Sitting a mere six feet from a 22-foot-high bluff flanking West Bouldin Creek in Austin, Texas, motorists driving on South Sixth Street couldn't see the eminent danger lurking at the base of the bluff. The bluff was being eaten away at a snail's pace by a varying flow of water. The creek followed a steady routine; undercutting the bank, causing a portion of the bluff to collapse, washing the collapsed debris away and then starting the cycle over again.

The problem began nearly 75 years ago, when railroad tracks were laid in the original bed of West Bouldin Creek. To accommodate the tracks, the creek had to be diverted to a manmade channel containing two 90-degree bends. The unnatural curves were no match for the water's tendencies, and nature began to slowly smooth out the bends.

Aiding in the ease of erosion, the base of the bluff was made up of easily erodible shale sitting below a layer of weathered and fractured limestone material. During heavy rain events, the shale would be scoured out creating a cantilevered bank.

Declaration of Emergency

According to Mike Kelly, Water Resource Engineer for the City of Austin, the deterioration of the bank had been spotted as early as 1996 by a citizen. "We have that in our complaint database," said Kelly. Following the complaint, the city performed a comprehensive erosion assessment of the watershed. The crew walked the entire channel, documenting and photographing all the problem areas.

"An outside engineer was consulted to design the project before it was threatening the road," said John Gleason, Landscape Architect for the City of Austin Development Services Watershed Protection Department. The engineer proposed a rock gabion design, a system requiring individual baskets to be filled with stone and then tied together with galvanized wire. The city was skeptical of the plan due to the aesthetics of the design, the planned alterations to the stream's environment and a doubt that the design would serve as a long term solution.

In the middle of 1999, the City of Austin deemed the situation an emergency project after heavy storms consumed half of the bluff, leaving only six feet from the Sixth Street's curb to the bluff's edge. Being declared an emergency project, the work could be done under a general permit, allowing the city to avoid the land development review process. This meant work could begin immediately.

A New Design

Kelly, the designer assigned to the West Bouldin Creek project, searched for new alternatives. With an emphasis on aesthetics, Kelly wanted a design having less impact on the environment. "The original design was going to protrude into the creek another six feet," said Kelly.

The city turned to Soil Stabilization Products Co., Inc. for advice. The company suggested a composite wall design using a geocell earth retention system and geogrid reinforcement to provide additional structural integrity and protection of the embankment from future erosion.

Kelly worked with two other geotechnical engineers to incorporate the bluff's composition into the design. "My argument was that because the bluff was standing nearly vertical on its own, there has got to be some inherent strength to the soil we should be able to give credit for," said Kelly.

"In other words, if you had a pile of sand, it's not going to stand up, it's going to recline to a natural angle of repose," Kelly explained. "This material was standing straight up so there was some inherent cohesion that we should be able to give credit to in the design."

Along with the assistance of the geotechnical engineers, Kelly was able to back-calculate what the strength of the hill was, reducing the footprint of the design on the stream's environment.

Securing the Site

The West Bouldin Creek Project began in August of 1999, and ended in March of 2000. Early stages of the project required removal of debris and securing the bank before construction could begin.

Before construction could begin on the embankment, the bluff had to be stabilized. First, the area had to be cleared of any debris. Piles of scoured shale cluttered the creek bed. In conjunction with clearing away the fallen shale, overhanging soil and limbs were brought down to avoid working under a loaded bank.

Geocells allowed the City of Austin to create an atmosphere more aesthetic and less intrusive than the rock gabion erosion control technique. Drought tolerant plants had to be selected for the wall since there is no irrigation. The process (shown clockwise starting with the construction detail) took about seven months to complete.

The top of the bank was then tested to determine the bank's strength. A large bucket from a brush truck was dropped repeatedly to reveal any tension cracks. "After doing that and seeing nothing more than a few pebbles come down, it increased our level of comfort that the slope itself was not going to come down on us while we were constructing it," said Kelly, who also acted as construction manager at the West Bouldin Creek site.

