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Freezing points play a crucial role in distinguishing between salt expansion and frost heave in frozen saline soil. Through a series of cooling experiments, the freezing points of sodium chloride and sodium sulfate saline soils were measured, considering various salt concentrations and ratios of c(Cl−)/2c( SO 4 2 - ). The results revealed a three-stage cooling process: rapid cooling, constant cooling, and decreasing cooling. The freezing point of saline soil decreases with increasing salt concentration ranging from 1.5 to 4.9 times, attributed to the presence of more hydrated water molecules. Furthermore, influenced by the solubility of sodium sulfate, the freezing point of saline soil decreases with an increasing ratio of c(Cl−)/2c( SO 4 2 - ) in the range of 0.7–4 times. The reduction of water content, on the other hand, enhances capillary action and soil particle adsorption, causing the freezing point of the soil to decrease by 0.5 to 1.7 times. To accurately predict the freezing point of saline frozen soil, a prediction model was proposed and validated, taking into account the effects of salt types, salinities, and water contents. This research serves as a valuable theoretical reference for predicting the freezing point of saline frozen soil. [ABSTRACT FROM AUTHOR]

This study employs a series of finite element models and field monitoring data to systematically investigate the deformation characteristics and protective measures of railway subgrades adjacent to excavation sites for groups of foundation pits. A baseline model with a single foundation pit is established based on a project to erect a viaduct in an area with soft soil, and the model is verified by field monitoring data. The effects of the number of foundation pits, the distance between adjacent foundation pits, and the distance between a foundation pit and an adjacent railway subgrade are studied. The results show that increasing the number of foundation pits, reducing the distance between adjacent foundation pits and decreasing the distance between a foundation pit and the railway subgrade lead to excessive settlement and horizontal deformation of the subgrade. Three kinds of subgrade protection measures, namely, increasing the thickness of the retaining structure, setting isolation piles, and adding grouting reinforcement, are proposed and analysed. Increasing the thickness of the retaining structure significantly reduced the horizontal displacement and settlement of the subgrade. [ABSTRACT FROM AUTHOR]

A series of soil cutting tests was firstly carried out on the Earth on a ground composed of Tongji-1 lunar soil simulant (TJ-1 simulant) to experimentally investigate the effects of cutting depth, blade width, rake angle and blade arc angle on the excavation force. Then the tests were numerically simulated with the Finite Element Method (FEM), where a constant velocity was applied to the soil in contact with the blade, namely the soil cutting method or a rigid blade was generated and moved to cut the soil, namely the blade cutting method. The results show that the FEM simulation using the soil cutting method can provide excavation forces close to the experimental data when the rake angle is 0°, and well capture the effects of cutting depth, blade width and blade arc angle except the rake angle. The FEM simulation using the blade cutting method can capture the effect of rake angle while the predicted excavation forces are much larger than the experimental data. Nevertheless, both the methods cut the soil to a small displacement beyond which the analysis is unable to meet the convergence requirement. [ABSTRACT FROM AUTHOR]

This study illustrates and discusses the physical properties, mechanical behavior, and microscopic characteristics of the South China Sea soft soil, and a comparison with the Bohai Sea marine clay, the Yellow Sea soft clay, and some typical marine soils is presented. In addition, the mechanisms of the difference in the geotechnical properties of these marine soils are revealed. Compared with the soils from other sea areas, the South China Sea soft soil presents remarkable characteristics of higher water content, larger void ratio, higher liquid limit, and higher content of fine particles. Besides, the South China Sea soft soil shows higher compressibility. As shown in the microscopic characteristics, the South China Sea soft soil contains lower content of clay minerals; in addition, due to the depositional and geological environments with higher salinity and higher temperature, there are more macropores, connected pores, and trellis pores in the South China Sea soft soil, so it presents a more open, flocculated structure rather than a granular-connection structure like the Bohai Sea marine clay and the Yellow Sea soft clay. The results presented in this paper will provide an important theoretical basis for revealing the engineering properties of marine soils, and improve the insights for ensuring the safety of ocean engineering during construction and service periods. [ABSTRACT FROM AUTHOR]

