ABSTRACT MEASURED deflections of tubing buried for five years in a silty clay (CL) showed that the soil pro-vides minimal side support against deflection. The Iowa formula was the best of four methods for predicting deflection but all methods generally underestimated measured deflections. Maximum installation depths were calculated for tubing up to 381 mm diameter, buried in silty clay soil. Unlike concrete or clay tile, corrugated plastic tubing is flexible, and deflects after installation under the weight of soil above it. Part of its ability to resist failure is derived from lateral support provided by the backfill when the pipe walls deflect outwards. Failure, either hydraulic or structural, is generally characterized by ex-cessive deflection (usually 20 to 30 percent) resulting in collapse of the pipe wall. Therefore, deflection should be predicted to ensure that tubing will perform satisfactorily under given installation conditions. The most widely accepted prediction method is the Iowa formula developed by Spangler in 1941, and modified by Watkins and Spangler (1958). Recently developed alternative methods such as those of Howe (1966) and Burns and Richard (1964) assume the soil-pipe system behaves elastically while others such as that of Watkins (1976) are statistical or empirical. Field measurements of deflection and standard laboratory tests provided data for this research, which had the following objectives: (a) refine estimates for the empirical modulus of soil reaction in the Iowa deflection formula; (b) compare measured deflections with those predicted by the four methods mentioned above; and (c) develop installation criteria for corrugated plastic drain tubing up to 381 mm in diameter.