Underground Space最新文献

The evaluation of urban underground space (UUS) suitability involves multiple indicators. Assigning weight to these indicators is crucial for accurate assessment. This paper presents a method for spatially variable weight assignment of indicators using the order relation analysis method (G₁-method), the entropy weight method, an improved grey relational analysis (GRA) and a set of spatial weight adjustment coefficients. First, the subjective and objective weights of indicators for engineering geological and hydrogeological conditions were determined by the G₁-method and entropy weight method, respectively, and their combined weights were then obtained using the principle of minimum discriminatory information. This study highlighted the impact of surface restrictions, such as buildings, on UUS, and the degree of the influence of these buildings gradually decreased with the increase in depth of the rock and soil mass in UUS, which resulted in changes in weights of indicators with depth. To address this issue, a coefficient was defined as the standardized value of the ratio of additional stress applied by restrictions to the self-weight stress of soil at the same depth to modify the combined weights so that all weights of indicators could vary in space. Finally, an improved GRA was used to determine the suitability level of each evaluation cell using the maximum correlation criterion. This method was applied to the 3D suitability evaluation of UUS in Sanlong Bay, Foshan City, Guangdong Province, China, including 16 evaluation indexes. This study comprehensively considered the influence of multiple factors, thereby providing reference for evaluating the suitability of UUS in big cities.

The pipe curtain structure method (PSM) is a novel construction method to control ground deformation strictly. Compared with the traditional pipe-roofing and pipe jacking method, the connection between pipes in large spacings using PSM is widely acknowledged as a unique construction procedure. Further study on this connection procedure is needed to resolve similar cases in that the pipes are inevitably constructed on both sides of existing piles. Cutting the steel plate during the connection procedure is the first step, which is crucial to control the safety and stability of the surrounding environment and existing structures. The deformation mechanism and limit support pressure of the cutting steel plate during the connection between pipes in large spacings are studied in this paper, relying on the undercrossing Yifeng gate tower project of Jianning West Road River Crossing Channel in Nanjing, China. A modified 3D wedge-prism failure model is proposed using the 3D discrete element method. Combined with Terzaghi loose earth pressure theory and the limit equilibrium theory, the analytical solutions for the limit support pressure of the excavation face of the cutting steel plate are derived. The modified 3D wedge-prism failure model and corresponding analytical solutions are categorised into two cases: (a) unilateral cutting scheme, and (b) bilateral cutting scheme. The analytical solutions for the two cases are verified from the numerical simulation and in-situ data and compared with the previous solutions. The comparative analysis between the unilateral and bilateral cutting schemes indicates that the bilateral cutting scheme can be adopted as a priority. The bilateral cutting scheme saves more time and induces less ground deformation than the unilateral one due to the resistance generated from the superimposed wedge. In addition, the parametric sensitivity analysis is carried out using an orthogonal experimental design. The main influencing factors arranged from high to low are the pipe spacing, the cutting size, and the pipe burial depth. The ground deformation increases with the increased cutting size and pipe spacing. The pipe burial depth slightly affects the ground deformation if the other two factors are minor. Cutting steel plates in small sizes, excavating soil under low disturbance, and supporting pipes for high frequency can effectively reduce the ground surface subsidence.

This paper presents a regional 3D geological modeling method based on the stacking ensemble technique to overcome the challenges of sparse borehole data in large-scale linear underground projects. The proposed method transforms the 3D geological modeling problem into a stratigraphic property classification problem within a subsurface space grid cell framework. Borehole data is pre-processed and trained using stacking method with five different machine learning algorithms. The resulting modelled regional cells are then classified, forming a regional 3D grid geological model. A case study for an area of 324 km² along Xuzhou metro lines is presented to demonstrate the effectiveness of the proposed model. The study shows an overall prediction accuracy of 85.4%. However, the accuracy for key stratigraphy layers influencing the construction risk, such as karst carve strata, is only 4.3% due to the limited borehole data. To address this issue, an oversampling technique based on the synthetic minority oversampling technique (SMOTE) algorithm is proposed. This technique effectively increases the number of sparse stratigraphic samples and significantly improves the prediction accuracy for karst caves to 65.4%. Additionally, this study analyzes the impact of sampling distance on model accuracy. It is found that a lower sampling interval results in higher prediction accuracy, but also increases computational resources and time costs. Therefore, in this study, an optimal sampling distance of 1 m is chosen to balance prediction accuracy and computation cost. Furthermore, the number of geological strata is found to have a negative effect on prediction accuracy. To mitigate this, it is recommended to merge less significant stratigraphy layers, reducing computation time. For key strata layers, such as karst caves, which have a significant impact on construction risk, further on-site sampling or oversampling using the SMOTE technique is recommended.

