Tibbs and Banta Landfill Riverbank Fill Slope
In the winter of 2002, the City of Indianapolis took action to address a critical erosion along a section of White River. Erosion along the river bank had exposed landfill debris and was threatening to expose a petroleum pipeline, jeopardizing public safety and private property.
In order to avoid potential action by the EPA and other environmental groups it was necessary to design, permit, and construct a cost-effective bank stabilization system as quickly as possible. The City chose Naren Patel, PE, of VS Engineering to design a solution.
VS Engineering first investigated a hard armor system and performed a rip rap design. The design indicated that D50 = 30” Rip Rap would be required to adequately armor the site. The budget for constructing the hard armor rip rap solution along the affected area would be $3 million dollars.
VS Engineering took a second look at the stabilization approach and contacted local CocoLogix distributor D2 Land & Water Resource to discuss the possibility of using reinforced vegetation and a hybrid soil biotechnical solution. Mr. Patel explained that environmental compatibility, cost effectiveness, the call for immediate action, the potential for quick permitting, and the need for public low bid work were all challenges to be considered. Additionally, the system would need to be built as quickly as possible, independent of seasonal changes in the weather and river hydrology. The bank stabilization system would need to be capable of surviving potential flood events during and immediately after construction.
The second analysis was conducted to determine if soils colonized by synthetically reinforced indigenous vegetation would adequately protect the landfill and pipeline from exposure. A detailed design analysis was performed and a hybrid system was proposed. The installed budget for the hybrid system was $2,200,000.
Considering the project’s challenges and budgetary analysis, VS Engineering, Inc. proceeded with the hybrid soil biotechnical design.
The hybrid design approach included a riprap wing deflector at the upstream end of the project as the affected river bank was approximately at a 30-degree skew to the river’s thalweg (deepest part of the main river channel).
A rip rap “aggregate raft” was constructed along the entire toe comprised of Tensar UX1500 Uniaxial Geogrid and Mirafi 1160N non-woven geotextile. The aggregate raft was utilized to minimize scour and differential settlement of the scour protection. It was constructed from three feet below the existing riverbed to an elevation two feet above normal pool. The top elevation coincides approximately with the two-thirds of the ordinary high water mark elevation, which is a typical transition point to the vegetated or soft armor components. A non-woven separation geotextile was laid on top of the 6” revetment rip rap leveling pad and aggregate raft to prevent the fill soils from migrating through the aggregate void space. A 2:1 reinforced soil fill slope was constructed over the aggregate raft in front of the landfill waste and the pipeline. The soil fill was reinforced with Tensar BX1200 Geogrids in 18” vertical lifts. Indigenous seeds were sown on the surface of the constructed slope and then covered with North American Green P-550 permanent turf reinforcement mat. A 20” vegetated CeresLog placed on top of the P-550 at an elevation coinciding with the top of the aggregate raft. The fill slope was contoured with 12” vegetated CeresLogs. Contours were placed to protect the overlap locations of the P-550 turf reinforcement mat. Vegetated CeresLogs were used to ensure a successful transition from hard armor to soft, protect the bottom anchor position and overlaps of the TRM, and to ensure the successful introduction and establishment of over twenty species of deep rooting indigenous plants.
The project’s installation began in April 2003 and was completed by the end of October 2003. The first flood took place less than thirty days after the first four hundred feet was installed (two thousand linear feet total). The last eight hundred feet was installed dormant waiting for Spring 2004 to germinate.
The system withstood six flood events during the 2003 installation, with inundation periods up to seven days and a record setting nine inch “Labor Day 2003” rain. It is of interest to note that root wads and lunker structures were included in the toe of slope design.
Incorporating fish habitat features into the design, along with the vegetative nature of the bank stabilization system, enhanced the project’s permitting process and the quick turnaround time of the permitting process. The system was last monitored in August of 2015 and continues meeting all expectations since installation with no maintenance.