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Background

Hello, and welcome to our Spring 2013 Appropriate Technology project at Humboldt State University (HSU). Our team is comprised of Ivan Diankov and Alex Kato. We will be designing and constructing an erosion control system on the slope behind the Yurt house at HSU's Campus Center For Appropriate Technology (CCAT). Erosion control and slope management come in many forms. From retaining walls to bio-engineered erosion control (BEC) techniques, there are various methods to maintain and control a hillside. The use of BEC techniques specifically provides avenues to enrich the surrounding area with more plant life and thus more stability than would a traditional retaining wall.

Problem Statement

The Campus Center for Appropriate Technology (CCAT) at Humboldt State University is a student run facility for both education and hands-on experience. On site are many examples of sustainable and appropriate building and living. The Yurt house, located on the hill adjacent to the main house, is one example at CCAT promoting a sustainable living household. Several problems have arisen from the location of the Yurt House.


  1. Complete access around the house perimeter is severely restricted due to the small space between the hillside and the side of the house and the large amount of vegetation that grows within and about that small space.
  2. There is concern over possible damage to the Yurt from erosion as well as concern over how the hillside is being currently used. As of January 2013, the area behind the Yurt comprises of two large fern patches crisscrossed with footpaths.

Our project is to clear a small space from the perimeter of the Yurt and to stabilize the surrounding hillside to prevent further erosion. The goal is to reinforce the hillside using bio-engineered erosion control techniques involving live stakes and branch packing.

Literature Review

Exploring Different Bio-engineered Erosion Control (BEC) Methods:

Branch Packing

Branch packing is a technique that introduces roots into small sloped areas. The layering of branches traps sediment flow and the growth of a root network solidifies the earth into a connected mass. A layer of crisscrossed live branches (0.5in-2.0in diameter) covers the surface. Live stakes are placed at intervals (2ft-3ft) throughout the area. A cover layer of soil no greater than 12in is applied and compacted. [1]

Live Stakes

Live stakes are a natural and locally sourced tool for erosion control. Stakes 2-3ft in length and 1-2in in diameter are prepared and then tamped into the ground 1 ½ -3ft. Stakes stabilize a slope by absorbing water from the soil and by providing addition root structure. By choosing a shrub or tree species that roots quickly, a slope can benefit from live staking within a season. For excessively wet environments, live staking alone is not sufficient for slope reinforcement. [2]

Live Fascines

Live fascines are bundles of live branches that are places at the bottom of a slope or at the line of a river bank. The perpendicular growth of the roots provides structural support at the base of a slope and can act as a sediment catchment system. Fascine bundles can also be constructed with dead branches to help control sediment and water flow. Fascine bundles are prepared as 6-8in diameter bundles that are fastened with twine. They are installed in shallow trenches that follow the contour of the slope. In our original ideas for this project, we envisioned using fascine bundles to line the bottom of the slope. Our final project did not incorporate them into the design. This was due to lack of materials and necessity. [1]

Other Retaining Systems

Log Retaining Wall

Logs are a popular material to use for retaining walls. Their intrinsic strength and durability make them appropriate resources for retaining up to large amounts of earth. Their natural aesthetic can add visual appeal to the landscape. An existing vertical log retaining wall at CCAT served as inspiration for our wall.[3]

Project Evaluation Criteria

The following Criteria will be used to assess the success of this project. These criteria were chosen based on the suggestions of the project coordinator as well as the diligent students who are working on the retaining wall. The scale (1-10) represents the importance level of meeting the constraint of each listed criteria.

Criteria Constraints Weight
(1-10)
Safety & Placement Over engineered for strength and durability
10
Eco Groovy Efficient use of recyclables and waste materials
10
Budget Must not exceed our budget of $300.00
9
Aesthetics Must be pleasing to the eye and look professional
8
Educational Aspect Must include an educational piece for community (something to explain or highlight the construction of the retaining wall)
7

Tentative Time Line

A proposed work schedule detailing the progress of this project. An unexpected change to the design has modified and extended our timeline.

Project Started Completion
Meet with CCAT to Finalize Design February 25 March 2
Sourcing of Materials February 23 March 2
Building Materials (as approved by CCAT) Tentative Tentative
Relocation of Vegetation March 2 March 12
Beginning Excavation March 12 March 23
Soil Erosion March 23 April 23
Testing and Error Correction Apr 23 May 4
Design Change by Client Apr 19 Apr 21
New Excavation Apr 19 Apr 26
Collection & Assembly of Logs Apr 26 Apr 29
Soil Erosion Control (live stakes) Apr 29 Apr 30
Tamping and Back-fill Apr 30 May 1
Chainsaw Apr 1 May 3
Finalize Project May 1 May 3

Basics

A well –built retaining wall should be firm, solid, and durable. A poorly built wall will lean, separate, and topple. A retaining wall need only retain a wedge of soil that lies in front of the plane of failure – the maximum slope beyond which the soil won’t stay put on its own. The undisturbed soil behind the failure plane has been naturally compacted for thousands of years, and if undisturbed, will stay where it is by itself for thousands more.

