C150016 Feasibilty Study 12/04/1997

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DISCUSSION The Idaho Springs Reservoir, located on Chicago Creek, provides storage for the reliable raw water supply to the City of Idaho Springs and the City's service area. The Office of the State Engineer, Dam Safety Branch, has progressively restricted the capacity of the reservoir due to the deteriorated condition of the dam, evidence of seepage, and structural damage of the spillway. Current restriction on the reservoir limits its storage to 120 acre-feet. The reservoir capacity is 270 acre-feet at the crest of the spillway. Parson's Engineering Science, Inc. of Denver, Colorado completed the feasibility study for the project, in November of 1997. The estimated project cost to address embankment seepage and to repair the spillway area was $1,100,000. Below is a brief summary of the major project elements and their estimated cost as presented in the original cost estimate: Planning and Design = $95,825 Dam Rehabilitation = $523,250 Spillway Rehabilitation = $350,000 Construction Insp./Contigencies = $130,925 Total: = $1,100,000 The dam and spillway rehabilitation work was bid for construction in the fall of 2000, and was awarded to Harrison Western Construction Company of Lakewood, Colorado, in August of 2000, at a bid price of $1,010,263. The Contractor mobilized on-site in August of 2000 and is currently 50% complete with the project. Prior to and during construction the Contractor has encountered cost overruns. These additional costs can be summarized as follows; cost overruns due to unforeseen changes encountered in the field, design changes incorporated into the project at the time of construction, and changes instituted by the State Engineer's Office not included in the original project cost estimate. Dam Embankment-Additional costs due to unforeseen changes encountered in the field: The awarded bid for the dam and spillway rehabilitation work was $137,013 over the project costs presented in the original cost estimate. The original concept for addressing seepage in the dam embankment was to seal the borrow pit area and other apparent seeps with gunite or concrete, perform hot asphalt injection grouting along the dam face, and to install a free draining rock berm along the toe of the dam. Upon uncovering the dam face, however, significant voids, piping, and organics were discovered within the existing embankment area, that required an alternate design be developed to adequately address the existing seepage. The modified design, reviewed and approved by the State Engineer's Office, was the placement of a Bentomat CLT Liner,placed in conjunction with various soil zones above and below the liner, extended along the dam face to the existing spillway abutment and past the existing outlet structure. Additionally, the existing rip rap layer over the existing dam face was removed, screened, and reset at a uniform 3-foot thickness across the dam face. This modified dam embankment design is reflected in the Harrison Western's additional construction cost of $137,013. Additionally, the City of Idaho Springs incurred an additional design cost of$35,000 to address the actual field conditions encountered in the dam face.