Features Vol 47 No 4 - Tackifiers Put to the Test: Erosion and Sediment Control Comes to a Natural Conclusion

Project Specifics
In the summer of 2001, a stretch of Highway 10 in Northwest Minnesota was repaved. To complete the highway project, Minnesota Department of Transportation (MnDOT) and the turf establishment contractor, Lawn and Driveway Service, were tasked with stabilizing more than 53 miles of east and westbound in-slope shoulders, with slope batters ranging from 1:6 to 1:10 and superelevated channel bottoms. As with most highway projects, construction had left the roadside shoulders void of vegetation and exposed to wind and water erosion. Such erosion can quickly expose the bituminous shoulder edge and cause safety problems when vehicles loose control while pulling to the side of the road for an emergency.

In addition to vegetation establishment, engineering specification and NPDES permit requirements, the poor in-slope soil conditions and gusty winds posed a challenge to minimizing the effects of erosion. The soil composition after construction lends little or no vegetative growth potential to roadside shoulders. This soil condition makes it even more difficult for seeding, mulching and tackifier products to properly adhere to the ground.

To properly maintain the project area, MnDOT and contractors, which included Cory Bernu of Chemstar, Vicky Dosdall of Lawn and Driveway Service and David Sherbrooke of Sherbrooke Turf, Inc. had to establish a three to five foot wide canopy of vegetation along the entire east and westbound shoulder areas. The vegetation would limit water and wind erosion, thus meeting the project goal: to establish a stable, aesthetically pleasing shoulder with 70 - 100 percent vegetation cover within 30 days of project completion. But, getting vegetation to grow and stay in place is very difficult in conditions such as those experienced at the Highway 10 project.

The Technology
Before MnDOT began the Highway 10 project, it was approached by Cory Bernu, Chemstar's Research and Development Manager, regarding the possibility of MnDOT testing the company's starch derived StarTak 100 tackifier in the field. Previous product tests with the California Department of Transportation were so successful that Bernu wanted to prove to MnDOT that its starch derived tackifier could flourish in the Midwest as well. Coincidentally, Dwayne Stenlund, CPESC for the Erosion Control Engineering Unit at MnDOT, was looking to field test nonguar based, naturally derived and locally harvested alternatives to guar and emulsified asphalt tackifiers.

"It was very important for us to find a viable substitute for emulsified asphalt," said Stenlund, MnDOT. "One of our goals during the project was to utilize a variety of different tackifier agents both guar and starch based. You have to keep in mind that emulsified asphalt is an excellent adhesive. However, there are a number of environmental concerns with putting that type of tackifier on the ground and into our environment. So, we made sure to test as many tackifier agents as we could in order to determine an appropriate alternative to guar and emulsified asphalt."

In all, the project utilized approximately 3,147 pounds of tackifier, including: four types of guar-based tackifiers (roughly 1,112 pounds in all) and the starch-based product (about 2,035 pounds) as a test to see how natural tackifiers held up against the proven petroleum-based tackifiers in the field. To properly judge the results, MnDOT monitored important factors, including: overall performance, chemical composition, mixing ratio, cost and ease of application.

"The performance of guar products is well-documented in our state," said Stenlund. "However, we never had the opportunity to test a starch-based tackifier in the field. So, we were extremely interested to see how this type of product would stand up against the traditional guar products in the conditions present in the Highway 10 project."

Windy conditions and insufficient mulch and tackifier coverage can lead to shoulder rill erosion patterns, which cause unstable shoulder areas.

Like guar-based products, a starch-based product is extremely versatile and can be used as a mulch tackifier, straw/hay overspray binder or as a soil stabilizer/dust abatement agent. However, the erosion control and turf establishment values of starch-based tackifier agents have additional benefits to those of its guar-based competitors. For example, starch tackifiers are a 100 percent active polymer that contains 7 - 8 percent natural nitrogen, which acts as a slow-release fertilizer to help increase the seed germination and plant establishment process. Such natural fertilization characteristics are often absent or at levels of less than 1.0 - 0.5 percent protein in guar, plantago and other synthetic tackifier products.

Another important factor regarding starch-based tackifiers is their high moisture retention characteristics. Starch-based products help improve seed germination rates and reduce irrigation requirements through the hydrophilic nature of their organic polymers. These polymers allow the tackifier products to readily absorb and retain water at the same time as they are helping to stabilize the mulch and hydroseeding layer from wind and/or water erosion possibilities.

