The Department of Defense (DoD) uses petroleum-based soil amendments for a number of engineering purposes. These petrochemical-based biopolymers have been shown to be effective for producing soils with increased strength and resistance to erosion. These soil characteristics are important for areas where steep earthen constructs cannot be protected from erosion. This project examined the use of a non-traditional soil additive, a biopolymer, as a substitute for the petrochemical-based synthetic polymers currently used in these applications. The biopolymer offers several advantages over the synthetic polymers including rapid re-vegetation and reduced transport of solids in runoff water. The use of synthetic polymers can be problematic from the standpoint of biodegradation, cost, availability, and logistics. The biopolymers examined in this study are a low density, natural material, which can be transported in a dry state and reconstituted with local water supplies.
The overarching objective of the demonstration was to validate soil erosion control by the biopolymer in the field at full-scale, and to transfer the technology to end users at Army industrial installations. The performance objectives were to:
- Maintain the original berm slope angle compared to an untreated control;
- Select the most effective soil application method;
- Establish treatment longevity; and
- Evaluate ease of use.
A technique has been developed through which R.tropici-derived biopolymer can be produced in an aerobic bioreactor. The polymer is separated from the growth media and derivatized in order to produce a non-reactive (non-crosslinking) material that can be used as a soil amendment. When wetted, the biopolymer will form a gel within the soil matrix. Individual soil particles are linked together within the biopolymer matrix, producing a soil in which individual soil particles have greatly reduced mobility and significantly reduced hydraulic conductivity. This change in the physical form of the soil, on a particle level, results in increased soil strength and decreased erodibility. The nature of the R. tropici biopolymer is to aid development of plant root systems. The enhanced root development also contributes to decreased soil erodibility by water and wind.
The earthen explosion protection berm at the Iowa Army Ammunition Plant (IAAAP) suffered from water erosion, slumping (loss of protective height), and was sparsely vegetated. The berm was mechanically recontoured and biopolymer was applied, along with grass seed, in three different ways in order to assess the effectiveness of each application method. Light Detection and Ranging (LIDAR) imaging was used to record the effects of biopolymer soil application on soil erosion. Visual inspection and plant collection evaluated re-vegetation efforts.
The use of biopolymer derived from R. tropici was evaluated as a soil modifier for erosion control, and sediment transport was evaluated through slope stability and surface soil durability studies at bench- and meso-scale. Simulated berms were constructed to evaluate erosion at the angle of repose characteristic on earthen berms and were used to empirically measure soil loss mass. Silty Sand (SM), Silty Clay (SC), and Silt (S) soil types were used in the experiments as these soil types represent the worst case for soil erosion. Soils were treated at dosing rates of 0.2% and 0.5% biopolymer (w:w) and compared to an untreated control of the same soil type. In addition, mesoscale rainfall lysimeters were used to evaluate the ability of the biopolymer to reduce soil erosion and the transport of sediment in both surface runoff water and leachate. Following a series of rain events equivalent to one year rainfall, the mass lost from each “berm” was measured. The untreated soils each lost the greatest soil mass. The Silt soil treated with either 0.2% or 0.5% biopolymer (w:w) and the Silty Sand treated with 0.5% biopolymer (w:w) each maintained a stable mass throughout a year of simulated weathering. The biopolymer-treated soils continued to demonstrate surface durability and resistance to erosion after 20 rain events, the equivalent of more than 2.5 years of weathering.
Sediment loads were measured in runoff water from treated and untreated Silty Clay soil during the slope stability experiments. Biopolymer amendment resulted in a 78% decrease in total suspended solids (TSS) in the runoff water, compared to the untreated control. Particle size analysis of treated and untreated soil demonstrated that the percentage of material in the >0.3-mm particle-size fraction increased by 22% in the biopolymer-treated soil. The biopolymer, performing its natural function as a soil binder, was very effective in this soil type at reducing the loss of sediment in runoff water.
