The research on early warning of gradual-change landslides has made great progress，and early warning and prediction have been realized in many cases. But the early warning and prediction of sudden-onset landslides are still impotent. Focusing on the internal and external response relationship between the stress release from the rupture of the internal rock and soil body and the coordinated deformation of the rock body on the slope surface in the evolution of landslides，we study the characteristics of the dynamic change of the stress-sliding force-deformation in order to obtain the precursor information of sudden-onset landslide destabilization. In the process of landslide from static deformation to dynamic slip，the potential slip surface is initially deteriorated by local rupture，and then accelerated by the rate effect after the overall penetration. Thus a parameter deterioration model considering the local rupture and accelerated deterioration of the slip surface is constructed. Based on the vector sum method，the landslide resistance and decline force are calculated，and a time-dependent degradation mechanical model considering the deterioration of the mechanical parameters of the slip zone is established. The model is used to calculate the force and deformation characteristics of the whole process of landslide formation and evolution，revealing the stage characteristics of stress-slip force-deformation in the process of landslide formation. The results show that landslide stress，slip force and deformation indicators show obvious stage characteristics，which can be divided into initial，isokinetic，accelerated and near-slip stages. When the landslide enters the near-slip stage，the sudden drop in slip resistance and stress is earlier than the rapid increase in landslide deformation，which is helpful for early warning and prediction of sudden-onset landslides.

The tailwater outlet slope on the left bank of Wudongde hydropower station，as a key slope shared by the diversion outlet，tailwater outlet and flood discharging outlet，is characterized by the steeply dipping and bedding structure with thin layers，the hidden passage of a long-growth fault and the crossing of tunnels. This paper firstly analyses the reasons for the sudden deformation of the slope-tunnel system caused by the key problems encountered in geology，design and construction，and the corresponding treatment methods are discussed systematically. Through the implementation of a series of comprehensive measures，including systematic reinforcement，support by concrete retaining walls on the open face，strict adherence to the excavation sequence of “slope before tunnel”，strict limitation of the mass vibration rate of blasting，and strengthening of the construction of external waterproofing system，the excavation of the tunnels through the slope has been successfully completed under controlled deformation monitoring，and the excavation stabilization of the slope-tunnel system is solved after the emergence of sudden deformations during the construction period. Secondly，it is verified by numerical simulation analysis that the dynamically adjusted support measures can meet the safety requirements of the slope during the construction and operation periods. Finally，the overall response strategy and specific process after the sudden deformation of the slope-tunnel system are systematically elaborated. The long-term monitoring results show that the tailwater outlet slope has been operating safely for more than nine years. The research results have important reference significance for the excavation reinforcement and stabilization of similar projects.

To investigate the influence of rock bridge length and water content on the mechanical properties and damage evolution patterns of sandstone，uniaxial compression tests were conducted on prefabricated double-fracture sandstone samples with varying rock bridge lengths under both dry and saturated conditions. Acoustic emission(AE) and digital image correlation(DIC) techniques were used to monitor the internal damage and surface crack propagation of the sandstone in real-time. The results indicate that the strength of the fractured samples is lower than that of the intact samples，and the longer the rock bridge length，the greater the uniaxial compressive strength of the samples. Saturated samples exhibited lower strength compared to dry samples. Fractured samples will show stress drops during the failure process，which corresponded to peaks in AE counts. As the rock bridge length increased，the first stress drop gradually approached the stress peak. The AE count patterns of the saturated samples were consistent with those of the dry samples. The AE localization events and DIC strain anomaly regions in both saturated and dry samples initially concentrated at the rock bridge area and then gradually developed along the prefabricated fractures towards the sample boundaries. These findings contribute to the monitoring and early warning of rock instability and failure in water-rich conditions.

