Repairing a Dry Stone Retaining Wall

Due to a number of structural problems, an eight-year-old, dry stone retaining wall in Hanover, New Hampshire was dangerously close to collapsing. I was asked by the property owner to remedy the situation. The rebuilding of a 6’x30’ section of retaining wall is often a straightforward business. But because this wall was in a well established, backyard garden with poor access and little room to store materials at the site, the build was a logistical puzzle. Concern was added to those challenges when, as work commenced, a municipal sewer line was discovered to be located scarily close to the back of the wall.

The first two days were taken up with dismantling and excavation. The stone and back-fill had to be backed out of the site on a steep, twisting path, in the bucket of a front-end loader. Twenty-four cubic yards of additional wall stone was ordered in, doubling the volume of wall mass. Stones that were laid in the original construction with their length along the face of the wall, were turned to run longwise into the wall.

I laid the foundation stones, and first few courses, with a radical downward pitch, from front to back. This slope-surface technique for retaining walls is something I learned from a team of French wallers I met in England this past spring. It resists the forces that allow stones, normally laid level-bedded, to slide forward. This is an especially valuable technique where heavy clay soils abound. The Connecticut River valley subsoil in the the Hanover area is pure clay to a depth of 1000’.

After six days of building that saw over 4” of rainfall, we were glad to finish up on a sunny day. Jared Flynn, Toby Bartles and I did the walling with support from Zak Grover, Quimby Mountain Stone, Chase Site Services, Stearns Septic, Hanover Highway Department and the gardening gang.