OnGrade Sewer Installations Using HDD By Richard Levings
October 27, 2011There are many current methods of installing sewer ongrade including open-cut, microtunneling, auger boring, pipe jacking and vacuum microtunneling, with open-cut being the most commonly used. Surface obstructions of all kinds often prohibit open-cut installations, which then shifts to a trenchless method.
Historically, it has been understood that when a trenchless method is chosen, cost will increase significantly. Since many of these projects are municipality-owned, funding can be limited. If one project requires an enormous amount more of limited funds, it restricts the number of overall projects a municipality can complete. For this reason, municipalities and sewer system owners are forced to look at alternative trenchless methods that offer a lower installed cost structure. Horizontal directional drilling (HDD) is one of those alternative methods.
HDD was developed starting in the mid-1970s for crossing rivers and large bodies of water. This migrated to developing smaller systems for replacement of failed electrical cables in the 1980s. The systems became commercially available in 1990 and after three to five years of slow growth, the industry blossomed in the mid-1990s. Technology and experience grew at an exponential rate.
For many years, however, it was believed that HDD was not accurate enough to install gravity sewer pipe. Even with this misunderstanding and reality in some instances, some sewer system owners began to use HDD on less critical grade installations. Today, HDD has become more accepted by the sewer industry.
What has changed to allow this to be a more viable installation method?
1. Users of HDD have a great deal of experience in their local soil conditions.
2. The drilling units and drill pipe are more suited for accurate boring.
3. Tracking electronics have greatly advanced capabilities to improve the accuracy of gravity installations.
4. Designers and inspectors are becoming more familiar and comfortable with HDD technology and its benefits and limitations.
5. Grade drilling packages have been developed specifically for HDD installations.
6. HDPE, restrained joint ductile iron, and restrained joint and fusible PVC pipe options have been developed and accepted by the sewer industry. These were necessary for HDD installations.
Obviously, HDD is not for every project. Critical grade installations below .5 percent can be challenging. They are not impossible as some have attempted and completed flatter grades but repeating these installations every time is unlikely.
So, where do HDD sewer installations add the most benefits?
A. When open-cut is not possible and/or it would be extremely costly to open-cut
B. When budgets are limited. HDD can allow a municipality to do more projects with the limited monetary resources it has in tough economic times
C. Shallow installations with HDD require minimal surface disruption, which reduces costly spoils removal and replacement
D. Installations that have longer distances from manhole to manhole (400 ft and further)
E. Pipe slopes at 0.5 percent and above
F. Curved radius installations, usually along a winding residential street
G. Environmentally sensitive areas requiring a low carbon footprint
What sewer installations are more challenging or not recommended for HDD?
1. Critical grade installations below 0.5 percent
2. Projects requiring slip joint or non-restrained pipe
3. Depths below 30 ft due to the accuracy of walkover tracking systems below that depth
4. Jobsites that have limited setback area for a surface launched system
5. Cobble rock, solid rock, broke rock, boulders and sand under a water table significantly decrease successful installations
One critical component for the success of shallow slope sewer installations using HDD is in the contractor and drilling unit operator. A successful HDD bore at a critical grade (<1 percent) takes more time to complete than a simple utility installation where less precision is required. The contractor for the job must be familiar with gravity sewer work and understand the principles involved in manhole tie-ins and maintaining correct flow lines and must be familiar with the soils in which they will be working. The drilling unit operator should be patient and methodical. The best methods for installing HDD should offer multiple depth checks along the bore path against a known standard and not rely on pitch readings from the drill head alone.
Using horizontal directional drilling for sewer installations requires a clearly defined and communicated plan.
Many of the early challenges of HDD installations came from the installer, the designer and the inspector not understanding each other’s expectations. A meeting of minds between all parties involved in the installation is recommended before work starts on the project. Many projects have been completed using HDD. Certainly some have failed, as well. However, advances in electronics, tooling and methods have positioned HDD as a cost-effective and viable option for many gravity sewer installations where open cut is undesirable.
Richard Levings is underground products manager for the Charles Machine Works Inc./Ditch Witch and is a member of the Drillmaster Advisory Board. All reports are review by the Drillmaster Advisory Board: Levings; Tod Michael, Vermeer Corp.; Frank Canon, Baroid Industrial Drilling Products; Ron Lowe, Myers-Aplex, a Pentair Pump Co.; and Trevor Young, Tulsa Rig Iron.
Comments are closed here.