Jackson Gulch Outlet Canal Rehabilitation Project

Transcription

1 Jackson Gulch Outlet Canal Rehabilitation Project Preliminary Budgetary Estimate for Rehabilitation February 2004 Prepared for the Mancos Water Conservancy District Jackson Gulch Reservoir County Road N Mancos, Colorado Civil, Structural & Geotechnical Engineers 222 South Park Ave. Montrose, CO (970) FAX (970)

2 Jackson Gulch Outlet Canal Rehabilitation Project: Preliminary Budgetary Estimate for Rehabilitation Introduction The Jackson Gulch Reservoir system consists of an off-line storage reservoir located in Jackson Gulch, which is a deep gulch with very little tributary drainage. The reservoir functions by diverting water from the West Mancos River and transporting it to the reservoir via an Inlet Canal. The stored water is used for hydroelectric generation, irrigation, and as a domestic water supply for the Mesa Verde National Park, Town of Mancos, and the Mancos Rural Water Company. The water treatment plant for the Mancos Rural Water Company is located just below the reservoir. The pipelines feeding the distribution systems are buried beneath the access road that parallels the Outlet Canal. Released water that is not diverted for consumption is returned to the West Mancos River at the end of the Outlet Canal. Buckhorn Geotech, Inc. has completed a detailed assessment of the Jackson Gulch Inlet Canal for the Mancos Water Conservancy District. This work was done to determine the cost and nature of restoration and rehabilitation work necessary to extend the life of the Inlet Canal. As a follow-up to this work, the District has asked that a preliminary budgetary estimate for rehabilitation of the Outlet Canal be completed. The following report presents our preliminary findings. Description The Outlet Canal begins at the stilling basin at the base of the Jackson Gulch Reservoir embankment and extends approximately 2.2 miles (see Vicinity Map). Its course starts in Jackson Gulch, crosses a narrow mesa, and then drops into the West Mancos River Valley where it discharges back into the river. With the exception of the 683-foot-long concrete drop chute at the end of the canal, the Outlet Canal was designed as an earthen embankment. However, landslide activity during construction forced conversion of about 1000 linear feet to a concrete bench flume. The capacity of the Outlet Canal system is 207 cubic feet per second (cfs). The attached drawings, sheets 1 through 10, show the course of the Outlet Canal with 100-foot stationing for reference. These sheets are at a scale of 1 inch = 100 feet and were produced from aerial photography collected in the Fall of 2003 by Aero-Graphics of Salt Lake City, Utah. Methodology Our detailed analysis of the Inlet Canal has provided insight into the geologic setting, slope stability problems, soil characteristics, concrete deterioration, and structural distress that are affecting the performance of the Outlet Canal. The knowledge and experience thus gained, combined with observations and testimony by District Superintendent, Gary Kennedy, has been Mancos Outlet Canal.doc Page 1 of 6

3 applied to discreet sections of the Outlet Canal to produce a package of preliminary recommendations for rehabilitation work. In each case, the recommendations are designed to address specific problems that have developed through the 55 year life of the existing Outlet Canal structure. These recommendations involve approximately 0.82 miles (37%) of the 2.2- mile-long canal. There is an additional 600 linear feet of off-canal ditch that must be lined to preserve slope stability. Findings The following narrative offers a section by section summary of existing problems and recommendations for rehabilitation. Station to Station This section of earthen canal is subject to seepage losses that are adversely affecting the stability of a large landslide area downslope of the canal as well as the soil materials supporting the foundation for the Mancos Rural Water Company s water treatment plant. Any significant movement of the hillside below the canal could result in rupture of the domestic water lines buried beneath the access road. Such rupture would terminate water flow to Mesa Verde National Park, the Town of Mancos, and the Mancos Rural Water Company. It is recommended that this section of canal be lined to prevent seepage losses and further deterioration of slope and soil strength. Drawing #2 shows the location of the proposed work and a typical section depicting how the lining may be completed. Station to Station The steeply sloping, denuded hillside above the canal has experienced slope failure. Sporadic movement of the soil mass requires continual maintenance. A large-scale failure of this unstable slope would block the canal, interrupt water delivery, and would result in significant off-site damage to the adjoining privately owned properties. It is recommended that detailed testing and analysis of this area be completed for design of appropriate slope stability enhancement measures. These measures may include use of tie-back anchors, retaining walls, or buttress structures. Drawing #2 shows the location of the proposed work. Station to This is another area of steep, open hillside above the canal. The hillside is experiencing accelerated soil creep as well as internal slumping which requires constant maintenance and threatens catastrophic failure. The instability of the hillside stems in part from excessive soil moisture. It is recommended that detailed testing and analysis of this area be completed for design of appropriate slope stability enhancement measures. Hazard mitigation will entail dewatering as well as enhancement of resistive forces. Recommendations include piping 600 Mancos Outlet Canal.doc Page 2 of 6

