Compost Research Project

Reviewing Compost Use In Construction and Modeling The Hydrologic Response of Vegetated Compost Blankets

Written by Corey Poland – August 2018

Poland_2018_Reviewing_Compost_Use_In_Construction_and_Modeling_The_Hydrologic_Response_of_Vegetated_Compost_Blankets

Abstract
The objective of this independent study is to evaluate the use of compost in large scale earthwork projects such as those conducted by Departments of Transportation. A literature review outlining how compost can be used for soil stabilization, runoff reduction, and vegetation establishment in construction sites shows that compost is viable for these purposes. Important design parameters related to compost were taken from scientific literature, as well as from recommendations by the American Association of State Highway and Transportation Officials and the U.S. Composting council, the authority on composting in the U.S.

Then each state’s latest DOT design specification document was assessed for if/how compost use is prescribed. Most states had some mention of compost, but specifications varied widely in scope. The particle size distribution requirements differed from state to state significantly. Many of the requirements outlined by the U.S. Composting Council and the American Association of State Highway and Transportation Officials were not incorporated design specifications for each state.

Modeling of the hydrology of vegetated compost blankets explored different vegetation scenarios, depths of compost, and types of compost in the one-dimensional domain. Fully established vegetation represents a significant sink of soil water from the root zone as transpiration or root water uptake. Since fully established vegetation covers exposed soil, it limited evaporation as well. Varying the depth of the compost layer had less of an effect on the change in soil water storage, evaporation and transpiration than changing the vegetation or type of compost. Different depths of compost did, however, affect the runoff depth more than changing the vegetation. Changing the hydraulic properties of the surface layer, based on hydraulic testing of various composts, had the greatest effect on runoff. While modeling different types of compost, we found the evaporation from the surface is not necessarily an indicator of the hydrologic effectiveness of a surface compost layer, as more water can infiltrate but is subject to evaporation, leading to similar cumulative evaporation.

Overall, the positive effects of compost have been demonstrated while DOTs and other construction companies would benefit from expanded understanding of how to use compost effectively on site. A way to model vegetated compost blankets is presented, which can help engineers determine how to best incorporate compost into design.

Here are a few key figures:

Picture1
Thickness requirements for a vegetated compost blanket for each of the 50 states. This is based on the rainfall amount and intensity. The depth requirements are based on The American Association of State Highway and Transportation Officials (2010). Standard practice for compost for erosion/sediment control (compost blankets), AASHTO designation: R 52-10.
Picture2
Map representation of how each state included compost in their latest design specification document. The maximum score was 15 out of 18.
Picture4
Conceptual diagram of an unvegetated compost blanket (left) and vegetated compost blanket (right) indicating different fluxes entering and exiting the control boundary
Picture6
Four important water balance parameters from scenarios were vegetation stage was varied and scenarios where compost depth was varied (in parenthesis). The model results indicate that thicker layers of compost increase the soil water storage and reduce runoff, as what would be expected. However, as more compost is applied, both evaporation and transpiration decreased because the negative potentials induced by these sinks were less able to access soil water.
Picture5
Different composts applied at 5 cm depths under the vegetation growth scenario had variable changes in storage, evaporation, runoff, and transpiration. The “No compost” run indicates no compost was on the surface, but vegetation was assumed to grow at the same rate as if compost were to be applied.

 

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