The tunnelling-induced ground deformation can be affected by the existing tunnel(s) in overlapped tunnels construction, especially when the existing tunnel is in operation. Therefore, the effect of subway train vibration loads on the ground movement induced by overlapped shield tunnelling is needed to further investigate. In this study, model tests and numerical simulations were conducted to investigate ground deformation caused by overlapped tunnelling while taking subway train vibration loads into account. The research results indicated that with increasing tunnel clearance, the settlement trough appeared to be a much steeper U-shape. At d/D = 0.5, the maximum ground settlement increased by 37.5% along the transverse section and 30.6% along the longitudinal section compared to the case of the vibration-free condition. As d/D increased from 0.5 to 1.0, the maximum value of ground settlement decreased by 34.1% along the transverse section and 31.25% along the longitudinal section. The ground lateral displacement appeared to be biconcave and flattened with increasing d/D. Based on the test results, a model was introduced to predict the ground settlement induced by overlapped tunnelling coupled with vibration loads in subway train. The findings in this study will help evaluate the ground deformation caused by overlapped tunnelling considering vibration loads of subway train in operation. [ABSTRACT FROM AUTHOR]

Recent research has shown that in a parallel twin-tunnel arrangement, the 2nd tunnel generates greater settlement than the 1st tunnel as the soil between the two tunnels softens. For other spatial twin-tunnel arrangements (twin tunnels with different cover depths), the pattern of settlement can be more complex and is inevitably affected by the order in which the two tunnels are constructed. Few studies have attempted to establish the relationship between the changes in soil stiffness and settlement behaviour. Hence, the settlement behaviour induced by twin tunnels is not yet well understood. A numerical model was created in this study, and a centrifuge model test was then used to verify the performance of this model. A series of comparative numerical analyses (e.g., inclined and overlapping twin tunnel arrangements) were performed to investigate the effect of the construction order and relative positions of the two tunnels. The relationship between the changes in soil stiffness and settlement behaviour was well established. The mechanisms by which the 1st tunnel affects the settlement behaviour produced by the 2nd tunnel were clarified. [ABSTRACT FROM AUTHOR]

The geotechnical behavior of clay is known to be influenced by its external environment chemistry. In this study, a series of experiments were conducted to investigate the geomechanical behavior, mineralogical behavior, and microscopic characteristics of Tianjin marine clay when subjected to an acidic environment. The micro-mechanisms of these phenomena were thus revealed. It is interesting to note that the increasing environmental acidity would promote the illitization in illite–smectite and conversion in part of the smectite component in illite–smectite to chlorite. In addition, our results indicate that a critical pH developed with the increasing environmental acidity, leading to phased variations in strength and compressibility. When the environment pH was greater than its critical value, the increasing environmental acidity led to an increase in strength together with a decrease in compressibility, which was mainly related to the reduced thickness of the diffuse double layer and improved intermolecular force between soil particles. However, with an environment pH lower than its critical value, the geotechnical behavior of Tianjin marine clay was deteriorated as a result of more open, flocculated microscopic structures and low intergranular cementation strength in the marine clay, such that the strength decreased while compressibility increased as the environment became more acidic. [ABSTRACT FROM AUTHOR]

Geotechnical investigation and design show that when evaluating the settlement of ground with soft soil ground, the use of a single stress path results in a larger error than in the case of a multistress path. To understand the influence of a multistress path on the deformation properties and microstructure characteristics of soil, a two-stage stress path (corresponding to three stress states: the in situ state, unloading-loading state, and grouting pressure state) of soil was examined by analytical solution. Then, a series of triaxial tests was conducted to analyze macroscopic deformation properties considering four influencing factors, i.e., the relative distance between the soil and tunnel, stress paths, type of silty clay, and drainage conditions. The results show that axial strain of silty clay caused by the two-stage stress path increases by 31.6% with the relative distance decreasing from 1.67R (R means tunnel radius) to 1.25R on tunnel side, while the volume strain increases by 80% with the relative distance decreasing from 1.21R to 1.06R on tunnel bottom. In addition, the deformation caused by the two-stage stress path (or reconstituted soil or undrained condition) is larger than that caused by the one-stage stress path (or undisturbed soil or drained condition). Finally, through scanning electron microscopy and mercury intrusion porosimetry, the pore size, distribution, shape, and arrangement of silty clay on the bottom and sides of the tunnel were investigated. The research shows that under the two-stage stress path, the number of large pores decrease and that of small pores increase. The probability entropy of pore orientation ranges from 0.834 to 0.971 when silty clay is subjected to a two-stage stress path. This illustrates that the pore order of the soil on tunnel side is better than that on the tunnel bottom. [ABSTRACT FROM AUTHOR]

