During excavation and unloading of underground chambers, the mechanical characteristics and crack propagation behavior of jointed rock formations are vital for project safety and stability. This study focuses on the jointed sandy slate in the underground powerhouse of the Kala Hydropower Station in Sichuan, and conducts series of tests on non-penetrating jointed sandy slate under triaxial loading and unloading conditions. We analyzed the mechanical properties and deformation characteristics under different stress paths, explored the applicability of various strength criteria, and investigated crack propagation mechanisms using fracture mechanics. It is found that: (1) As initial confining pressure increases, axial stress and strain at failure show an upward trend under different stress paths. (2) Compared to triaxial loading, unloading paths and jointed samples decrease load-bearing capacity. Unloading from axial stress to confining pressure increases cohesion by 4.1% and unloading from confining pressure to axial stress decreases it by 30.4%. The internal friction angle increases by 3.5% and 7.3% during axial compression, while jointed samples, compared with intact samples, show a decrease of 32.9% and 53% in cohesion and a decrease of 2.2% and 10% in internal friction angle respectively. (3) In terms of characterizing the strength properties of sandy slate throughout the loading and unloading processes, the Mogi-Coulomb strength criterion outperforms both the Mohr-Coulomb and Drucker-Prager criteria. (4) The theoretical critical angle increases with confining pressure, and biconjoint samples exhibit higher values than intact ones. The theoretical critical angles range from 55° to 60° under different loading paths.

The improvement of mechanical properties of granite residual soil is of great significance for engineering disaster prevention and control. In order to investigate the improvement effect of eco-friendly biopolymers on the mechanical properties of granite residual soil, xanthan gum (XG), guar gum (GG) and their composite gum (G-X) were used to modify granite residual soil, and the effects of biopolymer type, content, curing mode, and curing age on the improvement effect and its reinforcement mechanism were investigated through unconfined compression strength, triaxial shear strength and scanning electron microscopy tests. The results show that XG, GG, and G-X can improve the compressive strength of granite residual soil, and the compressive strength of improved soil under standard curing conditions increases continuously with polymer dosage but the increase slows down, while the compressive strength of improved soil under room temperature curing peaks, and the optimal dosage of polymer is about 1.0%-1.5%; The composite G-X improved soil has better compressive strength (up to more than twice that of untreated soil) and age stability, and the shear resistance is obviously improved (cohesion is about 2.4 times that of untreated soil), which combines the bonding effect of GG and the filling effect of XG, and presents a synergistic improvement effect.

Coral sand pile foundation engineering has attracted more and more attention in the construction of islands and reefs, and the influence of its occurrence environment on the bearing characteristics of pile foundation can not be underestimated. To study the shear mechanical properties of pile-coral sand interface in different environments, a self-developed temperature-controlled pile-soil interface triaxial test instrument was used to carry out triaxial drainage shear tests of coral sand taken from the South China Sea at different temperatures, confining pressures and salinities. The results show that the shear stress-displacement curve of the pile-coral sand interface has obvious strain softening characteristics, and the temperature has no significant effect on the surface characteristics of pile and coral sand, and the temperature has little effect on the shear characteristics of the interface between pile and coral sand. There is a linear positive correlation between the confining pressure and the shear strength of the contact surface. Under 100~300 kPa confining pressure, due to the rearrangement and fragmentation of coral sand particles in the shearing process, the friction angle of the contact surface decreases, and the residual strength occupies about 70%~80% of the peak strength, and the shear strength loss is more significant under higher confining pressure; compared with the fresh water environment, the salt water environment will weaken the shear strength of the contact surface to a certain extent.

In order to study the effect of seasonal freezing and thawing on rocks under in-situ stress, taking the sandstone on the bank slope of the Three Gorges Reservoir Area as the research object, a device which can carry out prestressed loading on standard samples is designed. The physical and mechanical properties of sandstone under different prestress levels and freeze-thaw cycles are tested. The variation rules of sandstone mass, porosity, wave velocity and compressive strength under prestress and freeze-thaw cycles are analyzed, and the influence mechanism of prestress and frost heaving force on rock mechanical properties is discussed from the perspective of stress distribution around pores. The results show that the mass loss rate, pore growth rate and longitudinal wave velocity attenuation rate of sandstone are directly proportional to the number of freeze-thaw cycles, and the application of prestress will significantly improve the above damage caused by freeze-thaw cycles; The uniaxial and triaxial compressive strength of sandstone under freezing-thawing increases sharply first and then tends to be stable with the increase of prestress. The frost resistance coefficient of sandstone with prestress can reach more than 75%. The prestress in the range of 0 ~ 3MPa can improve the frost resistance of sandstone more effectively, and the existence of Confining pressure can also weaken the deterioration effect of the freeze-thaw cycle on sandstone strength; Prestress changes the stress distribution of the pores in the sandstone and forms a compressive stress zone on both sides of the pores, which can effectively resist the tangential tensile stress formed by frost heave and inhibit the damage of freeze-thaw cycle to the sandstone. The research results can deepen the understanding of the law and mechanism of sandstone damage and deterioration under freeze-thaw cycles, and provide basis for rock mass engineering construction in cold regions.

