On Wednesday February 27, 2008 KISC received a telephone call from Kleinschmidt
Associates of Pittsfield, ME regarding a penstock failure that had just occurred at a
hydro power generating site in Pacolet, South Carolina. The owner, Lockhart Power, routinely contracted with Kleinschmidt as their source for hydro engineering requirements and was ultimately referred to the Pittsfield, Maine office due to their expertise in penstock structures.
Time was of the essence as this breach had resulted in Lockhart’s inability to generate
any power at the Pacolet site, as both generators were fed from this single penstock.
Kleinschmidt’s assessment of this “code red” situation concluded that the repair had to fit the following criteria. The owner was seeking a permanent repair as the Pacolet site generates year round and is profitable. The repair had to be started within one week and be completed as quickly as possible. The repair had to be be economical. The repair would preferably not result in a loss of water flow and reduce generating capacity. Lastly, the contractor would be required to work within strict safety and drug testing regulations adopted by Lockhart Corporate.
Kleinscmidt’s professional recommendation to Lockhart was to contact Knowles Industrial for a proposal to complete the repairs. KISC had repaired several similar failures in the past, engineered by Kleinschmidt, utilizing the structural shotcrete process; and ours was the only concrete repair method that fit all of the criteria. On the next day Thursday February 28, Knowles was contacted by Lockhart and discussions started concerning our assessment of the situation and how we could be of assistance in helping them resolve this problem quickly.By the following Tuesday, Knowles had submitted preliminary estimates, engineering sketches and a proposed schedule. A contract agreement was reached the next day with an agreement that Knowles would complete final engineering calculations, stamped drawings and a crew with equipment mobilized to the site by Monday March 10, to begin work and complete the project in 12 days by March 21. Engineering, planning and material procurement was completed by Friday, and the crew arrived in Pacolet, South Carolina to begin work on Monday March 10th as agreed, to work 12 hour shifts 7 days a week until complete.
Project Scope and Specifications
The penstock was a 10 ft. dia. X 117 ft long conduit of riveted steel construction. Built in 1922, original shell thickness was approximately 7/16 “ and was worn to about 1/8” on the bottom from corrosion and abrasion. The first 100 feet ran underground entering the powerhouse basement level where the last 17 feet was exposed all around and simply supported by two walls before entering the pressure case. Normal operating pressure was calculated to be 38 psi with a rejection and design pressure of 45 psi. The breach occurred in this 17 foot basement section.
For construction safety and to allow workers to cross the failed area, a system of cables and pressure treated trestle was designed and constructed. This was ultimately left in place, however it was not necessary for structural purposes as the calculations for the new structural liner neglected any remaining integrity in the existing steel shell.
Over 1000 ½ by 2 inch Nelson brand studs were installed by Knowles welders with modern automated stud guns on a predetermined pattern. The longitudinal reinforcing is tied behind the heads of the studs and pre-bent hoops tied to the longitudinals. Reinforcing design and alignment is critical to the strength of the new concrete pipe. The existing shell is simply used as a form at this juncture. Rebar is grade 60.2x2x12 gauge galv. Welded wire fabric is used for overhead shotcrete placement.
Knowles specified a pre-bagged blend of dry-mix shotcrete material manufactured by Quicrete of Charlotte, NC for the project. Design strength is 8000 psi F’c blended with 7% dry compacted silica fume for enhanced strength, durability and rapid early strengths. 70 cubic yards would be required for the liner fed to the placement equipment with a bulk hopper for high production.
Placement was made using a high production Allentown Meyco Picola rotary type gun capable of 4 cubic yards per hour.Design thickness was 4 inches nominal with a hard steel troweled finish to provide a Manning number of .012 to achieve 33% less friction loss than the existing .018 Manning of riveted steel.
The floor of the pipe is always placed first to facilitate removal of rebound.Sidewalls and placed next for the entire length. Streaking on the surface is curing compound. Test panels are prepared for eventual destructive and non-destructive testing.
This information will become a part of the permanent project records and confirm quality in accordance with ACI 506.2. All placement is performed by ACI certified nozzlemen who
undergo periodic retesting and recertification.Bright lights are used to achieve good visibility for proper finishing techniques.
Final shotcrete placement was made on Thursday March 20 one day ahead of the contract schedule. The owner resumed power generation the following Monday March 24, 2008. An immediate analysis and output index showed no loss in generating capacity. As has been shown in the past on other similar conduits, the cost of a reinforced shotcrete liner is typically 30% less than that of the next closest option which is a new slip lined steel liner with grouted annular space. What is more important from a cost standpoint is the down time required for the shotcrete option is less than half of the time required for slip lining or complete replacement which minimizes lost generation. Life expectancy of all available relining options available to owners is considered to be the same. Knowles can react quickly to an emergency relining requirement since there is not the 2 to 3 month lead time requirement for fabricated steel. Knowles can quickly prepare a detailed estimate based on your specific requirements.