An approach for predicting the stability of vertical cuts in cohesionless soils above the water table

• Limit analysis approach proposed for stability of excavations above water table.
• The approach was validated against simple small-scale column collapse tests.
• First validation of limit analysis for unsaturated soils (to authors’ knowledge).
• Approach successful in modelling stability of vertical cuts in pyroclastic deposit.
• Simple ‘conservative’ approach to assess rainfall impact on vertical cut stability.

Temporary vertical excavations in cohesionless (granular) soils pose a problem for conventional ‘two-phase’ soil mechanics theory since non-zero collapse height is not predicted using the classical ‘dry/saturated’ shear strength criterion, given that cohesionless soils above the water table are assumed to be dry. An extension of the classical shear strength equation to account for the effect of matric suction on the effective stress in partially saturated soil is presented here that is incorporated into the bound theorems of plasticity. A simple validation experiment is reported to test the concept following which, a case study is presented that explores the extent to which matric suction and its impact on shear strength can explain the large safe vertical cut height that is often observed in cohesionless pozzolan deposits in the field. Lastly, the impact of rainfall events and subsequent ponded infiltration is investigated using a very simple analytical technique based on the classical Terzaghi consolidation solution. The research presented here gives practitioners with no particular expertise in the mechanics of unsaturated soil, techniques to assess the stability of geostructures involving unsaturated cohesionless soils that are based on simple calculation techniques taught in undergraduate courses.