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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]

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]

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]

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. [ABSTRACT FROM AUTHOR]

It is important to assess the permeability characteristics of clays in the prediction of the postconstruction settlement of geotechnical projects on clay grounds. The coefficient of permeability in the vertical direction (kv) and horizontal direction (kh) of clays under different vertical effective stresses were measured using newly developed permeability test equipment to investigate the variations in the permeability and permeability anisotropy of reconstituted clays and their influential factors under one-dimensional (1D) compression. The evolution of the pore sizes and pore orientations was also investigated to clarify the corresponding micromechanisms. The test results showed that the permeability of the reconstituted clays was related to the mineral composition, clay content, void ratio, and clay fabric. The variations in kv and kh with the void ratio could be represented in terms of a linear e–log(k) relationship. Under the same strain, the values of kh were obviously larger than those of kv, and the permeability anisotropy ratio (kh/kv) values increased from 1 to 2 with increasing vertical strain. For all the tested reconstituted clays, both kv and kh generally decreased linearly with decreasing cumulative pore volume. For different clays with the same cumulative pore volume, kv and kh decreased with an increase in the plasticity index. In general, the anisotropy index of the pores increased, and the main orientation angle of the pores decreased with increasing vertical effective stress. For a certain clay, the increase in permeability anisotropy with vertical strain resulted from the gradual development of microstructural anisotropy. The findings of this study may assist in the accurate prediction of the consolidation behavior of reconstituted clays with vertical and horizontal drainage. [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 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]

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. [ABSTRACT FROM AUTHOR]

The compressibility and microstructural changes of four types of reconstituted clays, Tianjin, Shanghai, kaolin, and montmorillonite clays, were studied to investigate the effects of mineral composition and clay content on the compressibility and microstructure evolution of clays during one-dimensional (1D) compression. The scanning electron microscopy test results indicated that the clays were characterized by an aggregated structure with inter- and intraaggregate pores. A preferential orientation of the particles toward the horizontal plane was induced during the 1D compression. The mercury intrusion porosimetry test results showed that the cumulative pore volume clearly decreased and that the volume and size of the widely distributed interaggregate pores decreased with the increase of the vertical effective stress. For montmorillonite clay, particle reorientation was obvious due to the more loosened microstructure, and the volume of mesopores was much larger compared to other clays, which resulted in the largest compressibility. For kaolin clay, a distinct preferential orientation of the particles toward the horizontal plane was induced due to the more planar microstructure even under the lower vertical effective stress, and the decrease of the pore volume mainly arose from micropores compared to mesopores in other clays. For the tested Tianjin and Shanghai clays, the main minerals were the same, but the compressibility of Tianjin clay was larger. It seems that the higher the clay content, the larger the particle reorientation and the cumulative pore volume, the higher the compressibility. [ABSTRACT FROM AUTHOR]

Subgrade soft clays are subjected to intermittent changeable-frequency cyclic loading and exhibit complex dynamic reactions compared with those under single-frequency cyclic loading. This research proposed a novel testing scheme to take the influence of combined-frequency subway train speed and intermittency into account. The testing procedure consisted of five loading periods in which occurred undrained combined (low–high–low) frequency cyclic loading and drained static loading, and post-cyclic loading consolidation settlement. A series of triaxial tests on Tianjin soft clay were conducted under intermittent combined-frequency cyclic loading conditions. The pore pressure, deformation, shear modulus and damping ratio, and the post-cyclic-loading consolidation settlement were investigated. The pore pressure and deformation increment decreased greatly when N (the number of cycles in each loading period) = 1,000 (200 × 5), but smoothly when N = 500 (100 × 5) from the first to fifth loading period. The intermittency made the soft clays more resistant to degradation of shear modulus and the influence of subway train speed on the shear modulus and damping ratio should be considered in dynamic design. The post-intermittent combined-frequency cyclic-loading consolidation settlement presented a stable increasing pattern when the cyclic stress ration (CSR) = 0.10. The subway train speed had an evident influence on the post-intermittent combined-frequency cyclic-loading consolidation settlement of soft clays. [ABSTRACT FROM AUTHOR]

