Three full-scale tests with the imperfect ditch 1Tiethod are described. The imperfect ditch method involves installing a compressible inclusion above rigid culverts to reduce the vertical earth pressure. Superlight expanded polystyrene blocks are used as the compressible material. In the first test, the instrumented culvert was a 1.95-m diameter concrete pipe beneath a 14-m-high rockfill embankment. In the second test, a 1.71-m diameter concrete pipe was used beneath a 15-m-high rock fill, and in the third, the culvert is a cast-in-place concrete box culvert with a 2.0-m width ben. eath 11 m of silty clay. The culverts were built between 1988 and 1989, and the instrumentation measured earth pressure, deformation, and temperature. The full-scale measurements show considerable reduction in the vertical earth pressure: that on top of the pipes in the granular fill was reduced to less than 30 percent of the overburden and that on the box culvert beneath the clay fill was reduced to less than 50 percent of the overburden. The deformation of the expanded polystyrene was 27 percent in the rock fill and 42 percent in the clay. The long-term observations show that there is no increase in earth pressure on and deformation of the pipes beneath the rock fill. There is a slight increase in deformation of the expanded polystyrene in the clay. Use of this method in Norway has realized cost reductions of the order of 30 percent and has made it possible to use concrete pipes beneath higher fills.

The problem of earth pressure on buried structures is of great practical importance in constructing highway embankments above pipes and culverts. Both the magnitude and the distribution of earth pressure on buried culverts are known to depend on the relative stiffness of the culvert and the soil. Current design methods distinguish between a rigid culvert and a flexible culvert. The vertical earth pressure on a rigid culvert is greater than the weight of the soil above the structure (negative arching). The vertical earth pressure on a flexible culvert is less than the weight of the soil above the culvert (positive arching).