the unstable rock slope before and after reservoir impoundment. The results show that the safety factor is significantly smaller after reservoir impoundment. The safety factor can meet the safety control standards of specifications without considering the extreme conditions of combined rainstorm and seismic force under the proposed reservoir water level.

On May 12, 2019, a landslide occurred near a construction site of a reservoir dam along the left bank of the Wulong River, Qinghai Province, China. Video footage showed that the entire failure process of the landslide lasted for 15 min. Although an excavator that parked on the foundation pit was buried and the Wulong River was blocked, this landslide did not cause any casualties. Based on field investigations, 3D laser scanning, unmanned aerial vehicle observations, and borehole surveys, this study aims to reveal the cause and initiation mechanisms of the landslide. The landslide developed on a typical dip slope with thick-bedded conglomerates intercalated with a few mudstone layers. The slide mass mainly consisted of conglomerates with a dip angle varying from 20 to 40°, failing along bedding plane contacts with the underlying muddy intercalation. Numerous tension cracks had developed in the upper part of the slope due to stress relaxation during the process of bank slope evolution associated with rapid river incision. These cracks damaged the integrity of the rock mass and promoted rapid rainfall infiltration. The water infiltrated into the slope and deteriorated the mechanical properties of the rock mass, which resulted in long-term deterioration of the slope stability conditions. In addition, the temperature of Tongren County was below zero for 5 months in a year, hence the freeze–thaw cycle of the water filled in the cracks would have reduced the shear strength further. There were 10 days of continuous rainfall before the landslide occurred, and the cumulative precipitation in 2018 and 2019 was also significantly more than that of previous years. Three primary factors are involved in the landslide event: (1) thick-bedded conglomerates intercalated with a few mudstone layers with contrasts in strength, (2) development of numerous relaxation cracks, and (3) long-term rainfall accumulation.

An unstable rock slope called the Guobu landslide, observed in granite with a maximum cumulative displacement of 45m, located on the upper stream of the dam of the Laxiwa Hydropower Station, China, and directly threatening the safety of the dam and the people living downstream, was investigated. Detailed field surveys, geological structure investigations, remote sensing image analysis, and GPS displacement monitoring were carried out to investigate the deformation characteristics of the slope after reservoir impoundment, historical deformation, ancient landslide reactivation, the influence factors of the large deformation of the slope, and the mechanism. Based on the results of the above analyses, it was seen that there is a clear geological structural dependence of the failure mechanisms in the study area, and that the complex failure mechanisms include toppling, subsiding wedge failure, rockfalls, and tension cracking in deep-seated rock masses. Moreover, analysis of the remote sensing images indicated that the ancient landslide had been reactivated between 2005 and 2008. Detailed field displacement monitoring data showed that the whole slope was deforming significantly and continuously with the increase of the water level in the reservoir. The displacement rate had a positive correlation with the variation of the reservoir water level, increasing and decreasing as the the latter rose and fell, respectively. The vulnerable geological conditions, water infiltration, and the reservoir impoundment were the main factors causing the large deformation of the Guobu landslide. Water infiltration was the driving force resulting in the reactivation of the ancient landslide, and the reservoir impoundment accelerated the slope deformation. The mechanism of the large deformation of the rock slope was a combination of the upper pushing deformation induced by wedging and toppling, due to the ancient landslide reactivation, and the lower traction deformation due to the reservoir impoundment.

To investigate the effect of drag force on stability of residual soil slopes, a modified approach based on the limit equilibrium method is proposed by establishing a nonlinear coupled mathematical model to calculate the seepage velocities in soils and the runoff velocities at a fluid-solid interface. In this mathematical model, the Navier-Stokes equation and Brinkman-extended Darcy equation are used to describe the incompressible surface runoff and seepage in soils, respectively. Then, the drag force can be derived by adopting Newton's law of internal friction, and the factor of safety (FOS) of residual soil slopes considering drag force can be calculated based on the limit equilibrium method. The modified approach was applied to analyse the stability of rainfall-induced landslides in residual soils in Nanjiang, Sichuan, China, 2011. We found that the runoff depth can significantly control the FOS of the residual soil slopes and cause slope failures. The results of sensitivity analysis highlight the significance for considering drag force due to surface runoff when calculating the FOS of the residual soil slope. The FOS obtained from the modified approach is more reasonable, and this modified approach also can be used for evaluating or predicting rainfall-induced slope instability in residual soils.

Two rock topples with the opposite dip direction, observed in the Ribangliangzi hill and facing the China Provincial Highway S211, are located on the upper stream of the planned dam of Jinchuan Hydropower Station in China. The new site for submerged part of Highway due to reservoir impoundment is designed to be a tunnel crossing the Ribangliangzi hill. However, at this hill, the stratum facing the Dadu River dips southwestward, whereas the stratum facing the Xinzha Gully dips northeastward. Therefore, it’s regarded as an important slope structure that may lead to an occurrence of landslide directly threatening the future tunnel excavation stability and slope stability along the highway. This paper reports for the geologic structures of the overturned strata, formative processes and mechanism for toppling on cataclinal slopes. It was concluded that the toppling on cataclinal slopes was caused by a combination of toppling and sliding. Furthermore, to analyse the conditions for toppling on cataclinal slope, the cantilever beam model was established and analyzed the minimum strata dip angle required for toppling more prone to occur on cataclinal slope and the critical length of rock stratum. The results reveal that the toppling will be more prone to occur on cataclinal slope while the strata dip angle is greater than 60°even if under a very small external force or without the assistance of external forces.

滑坡堰塞坝是一类典型的地质灾害现象,溃坝将造成极为严重的影响.为了减少灾害的发生,滑坡坝的稳定性研究是关键,文章围绕滑坡坝的稳定性,总结了有关滑坡堰塞坝的形成条件、稳定性的影响因素及稳定性评价方法等方面的研究成果.将滑坡堰塞坝稳定性的评价方法分为定性和定量两大类,其中定量方法又分为统计学,物理模拟,数值模拟和其他等四种方法.但各种方法也明显存在诸多局限性,主要表现在评价方法不成熟,评价内容不全面等,最后针对存在的问题提出了滑坡堰塞坝稳定性研究方面的展望.

Landslide deposits have been observed to reactivate in association with water level fluctuations of reservoirs. The Shiliushubao landslide is an example of a mass movement where an accelerated deformation rate was triggered by the impoundment of a reservoir (the Three Gorges Reservoir in this case). This work investigated the relationship between the stability of this landslide and the variation in the reservoir water level. The stability of the landslide after reservoir impoundment was evaluated from February 2004 to December 2009 based on a sensitivity analysis of the hydraulic conductivity and the fluctuations in the level of the reservoir. The results suggest that the stability of the landslide is influenced by the rate of fluctuation in the level of the reservoir and the hydraulic conductivity. When the surface displacement monitoring data was integrated with the numerical modeling results, it became apparent that the stability of the Shiliushubao landslide increases as the reservoir water level is increased, whereas it decreases as the reservoir water level is lowered to avoid a dam breach during flood season.