Marine clays often need improvements before construction and how to stabilize such kind of soft soil has been a great challenge. To address this issue, a newly proposed additive, which is named anionic polyacrylamide (APAM), with lime was applied to the clay stabilization. The main objective of this study is to figure out the effect of APAM on lime-treated marine clay and the micromechanism of stabilization. The unconfined compressive strength (UCS) tests and oedometer tests were performed to investigate the mechanical properties of stabilized clays; the X-ray diffraction (XRD) tests were conducted for mineralogical analysis; the mercury intrusion porosimetry (MIP) tests and scanning electron microscopy (SEM) tests were carried out for microstructural study. The experiment results show that with APAM dosage increasing, the stabilization effect first enhanced and subsequently weakened. The mineralogical analysis reveals that no new compounds were detected as APAM was added. The microstructural study shows that the physical adsorption of APAM to clay particles was conducive to form a more stable clay structure. The Ca2+ produced by lime served as the cationic bridge, facilitating the APAM adsorption. However, the electrostatic repulsion between APAM and negatively charged clay particles and the inhibitory effects of APAM on pozzolanic reactions were the main factors weakening the stabilization effect. This study indicates that the APAM/lime treatment method is efficient for marine clay stabilization as long as a rational APAM dosage is selected. [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]

The causes, prevention, and control of clay disasters are of great concern in practical engineering applications. Recently, due to unprecedented economic development, increasing numbers of tunnels and subgrades of tidal flat areas have been built in Tianjin, China. Soft soil ground not only bears the vibration load during operation and construction but also receives seepage effects caused by the bursting of pipelines and variations in groundwater levels. Under the coupling action of dynamic and seepage loads, large settlement can occur in soft soil since the deformation is related to both cyclic stress and seepage. Therefore, it is significant to understand the dynamic deformation characteristics of soil to ensure the safety of engineering applications. In this study, a series of laboratory cyclic triaxial tests were conducted to study the deformation behaviours of Tianjin clays under coupled cyclic stress and seepage fields. To simulate the seepage field, water pressure is applied at the bottom of the specimen, and the water can be drained from the top of the specimen to form a head difference in the specimen during the tests. The effects of the seepage pressure, cyclic stress ratio (CSR), and a number of cycles (N) on the hydraulic conductivity, axial deformation, hysteresis curve and dynamic modulus of Tianjin clays were systematically investigated. The laboratory results show that the hydraulic conductivity (k) decreases with the increasing seepage pressure, CSR, and N, and its range is from 4.6 × 10−8 cm/s to 7.4 × 10−8 cm/s. The axial deformation of the soil increases with increasing CSR and seepage pressure. With an applied water pressure of 150 kPa at the bottom of the specimen and a CSR of 0.3, the maximum cumulative plastic strain reaches 4.78%, increasing by 53.2% compared with a CSR of 0.1. The hysteresis curve gradually becomes denser or thinner with increasing CSR and N; the hysteretic curve tends to a straight line for N of 5,000. The dynamic modulus increases with increasing N and depends on the CSR and seepage pressure. Empirical equations are suggested for calculating the dynamic modulus with different CSR values and seepage pressures. The results of this study can provide reference parameters for the construction and design of tunnels and subgrades of tidal flat areas in clay strata under the influence of water environments. [ABSTRACT FROM AUTHOR]

The mechanical behavior of clay subjected to cyclic loading is important to consider in the design of the foundations of many types of structures that must resist cyclic loading, such as subgrades and offshore foundations, because clay undergoes greater settlement under cyclic loading than under static loading. The difference in settlement between these two loading patterns due to creep behavior is affected by the cyclic frequency and the cyclic stress ratio. This study investigated the effects of the frequency and cyclic stress ratio of cyclic loading on the creep behavior of a natural clay in China using stress-controlled triaxial tests. The assessed the following parameters: three frequencies, four cyclic stress ratios, and six vertical stresses. The test results indicate that the soft clay displays accelerated creep behavior under dynamic loads. A specific “limit frequency” (in this case, 0.2 Hz) and a “safe load” at which the strain of the soft clay increases very slowly were observed. The effect of the effective axial stress on the creep behavior increases with the increase in the cyclic stress ratio. Based on the tests, the critical cyclic stress ratio is 0.267 at a certain effective axial stress and frequency. [ABSTRACT FROM AUTHOR]

Subgrade soils are subjected to coupled dynamic-static and changeable-frequency traffic loading. Considering different cyclic modes of single-frequency, coupled dynamic-static and intermittent combined-frequency loading, deformational characteristics of the Tianjin soft clay in each vibration period were analyzed by triaxial testing. Results show that the accumulative plastic strain and pore pressure have quite different development patterns under coupled dynamic-static and intermittent combined-frequency loading than single-frequency cyclic loading. The combined-frequency loading caused a considerable increase of pore pressure increment, but intermittency resulted in a decrease. During the loading of either coupled dynamic-static or intermittency, the static loading caused the soft clays to be more resistant to later undrained cyclic loading. The intermittency and combined-frequency loading have an integrated effect on the accumulative plastic strain development pattern and increment during each vibration period. The degradation index under the coupled dynamic-static loading decreased much less than that under single-frequency loading, especially with increasing cyclic stress ratio. • An experimental method to simulate the subway loading considering effect of intermittency and combined-frequency is proposed. • Deformation development pattern and increment of soft clays in each vibration period are investigated rather than the overall pattern under conventional cyclic loading. • The influence of intermittency and combined-frequency on soft clay pore pressure and deformation are compared with those under single-frequency cyclic loading. [ABSTRACT FROM AUTHOR]

