In order to study the effects of dry density and initial water content on the permeability characteristics of loess, the saturated permeability tests of undisturbed loess and remolded loess were carried out in Lanzhou New Area. The anisotropy of undisturbed loess under two factors is analyzed and compared, and the permeability difference between undisturbed loess and remolded loess is also analyzed. The results show that the structure of loess has an important impact on the permeability characteristics, and the dry density is an important structural parameter. The natural structure of undisturbed loess has obvious anisotropy. Under different pore conditions and water contents, it will show the anisotropic behavior of permeability. The influence of water content on permeability is mainly reflected in the influence on the properties of cement between soil particle skeletons. Under different dry density conditions, the influence of water content on permeability is different. When the pore structure is relatively compact, the influence of water content is more obvious. In practical engineering, these two factors need to be comprehensively considered in order to accurately describe the permeability distribution of loess. This study can provide some theoretical guidance for the construction of relevant loess projects.

In response to a series of engineering disasters encountered during the excavation and support construction of loess tunnels, considering the issues of water enrichment in surrounding rock induced by excavation disturbance and system bolt failure, drawing on the concepts of lime pile composite foundation and composite bearing arch, and based on the principle of the New Austrian Tunneling Method (NATM) that fully mobilizes and leverages the self-supporting capacity of surrounding rock, this study comprehensively considers the wetting and stress adjustment processes of the surrounding rock after excavation disturbance in loess tunnels. By adopting the technical principle of “water absorption and densification of shallow surrounding rock, suspension and anchoring of deep surrounding rock, and composite arch bearing”, a new type of water-absorbing, densifying, and anchoring bolt was developed that can reduce the water content of surrounding rock while enhancing its resistance. To further investigate the water absorption, densification effect, and pull-out bearing characteristics of this new bolt, laboratory model tests were conducted, examining the temperature, pore water pressure, densification stress of the soil around the bolt, as well as the physical properties of the soil in the consolidation zone. The test results indicate that a cylindrical heat source forms around the water-absorbing, densifying, and anchoring bolt, significantly inducing the thermal consolidation of the surrounding soil. The variations in temperature, pore water pressure, and densification stress of the soil around the bolt truly reflect the qualitative patterns of hydro-thermal–mechanical changes during the water absorption, curing, and exothermic reaction processes. The water absorption and densification segment of the bolt effectively enhances the density of the soil in the water absorption, densification, and consolidation zone, improving soil strength parameters. Compared to traditional mortar-bonded bolts, the water-absorbing, densifying, and anchoring bolt exhibits a greater pull-out bearing capacity. The research findings provide important guidance for the theoretical design and engineering application of this new type of bolt.

Tunnel excavation in water-rich and saturated loess layers often encounters a series of engineering disasters, including surface settlement, large deformations of surrounding rock, collapses, water inrushes, mud inrushes, and lining cracks. This paper presents an analogy of 16 cases of instability and collapse of surrounding rock during the excavation of water-rich loess tunnels in China’s loess regions. The weight of influence of various factors affecting the stability of surrounding rocks has been analyzed based on the Fuzzy Analytic Hierarchy Process (FAHP), addressing the engineering challenges encountered during the construction of the Tuanjie Tunnel. Measures such as deep well-point dewatering of the surface, reinforcement of locking foot anchors, and construction treatment with large arch feet are proposed. The effectiveness of these treatments is then monitored and analyzed. The results show that after 30 days of dewatering, the average water content of the surrounding rock decreased from 28.8% to 22.3%, transforming the surrounding rock from a soft plastic state to a hard plastic state. Phenomena such as mud inrushes at the tunnel face and water seepage through the lining are significantly reduced, and the self-stabilizing capacity of the surrounding rock is markedly improved. By optimizing the excavation method and enhancing support parameters, the construction progress rate for Grade VI surrounding rock has increased from 10–15 m per month to 40 m per month, validating the effectiveness of the proposed measures.

The Cambridge model reveals the basic characteristics of saturated remolded soil and establishes the constitutive equation of soil, which is one of the most important models in soil mechanics. However, the Cambridge model is still a phenomenological macroscopic model, which cannot consider the influence of soil microstructure. In this paper, based on the basic theory of complex material mechanics, a microscopic model of materials is established by point element method, and the governing equations of strain energy density function and state set function of material structure are obtained. Through simplified analysis, an envelope surface equation describing the set of structural states of the material is obtained. It is found that the envelope surface equation is similar to the Cambridge model critical state surface equation. This state envelope surface can not only reflect the similar characteristics of the Cambridge model, but also introduce the microscopic characteristic parameters of materials, so that the Cambridge theory describing the macroscopic properties of materials can be developed into a macro-microscopic method describing the properties of materials at multiple scales. More importantly, the Cambridge model is built based on experiments, and the macromicro equation obtained here is derived from the basic theory of complex material mechanics.