Building a Solid Foundation

In an effort to get a solid footing, the project called for a concrete foundation. A trench was dug along the base of the bank. The trench was filled with large rocks and then "grouted" in with concrete to create a level pad for the placement of the first layer of boulders. "Typically, it's like a lot of erosion projects, you need to have a good solid footing," said Gleason. "The base needed to be level and stable."

Once this was achieved, boulders were imported from an outside construction site. "We make periodic trips to construction sites where they are excavating for a road or a foundation," Kelly explained. "If the boulders are not going to be used as building materials, they are refuse that the contractor would have to dump somewhere. So they are often willing to let our guys come and pick them up."

After the boulders were placed on the foundation, a gravel and soil mixture was used as backfill. Layers of geogrid (a plastic uniaxial fabric) were also laid every two feet, which allowed the design to use less Geoweb®, reducing the cost of the project. The backfill was compacted and leveled in preparation for the first layer of geocells.

Building a Wall of Geocells

The first few layers of geocells were filled completely with gravel. This created a drainage alley to move any excess water downstream and divert it into the creek. Upon reaching the level of the boulder wall, filling the geocells took on a new formula. The back geocells continued to be filled with gravel, however, the front cells were filled with soil to accommodate the introduction of vegetation.

With the city running the project, the work crews consisted of city employees. The project used a system the workers were unfamiliar with, so there was a learning curve. To make the process a little easier, a frame was constructed to hold the geocells in place. The frame proved valuable not only in ease of use but in saving time as well.

Erosion-Proofing the Sewer Drain

An existing sewer drain pipe had to be worked into the new erosion-proof wall design. This meant water flowing from the drain would trickle down the geocell wall. If the geocells underneath the drain were filled with soil, it would only be a matter of time before those cells would be washed out.

"We filled the geocells directly underneath the immediate outfall where the water came out with concrete so that there was an un-erodible material and the water would bounce down to the creek," said Kelly.

Blending the Cells into the Environment

Having completed the geocell wall, there needed to be a way to incorporate the cells into the natural environment. The city found the best way to tie the wall into the slope was to use boulders.

"We used [boulders] at picking points in the landscape where the slope was more gradual so that we prevent any outflanking of the wall itself," Kelly said. The large boulders are tied into the slope at both the upstream and downstream ends of the erosion control technique.

Vegetating the Geocells

Besides the challenges of designing the wall, establishing vegetation in a xeric environment required a great deal of consideration. "The only thing that waters the plants is rainfall on the cells themselves," continued Kelly. "So you really have to have drought tolerant plants."

With those keys in mind, Gleason authored a plant list of 16 different species. The list included a wide variety of grasses and groundcovers. "During parts of the day, there is a lot of shade, so we had to consider that when choosing plant material," Gleason said.

On-Site Challenges

While working on an erosion control project the threat of rain becomes a serious issue. A sudden rain storm, depending on its severity, can create potential problems ranging from mud puddles to a more drastic event which could damage the work in progress. "There were a few rain events that occurred, nothing too big so we didn't lose portions of the project but it did slow them down," said Gleason.

Due to the steepness of the wall, the planting process proved to be a safety issue. The workers used nylon ropes tied to trees above and a rock climbing harness to work their way down the face of the wall. Furthermore, the crews were not accustomed to doing plantings, so there was more learning to be done.

With the project being in a residential neighborhood, the idea was to keep a natural look. Since you could incorporate plants into the geocell design, the cells were more desirable because it gave the stream bank a green and natural look.

The crews, after getting some help early on from the manufacturer of the geocell product, found the alternative erosion control technique much easier to construct than the rock gabion technique. "Once we customized [the frames] for the site they proved to be extremely effective in making this thing go quicker than anybody had imagined," said Kelly. "The crews even made the comment that they'd much rather work with geocells than build gabion baskets."

In addition to all the other benefits geocells had over the rock gabions, Kelly mentioned another positive feature. "This proved to be a very cost-effective