The properties of a soil, either in situ or at a hydraulic reclamation site, may change if influenced by the chemical status of the surroundings. Acidity, indicated by the pH, and magnesium (Mg2+), sometimes used to reinforce soil, are good indicators of the chemical environment. It is therefore worth investigating how acidity and Mg2+ affect the settling of dredged fill. In this study, a cylinder model test and ethylenediaminetetraacetic acid (EDTA) titration were used to investigate the mechanisms behind self-weight consolidation settlement. The results indicated that the settlement process of the dredged fill occurred in three stages and that the consolidation settlement process of the dredged fill varied as the pH and mass percentage of Mg2+ varied. As the pH increased from an initial low value, the settlement amount and settlement rate of the dredged fill approximated to a quadratic function trend and increased first and peaked when the pH was 4 and then decreased, reflecting the influence of the mineral composition of the soil and the Zeta potential of the soil particles. Under medium pH conditions, the settlement amount and settlement rate increased in an approximately linear manner, reflecting increases in the mass percentage of Mg2+ and changes in the double electric layers on the clay mineral surface. [ABSTRACT FROM AUTHOR]

Many uncertain factors in the excavation process may lead to excessive lateral displacement or overlimited internal force of the piles, as well as inordinate settlement of soil surrounding the existing bridge foundation. Safety control is pivotal to ensuring the safety of adjacent structures. In this paper, an innovative method is proposed that combines an analytic hierarchy process (AHP) with a finite element method (FEM) to reveal the potential impact risk of uncertain factors on the surrounding environment. The AHP was adopted to determine key influencing factors based on the weight of each influencing factor. The FEM was used to quantify the impact of the key influencing factors on the surrounding environment. In terms of the AHP, the index system of uncertain factors was established based on an engineering investigation. A matrix comparing the lower index layer to the upper index layer, and the weight of each influencing factor, were calculated. It was found that the excavation depth and the distance between the foundation pit and the bridge foundation were fundamental factors. For the FEM, the FE baseline model was calibrated based on the case of no bridge surrounding the foundation pit. The consistency between the monitoring data and the numerical simulation data for a ground settlement was analyzed. FE simulations were then conducted to quantitatively analyze the degree of influence of the key influencing factors on the bridge foundation. Furthermore, the lateral displacement of the bridge pile foundation, the internal force of the piles, and the settlement of the soil surrounding the pile foundation were emphatically analyzed. The most hazardous construction condition was also determined. Finally, two safety control measures for increasing the numbers of support levels and the rooted depths of the enclosure structure were suggested. A novel method for combining AHP with FEM can be used to determine the key influencing aspects among many uncertain factors during a construction, which can provide some beneficial references for engineering design and construction. [ABSTRACT FROM AUTHOR]

In this paper, the deformation response of the stratum in the vicinity of a tunnel is investigated based on a case study in Tianjin. A method to predict the surface settlement in the case of double-track curvature shield tunneling is also proposed. The research methodology consists of analytical calculations, finite element analysis as well as field monitoring. Straight shield tunneling and curvature shield tunneling are modeled, each with a single-track shield and a double-track shield, and their operational parameters are compared. The influence of overcutting and existing tunnel on ground movement is also discussed. The salient observations from the series of analyses are as follows: (a) The maximum settlement induced by curvature shield tunneling is much greater than that induced by straight shield tunneling, while the existing tunnel has little impact on stratum settlement in contrast, (b) the proposed method of predicting the surface settlement induced by double-track curvature shield tunneling fits well with the field-monitored data in the Tianjin case study, and (c) the location of the maximum ground settlement is deviated by the overcutting phenomenon, whereas the settlement of the soil above the excavated tunnel is reduced due to the presence of the existing tunnel. [ABSTRACT FROM AUTHOR]

Slope failure occurring could generate enormous loss to both human lives and properties, and stabilizing piles are broadly used for slope reinforcement as a significant anti-slip retaining structure. The performance of the stabilizing piles is strongly affected by different locations. Presently, most literature focuses on the location selection of single-row stabilizing piles. This paper introduces a classic slope and makes further stability analysis of the slope reinforced by single- and double-row stabilizing piles in different locations. According to the potential sliding surface, the distribution of the bending moment and the thrust, the results show that the failure modes of the slopes reinforced by single- and double-row stabilizing piles are different. As the single-row piles move towards the top of the slope, the failure mode of the piled slope takes on a "shallow-deep-shallow" trend. Moreover, as the pile location and spacing change, the failure modes stabilized by double-row piles can be divided into five categories which could be classified as shallow and deep sliding, and the thrust ratio increases with the increasing pile spacing. Regarding the safety factor, it can be concluded that the optimal location for single-row stabilizing piles within a slope is the lower-middle part of the slope, and the optimal locations for double-row stabilizing piles are proposed at the lower and lower-middle parts of the slope, respectively. The presented analysis could provide significant insight into the design of slopes stabilized by stabilizing piles. [ABSTRACT FROM AUTHOR]

