The soilbags reinforcement has been widely used for soft soil foundation improvement due to its high compressive strength and deformation modulus considering the time limit of many projects and the characteristics of the reclaimed soil. However, despite the strength and deformation properties of soilbags reinforcement, the drainage characteristics of soilbags reinforcement is a crucial factor that creates a large challenge to foundation improvement for soft soil. Thus, this study developed a four-staged surcharge preloading on soilbags-reinforced soft soil foundation and focused on its drainage consolidation effectiveness. The contrasting laboratory tests were performed in four identical experimental boxes with clayey soil from the Nanjing, China. Four-staged preloading were applied on the soilbags-reinforced testing model, respectively, the data of the settlement and water discharge during the test are monitored, and after the tests, the water content and shear strength at different positions are measured. And three contrasting tests considering the possible drainage channels of soilbags reinforcement were also conducted. The results show that the consolidation effect is achieved with the soilbags reinforcement in terms of the settlement, pore water pressure, water content and shear strength after consolidation.

PurposeThis paper aims to develop a probabilistic framework which combines uncoupled cofferdam stability analysis, random forest and Monte Carlo simulation for cofferdam reliability analysis.Design/methodology/approachThe finite element method and limit equilibrium method are used to calculate the seepage field and stability of cofferdam, respectively. Sufficient training and validating random samples are generated to obtain a random forest surrogate model with acceptable accuracy. The calibrated random forest model combined with MCS is used to conduct cofferdam reliability analysis. The proposed methodology is illustrated using a typical cofferdam model.FindingsThe numerical simulation results demonstrate that a larger pore water pressure leads to a lower stability of the cofferdam and vice versa. The increase in the slope angle significantly reduces the stability of cofferdam on the corresponding side, while the stability of cofferdam on the other side is mainly affected by the internal pore water pressure. The increase in the width and height of the reverse pressure platform significantly enhances the stability of cofferdam, and the changes in the angle of the reverse pressure platform affect the stability of cofferdam to some extent. The probability of failure (Pf) of cofferdam increases gradually with increasing vertical and horizontal scales of fluctuation, coefficient of variation, and cross-correlation coefficient when the degradation degree of soil properties is low. It is worth noting that the effect of vertical and horizontal scales of fluctuation, coefficient of variation, and cross-correlation coefficient on the Pf of cofferdam changes significantly when degradation coefficient decreases to a critical value.Practical implicationsA geotechnical engineer could use the proposed method to perform cofferdam reliability analysis.Originality/valueThe reliability of cofferdam can be efficiently and accurately studied using the proposed framework.

Solidified soil prefabricated pile (PPSS) is a new type of pile formed by extruding solidified soil with hydraulic equipment. The PPSS includes two parts: precast pile and core pile, which can be used to strengthen soft foundation. To study the deformation characteristics of PPSS under vertical load, the nonlinear mechanical behaviour of the double-contact interface of PPSS is analyzed by using the bond slip model and hyperbolic model. A settlement calculation method is proposed considering the displacement coordination of the doublecontact interfaces, e.g., interface between precast pile and surrounding soil, and interface between core pile and precast pile. The bearing characteristics of the double-contact interfaces are studied by using the numerical results. Based on the numerical results, the effects of elastic modulus ratio, diameter ratio, length and initial cohesion on the deformation characteristics of PPSS are analyzed.

The zoning excavation method is fully employed to control the deformation of foundation pits constructed in urban soft soil areas. However, the similarities and differences in forces and deformations between foundation pits excavated by the zonal method and those excavated by the conventional method still need to be further explored. In this study, the deformation was monitored and analyzed by taking the zonal excavation of a foundation pit of the 'New World' project in Hangzhou City as the research object. The measured results showed that the pre-built diaphragm wall for the first excavated foundation pit restricted the deformation of the first excavated diaphragm wall. The presence of extensive construction and unloading activities also changed the deformation pattern of the soil. Further, finite element simulations were carried out. The simulation results revealed that excavating the foundation pit first caused displacements in the pre-built diaphragm wall. The displacements transmitted by non-adjacent pits through the pre-built diaphragm wall were small and were concentrated at the junction of the two sub-pits. Adjacent foundation pits caused large displacements of the pre-built diaphragm wall with similar displacement patterns. The results of the study can provide effective guidance for foundation pit excavation in soft soil areas in the future.

The soil -structure relative stiffness is a key factor affecting the seismic response of underground structures. It is of great significance to study the soil -structure relative stiffness for the soil -structure interaction and the seismic disaster reduction of subway stations. In this paper, the dynamic shear modulus ratio and damping ratio of an inhomogeneous soft soil site under different buried depths which were obtained by a one-dimensional equivalent linearization site response analysis were used as the input parameters in a 2D finite element model. A visco-elasto-plastic constitutive model based on the Mohr -Coulomb shear failure criterion combined with stiffness degradation was used to describe the plastic behavior of soil. The damage plasticity model was used to simulate the plastic behavior of concrete. The horizontal and vertical relative stiffness ratios of soil and structure were defined to study the influence of relative stiffness on the seismic response of subway stations in inhomogeneous soft soil. It is found that the compression damage to the middle columns of a subway station with a higher relative stiffness ratio is more serious while the tensile damage is slighter under the same earthquake motion. The relative stiffness has a significant influence on ground surface deformation, ground acceleration, and station structure deformation. However, the effect of the relative stiffness on the deformation of the bottom slab of the subway station is small. The research results can provide a reference for seismic fortification of subway stations in the soft soil area.