Micropiles Supporting a Wraparound Addition, from the Basement-PPT Presentation

PPT Presentation

Pdf Summary

The project involved adding up to six stories of retail and office space around an existing 21-story concrete office tower in Midtown Atlanta, which had a two-level underground parking garage. The original foundation used drilled piers bearing on hard rock, generally designed for an allowable bearing pressure of 80 ksf.<br /><br />A new geotechnical review recommended a bearing pressure of 100 ksf, but proposed loads on existing parking-deck columns exceeded the capacity of some piers. The structural design was revised to add steel columns extending from street level through the garage, supported by new pile caps and micropiles. The design included compression-only micropiles rated at 160 or 210 kips and compression-tension micropiles rated at 160 kips in compression and 80 kips in tension. Permanent 7-inch casing extended to top of rock, with a center bar in the rock socket.<br /><br />Installation took place in the low-headroom, constrained garage. Equipment and grout supplies were staged at street level, with lines routed through ventilation shafts. Because the work occurred in an enclosed space, ventilation and dust-control measures were implemented; casing was flushed with air and water to control dust. A preproduction micropile was installed and tested in March 2020.<br /><br />The project installed micropiles beside existing columns and perimeter walls, with the general contractor cutting the slab and excavating to the pile-cap elevation. In total, the work included approximately 14,000 feet of micropiles. Piles ranged from 19 to 76 feet long, averaging 52 feet.

Keywords

Midtown Atlanta

retail office addition

existing office tower

micropiles

drilled piers

deep foundation

garage retrofit

steel columns

pile caps

low-headroom installation