J.L.K. Swee

Doctor of Philosophy, Oxford University, Hilary Term 1991

Pipebursting: Model Tests

Summary

Pipebursting is a pipeline installation technique which is in widespread use by the public utilities in Britain. Greater extent of use has been curtailed by the lack of understanding of soil displacements and the implicated risk of damage to adjacent structures as a result of operations. Fundamental research is necessary to fully understand the extent of its possible uses and limitations in terms of the ground deformation caused. Therefore, research was instigated to determine the extent and characteristics of the soil movement, during and after the use of the technique.

The problem was studied as a displacement-displacement (D-D) problem whereby the boundary conditions are only in terms of strains and displacements and only strains and displacements are observed. Small-scale model tests simulating the pipebursting process were carried out in saturated Speswhite kaolin clay, dense Leighton Buzzard 14/25 sand, and a sandy clay backfill. Tests in sand provided the upper bound of movements as dilation of the sand caused volume expansion while results from the clay tests defined movements at near constant volume deformation. The backfill tests gave a lower bound of movements because of soil compressibility due to a relatively high air voids content. Model moles of initial diameter D_o = 25mm to model the 4" pipe at 1/4 scale were used. Expansion ratios D_f/D_o ranging from 1.2 to 2.2. were used to simulate the upsizing process at several cover depths. Measurements of the ground movements were made on two planes of observation.

The ground movement mechanisms associated with the three types of soil were identified. Relationships between the ground movements and mole axis uplift associated with the geometrical effects of expansion ratio and cover depth ratio were established. A comparison of the model test results with the results predicted by the Flow Analysis (Sagaseta, 1987) suggests that the analysis may be used to provide an approximation of the ground movements caused by the technique in cohesive clay and backfill materials. Interpretation of some of the basic trends of the field measurements was made, based on the model test results. The numerical stress analysis (currently being developed) was also assessed and found to be sufficiently accurate in predicting ground movements caused by the pipebursting technique.

(no thesis available)