Pipeline projects are slowly but surely turning the trenchless way because of its non-disruptive nature. Highway and railway crossings and environmentally sensitive crossings, such as those passing through rivers, wetlands and wildlife habitats, can all be carried out using trenchless construction methods without interference unlike traditional trenching method.
While both are fraught with risks, benefits of trenchless technology far outweigh the risks associated with them, provided, proper planning, geotechnical investigation and correct method of installation are utilized; if not, the losses in a trenchless project can be phenomenal.
This is because trenchless projects are conducted out of sight beneath the surface and rely heavily on previous data obtained from municipality and through soil and geotechnical investigation. (Learn more with Getting Technical: Information Required in a Geotechnical Investigation Report.)
Proper planning is paramount to a successful bore for a trenchless project.
Factors to Consider for Planning the Bore
Trenchless technology offers different methods such as horizontal directional boring (HDD), horizontal auger boring (HAB), microtunneling and pipe jacking. Depending on soil conditions, available space and type of installation one of the methods is selected. (Learn more about soil conditions with Soil Conditions and Auger Boring: 5 Things You Need to Know.)
However; selecting a method is just one step in a procedure that should also consider the following factors in order to complete a borehole successfully.
Contractor Owner Communication
Prime to all projects is proper communication and paperwork between owner and the contractor who will carry out the project. All information regarding soil conditions along the bore path and a detailed geotechnical investigation report should be made available to the contractor.
Degree of accuracy or allowable tolerances, quality of material to be used and handling of unforeseen circumstances should all be discussed and laid down in advance to prevent last minute confusion and confrontation. Based on available data, the designer can plan the most efficient bore path. However; planned and actual bore paths may vary depending on existing sub-surface conditions.
An experienced contractor can use his expertise to study and provide his insight about potential problems that the bore may encounter prior to finalizing the planned bore path.
If a project is risky, it will inevitably cost more. Trenchless installation is completely underground and in some cases a last minute obstacle may lead to abandoning the bore and restarting again. This can prove very costly to the contractor if the contract agreement puts the responsibility on his shoulder.
To account for this, the contractor considers these risk factors while bidding for the project unless the owner agrees to share the cost associated with differing soil conditions. Proper procedures are laid down and should be followed in case such a circumstance arises in order to settle the matter amiably.
Location of Entry and Exit Pits
Some trenchless methods require a launch or entry shaft and reception or exit shaft. (Learn more about trenchless methods with Microtunneling Vs. Horizontal Directional Drilling: Understanding the Differences Between These Key Trenchless Methods.)
These shafts should be located in line with the planned bore path. Conflict with other utility lines has to be avoided so the utility lines passing through the area have to be identified prior to deciding the shaft location. The size of the bore pit should be adequate in order to install the required machinery and launch the pipe into the ground and also for retrieving the pipe on the other side.
Length of Bore
Bore length has to be adequate to account for loss of support and sloughing both during bore entry and exit. While crossing under roadways adequate length of the bore should be provided to protect the road and the side slopes from collapsing due to ground loss. The entry and exit shafts should be located sufficient distance away from the road curb.
Bore tolerance varies from project to project. A gravity sewer and a water force main don't require meeting the same tolerance because gravity sewers work on gravity and depend on grade tolerance unlike a force main. While designing a borehole, it has to be kept in mind that tight tolerances increase the boring cost because the equipment required to bore for tight tolerances are more complex and expensive. It also increases the risk for the contractor who works on a project with tight tolerances.
Where possible it is advisable for the designer to allow acceptable tolerances for grade while meeting all operational requirements for the installation. In projects that demand very high accuracy and tolerance, oversized casing pipes are used in order to provide space for the carrier pipe to be maneuvered for adjustment to correct line and grade.
What We've Learned
Many trenchless HDD projects don't require extensive planning, but it has to be kept in mind that trenchless technology is essentially out of sight. Saving a few hundred dollars to compromise on the planning stage is not worth the loss that will be incurred if the project encounters a problem. Target depth, setback distance, rise over run, dogleg severity etc. are also calculated in the design and planning phase that allows for as straight a bore as possible in order to have an easy pullback.
All trenchless installation methods should be well planned and discussed prior to launch to complete the project successfully.