The distribution of earth pressure behind the retaining wall is an important basis for the design of wall shape and reinforcement. In order to study the distribution law of earth pressure behind the retaining wall under dynamic compaction, the physical model tests of the retaining wall under self-weight, heap load and dynamic compaction load were carried out. Through the monitoring of earth pressure at different depths, the influence of the tamping drop distance and the number of tamping on the distribution of earth pressure behind the wall was analyzed, especially the influence effect of dynamic compaction on the loose area. Based on the equivalent static method, the theoretical calculation formula of earth pressure behind the wall under dynamic compaction was modified. The results show that the soil pressure behind the wall increases instantaneously to the extreme value and then gradually decreases to stability under single tamping. With the increase of tamping times, the soil gradually becomes dense, and the corresponding soil pressure behind the wall increases gradually. The shallow soil will form the impact loose zone under the action of dynamic compaction. The soil in the loose zone becomes loose and the soil pressure value is small. The soil pressure in the lower part of the loose zone will increase rapidly and then decrease, showing a “bulging” nonlinear distribution. The soil gravity in the loose area and the thickness of the loose area, especially the latter, have great influence on the theoretical distribution curve of the soil pressure behind the wall. The modified theoretical formula of the equivalent static method of the soil pressure behind the wall considering the influence of the loose area is more in line with the actual situation.

In order to reveal the mechanical properties and energy evolution law of unloading failure of sandy slate under different joint types, the conventional triaxial compression test and unloading test were carried out by using RMT-150C rock mechanics test system. The bearing capacity, failure mode and energy evolution characteristics of jointed slate were discussed, and the damage variable was proposed based on dissipation energy. The results show that: (1) The joint type has a great influence on the bearing capacity of the specimen, and the peak strength is mainly manifested as no joint specimen > double joint specimen > single joint specimen; (2) The macroscopic failure mode of the specimen is mainly shear failure. The unloading effect will cause the single joint specimen to produce transverse cracks at the joint end, while the double joint specimen will produce penetrating cracks. (3) The total energy and dissipated energy show the law of single joint specimen < double joint specimen < jointless specimen, and the damage variable (damage degree) of the specimen is also the same. The test results are helpful to understand the mechanical behavior and energy evolution characteristics of different joint types of rock, and provide the necessary basis for the structural stability of rock mass under unloading confining pressure.

Laying a flexible cushion or backfilling lightweight soil behind the retaining wall has important engineering significance for its optimized design and cost reduction. By carrying out model tests of retaining walls under different level of loads, the magnitude and distribution characteristics of the lateral earth pressure when laying EPS flexible cushion, backfilling lightweight soil and applying it simultaneously are analyzed, and the load-reducing effect is discussed. The result shows: when the retaining wall is paved with flexible cushion, backfilled with lightweight soil, or both applied, the lateral earth pressure exhibits a “bulging” nonlinear feature that increases first and then decreases with the burial depth, and the maximum value of earth pressure generally appears at half of the wall height; under the same loading, the load-reducing effect of backfilling lightweight soil is relatively weak, achieving a reduction rate of 10%-25%, the maximum load-reducing rate of laying a flexible cushion is increased by 1.5 times compared with lightweight soil, when both are applied, the load-reducing effect is the most significant, and the maximum load-reducing rate is increased by nearly 3 times; the load-reducing effect of the flexible cushion decreases with the variation of thickness, the foam content is more sensitive to the load-reducing rate of the backfill lightweight soil. In the engineering project, a medium-thickness cushion is an optimal choice, if the applied load of the backfill is large, the content of the backfill lightweight soil can be appropriately increased.