Search

Tunnel face stability is critical for tunneling safety, especially in constructing a tunnel adjacent to another. In tunnel construction, if there is an adjacent existing tunnel and the active failure of the tunnel face happens, there will be severe consequences. Therefore, the active stability of shield tunnel face should be studied considering existing tunnels nearby. In this work, a series of transparent clay model tests were conducted to investigate the active stability of a shield tunnel face adjacent to an existing tunnel based on the development process, failure mode, influence range and support force. In addition, discrete element method was used to study the shield tunnel face stability under various conditions. The role of existing tunnels on the shield tunnel face stability was discussed. The failure mode of the shield tunnel face is an overall failure mode and similar to a wedge shape and ladder shape along the longitudinal and transverse section, respectively. The critical support force ratio is approximately 4.5%. The relative vertical and horizontal distance have significant influences on failure mode, while the influence of burial depth and overlapping angles are not significant. Key words: active stability, tunnel face, transparent clay model test, numerical simulation, existing tunnel, 1. Introduction 1.1. Background With the arrival of the climax of the metro construction in China, the urban rail transit system has developed from a single line to a complex [...]

Air-booster vacuum preloading (AVP) without sand blanket and surcharge vacuum preloading (SVP) methods were used to treat two adjacent areas in a foundation reinforcement project on Donghai Island, Zhanjiang City, Guangdong Province, China. In this study, engineering characteristics of the AVP foundation were obtained through analyzing monitoring data. Results showed that the AVP method obtained superior foundation consolidation results compared to SVP method. This study proposed a new simplified mechanical model for AVP, with a single PVD (prefabricated vertical drain) centered and the air-booster pipes symmetrically distributed around the PVD, applied equivalent air-booster pressure to the drainage boundary. Furthermore, this study established a mathematical analytical model for predicting consolidation settlement based on the proposed simplified model. By comparing the theoretical settlement value with the measured value, this paper verifies the rationality of the analytical solution. Finally, this study analyzed the effects of well resistance, smearing effect, compression index, and permeability index ratio on consolidation characteristics. Land reclamation projects will generate a large amount of dredged ultrasoft soil foundation. The AVP method can achieve superior foundation treatment effects by injecting gas into soil mass through air-booster pipes. The results of this case study demonstrate that the AVP method has substantial advantages compared to the conventional vacuum preloading method. The AVP method can increase foundation settlement, shorten the time for foundation treatment construction, and improve the foundation reinforcement effect. In addition, the AVP method can alleviate PVD clogging and maintain soil permeability, while simultaneously avoiding the excessive use of dredging sand. Therefore, the AVP method can lead to potential economic benefits and protect the natural environment, and it is a method worth considering. Moreover, the mathematical model for predicting consolidation settlement of the AVP method was proposed based on new mechanical assumptions, which can be applied as a reference for settlement prediction in relevant engineering, and the rationality has been verified through comparison with measured settlement results. [ABSTRACT FROM AUTHOR]

A reclamation project was undertaken in Zhuhai, China, covering an area of 11.35 km2 with a clay-slurry thickness of 30 m. It is interesting to find a good reinforcement method, which is more suitable for strengthening such a large-scale dredged clay-slurry ground. Evidence from engineering practice shows that vacuum preloading or vacuum preloading combined with surcharge preloading method technology has certain advantages. However, further data is required to verify their performance in the field. This paper presents a case study using the conventional vacuum preloading and vacuum preloading combined with surcharge preloading method (vacuum pressure of 90 kPa, surcharge height of 3 m) to improve the thick clay-slurry ground for a subgrade engineering project in Zhuhai, China. A series of field monitoring, in-situ tests, and laboratory tests were performed to evaluate their treatment effect by analysis of vacuum degree, ground surface settlement, layered settlement, consolidation degree, lateral displacement, excess pore water pressure dissipation, and vane shear strength. The results showed that the ground treated via vacuum preloading combined with surcharge preloading method was more effective than conventional vacuum preloading. The vacuum degree of vacuum preloading combined with surcharge preloading method was slightly lower than that of conventional vacuum preloading, decreasing with depth. The settlement is increased by 55.56%, average consolidation degree is enlarged by 40.9%, lateral displacement can be augmented by 2.4 times, pore water pressure coefficient is less than 0.6, and undrained shear strength is amplified by 22.2%. These findings can be used to guide the design of the combined preloading method of the subgrade engineering project along the east coast of China. [ABSTRACT FROM AUTHOR]