As MnDOT geared up for the project, it had to properly map out the distribution of its five tackifier products. So, it split the 19.85 acres of in-slope shoulders into 25 sections of a little less than one acre each, with three replicates randomly assigned. This would provide a very good testing ground regarding the performance of the various tackifier agents. Now, the next challenge mixing ratio.

"Our specs for the state of Minnesota were well-documented with guar-based products," said Stenlund. "However, you need to understand that all guar products are not the same. The consistency from manufacturer to manufacturer is considerable. So, we had a rough idea of what guar in general could do at 100 - 200 pounds per acre, but no real indication of how a starch-based product would perform. Basically, we needed to see for ourselves if a starch-based product was worse than, comparable to, or better than traditional guar products."

Lawn and Driveway Service crimps the straw-mulch bed to make sure that windy conditions could not sweep away the mulch cover.

Bernu knew that prior applications of his company's starch-based tackifier demonstrated that the product remained extremely pumpable up to 12 bags of product per 3,000 gallons of water. This far outweighed the maximum of three bags per 3,000 gallons of water typically associated with guar products. Number of bags per truckload, or mixing ratio, was an important point because the more bags of tackifier a crew could mix per truck can save a lot of valuable project time and money. For example, the higher the mixing ratio, the less travel time from water source to spraying area. In addition, a higher mixing ratio allows the crew to apply more product in a shorter amount of time.

To ensure that the proper product specifications where followed during the project, Bernu was on hand to offer technical support, advise all parties of what to expect from the starch-based product and answer any and all questions regarding the mixing and application process.

The mixing ratios for the tackifier products were held at a constant. The starch-based tackifier was mixed at 8 fifty-pound bags per truckload and the guar averaged 3 fifty-pound bags per truckload. And, each product was tested at 100 and 200 pounds per acre. This would give MnDOT a good picture regarding how many pounds of tackifier, or concentration rate, were needed per acre for maximum mulch and soil adhesion. Thus, by monitoring and comparing the mixing ratios and concentration rates with the cost of product per pound, MnDOT could directly correlate how much time and money the state and its contractors could save by applying the least amount of product in order to achieve the greatest amount of adhesive impact.

However, in order for MnDOT's tackifier field test to show consistent results, it had to monitor more than just mixing ratios and concentration rates. For example, vehicle and application speed, nozzle velocity, water temperature and purity were all variables that had to be closely monitored and regulated to the best of the crew's ability.

Getting Started
On September 3, 2001, work began on the Highway 10 project. The crew's goal was easy: establish stable, green and aesthetically pleasing shoulders as quickly as possible. But, to do that the crew had to ensure vegetation cover could take root within 2 - 14 days after the project was completed. That, combined with the task of covering such a large area in just two days, would soon prove more challenging than first anticipated.

Even after the crimping process, the effects of the 10-25 mile per hour wind gusts were still evident. Strong winds forced the road crew to place 25% more straw mulch per acre than originally planned.

On-site preparation began on the long stretch of east and westbound shoulders with Lawn and Driveway Service thoroughly loosening the soil to a depth of three inches with a conventional agricultural field disk cultivator. By properly disking the seedbed area, the soil now stood a better chance at establishing vegetation growth. The area was then drop seeded with a common mix including part Kentucky bluegrass and smooth brome, followed by harrowing.

Next, Lawn and Driveway Service applied straw mulch at a rate of approximately 2 tons per acre. The combination of 10 - 15 mile per hour wind gusts on day one, 20 - 25 mile per hour wind gusts on day two, and constant vehicle traffic displaced a significant amount of the mulch from the shoulders and onto the road and surrounding landscape. Thus, it was necessary for Lawn and Driveway Service to add 25 percent more straw mulch per acre than called for in the plans to properly cover the project area. Normally, mulching would not be allowed under these wind conditions, but with attention paid to tight sequencing of operations and overrunning the quantity, the 90 percent soil coverage as per specifications was obtained.

Sherbrooke Turf, Inc. and MnDOT apply a top coat of tackifier to the straw-mulch bed to insure sufficient protecting from wind and water erosion.

After the straw mulch was applied to the shoulder areas, Lawn and Driveway Service then quickly crimped the straw mulch into place in order to minimize mulch movement and loss before the tackifier application. Once again, the job of crimping the mulch was made difficult by the all-too-familiar windy conditions and traffic speeds. These conditions forced the crew to accelerate production sequence timing so that the mulch would stand a better chance of staying in place before the wind whisked it away.

