CHEESMAN DAM OUTLET WORKS RENOVATION

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21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

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Underwater Construction Engineering 721

CHEESMAN DAM OUTLET WORKS RENOVATION UNDERWATER CONSTRUCTION ENGINEERING

Jeff Martin, P.E.1

Gordon Harbison, P.E.2

ABSTRACT

Denver Water is in the process of an Outlet Works Rehabilitation at Cheesman Dam. The outlet works consists of tunnels bored through the left abutment at elevations 6,780, 6,690, and 6,645, respectively referred to as the Auxiliary, Mid-Level, and Low-Level Outlets. The rehabilitation was accomplished by installing new stainless steel slide gates through underwater construction diving and at depths up to 210 feet. The Cheesman Upstream Control project is a phased project spanning several years of design and construction. The project begun in 2007 with the removal of the old balance valve at the low level intake and is anticipated to finish in 2011 with the removal of the old guard valves within the outlet tunnel and replacement of the Larner-Johnson Needle Valve with a new Jet Flow Gate. The entire project includes providing upstream control by means of new slide gates at the inlets to the outlet works tunnels, a new control building housing the hydraulic power for the gates, and finally removal of the guard gates within the outlet tunnel. The project design was based upon historical information including the original survey note books, asbuilt drawings, and photographs. The high costs of construction diving coupled with the likelihood of differing subsurface conditions required a good partnering effort between the Contractor, Resident Engineer, and Design Engineers. A fast-paced construction schedule (24-hours a day for approximately 15 weeks) required quick analysis of existing conditions and subsequent design changes to avoid lost production time and unnecessary costs.

FACILITY OVERVIEW

The Cheesman Dam is an on-stream facility located on the South Platte River in Jefferson and Douglas Counties, Colorado, in the Pike National Forest. The dam was constructed for storage of municipal water supply by the South Platte Canal and Reservoir Company, and was completed in 1905. At the time of construction Cheesman was the worlds highest dam. In 1973 Cheesman Dam was designated a National Historic Civil Engineering Landmark by the American Society of Civil Engineers. The dam structure is a gravity arch masonry dam constructed with solid granite blocks laid in cement mortar as facing over a core consisting of granite rubble in a bed of

1 Jeff Martin, Project Manager/Design Engineer, Denver Water Department, 1600 West 12th Ave, Denver, CO 80202 2 Gordon Harbison, Resident Engineer, Krech Ojard & Associates, 3580 Mount Hickory Blvd., Hermitage, PA 16148

21st Century Dam Design Advances and Adaptations 722

Figure 1. Site Layout

concrete. The dam is 221 feet in height and impounds 79,064 acre-feet of water, and is classified as a large high hazard dam. The outlet works tunnels are bored through the left abutment at approximate elevations 6,780, 6,690, and 6,645, respectively named within the drawing set as the Auxiliary Outlet, Mid-Level Outlet, and Low-Level Outlet. A fourth outlet tunnel at elevation 6734 was originally constructed, but was abandoned with the inlet portion being filled with concrete. The Auxiliary Outlet, built in 1925, is an independent tunnel outlet with no connection to the other tunnels and is controlled by the Larner-Johnson Needle Valve. The Low and Mid Level outlet tunnels combine to one tunnel approximately two-thirds of the way through the abutment. Both tunnels are controlled by a total of six 42-inch gate valves prior to the tunnel intersection, and then by various cone and free-discharge valves in the downstream valve house. The site layout is shown in Figure 1.

The only major work succeeding the original construction is a valve house constructed at the toe of the dam in 1971. The original waterway tunnel was extended with a 78-inch diameter steel pipe. The pipe is manifold to provide passageways for two 42-inch, one 24-inch, one 12-inch, and one 8-inch Howell Bunger type outlet valves. Each of the outlet valves are guarded by one or more ball valves.


The six 42-inch gate valves at one time controlled the release flows to the South Platte River, after the 1971 Valve-House addition the 42-inch gate valves act as guard gates to the valves located within the Valve-House. The 42-inch gate valves internal to the outlet works tunnels are equipped with hydraulic cylinders to operate the gate stem. The hydraulic system consists of an original Pelton Wheel connected to a triplex pump that furnishes hydraulic water pressure to the cylinders attached to the gate valves. The original Pelton Wheel, 42-inch gate valves, cylinders, and piping were used to operate the system, until completion and startup of the new slide gates as described in this paper. Figure 2 and Figure 3 the Primary outlet works and Auxiliary outlet works sections through the left abutment.