A well-built retaining wall will take into consideration three things:

  1. The bottom layer of the retaining wall should be buried one tenth of the height of the wall. This will prevent the soil behind it from pushing the bottom of the retaining wall out.
  2. The building material should be layered and stepped back with each layer. This enables the building material to use gravity to push back on the backfill. A perfectly vertical retaining wall will often topple as soon as it starts to lean.
  3. The base of the retaining wall should be highly compacted to ensure foundation stability.

Concerns

The major concerns with building our retaining wall have to do with the natural elements of Humboldt County; rain and seismic activity.

  1. Because damp soil weighs more than dry soil, proper drainage is extremely important for our success.
  2. Although our wall is small, it should be over engineered in case of earthquakes.


Designing interpretive materials

  1. Live Alder Stakes
  2. Branches (?)
  3. Logs (4' - 6' long)
  4. Flowers

Design:

Preparation

Fern Removal

Our site is covered in large fern patches that need to be removed in order to the stakes to be installed. These fern patches also contribute to the restricted access around the Yurt so their removal is necessary to the maintenance of the house. We plan to use shovels and pickaxes to remove the ferns. Once removed, we plan to relocate them beyond the CCAT property line. HSU Plant Operations expressed their desire to make the foot trails behind CCAT more discreet and we plan to replant some of the ferns along the trails to help mask them.

Slope Regrading and Shaping

After the site has been cleared of plant material, the slope will need to be regraded. One of the objectives of the project is to ease concern over soil erosion therefore lessening the slope steepness will contribute to that goal. We plan to remove soil from the slope starting from the back of the site and working toward the Yurt and relocate it off site. In addition, the slope will be pushed back a short distance (1 ft) from the Yurt. We will do this by shoveling the earth away from the Yurt and relocating it off site.

Live Stake Preparation

The alder trees that line the CCAT property border will be perfect resources for our live stakes. There are multiple large branches that are, with the aid of an extendable tree pruner, within cutting distance. We plan to cut large branches from these trees, strip them of the outshooting branches, and cut them to size (2-3ft). The smaller branches that are stripped off will be saved to use for the branch packing. Once the stakes are cut to size, we will cut one end into a pointed tip using a table saw. Once the stake preparation is complete, the stakes will be preserved in a water filled bucket for at least 24 hours prior to use.

Installation

Installing the Live Stakes and Branch Packing

In order to implement the branches for the branch packing system, we will remove up to 1 ½ ft of soil from the slope. Once the soil is removed, we will install the live stakes. The stakes will be spaced in a 2 ½ ft square grid and tamped into the soil to a depth of 2ft. Once all of the stakes have been put into place we will lay the live branches into position. The live branches will be laid in a crisscross pattern across the site, in between the stakes. Once a layer of branches is in place will be fill the site back with the 1 ½ ft of soil we initially removed. This soil will then be compacted and wet. An additional layer of live branches may be laid down and covered, but this depends on the amount of material we can gather. If any extra live branches are left over, it is an idea to make live fascine bundles. These bundles will then be placed in a shallow trench that lines the bottom of the slope.

Construction

Fern Relocation

The first step in our project was to relocate the existing vegetation on our hillside. There were two prominent patches of ferns directly obstructing our work space, at least a dozen moderate sized ferns in total. Removing the ferns proved to be a lot harder than we thought. The roots were deep, well established, and stubborn, as far as fern roots go. Relocating the ferns took a lot longer than we anticipated, and required additional hands. CCAT was generous enough to provide us with a few extra hands on volunteer Friday. Even with extra workers, this first phase of construction took several weeks. A lot of care was taken to preserve the ferns, and reduce any stress that they would receive. Half of the ferns were used to shape the footpath at the top of our soil erosion grid. The other half were planted in the surrounding areas. We thought it would be super eco groovy to incorporate some of the ferns into our brush layering technique, as a part of our soil erosion control. Two whole ferns were saved for later use during the brush layering in order to keep the branches alive as long as possible. These two ferns were stripped of their branches, which were incorporated into our brush layer, and their root masses were replanted in the surrounding area. The hope is that these branches would root and provide additional stability for our system.

Live Alder Stakes

Live stakes are crucial for our soil erosion control system, but are also very expensive. A survey of the surrounding area revealed a dozen mature alder trees, with a score of decent sized branches crowning each tree. We decided it would be much cheaper to source live stakes from the surrounding alder trees, rather than purchasing them from a local vendor. We used some rope to secure an old 20 foot aluminum ladder to each tree.

Hillside Excavation

about the digging

Retaining Wall

about the logs

Soil Erosion Control

about the alder stakes


Finalized Product

Overall view of the finished project

chainsawing, tamping, backfill, flowers, watering

References

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  1. 1.0 1.1 Eubanks, C.. "Soil Bioengineering Techniques." A soil bioengineering guide for streambank and lakeshore stabilization. San Dimas, CA (444 E. Bointa Ave., San Dimas 91773): U.S. Dept. of Agriculture, Forest Service, Technology and Development Program, 2002. 75 - 131. Print.
  2. Gray, Donald H., and Robbin B. Sotir. "Brushlayering." Biotechnical and soil bioengineering slope stabilization: a practical guide for erosion control. New York: John Wiley & Sons, 1996. 231. Print.
  3. "Patios and Courtyards." The complete backyard book. New ed. Sydney, NSW: Murdoch, 2002. 99. Print.
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