Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
water fills the soil pores but does not drain quickly, adding additional weight to the slope and
increasing the pore pressure. Both factors reduce the overall strength of slopes and increase
the likelihood that slope movement will occur when other conditions are right.
Soils with large interconnecting pores have a higher permeability than soils with smaller
pores that are less interconnecting. Soil textures that are well-graded (soils that have only
one particle size) typically have a higher permeability than poorly graded soils (soils having
a range of particle sizes from clays to small gravels). For example, poorly graded granitic soils
have low permeability rates because the different-sized particles are neatly packed together,
restricting the pathways for water flow. Well-graded soils, such as pure sands and gravels, have
high permeability because pores are large and interconnecting. Alternatively, compacted
soils often have low permeability rates because of the reduced or destroyed interconnecting
macropores.
Permeability – How to Assess
Simple field tests, such as percolation tests, historically utilized in assessing septic leach fields,
can be used for determining permeability rates. A small hole is excavated and water is poured
to a specified depth. The time to drain the water from the pit is measured in inches per hour.
These tests are run at different soil depths to determine if permeability rates change. Results
from percolation tests are subject to significant variability. Nevertheless, they can indicate the
relative permeability of different soil types or different soil disturbances.
Engineering laboratory tests for determining permeability include the constant head
permeameter for coarse-grained soils. In this test, a soil sample is placed in a cylinder at the
same density as the soils in the field. Water is introduced and allowed to saturate the sample. A
constant water elevation, or head, is maintained as the water flows through the soil. The volume
of water passing through the sample is collected and this provides a direct measurement of
the flow rate per unit of time. The test can be repeated at various densities to determine the
corresponding permeability. The test can also be repeated for various additions of organic
amendments and compaction levels to determine the effects of these treatments on permeability.
A soil lab should be consulted for how to collect and submit samples for these tests.
Where testing is not feasible, engineering and soil texts can give ranges for expected permeability
based upon the soil gradation and classification. A field assessment can also give some
indication of permeability rates. Subsoils or soils lacking organic matter that have a range of
soil particle sizes, from clays through small gravels, have a propensity for low permeability
rates, especially when they are compacted (see Section 5.3.3.1, Soil Structure – How to Assess).
Mitigating for Low Permeability
Tillage
Loosening compacted soil through tillage practices (see Section 10.1.2, Tillage) increases
permeability by creating large fractures or pathways for water to flow. However, tilled soils
often return to near-original permeability as the soils settle over time.
Organic Amendments
Long-fibered organic matter, such as shredded wood, tilled deeply into the soil will increase
the infiltration and permeability of the soil because larger, interconnecting pore spaces are
created (see Section 10.1.5, Organic Matter Amendments). Several studies evaluating the
incorporation of unscreened yard waste suggests that an optimum rate of organic matter
additions for increasing infiltration and improving soil structure is approximately 25 percent
compost to soil volume (Claassen 2006). In addition, incorporating organic matter can increase
slope stability because amended soils are lighter in weight than non-amended soils (mineral
soils can weigh 10 to 20 times more than soils amended with 25 percent organic matter). The
reduced soil weight lowers the driving forces that create unstable slopes.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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