Soft rock tunnels in red beds frequently encounter anchor failure issues. Improving the mechanical properties of the red-bed soft rock-grout interface is highly significant for ensuring the stability of anchor support. This study employs laboratory direct shear tests in combination with digital image correlation (DIC) technology to investigate the mechanical properties (peak shear strength zp, residual shear strength zr, and shear stiffness ks) of the grout-rock interface (GRI) among three typical soft rocks from red beds (red sandstone, mudstone, and grey sandstone) and two grout materials (ordinary Portland cement (OPC) and early high-strength cement (EHC)) under varying curing periods (6 h, 1 d, 2 d, and 7 d). The shear behavior and DIC-based failure mode are analyzed for red beds GRIs. The following key findings were observed: (1) As normal stress increases, both the shear strength (zp) and shear stiffness (ks) of the GRI also increase, following the trend: red sandstone > mudstone > grey sandstone. The cohesion values of the red sandstone, mudstone, and green sandstone with the EHC grout-rock interface at 2 days are 2.4, 0.9, and 1.2 MPa, respectively. The corresponding internal friction angles are 57.6 degrees, 38.0 degrees, and 27.0 degrees, respectively. (2) With an increase in curing age, both zp and ks at the GRI increase non-linearly. EHC exhibited superior bonding performance compared to OPC, with zp reaching 7.9 MPa at 6 hand 90% of the 7-day zp being achieved at 2 din red sandstone conditions. (3) The OPC-bonded specimens primarily exhibit grout failure near the GRI, whereas the EHC-bonded specimens show rock failure in red sandstone and mudstone conditions, and adhesive failure at the interface in grey sandstone. (4) An empirical model for the shear strength of the GRI in red bed soft rocks is proposed and validated, providing a framework for the rapid and reliable evaluation of anchorage strength in red bed soft rock tunnels.
Peaty soil,a distinct category of soft foundation soil,exhibits unique physical and mechanical properties that are strongly influenced by its microstructure.Its high water content,organic matter content,low strength and permeability often result in significant engineering challenges.Enhancing the mechanical strength of peaty soil has thus become a central focus in geotechnical engineering.Using slag-based geopolymer to synergize with cement for solidification,the mechanical properties of peaty soil before and after stabilization were examined through unconfined compressive strength and direct shear tests.The mechanisms of improvement were further analyzed through microscopic techniques,including scanning electron microscope(SEM),X-ray diffraction(XRD),mercury intrusion porosimetry(MIP),and Fourier transform infrared spectroscopy(FTIR).The results demonstrate that all three alkali activators contribute to the enhancement of the mechanical strength of the peaty soil,with NaOH showing the highest activation efficiency.Cement stabilization of peaty soil improves shear strength by reducing pore space and strengthening interparticle bonding via ion exchange,hydration product crystallization,and the formation of CaCO3 and calcium silicate hydrate(C-S-H).Four stages i.e.,dissolution activation,ion exchange,gel formation,and structural reorganization are identified in the reaction process of activated slag improving peat soil.The alkali activator facilitates the dissolution of the slag's vitreous phase,promoting ionic polymerization that leads to the formation of calcium-alumino-silicate-hydrate(C-A-S-H)gel.Simultaneously,organic functional groups in the peaty soil engage in ion exchange,forming CaSiO3 precipitates and establishing a"calcium bridge"structure.These reactions collectively contribute to the formation of a dense composite matrix,thus enhancing compressive strength.Grey relational analysis reveals that compressive strength is most strongly correlated with pore area,while shear strength shows the highest correlation with the shape factor.Modified soil specimens undergo five dry-wet cycles,with a minimum strength loss rate of 27%.These findings provide a theoretical foundation for the partial replacement of cement with alkali-activated slag in peaty soil stabilization,contributing both to soft soil improvement and the valorization of industrial byproducts.Furthermore,these results offer valuable insights for ground improvement in peat-rich regions,such as Yunnan,China.
