To: Members of the Castle Rock Water Commission
From: Mark Marlowe, P.E., Director of Castle Rock Water
Title
Resolution Approving a Construction Contract between the Town of Castle Rock and C&L Water Solutions for Woodlands Sewer Interceptor Manhole Rehabilitation Phase 2 [Woodlands Boulevard and Saddleback Drive]
Town Council Agenda Date: December 21, 2021
Body
________________________________________________________________________________
Executive Summary
Castle Rock Water is seeking Town Council approval of a resolution (Attachment A) approving a construction contract (Exhibit 1) in the amount of $960,095 with C&L Water Solutions (C&L) along with $96,000 (10%) staff-managed contingency, for a total authorization of $1,056,095 for the Woodlands Sewer Interceptor Manhole Rehabilitation.
Castle Rock Water Staff identified 25 corroded concrete manholes on the Woodlands Sewer Interceptor for rehabilitation construction in 2022. A request for proposals was published on the Rocky Mountain E-Purchasing System in November 2021. Five proposals for manhole rehabilitation were received in response to the advertisement. Staff rated C&L’s proposal the highest and the rehabilitation technique and products proposed will meet the project’s objectives. C&L’s proposal represents the best long-term value to the Town. Pre-construction activities will commence in early 2022. Construction is anticipated to begin in early summer 2022 and be completed before the end of the year.
Notification and Outreach Efforts
Some of the Woodlands Sewer Interceptor manholes selected for rehabilitation are in close proximity to residential properties in the Woodlands Development. There has been a history of sewer odor related complaints from adjacent residents. Castle Rock Water will notify adjacent property owners with information about the project and coordinate directly with residents regarding any project concerns.
This project will be an inconvenience to the adjacent homeowners as well as to residents who use the path that parallels the sewer. The pedestrian trail is heavily used and Water Staff is coordinating with Park Staff and will work with the contractor to keep the majority of trail safely open during construction. Some minor temporary closures and detours may be necessary for specific construction activities.
Communication with the adjacent neighborhoods and Homeowner’s Association will be an ongoing program before and during the project. We expect there will be some complaints during construction that will be addressed on a case-by-case basis.
Discussion
The Woodlands Sewer Interceptor is a 2.5-mile gravity pipeline installed in 1987 that conveys wastewater flows from development areas east of Founders Parkway/Ridge Road (Founders Village, Castle Oaks/Terrain, Castlewood Ranch, Bella Mesa), as well as the Woodlands development, back to the Plum Creek Interceptor. A significant portion of the Town’s wastewater flows through the interceptor pipeline. The interceptor is a critical component of the Town’s wastewater collection system.
Wastewater from areas east of Founders Parkway/Ridge Road are pumped uphill to the Woodlands Interceptor through long pressurized force main pipelines from various lift stations operated by Castle Rock Water. The two primary force mains (Castle Oaks and Mitchell Creek) are both greater than 2 miles in length. Some of the wastewater conveyed through these force mains has been previously conveyed by other smaller force mains. Wastewater flows by gravity to lift station wet wells where it is detained before being pumped through force mains. Thus, the majority of wastewater arriving at the upstream end of the Woodlands Interceptor has been detained in the sanitary collection system for a significant period of time. As wastewater is detained in a sanitary collection system and when it is pumped through long force mains, oxygen levels are depleted and anaerobic bacteria are proliferated. The increase in anaerobic bacteria ultimately leads to the production of hydrogen sulfide gas in the collection system. Hydrogen sulfide gas is the most common cause of odor complaints related to wastewater collection systems and is also the primary cause of sewer corrosion. Both of these have historically been issues for the Woodlands Sewer Interceptor.
The Woodlands Interceptor pipeline itself is constructed of polyvinyl chloride (PVC) which is highly resistive to the corrosive effects of sulfuric acid produced by hydrogen sulfide consuming bacteria in the system. Thus the interceptor pipeline itself is in good shape. However, the concrete manholes between pipe sections have not been as fortunate. Concrete and many metals are extremely susceptible to the corrosive effects of sulfuric acid. The high levels of hydrogen sulfide present in the Woodlands interceptor have led to severe corrosion of the concrete manholes. Recent inspections by Castle Rock Water staff have discovered deterioration of several inches of concrete from the manhole wall sections. This degrades the overall strength of the manhole structure and can lead to an eventual collapse of the manhole. Staff identified seven manholes in the upper portion of the Woodlands Interceptor that had extreme levels of concrete degradation. These manholes were selected as the most critical and rehabilitated in 2020. Corrosion-resistant fiberglass reinforced polymer manhole inserts were installed in the seven manholes for a final project cost of $278,395.