When tunnelling in difficult ground conditions, shield machine would inevitably produce significant ground loss and vibration, which may disturb the ground ahead of the tunnel face. In this paper, discrete element models calibrated by model tests were established to investigate the response of tunnel face under the coupling effects of unloading and cutterhead vibrations. The results show that the friction angle reduction under cyclic loading and vibration attenuation in the sandy ground are significant and can be estimated by the fitted exponential functions. Under cutterhead vibration, the tunnel face stability is undermined and the limit support pressure (LSP) increases to 1.4 times as that in the static case with the growth of frequency and amplitude. Meanwhile, the loosening zone becomes wider and the arching effect is weakened with the reduction of peak horizontal stress and the increase of vertical stress above the tunnel. Based on the numerical results, a pseudo-static method was introduced into the limit equilibrium analysis of the wedge-prism model for calculating the LSP under vibration. With an error rate less than 5.2%, the proposed analytical method is well validated. Further analytical calculation reveals that the LSP would increase with the growth of vibration amplitude, vibration frequency and covered depth but decrease with the increase of friction angle. This study can not only lay a solid foundation for the further investigation of ground loss, ground water and soft-hard heterogeneous ground under cutterhead vibration, but also provide meaningful references for the control of environmental disturbance in practice.

This paper concentrates on the sensitivity and dynamic simulation of randomly distributed karst cave groups on tunnel stability and connectivity extended ratio based on water–rock interaction using a novel contact dynamic method (CDM). The concept of karst cave group connectivity extended ratio during tunneling and water inrush is proposed. The effects of cave shape and spatial distribution on Qiyueshan tunnel are investigated. Tunnel deformation and damage index, and connectivity extended ratio with uniform random karst cave groups are evaluated. The results demonstrate that the connectivity extended ratio is verified as a crucial judgment in predicting the safe distance and assessing the stability of the tunnel with the karst cave group. CDM model captures the fracture propagation and contact behavior of rock mass, surface flow, as well as the bidirectional water–rock interaction during the water inrush of Qiyueshan tunnel with multiple caves. A larger cave radius and smaller minimum distance between the cave and tunnel increase the deformation and damage index of the surrounding rock. When the cave radius and cave area ratio increase, the failure pattern shifts from overall to local failure. These findings potentially have broad applications in various surface and subsurface scenarios involving water–rock interactions.

Discontinuity is critical for strength, deformability, and permeability of rock mass. Set information is one of the essential discontinuity characteristics and is usually accessed by orientation grouping. Traditional methods of identifying optimal discontinuity set numbers are usually achieved by clustering validity indexes, which mainly relies on the aggregation and dispersion of clusters and leads to the inaccuracy and instability of evaluation. This paper proposes a new method of Fisher mixed evaluation (FME) to identify optimal group numbers of rock mass discontinuity orientation. In FME, orientation distribution is regarded as the superposition of Fisher mixed distributions. Optimal grouping results are identified by considering the fitting accuracy of Fisher mixed distributions, the probability monopoly and central location significance of independent Fisher centers. A Halley-Expectation-Maximization (EM) algorithm is derived to achieve an automatic fitting of Fisher mixed distribution. Three real rock discontinuity models combined with three orientation clustering algorithms are adopted for discontinuity grouping. Four clustering validity indexes are used to automatically identify optimal group numbers for comparison. The results show that FME is more accurate and robust than the other clustering validity indexes in optimal discontinuity group number identification for different rock models and orientation clustering algorithms.

Shipwreck salvage is a risky, time-consuming, and expensive process. Although there are many sunken ships along coastlines and in the open seas, the salvage process of a sunken ship has rarely been reported. The integrated salvage of the “Yangtze River Estuary II” shipwreck used a novel method with 22 closely locked curved rectangular pipes to form a watertight base that wrapped the shipwreck inside. The basing was lifted out of the water using a powerful crane situated on an engineering ship. For the first time, the tunneling method was used in a shipwreck salvage project, significantly reducing the disturbance to the shipwreck and its stowage, thereby preserving the original state and integrity of the shipwreck to the greatest extent. In this study, the basic concepts of the salvage method and process are explained. Solutions to critical issues in the new salvage method are provided, including jacking force prediction and major considerations for the structural design of the salvage system. The design of the salvage system and salvage process of the “Yangtze River Estuary II” shipwreck are introduced. The monitored jacking force, pipe deformation, and observed water-tightness verified that the proposed method was effective and efficient. Other possible application scenarios for the proposed method are presented at the end.

