Soil stabilization can play a crucial role in erosion control. The implementation of suitable soil stabilization techniques — and when to use which techniques — is necessary for many erosion control professionals.
But what variables can influence soil stabilization approaches, and what approaches are there to choose from?
For the purposes of this article, soil stabilization means the modification of the properties of a soil to better control erosion, thereby preventing sediment runoff and protecting water quality.
Variables behind soil stabilization
There are several key variables to consider when applying soil stabilization methods. The soil qualities, weather and duration of stabilization are some of the most important factors: each one will decide the effectiveness of a stabilization technique.
Soil properties and qualities
Perhaps the most important variable in soil stabilization is the soil itself.
Can the soil sustain vegetation? Does it infiltrate effectively? Is it prone to erosion? These concerns are all decided, in large part, by the qualities of the soil itself.
Sandy soil will contain little organic content to support vegetation, while silty soil can promote water retention and contain more nutrients. Beyond a visual inspection, testing and sampling the soil can be useful in determining its specific qualities.
The location’s to soil can be critical for growing vegetation — one of the most important methods of stabilization.
“That is really critical when you're looking at what method you want to use for soil stabilization, because you have to have decent topsoil to grow vegetation,” said Andrew Earles, vice president of Wright Water Engineers Inc. “If you have really poor quality soils and you choose a soil stabilization strategy that relies on having to have vegetation grow, you may be waiting a long time for soil stabilization if you don't have suitable quality topsoil to support the growth of that vegetation.”
One helpful guide for topsoil management is “Topsoil Management Guidance” by the Mile High Flood District in Colorado. This guide provides a wealth of information — from soil sampling in the field to developing a topsoil plan and more.
Weather and seasonality
The question of “when” is also significant for soil stabilization. Weather can heavily influence the success or failure of stabilization methods, as can the time of year.
“You need to kind of be mindful of how weather and seasonality relate to your stabilization, especially for vegetative stabilization practices,” said Earles.
Vegetative stabilization is much more likely to succeed when seeds are planted during the right times — and much less likely during the wrong times. In addition, periods of heavy rainfall can interfere with the success of vegetation seeding and of some short-term stabilization methods.
“You just need to plan around the right seeding windows,” said Earles. “And then when you're outside of those seeding windows that's where you might use other things like surface roughening or slope tracking or temporary cover practices like mulch or erosion control blankets.”
Duration of stabilization
Just as important as “when” is “for how long?”
A construction site that will be idle for two weeks needs different soil stabilization methods than a site that will be idle for six months. The best soil stabilization method will vary depending on the duration of stabilization required.
“Really what you want to try to do is to select the right approach for the amount of time that you think that area is going to be idle,” said Earles. “And if it's not going to be reworked again, then get something there that's going to be permanent.”
Erosion control blankets can provide quick and effective results for large-scale, short-term stabilization, while vegetation can provide a more reliable long-term and eco-friendly solution.
Project requirements
Choosing a soil stabilization method will be heavily influenced by the project’s unique needs.
Often, the main reason for soil stabilization is to comply with local regulations or permits. The project itself might also have unique requirements for sediment management and acceptable/unacceptable stabilization practices. It is important to be familiar with all the project’s unique factors in order to choose an approach that best fits these needs.
Approaches to soil stabilization
With This article will explore five approaches to soil stabilization, considering the unique strengths of each.
Vegetation
“When I think about soil stabilization, the first thing that comes to my mind is trying to create a surface that's going to be resistant to erosion over the long term,” said Earles. “And most often the way that we do that on construction sites, if we want to retain some of the permeability of the ground, is through the use of vegetation.”
Planting grasses, shrubs or trees can create a natural, versatile form of soil stabilization. The vegetation can reduce the impact of raindrops, while the roots will increase the sheer strength of the soil. Well-established vegetation can stabilize a site indefinitely, sometimes requiring only minimal maintenance.
“Like many proprietary project products, I tend not to think of those as an alternative to vegetation, but I really view them as something that you use in conjunction with vegetation,” said Earles. “And those proprietary projects really give your vegetation a chance to get the jumpstart that it needs.”
When considering vegetation, it is important to keep in mind the site’s topsoil, seeding windows, and acceptable flora.
“Some construction projects and some contractors are really good about scraping off topsoil and saving it,” said Earles. “That can really save you a lot of money in the long run because the topsoil that has grown and developed in that location over thousands of years is going be the best thing for growing vegetation there.”
Sites where the topsoil has been lost will have a harder time establishing vegetation. In that scenario, it might be preferrable to invest in soil amendments.
Weather and seasonality can affect vegetative stabilization more than any other approach. Coordinating planting according to the flora’s seeding windows is essential, as is ensuring that the plant life receives the appropriate amount of water.
Because the availability of water for vegetation can change so drastically between locations across the U.S., native vegetation can be a reliable choice for stabilization.
With Wright Water Engineers Inc. being based in Colorado, Earles often uses native vegetation, since it can do well in the state’s semi-arid environment.
“Not only because it has lower lifecycle water requirements and you don't necessarily have to install a permanent irrigation system with it, but the way that native grasses in Colorado and other types of native vegetation have adapted is they have really deep roots,” said Earles. “And that deeper rooting is something that provides for much greater stability.”
Surface roughening
This process roughens the surface of soil on slopes, in a horizontally grooved pattern, to decrease runoff velocity. Roughening can include tracking, stair-step grading, or ripping and grooving.
This approach also helps increase infiltration. The practice both reduces erosion and helps with the establishment of vegetation. However, it only works best as a short-term solution.
Mulching
Mulching is the application of organic material, like straw, over the exposed soil. It protects against the impact of raindrops, promotes seed germination, and minimizes surface runoff.
“Mulching is honestly one of the best ways to reduce erosion on a temporary basis,” said Earles. “You can reduce the erosion rate by an order of magnitude or even two orders of magnitude with a good layer of mulch that's crimped in.”
Crimped straw mulch can work for measures that will only be needed for a few weeks. For longer applications without frequent mulch replacement, a heavier-duty solution might be necessary.
For measures that will only be needed for a few weeks, Earles recommends crimped straw mulch. For longer applications without frequent mulch replacement, he would go with a spray-on product.
Bonded fiber matrices
Bonded fiber matrices combine long fibers, like wood or straw, that can intertwine and lock the soil in place. The matrices also use a tackifier to stick itself with the soil. Like mulch, they protect against erosion and runoff while promoting seed germination.
Compared to mulching, bonded fiber matrices are a heavier-duty, longer-term solution.
Turf reinforcement mats
Turf reinforcement mats are a heavy and long-term solution. They are made from high-strength geosynthetics to provide soil stabilization under significant long-term water flows, like shorelines and riverbanks.
Turf reinforcement mats can be built according to specific project needs, such as different slopes and soil types. The mats can also feature openings that allow for vegetation growth.
Conclusion
Effective soil stabilization needs different approaches according to a variety of different variables. Paying careful attention to soil properties, weather and seasonality, the duration of stabilization, and the project’s requirements can help ensure that a chosen stabilization approach is the right one.
Jeremy Wolfe | Editor, Stormwater Solutions
Jeremy Wolfe is a former Editor for Stormwater Solutions.
