Volume 9, Issue 4 (December 1986)
Influence of Borehole Stabilization Techniques on Standard Penetration Test Results
The standard penetration test (SPT) is an extensively used and generally accepted field test for determining engineering characteristics of soils. However, there are many variables inherent in the performance of the test. The influence of some of the major variables on SPT results are examined under methods of borehole stabilization (drilling fluid, hollow stem augers, and driven casing); methods of raising and lowering the drive weight (cathead and rope and a mechanically released free-fall hammer); and nine different soil sites. Soil profiles at these locations were reasonably uniform with depth, and included sands, silts, and clays. Borehole depths ranged from 16 to 30 m, with ground-water levels varying from near surface to very deep. Standard penetration tests were performed by experienced drilling crews, using standard procedures as outlined in ASTM Method for Penetration Test and Split-Barrel Sampling of Soils (D 1586), at intervals of 1.5 m. Results were analyzed using standard statistical procedures and presented in both graphical and tabular summaries. Results indicate that use of casing gives SPT values different (higher above the ground-water level, lower below) than those obtained using drilling fluid or hollow stem augers in sands; whereas no significant difference was observed among the three methods in fine-grained soils. SPT values obtained using a mechanically released free-fall hammer were comparable to values obtained with the cathead and rope method using a level of care no higher than that routinely exercised in the field.