Knowles Wins ASA 2019 Outstanding Repair & Rehabilitation Project

(Left to Right:) Aaron Poole (ASA President), Lisa Maloney, Dan Maloney, Billy Roy, Aaron Elkins, Charles Hanskat (ASA Executive Director)

In early November of 2018, Knowles Industrial Services Corporation (KISC) was issued a contract by First Light Power Resources, Inc. (FLP) to perform a structural shotcrete liner within a steel-riveted penstock at the Falls Village Hydro Electric Plant in Canaan, CT. FLP’s request for bids permitted contractors to provide a design-build approach for a structurally self-sustaining system to be built within the penstock interior. The existing 9 ft (2.7 m) diameter, 360 ft (110 m) long penstock was buried in its entire length on a steep bank and crossed underneath a live highway. Penstock replacement by excavation proved to be too costly, as much of the existing penstock beneath the roadway was encased in reinforced concrete requiring significant demolition and interruption to traffic in this area.

BID REVIEW PROCESS

During the bid review process, three different repair methods were presented, which included hand-laid fiberglass mat systems, structural epoxy liners, and structural shotcrete. KISC provided a unique, low-cost reinforced shotcrete approach. It was imperative that the awarded contractor provide structural calculations demonstrating that their method would be self-sustaining and could withstand all static and water hammer pressures while in operation, 13.1 lb/in.2 (0.09 MPa) and 2.2 lb/in.2 (0.015 MPa), respectively. Calculations were not to consider any strength contribution from the existing riveted steel shell, whose structural integrity was questionable and impossible to conservatively define. With two professional engineers on staff specializing in penstock shotcrete and steel reinforcement design, KISC was able to secure the contract by providing a series of structural calculations and submittals that met the strength requirements for the new penstock when operating at full capacity.

THE DESIGN

KISC worked with Kleinschmidt Associates, Inc., the Engineer-of-Record, to design a shotcrete liner that met the structural demands of the environment without significantly reducing the existing pipe’s cross-sectional area, which Falls Village Penstock #1 Structural Shotcrete Rehabilitation Project By Billy Roy and Andrew Lawson would ultimately affect the overall power generation potential of the turbine units at the power station. In addition, a decrease in Manning’s coefficient was desired to promote increased laminar flow. It was determined the Manning’s coefficient for riveted steel is (0.19). KISC provided FLP and Kleinschmidt with substantial data proving that the new Manning’s coefficient for smooth concrete would be (0.012) and the eventual epoxy coating would further decrease the Manning’s coefficient to (0.010), ultimately providing less friction losses and higher water velocity to combat any losses due to reduced cross-sectional area. Each structural shotcrete penstock liner is unique. The design process is complex and requires attention to various details and site conditions. External forces, such as soil overburden loads, bending and shear stresses, and highway loads, must be considered in the design process as well as internal hoop stresses, such as static and water hammer pressures and flow geometry. This particular penstock design and construction was the tenth of its kind for KISC over the past 10 years. Over the years, KISC has worked with several engineering firms throughout New England to design and build reinforced penstock liners that have been approved and often favored by the Federal Energy Regulation Commission (FERC). KISC has assisted various owners and engineers on several pen - stock projects by compiling data and submitting packages for the shotcrete process which has, over time, become one of the preferred repair methods for water-filled pipes. The original bid documents indicated that FLP would supply the awarded Contractor with a 6-week canal outage to perform the work. The approximately 2000 ft (610 m) canal diverts water from the Hoosatonic River which sup - plies three 9 ft diameter penstocks to the 9 MW power station. Penstock #1 was one of three penstocks that would be entirely de-watered during this station outage. KISC indicated the project duration would require a 12- to 13-week effort from start to finish. This condition was unacceptable to the FLP at that time, as the cost of leaving the canal dry and keeping the entire station (all three penstocks) offline for more than 6 weeks was not feasible for the owner due to the lost revenue from potential power generation. Furthermore, FLP’s Safety Department was concerned with work being performed downstream of the existing Penstock #1 head gate if the canal was left at full capacity. KISC was charged with developing a work plan and method that could allow for the shotcrete work to be completed over a 12- to 13- week schedule without sacrificing lost power generation in Penstock Units #2 and #3. The KISC Engineering team, comprised of Senior Vice President Andrew Lawson and Project Manager Billy Roy, began to value engineer an approach that would provide the necessary schedule time while not sacrificing FLP revenue. KISC committed to designing a temporary, secondary safety bulkhead upstream of the Penstock #1 head gate. The 1-1/4 in. (32 mm) thick steel safety gate would provide a secondary level of protection in the event that the existing penstock intake gate failed under the hydrostatic pressure of a full canal during construction. This approach would permit safe working access into Penstock #1 for KISC workers for 13 weeks without requiring a total station outage. This permitted FLP to continue generating power in Units #2 and #3 without interruption during the construction in Penstock #1. Additionally, this process allowed for the flexibility to work in winter months during historically lower river flows when water availability is not abundant enough to sustain continuous operation of all three units. This value-engineered approach resulted in KISC being awarded the contract for Penstock #1 at the originally requested 13-week schedule.