In recent years, the artificial ground freezing (AGF) method has been widely used in subway construction, and the engineering properties of soil have changed significantly after freezing and thawing. In this paper, a series of dynamic triaxial tests were used to investigate the pore pressure development of freeze-thaw soft clay under different cyclic dynamic stress ratios, loading frequencies and freezing temperatures. The test results show that the types of normalization pore pressure development include developmental and stable. An empirical model, considering the effects of cyclic dynamic stress ratio, loading frequency, freezing temperature and number of cyclic loading, was proposed and compared with three representative models proposed by the predecessors to validate that the proposed method has the best estimation on the development of the pore pressure. • The type of pore pressure development in frozen-thawed clay contains two parts. • An empirical model for predicting pore pressure development is proposed. • Freeze-thaw cycle plays an important role on the pore pressure development. • The model can be used for stable and developmental type of pore pressure development. [ABSTRACT FROM AUTHOR]

Soft marine clay is susceptible to deformation under cyclic loading, controlled by microstructure variations. To investigate the microscopic characteristics of soft clay with the progression of deformation under cyclic loading, a detailed study has been conducted using consolidated undrained cyclic triaxial tests, as well as mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The relationships between the micro parameters and accumulated plastic strain of soft marine clay are discussed. Cyclic triaxial test results show that low cyclic stress ratio (CSR) induces stable deformation, while for greater CSR the deformation of soft marine clay presents a closely linear or exponential increase, which leads to the destructuration of the soil structure. The microanalysis shows that with vibration cycles under 0.15 CSR, the content of pore obtained by SEM decreases from 37% to 25%, and large aggregates are visibly crushed into smaller ones, leading to a 24% increase in the pore numbers, which shows the slow increase of the deformation. When the CSR is greater than 0.25, large diameter pores generate resulting in divergent development of accumulated plastic strain. An empirical model of accumulated plastic strain involving micro parameter is proposed for assessing the deformation of soft marine clay. [ABSTRACT FROM AUTHOR]

The artificial ground freezing (AGF) technique has been extensively employed in the construction of underground structures. Due to the existence of freeze-thaw cycle in the AGF technique, the properties of soft clay deteriorate and thus larger settlement is induced during its later operation phase. To investigate the strain behaviors of the soft clay undergoing freeze-thaw cycle and discuss the differences between the soft clay undergoing and without undergoing freeze-thaw cycle under dynamic cyclic loading, a series of dynamic triaxial tests with consideration of different influencing factors as dynamic stress amplitudes, freezing temperatures, and freezing temperatures were conducted. Based on the experimental data, an empirical model for predicting the accumulative plastic strain was proposed and validated. Besides, the microscopic structural characters of the artificially freeze-thaw soft clay were explored via mercury intrusion porosimetry (MIP) tests. The results show that the total accumulative plastic strain of specimens increases with the increase of the dynamic stress amplitude, while decrease with the increase of the loading frequency. The specimens undergoing freeze-thaw cycle produces larger plastic strain, and the lower the freezing temperature is, the larger the total accumulative plastic strain is. The empirical accumulative plastic strain model, which can synthetically reflect the effects of dynamic stress amplitudes, loading frequencies, freezing temperatures and number of cyclic loading is proposed and validated. The freeze-thaw cycle has a significant effect on the strain behavior of soft clay in a way decreasing the pore volume and increasing the pore size of the specimens. Results obtained in this paper may provide meaningful references for studying the deformation characteristics of the soft clay in engineering applications. • The effects of dynamic stress amplitude and loading frequency on the accumulative plastic strain of soft clay were discussed. • The effects of freeze-thaw cycle as well as various freezing temperature on the accumulative plastic strain behaviour of soft clay was investigated. • An empirical model for predicting the long-term strain of soft clay, can synthetically reflect the effects of dynamic stress amplitudes, loading frequencies, freezing temperatures and number of cyclic loading was proposed and validated. • The microscopic structural mechanism of dynamic stress amplitude, loading frequency and freezing temperature on affecting the accumulative plastic strain of soft clay was analyzed. [ABSTRACT FROM AUTHOR]