Three-dimensional numerical investigations into the synergistic mechanism of composite pile retaining systems consisting of large-diameter bored cast-in-place piles and small-diameter micro-grouted steel pipe piles remain limited. To address this gap, a refined three-dimensional finite element model was established using ABAQUS based on the Hutan Park Station excavation project of Dalian Metro Line 5. Five design cases were analyzed by varying pile diameter, pile spacing, and the presence or absence of micro piles. The main findings are as follows: (1) The maximum horizontal soil displacement in all cases remained below 8 mm, indicating satisfactory excavation stability. (2) The inclusion of micro piles promoted stress redistribution within the inter-pile soil and was associated with a more pronounced soil arching tendency, as indicated by the displacement contour patterns. (3) Based on the comparison between Case 3 and Case 5, where the spacing between bored cast-in-place piles increased from 2.0 m to 3.0 m while maintaining the composite support configuration, the maximum horizontal displacement and maximum bending moment decreased by approximately 12% and 9%, respectively. The inserted micro-grouted steel pipe piles participated in lateral load transfer, enhanced stress redistribution, and reduced local stress concentrations acting on the primary bored piles. Under the investigated geological and support conditions, the numerical results suggest that the introduction of micro steel pipe piles may improve deformation control and load-sharing performance in composite pile-supported excavations. The proposed support configuration shows potential for reducing the number of large-diameter bored piles while maintaining acceptable excavation performance and overall support stability.
Song et al. (Tue,) studied this question.
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