Estimation of Maximum Unit Side Resistance in Drilled Shaft Foundations from Partially Mobilized Load-Displacement Data-Paper

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The study develops a method for estimating the maximum unit side resistance of drilled shafts from load tests that do not fully mobilize shaft capacity. Such proof tests are common but do not directly reveal ultimate resistance.<br /><br />The authors analyzed 145 static and bi-directional load tests, producing 612 unit side-resistance curves. After excluding curves affected by data or construction issues, 454 were assessed using the Double Tangent method. This method estimates resistance where tangents to the initial and later portions of a load–displacement curve intersect. The ratio of this resistance to the peak measured resistance serves as a mobilization indicator: as resistance approaches full mobilization, the ratio approaches one.<br /><br />Of the analyzed curves, 52 showed fully mobilized resistance. These records were used to develop regression models relating mobilization ratio to maximum resistance. Models were examined by predominant soil type and construction method, as well as a global model using all data. The global cubic model had an R² of 0.95 and showed promising predictive performance. Soil-specific models generally predicted maximum resistance within about 10% of measured values, while construction-method models showed greater variability.<br /><br />Applying the global model to partially mobilized records produced estimated maximum resistance values that were, on average, higher than measured peak resistance, with relatively low variation. The authors recommend the global model for estimating maximum unit side resistance from partially mobilized load–displacement data. They note that the method does not account for residual soil behavior. Independent validation datasets were identified, but validation results were not presented in this paper.

Keywords

drilled shafts

unit side resistance

load tests

Double Tangent method

proof tests

mobilization ratio

maximum resistance

load-displacement curves

regression model

soil-specific models