From eroded to elegant
By James Chinchiolo
The site was originally owned by Scottish immigrant and journalist B.C. Forbes (founder of Forbes magazine), and it is now owned by a London family that splits its time between the U.S. and U.K.
Having found and bought such a dream property, the owners first had to deal with the issue of serious bluff erosion (Photo 2).Remediating the bluffside deteriorationÂ would become the largest and most complex shoreline stabilization and restoration project ever attempted in Lake Tahoe basin history.
The project site is on the western shore of Lake Tahoe on 4.8 acres with a vast 180-degree panorama overlooking Rubicon Bay.Â The propertyÂ is a gently sloping lot that abruptly drops 60ft down to a sandy beachÂ on the lakeshore.
The entire project involved removing an existing residence and constructing a new single-family home, tennis court, and guesthouse.Â This required placement of the main house just behindÂ the bluff’s edgeÂ and into a zone threatened by the unstable bluff face.
Initial concerns and discussions about the stability of the bluff face were prompted by the owner’s desire to both protect the new residence andÂ toÂ preserve that landscape with its magnificent view.Â There was also the requirement to create a safe and stable walking path down to theÂ privateÂ beach.
Surface erosion and sloughing of the bluff face at first look appeared widespread; furthermore, erosion of the bluff face was likely accelerated by the waves of Lake Tahoe undercutting the toe of the bluff during periods of high water.
In addition to structuralÂ considerations required to accommodate the new walking path and main house, the owners envisioned a masterfully restored facade toÂ recapture the much-admired native High Sierra ecology. Their vision paralleled the strict scenic assessment standards set by the Tahoe Regional Planning Agency (TRPA) andÂ aided in smoothly carryingÂ the stabilization project through a rigorous agency review process.
Traditional methods of mechanical slope stabilization such as benching, shotcreting, or retaining wall construction would have been effective at stabilizing the bluff face. Those approaches, however, would have substantially altered the appearance of the shoreline bluff facing and would have been rejected by the TRPA.
Project obstacles and solutions
The sandy silt bluff material coupled with the semi-arid Lake Tahoe environmentÂ was not an ideal restoration combination.
After considerable thought and discussion withÂ erosion control vendors and consultants,Â it was decided that a hybrid biotechnical solution was required—a smorgasbord of stabilization ingredients:
- about 500 soil nails launched into the bluffside (Photo 4).
- more than 10,000ft2 of turf-reinforcement mat and coated mesh materials (Photo 5).
- more than 10,000ft2 of custom-vegetated coconut coir confinement cells (Photo 6).
- more than 400 tons of 2ft-6ft–diameter native granite boulders.
The soil nails, installed at up to 200mph with a declassified British air cannon, provided slope stability and anchorage-plate points for the ensuing mesh layer.
The biotechnical surface treatment also included native granite boulders and thousands of Â plants. Boulders were placed within closely spaced arrays of the soil-nail surface plates at various heights to complement the boulders at the toe of the slope and bordering the pathway (Photo 7), which created a natural appearance.
More than 10,000ft2 of coconut coir cellular confinement was installed and hand-packed with a custom soil blend, then vegetated with plants and seed (Photo 8).A broad range of plant materials from grass and forbs to shrubs and trees, all native to the regional Tahoe Basin watershed, were selected to achieve maximum root anchorage and foliage cover.
Lake Tahoe shoreline stabilization and restoration
Location: Rubicon, Calif., west shore of Lake Tahoe
Design/build primary contractor: Landslide Solutions Inc.
Project engineer: John S. Black Consulting
Geotechnical consultants: Holdrege & Kull
Landscape design and installation: Native Plant Farms & Tree Movers
Civil and rock placement contractor: Manchester Enterprises Inc.
Permit specialist and project consultant: Jan Brisco
Architect: Faulkner Architects
Bluff stabilization: Soil Nail Launcher Inc.™
Surface treatments: MacMat® HDPE open-web mesh TRM
CellScape™ coconut coir cellular confinement (“CustomCoir™”)
Timeline: 10 weeks