Search Results (183)

The influence of foundation pit excavation on the bearing behavior of concrete tubular piles at the pit bottom remains unclear. Based on the Vesic cavity expansion theory, this paper proposes a method for calculating pile driving resistance, which takes into account the residual effect of vertical pressure changes on earth pressure during excavation. Furthermore, relying on the statistical regularity between Qu/Pu (ratio of ultimate bearing capacity to ultimate cavity expansion pressure) and L/d (length-to-diameter ratio), theoretical formulas for calculating the ultimate bearing capacity of tubular piles before and after foundation pit excavation are established, with their reliability and influencing factors analyzed. This method only requires determining the L/d of the tubular piles and the theoretical value of pile driving resistance. With its simple parameter requirements, it is suitable for estimating the ultimate bearing capacity of tubular piles affected by excavation. By comparing the computed penetration resistance, earth pressure, and driving resistance of tubular piles with field measurements, the computed results show good agreement with field measurements, and the accuracy of the proposed method meets the requirements of engineering design, verifying its feasibility as an empirical method. The fitting results of the Q_u/P_u ratios indicate that the deviations between the measured and computed values are 4.17% and 5.64% before and after excavation, respectively. Additionally, L/d and L/H (ratio of pile length to excavation depth) significantly affect the earth pressure, driving resistance, and vertical bearing capacity of monopoles. Smaller L/d and L/H ratios lead to greater earth pressure on the pile and more pronounced effects on driving resistance and vertical bearing capacity. The development of this method offers an approach for estimating the ultimate bearing capacity of tubular piles before and after foundation pit excavation during preliminary design, thereby holding substantial engineering significance. Full article

With the rapid urbanization and increasing development of underground spaces, foundation pit groups in complex geological environments encounter considerable challenges in deformation control. These challenges are especially prominent in cases of adjacent constructions, complex geology, and environmentally sensitive areas. Nevertheless, existing research is lacking in systematic analysis of construction sequencing and the interaction mechanisms between foundation pit groups. This results in gaps in comprehending stress redistribution and optimal excavation strategies for such configurations. To address these gaps, this study integrates physical model tests and PLAXIS 3D numerical simulations to explore the Nanjing Jiangbei New District Phase II pit groups. It concentrates on deformations in segmented and adjacent configurations under varying excavation sequences and spacing conditions. Key findings reveal that simultaneous excavation in segmented pit groups optimizes deformation control through symmetrical stress relief via bilateral unloading, reducing shared diaphragm wall displacement by 18–25% compared to sequential methods. Sequential excavations induce complex soil stress redistribution from asymmetric unloading, with deep-to-shallow sequencing minimizing exterior wall deformation (≤0.12%He). For adjacent foundation pit groups, simultaneous excavation achieves minimum displacement interference, while phased construction requires prioritizing large-section excavation first to mitigate cumulative deformations through optimized stress transfer. When the spacing-to-depth ratio (B/He) is below 1, horizontal displacements of retaining structures increase by 43% due to spacing effects. This study quantifies the effects of excavation sequences and spacing configurations on pit group deformation, establishing a theoretical framework for optimizing construction strategies and enhancing retaining structure stability. The findings are highly significant for underground engineering design and construction in complex urban geological settings, especially in high-density areas with spatial and geotechnical constraints. Full article

The construction of pit-in-pit has become increasingly challenging due to the bad geological conditions, particularly in collapsible loess strata. To understand its supporting characteristics and failure mode, it is necessary to study the composite instability mechanism in the loess strata. This research systematically investigates the interacting instability modes of pit-in-pit under a collapsible loess stratum, studies the effects of different reinforcement parameters through physical model tests, analyzes the significance level of each reinforcement factor, and monitors the displacement of the foundation pit during construction in a pit project in Zhengzhou. The result shows that the soil pressure distribution law of the pit in a collapsible loess formation is a complex function of soil parameters, the relative positional relationship between the inner and outer foundation pits, and the time of immersion. The model test shows that the width and depth of reinforced soil have the most significant influence. The reinforcement measures proposed in this paper can effectively control the displacement of each measuring point during the foundation pit excavation, which can provide a reference for similar projects. Full article