An experimental device for energy storage and seepage prevention in underground space was developed to study the deformation，damage characteristics and disaster-causing mechanism of the surrounding rock of an abandoned mine energy storage reservoir，and perform a safety assessment of the underground energy storage reservoir in the fissured rock body. The similarity function of the underground space?s energy storage and seepage prevention was reestablished based on similarity theory. The experimental design basis was obtained and the experimental setup was created using the modular design idea. The experimental device includes a large-tonnage loading frame，a specimen loading frame，hydraulic power source components，a water pressure loading module，an air pressure loading module and a monitoring and control system. Finite element software was used to simulate the large-tonnage loading frame and determine the design parameters. The large-tonnage loading frame with a hydraulic power source assembly enabled the simulation of the stress field of large-scale rock-type specimens(500 mm×500 mm×500 mm). It was equipped with the water pressure loading module that enabled the simulation of pumped storage processes in the underground space. Simulation of the whole process of compressed air energy storage in the underground space was performed using the air pressure loading module. The monitoring and control system realized the overall monitoring and control of the device. The experimental device was utilized to implement pumped energy storage tests in the underground space and compressed air energy storage cycle tests to verify the accuracy and reliability of the experimental device. The results show that the experimental device can be used in experiments on the mechanics of peripheral rocks under the action of cyclic air and water loading under complex stress conditions. It can provide theoretical guidance and technical support for the safe construction and operation of abandoned mine energy storage projects and serve as an important experimental test platform for the development and utilization of resources in abandoned mines.

In order to explore the influence of different CO2-load coupling effects on the mechanical properties and meso-damage and failure characteristics of coal mass，the uniaxial loading tests of raw coal under different CO2 pressures were carried out by using industrial CT scanning system，MTS 816 rock mechanics loading device and self-developed gas-solid coupling test device. The influence of CO2 pressure on the mechanical properties and failure mode of coal was analyzed. Based on image processing technology，the meso-damage evolution characteristics and crack propagation laws of coal bodies are explored from two-dimensional and three-dimensional scales respectively，and the mechanical response mechanism of coal bodies to CO2-load coupling is revealed from the macro-meso-level. The results indicate that：(1) the stress-strain curve of coal mass under gas-solid coupling can be divided into four typical stages，and both peak strength and elastic modulus exhibited a degradation trend with increasing initial CO2 pressure. (2) Coal fracture expansion can be divided into fracture adsorption and expansion stage，fracture compaction and closing stage，new fracture initiation and development stage，and fracture rapid expansion and penetration stage，the fracture volume，fracture rate and three-dimensional fractal dimension are positively correlated with CO2 pressure，additionally，influenced by the CO2 gas wedge effect，the spatial distribution of fractures inside the loaded coal mass becomes complex，and its failure form gradually changes from a single tension failure to a compound failure in which tension and shear coexist. (3) The damage variable was defined based on three-dimensional porosity，and it was found that its value gradually increased from 0 to 0.34，approximately exhibiting an exponential rise. By comparing the peak strength of the coal under gas-solid coupling with the theoretical strength obtained from the damage variable，the rationality of the damage variable was validated.

The mud pumping mechanism and dynamic characteristics of ballasted track subgrade under intermittent train load and rainfall wetting coupling are still unclear. A mud pumping model test in ballasted track subgrade under intermittent load-wetting coupling was conducted. The experimental results indicate that the cyclic loading has a promoting effect on the increase of volumetric water content and pore water pressure，and the promoting effect weakens with the increase of dry density. The accumulated deformation and particle migration mainly occur in the LS1-LS3 stages，and the changes are most significant in the LS2 stage because of the high saturation and low density of the soils. In the subsequent loading stages，only a small amount of accumulated deformation occurs due to the high density of the soils，and the phenomenon of particle migration is not obvious. A small resilience occurs during the intermittent period(IS4-IS7 stage). At the end of the experiment，the ballast void contaminant index(VCI) is 27%，which is close to the critical value of 40% for railway maintenance and needs to pay attention.

