Weathering and deterioration of geotechnical properties in time of groundmasses in a tropical climate

Weathering and degradation of slope materials govern erosion and stability of slopes over time. Weathering is often ignored or over-simplified in engineering due to absence of time-series of material properties and uncertainty on future environment. The research in this paper investigates the rate of deterioration of geotechnical properties due to weathering over time after excavation of cut slopes in completely weathered material and residual soil in a tropical environment. Three cut slopes are studied with different excavation dates but with similar geological, geotechnical and groundwater conditions, identical slope aspect, and land cover. Index and strength properties are determined by field and laboratory tests. The results show that the hydro-mechanical properties of completely weathered material in cut slopes evolve very rapidly to those of residual soil within time spans of 30 to 90 years in a tropical-humid environment. The properties reduce so much that instability of cut-slopes is inevitable if this reduction is not taken into account during design and excavation of the slopes. Change of strength properties is around 1.1 to 1.3%pt. per year from the original completely weathered material strength at the time of excavation. This is based on simplified linear and logarithmic relationships. The deterioration of geotechnical properties is likely the main reason for many slope failures in cut slopes after many years of stability in the research area of Yen Bai City in Vietnam.