Search Results (36)

Taking the shield construction of Xiamen Metro Line 2 tunnel side-crossing the Tianzhushan overpass and under-crossing the Shen-Hai Expressway as the engineering background, FLAC3D 6.0 software was used to examine the deformation of adjacent structures based on shield construction parameters in upper-soft and lower-hard strata. The reliability of the numerical simulation results was verified by comparing measured and predicted deformations. The study results indicate that deformation of the pile will occur during the construction of the tunnel shield next to the pile foundation. The shape of the pile deformation curve in the horizontal direction is significantly influenced by the distance from the pile foundation to the adjacent tunnel’s centerline, as well as by soil bin pressure, grouting layer thickness, and stress release coefficient. During the tunnel shield construction beneath the expressway, increasing the soil bin pressure, the grouting layer thickness, and reducing the stress release coefficient can effectively minimize surface deformation and differential settlement on both sides of the deformation joints between the bridge and the roadbed. The practice shows that, by optimizing shield construction parameters in upper-soft and lower-hard strata, the deformation of nearby bridges and pavements can be kept within allowable limits. This is significant for reducing construction time and costs. The findings offer useful references for similar projects. Full article

This study investigates the deformation characteristics and base stability of a circular diaphragm wall support system (external diameter: 90 m, wall thickness: 1.5 m) with pit bottom reinforcement for the South Anchorage deep foundation pit of the Zhangjinggao Yangtze River Bridge, which uses layered and partitioned top-down excavation combined with lining construction. Through field monitoring (deep horizontal displacement of the diaphragm wall, vertical displacement at the wall top, and earth pressure) and numerical simulations (PLAXIS Strength Reduction Method), we systematically analyzed the deformation evolution and failure mechanisms during construction. The results indicate the following: (1) Under the synergistic effect of the circular diaphragm wall, lining, and pit bottom reinforcement, the maximum horizontal displacement at the wall top was less than 30 mm and the vertical displacement was 0.04%H, both significantly below code-specified thresholds, verifying the effectiveness of the support system and pit bottom reinforcement. (2) Earth pressure exhibited a “decrease-then-increase” trend during the excavation proceeds. High-pressure jet grouting pile reinforcement at the pit base significantly enhanced basal constraints, leading to earth pressure below the Rankine active limit during intermediate stages and converging toward theoretical values as deformation progressed. (3) Without reinforcement, hydraulic uplift failure manifested as sand layer suspension and soil shear. After reinforcement, failure modes shifted to basal uplift and wall-external soil sliding, demonstrating that high-pressure jet grouting pile reinforcement had positive contribution basal heave stability by improving soil shear strength. (4) Improved stability verification methods for anti-heave and anti-hydraulic-uplift were proposed, incorporating soil shear strength contributions to overcome the underestimation of reinforcement effects in traditional pressure equilibrium and Terzaghi bearing capacity models. This study provides theoretical and practical references for similar deep foundation pit projects and offers systematic solutions for the safety design and deformation characteristics of circular diaphragm walls with pit bottom reinforcement. Full article

Soft clay soil is known for its high compressibility and low bearing capacity, making it one of the most challenging soil types. Sand columns and sand layers reinforced with geosynthetics are effective techniques to enhance the performance of foundations built on soft clay. Stone or sand columns improve load-bearing capacity by utilizing the natural lateral confinement of the soil. However, in very soft soil, a significant design challenge arises due to bulging in the stone columns, as the surrounding soil may not provide adequate confinement to support the required load capacity. This issue has been addressed by grouting the columns, resulting in highly stable and solid structures. Additionally, the grouting pressure enhances frictional resistance and fills any voids within the soil, contributing to increased overall stability. In the current study, soil improvement methods using ordinary sand columns and grouted sand columns were investigated and then compared with adding sand layers with geogrid reinforcement. The study demonstrated that grouted sand columns improved the bearing capacity by 90% over untreated clay. With geogrid reinforcement, sand columns achieved a 180% increase, while grouted columns with geogrid reinforcement reached a 260% improvement. Increasing the thickness of reinforced sand (H/B = 1.5) further raised capacity improvements to 300% for ungrouted and 420% for grouted columns. Full article