Figure 2. Primary Outlet Works


PROJECT INITIATION

The project need and initiation was bore out of several key factors: (1) An aging outlet works system dependent and operating on 19th century equipment and technology, (2) maintaining key infrastructure within the Denver Water system, (3) new sediment loading from the Haymen Fire3, and (4) a desire to provide upstream control of the outlet works tunnels for future work. The facility needs and the Denver Waters organizational goals and Mission Statement4 aligned and the project was selected for implementation. Outlet Works modifications studies and alternatives were initiated in 2005, followed by design concept selection in 2006. Early within the alternatives development and evaluation phase, an underwater construction approach was selected based on water storage risks associated with draining the reservoir. Analyses showed there was a significant probability of not filling the reservoir in a single run-off season. Specifically, the analyses showed a 30% probability of a storage shortfall in Cheesman Reservoir. A shortfall of the magnitude evaluated would have system wide impacts to the water supply system.

3 Colorados largest wildfire impacted the drainage area directly above Cheesman Dam 4 Denver Water will provide our customers with a reliable, high-quality water supply and excellent service. We will be responsible and creative stewards of the assets we manage. We will actively participate in and be a responsible member of our communities. We will accomplish this mission with a productive and diverse work force.

Figure 3. Auxiliary Outlet Works


DESIGN PLAN The Cheesman Upstream Control project is a phased project spanning several years of design and construction. The project begun as an Outlet Works Modifications study in 2005, with a selected design approach made in 2006. In 2007 Denver Water contracted with Rodney Hunt (RH) for fabrication of three hydraulically operated stainless steel slide gates and spools (tunnel liners/thrust restraints). The installation design began in 2007 and lagged the slide gate design, and was based upon the limited site knowledge and the pre-purchased slide gates designed and fabricated on a similar but earlier schedule. During the design phase it was understood that the project would be performed underwater and by a limited amount of divers; likely just one diver physically assembling the components of the design. The design and installation of the slide gates was based upon the interpreted size of the three inlet tunnel portal geometries. An interpretation of the geometry from historical data was made, and the size of the slide gates was maximized to maintain the current hydraulic capacity of the outlet works system. As the condition of the rock quality was not know at each of the inlets, a new tunnel liner, referred to as a spool, was designed for each tunnel inlet. The spool piece serves several key purposes, they are (1) Structural support to the tunnel portal area, (2) form for annulus grouting (seal) between the spool and tunnel, and (3) provide a clean connection flange for the gate. Basically, the spool acts as a traditional gate thimble, but also provides structural support to the tunnel portal area. Rock excavation, tunnel shaping, and spool placement and annulus sealing were the most critical steps for project success. Rock excavation and tunnel shaping contained the most contract risk, and a majority of the construction duration and all change order work was spent on these activities. It was known and understood by Denver Water that without a detailed physical hands on survey of all three areas, the tunnel portal geometry as related to the placement of the spools contained the most risk to the project. In order to maintain the current hydraulic capacity of the system the spool cross sections were maximized to within inches of the assumed tunnel geometry. This condition resulted in a minimal amount of room for the spool to fit within the tunnel. The hydraulic design criteria for the outlet works governed the spool size, and the risk of additional excavation and tunnel shaping was known and accepted by Denver Water prior to project bidding. The stainless steel slide gates and spools were designed specifically for each level, the Auxiliary Level is the largest at 8x8, while the Mid and Low Level gates are 4x7. The slide gates were designed to simply be attached with 52 or 72 bronze hex head bolts (depending on the size of the gate) secured to a flange piece on the front of the spool piece. The hydraulic tubing could then be connected to the gate, followed by the installation of the trashrack sections.


Through lessons learned from a different underwater project, the design team choose to drill inclined bore holes form atop the left abutment to the gate locations to house the hydraulic control lines. Two inclined rock borings were drilled, one to the Auxiliary Level, and one boring servicing both the Mid Level and Low Level Outlets. The inclined bore holes eliminated a great amount of risk to the contractor and provided a protected conduit to house the hydraulic control lines and bubblers air lines. Phase 2 of the Cheesman Upstream Control project includes removal and replacement of the Larner-Johnson Needle Valve with a new Jet Flow Gate, and removal of the six gate valves located in both the Mid-Level and Low-Level outlet tunnels. The opening left by the removal of the gate valves will be sealed with new tunnel sleeves. Bulkheads will be installed at this location for future access. In addition to the valve removals, new platforms, handrails, and ladders will be installed within the manways. Finally, a small tunnel air/fill line will be constructed so that the upstream control gates can operate under balanced head conditions. Phase 2 construction work is planned for spring/summer 2011.

