Study on Composite Support Technology of Micropile and Bored Pile Frame for Foundation Pits in Soil-Rock Strata

Abstract

The construction of foundation pits in soil-rock composite strata presents significant challenges due to the mechanical incompatibility between overlying soils and underlying rock masses, often causing abnormal stress redistribution and local instability in conventional pile–anchor support systems. This study proposes a micropile and bored pile frame composite retaining system, wherein reinforced concrete bored piles constitute the primary bidirectional spatial frame and grouted steel micropiles serve as auxiliary members for inter-pile soil retention. The collaborative load-transfer mechanism is analytically characterized through Winkler elastic foundation theory, yielding a stiffness ratio of η = 16.2 between bored piles and micropiles. Field validation was performed at Hou Pantao Village Station on Xuzhou Metro Line 3, wherein full-field strain evolution was monitored using BOTDR distributed optical fiber monitoring throughout staged excavation to a final depth of 17.06 m, with subsequent comparative verification against three-dimensional finite element computations (MIDAS GTS NX). Results demonstrate that: (i) structural deformation concentrates predominantly within the upper 5-m soil layer, with negligible displacement below the rock surface; (ii) micropile peak bending moment migrates downward with excavation, stabilizing near the soil–rock interface; (iii) overburden thickness dominates system response, with a critical threshold at 5 m, beyond which micropile horizontal displacement increased from 5.32 mm to 19.80 mm, matching the field-monitored range
(5.70–23.3 mm); (iv) reducing bored pile spacing from 9 m to 4 m decreases maximum micropile displacement by 42.1%, whereas increasing pile diameter yields only local stiffness enhancement without global deformation control.