Calibration & Evaluation of Different Methods with DeepFND
In 1993 FHWA in association with ADSC and ASCE released a report No. 41-30-2175. The report was prepared by Georgia Tech Research Corporation, from the Georgia Institute of Technology. The research study focused on the axial capacity of drilled piles in Piedmont residuum. Two 30-inch diameter drilled piles were installed and load tested to failure; a) one floating in the piedmont residuum at 55ft depth, and b) one reaching to bedrock at 72ft depth.
A very thorough geotechnical investigation was conducted that included a series of lab and in-situ tests. Soils at the site consisted of 15ft of fill, Piedmont residuum SM to 60 ft depth, partially weathered rock to 72ft depth, and Granitic Gneiss below. Ground water was encountered at 56 to 62ft depth.
Equipped with knowledge of the two load tests, we conducted a series of back-calibrations to examine what procedures are best suited to match actual load test results. All calibrations and investigations were performed with the DeepFND software by Deep Excavation LLC. In all cases the FHWA GEC-10 recommendations were incorporated. The following scenarios were examined:
a) Using lab test defined strength parameters for the residuum sands (RES) to calibrate the floating pile case. It was found that a friction angle of 35.5 degrees would accurately reflect load test data on the floating pile.
b) Calibrating the modulus of elasticity of the rock layer to match the observed end bearing pile response.
c) Using SPT values and different ways of estimating the friction angle to match observed response.
In case a) the geotechnical investigation reported a range of friction angles close to 35.8 degrees. We found though that equally important was simulating properly the elasticity of the side resisting soil springs.
In case b) The modulus of elasticity of the bedrock was calibrated to match the observed response. In case c) we experimented with various methods of estimating soil properties from SPT, especially the effective friction angle of the Piedmont residuum. Our analysis indicates that the Triaxial compression method for estimating the friction angle provided more realistic estimates of the effective friction angle vs. recommendations by Kullhawy-Chen (incorporated in FHWA GEC-10) for this case. The major differences though were obtained in the end bearing capacity where the Kullhawy-Chen approach estimated about 15% greater axial capacities.
Case A: 55ft pile load test vs. calibration with soil properties obtained from laboratory test data
Case B: 72ft load test on 30 inch drilled pile end bearing in Piedmont Gneiss
Case C: 55ft floating pile calibration vs. SPT estimated properties with Triaxial Compression Equation