Soil modification by the addition of biopolymer has also been demonstrated to reduce the production of fugitive dust by wind erosion, compared to commercially available, petroleum-based polymers. The lowest concentrations of respirable dust from a Silty Clay soil were produced when the soil was amended with 1% molasses-derived biopolymer applied in either a single or double application. The third best performance was given by the sorghum-derived biopolymer. A commercial, petroleum-derived polymer was the fourth most effective treatment.
Soil stability is also increased by enhanced plant root formation and development. Treatability studies in this area demonstrated that soil amendment with biopolymer encouraged rapid seed germination, enhanced root development (particularly of the fine root structure, thus increasing plant root density), and increased overall drought tolerance.
In summary, the treatability study on the use of biopolymer amendment to improve slope stability of bermed soil and reduce loss of sediment in surface water runoff showed that the biopolymer:
- Effectively maintained the slope stability of a simulated berm.
- Reduced transport of soil particulates in surface runoff.
- Performed effectively in soils with a high concentration of fines.
- Enhanced plant growth, particularly root development, to reduce transport of soil fines.
To achieve the objectives of the field demonstration, an explosion protection berm at IAAAP was reconstructed and treated with biopolymer and fescue seed, applied using a hydroseeder. Different application methods for the biopolymer were tested:
- Double at depth: two feet of soil was removed, biopolymer was applied, the area was re-covered, and then additional biopolymer and grass seed was applied.
- Double: a double surface spray application. The second application was made with grass seed approximately 24-hr after the first application.
- Single: a single surface spray application of biopolymer with grass seed.
- Control: the control received no biopolymer, only water and grass seed.
LIDAR was used to virtually survey the ground conditions of the berm following completion of the berm reconstruction, treatment and seeding, and again six months later. For berm change calculations, the data was decimated to 2 cm point spacing on an equal interval grid resulting in 2 cm vertically and 2 cm horizontally. All of the measurements and results were derived from this data sampling. Changes in berm slope and soil elevation were calculated from the differences in the pixels of each area.
Vegetative growth was collected from each treatment area and the control area. Below and above ground biomass was calculated and compared for treatments vs. control. In summary, the average biomass of fescue grass in the biopolymer treated areas increased 223% versus the untreated control area. The ratio of root mass to the above ground plant mass was approximately 7% for the treated areas and 5% for the untreated soil.
Following six months of weathering (October 2012 to March 2013), LIDAR imaging showed that the R. tropici biopolymer successfully met all performance objectives. The simplest and most effective application method, established by a change in surface roughness over time, was a single surface application of biopolymer and grass seed using a hydroseeder. The double application of biopolymer on the surface was next most successful, followed by a double application at depth—the first application at 1-2 feet bgs and the second on the surface. The double application at depth demonstrated greater soil compaction due to settling of the lower soil layer. All treated soils had greater biomass than the control area and higher root to above ground mass, adding to the soil stabilization.
The majority of the costs associated with the biopolymer are material cost (biopolymer production and delivery to the site) and labor. The quantity of biopolymer required for slope stabilization is based on soil type and size of the area to be treated. The biopolymer works well at low dosing rates for Silty Sand and Silt soil types. The biopolymer is less successful stabilizing soils with large, heavy grain sizes, such as Sand and Glacial Till, and requires higher dosing rates. A dosing rate of 0.5% has been successful with the majority of soils studied. Freight cost for delivery of the biopolymer to the site is dependent on the distance from the manufacturing plant, but biopolymer can be delivered in a dry state and reconstituted on-site. This should reduce shipping charges. Reconstitution does not require use of potable water supplies. The cost of treating a berm with a single application of biopolymer is approximately half (0.52) of what it costs for a traditional earthen berm over a 30-yr time frame.
There are no issues preventing implementation of this technology on DoD installations and facilities with soil erosion issues. There are no known regulations that apply to the use of this technology and no permits are required to implement this technology. The R. tropici bacteria are not added to the soil, just the processed biopolymer they produce. End users for the technology are installations and facilities with erosion control issues such as dirt roadbeds and berms. The biopolymer technology has been the recipient of ERDC Research and Development Awards, the USACE Green Sustainability Award, and the ESTCP Project of the Year.