Pile foundation engineering – When should uplift-resistant piles be installed? What are the tests for uplift-resistant piles?

When subjected to the buoyancy of groundwater, uplift-resistant piles rely on their own weight and the friction between the pile and the surrounding soil to provide uplift resistance. The uplift-resistant pile and the soil surrounding it together form a pile-soil system, through the interactions within which load transfer is accomplished. As the load at the top of the pile increases, the pile gradually moves upward, causing the soil surrounding the pile to move upward as well, which in turn leads to the upward movement of the soil in the surrounding area. As a result, shear deformation occurs in the soil surrounding the pile. When a solid-section uplift pile resists uplift, the uplift force is primarily provided by the frictional force at the side of the pile. In contrast, for a variable-section (flared) uplift pile, most of the uplift force comes from the flared portion; even when the frictional force at the side of the pile reaches its limit, the flared portion is still far from reaching its limit. Studies have shown that when the loading on uplift piles is low, the stress distribution within these piles and their calculations can be determined using traditional theoretical methods. However, when the loading reaches a certain level, the steel bars and concrete can no longer satisfy the conditions for deformation compatibility, and thus such methods are no longer suitable for calculating the stresses in uplift piles. Exploring the stress distribution characteristics of uplift-resistant piles is of great significance for studying the widespread application of such piles in floating resistance projects.

Different types of uplift-resistant piles can be selected depending on the geological conditions, and as the load at the top of the pile increases, the axial forces experienced by different types of uplift-resistant piles also vary. Chen Yang and others conducted model test studies on the distribution pattern of axial force in aluminum pipe piles embedded in calcareous sand. The tests showed that the axial force at the top of the model pile was the highest, and its value was roughly equal to the load applied. The axial force decreased as depth increased, with the rate of decrease gradually increasing; the axial force at the bottom of the pile was essentially 0. Yang Bai and others conducted on-site ultimate load tests on enlarged base pull-out piles for a certain project in sandstone geology; the distribution curve of the axial force within the pile also followed the aforementioned pattern. They proposed that the axial force in the pile consisted of the axial force in the rebar and that the axial force in the concrete, and it could be calculated using the rebar stress measured at different sections via strain gauges. The resultant value of the lateral friction force at two adjacent cross-sections of the pile shaft is determined by the difference in axial forces between those two cross-sections.

In areas with high groundwater levels, when the load from the superstructure is not sufficient to counteract the buoyancy of the groundwater, the structure as a whole or certain parts of it will experience an upward buoyant force. Pull-out resistance piles must be installed in places such as underground water tanks, the basement structures of buildings, and the biological treatment tanks in sewage treatment plants.