This paper reports the results of a full-scale field test that was conducted to assess the performance of the use of wet-milling cement and chemical compound grouting in the same hole to reinforce a flexural fracture zone. Wet-milling cement and chemical compound grouting methods were used to treat a layer of the flexural fracture zone with a thickness of 19 m. The procedures of the cement–chemical compound grouting method were described in detail, and the results of the normal water pressure test, fatigue water pressure test, failure water pressure test, and shear wave velocity test suggested that the working effects in the epoxy testing area were better than those in the acrylic acid salt test area, which further indicated that the cement–chemical compound grouting method was feasible. In addition, the improvement mechanism of the cement–chemical compound grouting technology was studied; this method is beneficial for solving the problem of the reinforcement effect not being ideal in practical engineering and further improving the compactness of dam structures. Full article

Polyurethane (PU) grouting materials are widely used in underground engineering rehabilitation, particularly in reinforcement and waterproofing engineering in deep-water environments. The long-term effect of complex underground environments can lead to nanochannel formation within PU, weakening its repair remediation effect. However, the permeation behavior and microscopic mechanisms of water molecules within PU nanochannels remain unclear. In this paper, a model combining PU nanochannels and water molecules was constructed, and the molecular dynamics simulations method was used to study the effects of water pressure and channel width on permeation behavior and microstructural changes. The results reveal a multi-stage, layered permeation process, with significant acceleration observed at water pressures above 3.08 MPa. Initially, water molecules accelerate but are then blocked by the energy barrier of PU nanochannels. After about 20 ps, water molecules overcome the potential barrier and enter the nanochannel, displaying a secondary acceleration effect, with the maximum permeation depth rises from 1.8 nm to 11.8 nm. As the channel width increases, the maximum permeation depth increases from 7.5 nm to 11.6 nm, with the rate of increase diminishing at larger widths. Moreover, higher water pressure and wider channels enhance the stratification effect. After permeation, a hydrophobic layer of approximately 0.5 nm thickness forms near the channel wall, with a density lower than that of the external water. The middle layer shows a density slightly higher than the external water, and the formation of hydrogen bonds between water molecules increases toward the channel center. Full article

Due to the impact of disordered mining activities in previous years, numerous abandoned roadways exist in the second mining district of the 13# coal seam in Chejiazhuang Coal Mine. The stability of the new roadway roof was analyzed under various distributions of abandoned roadways above. It was determined that the ultimate stable thickness of the coal layer between the new and abandoned roadways is 4.0 m. When the thickness between the two is less than 4.0 m, the roof becomes unstable after excavation, posing a risk of collapse. Advanced grouting reinforcement is required to enhance roof stability before installing U-shaped steel arches. Mechanical experiments were conducted on the polymer grouting consolidation of fractured coal, showing a significant increase in residual strength compared to intact coal. Furthermore, the uniaxial compressive strength of the polymer grouting consolidation partially recovered. On average, the consolidation coefficient and recovery coefficient were 5.28 and 85.51%, respectively. Grouting increased the ductility of the fractured surrounding rock, enhancing its resistance to deformation and plasticity. A polymer grouting consolidation technology for supporting fractured surrounding rock under the unstable roof of abandoned roadways is proposed, along with the design of corresponding support schemes and parameters. Monitoring the results of mine pressure indicated that the surrounding rock remained stable after roadway excavation, validating the effectiveness of the support schemes and parameters. Full article

