Stagecoach Spillway Replacement

A 100 year old wood and earth spillway structure operated by Xcel Energy required replacement to conform to federal standards. Engineering plans by Wheeler and Associates called for eleven post-tensioned rock anchors into solid granite bedrock to fix the structure in place.

The anchors were 4.5” diameter piles reinforced with #11 150ksi double corrosion protected bar with a bond length of 27’ and a 19’ free length for post tensioning. An expansive 7000psi grout was tremied to displace an average standing water depth of 17 feet.

Performance testing was performed on one third of the piles. Proof tests were performed on the remainder. Piles were loaded to 133% of design capacity, and were locked off at 165ksi. Although no creep was observed during testing, average bar elongation was 1.3” at lockoff.

Constructing below the water table required multiple corrosion protection measures including pregrouted and PVC encased reinforcement, expansive high strength grout and tremie grouting to force water from pile shafts.