Retaining walls in Biddeford, Maine.

Scantic soils are poorly drained, and their subsoil lets water through slowly. A wall holding back that kind of ground is holding back water as much as soil, and a wet clay backfill that freezes pushes hard against the face. Segmental block and granite veneer walls built on these lots get a compacted gravel footing, free-draining stone behind the face, and a perforated pipe at the base with somewhere for the water to go.

The city's Thatcher Brook watershed plan found that about 70 percent of that watershed is in hydrologic soil group D, meaning very low infiltration and fast runoff. Where a wall collects roof and driveway water, planning the outlet is part of the wall design.

Maine's shoreland guidelines treat a retaining wall that is not needed for erosion control as a structure, so it must meet the same setback from tidal water as a building, with narrow exceptions for low walls on existing lawn outside the 100-year floodplain. Soil disturbed for a wall within 75 feet of a coastal wetland also triggers a Natural Resources Protection Act permit from the DEP. Along the Saco, the Saco River Corridor Commission reviews development within 500 feet of the river, and its permits for construction are valid for two years. On waterfront lots at the Pool, Fortunes Rocks or along the Little River, those reviews shape where a wall can go before its height is decided.