In practical engineering, due to quality inspections of connections between prefabricated components and construction errors, reserved reinforcing bars in the transition layer may be partially insufficient or even completely absent. This defect significantly impacts the structural performance of sleeve connections, particularly under tensile or shear forces. This paper proposes a novel reinforcement method to address the connection issues caused by the absence of reserved reinforcing bars in the transition layer and verifies its feasibility through systematic experiments. To this end, this paper proposed a novel reinforcement method of grouting sleeve connection considering the absence of reserved bars in the transition layer, and 45 specimens with different reinforcement parameters were fabricated and tested under tension. Before verifying the reliability of the novel reinforcement method, nine specimens were fabricated and tested to verify the weldability of grouting sleeves and reinforcing bars. According to the test results, the fully grouted sleeves, including Grade 45 steel and Q345, showed good weldability with the HRB400 steel bars, while the ductile iron grouted sleeve showed poor weldability. When the single-sided welding length was greater than or equal to six times the diameter of the post-retrofitted connecting steel bar (D₂), the primary failure mode observed in specimens utilizing the novel reinforcement method was the fracture of the prefabricated steel bar. The novel reinforcement method could be used to repair the defect of the grouting sleeve connection considering the absence of reserved reinforcing bars in the transition layer. When the single-sided welding length was 4D₂, with a relative protective layer thickness of 2D₂, and using C60 grade reinforcement material, this combination of conditions represented the critical condition to avoid weld failure between the grouting sleeve and the post-retrofitted connecting steel bars. In practical reinforcement projects, it is suggested that the single-sided welding length should be 5D₂, the relative protective layer thickness should be 3D₂, and the reinforcement material strength should be C60. Full article

The grout injection forms a compacted backfill in the separation chamber, and most of the grouting water seeps into the overburden pores. The initial saturation of overlying rock has been found to affect the seepage distribution of effluent in overlying rock. In the actual grouting process, the flow direction of grouting water in overlying rock and the initial saturation of overlying rock may affect whether the bled water will leak to the working face. In order to investigate the influence of initial saturation on the seepage law of isolated overburden grout injection, the present paper studies the saturation change process of bled water seepage at three different initial saturations (S_r = 29%, 51%, 73%). The water leakage of the working face was monitored with the self-developed 3D visual simulation experimental system, without considering the overlying rock of the injection layer. The results show that the bled water flows mainly in the horizontal direction, and the horizontal seepage velocity is about 10 times the vertical seepage velocity. The higher the initial saturation of the overburden, the larger the seepage range of the overburden, and the closer the seepage boundary is to the working face, the easier the bled water will leak into the working face. Therefore, the grouting design and process control can be carried out better. When the thickness of the selected isolation layer is insufficient, the bled water may appear on the working face, so this problem can be improved by increasing the thickness of the isolation layer. Full article

A reasonable support parameter design scheme is the key to ensuring the stability of the roadway. This study established 217 models using FLAC3D to analyze the evolution characteristics of elastic strain energy and plastic dissipation energy of surrounding rock under different shotcrete and rockbolt support structures. Additionally, five single models (BP, DT, ELM, RF, SVM) were introduced to explore the application of machine learning in predicting the stability of the roadway. The study found that in the parameters of the shotcrete layer support structure, the energy evolution of the surrounding rock is more sensitive to isotropic and thickness; in the parameters of the anchor rod support structure, the energy evolution of the surrounding rock is more sensitive to Young’s modulus, cross-sectional area young, and grout stiffness. Additionally, the parameters of the shotcrete layer support structure are not necessarily the larger the better. When isotropic is 100 GPa, both the dissipated energy and the elastic strain energy are higher than that at 25 GPa. The results of the single model test indicate that machine learning is relatively accurate in predicting different shotcrete and anchor support structures. The runtime difference between traditional methods and machine learning models highlights the potential advantages of machine learning. Full article

