Yao Zhang

To investigate the failure characteristics of 3D-printed concrete utility tunnels under loading, a 1:25 scaled model was designed using similarity theory. Vertical loading tests were conducted under soil lateral confinement, and the load–displacement curves, discrete-point strain responses, and crack evolution process were obtained. The test results show that the structure successfully undergoes an elastic stage, a crack development stage, and a plastic failure stage. The incorporated polypropylene fibers exert a bridging effect, enabling the component to retain a certain load-bearing capacity after cracking. Crack distribution was highly heterogeneous: cracks were densest on the top slab, widest on the side walls, and multi-directional on the inner wall. A clear correspondence exists between strain response and crack distribution, with tensile strain zones highly coinciding with crack opening zones. The failure mode generally agrees with the “top slab compression–side wall tensile cracking” characteristic of traditional closed-frame structures. However, the wall thickness deviations induced by the 3D printing process are amplified during internal force redistribution in the statically indeterminate structure, resulting in markedly asymmetric failure of the left and right side walls.

Carbide lime (CL) and powdered glass (PG), as industrial by-products, possess significant potential as eco-friendly soil amendment materials. This paper presents a systematic investigation into the effectiveness and reaction mechanisms of a composite material comprising CL and PG for stabilizing dispersive soils. A systematic experimental program was designed with varying CL (0.5–6.5%) and PG (4–16%) contents, along with curing ages of 1, 7 and 14 days. Macroscopic properties, including dispersibility and permeability, were evaluated through pinhole, mud ball, and permeability tests, while phase composition and microstructural evolution were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results demonstrate a pronounced synergistic effect between CL and PG at optimal ratios: soil dispersibility is markedly improved when CL ≥ 2.5% and PG ≥ 8%, non-dispersive behavior is achieved at all curing ages with CL between 4.5 and 6.5% and PG between 4 and 16 permeability coefficient decreases significantly with increasing material content; for instance, increasing CL from 2.5% to 6.5% (at 16% PG) reduces the permeability coefficient by over 50%. Microstructural analysis reveals that CL supplies Ca²⁺ and an alkaline environment, whereas PG provides reactive SiO₂ and Al₂O₃. Their interaction facilitates ion exchange and pozzolanic reactions, leading to the formation of C–S–H and C–A–S–H gels. These cementitious products effectively fill pores and bond soil particles, thereby enhancing structural stability. This study confirms that the CL-PG composite is an efficient and sustainable soil stabilization material. It provides novel insights into the synergistic mechanisms and optimal dosage range, offering valuable theoretical and practical guidance for the resource utilization of industrial by-products in geotechnical engineering.

The construction of traditional underground utility tunnels faces prominent challenges, including high costs, long construction cycles, and limited workspace. Although 3D printing technology offers an effective solution to these issues, its practical application is largely constrained by key performance factors such as the printability, early strength, and interlayer bonding of concrete materials. This study aims to develop a 3D-printable concrete material specifically suited for the construction of underground utility tunnels. Through collaborative optimization of parameters such as the water–binder ratio, additives, and fiber content using single-factor and orthogonal tests, the optimal mix proportion was determined: a water–binder ratio of 0.30, a 10% dosage of rapid-hardening sulphoaluminate cement (R·SAC), a sand-to-binder ratio of 1.0, 20% mineral admixtures (15% fly ash + 5% silica fume), and a 1.0% volume fraction of polypropylene fibers. The results indicate that the fresh paste achieved a flowability of 192 mm, demonstrating excellent printability. Specimens printed using a sawtooth toolpath reached a 3-day compressive strength of 37.8 MPa, with 28-day compressive and flexural strengths increasing to 56.3 MPa and 7.8 MPa, respectively, and an interlayer bond strength of 3.5 MPa. Crucially, the compressive and flexural anisotropy coefficients were as low as 0.023 and 0.066, respectively, showing a preliminary exploratory trend superior to levels reported in some literature and suggesting the potential of printed components to improve structural performance consistency. This material system not only meets the requirements of 3D printing for early strength and workability but also, by introducing R·SAC to form a low-alkalinity binder system, provides a potential pathway for enhancing long-term durability in corrosive environments. This study offers a reliable theoretical and experimental basis for the application of 3D printing technology in underground engineering. Long-term durability will remain a primary focus of subsequent research.

