The existing Beijing Pingguoyuan Subway Station was extended through a extension project. The excavation for the extension was located directly above the existing station. Complex interactions exist between the existing structure and the retaining pile wall of the excavation. Based on this project, three-dimensional finite element models were established to investigate the mechanical characteristics of the embedded and non-embedded retaining pile walls. A parametric analysis was performed for both types of pile walls. The stress and deformation characteristics of the retaining pile walls and existing structures were analyzed. The results show that when the bottom of the non-embedded retaining pile walls are connected to the existing structure, the uplift of the existing structure is essentially constant; however, the maximum displacement of the pile is increased by approximately 2.7 times, and the bending moment of the pile is reduced to 57.1% of the connection condition. As the distance between the embedded retaining pile wall and the existing station increases, the uplift of the existing station increases linearly, whereas the soil between the pile and the station exhibits a non-linear increasing trend. The displacement of the embedded retaining pile wall increases as the inner force decreases. When the distance is greater than 4.7 m, the displacement and force of the pile remains essentially unchanged. The effect of the pile embedded depth on the force and deformation of the pile is mainly observed in the lower part of the pile. As the embedded depth increases, the maximum displacement decreases by approximately 16.9%, the maximum bending moment decreases, and the maximum negative bending moment increases. The key contribution of this research is to provide a prediction method for the mechanical behaviors of a expansion project. The findings from the study also provide industry practitioners with a comprehensive guide regarding the specific applications of the construction technology of a deep excavation structure overlying an existing subway station.

Experimental studies about the flexural performance of channel steel tube slab (CSTS) structure subjected to concentrated loads were carried out, with considering various spacing of adjacent tubes, longitudinal spacing between the channel steel and missing tensile channel steel. A self-designed flexural device equipped with particle image velocimetry (PIV) technology was used to monitor continuous structure deformation and concrete cracking characteristics. In this study, the mechanism and performance of CSTS structure serving in underground engineering were analyzed, and a theoretical model for the bearing capacity of the CSTS structure was proposed. The results show that the failure mode of the test specimens exhibits ductile failure with an average ductility coefficient 2.52. The large shear strain of the concrete was obtained from PIV speckle contour, and it shows the same propagation trend compared with the main cracks appearing in the tests. In addition, the tensile channel steel along with decreasing the spacing of adjacent tubes can increase the bearing capacity of the specimens. The proposed theoretical model shows a reasonable prediction of the ultimate bearing capacity of specimens compared with the experimental results.

Red clay with features of high liquid (plastic) limit, low permeability, medium-low compressibility and high strength is widely used in anti-seepage projects including roadbed, earth dam, tailings and landfill cover. This study investigates the hydraulic conductivity and propagation of desiccation cracks of compacted red clay in Dalian, China, considering the effect of freeze-thaw (F-T) cycles and saline intrusion. A series of compacted specimens were subjected to different F-T cycles at various controlled salt concentration of 0.2% and 4%. The surface cracking initiation and propagation process of compacted specimens under wetting-drying (W-D) cycles were monitored by Digital Image Correlation technique. The results indicated that permeability coefficient of compacted specimens increased significantly after the first F-T cycle regardless of specimens with variable dry density and salt concentration. The relationship between the number of F-D cycles and permeability coefficient can be expressed as the exponential function for Dalin red clay. Dry density and Saline inhibits the desiccation cracks of compacted specimens under W-D cycles. However, the F-T cycles have a modest promoting effect on crack propagation on the surface of saturated red clay. This study analyzes the underlying formation mechanisms of desiccation cracking-inducing geohazards and provides some guidance for the long-term performance of infrastructures upon saline intrusion and F-T cycles for red clay.

Microbially induced carbonate precipitation (MICP) has been utilized as a new method to improve loess soil strength. In this study, we investigated the influence of the main parameters on the shear strength of MICP-treated loess specimens. Initially, culture media with different formulas and pH values were examined to identify the most efficient medium for loess soil. To explore the shear behavior of MICP-treated loess under general stress levels, unconfined compressive strength (UCS) tests and triaxial tests relevant to the compression strength and vertical loads were performed on MICP-treated loess with different calcium sources, cementation concentrations, and curing periods. Subsequently, calcium chloride was selected as the optimal calcium source based on the ultimate strength of the MICP-treated loess. The effective cementation concentration in the loess soil was between 1.0 and 1.25 M. The ultimate strength of the MICP-treated loess was 3.6 times of the untreated loess. The stress-strain curves indicate that a higher cementing effect can be expected with an increase in the curing period. The formation process of calcium carbonate and the micromorphology of the MICP-treated loess samples were examined using scanning electron microscopy. In this study, we present an environmentally friendly technique for improving loess soil strength.