In 2021 staff completed inspections of an additional 25 manholes from Black Pine Drive to Woodlands Boulevard. The condition of the manholes in this section varies from high levels of corrosion and failing existing liners at the upper end near Black Pine (similar condition to those in Phase 1) to moderate and low levels of corrosion in the manholes close to Woodlands Boulevard. This is logical as the levels of hydrogen sulfide gas are typically highest where lift station force mains transition to gravity flow and often dissipate and decrease further downstream. This variability of existing manhole conditions in Phase 2 will require the use of multiple rehabilitation methods for the project. The proposed methods will require “fully structural” rehabilitation for the highly degraded manholes and “semi-structural” and “non-structural” rehabilitation for the less degraded manholes.
A request for proposals (RFP) was published on the Rocky Mountain E-Purchasing System in November 2021. Proposals were received from five contractors and ranged from $455,623 to $979,920. After review of the proposals, staff is of the opinion C&L’s proposal represents the greatest overall value and was the only proposal that fully met the project objectives and requirements outlined in the RFP. C&L’s proposed rehabilitation method for “structural rehabilitation” involves installation of fiberglass reinforced polymer (FRP) manhole “inserts” that are placed within the existing degraded sewer manhole. The advantage of this rehabilitation method is that the majority of the existing concrete manhole is replaced with an FRP manhole in its existing location with minimal removal of the existing degraded manhole. This minimizes the amount of overall excavation and corresponding construction footprint. The base of the existing manhole must be reconstructed and the top cone section removed before the FRP manhole with integral cone section is inserted within the existing failed manhole. The end result is a new FRP manhole constructed within the existing manhole. The FRP manhole is structurally rated to withstand all anticipated loads (independent of the remaining existing manhole) and has the advantage of being impervious to corrosion from sulfuric acid resulting from hydrogen sulfide. Semi-structural and non-structural rehabilitation methods will involve water and abrasive cleaning of corroded concrete followed by cementitious build-back and epoxy coating.
C&L has successfully installed over 150 manhole inserts for other major entities in the Denver/Metro area. C&L also successfully rehabilitated 7 manholes with FRP inserts for the first phase of the Woodlands Manhole Rehab project in 2020. The project was completed for approximately 30% ($125,000) less than the original contract amount. They have existing familiarity with the site constraints, flow conditions, and challenges that will be involved in this phase of the project.
C&L will begin pre-construction work in early 2022 with more detailed “manned entry” inspections of the 25 manholes. This will provide specific measurements needed for fabrication of the FRP inserts as well as any adjustments to the scope of work that could be needed due to findings. Actual construction will begin in early summer due to weather constraints of the proposed rehabilitation methods and materials. Construction duration will be 180 calendar days with completion by the end of 2022. Rehabilitation of the manholes must occur with the line in service and Caste Rock Water staff is working with the contractor on a plan to manage wastewater flows during construction.
Budget Impact
Funding for this project will come from the Sewer Line Rehab budget as shown below.
|
Project |
Account Number |
2021 Budget |
Available Balance |
This Authorization |
|
Sewer Line Rehab |
213-4575-445.75-62 |
$2,400,000 |
$ 2,035,682 |
$1,056,095 |
Staff Recommendation
Staff requests Council approval of a construction contract in the amount of $960,095 with C&L, along with contingency of $96,000, resulting in a total project authorization of $1,056,095 for the Woodlands Sewer Interceptor Manhole Rehabilitation Phase 2 Project.
Proposed Motion
“I move to recommend Town Council approval of the Resolution as presented”
Attachments
Attachment A: Resolution (Not Attached)
Exhibit 1: Construction Contract with C&L Water Solutions (Not Attached)
Attachment B: Site Location Map