The Water System Mapping Project plays a vital role in maintaining the infrastructure for DeKalb County.  The GPS / GIS / Data Management Department field locates, verifies, and surveys all of the water mains, valves, hydrants, meters, pump stations, tanks, etc. to create a highly accurate comprehensive water system map.  All of the features of this map are accurate to the nearest centimeter.

The water system map is essential when planning future development as well as a valuable resource when natural disaster or other emergencies arise.  The water system map replaces 65 years of paper records and as-builts that are difficult to maintain and analyze.

The water system map is also built with water modeling nodes in place.  This allows an easy integration into modeling software so DeKalb County can plan for a bright future and continue delivering safe, clean drinking water to our citizens.

DeKalb County Water Uses GPR to Build Massive Digital Mapping Project

Above the surface, DeKalb County, Georgia, is home to Stone Mountain Park, where the skyscraper-tall images of confederate Civil War Generals Robert E. Lee, Jefferson Davis, and Stonewall Jackson on horseback form the world's largest relief carving on the world's largest mass of exposed granite.  Referred to as "the Eighth Wonder of the World," Stone Mountain Park is one of the most popular attractions in the country because of its 3,200 acres of breathtaking scenery and unique recreational attractions.

Underground lies an unseen infrastructure - a dark maze of water lines invisible to residents and the Stone Mountain Park's 4 million visitors.  Engineers and technicians at DeKalb County Department of Watershed Management are converting the county's sprawling water system into a paperless map using alternative technologies.  Only a fraction of the county's 2,500-mile digital mapping project has been completed using conventional utility locating techniques, but with the aid of cutting-edge ground penetrating radar (GPR) systems that can "see" subsurface infrastructure to assist with locating the water lines, procedures can be accelerated with no loss of accuracy or precision.

DeKalb County is Georgia's second largest metropolitan county in population, covering 275 square miles.  DeKalb's Scott Candler Filter Plant delivers an average of 80 million gallons of water per day (MGD), and has the capacity for 128 MGD to service a population of approximately 600,000 people in eight principal municipalities.  DeKalb County's water supply is delivered to 97 percent of the county except for a small area located within the southeastern edge of Atlanta.

In the office of DeKalb County Water Engineer Paul West, new and old technologies square off.  One corner of the office houses a floor-to-ceiling bookshelf lined with hard-bound maps of water records.  In the far corner sits a computer equipped with mapping software that allows West and colleague Darren Eastall to integrate survey data with global positioning systems (GPS) and geographic information systems (GIS) to produce a state-of-the-art water system map.

          

Although DeKalb County Water has been installing water lines since the turn of the century, the oldest records available date back to 1909.  The process of documenting information in the form of as-built maps started in the late 1940s but their accuracy has remained questionable until recently.

Engineers West and Eastall report to Supervising Engineer Dave Eller who heads up a team of 25 employees in the GPS/GIS/Data Acquisition Department whose responsibility is securing accurate coordinates of water mains, valves, hydrants, meters, and manholes.  What started in 1990 as a department with three men, a shovel, and a tape measure, is now known throughout the state, and most of the Southeast, as a pioneer for its ability to identify and implement locating and mapping technology.

To date, the department has mapped about 400 miles of the approximately 2,500 miles of water mains in their territory using various pieces of equipment to confirm water line locations including traditional pipe locators, sound locators, dowsing rods, metal detectors, potholing, and probing.  GPR recently joined the ranks as a viable locating method in ground conditions that range from granite to piedmont to red Georgia clay.

The current phase of the aggressive mapping project involves locating non-ferrous, polyvinyl chloride (PVC), asbestos-concrete (A/C), and polybutelene (PBC) pipes used in older subdivisions developed between the 1960s and 1980s.  “We knew we were going to start getting into a lot of non-ferrous pipe and smaller mains that would not be easy to radio, that’s why we started looking for alternatives,” said Eastall.  The alternative technology Dave Eller chose was the Vermeer Interragator ground penetrating radar (GPR) system.

When used with conventional locating technologies, the Interragator can assist in identifying underground metallic and non-metallic pipes and cables. “The Interragator is very useful to help locate non-ferrous water mains, but it’s also useful for locating covered or buried valve boxes, manholes, and eventually sewer force mains.  It’s easy to use, very mobile, and produces a printout and a file to document each job,” said West.

