Deep Excavation for Loop Pit-Paper

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The paper describes the design and construction of an approximately 12 m deep loop pit inside an operating, nearly 80-year-old steel-plant building. The pit was needed for a new slitting line, but limited working space, nearby equipment and fragile brick-masonry column foundations made excavation particularly challenging. Subsurface conditions included fill and clay over mica-schist, ranging from completely to moderately weathered, with groundwater levels varying by season.<br /><br />Initial proposals for retaining the excavation—contiguous bored piles with bracing, followed by protective micropiles—proved impractical because of the rock conditions, cost, construction time and interference with an operating overhead crane. The team therefore revised the plan to use open excavation with sloped sides. Dismantling a low dado wall provided extra room beside the building columns, and a bench was added to improve slope stability.<br /><br />Slope stability was assessed using limit-equilibrium methods, including Fellenius, Bishop, Janbu and Spencer, with groundwater and potential seepage considered. Calculated factors of safety ranged from 1.159 to 1.331, which the authors considered acceptable for temporary works. Geotextile and two layers of sandbags protected the excavated face, added surcharge and helped manage seepage; a sump and pump kept the rock as dry as practicable.<br /><br />The modified approach enabled excavation and pit construction to proceed safely and on schedule. The case illustrates how site-specific engineering and reassessment can overcome constraints when conventional retaining systems are unsuitable. The authors note that slope movement and pore pressure were not instrumentally monitored, and recommend such monitoring for improved safety in future projects.

Keywords

loop pit excavation

slitting line

slope stability

limit equilibrium analysis

retaining excavation

micropiles

steel plant construction

groundwater seepage

bench excavation

factor of safety