Field Monitoring of Secant-Pile Supported 11 m Excavation in Residual Soil: A Case Study from Bogor

Abstract

To maximize usable construction area in contoured terrain, cut-and-fill operations are often unavoidable. This paper presents a two-tier excavation, the deepest cut being 11 m + 4.7 m and the shorter cut being 5 m + 4 m. The cut was used to make space for a residential house. Subsurface investigation revealed a stratigraphy of medium silt (average NSPT ≈ 5) to a depth of 6 m, underlain by 4 m of stiff silt (average NSPT ≈ 12), and a hard silt layer extending beyond the investigation depth. The deepest excavation till mid-excavation was retained by 22 m long secant pile, while the mid-excavation till the shallowest excavation was retained by 16 m long secant pile. Excavation support was provided by secant piles: a 22 m long system for the upper excavation and a 16 m long system for the lower excavation, comprising 0.8 m diameter secondary piles and 0.6 m diameter primary piles. Pre-construction analysis predicted a maximum deformation of 2.3 cm at the top of the secant pile for the deepest cut. To ensure deformation was within limits, inclinometer monitoring was conducted daily over 12 days during excavation. Surprisingly, no measurable movement was recorded. This could be attributed to the saturated soil parameters used for analysis, whereas the soil was in unsaturated state during excavation. While adopting unsaturated parameters may yield results closer to observed behavior, such an approach is less conservative. Given Bogor’s high rainfall, conservative design using saturated soil parameters remains prudent to ensure safety, particularly in residential construction.