Cylindrical retaining structures are widely adopted in intercity railway tunnel engineering due to their exceptional load-bearing performance, no need for internal support, and efficient utilization of concrete compressive strength. Measured deformation data not only comprehensively reflect the influence of construction and hydrogeological conditions but also directly and clearly indicate the safety and stability status of structure. Therefore, based on two geometrically similar cylindrical shield tunnel shafts in Shenzhen, the surface deformation, structure deformation, and changes in groundwater outside the shafts during excavation were analyzed, and the deformation characteristics under the soil–rock composite stratum were summarized. Results indicate that the uneven distribution of surface surcharge and groundwater level are key factors causing differential deformations. The maximum horizontal deformation of the shafts wall is less than 0.05% of the current excavation depth (H), occurring primarily in two zones: from H − 20 m to H + 20 m and in the shallow 0–10 m range. Vertical deformations at the wall top are mostly within ±0.2% H. Localized groundwater leakage in joints may lead to groundwater redistribution and seepage-induced fine particle migration, exacerbating uneven deformations. Timely grouting when leakage occurs and selecting joints with superior waterproof sealing performance are essential measures to ensure effective sealing. Compared with general polygonal foundation pits, cylindrical retaining structures can achieve low environmental disturbances while possessing high structural stability. Full article

This study established a numerical model for a foundation pit at the Zhongyilu Station of the Wuhan Metro Line 12, using Plaxis3D version 2021 finite element software to examine the horizontal displacement of the diaphragm wall, ground surface settlement, and differential settlement between the diaphragm wall and the lattice columns across various construction stages. A comparison with the cut-and-cover method prompted the adoption of a strategy that integrates segmental pouring of the main structure and the installation of internal supports to optimize the original scheme. The results indicated that as the foundation pit was excavated, both the horizontal displacement of diaphragm wall and the ground surface settlement gradually increased, while the differential settlement between the diaphragm wall and the lattice columns shows exhibited an initial decrease followed by an increase. In comparison to the cut-and-cover method, the cover-and-cut method demonstrated greater efficacy in controlling foundation pit deformation and minimizing disturbances to surrounding environment. As the number of segmental pouring layers and support levels increased, the overall deformation of the foundation pit showed a gradual decreasing trend, and the differential settlement between the diaphragm wall and the lattice columns continued to fluctuate. When each floor slab was poured in three layers with two supports placed in the middle, the maximum horizontal displacement of the diaphragm wall could be reduced by 22.47%, and the maximum ground surface settlement could be decreased by 19.01%. The findings in this research can provide valuable basis and reference for the design and construction of similar projects. Full article

This study addresses the water basin effect in the underwater sand layer of steel pipe pile cofferdams by integrating the concept from building foundation pits to cofferdam foundation pit analysis. A theoretical derivation is presented for the deformation evolution of steel pipe piles and bottom seals within the cofferdam pit. The cofferdam construction dewatering process is divided into four stages: riverbed excavation for bottom sealing, dewatering to the second support, dewatering to the third support, and dewatering to final bottom sealing. The steel pipe piles are modeled as single-span or multi-span cantilever continuous beam structures. Using the superposition principle, deformation evolution equations for these statically indeterminate structures across the four stages are derived. The bottom seal is simplified to a single-span end-fixed beam, and its deflection curve equation under uniform load and end-fixed additional load is obtained via the same principle. A case study based on the 6# pier steel pipe pile cofferdam of Xi’an Metro Line 10 Jingwei Bridge rail-road project employs FLAC3D for hydrological–mechanical coupling analysis of the entire dewatering process to validate the water basin effect. Results reveal a unique water basin effect in cofferdam foundation pits. Consistent horizontal deformation patterns of steel pipe piles occur across all working conditions, with maximum horizontal displacement (20.72 mm) observed at 14 m below the pile top during main pier construction completion. Close agreements are found among theoretical, numerical, and monitored deformation results for both steel pipe piles and bottom seals. Proper utilization of the formed water basin effect can effectively enhance cofferdam stability. These findings offer insights for similar engineering applications. 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

In this paper, the reinforced cement soil nailing support technology is adopted, and the soil nailing and surface layer of loess-based composite slurry are prepared by using loess and cement. A scale model box test is conducted to examine the changes in surface layer displacement and axial force in the soil nailing during excavation and loading. The step-by-step excavation process of the foundation pit, reinforced with a loess-based composite slurry soil nailing wall. It was simulated using ABAQUS finite element software (MATLAB R2022b). The results show that as the depth of the foundation pit continues to increase, the displacement of the surface layer increases first and then decreases, and the peak displacement appears in the middle of the foundation pit. During excavation, the axial force at the middle of each row of soil nails is greater than the axial force at the end, and the axial force will increase with the increase in depth. Throughout the loading process, the axial force in the soil nail diminishes as the depth of the foundation pit increases. Initially, the change is slow, but later it escalates considerably. As the excavation depth of the foundation pit increases, the safety factor of the foundation pit will gradually decrease, and finally stabilize at about 2.4, indicating that the loess-based cement slurry soil nailing wall support has high safety. Full article