The Loess Plateau region faces persistent geohazards that critically threaten both ecological stability and human safety, largely due to the distinct metastable microstructure of loess, which evolves under changing environmental conditions and governs its macro-mechanical behavior. Unlike traditional models that rely solely on macroscopic parameters, this study introduces the ratio of large and medium pore volume (PLM) as a key predictor alongside moisture content (w) to establish a novel empirical model for loess shear strength. Laboratory tests were conducted on undisturbed loess specimens with controlled moisture content and pore structure characteristics. The results demonstrate that increased moisture content and a higher proportion of large and medium pores significantly reduce shear strength, revealing the critical role of pore space morphology in mechanical degradation. A new method for accurately predicting the shear strength of unsaturated loess using easily measurable parameters has been developed based on mechanical analysis and experimental data. This method provides a practical, microstructure-based tool for evaluating the stability of surface slopes and foundations in loess areas, and has wide applicability in geotechnical engineering.

Loess, as a typical structural soil, exhibits shear strength characteristics significantly controlled by mesostructural evolution. This study investigates as-built loess in Xi 'an's Chang'an District through systematic triaxial shear tests combined with mercury compression and particle analysis. The research quantitatively reveals the macro-meso mechanical response mechanism of loess under coupled water content and confining pressure. Results demonstrate that as water content increases from 5% to 25%, the shear strength of loess decreases markedly: cohesion drops sharply from 92.52 kPa to 23.76 kPa (a 74.3% reduction), while the internal friction angle decreases from 26.57° to 25.22° with relatively gradual changes. Mesostructural analysis shows that the volume proportion of large and medium pores (diameter ≥ 0.38 µm) decreases significantly with increasing water content. Under 400 kPa confining pressure, this proportion drops from 77.6% at low water content to 50.5% at high water content, while small pore volume increases correspondingly, revealing the structural reconstruction process under combined loading and moisture effects. Based on fractal theory and pore distribution characteristics, a macro-meso correlation model integrating water content, shear stress, and large/meso-pore volume proportion was established. The model achieves an R² value exceeding 0.94 for experimental data fitting, accurately predicting shear strength evolution under various conditions. This study establishes a quantitative mapping relationship between macroscopic strength and mesostructure through physical mechanisms, providing reliable theoretical basis and computational tools for precise prediction and prevention of loess engineering disasters.

To overcome the limitations imposed by the anisotropy and heterogeneity of natural loess, this study establishes a novel quantitative macro–micro correlation framework for investigating the deformation mechanisms of artificial structural loess (ASL). ASL samples were prepared by mixing remolded loess with cement (0–4%) and NaCl (0–16%), followed by static compaction (95% degree) and 28-day curing (20 ± 2 °C, >90% RH) to replicate the structural properties of natural loess under controlled conditions. An integrated experimental methodology was employed, incorporating consolidation/collapsibility tests, particle size analysis, X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). A three-dimensional nonlinear model was proposed. The findings show that intergranular cementation, particle size distribution, and pore architecture are the main factors influencing loess’s compressibility and collapsibility. A critical transition from medium to low compressibility was observed at cement content ≥1% and moisture content ≤16%. A strong correlation (Pearson |r| > 0.96) was identified between the mesopore volume ratio and the collapsibility coefficient. The innovation of this study lies in the establishment of a three-dimensional nonlinear model that quantitatively correlates key microstructural parameters (fractal dimension value (D), clay mineral ratio (C), and large and medium porosity (n)) with macroscopic deformation indicators (porosity ratio (e) and collapsibility coefficient (δ_s)). The measured data and the model’s output agree quite well, with a determination coefficient (R²) of 0.893 for porosity and 0.746 for collapsibility, verifying the reliability of the model. This study provides a novel quantitative tool for loess deformation prediction, offering significant value for engineering settlement assessment in controlled cementation and moisture conditions, though its application to natural loess requires further validation.

