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

  • Xuan Man Nguyen
  • Thi Them Do


Together with the excavated width, the depth of trench play a
vital role in stability as well as economics effectiveness of a trench
constructed by open trench method. It is widely recognized that a steeper
angle of an open trench, a smaller amount the excavated soil required;
however, the trench is unstable. In this case, an external supporting structure
system is obliged to prevent the trench from falling into the excavation zone.
In addition, an open trench is normally located above the water table, hence
under unsaturated condition. One of the most important characteristics of
unsaturated soil is negative pore water pressure which brings about the
matric suction, higher negative pore water pressure, higher matric suction;
consequently, higher shear strength of soil. On the other hand, there is a few
theories and research works have been reported on the method of
determining a suitable depth of a trench under unsaturated condition.
Previous works tend to assume that the distribution of matric suction is
either constant or linear with depth; as a result of this, the designed results
are often overestimated compared to practical results. In this paper, the
effect of distribution of matric suction was taken into account to propose an
equation to estimate the depth of a trench that can be applied without
supporting structure. Finally, an example of numerical calculation of a depth
of the open trench was described.