In deep foundation pit engineering, the rational arrangement of internal struts plays a crucial role in controlling diaphragm wall displacement and minimizing environmental impacts. This study investigates the effects of servo steel struts through model tests, analyzing diaphragm wall displacement, bending moment, surface settlement, and surrounding soil pressure during both excavation and active servo control phases. The results show that installing servo struts near the pit bottom significantly improves deformation control, whereas strut placement in shallow zones more effectively mitigates surface settlement. The servo system dynamically adjusts strut displacements, thereby inducing internal force redistribution in the diaphragm wall and modifying the stress field in surrounding soils. This mechanism leads to an increase in positive bending moments on the wall’s backside, which may necessitate the localized reinforcement of the diaphragm wall at servo strut connections to ensure structural integrity. The lateral wall and surrounding soil pressure exhibit further increase, effectively compensating for the pressure loss induced by excavation unloading. Notably, the influence on soil pressure demonstrates a dissipating trend with an increasing distance from the excavation. Full article

The unloading effect induced by foundation pit excavation leads to soil deformation, which may adversely affect the underlying tunnel. Foundation pit excavation is a three-dimensional (3D) deformation process, whereas most existing methods are based on a two-dimensional (2D) plane assumption. To improve conventional 2D analysis methods, this study considers the influence of the actual construction sequence on tunnel deformation. A 3D analytical method for evaluating tunnel deformation and stress induced by foundation pit excavation is proposed, based on the image source method and the rotational dislocation-coordinated deformation model. The proposed method is validated through comparative analysis with other methods using monitoring data from three engineering cases. Furthermore, the study examines and discusses the impact of excavation sequences on the final longitudinal displacement of the tunnel. The results indicate that the proposed method provides more accurate predictions of tunnel deformation induced by foundation pit excavation in actual projects. Staged and segmented excavation reduces bottom heave of the foundation pit, thereby mitigating its impact on the underlying tunnel. When the segmentation efficiency is positive, increasing the number of excavation blocks contributes to better tunnel deformation control. However, when the segmentation efficiency is negative, an increase in excavation blocks has an insignificant effect on deformation control or leads to excessive construction workload. Full article

The Yellow River Basin features a unique geographical environment with challenges like seawater erosion and soft soil. In this context, the construction of foundation pits can significantly impact the settlement of adjacent structures. Grounded in a real-world project, this study employs the finite element software Midas GTS to construct a 3D interaction model between foundation pit excavation and nearby buildings. Through this model, we analyze the settlement patterns of adjacent buildings influenced by variables such as foundation soil strength, slope gradient, and construction sequence. By integrating orthogonal experimental design and range analysis, we identify the sensitive factors affecting the settlement deformation and stability of foundation pits. Our analysis reveals that among the factors significantly influencing settlement deformation at the foundation pit base, groundwater levels and internal friction angles are the most critical. Slope gradient and soil cohesion also play substantial roles, whereas the compressive modulus of soil shows relatively less impact. However, a comparison with actual engineering data indicates that groundwater factors considerably affect slope deformation, underscoring the necessity for stringent control of groundwater level fluctuations. Full article

The excavation of upper shallow foundation pits may cause the uneven deformation of existing tunnels buried below a shallow depth. Improper control measures may lead to a series of diseases, such as local cracking or breakage of the tunnel lining, which threaten the safety of tunnel operations. Regarding the safety of the existing tunnel affected by the construction of the foundation pit, cases of the application of portal frame anti-heave structures in upper foundation pit projects of existing tunnels in Shenzhen have been documented, and the main influencing factors have been analyzed and summarized. Taking the Qianhai Ring Water Corridor Project as an example, numerical orthogonal experiments were conducted to analyze the deformation response patterns in the depth of existing tunnels and the effectiveness of control measures in the upper shallow of foundation pit engineering. The roles of portal frame anti-heave structures are analyzed in detail using measured data. Studies indicate that the deformation of the existing tunnels mainly occurs during the top and immediately adjacent block excavation stages, and stabilizes after the uplift-resisting piles and anti-floating slabs form an effective frame structure. The portal frame anti-heave structures, combined with measures such as block excavation, jet grouting interlocking reinforcement, backfilling, and surcharge loading, have extremely strong deformation control capabilities. However, the construction costs are relatively high, leaving room for optimization. Full article