UNDERWATER CONSTRUCTION Contract Global Diving and Salvage (Global) was contracted through a quality based selection process to perform the underwater diving work and manage other ancillary conventional construction that had impacts to the diving work and schedule. Prior to executing the contract, Global and Denver Water negotiated a predetermined daily rate for anticipated change order work and standby (idle) time. The pre-negotiated rate for surface diving was approximately $27,000/day and for saturation diving was approximately $80,000/day. The surface air diving was based upon 12-hour days and would be implemented at the Auxiliary Level while the saturation diving schedule was based upon 24-hour days and would be used at both the Mid Level and Low Level with continuous operation until the project was complete. Schedule Due to the high contractor overhead costs for the dive barge, crane, saturation diving life support system, and short weather window, the schedule was aggressive and fast paced. The sequencing of the installation work and owner restrictions for continued operation of the facility, created a schedule with little to no float and a series of activities dependent on finish-to-start relationships. In order to continue with operation of the facility and bypass critical water calls on the river, concurrent work was not allowed; limiting the contractor to work on one level at a time and to completion. With no schedule float, it was recognized at the beginning of the project that a teaming effort between the contractor, resident engineer, and the owner would be required to tackle project issues in an effective and decisive manner. All project team members strove for zero lost time production.


Construction Sequencing The general sequencing of construction was similar to what Denver Water had anticipated and designed. All three levels followed the general sequencing shown in Figure 4.

Figure 4. Construction Sequencing (Typical for All Three Levels)

CONSTRUCTION ENGINEERING Construction Engineering Team

The very nature of an underwater capital improvement project at a 110-year facility contains risk. In this projects case the greatest risk was changed underwater physical conditions of the tunnel areas as related to pre-purchased owner furnished equipment. The high mobilization fee and overhead for the specialized diving equipment resulted in large daily operation costs for the contractor. Cost of operation and cost of production was almost identical, resulting in the same daily rate charge in the event of change order work or standby time. In the event of a change of conditions, the Contractor may be required to standby until an approved owner solution was provided for the implementation. In this event the contractor would be compensated at the predetermined daily rate. Again, the daily construction/standby time daily rates were approximately $28,000/day ($2,340/hour) for surface/air diving and $80,000/day ($3,325/hour) for saturation diving. Construction engineering as related to changed field conditions played a vital role in the success of the project.


This construction engineering team consisted of the contractor, resident engineer, and the owner. The contractor provided a full time project manager and project engineer to develop work plans and detailed submittals outlining the construction sequencing. In addition, the contractor staffed a construction superintendent who reviewed and revised the work plans for underwater constructability. Denver Water contracted Krech Ojard and Associates (KOA), through HDR, Inc., to provide Resident Engineering (RE) services for the underwater diving construction. The purpose of the Resident Engineer was to monitor the diving construction progress and quality through a live feed closed caption video mounted atop of the divers dive helmet. KOA monitored the entire underwater construction process, including 3 months of air/surface diving, and 101 days of 24-hour saturation diving. The Resident Engineering position was critical to the success of the project, by locating the RE onsite in close contact with the contractors project manager and project engineer, the RE was able to assist in submittal development, submittal review, and design construction when issues were identified. The original Denver Water design team and structural consultant5, URS, maintained close contact throughout the project to ensure issues were identified and resolved according to the original design criteria of the new slide gates.

Risk Management and Issue Resolution

Early identification and timely resolution of project issues was a key factor for project success. In order to manage the project issues that would arise during execution, the construction engineering team continuously evaluated potential project risks through a risk management plan and created response plans for risks that could seriously impact the project. The risk management plan was developed by the project team and updated by Denver Water, prior to each construction phase the construction engineering team met and brainstormed potential project risks and issues that could impact the project. The risk register outlined project risks that could impact both the owner as well as the contractor. The project team worked on these risks together for the common success of the project. The risk register was constantly updated, and most importantly, lessons learned from previously completed activates were incorporated within the risk register to minimize the impact of the same issue occurring multiple times. The risk management plan also outlined a response plan in the event a risk item occured. The risk response plan would generally consist of having key decision makers available to review changed conditions and design modifications, decreasing the owner response and direction time to the contractor. The risk response plans proved valuable for several of the risk scenarios, and allowed project issues to be resolved in an expedited and pre-defined manner. The risk register and notification to stakeholders of the potential risk was extremely valuable to expediting the response plan.

5 URS designed the trashrack structures and grout mix used for the spool annulus grouting. URS also provided structural design analysis and review at the Mid Level spool cover structural block.


