Prediction of jacking forces for microtunnelling operations

This paper outlines methods for estimating the jacking forces associated wi th different types of microtunnelling operations. These methods have been d eveloped using probably the most extensive database of microtunnelling jack ing forces assembled to date. These data were collected by questionnaire in Japan as part of an initiative by the International Society for Trenchless Technology (ISTT) and coordinated by the Japanese Society for Trenchless T echnology (JSTT). Methods for predicting jacking force are produced for slu rry, auger and push-in type microtunneliing operations. Separate methods ar e suggested for these techniques as it was found that the jacking force is sensitive to the method of installation. The methods can also take into acc ount the soil type found on a particular project. However, sensitivity of t he measured jacking force to other factors, such as soil strength and depth of installation, were not included in these predictive methods as no disce rnible relationships could be established due to the variability in the dat a. Examples from two case histories, one involving a 1.0 m nominal diameter sl urry microtunnelling machine in dense silty sand and the second involving a 500 mm nominal diameter microtunnelling machine in sand and gravel, are pr esented which use the equations proposed in this paper. The results from th ese examples show that the predicted jacking forces are comparable to those measured in the field. This paper therefore presents practical and reliable methods of predicting jacking forces associated with microtunnelling projects. (C) 1999 Elsevier Science Ltd. All rights reserved.