The analysis of the impact of the construction of the subway protection zone on the adjacent subway tunnel has become the premise on which to ensure the safe operation of the tunnel. The need for expert members to carry out safety assessments based on specific calculations to determine the impact of construction on the safety of protected tunnels is extremely inconvenient for safety management and significantly reduces management efficiency. This paper analyzes and qualitatively judges the influence range and disturbance size of pile foundation construction, shallow foundation engineering, and foundation pit excavation. Based on relevant research results from scholars and numerical simulation methods, quantitative analysis and comparison are performed on the parameter sensitivity of pile foundation engineering, shallow foundation engineering, and foundation pit engineering along the subway line, and the influence of multi-factor combination is studied and discussed to obtain the influence sensitivity of each factor. The results show that the increase in pile spacing can effectively reduce the pile group effect. The sensitivity of subway tunnel settlement displacement is mainly controlled by the settlement displacement value. The larger the settlement displacement is, the stronger the sensitivity is. The loaded pile foundation arranged along the direction of the subway tunnel has more obvious disturbance to the subway tunnel than that arranged perpendicular to the direction of the subway tunnel. Full article

Dewatering and excavation are fundamental processes influencing soil deformation in deep foundation pit construction. Excavation causes stress redistribution through unloading, while dewatering lowers the groundwater level, increases effective stress, and generates seepage forces and compressive deformation in the surrounding soil. To systematically investigate their combined influence, this study conducted a scaled physical model test under staged excavation and dewatering conditions within a layered multi-aquifer–aquitard system. Throughout the experiment, soil settlement, groundwater head, and pore water pressure were continuously monitored. Two dimensionless parameters were introduced to quantify the contributions of dewatering and excavation: the total dewatering settlement rate ${\eta }_{dw}$ and the cyclic dewatering settlement rate ${\eta }_{dw,i}$. Under different experimental conditions, ${\eta }_{dw}$ ranges from 0.35 to 0.63, while ${\eta }_{dw,i}$ varies between 0.32 and 0.82. Both settlement rates decrease with increasing diaphragm wall insertion depth and increase with greater dewatering depth inside the pit and higher soil permeability. An analytical formula for dewatering-induced soil settlement was developed using a modified layered summation method that accounts for deformation coordination between soil layers and includes correction factors for unsaturated zones. Although this approach is limited by scale effects and simplified boundary conditions, the findings offer valuable insights into soil deformation mechanisms under the combined influence of excavation and dewatering. These results provide practical guidance for improving deformation control strategies in complex hydrogeological environments. Full article

Tunnel Boring Machine (TBM) excavation materials were recycled by sieving and separating particles into sizes 5–10 mm (coarse aggregates) and below 5 mm (manufactured sand) to explore their potential as aggregates in shotcrete production, with the aim of reducing environmental harm from waste disposal. Mix proportion experiments were conducted to evaluate the mechanical properties—including failure patterns, compressive strength, flexural strength, and deflection—of the shotcrete specimens through cubic axial compression and four-point bending tests; furthermore, rebound tests were conducted on shotcrete mixed with the recycled TBM aggregates in foundation pit engineering. These tests assessed the effects of key parameters (water–binder ratio, sand ratio, fly ash content, synthetic fibers, and liquid alkali-free accelerator) on shotcrete composed of recycled TBM sand and gravel. The results indicated that crushing and grading flaky TBM-excavated rock fragments, and subsequently blending them with pre-screened fine aggregates in a 4:1 ratio, yielded manufactured sand with an optimized particle gradation and controlled stone powder content (18%). Adjusting the water–binder ratio (0.4–0.5), fly ash dosage (mixed with 0–20%), and sand ratio (0.5–0.6) are feasible steps in preparing shotcrete with a compressive strength of 29.1 MPa to 50.4 MPa and slump of 9 cm to 20 cm. Moreover, the rebound rate of the shotcrete reached 11.3% by applying polyoxymethylene (POM) fibers with a 0.15% volume fraction and a liquid-state alkali-free setting accelerator (8% dosage), demonstrating that the implemented approach enables a decrease in the rebound rate of shotcrete. Full article