Effect of Bentonite Slurry Pressure on Interface Friction of Pipe Jacking

Pipe jacking is a technique used to construct tunnels by pushing prefabricated pipes through the ground from an entrance shaft to an exit shaft. This technique is referred as microtunneling in U.S. terminology, therefore, in the rest of the text, pipejacking and microtunneling will be used as having the same meaning. Pipe jacking is a more economical alternative to segmental lined tunnels. However, the main limitation of pipe jacking is the jacking distance, which is directly dependent on the friction between the pipe and the soil. Therefore, reducing the friction between the pipe and soil is a critical issue in terms of economy and construction speed. In pipe jacking, in order to reduce the friction between pipe and soil, a bentonite injection is applied. In this paper, a new laboratory test setup was prepared to simulate pipe jacking. The main aim of this test setup was to find the effects of bentonite pressure on the lubrication efficiency, which has not been clarified in the literature yet. In order to verify the test method, results of laboratory tests were compared with measurements of the friction between jacked pipe and soil in several real life projects. The measurements were compared with the coefficient of friction values obtained from three different field measurements conducted in different projects. According to the results, it can be stated that by applying continuous bentonite injection even under very low pressure, an interface can be created between bentonite slurry and a concrete pipe, and this bentonite interface is able to reduce the friction coefficient tremendously to approximately 10% of the sand–concrete pipe interface friction coefficient. But when the injection is not applied continuously, the application of bentonite slurry injection decreases the coefficient of friction to half of that in the case of no bentonite injection.