Observations and Impact of Field Logging and Drilling Procedures on Site Characterization of Cambridge Argillite in Somerville, MA-Paper

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The study examines how field logging, drilling, and sample handling affect characterization of Cambridge Argillite at a 15-acre site in Somerville, Massachusetts. The investigation included seven SPT boreholes, rock coring, detailed field classification, and laboratory index, direct shear, slake durability, and compressive-strength tests.<br /><br />The subsurface profile and bedrock quality varied substantially between boreholes and with depth. Core recovery ranged from 0% to 100%, and RQD from 0% to 79%. Drilling sometimes remolded decomposed argillite into a clay-like paste, potentially reducing its shear resistance—the behavior relevant to drilled-shaft side resistance. Direct shear tests showed that cohesive decomposed material could be significantly weaker after remolding than SPT-based correlations imply. Such correlations should therefore not be used for cohesive weathered material without verification; index testing and remolded direct shear testing are recommended.<br /><br />Intact argillite was also highly moisture-sensitive. Air-dried samples had unconfined compressive strengths 1.7 to 13.3 times those of nearby as-received samples, while repeated wetting and drying can promote weathering. Core moisture must be preserved during collection, storage, and transport to obtain representative test results.<br /><br />Recommended practices include careful visual-manual soil classification verified by laboratory testing, recording coring times and noting core loss or barrel jams, and using appropriate core barrels to improve recovery and reduce disturbance. Because the formation is highly variable, the authors recommend borehole-specific characterization for drilled shafts rather than relying on a single site-wide stratigraphy. Full-scale load testing, where feasible, can further validate design assumptions.

Keywords

Cambridge Argillite

Somerville Massachusetts

field logging

drilling disturbance

rock coring

SPT boreholes

direct shear testing

slake durability

unconfined compressive strength

drilled shaft design