4 linear feet of an up-slope irrigation ditch as well as possible use of tie-backs, retaining walls, and buttresses. Drawing #3 shows the location of the proposed work and a typical section depicting how the ditch piping may be completed. Station to Station This section of earthen canal has experienced destructive slope failure along the access road and adjacent canal embankment. The ground movement ruptured the domestic water lines which created a major inconvenience and potential health problem in the service areas. The following photograph shows the damage that has occurred in this area. Continuing seepage losses are contributing to active movement within a downslope landslide area. Although the domestic water lines have been rerouted from beneath the access road, they still traverse the lower hillside where they could be impacted by future slope movement. It is recommended that this section of canal be lined to prevent seepage losses and catastrophic slope failure. Additionally, the failed slope below the canal must be repaired and stabilized to restore global support to the hillside, stabilize the waterline route, and allow permanent repair of the access road. The recommended work includes detailed testing and analysis of the failed slope as well as use of a mechanically stabilized earth fill and other measures as needed to buttress the road and stabilize the hillside. Drawing #4 shows the location of the proposed work. Station to Station This section of the Outlet Canal traverses a steeply sloping, permeable hillside that was destabilized by construction grading and moisture invasion. A slope failure through this area in 1947 forced conversion of the earthen canal to a concrete bench flume. Although the flume has performed well, aging and concrete deterioration has resulted in joint failures and seepage losses Mancos Outlet Canal.doc Page 3 of 6

5 that are now threatening the stability of the hillside below the canal. Movement of the hillside would cause severe damage to the flume, access road, and the domestic water lines. Additionally, erosion and slope movement on the hillside above the canal has created intensive lateral pressures that push against the upper wall of the flume. Since the flume has no lid or other means of resisting the lateral forces, the wall is leaning inwards and is in danger of rupture. Recommendations for rehabilitation of the concrete bench flume include removal of encroaching slope material, buttressing an exposed section of the downslope wall, patching and lining the flume floor and walls, and installation of a network of cross-braces to resist lateral bucking. Also, two bridges over the canal will be needed to complete this work. Drawing #4 shows the location of the proposed work and a typical section depicting how the slope can be modified, the flume lined, and the lateral bracing installed. Station to Station The canal at the end of the concrete flume section is cut into the flank of a steep hillside. Naturally occurring erosion and sloughing of the glacial material comprising the hillside cascades into the canal reducing its capacity, adding turbidity, and creating a major maintenance burden. It is recommended that this section of the earthen canal be replaced with an arched pipe. Drawings #4 and #5 show the location of the proposed work and Drawing #5 also shows a typical section depicting how the pipeline can be installed within the canal embankment. Station to Station As the Outlet Canal departs the eroding hillside, it encounters an outcrop of Mancos shale. This highly fractured, thinly bedded, and partially weathered shale material is notoriously expansive. Repeated cycles of wetting and drying within the canal have accelerated weathering and enhanced permeability. The result is excessive seepage losses, saturation of subgrade materials, and loss of internal strength which may result in catastrophic slope failure. It is recommended that his section of the earthen canal be lined to prevent further seepage losses and weakening of the hillside. Drawing #5 shows the location of the proposed work and a typical section depicting how the canal lining work may be completed. Station to Station As the Outlet Canal approaches the rim of the West Mancos River canyon, it again crosses a deposit of glacial material. Seepage losses along this section are migrating towards the steep canyon walls and contribute to slope stability problems in the vicinity of the drop chute. It is recommended that this section of the earthen canal be lined to prevent further seepage losses and adverse impacts to the drop chute. Drawing #9 shows the location of the proposed work and a typical section depicting how the canal lining work may be completed. Mancos Outlet Canal.doc Page 4 of 6

