Methods of Construction

The construction of a subsurface drain involves trench excavation, dewatering, wall stabilization, pipe installation, and backfilling. Trench excavation is the most significant step in the construction of a subsurface drain. A variety of excavation equipment can excavate the trench; the optimum is determined by the depth, width, length of the trench, and the type of material being excavated. Dewatering can be performed by open pumping, predrainage using well points, or groundwater cutoff. Wall stabilization methods include shoring for deep excavations or open cuts for shallow excavations. Continuous trenching machines can accomplish all excavation and pipe installation operations simultaneously (Oakley et al. 1994); however, this machinery is limited to small diameter and relatively shallow subsurface drains.

Another important aspect in subsurface drain installation is the placement of filter and envelope materials around the pipe to prevent soil particles from entering and clogging the pipe. Geotextiles and well-graded sand and gravel can be used as filter materials. The general requirement for envelopes is that their hydraulic conductivity is higher than that of the base material. Design procedures for filters and envelopes are in the Soil Conservation Service (1973).

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