The lower guidewall at Locks and Dam 15 on the Mississippi River was originally constructed in the early 1930s. For more than 80 years this concrete wall has been used by commercial vessels to approach and depart the lock. Now, after many years of standing strong, the wall is showing wear and the Rock Island District dive team was called in for a closer look.
The guidewall, which extends 1,000 feet below the main lock chamber at Locks and Dam 15, is made of concrete and sits on top of a wooden framework of timbers known as cribbing. Recently a portion of this wall has started inching its way into the river, separating away from the rest of the wall. Operation of the lock has continued despite the wall shifting; however commercial traffic has needed to alter the way they approach and depart the lock.
In an effort to get a better understanding of what was happening with the wall, a multibeam sonar was used to scan the underwater portions of the wall.
"Although the multibeam is a good tool, in this situation it was not successful in helping us determine the cause of the wall’s movement," said project engineer, Brent Anderson. "That is why we called on the District’s dive team to help us get a better look."
The District’s dive team is made up of specially trained lock and dam personnel and maintenance employees who temporarily leave their regular job and serve as a member of the team when a dive is needed. Divers Nate Gorham and John Snell volunteered to help with the dive at Locks and Dam 15. Gorham, who is assistant lockmaster at Locks and Dam 15, was very interested in assisting with the dive since he is very familiar with the facility and works there every day.
"Although I’ve made multiple dives at Locks and Dam 15 it is always nice to see firsthand what’s going on below the surface of the water," said Gorham.
Since this dive was taking place during the main river navigation season, special permission was granted to the dive team to close the lock to traffic for a two-hour timeframe to give the divers the best chance at gathering data. To assist the divers in getting a better look, engineers from the District’s geotechnical branch offered the use of an underwater sewer camera.
Inside the dive boat, which houses air tanks, communication equipment and additional dive team staff, the engineers set up a video viewing station where imagery from the sewer camera could be watched live while the divers were underwater. They also did screen captures of the underwater images to save for future reference.
"Because the water below the locks and dams is so muddy, it can be very difficult for the divers to get a good view of the areas they are inspecting," said geotechnical engineer Bill Tague. "We thought using the sewer camera would give more people the chance to look at the images and make a clearer diagnosis of the problem."
Radio communications are typically the only thing used during a dive to relay information about what the divers are seeing underwater. It is a detailed process that involves divers moving inch by inch, describing everything they see and in many cases what they feel since visibility is so low. On the surface, team members inside the boat monitor the location of the divers by feeding out air line and watching the tiny bubbles coming from the divers below.
"Its intense work," said Gorham. "But it’s something most people never get the chance to do."
In addition to visual inspection and using the underwater camera, divers also incorporate a variety of different tools to aid in the inspection process. For this inspection the divers used a tool called a story stick to determine if riprap was missing from within the cribbing structure. The story stick marked with predetermined measurements is a good tool for the divers to use as the numbers can quickly be read back to the recorder on the boat and a large number of measurements can be taken in a short amount of time.
Although the dive at Lock 15 was for inspection purposes, the dive crew also has the capability to do actual construction underwater such as core drilling, installing anchors, cutting metal and concrete removal. Recently, the dive team played a pivotal role in the installation of downstream bulkhead recess sill beams at Locks 20 and 22, which required underwater placement and anchoring of the sill beams to the existing sill.
"Having our own internal dive team is a real benefit," said Anderson. "There are many times when being able to get under the water has saved the District a lot of time and money when it comes to structural maintenance."
During the two-hour closure, the dive team, along with help from the engineers, gathered as much data as they could. The team worked together to ensure the safety of the divers as well as get the best inspection data possible. Now that the dive has been completed, the engineers will take the information, along with data gathered from the multibeam sonar and recent geotechnical investigations to determine a course of action for the repair of the guidewall. Without the information gained from the dive operation, the designers would be missing a key element in determining that course of action.