Performance of Drilled Shaft Under Combined Vertical and Horizontal Loading

Date
2020
Authors
Malla, Sandeep Kumar
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Abstract

The p-y method has been widely used to design the laterally loaded drilled shaft and driven piles, which has been a reliable and predominant method of analysis for the past decades. Research and practical data provide enriched data that have been made continuous updates possible. So far, the developed p-y curves for different soils have been successfully incorporated into numerous software to aid the design of laterally loaded drilled shafts and driven piles. However, up to date, the design of drilled shafts/driven piles cannot consider the interaction between vertical and horizontal loads, if both loads are present. Namely, vertical and horizontal loads are considered independently in the current design practice. This assumption works reasonably well when either vertical or horizontal load is the dominant one; however, if both vertical and horizontal loads are significant, the validity of the existing p-y curves under these circumstances has not been systematically verified.

This research concentrates on the behavior of drilled shafts under combined vertical and lateral loads. This study is conducted in two stages, i.e., experimental and numerical, to investigate the effect of vertical loads on a laterally loaded pile/shaft. The experimental study involves laboratory tests of soil to find the soil parameters and preparing soil bed to perform the lab-scale test of the drilled shaft. The testing chamber was built by placing the wooden planks together with the help of a wooden beam, filling the chamber with the soil, and constructing a drilled shaft. The drilled shaft was instrumented with the strain gauge at three different elevations from top to bottom. Then the vertical load was applied through the metal weights, and the lateral load was applied with the help of a hydraulic jack. The deflection was monitored with a digital dial gauge and applied lateral force was recorded by a proving ring. The results from the lab-scale test were then used to calibrate a numerical model that was utilized to perform the parametric analysis to study the effects of many possible factors such as drilled shaft geometry, soil friction angle, and soil stiffness. The shaft length considered were 9 m, 12 m, and 15 m; the diameter of the shaft was 0.9 m, 1.2 m, and 1.5 m. The soil friction angles were 25o, 29o, and 35o; the soil stiffness was represented by a compressibility index of Modified Cam Clay (MCC) model, which varied from 0.026, 0.079, to 0.109 in this study. During the numerical modeling, the vertical load, 80% of the ultimate load-carrying capacity of the shaft, was applied to the shaft head and the horizontal load was applied through the velocity of 5×10-7 meter per step until a lateral displacement of 10 cm was reached. The simulation results showed that the effect of the vertical load is significant on a laterally loaded drilled shaft, and the effect of the vertical load is less as the length of the shaft is increased.

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Keywords
Combined loading, Drilled Shaft, lateral deflection, Modified Cam Clay
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Department
Civil and Environmental Engineering