Specifying a deep basement or cut-and-cover tunnel in Cambridge without a solid geotechnical excavation monitoring plan remains the single most common root cause of cost overruns on constrained urban sites. The Gault Clay and the overlying river terrace gravels behave predictably — until they don’t. A minor change in groundwater pressure behind a sheet pile wall, or a sub-millimetre movement in an adjacent listed college building, triggers Section 61 notices and programme delays that no contractor can absorb. The instrument array must match the ground model: vibrating wire piezometers in the gravel lenses, in-place inclinometers through the clay, and precise level surveys on every sensitive structure within the zone of influence defined by BS EN 1997-1:2004. When the geology demands it, we also integrate CPT test data to refine the soil stiffness profile before excavation begins, ensuring the observational method has a reliable baseline.
In Cambridge’s Gault Clay, heave recovery can continue for 90 days after excavation stops — monitoring must match that timeline.
