The Spatial Variability of the Cone Tip Resistance of Weathered Mudstone Profiles from CPT Testing

Cone penetration test (CPT) data were used to explore the vertical spatial variability in weathered layers of the Charmouth Mudstone Formation (Lias Group) at a site in Oxfordshire, England. The Charmouth Mudstone Formation is a stiff, fissured Jurassic mudstone that was formed 183-199 million years ago and weathered by glacial and periglacial conditions during the past 200,000 years. Forty-six CPT profiles were considered in the variability assessment. The vertical variability of the cone tip resistance was characterized by determining the vertical scale of fluctuation analyzed within a homogenous layer via the random field theory. The results showed that the profiles of weathered mudstone had a low scale of fluctuation, which translates to high variability among the property values. The vertical scale of fluctuation of q_c for the weathered mudstone profiles varied between 0.10 m and 0.69 m, with an estimated theoretical average of 0.30 m and a coefficient of variation equal to 42%. A probability histogram of the vertical scale of fluctuation of q_c showed higher probability of occurrence between 0.11m and 0.49m. The vertical scale of fluctuation q_c did not show a distinction between the weathered mudstone layers and the underlying, unweathered material.

Keywords: Spatial variability, Weathering, Scale of fluctuation, Uncertainties, Mudstone.