A foundation pit is constructed in the floodplain of Yangtze River, and a deep and thick layer of large-particle pebble gravel exists below the base slab, thus forming a connected supply channel with the adjacent Yangtze River. The large water volume, high water pressure, and strong permeability of this layer bring great risks to the foundation pit construction. In view of the fact that conventional waterproof curtain construction technologies such as the deep mixing column and high-pressure jet grouting column cannot meet the engineering requirements under these kinds of geological and environmental conditions, a new waterproof curtain construction technology that combines the trenching technology of the diaphragm wall with the TRD (Trench cutting Remixing Deep wall) technology is proposed, i.e., the trenching-and-replacing-style TRD technology, as well as the construction process of this technology, is presented. After the waterproof curtain is built using the proposed technology, the strength, integrity, uniformity, and service performance of the waterproof curtain wall are tested and evaluated by the comprehensive methods of coring, borehole television imaging, resistivity CT, and a group well pumping test. The results show that the proposed technology overcomes the adverse effects of underlying large-particle pebble gravel layer, and the waterproof curtain built by it effectively cuts off the hydraulic connection inside and outside the pit. The technical proposal can provide useful references for similar projects. Full article

As transportation networks continue to expand into mountainous regions with high seismic activity, ensuring the seismic safety of tunnels crossing active faults has become increasingly crucial. This study aimed to enhance our understanding of the impact of fault zones on the seismic behavior of tunnels and to provide optimized seismic design recommendations through a comprehensive experimental and numerical investigation. The focus of this research is the Xiangyangshan Highway Tunnel in China, which intersects a significant longitudinal fault. Large-scale shake table tests were performed on 1:100 scale physical models of the tunnel to analyze the seismic responses under various ground motion excitations. Detailed three-dimensional finite difference models were developed in FLAC3D and calibrated based on the shake table results. The tests indicated that strains, earth pressures, and accelerations experience localized amplification within 10–20 m of the fault interface compared to undisturbed ground sections. Common seismic mitigation measures, such as rock grouting, seismic joints, and shock absorption layers, were observed to effectively reduce the amplified seismic demands. Grouting, in particular, led to an average reduction of up to 56.3% in circumferential strain and 38.5% in earth pressure. It was concluded that 6 m thick grouted zones and 20 cm thick rubber interlayers between tunnel lining shells provide optimal structural reinforcement against the effects of fault zones. This study provides valuable insights for improving the seismic resilience of underground transportation corridors in seismically active regions. Full article

The construction of underwater shield tunnels under high water pressure conditions and seepage action will seriously impact the stability of the surrounding rock. In this study, an analytical model for the strength of the two-lane shield tunneling construction under anisotropic seepage conditions was established, and a series of simulations were carried out in the engineering background of the underwater section of Line 2 of the Taiyuan Metro in China, which passes through Yingze Lake. The results show that: (1) the surface settlement has a superposition effect, and the late consolidation and settlement of the soil body under seepage will affect the segment deformation and the monitoring should be strengthened; (2) under the influence of the weak permeability of the lining and grouting layers, the pore pressure on both sides of the tunnel arch girdle is reduced by about 72% compared with the initial value, with a larger hydraulic gradient and a 30% reduction at the top of the arch; (3) within a specific range, the tunneling pressure can be increased, and the grouting pressure and the thickness of grouting layer can be reduced to control the segment deformation; (4) the more significant the overlying water level is, the larger the maximum consolidation settlement and the influence range of surface settlement. This study can provide a reliable reference for underwater double-lane shield tunnel design and safety control. Full article

