Descending into the Depths
October 1, 2006
When the Tennessee Department of Transportation (TDOT) began surveying in 2002 for a proposed highway in eastern Tennessee, it stumbled onto a deep problem. The proposed highway, Interstate 475, is set to connect Interstate 75 in Loudon and Anderson Counties. But during environmental and archeological investigations associated with the I-475 project, it became evident that the presence of caves would have a major impact on the proposed location of the highway. The limestone bedrock of East Tennessee has created irregular underground streams and caves throughout the region, known as karst topography. These cavernous depressions can be a barrier to construction, so TDOT contracted Vaughn & Melton, an engineering firm headquartered in Middlesboro, Ky., to map all of the caves in the project's alignment.
"Since some of the caves were very extensive, TDOT did not want a bulldozer to find any of the caves by surprise," says Bill Davis, survey manager for Vaugh & Melton. "[The department] realized the importance of identifying each cave to ensure safety for construction crews and to prevent damage to the caves and wildlife."
As lead consultant on the project, Vaughn & Melton was responsible for finding the exact location of the caves as well as the extent of the cave passages as related to the proposed designs. Initially, the team was charged with mapping three known caves on or near the proposed route of I-475, but the scope of its responsibilities grew as the project progressed.
Planning the ProjectVaughn & Melton and its subconsultant, AMEC, an environmental and geotechnical firm located in Nashville, Tenn., met with TDOT to determine the methods that would be used throughout the project. Vaughn & Melton provided all surveying and mapping services while AMEC used its expertise in cave exploration to provide a safety plan. The joint team of Vaughn & Melton and AMEC decided the mapping project would be completed in two phases. Phase I called for a line drawing of the cave passages; Phase II included a full three-dimensional survey.
Before work began, Vaughn & Melton's crew visited each cave to become familiar with the landscape. The first cave, called Cherokee Caverns, was a commercial cave that is operated on Halloween and by tourist appointment. In the main passage of the cave, which had high enough ceilings for people to walk upright, sidewalks had been constructed for the tourists. The rest of the cave, known as the "wild" portion, had not been developed for walking traffic and was made up of mud. The second cave was at the bottom of a large sinkhole. This cave's main passage had 90- to 100-foot ceilings and was about 40 feet wide with very little mud. The third cave was very small with a 12-foot vertical drop from the entrance.
After preliminary investigation of the caves, Vaughn & Melton chose to use the following tools for the project: a reflectorless Trimble (Sunnyvale, Calif.) TS 330 total station, a Topcon (Livermore, Calif) GTS 311 total station, a Tripod Data Systems (TDS, Corvallis, Ore.) Recon data collector, a Garmin (Olathe, Kan.) 12 GPS unit and handheld equipment including a Brunton (Riverton, Wyo.) compass, fiberglass tape and inclinometer. (Although Vaughn & Melton considered the use of a laser scanner to save time and increase effciency, its size made it impossible to use in the small cave passages.) The crews used a combination of all the tools to accurately and efficiently survey the caves. Vaughn & Melton performed most of the field surveying and created all the computer models in Bentley (Exton, Pa.) MicroStation using GeoPak software.
Squeezing into the PassagesThe crew started out working in the commercial cave since it had some existing lighting and was large enough for total station use. After TDOT set a pair of GPS points near the entrance of the cave, the crew ran a traverse into the cave and around the sidewalks with the Topcon GTS 311 total station and a TDS Recon data collector. The crew set points at the entrance of the small passages to work from later.
As with any project of this nature, safety was a top priority. Vaughn & Melton met with AMEC's team to learn about caving and safety procedures, and safety meetings were held each time a team went down into a cave. One obstacle in the caves that affected the crewmembers' safety and their ability to work was the limited lighting. The crew overcame this by carrying multiple lights. Each crewmember's helmet had a light that aided in seeing the instrument, and each had a handheld light to see the point that was being set up.
After the initial traverse loop had been run, Vaughn & Melton surveyors returned to the small passage entrances. As expected, some passages leading from the cave were too small to set up a total station inside. In these instances, the crew members used their Brunton compass, inclinometer and fiberglass tape.
The entrance to the first small passage leading off from the main portion of the cave was barely one foot high and three feet wide. Davis, as team leader, volunteered to take the first run. "As the leader, and because I was the smallest, I felt inclined to take the first passage since it was so tight," Davis says. "It was a very tight space, but fortunately I am not claustrophobic."
According to Davis, the small passages were lined with a fine silt mud. "Since some of the passages were so low and narrow, at the end of the day I was covered in mud," Davis says. The dirty conditions made it hard to keep the field book clean, but this was solved by storing it in a Ziploc bag.
Davis and his crew pushed into the passages as far as they could. The limit was usually an entrance of one foot high by two feet wide. Running the traverse and completing Phase I for the first cave took four days to cover 2,005 feet. The crew then moved on to perform the Phase I surveys of the second and third caves. "The team moved more quickly with each cave," Davis says.
After completing Phase I, another meeting was held with TDOT to discuss the Phase II survey of the caves. At this time, a fourth cave was found and added to Vaughn & Melton's list. The four caves were close enough to the proposed alignment of I-475 and I-75 to necessitate the Phase II three-dimensional survey.
Building the 3D ModelTo begin Phase II, the team went back to the commercial cave and began shooting the floors, walls and ceilings using the reflectorless TS 330 total station. The crew took numerous shots of the interior so that a good representation of the interior surface could be shown in the final deliverable. Since GeoPak software does not support entering two surfaces on top of each other, the surveyors developed a coding system for telling the CADD operator where the wall changed slope direction. Different codes were used for the floor, ceiling and point of slope direction change. In a simple case, the team made separate TIN files for the floors and ceilings, with the wall shots common to both files. After this was complete, the two files were merged for a continuous model.
Including the CADD operator in this process proved to be helpful because it allowed the operator to see the actual surfaces. TDOT also requested a fly-through of the 3D model. Vaughn & Melton does not traditionally produce fly-throughs, but agreed to provide it as part of this contract. As a result, the team endured a learning curve to build the fly-through using GeoPak software, but the end result impressed TDOT because of the accuracy of the model.
Growing Project ScopeAfter TDOT held more public meetings with residents, Vaughn & Melton learned of 23 additional caves in the area. Overall, its crews investigated a total of 25 caves. The team visited each new cave to secure GPS coordinates at the entrance of each with a Garmin 12 handheld unit, and then reported to TDOT how close the cave was to the proposed alignment. TDOT followed up by making the decision on whether a survey was needed. In the end, 11 caves received a Phase I survey and five received a Phase II survey. The remaining caves were not close enough to the alignment to affect the design.
All of the caves, except the last one found, were known to the local caving society. The last of the 25 caves was discovered when an environmental team encountered a deer hunter who told them of the cave. The deer hunter's location for the cave was passed onto the Vaughn & Melton team, which then surveyed 1,100 feet of passages in the final cave. The total length of all passages in all caves surveyed was 7,745 feet.
Surveying UndergroundAs they worked on this project, crew members were often asked by friends and colleagues about the presence of wildlife in the caves. In general, minimal wildlife was found. The most frequent wildlife encountered by the team were salamanders and bats. In fact, one cave was home to a maternal bat colony where several hundred bats raised their young. Another cave had a large lake with 50-degree water.
Upon completion of the surveying, TDOT used Vaughn & Melton's 3D models in the design of Interstate 475 to prevent and minimize damage to the caves and associated wildlife. The cave mapping project has proved to be invaluable for the construction process because of the protection it provides to construction crews and equipment working on the I-475 highway.