Marshfield Penstock Evaluation and Repair

Project Name: Marshfield Penstock Project

Summary: Evaluation, inspection, modeling, and design services

Client: Green Mountain Power Corporation

Project Location: Montpelier, VT

Project Size: 5 MW

Services Provided:
•  Penstock Evaluation and Inspection
•  Penstock Preliminary and Final
•  Opinion of Cost
•  Bid Specifications and Drawings
•  Construction Services

Project Timeframe:
2006 - 2011

Relationship with Client: Since 1980’s

Client Needs:
Green Mountain Power’s (GMP) 8,300 foot Marshfield penstock was constructed in 1927 and consists of three primary sections: 400 feet of buried pre-cast concrete under the earthen dam, 6,800 feet of above-ground wood stave pipe, and 1,400 feet of buried, riveted steel which descends to the powerhouse. Over the years, GMP compiled concerns about the penstock’s structural integrity.
Kleinschmidt Solution:
Kleinschmidt conducted a penstock evaluation including a close-up field inspection, computerized hydraulic modeling of the transient hydraulic pressures, and a penstock stress analysis based on the field measurements. The evaluation showed that the penstock was structurally sound, but multiple severe leaks were occurring along the length of the wood stave penstock, at the surge tank, and the joints of the pre-cast concrete penstock. Based on the analysis, GMP decided to replace the wood stave penstock upstream of the surge tank and the concrete penstock up to the toe of the earthen dam with a new steel penstock in a programed manner over a five year period. Kleinschmidt provided feasibility studies, preliminary design, opinions of cost, final design and construction assistance for the sequential multi-year replacement of 6,850 linear feet of the wood stave penstock with new steel pipe. Repair also included application of watertight membrane liner to internal section of concrete pipe left in place under the earthen dam.
Client Benefits:
Kleinschmidt’s design resulted in most of the replacement pipe being buried and utilized ring girder and simple support spans for several above ground sections to cross gullies without the use of costly support structures.