PROJECT: PROSPECT PENSTOCK ROCKFALL MITIGATION, Prospect, Oregon


Powerhouse and Rockfall Protection Measures

PacifiCorp Energy's Prospect 2 Penstock is comprised of two, side-by-side, 6.5-foot O.D. pipes.  The penstock is roughly 850 feet long with an elevation drop of 484 feet.  The facility was constructed in 1927.  Early photographs of the site prior to penstock construction show a completely cleared slope with no vegetation. The slope is steep (nearly vertical) near the top and flattens as it nears the powerhouse.  Overall, the slope is bowl-shaped with the penstock located in the center.  The drawback to this arrangement is that rockfall from the surrounding slopes is funneled towards the penstock powerhouse below.

The initial upslope inspection revealed the presence of a failing 125-foot long, tied back, timber crib wall aligned roughly perpendicular to the penstock.  The wall was in such a secluded location that its existence had passed from the knowledge base of current employees.  The wall was extremely deteriorated and there was a reasonable concern that it could completely fail releasing a large quantity of rock backfill. If this occured, it would likely damage to the penstock and possibly the powerhouse.

Following the reconnaissance, LT recommended removal of the actively failing timber crib wall. To do this without damage to the facility required construction of two Brugg rockfall fences and suspending two draped mesh curtains followed by slope scaling and careful demolition of the timber crib wall. This approach allowed the timber wall to be removed and prevented any damage to the powerhouse, transformers, or penstocks occurring due to rockfall.