High-pressure waterjet-assisted tunnel boring machine (WTBM) is an efficient method for improving the tunneling performance of a tunnel boring machine (TBM) and reducing the wear of its disc cutters in hard rock with high geostresses. Confining pressure directly affects the efficiency of rock breaking and the configuration of the disc cutters. In this study, we evaluated the effect of confining pressure on WTBM rock breaking by developing a self-designed and manufactured experimental system, including confining pressure loading, TBM disc-cutter penetration, and high-pressure waterjet. The macro fracture, acoustic emission (AE), peak normal force drop, and specific energy (SE) were analyzed for four different confining pressures (10, 20, 30, and 35 MPa). The results showed that the cutting depth of the waterjet increased linearly as the waterjet pressure increased and decreased with the gradual increase in the nozzle moving speed. The expansion and development of cracks formed rock debris, and the size of the rock fragments decreased with an increase in confining pressure. When the waterjet pressure was 280 MPa, the nozzle moving velocity was 800 mm/min and the kerf space was 75 mm, which indicated that the confining pressure, which was 23.16 MPa, minimized the cutting SE under this condition. However, regardless of the confining pressure, the maximum normal force of WTBM was less than that of a TBM, whereas the SE of WTBM was less than that of complete TBM cutting mode (CTCM). The average force drop and average drop rate of SE were approximately 25%, and 80%, respectively. The results of this study can inspire the design and mechanism of a TBM assisted by a high-pressure waterjet.

In large-diameter shield tunnels, applying the double-layer lining structure can improve the load-bearing properties and maintain the stability of segmental lining. The secondary lining thickness is a key parameter in the design of a double lining structure, which is worth being explored. Based on an actual large-diameter shield tunnel, loading model tests are carried out to investigate the effect of the secondary lining thickness on the mechanical behaviours of the double lining structure. The test results show that within the range of secondary lining thicknesses discussed, the load-bearing limit of the double-layer lining increases with growing secondary lining thickness. As a passive support, the secondary lining acts as an auxiliary load-bearing structure by contacting the segment. And changes in secondary lining thickness have a significant effect on the contact state between the segment and secondary lining, with both the contact pressure level and the contact area between the two varying. For double-layer lining structures in large-diameter shield tunnels, it is proposed that the stiffness of the secondary lining needs to be matched to the stiffness of the segment, as this allows them to have a coordinated deformation and a good joint load-bearing effect.

A novel coupled model integrating Elman-AdaBoost with adaptive mutation sparrow search algorithm (AM-SSA), called AMSSA-Elman-AdaBoost, is proposed for predicting the existing metro tunnel deformation induced by adjacent deep excavations in soft ground. The novelty is that the modified SSA proposes adaptive adjustment strategy to create a balance between the capacity of exploitation and exploration. In AM-SSA, firstly, the population is initialized by cat mapping chaotic sequences to improve the ergodicity and randomness of the individual sparrow, enhancing the global search ability. Then the individuals are adjusted by Tent chaotic disturbance and Cauchy mutation to avoid the population being too concentrated or scattered, expanding the local search ability. Finally, the adaptive producer-scrounger number adjustment formula is introduced to balance the ability to seek the global and local optimal. In addition, it leads to the improved algorithm achieving a better accuracy level and convergence speed compared with the original SSA. To demonstrate the effectiveness and reliability of AM-SSA, 23 classical benchmark functions and 25 IEEE Congress on Evolutionary Computation benchmark test functions (CEC2005), are employed as the numerical examples and investigated in comparison with some well-known optimization algorithms. The statistical results indicate the promising performance of AM-SSA in a variety of optimization with constrained and unknown search spaces. By utilizing the AdaBoost algorithm, multiple sets of weak AMSSA-Elman predictor functions are restructured into one strong predictor by successive iterations for the tunnel deformation prediction output. Additionally, the on-site monitoring data acquired from a deep excavation project in Ningbo, China, were selected as the training and testing sample. Meanwhile, the predictive outcomes are compared with those of other different optimization and machine learning techniques. In the end, the obtained results in this real-world geotechnical engineering field reveal the feasibility of the proposed hybrid algorithm model, illustrating its power and superiority in terms of computational efficiency, accuracy, stability, and robustness. More critically, by observing data in real time on daily basis, the structural safety associated with metro tunnels could be supervised, which enables decision-makers to take concrete control and protection measures.