Re-building Bayou Bourbeux
Contractor paves steep Opelousas canal with low slump mix

By Sam Barnes

A 1.5-mile steep-sloped canal passing through Opelousas is getting some much needed cosmetic surgery that will accelerate stormwater flow beginning next year.

The $7.1 million Bayou Bourbeux contouring and paving project is part of a Natural Resources & Conservation Services flood control and restoration program and is expected to alleviate flooding problems for southwest parts of the city.

"This is the last phase of a four-phase flood control initiative in the Opelousas area," NCRS resource conservationist Dexter Sapp said. "Preliminary research identified the canal as a major obstacle to flood control."

NCRS provided $7 million for the canal's construction and the state provided the remainder through its Capital Outlay Program.

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While design of the project was performed by NCRS engineers, many decisions are being made "in the field" by Circle Inc. of Belle Chasse to accommodate changing conditions.

Project manager Matt O'Brien said his crew broke ground at the weather-plagued jobsite in November, so contending with a jobsite soaked in wintertime rains was the immediate concern.

"Since finishing clearing and grubbing, we've had nothing but rain," O'Brien said. "This is especially problematic when working in a drainage canal. A sudden rainstorm can wash away your sand and gravel bedding material and you just have to write that off as a loss."

To make matters worse, contract specifications required the contractor start on the downstream end of the canal.

"We're supposed to be pouring 90 ft. a week, and from January until early March we've poured a total of only 180 ft.," O'Brien said.

This forced the contractor back to the drawing board.

"To speed the concrete placement, we're looking into different things," he said. "We're pumping it right now, but it's a very slow process because we must have a special low-slump mix to flow through the pump. It's taking us eight hours to make a 30 ft. pour."

The contractor will likely begin placing the concrete with a crane and bucket.

"Bucketing the concrete is a little more labor intensive but it will speed production and help us make up some lost time," he added.

Jobsite superintendent Keith Lua said the crew has also been "blowing" a ¼-in. layer of gunite onto the canal bottom and slopes to keep the bedding from washing away during rain events.

"It gives us a protective covering over the bedding until we pour concrete," Lua said. "With the rain we've had, the fill can wash away easily. We're prepping the bottom with gunite, then coming back and pouring the slabs." Gunite, also known as shotcrete, is a dry concrete mix distributed through a rubber gunite hose and placed by air pressure.

The contractor's next major hurdle will come this summer.

"The canal goes through some tightly-compacted residential areas," O'Brien said. "The most difficult part of that will be getting equipment access from both sides of the canal."

For one 1,700-ft. section of the canal, access to both sides will be impossible.

"The close proximity of residential properties will force us to work from one side, so it's going to get crowded," he said. "Once we get to that area, we'll have to bring in some larger equipment like a 14-ton cherry picker and long-reach excavators to be able to reach the entire canal from one side."

Ultimately, the meandering Bayou Bourbeux will be straightened, deepened, contoured and paved, which should reduce friction and speed the flow of drainage down the canal.

The canal will also be nearly 1 ft. deeper.

"We're digging out the bottom, contouring the slopes, then placing a layer of bedding material on the bottom and some earthen fill on the slopes," O'Brien added. The canal bottom is 10 ft. wide and the canal top is 45 ft. wide.

A 12-in.-deep layer of bedding material on the canal's bottom consists of gravel and pure sand and a 6-in. layer of slope fill consists of "a finer sand and a smaller stone," all supplied by TXI of Baton Rouge.

"The finer sand compacts better, which is critical on the steep slopes," Lua said.

PVC drain pipe runs lengthwise through the canal bottom fill material and "branches out every 25 ft., goes up the slope 6 in. and out the concrete wall" to provide hydrostatic relief.

Fabric is placed on top of the bedding and the protective layer of gunite is distributed.

"We then place a single mat of reinforcing steel followed by 6 in. of concrete for the bottom and the slopes," Lua said. The 4,000 psi concrete was pumped up until early March and had a 5-in. slump.

"We could go to a higher slump, depending on what we decide to do."

During the concrete pours, crews form and pour the bottom slab, then use the slab as the bottom support for the slope concrete.

"Then we just start placing concrete at the bottom and work our way up the slope. Everything just ties into the canal bottom," he said. A 30 ft. trailer-mounted screed is wenched up the slope as the concrete is placed.

"As the screed turns it pushes the concrete up and finishes it," Lua said. "It has to be screeded several times."

Throughout the process, a sheet pile dam is constructed at 500-ft. intervals and two 6-in. pumps redirect stormwater around the work area to the other side.

Other work at the site required the extension of corrugated metal cross drains at several locations along the canal to connect to the city's existing drainage system, and the demolition of "a couple hundred feet" of existing paved canal areas.

Also, a 60-ft. downstream section of the canal is being lined with a 2.5-ft.-deep layer of 300-pound limestone riprap supplied by Diversified Materials of New Iberia.

O'Brien said the canal paving project will likely provide more obstacles for the contractor before it's completed in fall 2005, but the reward will be an "aesthetically pleasing design" in a high-visibility area of Opelousas.

"A lot of contractors didn't bid this job because of the difficult slope," he said. "But in the end I think it will be a rewarding experience."

Useful Resource:

For more project detail, go to: http://www.la.nrcs.usda.gov/