The final step in the Highway 10 project involved the tackifier application.To do this, Sherbrooke Turf, Inc. utilized a 2,500 gallon Finn Hydroseeder® to mix each tackifier product with water. In a perfect world, the water utilized for the Hydroseeder would have been of constant temperature and composition. However, such a scenario was not available to the Highway 10 crew.

The project crew used a single cold water source with a temperature around 55 degrees from a centrally located stream throughout the two-day project. The water could have different amounts of bacteria, algae and other impurities than might be expected on other projects and different parts of the state. The crew was aware that all of these variables could affect the mixing rates and mixing times for each batch of tackifier. The crew also knew the results would not be too far off from those obtained by using other natural water resources.

As each tank was filled with water, careful records of the number of tackifier bags placed into each tank was made. Once the tackifier was added, it was allowed to mix for at least 15 minutes, which was roughly the travel time from water source to spray site. This allowed the tackifier ample time to thoroughly disperse and hydrate in the water solution leading to a more consistent spray and adhesion rate.

Staying true to form, the tackifier application process also was altered due to windy project conditions. Normally the contractor would use a back mounted chair and hose to directly overspray the mulch under low pressure. Due to the requirement of tacking both sides of the road shoulder, and moving with the flow of traffic, the top turret nozzle was used. The pump pressure was increased to minimize over spray onto the bituminous surface and outside of the shoulder. With increased nozzle velocity, the straw mulch tended to be displaced and spread away from the shoulder areas. In addition, winds made it challenging for the hydroseeding crew to hold and establish a five foot shoulder spray pattern. The rolling terrain made it difficult to maintain constant travel speed.

Now, to make things even more challenging, MnDOT had to make sure that the straw mulch, mulch crimper, hydroseed and tackifier applications were all done in perfect succession. This would ensure that the mulch and soil would remain in the shoulder areas and not become dislodged by variations in wind and/or traffic speed. With greater adhesion and coverage comes more robust germination rates and thicker vegetation cover. In addition, MnDOT has a rule that no contractor or subcontractor shall be paid for their services if mulch and seed are not spread uniformly and/or remain properly adhered to the seedbed areas within 48 hours of application. So, everybody involved in the project had added incentive to make sure that their job was done and done well.

"Making sure all of the applications were done in concert was quite a production of timing," admits Stenlund. "In the end, timing was everything. We had to make sure that all the applications were done at the same time and slow enough so that I could accurately measure the speed of the vehicles and rates of application in order to have sound test results."

The Results
On September 4, 2001, the Highway 10 mill and overlay and culvert replacement project was completed. Within two days, the project crew had utilized more than 3,100 pounds of tackifier over straw mulch to cover more than 19 acres of in-slope shoulders. Now came the moment of truth: how did the various tackifier products hold up in the field?

Two weeks after the conclusion of the project, Stenlund returned to the Highway 10 area to survey the test results. He closely monitored the mulch cover and corresponding vegetation establishment at various points throughout the 11.7 miles of westbound and 15.1 miles of eastbound shoulder areas (both sides of the road). During his survey of the shoulder areas, he relied on a rating system he created in order to judge the percentage of mulch cover remaining on the shoulder bed. The rating system (see Table 1) was based on a number of factors, including:

Percentage of mulch cover is directly proportionate to the success rate of vegetation growth - If there is less than 50 percent of mulch cover remaining after two weeks, vegetation establishment is next to impossible because of wind/water erosion factors. So, the higher the percentage of mulch cover, the better the chance of a thick and rich vegetation cover.
Lawfulness - Less than 70 percent perennial vegetation cover is a violation of the Clean Water Act. Thus, it is extremely important to be with in 70-100 percent vegetation growth for all highway roadside projects.
MnDOT Specs - According to MnDOT specifications, no contractor will be reimbursed for a project if there is less than 90 percent mulch cover within 48 hours of project completion. So, adherence of a thick mulch cover is extremely important to both contractor and subcontractor.

By utilizing his "mulch cover" rating system, Stenlund had the opportunity to thoroughly document the bonding properties of each type of tackifier and see how those properties helped promote stable and green in-slope shoulders via accelerated germination rates and vegetation cover. Stenlund's rating system concluded that the starch-based tackifier well outperformed its four guar-based competitors. Overall, the starch based tackifier performed in the B+ to C+ range while the four guar-based tackifiers performed in the C+ to D categories.