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...

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