As part of the contract specifications, KISC was tasked with providing design calculations for the shotcrete reinforcing steel required to withstand the static and hammer water pressures in this penstock. The KISC Project Manager, Billy Roy, determined that the reinforcing bar configuration would be #4 (#13M) longitudinals spaced at 12 in. (300 mm) radially and #5 (#16M) prebent radial hoops spaced at 6 in. (150 mm) longitudinally. The reinforcing bar mat would be tied to a series of 1.5 in. (38 mm) slab bolsters welded long - itudinally down the length of the penstock and spaced at 4 ft (1.2 m) radially. As with all water delivery pipes for power generation, it is imperative to design a shotcrete product that provides sufficient cover over the embedded steel while maintaining a small loss in overall cross-sectional area of the pipe. KISC elected to use Quikrete Shotcrete MS at 4 in. thickness around the circumference of the pipe. Rebound removal was difficult on this site due to limited access in and out of the work area, so a higher concentration of silica fume was desirable to minimize rebound and unnecessary cleanup labor.

CONSTRUCTION

KISC mobilized to the site on October 15, 2018. Construction began by excavating two areas through the soil over - burden to expose the tops of the existing penstock to pro - vide various points of worker access as well as to provide means of expelling shotcrete dust. Two large holes were cut in the existing penstock steel.

Once the pipe was entered, the entire substrate was pressure washed with 5000 lb/in.2 (34 MPa) water blasting. All areas to receive 1.5 in. welded slab bolster were ground for cleanliness and to promote weld strengths. The project consisted of welding 2100 ft (640 m) of 1.5 in. continuous slab bolsters, installing 6.5 miles (10.5 km) of reinforcing steel, and applying 150 yd3 (115 m3) of Quikrete Shotcrete MS. Smoothness of the finished shotcrete was incredibly important on this project. As with all hydroelectric utilities, great care is taken to prevent any power generation reduction due to friction losses anywhere within the system. To maintain Manning’s coefficient of (.012) for smooth concrete, cutting and finishing was a labor-intensive and necessary process. This would ultimately lead to increased laminar flow, higher water velocity, and subsequently higher generation capacity. KISC crew members spent considerable time finishing the entire 10,200 ft2 (950 m2 ) interior of Penstock #1.

SHOTCRETE INSPECTION

Specifications required the involvement of a third-party shotcrete consulting firm to provide unbiased commentary and inspection of the shotcrete process. Ray Schallom of RCS Consulting & Construction Co. Inc. was contracted to provide insight, education, and inspection services to assure that all shotcrete work and techniques were aligned with the project specifications. Among the topics inspected by Schallom were reinforcing bar layout and cover, silica awareness and mitigation, shotcrete finishing, and curing procedures. Final reports submitted by RCS Consulting reported no deficiencies in the shotcrete pro - cess implemented by KISC.

PROJECT CHALLENGES

Some of the challenges faced on this project included confined space entry and rescue, winter conditions with cold temperatures, shotcrete curing, and transporting large quantities of construction materials at great distances by foot. KISC provided all confined space permitting, air monitoring, attendants, and rescue services. To combat winter temperatures, all gunning and pre-dampening operations were confined to heated enclosures built near the work area. KISC crews exercised great diligence in protecting air-powered equipment from frequent freezing and provided ample protection for shotcrete materials stored outdoors at the site.

CONCLUSIONS

The project was completed on time in the original 13-week schedule. KISC crews worked 50 hours/week, including several weekends on and around holidays. The temporary safety gate was removed on January 4, 2019, and the penstock was placed back into operation later that afternoon with no startup issues. The owner expressed gratitude for the timely, cost-effective approach engineered by KISC which created very little power generation losses throughout the duration of the project.

In July 2019, KISC crews returned to Falls Village Hydro during the annual outage. The shotcrete liner was inspected after operating for approximately 7 months. No defects were found. FLP was pleased to announce that after col - lecting power generation data over that time period, no generation losses were reported due to the installation of the shotcrete liner. KISC finished the project by abrasive brush blasting of the entire 10,200 ft2 penstock shotcrete liner and applying two 30 mil (0.76 mm) spray coats of high-build epoxy. The intent of the epoxy was twofold; to extend the service life for the shotcrete and to provide a lower Manning’s coefficient to promote more laminar flow. The epoxy lining project was completed in the scheduled 2-week timeframe.