Drilled Shafts · Acton, MA
Rock-Socketed Piers for a Solar Carport, Acton
Eleven 36 in drilled concrete piers for a solar carport, on a site where the borings showed hard bedrock from about 7.5 ft down, boulders, and groundwater between about 6 and 13 ft.
The problem
Eleven 36 in drilled concrete piers for a solar carport, on a site where the borings showed hard bedrock from about 7.5 ft down, boulders, and groundwater between about 6 and 13 ft.
The engineered approach
Temporary steel casing held the overburden where it was wet or loose, then each shaft was cut into the rock to its design embedment of 9 to 12.5 ft. Each pier then got its reinforcing cage and an anchor-rod group set to a template before 5,000 psi concrete went in.
Outcomes for the GC
- 11 piers drilled and poured, 9 of them socketed into rock
- Rock came in as shallow as 3 ft, shallower than the borings showed
- Every shaft logged by an independent geotechnical inspector
How we built it
A schematic section of the drilled shafts system for this Acton, MA job. Scroll to build it in construction order, or select a callout to read the detail.
Callouts
Select a numbered part below to read what it is and what it does.
- The borings put hard rock at about 7.5 ft and deeper; the independent drilled-shaft logs found it in 9 of the 11 shafts, as shallow as about 3 ft. An independent geotechnical inspector logged every shaft.
- Where the overburden was wet or loose, temporary steel casing held the hole open down to rock. It was used at 6 of the 11 piers and pulled as the concrete went in.
- Above the rock sat a short run of fill and glacial till with boulders. The borings put groundwater at about 6 to 13 ft, so where rock was deeper the overburden could be wet.
- Each shaft was cut into hard rock to its design embedment of 9 to 12.5 ft. Where rock came in shallow, the socket made up most of the pier.
- A reinforcing cage went into each finished hole, from near the bottom of the socket up into the stick-up.
- The anchor-rod group was set to a template held on a frame over the hole, so the rods are cast in position and level for the canopy steel.
- Each pier was poured with 5,000 psi concrete up into a formed stick-up above grade. On the cased piers the casing came out as the concrete went in.
- The solar canopy columns and steel, drawn dashed, are by others. ESC's scope ends at the finished pier with its anchor rods cast in.
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