This project included a non-traditional biotechnical stabilization and restoration approach using launched soil nails, galvanized steel mesh, and native plants in an area prone to rotational slope failures along the shore of Lake Tahoe, California. Working within stringent Tahoe Regional Planning Agency (TRPA) guidelines, the project produced a restored shoreline face with a walking path through a newly stabilized high bank waterfront property. In addition to preventing further rotational failures, the approach mitigated widespread surface erosion and sloughing of the shoreline face.
Bluff erosion and stability is an ever present problem along the shoreline of pristine Lake Tahoe and one such area presented itself during the planning phase of an estate reconstruction project along the west shore of Lake Tahoe. The inherent problem of an eroding unstable bluff is quite obvious. In this situation the problem was intensified by the desire to build a high end housing structure just feet from the top edge of the bluff while planning to restore the bluff face and construct a walking path to the beach below. In addition, because of tough environmental regulations, most of the work would need to take place from the top of the bluff thus requiring specialized equipment and construction techniques.
These factors were then made even more difficult by stringent Tahoe Regional Planning Agency (TRPA) and local county permit requirements. Permit requirements limited construction from May 1st to October 15th and required a massive amount of BMP’s prior to any rains and / or before October 15th. The ultimate goal of the project was to protect, strengthen, support, preserve and provide global stability to the existing bluff surface all without noticeably changing the appearance of bluff.
If bluff surface erosion was not corrected at this site the bluff would have continued to erode back thus reducing usable property at the top of the bluff. If overall global stability of the slope was not corrected the housing structure would have had to be placed on a pilling system sufficient enough to support the house should the bluff continue to fail.
Several stakeholders, agencies and industry professionals all worked in harmony to develop the final bluff erosion and stability plan. Those entities included the owner, project architect, project civil engineer, permit specialist, geologist, landscape design / build contractor and specialty slope stabilization design / build contractor.
After considerable thought and consultation with erosion control vendors and consultants, it was decided that a hybrid biotechnical solution would be required – a smorgasboard of stabilization techniques. Ultimately, a combination of launched soil nails, steel SuperMesh™ and a specialty custom vegetated CellScape™ coir cellular confinement was chosen.
The launched nails were primarily chosen because of the ability to install them with minimal impact to the bluff, faster production rates relative to traditional nails and the ability install the nails from the top of the bluff (no need for equipment on the beach). Cost was also an important factor with regards to the final decision to utilize launched soil nails.
Launched soil nails are long steel or fiberglass rods installed to reinforce or strengthen the existing ground. Soil nails are inserted using high-pressure air approaching 2500 psi (17.2 MPa) by a launcher that can be mounted on a hydraulic excavator. As the soil nail passes into the soil, the ground around the nail is displaced by compression at the nail tip. This forms an annulus of compression which reduces the soil drag on the nail. As the nail comes to rest, the soil rebounds onto and bonds with the nail. The soil nails reinforce the locally unstable soil mass by transferring the nail’s tensile and shear resistance through the failure plane of the sliding soil. The nails maintain the resisting force because they are anchored beyond the slip plane.
In order to determine the length and spacing of the launched soil nails both a slope survey and a soils report were used. These documents were provided by the project engineering geologist (Jake Hudson of Holdredge & Kull) and information gleaned from them was imported into a commercially available slope stability software tool. The tool then calculated probable slide planes under varying circumstances at a defined Factor of Safety (FoS). An appropriate FoS at this site was determined to be 1.5, effectively meaning that the repair is capable of handling 50% more than the maximum expected load. Without any stabilization measures the slope was approximated at a FoS 1.0. With the installation of launched nails on 5-ft x 5-ft spacing it was determined the slope could be augmented to reach the targeted FoS 1.5.
Galvanized wire mesh was then attached to the tips of the nails with the purpose of providing cohesiveness to the launched soil nail system, a medium to attach the coir cellular confinement and a last defense for erosion in the event of localized coir failure. The mesh was carefully tensioned across the entire slope face. At the bottom of the slope native rocks were hand selected and placed to provide slope buttressing and to protect against high water wave action. The wire mesh was installed prior to the placement of the rocks thus the rocks provided another source of tension as they were set in place.
The success of the entire stabilization system depended in large part on the erosion control and beatification provided by the vegetated surface treatment. The installation of custom vegetated CellScape™ surface treatment was installed immediately after the placement of the steel mesh and native rock. Vegetation selected for planting within the CellScape™ included; large deeply rooted woody shrubs, forbs, sub-shrubs, ground-cover and grasses that were chosen based on esthetic appeal and ability to retain soil. In order to support these species amended soil was imported and placed within six inch deep honeycomb-configured coir cellular confinement (CellScape™). This biodegradable six inch deep blanket was securely anchored to the SuperMesh™ and stretched in six foot stripes from top to bottom of slope forming a continuous three dimensional form covering the slope.
Soil sources were tested and a biologically active green-waste composted soil formula was selected. The soil needed to hold and distribute moisture, rapidly germinate seed, grow healthy plants, texture and color to blend with native soil and adhesiveness. Soil was conveyed and packed into the coir layer. A diverse mix of native plants was specified, ranging in size from 4” native grass plugs to 44” ball and burlap native trees. Larger tree species, Incense Cedar and Jeffery Pine were placed lower on slope to avoid being blown over by the wind. Smaller scale, slower growing native trees such as Mountain hemlock (Tsuga mertensiana) and Subalpine fir (Abies larsiocarpa) and other native shrubs were placed higher on the steepest exposed areas of bluff. The native shrubs provide shade, bird habitat and forage for foul and small mammals. Boulder groupings pinned periodically in the slope with Launched Soil Nails™ also provided an opportunity for small mammal colonization and complimented the boulders throughout the rest of the property. Native ground covers and forbs like Squaw Carpet (Ceanothus prostratus) and Mountain Pride Penstemon (Penstemon newberryi) added color, attracting beneficial insects. Additionally, small burnet (Sangusorbia minor) provided nitrogen fixing.
Five gallon and larger plants were installed in small openings cut through the CellScape™ and SuperMesh™. Inserting the root mass of these larger plants into the subsurface soil allowed space for root growth and trunk flare. A final mulch layer formulated with a matrix of long, medium and fine natural fibers was distributed throughout the entire area. The mulch layer was composted with a small amount of formulated soil for one week to activate microbial activity, and then firmly felted on the surface. After mulching a seed mix of shrubs, forbs and native grass was broadcasted evenly over the mulch. A three foot wide path was constructed and surfaced with Vast™ recycled rubber composite pavers. The construction process was completed by September 15, 2010. By October seed had begun to germinate and plant materials already exhibited growth. The entire planted area is now irrigated and controlled by a sophisticated irrigation controller that automatically turns the entire irrigation system off if any irrigation line breach is detected. The system controls an in-line drip irrigation system providing low volume, low impact supplemental irrigation for the first three growing seasons.
Late October 2010 brought an unusually strong rain event (4-inches within 24 hours). The stabilized area weathered the storm without an issue. Project monitoring and maintenance will continue through October 2015.
From conception to completion this project took more than three years. Construction took two and one half months with more than two years in planning and permitting negotiations. The project has met the expectation of all entities involved and provides an attractive natural looking foot path to the beach, stabilized slope suitable for a building above and a restored view from land and water.
A variation of karst sinkhole grouting is the grouting of mined coal seams and workings located below existing infrastructure that exhibit the potential to propagate to the surface and cause sinkholes and/or subsidence.