Six Rivers Charter High School (abbrev. Six Rivers) in Arcata, California has a garden and outdoor learning space located at the bottom of a hill below the main classrooms. The staircase originally connecting the space to the rest of the school had fallen deep into disrepair and sorely needed to be replaced this year (Fall 2019). The team spent a few weeks researching pathway and stair design, slope stabilization, retaining walls, pathway construction, erosion control, and native plants before meeting with Six Rivers' Principal Ron Perry to establish their personal criteria for the design. The team was tasked with designing a pathway that is safe, wheelbarrow accessible, widely plantable, cost effective, ecologically sound and aesthetically pleasing. The primary objective of our design was on functionality and durability.
PDF - Full report of SRCHS pathway project
Problem Statement and Criteria[edit | edit source]
The top priorities for any trail/pathway are safety and accessibility, and the importance of other criteria is situation specific. For this project labor intensity and cost are very important because without prioritizing these the project may not have been able to reach completion in time. Remaining criteria is valued with respect to education.
|Criteria||Weight (out of 10)|
|Safety and Accessibility||10|
Prototyping[edit | edit source]
The first prototype was constructed out of drywall compound, chicken wire, and magazines on a plywood frame. The clam-shell cement blocks are represented by the colorful foam blocks, the urbanite retaining wall is represented by the black foam blocks, and the compact earth tire wall is represented by the slices of wine cork. The first problem posed was the sharp inside angle of the switchback. The fact that this would present an issue was confirmed by the second prototype which involved carving into the construction site. As a result of the prototyping process, the team chose to extend the landing section of the trail farther back into the hillside to allow for a gentler inside angle on the switchback. The prototype was tested by showing it to Cal Poly Humboldt engineering students who gave their opinions of what could be improved on and what could be potentially problematic or unsafe.
Description of Final Design[edit | edit source]
Final Design[edit | edit source]
The final design shown below addresses the client criteria and insight gained through the prototyping process. The full length of the pathway is approximately 90' and the area of the landing is approximately 35 square feet. The bottom of the pathway to the landing is supported by a 66' urbanite retaining wall that expands upon a pre-existing concrete block retaining wall. The landing is supported by a compact earth tire retaining wall measuring approximately 54" high by 13' wide, and comprised of 24 tires arranged hexagonally. The portion of the pathway between the top and the landing does not require reinforcement. The pathway surface is composed of three layers: tamped earth as the base, a mixture of sand and gravel in the middle, and mulch on top. Outsloping of the pathway and Coweeta dips were used for drainage, and a combination of ripraps, straw, and grass seed were utilized as temporary erosion control for remaining bare slopes. All elements of the design were produced as planned to the satisfaction of the team as well as the client.
Costs[edit | edit source]
|Quantity||Material or Service||Source||Cost ($)|
|As needed||Materials Transportation||Father of Dustin||46.91|
|1||Hay Bale||Three G's Hay and Grain||8.62|
|1 bag||Grass Seed||Mad River Gardens||3.76|
|As needed||Recycled Concrete||Donated by: Figas Construction||0|
|As needed||Tires||Donated by: Anonymous||0|
|1 cubic meter||Wood Chips||Free pile located at the Indianola Cuttoff||0|
How to build[edit | edit source]
Compact Earth Tire Retaining Wall[edit | edit source]
Place the first row of tires neatly in a line and use a level to flatten them as evenly as possible. Place cardboard inside tire to cover the bottom hole. This prevents dirt from spilling out through the bottom as it is filled. The photo to the left demonstrates measuring the cardboard to be cut before it is inserted into the tire.
Pack the first row! It is most efficient to have one person shoveling scoops of earth into the tire while one person packs it using a tamper, sledge hammer, or similar tool. If the local soil is particularly dry it is best to lightly dampen the soil to help it bind to itself.
Note 1: Minimize plant/organic matter from mixing into the soil. Organic matter will decompose over time and this can cause the fill to shrink which could compromise the strength of the tire.
Note 2: Be sure to pack in the sides of the tire. As you fill the tire it will be come increasingly difficult to compact the sides so a handheld sledge is recommended for this.
Fill the second row by repeating steps 3 and 4.
Optional: Use your tires as planters by shallowly digging into them. The tires function well as planters and roots will add additional strength, but digging too deeply into them can compromise the strength of the soil that has been packed in. Placing above ground bottomless planters on the center hole of a tire is the recommended method.
Urbanite Retaining Wall[edit | edit source]
The urbanite retaining wall described below is constructed from recycled concrete. Pieces of concrete with one flat side such as those sourced from old sidewalk are ideal, but not critical. This type of retaining wall is known as a gravity wall because it does not have anything anchoring the wall to solid ground. The strength and stability of this design is rooted in it's sheer weight, so it is important to remember that the sum mass of urbanite used is proportionate to the overall strength of the wall.
Create a foundation layer by digging out all loose dirt, tamping firm the earth that your first layer of urbanite will be placed on, and level it.
Repeat steps 4 and 5 until the first layer is complete.
Note: It is highly recommended that each block be firmly stabilized before moving on to subsequent blocks, rather than completing the entire row and returning to check stability of individual blocks afterwards. Check each block's stability by tapping it with your foot. If it wiggles in place then it has not been set firmly enough and must either have additional side/backfilling, or increased lateral support by adjusting the placement of adjacent blocks. This is difficult to do once the entire row is complete because adjusting the placement of any one piece will alter the placement of the blocks touching the one being adjusted.
Trail Placement[edit | edit source]
Path Surface[edit | edit source]
Maintenance[edit | edit source]
This section details the known and anticipated maintenance that this design will require to remain functional and cosmetically intact. It is designed with the intent of being maintained by the students alone.
Schedule[edit | edit source]
- Plant care: watering and fertilizing as needed.
- Remove dirt knocked onto pathway by gophers, pathway users, etc.
- Analyze retaining walls for points of soil erosion that could affect the long term stability of the supporting structure.
- As necessary
- Add mulch or gravel to the top layer of the pathway at points where it has become muddy or uneven.
Suggestions for future changes[edit | edit source]
Team AZDC recommends the following as future changes to the project:
- A handrail placed on the west side of the path running from the landing to the bottom.
- Planting an assortment of edible fruiting plants that would also serve as additional slope stabilization such as:
- Snow Berries
- Goumi Berries
- Chilean Guava
- A cob bench embedded into the uphill side of the landing to provide additional slope stabilization as well as an aesthetic and practical feature.
- A roof for the cob bench so that it will survive the Humboldt county winter.
About the Team[edit | edit source]
Environmental Resource Engineering students at Cal Poly Humboldt, Fall 2019.