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Avoiding Damages to Facilities When Using Horizontal Directional Drilling

drill rigs due to the rather inconspicuous nature of the technology compared to traditional open cut equipment. Often, HDD is given a black mark from negative press coverage as a result of isolated incidences of striking existing utilities. Hit an electrical line and the neighborhood loses power. Hit a fiber optic line and critical communication may be lost. Hit a water line and a sinkhole may result. Hit a wastewater line and sewage will leak into the surrounding soil. Hit a natural gas line and an explosion may occur with potentially catastrophic results.
The bottom line is that, due to operating in congested urban settings, it is imperative that public safety considerations be undertaken. Each party (i.e. designer, contractor, and owner) involved in a project must carefully evaluate all potential risks. Risk assessment is common on construction projects to minimize the potential for failure. Evaluation of risks may even reveal the project to be unsuitable for HDD. Isn’t it better to not even attempt a project that is doomed for failure? Why try and fit a square peg into a round hole? The results will likely be a major impact to the contractor’s pocketbook due to costly litigation and negative exposure to HDD.
So, what are the major damage risks associated with Horizontal Directional Drilling? One cannot even attempt to predict what could occur during any type of construction operation; however, the major potential incidences are as follows.

Damage to Underground Facilities
Perhaps the situation that creates the most negative press is when existing underground utilities are hit. We often hear stories about the wastewater line that was struck by the HDD contractor installing a fiber optic line. In some cases, a contractor strikes an unmarked or mis-marked utility. There is not much that can be done in such a situation; however, contractors should perform a pre-installation checklist of likely encountered utilities prior to any installation. Confirm those that have been identified by the one call markings by exposing them using soft dig technology and further question those that might be missing. During the backreaming process, contractors should visually observe the backreamer passing an existing utility during pullback to ensure that potential reamer migration does not occur. This practice dramatically reduces the likelihood of damage to an existing utility that crosses the path of the installation.

 
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Prior to initiating boring activities, contractors should always conduct a site walkthrough. This provides an opportunity to perform a risk evaluation of the site to explore for possible overhead, surface, and subsurface hazards that may be encountered during drilling. Evaluation of structures such as gas meters, electrical transformer boxes, and manhole covers can provide valuable reconnaissance information. It is surprising what one might find. For example, if an electrical transformer box is present, then logic suggests that some electrical lines would also be present. Additionally, a natural gas meter in the vicinity is a likely indicator of a natural gas line.

 

There are numerous documented cases of catastrophic injury resulting from explosions from natural gas distribution lines cross-bored into sewer service laterals in private homes and public buildings. Unfortunately, current underground facility laws in many States are still unclear about the responsibility of marking these sewer laterals. Great progress is being made by grass roots organizations working with legislators to mandate responsibility for marking. In my opinion, the entity receiving revenue from the sewer lateral is really in the best position to identify and mark them. It is unreasonable to expect contractors or home owners to perform this critical task.
The engineering community also has a responsibility to design projects that can actually be installed using HDD, rather than testing the limits of the technology. Furthermore, they must ensure that all existing utilities are marked on the plans, including abandoned ones. On rare occasions, live utilities have been accidentally hit because, in doing their line marking, the utility owner inadvertently marked an older, abandoned utility. Subsequently, while following suggested good practices, the contractor
exposes this abandoned utility and unknowingly strikes the “unidentified” live utility. Keep good records! That is the message all utility owners must adhere to in providing a safer environment for future underground utility construction whether it be performed using traditional open cut or a form of trenchless technology like HDD.

Damage to Surface Facilities
Sometimes HDD installations result in damage to surface facilities such as heaved roadways, sidewalks, transformer boxes, swimming pools, and foundations. What causes this to occur? During the drilling and product installation process, drilling fluid is pumped through the drill string serving a number of vital functions. Helping to keep the borehole open, cooling the locating transmitter, and aiding in transporting cuttings from the borehole to the surface are just several important functions of drilling fluid. The borehole is pressurized and maintains its integrity as long as the internal borehole pressure does not exceed the external confining pressures. Failure will result in a discharge of drilling fluid to the surface and is commonly referred to as a “frac-out”. Frac-outs often result in damage to surface facilities. The excessive buildup of downhole pressures can be attributed to a number of factors including inadequate fluid mixtures (i.e., lack of necessary additives, high mud weights, etc.) or drilling/backreaming at a rate faster than the fluid can be transported (i.e., outrunning your fluid). Currently, pressure monitoring devices are available to monitor downhole pressures. These devices aid the contractor in reducing frac-outs by providing valuable downhole information. At a minimum, the rig operator should continuously monitor the returns. As long as fluid is returning to the entrance pit, a frac-out should not occur. Conversely, lack of returns means that the fluid is travelling somewhere, which is likely to the surface.
Conclusions
Horizontal Directional Drilling (HDD) is an innovative and novel technology for underground utility installations. Today, the use of HDD is commonplace in communities worldwide. The benefits of minimal excavation are critical to preserving surface disruptions essential in urbanized areas. Stakeholders employing the technology should follow prudent practices to minimize the occurrences and impacts of possible damages to both underground and surface facilities.

About the Author • Samuel T. Ariaratnam, Ph.D., P.E. is a Professor in the Del E. Webb School of Construction at Arizona State University in Tempe, Arizona
• He currently serves as the Vice Chairman of the International Society for Trenchless Technology (ISTT) and lectures internationally on issues related to trenchless construction methods. Additionally, he has published over 150 technical papers.
• Sam is the co-author of the Horizontal Directional Drilling Good Practices Guidelines and the Pipe Bursting Good Practices books that have been published in several languages.
• He can be reached at (480) 965-7399

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