Abstract
<title>Abstract</title> <p> This paper investigates the spatial variability of a soft lacustrine clay deposit in Tubarão, southern Brazil, via piezocone (CPTu), piezoball, and piezo-T full-flow penetrometer tests. Vertical scales of fluctuation (δ <sub>v</sub> ) and inherent variability coefficients (COV <sub>w</sub> ) were derived from seven in situ soundings at two adjacent sites. The random field theory framework was applied, incorporating stationarity verification via Kendall's tau test and fitting four theoretical autocorrelation models (SNX, SQX, CSX, SMK). The single exponential model provided the best fit for most profiles, with δ <sub>v</sub> values ranging from 0.12 to 0.43 m for penetration resistance, with is consistent with typical ranges reported for marine and lacustrine clays worldwide. The full-flow penetrometers exhibited lower penetration resistance than did the piezocone but yielded consistent undrained shear strength estimates after site-specific bearing capacity factors were applied (N <sub>kt</sub> = 15.5, N <sub>b</sub> = 13.0, N <sub>t</sub> = 11.8). Probability density function analysis revealed that the Weibull distribution best represented the grouped datasets. The findings confirm the applicability of full-flow penetrometers for spatial variability characterisation of soft clay deposits and provide useful statistical parameters for reliability-based geotechnical design, reinforcing the importance of careful preprocessing and model selection in random field analyses. </p>