3D printing has emerged as a valuable tool for studying the mechanical behavior of rock replicas under various stress-strain states.This technique enables the creation of an unlimited number of replicas with predetermined properties and a homogeneous structure.Among various 3D printing methods,liquid crystal display(LCD)-based printing offers a cost-effective and high-quality approach for rapid prototyping of rock samples.This study investigates the feasibility of using 3D LCD printing to create rock analogs for geomechanical investigations.We evaluate the microstructure of LCD-printed samples and its influence on their elastic and mechanical properties.To assess these properties,we subjected cylindrical samples to elastic wave propagation and uniaxial compression tests.Our results demonstrate that LCD-printed samples exhibit high homogeneity of elastic properties.The velocities of elastic wave propagation across and along the layers are essentially identical,differing only by the error value.Moreover,Young's moduli obtained under uniaxial loading are in good agreement with non-destructive test results,indicating a high degree of homogeneity in elastic properties up to 20 MPa.These findings suggest that 3D LCD-printed rock analogs are well-suited for investigating processes in rocks under purely elastic loading.Additionally,the technology's versatility allows for the creation of rock replicas with various features,providing researchers with the ability to study the mechanical behavior of rocks with specific characteristics.We demonstrate the potential of 3D LCD-printed rock analogs through a case study investigating the impact of cyclic deformations on the conductivity of thin capillaries in a porous medium.Our results provide a strong foundation for utilizing 3D LCD printing to advance our understanding of geomechanical processes in rocks.
Open-ended precast hybrid reinforced concrete (PRC) piles exhibit distinct vertical bearing behavior compared to closed-ended counterparts, primarily due to soil plug formation during installation. To quantitatively assess this difference, fiber Bragg grating sensors were embedded during pile fabrication, and vertical static load tests (SLTs) were conducted on piles P1-P6 with two end configurations and pile lengths. Experimental results for piles P1-P3 were validated through numerical simulations, and length optimization was performed. A parametric study was conducted to evaluate the effects of key geometric parameters on vertical bearing capacity. Results showed that, under identical pile length and stratigraphic conditions, open-ended piles exhibited lower ultimate bearing capacity (UBC), top settlement, and rebound rate than closed-ended piles. However, longer open-ended piles demonstrated significantly greater settlement and rebound than shorter counterparts. Optimization analysis indicated that the closed-ended pile could be reduced from 40 m to 35 m. With constant concrete volume, the D800t130 pile type yielded optimal performance, achieving the highest compressive coefficient (0.42) and material utilization rate (783.1 kN/m3). Both UBC and end resistance ratio increased with pile diameter. For closed-ended piles, diameter significantly influenced axial force distribution and side resistance, while wall thickness had minimal effect on end resistance but reduced side resistance. In contrast, open-ended piles exhibited greater sensitivity to both diameter and wall thickness in terms of axial force and lateral resistance. Inner wall friction was concentrated within twice pile diameters above the soil plug base, although its magnitude remained low. The height-to-diameter (h/D) ratio of the soil plug critically affected vertical bearing behavior. Compared to closed-ended piles, open-ended piles showed reduced lateral friction, with reduction coefficients ranging from 0.78 to 0.92. Notably, when diameter increased from 600 mm to 800 mm, open-ended piles outperformed as closed-ended piles in stiff plastic silty clay.
The application of buffer layer provides an effective solution to the problem of secondary lining failure.Different buffer layer materials exhibit significant variations in mechanical properties,making it essential to establish a widely applicable theoretical model.For this purpose,this study firstly divides the nonlinear compressive stress-strain curve of buffer layer materials into n deformation stages,and the deformation characteristic of each stage is described by replacing with a straight line.Secondly,an interaction mechanical model between rheological surrounding rock and support considering the effect of the buffer layer is established.By using the deformation coordination in both the surrounding rock-buffer layer interface and the buffer layer-secondary lining interface during the whole interaction process,analytical solutions for tunnel displacement and contact pressures at different interfaces during various deformation stages of buffer layer materials are presented.Furthermore,the effectiveness of the proposed theoretical model is validated by comparison with previous studies and numerical results.Finally,a parametric analysis of the mechanical responses of tunnels with polyurethane and polyethylene foam buffer layers(with differing deformation characteristics)is carried out based on the theoretical model.The results show that the proposed model is applicable to different buffer layer materials.The division of deformation stages in buffer layer materials significantly impacts prediction outcomes.For polyurethane foam buffer layer,the prediction result of the secondary lining pressure without considering the deformation stage division is 35.2%higher than that under consideration,while for polyethylene foam buffer layer,the predicted value is even 96%higher than that considering the deformation stage division.For a given tunnel,the thickness of buffer layer has a reasonable range,with an optimal thickness of 25 cm for both polyurethane and polyethylene foam buffer layers under these conditions.Installing a buffer layer is more beneficial for tunnels subject to significant long-term deformation,as it effectively reduces secondary lining pressure and ensures long-term safety.
