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Q.Our remodeling company has contracted to do a room addition that will be elevated over an existing concrete driveway. We will need to install a concrete footing at the edge of the existing driveway slab. I’m worried that when we excavate for the footing, the fill under the slab will cave into the trench, leaving a void. How should we repair the undermined area?

A.Jay Meunier, contracting specialist at S.T. Griswold and Co. in Williston, Vt., responds: The amount of erosion or undermining will depend on the soil type. While a heavy clay soil may stay in place, a light sand will easily erode.

It is important to take steps to minimize undermining soon after excavating for the new footing. Provided there are no space limitations, rip-rap blocks — large one- or two-ton concrete blocks — can be used to contain the material under the concrete driveway. A more expensive option would be to pour a retaining kneewall, separate from the new foundation, against the exposed soils. This requires building a one-sided form.

You want to avoid any undermining that causes structural problems to the driveway. If some undermining does occur, flowable fill can be used to fill voids and provide support to the driveway slab. Flowable fill, however, cannot correct extreme undermining that has threatened the integrity of the driveway.

Inserting flowable fill will be easier if there is at least 6 inches between the existing driveway and the new foundation, allowing room for the hose. If the undermined area is not accessible any other way, it is possible to core holes in the slab, every 4 feet above the undermined area, for inserting flowable fill.

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