This Biotechnical Soil / Slope Stabilization used to protect top of slope recession on the Bluffs above Redding’s Sacramento River.
This VMSE detail, along with widely referenced design and construction guidance, comes from BioDraw3.0 and ESenSS Manuals with CAD-ready digital files. See Store
Fig 1 – The bluffs are about 200’ high. We decided to start the structure at a stable platform approximately 25’ vertical below top of bank.
Fig. 2 The “toe” of the structure utilized Coir Logs anchored with Gripple anchors and wrapped in Biaxial Geogrid
Site goals, opportunities and constraints were:
1. There was no equipment access between the condos, wheelbarrow only.
2. The desired outcome was to return the slope to restore some of the contours. That would require an average of roughly 3-4ft of fill horizontally.
3. A soldier pile wall or adding compacted layers of engineered fill, 0.5:1 slope over 20-ft high was not feasible.
4. A lighter fill comprised of 50% sandy/gravelly loam mixed with 50% composted OM, reinforced with geogrid and roots seemed possible given the wheelbarrow transport and hand shovel restrictions.
5. A rule of thumb for bioengineering on steep adverse soils is to compensate for lack of compaction (engineered soils) with lots of reinforcement (geogrids and geotextiles initially and variable roots long term) and use relatively free draining soils with added cohesion (in the form of fungi, humus, and organic matter).
6. Compost-filled socks were selected for both reinforced fill and an excellent growth medium. We relied on compost socks for over half of the needed fill, and was also used as the new slope facing. The compost was relatively light so we weren’t putting an inordinate amount of weight on the steep slope and the “encapsulated” compost socks allows us many opportunities to “anchor” the structural elements to each other and to the slope.
7. The Filtrexx Siltsoxx and Growsoxx proved to be perfect product components for this project. Growsoxx are produced to grow vegetation really well. The compost filling them is certified compliant with the US Compost Council and there are many successfully documented projects using the Growsoxx to establish “living walls”. The Siltsoxx, on the other hand, are particularly designed to filter sediment-laden water. Because they are filled with a larger gradation of composted chips and stems, averaging 5/8” diameter, they allow free drainage but and are thought to be less effective as a growing medium.
8. I used some specific rationale in selecting both Siltsoxx and Growsoxx for this job. The material enveloping the Siltsoxx (green geotextile) appears stronger and longer lasting. If we use the Siltsoxx along the horizontal areas “lifts” where we plant / install willow branches (brushlayering) then having a super growth medium on the slope surface is not that important. Remember the willows will root from deeper into the slope. And the bigger wood chip gradations will last longer.
9. I further theorized that the Growsoxx would be better used behind the Siltsoxx, deeper in the slope, when brushlayering is used. That is where the willow roots will need good growth medium. Conversely, the Growsoxx will be more effective on the slope outer face when we are relying on native grasses and grass plugs to take over.
Here are some photos depicting fire rehab for a landowner protecting their property near Keswick this week. See the last blogs, but the purpose was to establish CA native grasses to kick start natural succession (healing) the way nature intended! To get CA native grasses instead of star thistle, noxious annual grasses and ultimately the fire-prone Knobcone Pine (climax species ??) we need to add the symbiotic mychorrizae fungi, a slow release biotic fertilizer (BioSol and Sustane are examples), AND the proper selections of native grass species.
Even two of my favorite species, Elymus glaucus and Nassella pulchra, that grow in the same area but one (NP) likes hot dry, south-facing slopes and EG proliferates on slopes with a little shade occasionally.
For this property (fortunately the house was spared) I developed two different mixes, one for south-facing and one for north-facing slope.
Measure, weigh, and calibrate – especially if you seed by hand. I like seeding by hand because I have more control. See the prior videos.
Straw Mulching can be difficult, the bales weigh 60-80lbs
we also had a mini excavator on hand, helping “toss” some bales down the slope!
Drainage drainage drainage. Now is the time to treat old skid roads or ? that can turn into gullies. After a fire, these historically benign drainage features can become big problems.
I used a small excavator to “remove” and re-contour, thus correcting the drainage anomaly.
The Straw Blower, FINN
We learned from previous blogs that seeding with CA Native Grasses will be beneficial for erosion control and long-term soil stabilization (the roots are extremely fiberous and long), and native grasses will help ” set the stage” for a more natural succession of native species and vegetation that will become less “fire prone” in the future.
The information here will describe some seed mixes and seeding rates, specific for the areas in western Shasta County.
Here are some seed mixes developed by myself and Pacific Coast Seed, Livermore, CA – my “go to” seed specialists and materials provided.
Here is the seed mixes developed by CalTrans for the 299W Highway corridor
You will notice that the total rates are different. My seed recommendation is about 30#/ac and the Caltrans rates are about 50#/ac. The reason is two-fold. My rates are going to be applied by hand-broadcasting, similar to the video. The CT rates will be applied with a hydroseeding/hydromulching machine. A general rule is that adding/applying seed with a hydromulcher requires more seed – just a general rule, many other factors come into play, e.g. the ability to protect the seed from bird predation by mulch and if the seed can be applied with close contact with the soils (sometimes Hydromulch is applied in two stages, the first with seed and little hydromulch to ensure good soil contact followed by the rest of the specified mulch for Erosion Control cover).
I will present more on Hydroseeding and Hydromulching (really one in the same thing) in the near future.
But note that once the seed in down it really must be covered with MULCH. The seed mix above that I am using for hand application will be followed by Straw Mulching.
The straw mulch I have chosen is rice straw (see previous Straw Talk video) and we will apply is at about 1.5 T /ac which relates to about 1 bale per 1000sf.Straw bales generally weigh between 70-90#/bale. Therefore a burned area of 6000sf would require approximately 6 bales. The straw shall be spread uniformly over the ‘disturbed soil areas’ such that 70%-90% of the soil is covered with an average of 1″-2″ thick straw. For small area rehabilitation on up to 2:1 steep slopes I seldom apply tackifier – the first rains will “lay the straw down” and prevent it from blowing away – general rule!