Numerical Exploration and Optimisation on Pullout Capacity Behaviour for Under-Reamed Piles in Dry and Partially Saturated Sand

The pullout capacity behaviour of straight and underreamed piles in two soil layers was investigated in the current work using Plaxis 3D 2020. The layered soil includes one layer of homogeneous sand, dense sand topped with stiff clay, and stiff clay topped with dense sand. The piles were selected per the Indian Code Specification (IS 2911). To depict the impact of bulb numbers and their positions on the ultimate pullout load of the pile, three types of large-scale concrete pile models were employed: uniform pile shaft (SP), pile with one bulb (SURP), and pile with two bulbs (DURP). Under-reamed piles measure 0.3 m in stem diameter by 4.5 m in height, with a bulb diameter of 0.75 m. The relative density of the soil is varied to understand its impact on pile behaviour, with the internal friction angle (φ) in sand varying between 30 and 39 degrees. In this regard, the soil was assumed to obey Mohr-Coulomb's failure criteria. The findings revealed that multiple under-reams led to more soil-structure interactions, resulting in increased mobilisation of soil resistance. Compared with a conventional pile under identical conditions, a single under-ream increased the ultimate pullout pile load by about 3.5 times. For the double under-ream, it was increased by about 4times. For all types of piles, the ultimate pullout load of a pile embedded in dry sand is often greater than that fixed in fully or partially moist soil. The ultimate pullout carrying capacity was the largest when the two under-reamed piles were embedded in stiff clay resting on dense sand, rather than in dense sand in the top layer or in a single layer of homogeneous dense sand.