The stability of the shield tunneling face is an extremely important factor affecting the safety of tunnel construction. In this study, a transparent clay with properties similar to those of Tianjin clay is prepared and a new transparent clay model test apparatus is developed to overcome the "black box" problem in the traditional model test. The stability of the shield tunneling face (failure mode, influence range, support force, and surface settlement) is investigated in transparent clay under active failure. A series of transparent clay model tests is performed to investigate the active failure mode, influence range, and support force of the shield tunneling face under different burial depth conditions, whereas particle flow code three-dimensional numerical simulations are conducted to verify the failure mode of the shield tunneling face and surface settlement along the transverse section under different burial depth conditions. The results show that the engineering characteristics of transparent clay are similar to those of soft clay in Binhai, Tianjin and satisfy visibility requirements. Two types of failure modes are obtained: the overall failure mode (cover/diameter: C/D ⩽ 1.0) and local failure mode (C/D ⩾ 2.0). The influence range of the transverse section is wider than that of the longitudinal section when C/D ⩾ 2.0. Additionally, the normalized thresholds of the relative displacement and support force ratio are 3%–6% and 0.2–0.4, respectively. Owing to the cushioning effect of the clay layer, the surface settlement is significantly reduced as the tunnel burial depth increases. [ABSTRACT FROM AUTHOR]

Core samples to a depth of approximately 50 m and undisturbed soil samples were retrieved from eight locations in the Saga Plain, Japan, to investigate the effects of the depositional environment and post-depositional processes on the engineering properties of Quaternary clays in the Saga Plain. The engineering properties of these soil samples were measured, and the test results showed that the clays in the Saga Plain have a high void ratio, high water content, high compressibility, high sensitivity, and low stiffness. The one-dimensional compression curves plotted above the sedimentation compression line proposed by Burland, and the compression index was high, which indicated that the clays had been cemented to some degree. The clays have high strength sensitivity and high stress sensitivity, indicating that they have strong structures. It was considered that a considerable amount of volcanic ash from the four large volcanic eruptions that occurred during the deposition of these clays settled into the clays, resulting in a certain degree of cementation, which was attributed as the main driver of the strong structure in these clays. Comparisons of the NaCl concentration, pH, and oxidation-reduction potential (ORP) of the clay samples representing the initial depositional environment and the current ground environment showed that salt leaching occurred in the Ariake clay formation, and that the ground environment changed. When the NaCl concentration was less than approximately 2 g/L, the strength sensitivity of the Ariake clay samples with low-activity clay minerals significantly increased due to the decrease in the liquid limit and subsequent significant decrease in the remolded undrained shear strength. Therefore, salt leaching was probably a main cause of the high strength sensitivity of the clays in the Saga Plain. The depositional environment at some locations changed from an alkaline, reducing environment, to an acidic, oxidative environment, with the potential to change into a strongly acidic environment, which could have deteriorated the strength of the cement/lime-treated clays in the Saga Plain. [ABSTRACT FROM AUTHOR]

Ultra-soft soils have high water content, low strength, and high compressibility. Staged preloading has been commonly used in China to improve ultra-soft soils during land reclamation before highways and other structures are constructed. In this study, seven series of one-dimensional consolidation tests were performed on undisturbed, remolded, or reconstituted specimens of three types of soils subjected to simulated, staged loading. Test results showed that the compression curves had two straight lines, which intersected at a yield stress. Burland's concept of the intrinsic compression line was adopted to present the compression curves of reconstituted clays at different initial water contents. The intrinsic compression line was modified to better fit the test data for the stresses lower than 40 kPa. The coefficient of soil consolidation increased with an increase of the effective vertical stress as a result of the rate of increase in constrained modulus higher than that of decrease in permeability. The maximum coefficient of secondary consolidation, C, was correlated with the ratio of the initial void ratio to the void ratio at yield stress by a unique curve, which was proposed to distinguish the pre-yield stress from the post-yield stress state. Test results showed that the secondary compression characteristics depended on the applied stress and the initial void ratio. [ABSTRACT FROM AUTHOR]