Deep enclosing rocks exhibit non-homogeneous characteristics of dense joints and fissure development. Soft rocks subjected to multiple factors, such as high stress and hydrostatic pressure, are prone to damage and significant deformation, which can lead to instability of the surrounding rocks, failure of the supporting structure, and other accidents. In this study, we investigated the creep behaviour of deep soft rocks and siltstone-like materials subjected to different unloading confining pressures coupled with a high stress field and seepage. Subsequently, the laws and behaviours of parameters, such as the transient strain, creep deformation, and creep rate were derived and analysed under various conditions. The results indicate that the radial creep curve exhibits a variation pattern similar to that of the axial creep. However, the extent of radial creep exceeds that of axial creep in soft siltstone-like rocks under unloading confining pressure conditions. We derived expressions for the constitutive relation of siltstone-like specimens under various unloading confining pressure conditions in the presence of seepage using an improved viscoelasticmodel that considered the coupling effect of fissure and seepage flow. The correlation coefficients of the calculated model values with the experimental values, as obtained by the non-linear least-squares fitting, were all above 0.9178, indicating that the proposed model can accurately characterise the creep process in fissured siltstone.

The strength of the roadbed composed of soft rock decreases following water immersion. Cement slurry is mixed with a soft rock to strengthen the soft rock roadbed to address this problem. The uniaxial compressive strength was determined, and the triaxial compression tests were conducted to analyze muddy sandstones mixed with different amounts of the cement slurry of the same composition. Some of the cured samples were soaked in water, and the samples were analyzed. The results were compared with the results obtained by analyzing unsoaked samples. The MIDAS/GTS NX software was used to develop a three-dimensional finite model to simulate the settlement patterns of roadbeds developed in the presence of natural water, saturated water, and muddy sandstone reinforcements. The experimental results have been presented and discussed. (1) When the filling rate corresponding to the cement slurry is 32%, the uniaxial compressive strength of muddy sandstone containing natural water reaches 3.32 MPa, which is approximately 5 times higher than the strength of muddy sandstone devoid of cement slurry. (2) The water absorption energy of the reinforced muddy sandstone was analyzed from the perspective of saturated water content. The water plugging effect observed after the addition of the cement slurry was studied, and the equations defining uniaxial compressive strength, elastic modulus, and free saturated water content were established. (3) An increase in the content of the cement slurry admixture resulted in a change in the nature of the partial stress–strain curves generated for muddy sandstone. The curve change from strain-hardening to strain-softening type and the deformation modulus and extent of cohesion realized increased exponentially with an increase in the cement slurry filling rate. A positive linear correlation was observed between the internal friction angle and the cement slurry filling rate. (4) An increase in the cement slurry filling rate corresponding to the saturated muddy sandstone resulted in an exponential decrease in the settlement deformation value. The vertical displacement decreased to 0.3497 mm when the grouting rate reached 32% under conditions of saturation. The deformation tended to converge and stabilize, indicating that the introduction of the admixture consisting of the cement slurry could help in effectively controlling the property of settlement deformation observed in the fully weathered muddy sandstone roadbed. The results helped in the scientific evaluation of the reinforcement effect. The reported results also helped develop a theoretical basis for the development of soft foundation reinforcement.