Leak location survey at the Columbus Upground Reservoir (CUGR)
By Abigail Beck
Once construction is complete later this year, the Columbus Upground Reservoir (CUGR) will be the largest continuously-lined lagoon on the planet, nearly twice as large as the lagoon currently holding that title. The sheer enormity of the project has been compounded with difficulties from the beginning of construction to make it one of the most challenging geomembrane-lined structures ever built.
Battling the extremes of desiccation and flooding, high winds and subterranean gases, each construction phase seemed to open the door to new challenges. Applying the latest quality control technology, a post-construction electrical liner integrity survey, posed additional demands on both the general contractor and the liner installer.
This type of survey, where electricity is applied to the soil covering the geomembrane and grounded to the subgrade beneath it to track the path of electrical current through leak locations, is typically performed on landfill expansions or impoundments where the main concern is contaminant leakage to the environment.
Containment facility projects are rarely larger than 50 acres, which makes it fairly easy to finish the survey within two weeks. The liner integrity crew mobilizes to the site once the project is complete, performs the survey without interaction with the construction crew, and then the survey crew is free to demobilize once the leaks are located. The 850-acre CUGR project required a completely different approach.
With construction ultimately spanning three years, the plan was to build the lagoon in discrete phases according to site geometry and construction logistics. The first 9-acre portion was checked for leaks before continuing with the project to ensure that the cover material and placement methods would prove protective of the geomembrane.
During the first year of construction, only 33 acres were placed, mainly due to extreme weather. A large rain event could shut the site down for weeks, with poor drainage and fat clay making it impossible for vehicles to drive on-site. With good weather the exception rather than the rule, performing surveys over large areas at the CUGR project included as much time standing around in mud puddles as actually surveying.
To electrically isolate the cover soil for each phase, a trench was left open around the entire phase. When it rained, the trenches would fill up with water and it would often take days to pump out the water and re-create the electrical isolation. Eventually, rain flaps were incorporated into the isolation trenches so the flap would restrict current flow even if the trench was full of water. However, the depth of the water in the trenches was often deep enough to overwhelm the flap, which is partially submerged in Figure 1.
The survey area itself became too soft to drive on. After several vehicles got stuck in the mud of the pad, a mud buggy became the salvation of the leak surveying team until it, too, succumbed to the mud (Figure 2). Old-fashioned technology was used to free the buggy—more than a dozen men and the old heave-ho.
When the site wasn’t swamped, the clayey cover soil would rapidly desiccate to the point where backpack sprayers were used to rehydrate the soil because it was too thin for water trucks to safely drive on the pad. With the heavy rains, interrupted by periods of clear skies, the portions of the cover soil carrying seeds from the loamy patches of earth sprouted thick weeds. The weeds grabbed onto the legs of the dipole as the operators walked, making it impossible to pass through unless the dipole was lifted overhead to avoid getting caught on the weeds (Figure 5). The contractor graciously located a lightweight tractor to mow the weeds ahead of the survey.
Completing the survey
In spite of the environmental difficulties, the surveying crew worked tirelessly to collect data during the spells of decent weather. The rapid data collection was then downloaded into a software program to map the voltage potential measurements throughout the survey area.
The mapping technique is unique to the double dipole used to record the data. The data acquisitioner calculates the voltage potential between the two sets of dipoles in two directions simultaneously, both parallel and perpendicular to the direction of the survey progression. This produces two voltage maps of the survey area—one oriented north-south and the other oriented east-west. By cross-referencing the two maps, hole locations can be positively identified because the voltage field is disturbed in both directions in the case of a leak.
Figure 6 shows the voltage mapping of a portion of one completed construction phase. The current injector electrode shown in the figure is blocked out because a survey cannot be done too close to the current injector electrode; that circular area is surveyed while the current injector electrode is placed elsewhere. A unique voltage map is created for each position of the current injector electrode.
The signature of a leak location for this type of voltage mapping using dipole measurements is closely spaced voltage potential contours between a high and a low peak, with the peaks oriented in the direction of the dipole measurement.
In Figure 6, the dipole direction is north-south for map 6a and east-west for map 6b. The positive voltage measurements are indicated by green and yellow, with yellow being the greater positive value. The negative voltage measurements are represented by red and blue, with blue being the greater negative value.
The leak locations have the mirror opposite pattern in the voltage field as the current injector electrode because current is exiting from the current injector and disappearing into the leak locations. In terms of energy flux, think of the current injector like a star, radiating current, whereas the holes are like black holes, sucking up the current.
An artificial leak, in this case with a ⅛-in. diameter, is positioned in the survey area at the “worst case” position, meaning as far away from the current injector electrode as possible. The color settings of the voltage mapping are adjusted so the ⅛-in. hole is clearly seen in the data in both directions.
Analyzing the data
After reviewing the voltage potential maps, the survey crew lists potential hole locations and uses the equipment’s precision GPS to go back to those locations and either pinpoint the leak for excavation or confirm that it was just a voltage anomaly in the data. This anomaly can be caused by poor contact between the dipole probe and the soil surface.
Any area of the data that looks suspicious is rechecked within about a 20-ft radius of the anomaly location with closely-spaced dipole measurements. Leakage from the perimeter isolation trench is seen on the voltage map. The current leakage on the perimeter is also tracked during the leak location phase to make sure the electrical leakage is coming through the anchor trench flap and is not a leak through the lining system.
The survey crew located 69 leaks in the 589 acres completed by October 2012, with sizes ranging from about the diameter of a pencil tip to seam splits and equipment damage a few feet long. A number of holes were located after leaks were repaired and resurveyed, since large holes can mask smaller holes in the vicinity due to the preferential current flow path created by a large hole.
Many of the holes (28%) were due to oversized particles puncturing the geomembrane, as seen in Figure 7, 19% of the damage was due to knife slices, 16% from punctures by unknown objects, 13% from large equipment damage, 10% from rips or scrapes, 7% from bad seams, and 7% was uncategorized. The uncategorized damage came from odd causes such as a fence post through the liner or a cluster of tiny pinholes that appeared to be burns.
Major damage was at the location of a temporary access road, where the constant traffic put the geomembrane into tension on one side, ultimately causing it to rip at one point. The geomembrane had been stretched thin over a large area and had to be replaced in an area measuring about 60ft × 100ft.
With the uncovering of each leak location, the installation project manager studied the cause of the damage so the site could learn from its mistakes. With each phase constructed, the number of leaks per acre generally went down as mistakes were remedied and construction methods improved. The reservoir is scheduled for completion by autumn 2013, when it will officially break the world record for the largest continuously-lined lagoon on the planet.