Directional pre-splitting technology is conducive to the precise caving of rock mass and the control of stope stress distribution. Existing directional fracture technologies mainly include directional water jet，directional hydraulic fracturing and convergence energy blasting，but these methods suffer from small fracture scales and poor directional effects. In this paper，a directional roof slitting by composite blasting(DRSBCB) based on “jet + detonation” was proposed. A continuous directional fracture surface was formed by utilizing the perforating charge?s shaped charge jet at the microsecond level and the explosive's high-pressure splitting effect at the millisecond level，achieving precise fracture creation. The key equipment and process flow of the DRSBCB were introduced. The mechanical model of shaped charge jet and secondary blasting was established. The mechanism of the directional jet penetrating rock and the secondary blasting high pressure splitting was obtained. The theoretical calculation formulas of the directional perforation depth of shaped charge jet was provided，and the theoretical expression of the directional crack propagation length during secondary blasting process was given. The mechanism of the DRSBCB was revealed. The numerical model of jet penetrating rock was established by using LS-DYNA numerical simulation software. The jet channel expansion law and rock damage characteristics of the whole penetration process were obtained，and the evolution law of jet morphology，jet velocity and jet channel length in different periods was revealed. Based on the jet penetration model，the numerical model of secondary blasting was established，and the whole fracture propagation process of high pressure splitting in secondary blasting was clarified. The evolution law of crack length，crack propagation speed and rock damage with time was obtained. The characteristic of whole fracture propagation process of the DRSBCB was revealed. Additionally，the feasibility of the DRSBCB was verified through ground concrete target test，and industrial experiment was carried out in mine underground roadway. The directional fracture rate of 91%，89% and 61% in the hole was obtained by borehole peeping. Through the research results of the paper，the feasibility of the directional fracture theory of the DRSBCB was proved. This approach offers a new technical method for precise control of coal and rock mass in coal mines.

A thorough investigation into time-dependent water-rock interactions is of significant importance for the prevention and control of engineering instability disasters. In this study，laboratory uniaxial compression tests，Brazilian splitting tests，and direct shear tests were conducted on sandstone，marble and granite specimens with immersion times of 30，90 and 180 d，respectively，to explore and reveal the characteristics and mechanisms of the effects of different immersion times on the strength and failure characteristics of rock masses. The results show that after immersion，the mechanical strength of sandstone specimens decreases significantly，and the decreasing trend gradually slows with increasing immersion time. In contrast，the compressive strength of marble and granite specimens decreases significantly after 30 d of immersion，but shows an increasing trend after 90 d of immersion，followed by a subsequent decrease. Similar trends are observed for their shear strength. These changes may be due to the dissolution and weakening of the cement between mineral particles inside the rock，coupled with the local strengthening of strength under non-draining conditions of water-saturated pores inside the rock mass. Analysis of the energy characteristics of the specimens revealed that the total input strain energy and elastic strain energy(stored energy capacity) of sandstone and granite specimens both show a decreasing trend after immersion，while the change in marble specimens is not significant. This suggests a reduced severity of ejection failure in granite specimens after immersion. Based on these findings，a shear brittleness index(Bs) was proposed to evaluate the brittleness characteristics of rock joints. The study found that with increasing immersion time，the brittleness of sandstone，marble and granite joints all show a decreasing trend. The results of this study can provide important references for evaluating the stability of engineering rock masses under water-rock interactions.

When the frequency of external disturbances approaches the natural frequency of a block rock mass，a quasi-resonance phenomenon occurs. This phenomenon results in a sharp change in the strain of the weak interlayers between rock blocks，accelerating the dynamic damage and fracture of these interlayers. To investigate the dynamic tensile-compression damage and fracture patterns of weak interlayers caused by quasi-resonance in block rock masses，the deformation characteristics of weak interlayers under quasi-resonance conditions were analyzed based on a block rock mass model. Combining the rock dynamic damage fracture criteria，a theoretical and computational analysis of the dynamic damage and fracture of weak interlayers under quasi-resonance was conducted，further studying the dynamic failure patterns. Numerical simulations revealed the damage evolution characteristics of weak interlayers in block rock masses under quasi-resonance conditions. The results indicate that during quasi-resonance in block rock masses，the relative displacement between rock blocks sharply increases，significantly enhancing the strain and damage degree of the weak interlayers. The damage degree of the weak interlayers is highest at low-order quasi-resonance frequencies and is influenced by external disturbance forces and system parameters such as the stiffness，viscosity，and thickness of the weak interlayers and the mass of the rock blocks. When quasi-resonance occurs in block rock masses，the fracture stress increases，but the time from fracture to complete failure is significantly shortened，indicating a substantial decline in residual bearing capacity. This study provides a valuable reference for understanding the dynamic fracture behavior of block rock masses.

