Drained Elastic Behavior of Drilled Shafts in Cohesionless Soils

Abstract

The drained elastic (small-strain) behavior is evaluated for drilled shafts in cohesionless soils. A database of load tests is evaluated using the LI-L2 interpretation method, in which Lt is the elastic limit, defined at a specific load (QL1) and corresponding displacement (PL0. Values of drained elastic soil modulus (Ed) are back-calculated from QLI and pLI and then are normalized by the unit side resistance (f) and cone tip resistance (qc). For the database, the back-calculated Ed/f and Ed/qc ratios had coefficient of variation (COV) values on their means of approximately 70 to 90% in uplift and 90 to 100% in compression. These COVs are consistently larger than those for pL1 alone. Comparisons also are given to results for drilled shafts in cohesive soils. Overall, the results demonstrate that elastic behavior can be quantified using either normalized 9L1 or Ed values, but large COVs are inherent for subsequent displacement predictions. Introduction A database of axial load tests (Cushing, 2001) was used to examine the drained elastic (small-strain) behavior of drilled shaft foundations in cohesionless soils. The slenderness ratio [depth (D) / diameter (B)] for the shafts ranged from 2.5 to 56, and the diameters ranged from 0.24 to 2.00 m. Unit side resistance (f) values ranged from 4 to 306 kN/m 2. Overall, more than 75% of these load tests had D/B less than 25, B between 0.31 and 1.0 m, and fless than 150 kN/m 2. This paper consists of two parts: (a) an assessment of the Lt-L2 interpretation method for drilled shafts in cohesionless soils and (b) an evaluation of back-calculated drained elastic soil modulus (Ed) values from this method and subsequent normalization using unit side resistance (f) and cone tip resistance (qc). Where appropriate, comparisons are made to the results of a companion study for drilled shafts in cohesive soils (Cushing and Kulhawy, 2001). Assessment of L1-L2 Method Background. Hirany and Kulhawy (1988, 1989a,b) proposed quantitative guidelines for drilled shaft load test data interpretation. They noted that load-displacement curves generally can be described by three distinct regions: initial linear, transition, and final linear. The end of the initial linear region is the elastic limit (L0, while the beginning of