Goose Pasture Dam Spillway

The Problem

Goose pasture dam, located near Breckenridge Colorado has a spillway crest elevation of 9886 feet. The dam is an embankment with a concrete spillway and RCC emergency spillway built adjacent to the main chute. The dam was originally constructed in 1964. The main spillway chute is 152 feet long and averages approximately 40 feet wide. Typical maximum daily flows are 300 cfs, however, the spillway is sized for larger flows. Over the years, several locations of concrete spalling occurred near two spillway slab joints. Water was seen leaking from under the slab through the spalls. Phreatic surface readings were taken and determined to be between 12 feet and 18 feet below the elevation of the spillway slab. Concrete cores were removed and a metal rod was used to probe the base material below the spillway slab. As expected, soft materials and voids below the slab were found. Two potential dam failure mechanisms were possible under the current condition. First, water could create uplift pressure failing the spillway and dam. Second, the very pliable and wet soil could lose enough friction to allow the spillway to slide down the 3:1 embankment causing failure. State Engineers assessed the data and required spillway remediation.

The Solution

Replacement of the entire spillway with removal and replacement of soft materials was found to be expensive. Restruction Corporation, acting as the prime contract holder initially installed forty three 15-foot long earth anchors with a 7 foot long bonded length stressed to 8-kips as a spillway tie down. A total of 35 cubic yards of cement grout was pumped under low pressure into voids below the apron slab, spillway crest and spillway slabs. The grout was a mix of cement, fly ash and flow aid, measuring a maximum 19 seconds on a flow cone. The grout was fluid enough to fill the void system under the pressure of the head only. This low pressure grouting technique minimized any potential uplift pressure on the spillway slab. The original dam construction did not include an under-drain system. At two joint locations, portions of the spillway slab were removed in phases and new under-drain bedding and piping were “stitched” into place. Cracked and delaminated portions of the apron slab and spillway slab were removed and replaced with rebar dowels, new reinforcing and 5000-psi compressive strength concrete. The existing spillway concrete was roughened using a scarifier. Epoxy rebar dowels were set into the existing spillway concrete at 2 foot centers and a mat of #3 epoxy rebar placed at 6-inch centers was tied in preparation for a concrete overlay. Six separate concrete overlay placements were completed averaging 10-inches thick over the entire 6250 square foot spillway surface. Conventional PVC water stop was installed at each joint location. Shotcrete was utilized to complete the 10-inch thick overlay on the crest face. The crest of the dam was raised slightly, increasing the storage capacity of the reservoir but likely compensating for sedimentation loss since original construction. The parshall flume was set in a different location and some minor modifications to the toe drain completed the project.

Project Images

Overall View of Project

The first row of earth anchors and roughened concrete surface near the crest

Excavating for the new under drain system

Drilling for earth anchors

The second concrete overlay placement

Overall View of the Lake with the project site in the lower right hand corner

Grouting under the spillway slab

Placement of the apron slab repairs

Preparation for the final concrete overlay placement