Land Assessment/Verti-Block Wall Project
The Tyson residence is perched along the side of a low South-facing ridge above Asheville, N.C.
The home was built in the early 1980’s before stringent steep slope ordinances. During the extremely rainy 2013 we received numerous calls regarding erosion and site failure. The Tyson’s was one of the worst. The house had been correctly constructed on virgin subsoil of reasonable quality, however, the upper drive and parking area had been built of fill materials and retained with gabion baskets. Gabions are large rectangular wire mesh baskets filled with ballast or larger stone. They are often used for bank stabilization but were poor choice for the application. Following a massive rain event a 100’ section of the structure collapsed and slid into a large pile 30-50’ down the steep grade. To make matters worse the house is at the end of a narrow paved drive that wraps the face of the ridge with no opportunity for staging along the way. It was clearly going to be a technical and challenging project.
Education with Regard to Verti-block Wall Project
The education process began immediately with multiple site visits. All senior V&V operators visited the site for an initial assessment. Possible solutions were presented to Mr. Tyson regarding both new wall construction and staging. A former military test pilot and engineer, he was a quick study, speeding the process along with thoughtful questions and a steady flow of information. It was immediately apparent that an MSE (Mechanically Stabilized Earth) wall of substantial proportions would be required. A wall comprised of Verti-block, an interlocking concrete block system, and layers of geo-textile fabric was chosen as the solution. The materials, though more expensive than smaller proprietary block systems, provides better structural retention and can be assembled faster by a knowledgeable and experienced crew. These ideas, as well as a request for boring samples and analysis, were presented to local engineer Will Gentry so accurate design and planning could begin.
Planning a Verti-Block Wall
Several stages of planning and permitting were required before the Tyson project could be initiated. It was decided that a DEM (Digital Elevation Model) provided by a surveyor and a fully developed site plan would be needed. On less technical sites engineer-approved wall specifications can sometimes be presented for permitting without a customized site plan. This was not such an occasion.
Within three weeks surveying and geo-technical borings were completed and wall design was well under way. V&V office staff completed all necessary documentation prior to design submission so the permitting process could proceed more rapidly. Once the wall design was finished it was time for a few site specific modifications. Meanwhile, the staging dilemma was solved. We would excavate around the hillside and construct the wall on “the way out” on a series of stair-stepped footings designed to reduce material and excavation costs – stair-stepping footings requires greater excavation skill but reduces overall wall depth, and thus cost. In addition, the wall was to be constructed approximately six feet beyond the perimeter of the old wall allowing for vastly improved access, parking and drainage. With all parties on board with the new wall design it was time to begin site clearing, demolition and prep work.
Consolidation of Tasks
The consolidation component was crucial to the success of the Tyson project. Several decisions contributed mightily to this, the first of which was staging. Keeping the staging points within the immediate impact area of the wall failure reduced costs by minimizing site disturbance and clean-up. In addition material deliveries were choreographed to allow for necessary access and egress (Mr. Tyson’s wife was unwell and required around the clock nursing during the entirety of the project). Also important was the decision to use brush braids comprised of material removed from the impact area in place of silt fencing. We were thereby able to move or augment our erosion control as site conditions demanded. Finally, we were allowed to steepen the profile of the excavated cut behind the wall after proving soil conditions were stable and safe. This generated savings by reducing the volume of gravel required to backfill behind the wall.
In spite of inclement spring weather the wall was completed and site, dressed, matted and re-vegetated in two months. The new wall, though an unintended purchase, adds considerable structural integrity, value and better access to the Tyson residence. We were very proud of our dedicated wall crew on this one, with no small amount of thanks going to Jim Tyson who was an ideal client in every regard.