Most serious issues on this project were the result of a differing underwater physical condition that was different than what was shown within the construction drawings. Some of the issues were small and others were large. Regardless, any changes to the planned work as related to the contract drawings resulted in cost increases to the contract. Along with the pre-negotiated daily rates, a clear division was made between contract work and change order work. Prior to implementation of any changes a meeting between the contractor, owner, and resident engineer was had to review the change order work and delineate a scope line for change order work and what would be eligible for additional contract charges. Because all change orders were based upon a time and material rate it was very important to monitor true change order work versus contract work. A good example of this is the rock bolts used to secure the spool in place. At the Mid Level location, the required length of the rock bolts was typically about 3 feet in length according to the contract drawings. After, identification of changed tunnel geometry, the required length of the bolts was between 5 feet and 6 feet in length. In this case, the base contract covered the first 3 feet of drilling for the bolts, while the remaining 2-3 feet of drilling was completed as time and material with the resident engineer tracking the additional time. By agreeing to a clear delineation of the scope line there was very little disagreements between the owner and contractor regarding the costs for change order work.

Figure 5. Issue Flow Chart


Auxiliary Level Tunnel Widening: The first project issue requiring field construction engineering related to the Auxiliary Outlet tunnel. The Contractor identified a differing underwater physical condition within the Auxiliary Outlet tunnel. The differing underwater physical condition consisted of differing tunnel geometry within the Auxiliary Level resulting in the tunnel being too narrow to allow the pre-purchased spool piece to physically fit within the tunnel, requiring additional widening of the tunnel annulus. In order to minimize the cost and schedule impacts of the additional work, the Contractor, Resident Engineer, and Owner reviewed several alternatives, shown in Figure 6, for relocating the spool alignment to reduce rock excavation. The final chosen alignment (Alternative 2) reduced the amount of excavation by adjusting the bearing and moving the entire spool south. The re-alignment resulted in approximately 20% less tunnel excavation, and a cost savings of about $76,000.

Figure 6. Auxiliary Level Widening

Auxiliary Level Trashrack: While installing the second trashrack section, interference between the existing tunnel wing walls and the trashrack was encountered. Either the wing wall or the trashrack required modification to fit the trashrack assembly properly around the gate. It was decided to remove the trashrack from the water and make structural modifications in the dry. The contractor had concurrent work to perform on the hydraulic piping, and continued with their underwater operations and adjusted the schedule for the trashrack to be installed 3 days later. The trashrack manufacturer was immediately contacted and requested to send a crew to the site along with material that was anticipated for the construction modifications. The existing wing wall was then resurveyed. The data was used to design the modifications necessary to ensure the trashrack would fit the existing condition. The Resident Engineer and Owner reviewed the interference areas and designed a new cut out area within the trashrack that would allow the rack to fit around the wing wall. The proposed modification design was then reviewed and approved by the structural engineer.


Figure 7. Mid Level Spool Post Blast Void

The design was based upon immediately available materials on hand at the jobsite and those available for local pickup by the fabricators en route to the jobsite. The redesign of the trashrack section took approximately 24 hours to complete while the modifications to the rack itself took an additional 48 hours. Topside modifications to the trashrack as opposed to additional wall excavation saved the project approximately $40,000 and did not impact the project schedule. Mid Level Tunnel Geometry: The existing Mid level tunnel geometry and physical conditions was identified as a risk item early in the design phase of the project. The as-constructed drawings and histrocial photographs showed conflicting information, and recent side sector scan survey information continued to disagree with the historical information. The design team choose to show the most conservative interpretation of the data, but assumed the existing conditions would be different than shown within the contract documents.

As anticipated, the actual tunnel geometry was different than what had been shown in the contract drawings. The existing tunnel geometry was too narrow to allow the pre-purchased spool piece to physically fit within the tunnel, requiring additional widening on the south tunnel wall. The widening was accomplished by drilling additional blast holes and limited hand excavation similar to the Auxiliary Level. In addition to the tunnel geometry issue, a larger void space was identified along north wall adjacent to the spool. A small cavity was shown the project drawings that anticipated this feature, the feature turned out to be approximately 30% larger than anticipated. The cavity was filled with additional reinforcement consisting of No. 6 rebar doweled into the tunnel and fill grout. Finally, interference outside of the tunnel between the rock and trashrack was identified. Additional minor rock excavation would be required to fit the trashrack. In order to reduce costs, the entire spool, gate, and trashrack were adjusted to the south and vertically. The realignment resulted in additional tunnel shaping, but reduced the exterior rock excavation. This proved to be a cost effective alternative because the additional tunnel shaping could be performed as part of the contract underwater blast, while the exterior hand chipping would be laborious and time consuming. The estimated project savings in changing the original alignment are estimated at $120,000. Mid Level Rock Fall: A large rock mass above the tunnel portal released during the blast operation, resulting in a large void space above the gate spool. Through a post blast visual inspection of the area, it was reasonably concluded the rock mass


