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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]

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]

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]

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]

Deep mixing method has been widely used to improve soft soils by increasing bearing capacity, reducing total and differential settlements, and enhancing stability. Experimental and numerical studies have demonstrated that the inclusion of deep-mixed (DM) columns in soft soils (e.g., clays and silts) could accelerate the consolidation of soft soils. Existing theoretical solutions for calculating the consolidation degree of DM column-improved soft soils are mostly complicated and not easy to use for practical applications. Therefore, a simple analytical solution is needed to calculate the consolidation degree of DM column-improved soft soils. In this study, a unit cell concept was used to develop a simplified method for calculating the consolidation degree of DM column-improved soft soils under a rigid footing. The simplified method considers the stress transfer between columns and their surrounding soil under elastic conditions and the vertical drainage of columns and their surrounding soils. The proposed simplified method is verified after compared with the numerical analysis considering four key influence factors on the consolidation degree of DM column-improved soft soils. The key influence factors include the soil thickness to unit-cell diameter ratio, the area replacement ratio, the modulus ratio of DM columns to soil, and the permeability ratio of DM columns to soil. The proposed simplified solution also captures the characteristics of the consolidation behavior of DM column-improved soft soils by considering stress transfer between columns and soil during consolidation and is easy to use in practice. Three existing analytical solutions are used for the comparisons with the proposed method. Among these methods, the proposed method has the best estimation of the consolidation degree for the DM column-improved soft soils under most conditions. It is recommended that the proposed method should be used for the condition where column permeability is close to soil permeability. [ABSTRACT FROM AUTHOR]

Tunneling inevitably induces changes in soil stress and, hence, may adversely affect the surrounding ground and nearby existing tunnels. A model test and a series of numerical analyses were carried out to investigate the response of the ground and existing tunnels in sandy soil to nearby tunneling according to the relative spacing. The test results showed that increasing the distance-to-diameter ratio (d/D) changed the settlement trough from W-shaped to U-shaped along the transverse section. In addition, the major settlement area induced by each advancing step along the longitudinal section was within 0.75D in front of the tunnel face. The numerical results showed that the overall downward movement of the existing tunnel was due to the combined action of soil pressure and excavation unloading. In this article, the mechanism of the W-shaped surface settlement is discussed; a modified Peck formula is used to describe the W-shaped settlement trough, and the relation of typical parameters to d/D is analyzed. [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]

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]