Failing walls compromise storage capacity and water quality of a coastal detention system.
In most coastal communities, stormwater treatment is extremely important to the overall health of streams, lakes, coastal waterways, and ultimately the ocean. Collier Canal is the primary stormwater detention area for more than 50 percent of the city of Sebastian along the east coast of Florida. The canal directly discharges into the Sebastian River, which flows into the Intracoastal Waterway and subsequently into the Atlantic Ocean.
During the last decade, the service life of the transite concrete walls had reached a critical state and started failing. The city realized it needed to take action to preserve its water quality. Many of the bulkhead sections along the 2.1-mile stretch of Collier Canal had failed and literally fallen into the canal. Silt had filled in the canal and reduced water storage capacity and compromised the water quality. The city of Sebastian concluded that it needed to dredge and dispose of 30,000 cubic yards of silt and stabilize 2.1 miles of canal bank.
The city had concerns about the service life of traditional materials and asked its consultant to consider the use of vinyl sheet piling as the primary material for stabilizing the canal banks. However, the initial consulting firm thought a stone rip rap solution would be more economically feasible. The project was then designed and bid as a rip rap system. However, during the advertisement period, vinyl sheet pile systems were again proposed by many of the contractors as an alternative. S.E. Cline Construction offered the vinyl sheet pile seawall option as a value engineer solution. This resulted in the city asking its consultant to rebid the project as a design-build vinyl sheet pile project with the consultant establishing the performance criteria for the bulkhead structure.
After months of design iterations, political protocols, and proposal reviews, a design-build team eventually was selected. S.E. Cline Construction of Palm Coast, Fla., was the successful general contractor with Everlast Synthetic Products LLC of Woodstock, Ga., manufacturing the vinyl sheet pile material and providing engineering design of the wall. Out of the five proposals provided to the city, the Cline/Everlast proposal was the only one to receive the highest marks from the evaluation committee in all categories.
Permitting and engineering
The project was permitted through the St. Johns River Water Management District at the state level and the U.S. Army Corp of Engineers at the federal level. Performance-based design criteria were established by the city’s consultant requiring the wall design to be considered two ways:
1) an extreme condition that contained a differential water level during a potential flood event, and
2) a common condition with a 250-pound-per-square-foot surcharge.
Another challenge for the Everlast Engineering Team was that traditional deadman systems could not be constructed because of confined space and the location of pools, garages, and various other structures along the 11,100 linear feet of wall. This challenge was overcome by using 940 driven earth anchors. Each earth anchor was proof loaded and documented to the specified minimum load of 9.9 kips. These anchors were connected to a reinforced concrete cap designed to support the top of the sheet pile and to span the distance between the tie-rod/earth anchor system.
The bridge along Fleming Street required special sheet pile and anchor designs to account for confined spacing and alignment of abutment end-bents. Design modifications were required in certain areas when S.E. Cline’s crew discovered soft soils. These modifications mandated the use of longer vinyl sheets and driven W-sections for deadmen. Unexpected drainage pipes and culverts required special sheet pile designs for the tie-in as well. All of these unforeseen obstacles were solved successfully by the Cline/Everlast Team.
The 2.1 miles of canal bank stabilization would have required dumping 1,000 trailer loads of rock, while the vinyl sheet pile wall only required 10 flatbed trucks. With most sheets weighing less than 50 pounds, this allowed Cline to use lighter and smaller equipment on the job. The sheets were easily offloaded and stored in designated staging areas. With limited project site access, this helped alleviate a major concern of impact to the 500 homes adjoining the site.
The sheets were driven with a modified APE 6 vibratory hammer mounted on an excavator. Manta Ray earth anchors by Foresight Products were driven using the same vibratory hammer with a slight modification to the drive steel. Areas near the Fleming Street Bridge required higher-capacity anchors and tighter spacing to account for the larger load. Because of the skew of the abutment in relation to the wall alignment, Cline had to work with not only the horizontal angle of driving the anchor, but also the pitch.
After installation of the earth anchors and the vinyl sheet pile, tie-rods from the anchors were secured to the tops of the sheet by using two steel channels bolted on the front and rear of the vinyl sheets. This provided structural support as a temporary wale while backfilling took place. Once fill was placed and compacted, minor alignment and adjustments were made just prior to forming and pouring the concrete cap.
Quality control and assurance
The city’s chief building inspector, Cline, and Everlast worked together closely to develop a detailed QA/QC program. Some of the procedures implemented included:
- collecting and testing a minimum of three concrete cylinders for each of the 200 pours for the concrete cap;
- making every bundle of vinyl sheets with a production code for manufacturing accountability; and
- proof loading and documenting each of the 940 earth anchors.
Some other common inspection items inspected included certifying that sheets were driven to proper grade, attesting to the rebar location in the cap prior to pouring, ensuring the drainage stone and geotextile were placed in the correct location, and verifying EverDrain weep-hole placements.
The city of Sebastian gained additional cost savings by using its own inspection team to carry out the QA process. This, coupled with the city procuring materials directly from Everlast Synthetic Products, yielded an additional savings of $60,000.
Challenges and results
Some of the challenges faced by the construction team included undetected drainage pipes, culverts, bridges, unpredictable soils, debris, and alligators. For example, the Barber Street Bridge only had 7.5 feet of head room, which prevented the sheet piling from being driven vertically with any kind of hammer. Water-jetting of the entire wall as a single unit was the only way to install the sheets adjacent to both bridge abutments. In spite of these challenges, the project was completed in less than eight months from issuance of the Notice to Proceed.
The goal of improved water quality and increased storage capacities was met, while the construction had minimal disruption to the 500 homes. Many of the residents commented on the aesthetically pleasing structure the Everlast vinyl sheet pile wall offers compared with traditional materials. Not only does the vinyl sheet pile bulkhead provide for a less expensive structure, the life expectancy is far longer than traditional materials. Nearly all of the work was performed by local personnel, which economically benefited the community.
Carl A. Hazenberg, P.E., is chief engineer for Everlast Synthetic Products LLC. He can be contacted at firstname.lastname@example.org