The foundations for a 200-foot tall communications tower would require additional capacity to accommodate future equipment additions. The four tower legs were originally supported on 2.5-foot-diameter drilled pier foundations, extending approximately 27 to 32 feet below grade. The existing tower foundations would require reinforcement to resist increased compression and tension loading caused by new equipment on the tower. The additional foundation support would be installed in tight working conditions around the existing tower to avoid large cables, an adjacent equipment station, ice bridge supports, and perimeter fencing.
The proposed foundation detail included two helical piles at each drilled pier location. The helical piles would be cast into concrete pile caps, doweled into opposing sides of the drilled piers.
The helical pile configuration consisted of Model 287 (2.875-inch OD by 0.203-inch wall) round shaft with an 8”-10”-12” triple-helix plate lead section. A 40 kip ultimate capacity, in both compression and tension, was specified for each helical pile to satisfy the deflection criteria under the anticipated loading conditions. The helical piles were advanced to depths from 16 to 22 feet to achieve the specified torque-correlated ultimate capacity. A mini-excavator was used to install the piles within the limited working areas. Custom new construction brackets were field-welded to the tops of the installed piles and sprayed with a zinc-rich coating for corrosion protection. Steel reinforcing bars were placed around each helical pile and doweled into the existing drilled piers.