Figure 8. Low Level Masonry at Tunnel Brow

contained a joint and fracture pattern that was present prior to the blast operation. The removal of the underlying rock (as required by the contract documents) and the joint and fracture pattern of the rock caused the rock mass to fall. The result of the rock fall left the spool piece with no cover above it as planned within the design package, as seen within Figure 7. In order to meet the original design criteria of at least one tunnel diameter of solid competent rock cover above the spool, a new monolithic reinforced concrete block was designed to provide structural cover over the front half of the spool, and increase length of potential seepage pathways. The design of the new cover block was developed by the Resident Engineer and Owner and was based upon survey information developed by the Contractor. A concept design was developed and submitted to URS Corporation for structural analysis review. The embedment lengths of the bar and the bar spacing was checked and approved. Upon value engineering review with the Contractor, the rebar sizing was increased to reduce the overall length of rebar embedment within the rock. Through contract work the contractor had learned drilling in the native granite rock for rebar embedment was a laborious process, and reducing the length and quantity of embedments would reduce the change order cost. The reinforcement size was increased, decreasing the amount of drilled hole for embedment within the rock. This exercise was estimated to save approximately $60,000 in installation costs. Low Level Masonry Removal: During the contract blasting and construction activities at the low level tunnel portal a differing site condition was identified. Unlike the previous differing conditions, the low level tunnel geometry was as expected and could fit the

spool without further modifications. However, it was discovered that the tunnel brow and ceiling areas consisted of a poor quality masonry material (See Figure 8), not hard competent granite as shown in the historical documents. The unstable masonry brow posed several challenges to the project. The first was diver safety, the area was unstable and exhibited signs of potential failure and rock fall. This condition posed direct safety concerns to the construction divers. The second was the portal stability above and behind the spool piece, if the area was masonry and not granite rock, the


entire system would depend on this material with unknown structural and seepage characteristics. The original design had depended on strong competent granite to lie directly over the spool and tunnel just downstream of the spool. It was clear the original design intent and design basis criteria would not be achieved if the masonry was left in place above the spool. In order to best ensure the new installation was structurally sound, it was decided to remove identified masonry material and reposition the spool further within the tunnel. Additional surveying was required to identify the vertical control of the spool based on the rise of the original tunnel floor. Proper placement was required to minimize any additional material removal and maintain the hydraulic capacity of the tunnel. The reposition of the spool met the original design intent and design basis criteria. The repositioned spool provided over 15 feet of vertical granite rock above the back end of the spool. This cover material was believed to be good quality competent granite rock that could provide the required structural stability for the tunnel ceiling. The repositioning required three addition underwater blasts to remove the material to the desired area. The addition work took approximately 18 days and resulted in a change order close to $1,400,000. The construction engineering was vital in identify the issue and to design a new alignment that met the criteria of the original design. If the masonry condition was not discovered by the contractor, this issue would likely have caused failure of the tunnel ceiling area behind the downstream end of the spool, leading to major impacts to the facility and Denver Waters operation.

PROJECT RESULTS AND CRITICAL SUCCESS FACTORS

Project Results The project was a success in providing new upstream control at the Cheesman Dam. The new stainless steel slide gates were operated successfully during the startup and commissioning of the system. Most notably, the combined tunnel seepage and gate leakage results were extremely low, and are shown in Table 1. When consideration is given to the fact that these gates were installed underwater at depths of 200 feet, and that there are significant portions of the outlet works tunnels that are unlined (allowing reservoir seepage), the minimal seepage observed is considered to be a success.

Table 1. Combined Gate Leakage and Tunnel Seepage Rates Auxiliary Level 2.8 gpm Mid Level 45 gpm Low Level 7 gpm

Critical Success Factors The project can be considered a success for many reasons, from no lost time injuries or incidents to a working system with extremely low seepage/leakage rates. Some of the predefined critical success factors that were accomplished are listed below.


The new slide gates and upstream location meet present day standards. The new slide gates provide guard function to the downstream valves. The new slide gates were installed almost exactly as designed. The new slide gates provide new flexibility in operating the facility. The Low Level slide gate trashrack has the ability to be simply modified in the

future to store approximately 10 feet of sediment. Contract modifications (negotiations) were fair and managed on a current time basis,

with no issues left to the end of the project. The risk register and response plans created a teaming effort between the

construction team to tackle issues in an efficient and predefined manner. No standby time was incurred for the project. All change order work resulted in high

production rates. Very minimal disruption to the operation of the water supply system.

CHEESMAN DAM OUTLET WORKS RENOVATION

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