Polyurethane (PU) composite is increasingly used as a repair material for civil engineering infrastructure, including runway, road pavement, and buildings. Evaluation of polyurethane grouting (PUG) material is critical to achieve a desirable maintenance effect. This study aims to evaluate the flexural behavior of normal concrete repaired with polyurethane grout (NC-PUG) under a three-point bending test. A finite element (FE) model was developed to simulate the flexural response of the NC-PUG specimens. The equivalent principle response of the NC-PUG was analyzed through a three-dimensional finite element model (3D FEM). The NC and PUG properties were simulated using stress–strain relations acquired from compressive and tensile tests. The overlaid PUG material was prepared by mixing PU and quartz sand and overlayed on the either top or bottom surface of the concrete beam. Two different overlaid thicknesses were adopted, including 5 mm and 10 mm. The composite NC-PUG specimens were formed by casting a PUG material using different overlaid thicknesses and configurations. The reference specimen showed the highest average ultimate flexural stress of 5.56 MPa ± 2.57% at a 95% confidence interval with a corresponding midspan deflection of 0.49 mm ± 13.60%. However, due to the strengthened effect of the PUG layer, the deflection of the composite specimen was significantly improved. The concrete specimens retrofitted at the top surface demonstrated a typical linear pattern from the initial loading stage until the complete failure of the specimen. Moreover, the concrete specimens retrofitted at the bottom surface exhibit two deformation regions before the complete failure. The FE analysis showed good agreement between the numerical model and the experimental test result. The numerical model accurately predicted the flexural strength of the NC-PUG beam, slightly underestimating Ke by 4% and overestimating the ultimate flexural stress by 3%. Full article

In soft soil areas, to compare the load-bearing characteristics of bag grouting piles and cement mixing piles and study the load-bearing mechanism of bag grouting piles, field tests are conducted in this study, including the comparative compressive test of bag grouting piles and cement mixing piles, and the analysis of pile axial force, pile side friction resistance, and pile end resistance. Moreover, a numerical simulation is developed using ABAQUS 2020 (finite element analysis software) for three-dimensional modeling. The numerical simulation results are compared with the field test results to verify the reliability of the numerical simulation. Furthermore, the influences of five factors are studied; namely, pile length, pile diameter, pile spacing, the thickness of the bedding layer, and grouting pressure are studied for their effects on the compressive bearing characteristics of the bag grouting pile. The results show the following: (1) For composite foundations, bag grouting piles are more effective than cement mixing piles in soft soil areas, and the former provide an 8.8% increase in the bearing characteristics. (2) With an increase in the load, the bag grouting pile experiences greater compression in the middle of the pile body, and the pile side friction resistance is increased; therefore, the pile side friction resistance can be fully developed, and the bag grouting piles have the ability to transfer the load from the top of the pile to the soil at the bottom of the pile. (3) When the external load is maximized, the sharing ratio of pile side friction resistance reaches 96.3%, which shows the excellent frictional performance of bag grouting piles. (4) Among the five factors mentioned above, the most important one is the pile diameter, followed by the pile length and pile spacing, the thickness of the bedding layer, and finally the grouting pressure. The optimal combination in this paper is a pile length of 18 m, pile diameter of 0.4 m, pile spacing of 1.0 m, bedding thickness of 0.3 m, and grouting pressure of 0.6 MPa. Therefore, changing the pile diameter can be given priority during the construction design. The findings in this paper can provide valuable insights and practical experience for the design of similar engineering projects. Full article

The support pressure on an excavation surface is a critical factor in the ground deformation and excavation stability of a submarine shield tunnel. The shield tail gap and the disturbance zone of grouting behind the tunnel wall are also important influencing factors. However, the effects of these factors on excavation stability are difficult to quantify. Consequently, a homogeneous, elastic, and annular equivalent layer is employed to simulate the thin layer behind the tunnel wall. Using COMSOL Multiphysics software, the effects of the water level depth, the thickness of the equivalent layer, the diameter of the shield tunnel, and the internal friction of soil and tunnel burial depth on the excavation deformation and ground surface subsidence of a submarine tunnel are considered with regard to the fluid–solid coupling effect. The result show that the surface subsidence of the case with respect to the fluid–solid coupling effect and the equivalent layer is larger than that without interstitial fluid and the equivalent layer, indicating that the present model can better simulate the stability of tunnel excavation. Therefore, it is important to consider the impact of the fluid–solid coupling effect and the equivalent layer on the deformation of the excavation face and ground surface subsidence. Full article