Investigating the effects of the use of compressible bedding on the behavior of buried flexible pipes

A buried thermoplastic pipe installed with conventional method deflects vertically downwards under soil load. Because the displacement of the pipe crown is more than that of the sidefill soil, differential settlements in the soil medium over the pipe crown level take place. Differential settlements lead to the development of a mechanism called "positive soil arching" within the soil medium and, consequently, a vertical stress smaller than the geostatic stress acts on the crown of the thermoplastic pipe. The degree of arching depends on the magnitude of the differential settlements. Therefore, the degree of arching can be increased by placing compressible materials, which will increase the magnitude of the differential settlements, around the thermoplastic pipe when the pipe is being installed. The increase in the degree of arching will provide less vertical stress on the pipe crown and an improved pipe behavior. In this thesis, the effects of compressible materials placed around the pipe on the thermoplastic pipe behavior was investigated by 1:1 scale laboratory tests and numerical analyses performed using finite element method. Five different pipe-compressible zone configurations are designed to investigate the effects of compressible zone geometry on the pipe response. Expanded polystyrene (EPS) was selected as the compressible material for the reasons that it's a manufactured material and minimum requirements for its engineering properties are specified by standards. In order to investigate the effects of the density of the compressible material on the pipe response, tests were performed using EPS products with two different densities. A comprehensive instrumentation was implemented both for the soil medium and the test pipe. Soil medium was instrumented with settlement plates and earth pressure cells in order to measure soil settlements and soil stresses, respectively. Test pipe was instrumented with potentiometric displacement sensors and biaxial strain gauges in order to measure pipe deflections and pipe wall strains, respectively. Numerical analyses were performed for each test condition by using PLAXIS 2D software, a wellknown finite element code. Measurements collected from laboratory tests were compared with calculated values obtained from numerical analyses. It's concluded that the use of compressible bedding material around the thermoplastic pipe provides the degree of positive soil arching on the pipe to increase regardless of the vertical deflection of the pipe and, consequently, relatively very small pipe deflections to occur. The configuration that provides the greatest improvement in pipe behavior provided a reduction in vertical soil stress on the pipe crown up to by 77%, in vertical deflection of the pipe up to by 89%, in horizontal deflection of the pipe up to by 96% and in the maximum bending moment around pipe wall up to by 78%.