Overcoming the Quagmire with Vibratory Caissons

Arkansas Electric Cooperative Corporation (AECC) engaged Allgeier, Martin and Associates to design a new 69kV line in southeast Arkansas.

Challenges

From a wide view, this appeared to be a straightforward design. Over the length of 23 miles, the elevation of the ground only varied within a 20-foot range, and the location of the line was in a rural area along farm fields and county roads.

However, in three areas along the route, the landscape changed suddenly from farm fields to swamps. One stretch was about 1.5 miles of thick, gator-infested swampland with green, mossy water of indeterminate depth.

On dry, accessible land, pole setting is routine and straightforward. Here, the permanent standing water made it much more difficult. Additionally, the only access to the area from one side was along an old, abandoned railroad bed, just wide enough for a vehicle. Near the middle, an old, broken-down trestle halted travel. From the other side, a 20-foot-wide ditch prevented access.

We found the swampy quagmire we were overcoming was a befitting description of the quagmire we found ourselves in from a design standpoint.

Options

A few options were considered to get past the swamp:

• Avoiding it: Re-routing the line was not viable due to time constraints. Additionally, the swamp was very large, so avoiding it would mean moving the line miles away.

• Placing poles on the old railroad bed: This was not viable, because we needed the old railroad bed for the construction equipment to traverse. 

• Casing the holes: Installing casing, digging the holes pumping the water out, and using slurry. This was thrown out due to difficulty and anticipated high costs.  

Vibratory caissons were the chosen method since it would allow the construction equipment to utilize the old railroad bed to travel up and down the swamp while installing the poles in the swamp to the side without the need to dig the pole holes. 

Vibratory Caissons

A vibratory caisson is essentially a straight steel pole section with no bottom that is driven into the ground with a vibratory hammer. The caisson is attached to the hammer, lifted into place, and dropped until it contacts the ground. Then, the hammer vibrates at a high frequency while applying a downward force. This causes the caisson to sink into the ground. Once it has dropped to its designed elevation, the hammer is removed. The pole is later installed on top by connecting flange plates with bolts.

The design was modeled using PLS software. First, structures were spotted with the assumption they would be placed about 2 feet above the waterline, which is where the top of the caisson would be.

To accurately design the caissons, knowledge of the soil properties in the area was needed. Soil borings were taken along the line to obtain this information. Additionally, the approximate depth of the water at each structure location was found because the caissons needed to be deep enough in the soil to be structurally sound, but they also needed to end up with the top at a precise elevation. To obtain the water depths, a survey crew used a boat and a leveling rod.

Dis-Tran Steel was selected to manufacture the steel poles and caissons. They joined the design team in meetings to discuss how the caissons are manufactured and what information was needed. The caisson depths were designed using PLS-CAISSON. Dis-Tran was then given the necessary information to design and manufacture the caissons and poles. This information included caisson depths, top to ground distance, pole height, pole loading, and pole and caisson attachment requirements. 

Every caisson had a unique loading, water depth, and soil profile. To simplify the process, the caissons were grouped. The necessary depths and sizes were adjusted slightly to fit them all into only three variations. This simplified the design and construction.

Vibratory Caisson

Construction

The construction contractor was able to use the old railroad bed to reach each structure location and install the caissons and poles. Matting was placed in some areas. They worked from one side until they reached the trestle. From the other side, a makeshift bridge made from an old flatbed truck trailer was placed over the ditch from the north side allowing the equipment to cross.

Conclusion

After weighing the options, vibratory caisson footings provided a solution to the issues with pole placement in the swamp and to utilize the old railroad bed as equipment access. With this solution, we were able to overcome the quagmire.