To investigate the impact of high-temperature water on gneiss characteristics，the uniaxial compression and Brazilian splitting tests under various high-temperature water bath conditions were performed. Subsequently，the evolution mechanisms of mechanical properties and macroscopic failure characteristics of gneiss were analyzed. Meanwhile，the acoustic emission(AE) monitoring technology，X-ray diffraction(XRD) and scanning electron microscopy(SEM) were utilized to explore the microscopic mechanism of rock failure. The experimental findings reveal that high-temperature water baths significantly diminish the mechanical properties of gneiss，with a more pronounced effect observed with increased water bath time and temperature. Additionally，the failure mode of rocks is also impacted. As the water bath time increases，the failure mode of gneiss transitions from single oblique shear failure to compressive conical shear failure and eventually to shear failure accompanied by tensile cracks，under the conditions of no water bath at room temperature，water bath at 60 ℃ and 92.6 ℃. Moreover，high-temperature water accelerates the weakening effect of the mechanical properties of gneiss compared with room temperature water immersion. This research provides essential theoretical support and engineering guidance for tunnel construction and disaster prevention in high-temperature water environments.

Strength theory serves as the foundation for disaster prevention and control in geotechnical engineering，but the applicability of classical strength theories under complex stress states still has certain deficiencies. Focusing on the evolution mechanism of rock energy storage limit，and based on the existing theory，the evolution model of rock energy storage limit under complex stress state is established by introducing parameters ? that can reflect the influence of Lode angle. The analysis of four special stress states reveals the phenomenon of unequal tensile and compressive strength of rocks and possible failure under hydrostatic pressure. Based on the rock energy storage limit evolution model，a new rock strength criterion is established，which can be degenerated into the D-P strength criterion，and effectively addresses the deficiencies of the D-P strength criterion，namely the lack of Lode angle effect and the oversized tensile-shear region. Verification using true triaxial experimental data of four types of rocks shows that the newly established strength criterion exhibits high calculation accuracy，with an average error ranging from 2% to 12%，while the D-P strength criterion exhibits an average error ranging from 18% to 51%. Compared with other strength criteria，the strength criterion established from the perspective of rock energy storage limit has clear physical meaning and high accuracy，which holds reference significance for the study of rock failure behavior.

Fracture roughness and spatial distribution significantly influence the anisotropic behavior of rock masses. Traditional fabric tensor models often simplify fractures as planes，whereas natural fractures exhibit rough surfaces. This study aims to modified the existing fabric tensor and derive a detailed method for calculating the equivalent elastic mechanical parameters of rough discrete fracture networks(RDFN). By introducing fractal theory，a case model is constructed，and a comparative analysis between planar discrete fracture networks(DFN) and RDFN is conducted. To further enhance the reliability of the study，a simple model is generated using numerical software，and both analytical and numerical solutions of the model are computed. The results indicate that：(1) the modified method that considers roughness effectively mitigates the limitations of the fabric tensor. (2) The RDFN case model constructed based on the W-M fractal function exhibits significant rough characteristics，with the fractal dimension (D) and fractal roughness(G) greatly impacting the surface roughness of fractures. (3) Compared to the DFN model，the inherent roughness influences the equivalent elastic mechanical parameters of RDFN models by approximately 5% to 15%. (4) The numerical solutions of the simple numerical models closely match the analytical solutions，with minimal error，validating the accuracy of the constitutive relationship based on the modified fabric tensor. These findings provide valuable insights for analyzing the equivalent elastic behavior of rock masses with rough fractures.

