Use of Socketed Micro Piles for Excavation Support in Heterogeneous Fill Material-Paper

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This case study describes the use of socketed micropiles to support temporary excavations for new pipe-rack trestle foundations inside an operating steel plant. Excavation was required beside a busy plant road, existing shallow foundations, trestles, and underground utilities, leaving little room for open slopes or conventional support systems.<br /><br />Ground conditions were highly variable: up to 4 m of heterogeneous fill containing slag, fly ash, soil, bricks, concrete debris, and boulders overlay residual soil and weathered mica-schist. Excavations were generally 3–4 m deep. Because fill thickness and quality varied across the site, micropile lengths and embedment had to be adjusted during construction.<br /><br />The support system used steel pipes, central reinforcement bars, and cement-based grout. Wagon-mounted drilling equipment formed approximately 150 mm holes for micropiles around 100 mm in diameter. Piles were typically 5–6 m long and spaced 0.3–0.5 m apart, with their bonded lengths socketed into weathered rock or grouted slag-filled zones. Pressure grouting was also used to fill voids in the loose fill before drilling and excavation. Casing, supplemental steel sections, and inclined supports were added where ground conditions or pile spacing required them.<br /><br />Design combined limit-equilibrium slope-stability analyses with finite-element assessment. For temporary works, target factors of safety were 1.2–1.3. Analyses showed that unprotected vertical excavation was unstable; installing micropiles raised calculated factors of safety to 1.257 beside the road and 1.433 beside an existing building. Field measurements confirmed pile deflections within a 5 mm limit.<br /><br />The system enabled safe excavation while minimizing disruption to plant traffic and protecting nearby infrastructure. The authors conclude that micropiles can provide an adaptable, practical solution for constrained brownfield projects with uncertain fill and limited access, provided design and construction respond to site-specific conditions.

Keywords

socketed micropiles

temporary excavation support

steel plant foundations

brownfield site

limit equilibrium analysis

finite element analysis

pressure grouting

weathered mica-schist

construction constraints

slope stability