The deformation of soft clays under cyclic loading is controlled by microfabric changes. Cyclic triaxial tests, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted to investigate the deformation and the corresponding micromechanism under cyclic loading. The correlations between the microparameters and deformation of Tianjin soft clay are discussed. The deformation increases linearly in the initial compression stage (number of cycles, N < 500), at a decreasing rate in the later shearing stage (N < 5000), and eventually stabilizes when the cyclic stress ratio (CSR) is 0.30. The probability entropy of both the soil particles and pores have a weak correlation with the deformation on the whole, reflecting a fluctuation within a small range in the initial compression stage. The correlations between the median pore diameter, median particle diameter, and specific surface area and the deformation are validated by using the gray correlation method. More specifically, the specific surface area and the deformation are highly correlated in the initial compression deformation stage. The median pore diameter and median particle diameter are highly correlated with deformation in the later shearing deformation stage. Key words: soft clays, deformation, micromechanism, gray correlation method. La deformation des argiles tendres sous charge cyclique est controlee par des changements de la microstructure. Des essais triaxiaux cycliques, la microscopie electronique a balayage (MEB) et la porosimetrie au mercure (PM) ont ete effectues pour etudier la deformation et le micromecanisme correspondant sous une charge cyclique. Les correlations entre les microparametres et la deformation de l'argile tendre de Tianjin sont discutees. La deformation augmente lineairement dans la phase initiale de compression (nombre de cycles, N < 500), a un taux decroissant dans la phase ulterieure de cisaillement (N < 5000) et se stabilise finalement lorsque le rapport de contrainte cyclique (<< CSR >>) est de 0,30. L'entropie de probabilite des particules du sol et des pores a une faible correlation avec la deformation dans son ensemble, refletant une fluctuation dans une petite fourchette dans la phase initiale de compression. Les correlations entre le diametre median des pores, le diametre median des particules et la surface specifique et la deformation sont validees en utilisant la methode de correlation des gris. Plus precisement, la surface specifique et la deformation sont fortement correlees dans la phase initiale de deformation par compression. Le diametre median des pores et le diametre median des particules sont fortement correles avec la deformation au stade ulterieur de la deformation par cisaillement. Mots-cles : argiles tendres, deformation, micromecanisme, methode de correlation des gris., 1. Introduction The dynamic responses of soft clays under cyclic loading are affected by the confining pressure, number of cycles (N), cyclic stress ratio (CSR), and vibrational frequency (f). The [...]

AbstractThis case study proposed a novel electro-osmosis PRD vacuum preloading method to solve dredging sludge treatment issues: difficulty in draining from soil showing large volume, non-uniform settlement, and low strength. To verify the effectiveness of the new method, four kinds of physical model tests integrating particle image velocimetry (PIV) technique of traditional vacuum preloading (VP), prefabricated radiant drain vacuum preloading (PRD-VP), electro-osmotic vacuum preloading (EO-VP), and EO-PRD-VP methods are conducted. The water discharge, average surface settlement, pore water pressure, water content, and undrained shear strength after treatment, clogging range, relationship between clogging range and water discharge rate, and relationship between clogging range and average surface settlement are investigated. For those model tests, it is demonstrated that EO-PRD-VP method has the best advantage in volume reduction, uniform settlement, and strength improvement. Water discharge is enlarged by 13–33%. The differential settlement can reach 2.3 cm, decreased by 28–56%. The undrained shear strength can reach 12 kPa, increased by 1–2 times. In addition, the clogging range development is described, for the given water discharge rate and average surface settlement, clogging range of EO-PRD-VP method is the minimum. The empirical equations between clogging range and water discharge rate, clogging range, and average surface settlement are established to predict the clogging range, which can lay the foundation for developing the consolidation theory of EO-PRD-VP method. [ABSTRACT FROM AUTHOR]

Dredged fills have been widely used as a filling material to meet the demand for land resources during land reclamation in coastal areas. When the conventional vacuum preloading (CVP) method is used to reinforce dredged fill foundations, clogging and accumulation of a dense silt layer significantly reduce the lateral soil permeability of the soil surrounding prefabricated vertical drains (PVDs), leading to a poor reinforcement effect. To address these issues, this paper presents a laboratory model study to evaluate the reinforcement effect of a new method, the stepped-alternating vacuum preloading method (SAVP), in comparison to the CVP method. A series of laboratory model tests were conducted to compare the results of the new SAVP method and the CVP method. The model tests used water discharge, discharge rate, and surface settlement to compare both methods. Additionally, vane shear strength and moisture content were measured after reinforcement. This study evaluates the performance of the SAVP method and proposes an optimal preloading pattern that demonstrates best performance in delaying clogging formation and alleviating the accumulation of a dense silt layer on the surface of the PVDs. [ABSTRACT FROM AUTHOR]

The long-term deformation characteristics and damage degree of soft marine-deposited clay must be evaluated to safely design seawalls. According to abundant field evidence, soft marine-deposited clay tends to experience coupled dynamic loading and seepage loading. Determining the critical load and damage index for these complex loading conditions is imperative. With Tianjin marine-deposited clay as an example, we employed an improved cyclic triaxial instrument to analyse the cumulative plastic strain, water discharge and stress–strain relationship of the clay to understand its long-term deformation development. Based on shakedown theory, the critical cyclic stress ratio (CSR) was determined. A multi-factor damage variable method was proposed to evaluate the damage degree. Its comparison with a traditional mono-factor damage variable method illustrated that this method is effective for damage analyses of marine clay around seawalls under coupled dynamic-seepage loading. [ABSTRACT FROM AUTHOR]