In view of the shortcomings of most rock unsteady creep models under conventional triaxial compression conditions，such as complex functions，numerous parameters and difficulty in determining，based on 31 sets of unsteady creep test data of different types of rocks，the strain functions and time functions were constructed，and the linear relationship between the two was statistically analyzed. Based on this，a new empirical model for unsteady creep of rocks was established，using creep test data of shale，marble，mudstone and sandy shale at different stress levels to verify the accuracy and rationality of the model. The results show that the empirical model function of rock unsteady creep based on statistical laws is a unified expression containing only four model parameters. Its creep curve is approximately an inverse S-shape，and the creep velocity curve is approximately a positive U-shape，both of which are consistent with the actual rock creep laws. The creep test results of shale，marble，mudstone，and sandy shale at different stress levels are highly consistent with the theoretical curve of the empirical model. Its rationality and accuracy have been verified. The model can not only describe instantaneous elastic strain，attenuation creep，and constant velocity creep，but also describe accelerated creep with particularly obvious nonlinear characteristics.

Differences in meso local expandable displacements within the specimen after heating of the plunger rock samples lead to significant microcracking non-homogeneity，so local expandable displacement constraints are also one of the necessary considerations when investigating the mechanical behavior of rocks after heating. Using sandstone as an example，the crack features by multi-point measurements of heated rock slices were determined first. Furthermore，digital cores are reconstructed using the polycrystalline discrete element method，and thermal damage simulations are carried out to investigate the mechanism of microcrack formation in sandstone as well as the variation characteristics of mechanical parameters under the combined influence of displacement constraints and temperature. The research findings are as follows. (1) Heating temperature and expandable displacement influence the development of intragranular cracks，resulting in preferential orientations along cleavage planes，conjugate shear cracks in a crossed pattern，and increased crack density within small particles. (2) After ? 25 mm×50 mm rock samples were heated at 200 ℃–1 000 ℃，the maximum expandable displacement increased linearly with temperature，but the effect of temperature on the dispersion of expandable displacement values was not significant. The number of cracks increased linearly with the expandable displacement at the beginning，and then slowed down to a stable level. 800 ℃ heating，the number of intra-grain tensile cracks exceeded the inter-grain tensile cracks in the highest proportion，while the intra-grain shear damage almost did not occur below 1 000 ℃. (3) Higher temperatures inhibit the promotion of microcrack development by expandable displacements. For example，after heating at 400 ℃，the microcrack growth rate is 668.3%，but it drops to 12.8% after heating at 1 000 ℃. As the expandable displacement increases，the non-uniform development of microcracks makes the rock macroscopically non-homogeneous，and the structure is more deteriorated leading to a decrease in mechanical parameters.

The explosive replacement method(ERM) is one of the effective methods to treat the deep soft soil in coastal area. The model tests of explosive replacement were carried out to investigate the post-explosion settlement evolution pattern，and a integrated rockfill was used to represent the crushed rock to reduce the measurement error caused by the randomness. The settlement behavior of the rockfill with different rockfill sizes and blasting design parameters was analyzed. Then，the settlement prediction methods were discussed. The research results indicate that the wider or higher the rockfill cross-section，the greater the post-explosion settlement of the rockfill，and the deeper the treated silt depth，but the longer the settlement stabilization delay. The greater the depth of explosive burial，the post-explosion settlement of rockfill tends to decrease. There is an optimal depth of explosive burial. Combining the results of field measurements of a subgrade project of S201 Trunk road of Nation and Province in Ningde，Fujian Province，China，the predicting method of post-explosive settlement of the subgrade was proposed by taking the coefficient of determination，prediction error，and final settlement as the indicators of the method validity assessment based on on-site measurements. The field and experimental settlement results were compared and analyzed to verify the reasonableness of the prediction model. The study can provide technical guidance for relevant engineering practice.

