Analyzing GCO probe test data of the CHASE study in the context of weathering grades and index test data : a pilot study

Abstract

The relationship between high decomposition grades (Grades IV to VI) and engineering properties is critical in Hong Kong, where these weaker materials often govern slope stability. Although GCO Probe test is routinely used to evaluate fill material compaction, its application in cut slopes of weathered rocks remain underutilized. While earlier studies suggest soil indexes can enhance the reliability of material descriptions, practical applications are scarce. The extensive CHASE dataset has presented a unique opportunity to investigate relationships between GCO Probe test data, soil indexes and decomposition grades (IV to VI). To minimize confounding factors and enhance comparability, 53 slopes of Hong Kong Granite are selected from the 183 slopes for analysis. Despite limitations in CHASE data collection and layer boundary assumptions, the following potential geotechnical applications are revealed. Although definitive blow count ranges for specific decomposition grades are not established, blow counts systematically increase as the GCO Probe transitions from Grade VI to Grades V/IV materials. While blow counts alone cannot reliably distinguish decomposition grades, depth-profile patterns differ significantly among Grades VI, V, and IV. These characteristics provide a broad criterion for differentiation, especially when supplemented with drillhole and trial pit data. Among the nine soil indexes analyzed, the most effective indicators for distinguishing decomposition grades are Percentage Fines, Schmidt Hammer rebound values (L-type and N-type), Slake Test results, and Feldspar Strength. Supplementary indicators, which lack standalone reliability due to overlapping values or classes, are Hand Penetrometer strength, Percentage Sand, and Field Test Strength Estimate. Microfracturing does not directly differentiate decomposition grades but critically influences other indexes. Optimal distinction is achieved through combined use of soil indexes.