Anchored Hollow Bar Tangent Pile System For Underpinning-PPT Presentation

PPT Presentation

Pdf Summary

The project involved underpinning the foundation of a two-story hospital during construction of an addition. Excavation extended 5–15 feet below the existing structure and was enclosed on three sides by the hospital. Although the original design specified staged pit underpinning, clean, loose sands and gravels made it difficult to maintain stable excavation faces. The contractor observed ground loss and raised concerns about damaging building movement.<br /><br />Engineers and contractors evaluated permeation grouting as a way to stabilize the ground, but field tests showed limited grout intake in finer soils and uneven results. They instead adopted a composite system combining closely spaced, grouted hollow-bar micropiles with a shotcrete soil-nail wall. The micropiles were drilled through slots in the existing footing and installed in alternating sequences. The soil-nail wall was built in five-foot lifts, with soil nails spaced five feet apart horizontally and vertically.<br /><br />Construction took place in a constrained work area and required measures to address cold weather. Monitoring included verification and performance tests of the nails and micropiles, as well as daily surveys of the building. Pullout-test results exceeded the required capacity by at least five times, with little or no creep. Maximum measured building movement was 0.6 inches vertically and 0.13 inches horizontally.<br /><br />The system achieved the original underpinning objective while reducing construction risk and time compared with pit underpinning. The authors recommend using a prequalified underpinning subcontractor on similar projects.

Keywords

underpinning

micropiles

soil nails

shotcrete

foundation stabilization

hospital addition

excavation support

grouting

ground movement monitoring

construction risk reduction