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Reinforced Soil Raft Foundations

By Eric R. Kizak, P.E.

At first glance the replacement of a slab-on-grade appears to be a straight forward process, and in many cases it is.  The replacement of the apparatus slab at the Woodlawn Street Fire Station in Lynn, Massachusetts was not one of those cases. 

 The slab was scheduled for replacement due to settlement and cracking as a result of the increased weight of the current equipment.  Upon initial investigation we found the fill material below the slab to be a cinder and ash mixture, which is a poor bearing material.  We could have followed a “standard” approach, remove all of the unsuitable material and backfill with good material, but that approach would require removing and replacing over 400 tons of material as well as supporting the existing under-slab plumbing; a costly solution. 

An alternative solution was to use a reinforced soil raft foundation.  Construction of this type of foundation is relatively basic and saved the Owner considerable time and money. 

         

Soil raft foundations consist of several layers of compacted structural fill on a geotextile fabric; the raft is then capped with clean gravel and the concrete slab.  The layers of fill and geotextile fabric effectively form a “raft” that spreads the loads on the slab over a greater area. 

 This project represented a common construction and design problem.  An apparently simple fix can get complicated by the site conditions but can be brought back on budget and schedule by applying creative solutions.