A case study on the determination of the excavated trench depth in unsaturated soil constructed by trench method without supporting structures

Together with the excavated width, the depth of trench play avital role in stability as well as economics effectiveness of a trenchconstructed by open trench method. It is widely recognized that a steeperangle of an open trench, a smaller amount the excavated soil required;however, the trench is unstable. In this case, an external supporting structuresystem is obliged to prevent the trench from falling into the excavation zone.In addition, an open trench is normally located above the water table, henceunder unsaturated condition. One of the most important characteristics ofunsaturated soil is negative pore water pressure which brings about thematric suction, higher negative pore water pressure, higher matric suction;consequently, higher shear strength of soil. On the other hand, there is a fewtheories and research works have been reported on the method ofdetermining a suitable depth of a trench under unsaturated condition.Previous works tend to assume that the distribution of matric suction iseither constant or linear with depth; as a result of this, the designed resultsare often overestimated compared to practical results. In this paper, theeffect of distribution of matric suction was taken into account to propose anequation to estimate the depth of a trench that can be applied withoutsupporting structure. Finally, an example of numerical calculation of a depthof the open trench was described.