Raft Foundation Engineering for Madison's Glacial Soils

For a typical commercial or mid-rise residential project in Madison, the geotechnical investigation and engineering design for a raft/mat foundation generally ranges from US$960 to US$4,600. The final fee depends on the building footprint, the depth of the soft lake-bed deposits, and the number of borings or CPT soundings required to characterize the site variability.

In Madison's Yahara River valley, the compressible organic silts can extend 50 feet deep before hitting competent till. End-bearing piles require driving through that soft zone to reach the dense till, which is expensive. A raft/mat foundation design uses the near-surface soil as a distributed support system. It avoids the cost of deep pile rigs and handles the variable thickness of the soft layer by floating over it as a rigid unit.

We typically design mats no thinner than 12 inches for residential loads and 18 to 36 inches for commercial structures. The thickness is driven by the required rigidity to span weak pockets in the glacial drift. A thinner mat cannot provide the necessary beam action and risks excessive differential settlement when the underlying peat lenses compress unevenly under the building's dead load.

The water table in central Madison often rises to within 2 to 4 feet of the surface. For a raft/mat foundation design, we account for buoyancy and hydrostatic uplift forces. The mat's dead load must exceed the uplift pressure, or we must install a sub-slab drainage layer and sump system to relieve pore pressure. We also specify waterproofing admixtures in the concrete and consider sulfate attack if the groundwater chemistry warrants it.

Madison is in a region of low to moderate seismicity, but the deep soft clay deposits can amplify ground motion. We design the raft/mat foundation using the spectral response accelerations from ASCE 7 and the IBC. The mat's large footprint and mass provide excellent base shear transfer. Its rigidity prevents the differential horizontal movement that would rack a conventionally framed foundation on isolated footings during a long-period event.