In order to investigate the strength loss and strength recovery characteristics of the sludge solidified soil after crushing and remodeling，a series of unconfined compressive strength(UCS) tests were conducted. The effects of curing agent content，initial water content of sludge，pre-curing age，two-stage solidification method，crushing degree and post-curing age on the strength and deformation characteristics of the remodeled solidified soil(RSS) were studied. In addition，the comparative analysis was carried out with that of solidified soil. The results show that the strength of the initial RSS tends to be stable as the strength of the solidified soil is greater than 600 kPa. Shortening the pre-curing age and increasing the amount of curing agent are conducive to the recovery of the strength of the RSS in the later stage，and the final strength of the RSS can be restored to 40%–70% of the strength of the solidified soil. For the two-stage solidified RSS，the strength is generally smaller than that of one-stage solidified RSS when the total amount of curing agent kept the same，and the higher the agent content used in the first stage solidification，the more obvious the strength decrease. For the influence of crushing degree，the initial strength of the two-stage solidified RSS after strong crushing is less than that of the normal crushing，but the later strength increases with the increase of the agent content used in the second stage solidification，and even be greater than that of the later strength after normal crushing. For the relationship between UCS and failure strain of the RSS，the failure strain decreases with the increase of strength as the UCS of the RSS is less than 200 kPa. As the UCS of the RSS is greater than 200 kPa，the failure strain is close to that of the solidified soil between 1%–2%. Finally，the relationship between the deformation modulus and the compressive strength of RSS and solidified soil is basically the same，which agrees linear relationship of E50 = 73.8qu.

This study evaluates the effects of microbial processes in improving the swelling and shrinking properties of expansive soil. Elemental microanalysis，X-ray diffraction，Fourier-transform infrared spectroscopy and soil swelling-shrinkage tests were conducted to explore the changes in composition，content and structure of the mineral aggregate of expansive soil in bio-mediated treatment process，as well as the effects of microbial-induced mineral phase transformations on the swelling and shrinking characteristics of the soil. The experimental results indicate that cations Si4+，Al3+，K+，and Mg2+ in the microbial-soil reaction liquid continuously leached out over time. Notably，the concentration of K+ exhibited an initial increase followed by a decrease，reaching a peak at 14 days. The Si-O，Al-O and -OH bonds within montmorillonite crystals were also affected. It was suggested that K+ ions entered the interlayers of montmorillonite crystals，disrupting their structures. After 14 days of microbial activity，an illite-smectite mixed-layer peak appeared at 2θ = 9.882° in the expansive soil samples，and an illite peak was observed at 2θ = 48.07° after 28 days. Calculations using the K-value method proved a decrease in montmorillonite content and an increase in illite content. The free swelling rate and load swelling rate of the expansive soil decreased by 69.01% and 24.04%，respectively；the linear shrinkage rate and volumetric shrinkage rate decreased by 32.9% and 30.98%，respectively. The study reveals that Paenibacillus mucilaginosus effectively promotes the transformation of montmorillonite to illite and improves the swelling and shrinking properties of expansive soil，which provides a novel approach towards modifying some engineering properties of soils.