The large amount of dredged soil produced annually by rivers, lakes, seas, and oceans affects the ecological environment. Dredged material such as soft clay, which has high fluidity and high water content, is often utilized for building ground in engineering practice. Conventional vacuum preloading can effectively treat such soil but has a deprived consolidation effect. To improve the treatment effect, in this study, prefabricated radiant drain (PRD) vacuum preloading technology is proposed innovatively by adding prefabricated horizontal drains (PHDs) based on a prefabricated vertical drain (PVD). And a four-way connector is designed to connect PVD and PHDs, PHDs are attached to PVD to transmit vacuum pressure. A series of laboratory tests are run to compare and analyze the differences between conventional PVD and PRD vacuum preloading. The water discharge, average surface settlement, water content, and undrained shear strength values are analyzed. Then the parametric analysis of PHD spacings and length-width ratios (characterizing PHD length) are investigated. Overall, with the new PRD method, the consolidation effect of soft clay ground with a water content of 80% is improved by approximately 15%. [ABSTRACT FROM AUTHOR]

It is essential and important for artificial ground freezing (AGF) engineering design and maintenance to systematically investigate the mechanical properties of artificially frozen soil. Based on a series of low-temperature triaxial shear tests, the mechanical properties of artificially frozen clays under different freezing methodologies (including freezing mode, freezing temperature, and confining pressure) were investigated and compared in this study. Importantly, the commonly used mechanical parameters of artificially frozen soil do not consider the effect of pressure during freezing, and the results presented that the stress–strain and shear strength behaviors of the artificially frozen clay freezing under pressure were better than those bearing freezing without pressure. In addition, both the lower freezing temperature and the higher confining pressure could lead to a notable improvement in the shear strength behavior of the artificially frozen clay. Furthermore, the significance of freezing factors was analyzed and revealed in this study, and the results demonstrated that the mechanical properties of the artificially frozen clay were more sensitive to the changes in freezing temperature, while the higher confining pressure would weaken the influence of the freezing temperature on mechanical properties. This could motivate, considering the effect of freezing conditions, especially the pressure during freezing, the development of a more practical design in AGF engineering. [ABSTRACT FROM AUTHOR]

The high-water-content sludge produced through dredging seriously restricts marine ecological environments. Vacuum preloading methods (involving prefabricated vertical drains (PVDs)) are widely used to dewater and consolidate sludge ground, but these methods require long treatment periods and have deprived reinforcement effects. To overcome the limitations of PVD vacuum preloading, a prefabricated radiant drain (PRD) vacuum preloading method is proposed is this study by adding a prefabricated horizontal drain (PHD) to the PVD framework. Physical model tests are conducted under PVD and PRD vacuum preloading conditions, and a large-strain three-dimensional finite element (FE) model is established and validated against the physical model test data. FE parametric analyses are also conducted to evaluate the key factors influencing the PRD, i.e., the PHD length and spacing. The results show that the reinforcement effect of PRD vacuum preloading is better than that of PVD vacuum preloading. The ground settlement effect increases as the PHD length increases and the PHD spacing decreases. The settlement and pore water pressure metrics show elliptical distributions with an increasing PHD length. Through this work, an applicable sludge ground treatment design procedure is suggested based on parametric analyses; these findings can provide a reference for designing sludge ground treatments in the future. [ABSTRACT FROM AUTHOR]

In a full-scale case study, the performance of two methods—the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)—were compared in terms of the dredged soft soil ground of the Xiang'an International Airport pavement reinforcement project in Xiamen City, China. The pore water pressure dissipation, settlement, lateral displacement, and consolidation degree were measured in the zones reinforced using the two methods. The direct shear test and field vane shear test were conducted before and after improvement. The S-VP and A-VP methods were loaded for 308 and 228 days, respectively. The results show that both reinforcement methods significantly improved the shear strength and foundation bearing capacity of the soft soil ground. The S-VP method has about 30% higher final vane shear strength and more uniform settlement compared to the A-VP method. However, the dissipation value of the average pore water pressure and the maximum foundation settlement were 5.6% and 11% higher, respectively, in A-VP than in S-VP. Compared to the S-VP method, the A-VP method decreased the maximum lateral displacements by 15%–20%, and increased the maximum reinforcement depth by 11%. Finally, compared with the use of sand cushion and vacuum pump, the A-VP method was more economical in terms of cost and energy use. Land reclamation projects will produce a large area of ground composed of ultrasoft soil, which has very low strength and cannot be used for construction activities directly on such ground. For such ultrasoft soil ground, the vacuum preloading method is generally used to strengthen the ultrasoft soil. In this study, two different vacuum preloading methods [the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)] were used to strengthen the ultrasoft soil, and the final reinforcement effect and cost of the two methods were compared. The results show that the final reinforcement effect of the A-VP method was close to that of the S-VP method. Furthermore, the A-VP method saved more than 3 million Chinese currency (CNY) and reduced the power consumption by 80% in this project. In addition, the implementation of the A-VP method does not require a large amount of sand that is difficult to obtain in coastal areas, which is good protection for the natural resources in coastal areas. Therefore, in the ultrasoft soil reinforcement project, the A-VP method is a method worth considering. [ABSTRACT FROM AUTHOR]