In Cambridge, the ground rarely gives you a straight answer. Between the stiff Gault Clay that swells with the seasons and the pockets of loose terrace gravels along the Cam, designing a raft foundation here means anticipating what the soil will do three winters from now, not just what it does today. Our team has worked on enough extensions in Chesterton and new builds near Addenbrooke’s to know that a standard ‘uniform bearing pressure’ assumption falls apart when the upper metre of made ground sits over a desiccated clay crust. We approach every raft/mat foundation design by mapping the stiffness profile first — using in-situ test data rather than textbook correlations — because in East Anglia the difference between a 150 mm and a 300 mm thick slab often comes down to one soft lens nobody spotted. When the stratigraphy is patchy we’ll combine the raft analysis with CPT testing to pin down the transition between the weathered zone and intact clay without the disturbance you get from borehole sampling.
A raft foundation in Cambridge does not just carry the building — it bridges the uncertainty between the gravels, the Gault Clay, and whatever the Fenland edge left behind.
