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A Lake Tahoe bluff-restoration story

June 1st, 2011 / By: / Erosion Control, Erosion Control Materials, Feature

From eroded to elegant

Introduction

This project took place on a scenic high-banked lakefront property in Rubicon, Calif., with a 60-ft-high unstable and badly eroded shoreline bluff facing (Photo 1, below).Photo 1 The story starts with a badly eroded Lake Tahoe shoreline bluff.

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).Photo 2  Issue #1—serious bluffside erosion.Remediating the bluffside deterioration 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. The property is a gently sloping lot that abruptly drops 60ft down to a sandy beach on the lakeshore.

The entire project involved removing an existing residence and constructing a new single-family home, tennis court, and guesthouse.  This required placement of the main house just behind the bluff’s edge and into a zone threatened by the unstable bluff face.

Bluffside restoration

Bluff stabilization was an obvious and immediate objective (Photo 3).Photo 3  Bluff stabilization was an obvious and immediate objective.

Initial concerns and discussions about the stability of the bluff face were prompted by the owner’s desire to both protect the new residence and to preserve that landscape with its magnificent view. There was also the requirement to create a safe and stable walking path down to the private 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 structural considerations required to accommodate the new walking path and main house, the owners envisioned a masterfully restored facade to recapture the much-admired native High Sierra ecology. Their vision paralleled the strict scenic assessment standards set by the Tahoe Regional Planning Agency (TRPA) and aided in smoothly carrying 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 environment was not an ideal restoration combination.

After considerable thought and discussion with erosion control vendors and consultants, 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).Photo 4 About 500 soil nails were launched into the bluffside
  • more than 10,000ft2 of turf-reinforcement mat and coated mesh materials (Photo 5).Photo 5 An area of more than 10,000sf was covered with mesh-reinforcement materials.
  • more than 10,000ft2 of custom-vegetated coconut coir confinement cells (Photo 6).Photo 6 A coconut coir cellular containment system was installed over the mesh to accommodate the soil infill and custom vegetation.
  • 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  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.Photo 7 Indigenous boulders and native plantings covered the rejuvenated bluffside

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).Photo 8  The degradable confinement system cells were hand-packed with a blend of custom soils and native vegetation.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.

Lateral low-volume drip lines (Photos 8 & 9) were placed throughout the facing and were required only temporarily to help establish the native plant species.Photo 9 Drip lines were needed initially to establish the plantings; the new bluffside path leads to the beach.

Project highlights
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
Galfan®-coated SuperMesh™
CellScape™ coconut coir cellular confinement (“CustomCoir™”)

Timeline: 10 weeks

Cost: $120/sf

James Chinchiolo is the CEO of Landslide Solutions Inc., based in Elk Grove, Calif.

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