6 Station to Station Concrete aging, joint deterioration and structural distress throughout the length of the drop chute have resulted in seepage losses, saturation of subgrade materials, and slope instability similar to that being experienced at the drop chute at the end of the Inlet Canal. It is recommended that a 30-inch diameter welded steel pipeline be placed inside the concrete chute. This will eliminate seepage losses, increase safety, and aid in stabilizing the hillside. Construction will include piping, the drop chute, installation of a trash rack and transition structures, and pouring a new concrete floor in the stilling basin. Drawing #10 shows the location of the proposed work and typical section depicting how the pipeline may be installed. The following photograph shows a view of the drop chute as viewed from below. Preliminary Cost Estimate A preliminary estimate of the cost of the above recommended rehabilitation work has been prepared using criteria developed during our analysis of the Inlet Canal. It is cautioned that this assessment and cost estimate is based on experience gained from detailed study of similar canal conditions and has not been verified through inspection, testing, and analyses. The estimated cost of the recommended work identified for rehabilitation of the Outlet Canal sections described above totals $ 1,363,194. Attachments: Vicinity Map, Drawings 1 through 10 Mancos Outlet Canal.doc Page 5 of 6

7 Mancos Outlet Canal.doc Page 6 of 6 MANCOS WATER CONSERVANCY DISTRICT JACKSON GULCH OUTLET CANAL BUDGETARY COST ESTIMATE February-2004 Unit Quantity Unit Price Total Subtotal Canal Lining of Earthen Sections Sta 0+00 to Sta 3+50 L.F. 350 $ 90 $ 31,500 Sta to Sta L.F. 250 $ 90 $ 22,500 Sta to Sta L.F. 450 $ 90 $ 40,500 Sta to Sta L.F. 225 $ 90 $ 20,250 Canal Lining of Earthen Sections Subtotal $ 114,750 Concrete Bench Flume Rehabilitation Sta to Sta Concrete Restoration L.S. 1 $ 30,000 $ 30,000 Flume Lining L.F. 950 $ 66 $ 62,700 Upslope Over-burden Removal L.F. 950 $ 40 $ 38,000 Downslope Buttress L.F. 300 $ 20 $ 6,000 Steel Cross Braces Each 100 $ 200 $ 20,000 Bridge Crossing Each 2 $ 12,500 $ 25,000 Concrete Bench Flume Rehabilitation Subtotal $ 181,700 Slope Stabilization Sta to Sta L.F. 200 $ 400 $ 80,000 Sta to Sta L.F. 300 $ 100 $ 30,000 Sta to Sta L.F. 250 $ 100 $ 25,000 Slope Stabilization Subtotal $ 135,000 Pipe Upslope Irrigation Ditch Sta to Sta New Pipe L.F. 600 $ 50 $ 30,000 Transition Section Each 2 $ 5,000 $ 10,000 Pipe Upslope Irrigation Ditch Subtotal $ 40,000 Pipe Canal Section Sta to Sta Arch Pipe L.F. 650 $ 500 $325,000 New Transition Each 1 $ 8,000 $ 8,000 Pipe Canal Section Subtotal $ 333,000 Drop Chute Sta to Sta inch Diameter Steel Pipe L.F. 650 $ 350 $227,500 Concrete Encasement C.Y. 50 $ 200 $ 10,000 Inlet Transition Each 1 $ 10,000 $ 10,000 Outlet Transition Each 1 $ 10,000 $ 10,000 New Concrete Floor in Stilling Basin L.S. 1 $ 20,000 $ 20,000 Trash Rack L.S. 1 $ 10,000 $ 10,000 Drop Chute Subtotal $ 287,500 Total Construction Cost $ 977,200 15% Engineering $ 146,580 20% Contingency $ 195,440 3% A YEAR ESCALATION FACTOR YEAR Assumes construction duration two years 2005 $ 29,316 with each year's construction value equal to 2006 $ 14,658 1/2 the total construction, beginning in 2005 Total Budgetary Project Cost $ 1,363,194

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