Xiao Feng

The construction of foundation pits in soil-rock composite strata presents significant challenges due to the mechanical incompatibility between overlying soils and underlying rock masses, often causing abnormal stress redistribution and local instability in conventional pile–anchor support systems. This study proposes a micropile and bored pile frame composite retaining system, wherein reinforced concrete bored piles constitute the primary bidirectional spatial frame and grouted steel micropiles serve as auxiliary members for inter-pile soil retention. The collaborative load-transfer mechanism is analytically characterized through Winkler elastic foundation theory, yielding a stiffness ratio of η = 16.2 between bored piles and micropiles. Field validation was performed at Hou Pantao Village Station on Xuzhou Metro Line 3, wherein full-field strain evolution was monitored using BOTDR distributed optical fiber monitoring throughout staged excavation to a final depth of 17.06 m, with subsequent comparative verification against three-dimensional finite element computations (MIDAS GTS NX). Results demonstrate that: (i) structural deformation concentrates predominantly within the upper 5-m soil layer, with negligible displacement below the rock surface; (ii) micropile peak bending moment migrates downward with excavation, stabilizing near the soil–rock interface; (iii) overburden thickness dominates system response, with a critical threshold at 5 m, beyond which micropile horizontal displacement increased from 5.32 mm to 19.80 mm, matching the field-monitored range(5.70–23.3 mm); (iv) reducing bored pile spacing from 9 m to 4 m decreases maximum micropile displacement by 42.1%, whereas increasing pile diameter yields only local stiffness enhancement without global deformation control.

The seepage diffusion of cement grouting materials into a sandy medium is influenced by the skeleton’s adsorption and the pore channels’ tortuosity, resulting in heterogeneous retention of cement particles during migration. This study established a theoretical model for the filtration coefficient based on the mass balance equation and linear filtration law. Grouting tests were conducted to determine the density of the cement slurry at various diffusion positions, and the filtration coefficient was calculated using the theoretical model. Results indicate that the filtration coefficient varies dynamically along the diffusion distance rather than remaining constant. The surface filtration range of Grade 42.5 Portland Cement slurry in sample S1 is approximately 30 cm, with a final diffusion distance of 190 cm. In contrast, the surface filtration ranges for the 800 mesh superfine cement in S2 and the 1250 mesh superfine cement in S3 are less than 10 cm, resulting in final diffusion distances of 69 cm and 87 cm, respectively. This demonstrates that a longer surface filtration range in the sand sample corresponds to a farther final diffusion distance of the slurry. Additionally, a larger ratio of sand pore diameter to cement particle size results in a smaller filtration coefficient and a greater slurry diffusion distance. Under a constant water–cement ratio, smaller cement particle sizes are associated with decreased slurry fluidity, which reduces the diffusion of cement slurry within the sandy medium. The research findings provide valuable insights for designing borehole spacing in grouting treatment for sandy media.

Superfine cement is widely used in building reinforcement and repair, special concrete manufacturing, and environmental protection engineering due to its high toughness, high durability, good bonding strength, and environmental friendliness. However, there are some problems in superfine cement slurry, such as high bleeding rate, prolonged setting time, and consolidated body volume retraction. In this article, on the premise of using the excellent injectability of superfine cement slurry, the fluidity, setting time, reinforcement strength, and volume expansion rate of novel expansive superfine cement slurries with varying proportions were analyzed by adding expansion agent UEA, naphthalene-based water reducer FDN-C, and triisopropanolamine accelerating agent TIPA. The results show that under most mix ratios, the bleeding rate and fluidity of the novel superfine cement slurry initially increase and decrease with rising water-reducing agent dosage. The initial setting time generally decreases with accelerating agent dosage, reaching a minimum value of 506 min, representing a 33.68% reduction compared to the benchmark group (traditional superfine cement). Under normal conditions, the compressive strength of the net slurry consolidation body is positively correlated with expansion agent dosage, achieving maximum strengths of 8.11 MPa at three days and 6.93 MPa at 28 days; these values are respectively higher by 6.7 MPa and 2.6 MPa compared to those in the benchmark group. On the seventh day, the volume expansion rate of the traditional superfine cement solidified sand body ranges from −0.19% to −0.1%, while that for the corresponding body formed from the novel superfine cement is between 0.41% and 1.33%, representing a difference of 0.6–1.43%. After the on-site treatment of water and sand-gushing strata, the core monitor rate of the inspection hole exceeds 70%. The permeability coefficient of the stratum decreases to a range between 1.47 × 10⁻⁶ and 8.14 × 10⁻⁶ cm/s, resulting in nearly a thousandfold increase in stratum impermeability compared to its original state. Hence, the findings of this research hold practical importance for the future application of such materials in the development of stratum reinforcement or building repair.

In the treatment of goafs in traffic engineering, technical problems such as those related to large-volume grouting and the precise control of material properties are often encountered. To address these issues, we developed a new composite material comprising cement-fly ash-modified sodium silicate (C-FA-MS). The setting time, fluidity, unconfined compressive strength, and microstructure were varied for different proportions of cement-sodium silicate (C-S) slurry, cement-fly ash-sodium silicate (C-FA-S) slurry, and C-FA-MS slurry, and their performances were compared and analysed. The experimental results showed that the initial setting time of the slurry was the shortest when both the original sodium silicate volume ratio () and modified sodium silicate volume ratio () were 0.2. The final setting time of the C-S and C-FA-S slurries tended to decrease but then increased with decrease in , while that of the C-FA-MS slurry increased with lower . The fluidity of the C-FA-S and C-FA-MS slurries decreased with decrease in or at different fly ash admixture ratios. The consolidation compressive strength of C-S increased with decreasing , while that of C-FA-S showed a considerable increase only when decreased from 0.4 to 0.2. Meanwhile, the compressive strength of the C-FA-MS concretions first increased and then decreased with decrease in . Microstructural analysis revealed that there were more cracks in the C-S agglomerate, the fly ash in the C-FA-S agglomerate reduced the relative density of the skeletal structure, and the stronger cross-linking in the C-FA-MS agglomerate improved the strength of the agglomerate. Under the condition of unit grouting volume, the cost of the C-FA-MS slurry was approximately 44.7% and 31.3% lower than that of the C-S and C-FA-S slurries, respectively. The new C-FA-MS material was applied for the treatment of the goaf in the Wu Sizhuang coal mine. Core drilling detection and audiofrequency magnetotelluric survey revealed that the goaf was sufficiently filled.

The migration velocity model and seepage pressure model of suspension grout with plane-parallel flow state in porous media were proposed based on mass conservation equation, seepage flow continuity equation, linear filtration law, and pressure gradient equations. The effect of the particle filtration process of suspension with distance on the migration velocity, pressure gradient, and seepage pressure was analysed. By using the independently developed monomer combined type osmotic grouting device, the diffusion velocity field and pressure field of cement slurry in sand layer were studied. Then, the value of model test was compared with the filtration theoretical values and without filtration theoretical values. Results show that the difference between the value of migration velocity with neglecting filtration effect and that of the model test was 1.2~17 times. With the decreased of K _c/K ₀, the theoretical value and the experimental value of filtration velocity decreased along the path, and the rate of both was positively correlated with the diffusion distance. At a certain diffusion distance, the model test value and the theoretical value of seepage pressure increase with time, but the rate was different. The difference between the model test value of seepage pressure and the theoretical value considering filtration was -23~84kPa, and the deviation rate was -70%~58%. The plane-parallel flow pressure model of slurry can accurately reflect the law of seepage field of cement slurry.