Pipe jacking forces in sand
In this research work, a comprehensive review of investigations into pipe jacking operations, and in particular the forces generated during the process, is presented. The factors affecting these forces were examined and their influence was studied in order to give a clear indication on the their magnitude and behaviour. A model scale testing facility, based on a 1.8m X 1.5m X 1.5m high steel tank, has been developed together with hydraulic jacking facilities and scale model pipes of 200mm diameter having both steel and concrete surfaces. In this facility the actual pipe jacking operation can be simulated, so that a study of the jacking forces could be made with the use of appropriate instrumentation techniques and computerised data recording. A series of tests was conducted to examine the influence of several parameters on the jacking forces, such as overcut ratio, type of soil, type of pipe surface, overburden pressure, and the rate of jacking. These parameters were in relation to the distance jacked. Two granular soils, uniformly graded Leighton Buzzard sand and well-graded River sand, were used as the material surrounding the pipeline, that is the material through which the pipes were jacked. Laboratory analysis on the soil materials was conducted and the relationship between results obtained from these tests and scale model testing were established. The interface friction between the surface of the pipeline and the surrounding soil was determined. The results show a peak in the jacking forces at the start of the jacking process, followed by a decrease and leveling off as the jacking continued. The graph is very similar to the stress/strain relationship of a typical granular soil. Both steel and concrete pipes were used in the tests to detennined the influence of pipe surface, and to a lesser degree pipe joint, characteristics. Higher forces were recorded for concrete pipes. The jacking rate was kept constant and consistent for all tests. It was found to have little effect on the magnitude of the jacking forces. The jacking forces were measured automatically using a computer. The results show a clear trend of increase in the jacking forces when the overburden pressure and jacking length were increased. Tests using different overcut ratios have indicated that a minimum jacking force might be achieved at a specific overcut ratio. The results of this research show that the jacking forces are influenced by a number of parameters. These include the overburden pressure, overcut ratio, pipe diameter, jacking length and the influence of the surrounding material. This is fully demonstrated by establishing a method of jacking forces prediction, termed herein the Loughborough Method.