Cutoff Trench

A dam can lose water three ways: over the top, through the wall, or underneath it. A cutoff trench deals with the last. Before the bank is raised, a trench is cut along the line the dam will follow, down through loose porous ground to a layer water cannot pass, and refilled with heavy clay packed hard. That buried wall ties the dam to solid ground, leaving no seam for water to creep along. Skip it and water seeps out under the dam.

None of it can be designed from a map. Test pits 1.2 to 1.8 m (4 to 6 ft) deep, dug first, show where the tight layer sits and whether there is one; depth needed runs from about 1.2 m to 6 m (4 to 20 ft). It is keyed at least 30 cm (12 in) into that layer, with a floor at least 2.4 m (8 ft) wide - a width set by the machine that must compact inside it, not by the water - and sides no steeper than one across for one down, or the open trench caves in. It is carried up the valley sides wherever leaky ground continues: a trench stopping at the foot of the slope leaves water a way round.

Everything turns on the moisture of the clay going back in. Soil too dry or too wet will not pack down however hard it is worked, and the refill goes in thin layers, 15 to 20 cm (6 to 8 in) at a time, each roughened before the next so they bond into one mass rather than stack as slabs with gaps between. That is the trap: a badly backfilled trench looks finished, passes inspection, and shows up only when the pond will not hold. Once the dam is built it cannot be reached, so it has no upkeep and no life of its own; what needs watching is what reaches it from outside, as tree roots push through the core in time and burrowing animals tunnel into banks. Stripping topsoil and roots from the whole footprint, and collars on any pipe through the wall, answer the same problem elsewhere; none rescues a dam whose fill is badly packed round that pipe, where most dams fail.