## 2 The Structure and Working Mechanism of Water-Absorbing and Compaction
### Working Mechanism  
The development of the water-absorbing and compaction anchoring bolt is inspired by the concepts of lime pile composite foundation [31; 32; 33; 34; 35] and composite bearing arch [36; 37; 38]. Based on the principle of the New Austrian Tunneling Method (NATM) that fully mobilizes and leverages the self-supporting capacity of the surrounding rock, it comprehensively considers the wetting and dynamic stress adjustment processes of the surrounding rock after tunnel excavation disturbance. By adopting the technical principles of \"water absorption and compaction of shallow surrounding rock, suspension and anchoring of deep surrounding rock, and composite arch bearing\", it achieves the goals of water absorption, compaction, and consolidation of the shallow surrounding rock around the tunnel, the suspension and anchoring of the deep surrounding rock, and the collaborative bearing of the shallow and deep composite arches. This effectively improves the mechanical strength parameters of the loess surrounding rock, enhances the stress state of the surrounding rock, and addresses the deformation control issues of the surrounding rock. Figure 2 shows a schematic diagram of the support provided by the water-absorbing and compaction anchoring bolt for the tunnel surrounding rock. The working mechanism of the water-absorbing and compaction anchoring bolt is mainly embodied in the mechanisms of water absorption, expansion, and compaction, consolidation and drainage, and composite bearing in the reinforced zone. After the construction of the water-absorbing and compaction membrane bag, the quicklime, which is the water-absorbing medium filled in the geotextile membrane bag, chemically reacts with the moisture in the surrounding rock and soil mass around the tunnel. The hydration reaction of quicklime causes the membrane bag to radially expand, and the radial expansion pressure produces a compaction effect on the surrounding soil. The heat released from the hydration reaction is transferred and diffused to the surrounding soil, causing the temperature of the surrounding soil to rise, which further increases the permeability coefficient of the soil and enhances the drainage and consolidation effect of the soil. After drainage, compaction, and consolidation, the soil around the membrane bag  
Figure 1: Schematic diagram of the structure of the water-absorbing and compaction anchoring bolt. effectively improving the density and strength index of the soil. After the quicklime hydration reaction is completed, the tunnel circumferential expansion, compaction, consolidation, and reinforcement zone, together with the deep anchoring zone, form a composite bearing zone to jointly bear the surrounding rock load. Compared to traditional mortar bolts, the water-absorbing and compaction anchoring bolt adopts a segmented structure of \"water absorption around the tunnel and deep anchoring\", effectively solving the problem of insufficient compressive resistance or effect of the anchoring system in loess tunnels. It achieves water absorption, compaction, consolidation, and reinforcement of the surrounding rock around the tunnel, as well as the suspension and anchoring reinforcement of the deep surrounding rock, maximizing the self-bearing capacity of the surrounding rock. Its structural advantages are mainly manifested in:  
1. Novel structure: Compared to traditional bolts, the water-absorbing and compaction anchoring bolt has an ingenious design that combines anchoring support, physico-chemical improvement, and other technologies. It comprehensively considers the deformation characteristics of the surrounding rock strength under the influence of water and force, maximizing the of the surrounding rock's self-stabilization and support structure. This structure can select different water-absorbing media and anchoring agents based on different basic parameters of the surrounding rock.