The purpose of the research reported in this bulletin is to provide knowledge concerning the structural behavior of the flexible types of culvert pipe. such as those fabricated of corrugated metal. The ability of this relatively light-weight type of pipe to carry surprisingly high fill loads has been recognized for many years, but until now no rational analysis of its behavior, based on accepted principles of mechanics has been available. Laboratory studies were conducted in which corrugated-metal pipes were loaded at diametrically opposite elements and the vertical and horizontal deflections of each pipe were measured. The close correlation between measured deflections and those calculated by the thin-ring elastic analysis established the validity of the thin-ring analysis within the elastic limit of the material, even though the pipes are flexible and the changes in curvature under load are relatively large. With this fact established, a hypothesis of the action of such a culvert under an actual earth embankment was devised and each loaded pipe was analyzed to determine its horizontal and vertical deflections, its bending moments, and its tangential thrusts. The hypothesis assumes a uniformly distributed vertical load and vertical reaction, and a passive pressure against the sides of the pipe distributed approximately parabolically over the middle 100 degrees of the pipe sides. The passive pressure is postulated to be proportional in magnitude to the lateral movement of the sides of the pipe. Extensive full-scale experiments on flexible culverts ranging from 36 to 60 in. in diameter were conducted. The pipes were loaded by embankments of several kinds of soil 15 and 16 ft. high, without traffic loads, and the loads, deflections, radial pressures, settlements and other pertinent data were observed. These experiments amply verify the design formula developed from the load hypothesis. One of the experiments has been in progress for more than 13 yr. and valuable data concerning long-time deflection increases of this type of culvert pipe have been obtained. Soil tests were made to measure the density of the sidefill surrounding the pipes in an attempt to determine the relationship between passive pressure and soil density. Maximum Proctor densities of these soils were also determined.