Aiming to investigate the gas permeability of unsaturated soil and its underlying micro-mechanism，the gas permeability coefficients of compacted loess under various initial states，along wetting/drying，and constant stress ratio compression paths were examined in this study by utilizing a modified gas permeation device. The Mercury Intrusion Porosimetry(MIP) technique was employed to further examine the microstructure changes of compacted losses，thereby analyzing the micro-mechanism of air permeation. The test results indicate that the gas permeability coefficient(keff) of compacted loess varies within the range of 10－12 to10－15 m2 in response to the increasing compaction saturation degree(Sr0). At low dry densities，keff exhibits an initial rise followed by a decline as Sr0 increases，whereas at high dry densities，a nonlinear decrement is observed. Wetting significantly reduces keff of compacted loess by up to three orders of magnitude，with a rapid decrease after the wetting saturation degree reaches 0.65. Conversely，drying improves gas permeability，but its impact is much less significant than that of wetting. Under constant stress ratio compression，keff decreases exponentially with increasing stress，and the decrease is more pronounced at lower stress ratios. The MIP test results reveal that the macro porosity first increases and then decreases with the increase of Sr0. Wetting has a minor effect on the pore size distribution curve(PSD)，while drying can increase macro porosity. Under similar state，the as-compacted soil exhibits more macropore structures compared to the after-wetting soil，while less macropore structures compared to the after-drying soil. The constant stress ratio compression results in a reduction of macropores，with a greater reduction at lower stress ratio. According to the variation of keff，the gas permeability of unsaturated compacted loess depends on the amount of the interconnected macropores，the more macropore structures，the better pore connectivity，and the better gas permeability. A pore structure paramesed，and a micro-scale air permeation model considering the pore structure parameter is established. The applicability of the proposed model is then verified through experimental data.

To study the influence of the thickness of the underlying karst cave roof on the bearing characteristics of the rock-socketed pile，the load test of the model pile under different roof thickness conditions(1D–5D) was carried out. The failure mode of the karst cave roof，the load-displacement curve，the distribution characteristics of the axial force and resistance of the pile，and the variation of the ultimate bearing capacity with the roof thickness were analyzed. The calculation model of the ultimate bearing capacity of the karst cave roof is established. Based on the roof failure mode and the H-B criterion，the calculation formula is derived. The results show that：(1) when the roof thickness is 1D and 2D，the roof punching failure；when the roof thickness is 3D and 4D，the roof is bent-pulled-punched combined failure；when the roof thickness is 5D，the roof is flexural failure；the critical thickness of the underlying karst cave roof is 5.9D. (2) The load-displacement curve of the test pile is a steep change type，and the load-displacement relationship is linear before the steep change point appears. The pile top displacement corresponding to the steep change point of the load-displacement curve is(0.215‐0.3)D. The code takes 0.05D displacement corresponding to the load as the ultimate bearing capacity of the pile is conservative. (3) For every 1D increase in the roof thickness，the ultimate bearing capacity of the test pile is significantly improved，but the increasing extent is gradually reduced from 158% to 35%. When the roof thickness exceeds 2D，the bearing ratio of the pile end exceeds 90%. (4) The proposed calculation method of the ultimate bearing capacity of the pile of the underlying karst cave is compared with the experiments. The error range of results is 3.29%‐15.62%. They are in good agreement.

The bearing performance of pile foundations in marine environment island and reef engineering construction is often closely related to the fragmentation characteristics of coral sand. It is of great significance to clarify the impact of coral sand particle fragmentation on the bearing capacity of pile foundations. Based on this，a series of HSCA expansion pile model tests and field tests were conducted to explore the effects of the normal stress of the expansion pile and the compactness of the coral sand on the improvement of pile side friction resistance. The research results show that：(1) incorporating HSCA high-strength expansion agent can significantly improve the normal stress on the pile side and the compactness of coral sand around the pile，thereby greatly improving the pile side friction resistance and overall pile bearing capacity. When the expansion agent dosage reaches 15%，the ultimate lateral resistance of the expansion section of the pile increases by 69.7%，and the ultimate bearing capacity of the entire pile increases by 38.5%. (2) Considering the cumulative stress loss effect due to particle breakage，a stress transfer model for the expansion of coral sand on the side of expansion piles considering particle breakage is proposed. Verification analysis demonstrates that the model accurately reflects the transfer law of expansion stress in coral sand. (3) The expansion pile significantly improves the compactness of coral sand on the pile side. The self-expansion of the pile body effectively fills the shrinkage space in coral sand caused by particle breakage，thereby enhancing the bearing capacity of the coral sand foundation around the pile. (4) A prediction model for lateral friction resistance of expansion piles in coral sand foundations was established and its engineering applicability was verified through field experiments. The findings of this research can serve as a reference for the construction design of expansion piles in coral sand foundations.