GPR Resolves Directional Boring Conflicts from Beyond the Meter

GPR - essential to survey and markAny mariner, aviator or expedition leader today finds it harder to venture into uncharted territory and make headlines. It just seems every corner of the globe is mapped to some degree diminishing the drama a couple of octaves.

Amazingly, directional boring contractors face the prospect of making headlines they never intended since the underground world of buried utilities is akin to 1492 when the Spaniards set out to discover the West Indies — particularly when boring on private property. In this case, the great mystery is not what lies beyond the horizon of the Atlantic Ocean but rather what lies beyond the meter. In the United States, the system in place through various one-call centers, while not perfect, has reduced the risk of utility conflicts to HDD contractors or others involved in underground construction activity by large margins in public rights of way.

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However, what is the solution beyond the meter? Documentation is not required for buried utilities at private facilities and is seldom available. In fact, it is very likely a private entity whether a manufacturing facility, industrial plant, resort, office park, etc. possesses original blueprints that are quite accurate I might add of the building(s) and even landscaping but underground facilities-no such luck.

The Situation
This is the exact problem recently faced by an HDD contractor at an established condominium complex in Hutchinson, Fla. A new electric feed installed with a directional bore was proposed for the complex and a lack of underground information was a major concern for the HDD contractor. The list of unknowns included water, sewer, fire protection line for the hydrant system, and as well the original major electric feed. To make matters even more challenging the nearest manholes were several feet away from the site and both the sanitary and water lines were constructed of plastic and the public utilities do not provide locate services beyond the meter.

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GPR can provide reasonable depth estimatesSo where to start?  
The HDD contractor hired Ground Hound Detection Services Inc. of Boynton Beach Fla., to provide the underground details needed along the proposed boring corridor. Ground Hound is quite familiar with private locates beyond the meter and in fact, according to Ground Hound president Jeff Poppe, the vast majority of their locate expertise involved these scenarios for the past two decades. In addition, Ground Hound employs a full suite of technology including ground penetrating radar (GPR).

One of the biggest challenges in the underground is non-metallic non-conductive utilities whereby conventional EM location tools simply cannot be used to locate them. GPR is now an established tool and in all but a few circles can now be considered conventional. According to Poppe, GPR — like the latest offering from MALA, the Easy Locator with HDR — provides incredible resolution, as well as simple user interfaces not found in systems in the past. Poppe should know since he was a pioneer in the use of GPR for utility locating when systems were expensive and the user interfaces were designed for scientists not locators. As well, the cost of GPR systems, such as the Easy Locator HDR and others, is one-third of the systems on the market just 10 years ago. Many systems have seamless integration with GPS today and optional mapping tools built into the user interface.

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The strength of GPR is two-fold; it is the only non-destructive/non-intrusive method to locate plastic, terracotta, ACP or other non-conductive (tone-able) facilities; and it is an extremely effective tool for wild catting or blind searching an area for unknowns. GPR does not need to be physically coupled to receive a signal as is the case with EM tools.    

In this instance, GPR was essential to survey and mark out the corridor by scanning the entire area to look for conflicts. According to Sean Halsey, director of South Florida operations at Ground Hound, the field crew located a 4-in. PVC water line and an 8-in. PVC gravity sewer line directly in the HDD trajectory with GPR. However, by gridding the corridor and performing several GPR scans the field crew noticed an additional utility not anticipated in the proposed HDD path. Using the GPR, the field crew located the utility back to its source off the property and discovered it was the existing main electric supply to the condominiums. This was a shock — no pun intended — since the power company graciously marked out the line on the ground surface on the private portion of the property. Unfortunately, the line was marked 8 ft off from its actual location.  As well, access to the metallic fire protection line was so far from the HDD path, conventional EM tools could not transmit a carrier signal where it was needed to provide an accurate locate. So in this instance GPR was also needed to locate a utility that EM tools are normally very effective at detecting.

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Although GPR can provide reasonable depth estimates by most standards it is always good practice to “pot hole” an area when HDD activity is involved. According to Halsey, all targets were verified with vacuum excavation including the erroneously marked underground power line.

As with any non-intrusive technology, there are limitations to the GPR method HDD contractors should understand. Soil conditions, particularly moisture content, is highly variable regionally and in some cases locally. The moisture content and the soil type affect the velocity of the GPR signal and as a consequence depths can normally be constrained to no more than plus or minus 10 percent. In addition, not all soil types are ideal for GPR. Clean Florida Ocala sands are undoubtedly at the top in terms of GPR performance whereas downtown Houston, gumbo clay probably ranks equally at the bottom. One can expect depth penetration in clean sands beyond 20 to 30 ft; alternatively penetration in clay may be as low as 1 ft.  
The case presented is a classic example of how GPR provides value as an integrated locate technology. Moreover, since private property seldom has documented underground information, and, as well as a paucity of aboveground clues such as valve boxes and meters beyond those at the edge of the public/private domain, it is essential to have a tool that can be used in a  manner that can survey areas blindly with no preconceived information.

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Surely even Columbus would have appreciated some advanced tools as he looked over the Atlantic horizon and perhaps actually located his chosen objective. For HDD contractors it is no longer that daunting and it is certainly good practice to avoid making the headlines.

Matthew J. Wolf is president of MALA Geoscience USA Inc.

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