Mounted on a cart, the Interragator consists of two components that can locate underground objects to a depth of 10 feet with a 400 MHz antenna, or to a depth of 20 feet with a 200 MHz antenna depending upon soil conditions.  The unit transmits high frequency electromagnetic waves into the ground through an antenna or transducer.  The energy is reflected from various buried objects or distinct properties of material.  Operators can see the detected utilities in the form of hyperbolic or bell-shaped response curve, which is stored in the control unit for printing or downloading into mapping systems.

          

GPR has been used for a number of years in construction and mining applications, environmental audits and site assessments, as well as geological and archaeological studies.  Providing a continuous profile of the subsurface, GPR has been helpful in locating underground utilities, storage tanks, old foundations, abandoned mineshafts, and sinkholes.

DeKalb County Water’s GPS/GIS/Data Acquisition Department has 11 Senior Engineer Technicians locating with the ground penetrating radar system.  According to West and Eastall, the crews were operational and productive with the units within two weeks of a comprehensive two-day training session sponsored by Vermeer factory personnel and staff from the dealership, Vermeer Southeast Sales & Service located in Marietta.

“Our field crews seemed to learn to operate the Interragator very quickly, says West.”  The field crews are responsible for locating and surveying designated areas, and providing the data to West and Eastall who manipulate the data into a usable form.

Prior to teaming GPR with potholing or probing, field crews used a combination of conventional locating and verification methods.  Eastall explained that in residential areas to locate non-ferrous water mains, crews used to connect a conventional locating device to each house’s water service line.  The crew would radio a signal from the service back to the main, and pothole or pound a “T” shaped metal probing rod into the ground to verify lines.  Then, move down to the next house and do it all over again.  “In the meantime, you’re down the street with the Interragator – it just saves so much time,” he added.

Crews partner the GPR system’s 200 MHz antenna with existing techniques.  The newfound technology adds more accuracy and has expedited the process.  West and Eastall say preliminary estimates show the department as a whole yielded between 500-1000 feet of verified utility per day with the labor-intensive process.  Since putting their three Interragator units to work in January, GPR has helped produce approximately one-half mile or 2,500 feet of verified water line per day.

“In terms of accuracy, the Interragator with potholing gives very accurate locations, and fits the parameters the department has set for it’s locating equipment.  Without GPR, it’s just an educated guess,” said West.  “But, GPR does not replace conventional verification methods.  You don’t just go out and run this unit over the ground and stop.  We make sure the guys still go back and probe or pothole to verify what they’re seeing,” added Eastall.

          

Typical residential water lines run about five feet deep, manholes vary in depth, and transport mains are less than 10 feet deep, but they can vary.  Since depth is critical in this type of work, operators rely on the Interragator’s ability to read vertical and horizontal distance to the utility on the screen.  Recently, the Interragator helped the department avoid a potentially costly rehabilitation at a utility intersection.  A crew used the GPR system to locate a 30-inch water main at a depth of 7 feet over a 15-inch sewer main 10 feet deep under a five-lane road.  The sewer main was scheduled to be pipeburst and upsized to 21 inches.  The unit’s scans revealed that the lines were too close to each other to enlarge the sewer line with the bursting technology without damaging the water main.

At another site on busy Memorial Drive, a six-lane main thoroughfare that runs from Stone Mountain into downtown Atlanta, GPR helped crews locate the coordinates of a 150-foot section of a 24-inch sewer main.  As a result of a road-widening project several years before, three manholes were covered up by the traffic way built on the crown of an embankment over 20 feet high.  Crews using GPR were able to find two of the manholes, but the third one was difficult to locate due to the depth.  The Interragator system was used to determine a possible location of the manhole, which was later verified by employing a remote underground television system.

In 1999, private development installed 37 miles of water mains in the county, while DeKalb’s crews installed 7.3 miles primarily for maintenance.  “The county relies on contractors to produce the as-builts, and many times they’re not accurate or of survey quality.  We have to go back in and do it ourselves; to make sure it’s right.  Our goal is to get to the point where contractors provide us with a digital survey we can download directly into our system,” said West.

Until that day, the teamwork of GPR and verification is helping to get the job done faster with no loss of precision or accuracy when compared to conventional methods of locating by the GPS/GIS/Data acquisition department in DeKalb County.