The collapsibility of loess in the northwest region poses a significant threat to infrastructure stability. Current research predominantly separates macroscopic mechanical behaviour from mesoscopic structural characteristics, lacking a systematic methodology to quantify their interdependence. This study integrates consolidation tests, laser particle size analysis, mercury intrusion porosimetry (MIP) tests, and fractal theory to propose a multi-scale evaluation framework for assessing the structural potential of collapsible loess in strength, with on-site verification conducted. This framework quantitatively links grain size, pore potential, and connection potential to the collapsibility of loess. The experimental results indicate that loess’s high compressibility and collapsibility are primarily governed by grain size and pore potential. In contrast, the connection potential of soluble salts mitigates structural instability through ionic bonding. Field verification demonstrates a strong correlation between the three structural potentials and the subsidence coefficient (R² = 0.92, p < 0.01), validating the framework’s effectiveness in evaluating structural stability. A ternary evaluation system has been established based on fractal dimension, void ratio, and soluble salt content. These research findings provide predictive tools for managing collapse risks in loess-related projects, enabling fixed-point design of loess foundations and early warning of collapse risks.

The structure, strength, and deformation characteristics of artificial structural loess can be manually controlled, which has significant advantages in scientific research on loess. By preparing and testing artificial structured loess, the natural properties of structured loess can be better investigated and studied. In this paper, the influence of varying moisture contents and additive dosages on artificial structured loess strength characteristics through triaxial shear tests were analyzed. The moisture content and additive dosage reflecting the structural properties of natural loess were obtained. Based on the microscopic test results, the mineral components, micromorphology, and pore characteristics of artificial structural loess were analyzed, and the mechanism of the structural evolution of loess under mechanical action was revealed. The results show that the minimum differences in the peak strength between W₁₆-Y_2.0C_2.0 and undisturbed soil under confining pressures of 50, 100, and 200 kPa are 6.481 kPa, 7.676 kPa, and 4.912 kPa, respectively. The minimum differences in the cohesion and inner friction angle between W₁₆-Y_2.0C_2.0 and undisturbed soil are 2 kPa and 0.2°, respectively, indicating that W₁₆-Y_2.0C_2.0 is the optimal structural soil with a structural strength closest to that of undisturbed soil. Compared with the undisturbed loess, the content of calcite in the artificial structure loess increases from 9.8% to 11.2%, the proportion of plagioclase decreases from 20.5% to 17.4%, amphibole is consumed completely, and 2.1% of halite is generated. Furthermore, the pores of structured soil exhibit a three-peak distribution and are divided into four types, including micropores (≤0.02 μm), small pores (0.02~0.21 μm), medium pores (0.21~13.5 μm), and large pores (≥13.5 μm). When the pressure increases from 50 kPa to 200 kPa, micropores increase by 4.67%, small pores increase by 4.97%, medium pores decrease by 2.4%, and large pores decrease by 7.24%. The trend of pore structure changes in W₁₆-Y_2.0C_2.0 is similar to that of undisturbed loess. The research results provide a reference for preparing and applying artificial structural loess.

Saturated sand foundations are susceptible to liquefaction under dynamic loads. This can result in roadbed subsidence, flotation of underground structures, and other engineering failures. Compared with the traditional foundation reinforcement technology, enzyme-induced calcium carbonate precipitation technology (EICP) is a green environmental protection reinforcement technology. The EICP technology can use enzymes to induce calcium carbonate to cement soil particles and fill soil pores, thus effectively improving soil strength and inhibiting sand liquefaction damage. The study takes EICP-solidified standard sand as the research object and, through the dynamic triaxial test, analyzes the influence of different confining pressure (σ₃) cementation times (CT), cyclic stress ratio (CSR), dry density (ρ_d), and vibration frequency (f) on dynamic strength characteristics. Then, a modified dynamic strength model of EICP-solidified standard sand was established. The results show that, under the same confining pressure, the required vibration number for failure decreases with the increase in dynamic strength, and the dynamic strength increases with the rise in dry density. At the same number of cyclic vibrations, the greater the confining pressure and cementation times, the greater the dynamic strength. When the cementation times are constant, the dynamic strength of EICP-solidified sand decreases with the increase in the vibration number. When cementation times are 6, the dynamic strength of the specimens with CSR of 0.35 is 25.9% and 32.4% higher than those with CSR of 0.25 and 0.30, respectively. The predicted results show that the model can predict the measured values well, which fully verifies the applicability of the model. The research results can provide a reference for liquefaction prevention in sand foundations.