A pragmatic risk-based approach to deep excavations and dewatering in an urban environment

This paper describes the design and construction of temporary works to enable the repair of a broken deep foul sewer on the outskirts of Glasgow, UK. Clyde Alluvium soils, composed primarily of soft, liquefiable sandy silts and silty sands, were prevalent at the site. Shallow groundwater resulted in extensive temporary works, including sheet piles, dewatering and grouting, being required to facilitate the repair. The temporary works design was complex given the urban environment of the site and, in particular, the close proximity to existing sewerage assets and residential properties. Settlement analyses included estimations of residual settlement tolerances of the adjacent residential properties, following settlements incurred by a previous phase of ground improvement works at the site. Back-analyses from previous works were undertaken to assess anisotropic conditions relating to compressibility and permeability, which informed the risk-based design approach of the temporary works. Sheet pile settlements for a low-energy hydraulic pressing technique were estimated using an extrapolation of findings of previous studies. This paper examines how the monitored settlements compared with the initial estimates and how the findings can be used for future settlement predictions in similar conditions.