Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
the long vertical roots (taproots) that can cross failure planes and bind the soil
strata together (Figure 5-62).
Soil Strength – How to Assess
The designer will probably not perform engineering tests for soil strength, yet
knowing a little about these tests could be important, especially if the designer
is making recommendations that soils on potentially unstable sites be deeply
tilled or amended with organic matter.
A common method engineers use to estimate soil strength is to correlate soil
classification (from sieve analysis and the characteristics of clay particles) with
published literature values. Shear vanes or cone penetrometers are good
methods to approximate the strength of fine-grained soils in the field, and
published research is used to correlate these readings with laboratory shear
strength test results.
The triaxial shear test is a more precise laboratory method to determine shear
strength of soils. In this test, a long cylinder of the soil is placed in a latex
membrane and submerged in a clear plastic cylinder filled with water. A vertical
pressure is applied to the cylinder at a slow rate until the soil sample shears. Very
sensitive strain gauges measure the soil displacement, applied forces, and any
pore water pressures that develop. Various water pressures are applied to the
cylinder to simulate the confining pressure of soil depth (Brunsden and Prior
1984). This test can be used to determine the shear strength of soils that have
been amended with organic materials.
Mitigating for Low Soil Strength
Biotechnical Engineering Techniques
Many biotechnical slope stabilization techniques use vegetative cuttings from willows or other
easy-to-root species to structurally reinforce the soil. As these materials root, they add further
stabilization to slopes through interconnecting root systems and soil moisture withdrawal.
These practices include stake planting, pole planting, joint planting, brush layers, and branch
packing (see Section 10.3.3, Installing Cuttings).
Shrub and Tree Seedlings
On drier sites, where willow cuttings are less likely to survive and grow, shrub and tree seedlings
can be used. While these species are slower growing, they usually have deeper root systems
and persist longer once they are established. Grass and forb species can quickly establish on
Figure 5-62 | Plant roots
and slope stability
Plant roots and stems increase slope
stability by (A) reinforcing the surface
horizon through a matrix of roots, (B)
anchoring surface horizons to rock or
subsoils, and (C) stems supporting the
soil upslope.
Figure 5-63 | Effects of roots
on slope stability
This debris slide (noted by its shallow, steep
appearance) took place two years after construction
when the grass and forbs were fully
established. Establishing shrub species, rather
than grass and forb species, on steep, potentially
unstable slopes, would be better for long
term stability because shrub species are deeper
rooting and have higher root tensile strength.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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