Within the framework of a national project undertaken in Belgium (2003-2009), investiga-tions on the stability of temporary trenches in unsaturated soil are carried out by means of theoretical and ex-perimental approaches. It is well known that soil matric suction is an essential stress variable that influences the shear strength of unsaturated soil and consequently slope stability. In this contribution, the characterisation of the shear strength of a partially saturated quaternary loam is achieved by using simple unconfined compression tests. The derived shear strength is then used for numerical simulations of a 5m depth full-scale experimental trench, executed and monitored in the unsaturated loam at the site of Belgian Building Research Institute (BBRI). Comparison of numerical investigations and practical observations reveals that the adopted procedure remains on the safe side. This paper includes descriptions of the laboratory investigations, full scale experi-ments and numerical simulations.