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 qc 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 qc showed higher probability of occurrence between 0.11m and 0.49m. The vertical scale of fluctuation qc did not show a distinction between the weathered mudstone layers and the underlying, unweathered material.
González et al. (2022) studied this question.