Thermal properties of soil and rock in Hong Kong and their implications on temperature field in bored piles

Abstract

Hong Kong is a city with extremely limited land resources, so large-diameter bored piles (LDBP) are commonly used as pile foundations for constructing super-high-rise buildings to address land shortages issue. In the construction industry, bored piles are widely adopted for pile foundation construction due to their numerous advantages: they are suitable for various geological conditions; they provide exceptional load-bearing capacity for high-rise buildings, bridges, and other engineering projects; their construction employs concrete pouring technology, with pile diameter and dimensions adjustable according to specific project requirements; and offers a high cost-effectiveness ratio. Currently, research on LDBP has primarily focused on the structural stability of such piles, including their load-bearing capacity, compressive strength, and resistance to deformation. However, thermal studies during the construction process of LDBP have been relatively scarce. The construction of LDBP primarily involves the pouring of self-compacting concrete (SCC). During the pouring process, SCC undergoes hydration exothermic reactions, causing the pile's temperature to continuously rise. The heat generated by these reactions is transferred to the surrounding geological environment (the soil layers and bedrock layers where the pile is embedded). During the pouring of SCC-LDBP (self-compacting concrete large-diameter bored piles), if the temperature difference generated during the hydration exothermic reaction is significant, thermal cracks may form inside the pile. The presence of thermal cracks may affect the structural performance of the pile, such as its bearing capacity. Therefore, the thermal properties of the soil layers and rock layers into which the SCC-LDBP are embedded play a crucial role in the thermal conduction of heat generated by the hydration exothermic reaction of the concrete piles. The primary soil layers in Hong Kong consist of completely decomposed granite soil (CDG), with the main bedrock layers being granite. Based on the construction environment of LDBP in Hong Kong, this project used ABAQUS finite element software to establish two models: axisymmetric models of a SCC-LDBP with/without bell-out—CDG soil—granite bedrock system, simulating the temperature field of hydration exothermic heat during the pouring process of LDBP; based on the established pile-soil-rock model, sensitivity tests were conducted on the thermal properties of the soil layer and rock layer respectively. The simulation results obtained using ABAQUS software were as follows: the temperature fields of SCC-LDBP with and without bell-out were similar. The center point of the pile reached its temperature peak approximately 70 hours after concrete pouring is completed; in the sensitivity tests, it was found that the thermal conductivities of the CDG soil and granite bedrock were the most sensitive parameters. Changing the thermal conductivity resulted in the largest temperature difference between the upper and lower boundaries of the pile. In summary, this project established a numerical model of the pile-soil-rock system commonly found in Hong Kong using ABAQUS software, providing a better understanding of the thermal behavior of SCC-LDBP during the pouring process. Furthermore, the numerical model was used to simulate and predict the thermal behavior of SCC-LDBP foundations during the construction process.