Modeling Test and Numerical Simulation of Vertical Bearing Performance for Rigid-Flexible Composite Pouch Piles with Expanded Bottom (RFCPPEB)

Rigid-flexible composite pouch piles with expanded bottom (RFCPPEB) are generally considered as new symmetrical piles in practical engineering, but their bearing characteristics and design method are still not completely understood. The objective of this study is to investigate the vertical bearing performance and the optimal design scheme of RFCPPEB. Hence, laboratory modeling tests for this symmetric structure and an ABAQUS three-dimensional (3D) numerical simulation analysis were used to study the vertical bearing characteristics on bottom-expanded piles and rigid-flexible composite piles with expanded bottom. The vertical bearing capacity, shaft resistance, pile tip resistance distribution rule, and load sharing ratio of RFCPPEB were analyzed and verified using different bottom expansion dimensions and cemented soil thicknesses. The results revealed that the optimal bottom expansion ratio of rigid bottom-expanded piles was 1.8 when the ratio of pile body to bottom-expanded pile head was 9:1. When the bottom expansion ratio (D/d) was increased, the bearing capacity of bottom-expanded piles was significantly increased at D/d = 1.4 and D/d = 1.8 compared to that of D/d = 1.0, reaching 1.67 and 2.29 times, respectively, while for D/d = 1.6 and D/d = 2.0, the ultimate bearing capacity remained unchanged. Besides, shaft resistance played an important role in the bearing process of the rigid bottom-expanded piles and RFCPPEB. When the shaft resistance was increased, the ultimate bearing capacity of the pile foundation was significantly improved. The shaft resistance of RFCPPEB was increased with increasing cemented soil thickness. The increases in the shaft resistance and thickness of the cemented soil showed a nonlinear growth, and the maximum shaft resistance was approximately 75 cm from the pile top. When the diameter of the expanded head was 1.8 times the diameter of the pipe pile and slightly larger than the thickness of the cemented soil (0.5 times the diameter of the pipe pile), the optimal amount of concrete 425.5 kN/m3 required for per unit volume around piles was obtained, with the RFCPPEB ultimate bearing capacity of 7.5 kN. For RFCPPEB, the soil pressure at the pile tip was directly proportional to the pile top load under small load and was decreased in the form of a half quadric curve under large load. It reached the most reasonable position where the slope of the quadric curve was the largest when the thickness of the cemented soil was larger than 0.5 times the diameter of the pipe pile.

Physical and Mechanical Properties of Cemented Soil Fill At Liquid and Hardened States

Cemented soil fill is a new backfilling technology developed for the problems of narrow foundation trenches and uncompacted backfilling. It has good fluidity before solidification and higher strength and stiffness after solidification. This type of fill materials makes full use of the waste soils. The proportioning test was carried out on excavated soil on a construction site. Liquid property tests and unconfined compressive strength tests was carried out. The results show that the cemented soil fill can meet the requirement of foundation trenches backfilling, which has great prospect for future applications.

Material Preparation and Geotechnical Properties of Transparent Cemented Soil for Physical Modeling

The preparation of transparent materials suitable for simulating different rock and soil masses is the foundation for image-based physical modeling tests in studying deformation and failure mechanisms in geotechnical media. A transparent cemented soil (TCS) with similar geotechnical properties of natural soil and soft rock was prepared using fused quartz as the skeleton, hydrophobic fumed silica powder as the cement and mixed mineral oil of 15# white oil and n-dodecane as the pore fluid. Eleven groups of TCS samples with different shear strengths were synthesized by adjusting the content or mass ratio of the cement and particle size or gradation of the skeleton. Contrasting tests of unconsolidated-undrained triaxial compression were carried out and the mechanical characteristics of TCS were analyzed, showing that the stress-strain relationship, shear strength and failure mode of TCS are similar to those of natural soil. The mechanical parameters of TCS undergo complex variation with the factors, and the mesoscopic mechanism of the changes therein was revealed with the help of optical microscope photos. The similarity ratio of TCS to soft rock was derived according to geometries and stress conditions of laboratory model tests, demonstrating the feasibility of using TCS as similar materials to soft rock. Moreover, empirical formulas for the change of shear strength parameters with the factors were fitted to facilitate the preparation of TCS with target shear strength in the future. The findings can provide a basis for preparing transparent similar materials to natural soil and soft rock in physical modeling tests.

Bio-Cementation for Improving Soil Thermal Conductivity

A promising technology for renewable energy is energy piles used to heat and cool buildings. In this research, the effects of bio-cementation via microbially induced calcite precipitation (MICP) using mixed calcium and magnesium sources and the addition of fibres on the thermal conductivity of soil were investigated. Firstly, silica sand specimens were treated with cementation solutions containing different ratios of calcium chloride and magnesium chloride to achieve maximum thermal conductivity improvement. Three treatment cycles were provided, and the corresponding thermal conductivity was measured after each cycle. It was found that using 100% calcium chloride resulted in the highest thermal conductivity. This cementation solution was then used to treat bio-cemented soil samples containing fibres, including polyethylene, steel and glass fibres. The fibre contents used included 0.5%, 1.0% and 1.5% of the dry sand mass. The results show that the glass fibre samples yielded the highest thermal conductivity after three treatment cycles, and SEM imaging was used to support the findings. This research suggests that using MICP as a soil improvement technique can also improve the thermal conductivity of soil surrounding energy piles, which has high potential to effectively improve the efficiency of energy piles.