Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
Mitigating for Restrictive Layers
Tillage
If restrictive layers are within several feet of the soil surface, site treatments that break up
portions of this layer can increase permeability, which will increase stability. Site treatments
that accomplish this are bucket tillage, deep ripping, spading, and fill cut (benching and
backfilling), which are discussed in Section 10.1.2, Tillage. The drawback to tillage is that it will
reduce soil strength in the short term until roots occupy the soil and increase soil strength.
Temporary irrigation systems have been installed on sites where quick establishment of grass
cover during summer and fall is essential for slope stabilization prior to winter precipitation
(Hogan 2007). Tillage techniques that leave a deep uneven subsurface profile reduce the
potential for downslope debris slides.
Organic Amendments
Where possible, the incorporation
of organic matter will help keep the
restrictive layer from returning to its
original soil density and allow more
time for roots to establish (see Section
10.1.5, Organic Matter Amendments).
As described above, soil strength is
reduced until vegetation becomes
established, but the negative effects
of reduced soil strength are offset
by the increased permeability and
reduced soil weight of the organic
amendments.
Key-In Surface Material Installations
Installation of straw wattles, staked in place on contour, prior to installation of topsoil or
compost can hold permeable surface layers in place long enough for plant root systems to
knit the layers together. Fully biodegradable wattles should be used.
Live Pole Drains
Live pole drains are constructed to intercept water from seepage areas and remove it through
a system of interconnecting willow bundles to more stable areas, such as draws, ditches, or
other waterways (Figure 5-61). The interception and flow of water encourages the establishment
and growth of willow cuttings along the length of the live pole drain.
5.7.3 WATER INPUT
Water, which is the driving force behind most landslides, comes through rainfall, groundwater
flow, snow melt, or diverted from other areas through road drainage. Landslides often occur
after a series of strong winter storms have delivered a high amount of rainfall over a short
period of time. Under these conditions, soils have not had sufficient time to drain before the
next storms arrive and water in the soil builds up to a very high pressure (Figure 5-58). With
additional storms the combination of increased pore water pressure and additional weight of
water in the slopes eventually leads to slope failure. Landslides can occur where water from
road ditches or other road features drain water onto marginally unstable slopes. Major site
factors that affect water input are rainfall duration and intensity, rain-on-snow events, and
road drainage.
A
B
Figure 5-61 | Live pole drains
Slumps are characterized by scarps,
cracks, and benches. Water collects
on benches and in cracks where it is
transmitted into the slide mass, creating
continued instability. Slopes can be
stabilized by removing water through
a series of hand-dug surface ditches.
Using the cracks as guides for the
location of ditches, they are filled in
with soil and dug wide enough for a
willow fascine (see Section 10.3.3.4, Live
Fascines) to be placed in the bottom (A).
Called “live pole drains” (Polster 1997),
these structures not only quickly move
surface water from the slide to more
stable areas, but the willow cuttings
in the fascines, encouraged by the
presence of high soil moisture, grow
into dense vegetation (B) that stabilize
the slide through the deep rooting and
dewatering.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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