A Novel in Situ Test for the Design of Drilled Foundations-Article

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The paper introduces the Borehole Plug Test, an in situ method intended to improve the design of drilled foundations whose capacity depends largely on side shear, including micropiles, ground anchors, and drilled shafts. A short grout plug is cast at the bottom of a standard geotechnical borehole, using installation methods intended to represent those of the full-scale foundation. The plug is then pulled out while load and movement are measured. These measurements provide the grout–ground bond strength and a load-transfer (t–z) curve that can inform estimates of foundation capacity and settlement.<br /><br />A field trial at a Rhode Island bridge site tested a plug in very dense silty fine sand. Its measured bond strength was 183 kPa, close to the value assumed in the micropile design. Researchers used the plug’s normalized load-transfer curve to simulate a nearby micropile load test. The predicted and measured responses agreed well at lower loads, but the simulation was stiffer at higher loads. The full-scale test pile did not reach geotechnical failure, limiting direct comparison of ultimate capacities.<br /><br />The test can provide site-specific design information early in a project and may reduce reliance on conservative assumptions or costly full-scale load tests. It can be incorporated into site investigations and requires relatively simple equipment. However, its accuracy at full scale remains uncertain, particularly because of differences in plug geometry, foundation size, soil disturbance, and ground type. The authors conclude that further fundamental research and field testing against full-scale load-test data are needed before the method’s broader design use can be established.

Keywords

Borehole Plug Test

grout-ground bond strength

load-transfer curve

t-z curve

micropile design

drilled foundations

side shear

in situ testing

ground anchors

settlement prediction