In the late 1980s a good friend and competitor was awarded a large 18-in. sewer project near a lake. The soils were a silty sand with a high water table that made excavation in the quicksand conditions very difficult. In the end he was unable to complete the project and as a result lost his business. Had my friend been able to use the latest trenchless technology available today he would have rescued his company and most likely made a decent profit on the project.
An ongoing project near the same lake with similar soil conditions illustrates the advances that trenchless technology has made in becoming a very cost-competitive alternative to open-cut sewer installations. The project is the first phase of a large sewer system expansion by the City of Provo, Utah, to service an area of rapid development.
Bids received for 15- and 18-in. sewer mains ranged from $2 million to $2.5 million with the low bidder 25% lower than the next bidder. A proposal for trenchless installation of the sewer was discussed with the low bidder, which would reduce his bid by over $200,000. Because this phase of the project was required to start and finish with very tight deadlines, the lead time required for the trenchless option made it impractical for use on this first section. Here are the original bid results and comparative bid results had the trenchless option be available to each bidder.
Why is the open cut option so much more expensive?
Over the last two decades the cost of open-cut sewer projects has increased significantly. Factors that have caused the increases relate to more stringent design, environmental, safety and public nuisance issues:
- Public nuisance issues – Work hour limitations; Increased traffic control; Increased truck traffic issues; Work hour restrictions during high traffic volumes and special events; Night work only with noise limitations; Winter and weather shutdown requirements.
- Environmental Requirements – Noise mitigation; Storm water runoff restrictions; Equipment pollution such as mud on roads; Strict dewatering, treatment and disposal requirements; Wetlands or animal disturbance issues.
- Design Requirements – Off haul of all excavation; Imported gravel for all backfill; Increased pavement restoration to entire lane or road width; Existing utility damage prevention.
Increased Safety Requirements – More robust trench shoring requirements; Increased OSHA requirements such as dust disturbance mitigation.
The cumulative effect of these more stringent requirements results in substantially increased costs and risk to the contractor. Overruns on gravel backfill alone, due to sloughing soil conditions, can cause an otherwise profitable project to suffer substantial losses.
Why is the trenchless option so much less expensive?
With the relatively small footprint at manhole locations, the trenchless technologies eliminate 95% of the issues with the open-cut work mentioned above. The cost of compliance with these issues is simply not incurred. The attractive advantages of trenchless methods result in less cost, safer projects with less risk and more profit for the contractor and less cost to the project owner:
- Public nuisance issues – Work around the clock is normally permitted with a noise attenuated generator as the main power source; Work can proceed during inclement weather and cold winter conditions; Public inconvenience and traffic control are minimal; No truck traffic except for excavation and backfill of shafts at manhole locations.
- Environmental – No dewatering of the soil is required therefore no water treatment and disposal issues arise; Storm water mitigation is limited to manhole locations; Equipment pollution is limited to off haul of excavation at manhole/shaft locations; Environmental disturbance is eliminated by tunneling under sensitive areas.
- Design Requirements – Imported gravel backfill is limited to manhole locations; Surface restoration is eliminated and limited to manhole locations; Existing utility damage from exposure during excavation is eliminated by tunneling under or over the utility.
- Safety – Trenchless methods are inherently safe – no trench shoring is required; Shoring is required only at manhole/shaft locations; Safety exposure is limited as crew sizes are commonly 4-6 persons; The presence of water in soils is not a safety issue – water is a benefit providing lubrication and tunnel face stability.
The introduction of new technologies to the U.S. market have made trenchless installations much more competitive. Until recently, trenchless technologies for gravity sewers such as pilot tube, slurry and auger microtunneling methods have been more expensive than open-cut methods. The high cost of shafts has been a major factor with shafts often costing more than the tunneling itself.
Recently, rapid shaft technology has been introduced in the United States, reducing the cost of shafts substantially. The introduction of pilot tube and auger microtunneling equipment to install PVC and ADS pipes as well as all other sewer pipe types also reduces the cost of trenchless installations. Pilot tube and auger microtunneling systems in the 8- to 30-in. pipe sizes now have a much wider range of soil types where installation is possible.
Where it is impractical to install pipe in fluid soils and hard rock conditions by open-cut, it is well within the capabilities of trenchless methods. Current pilot tube and auger microtunneling systems have been reported to have installed pipe in hard rock of up to 22,000 psi unconfined compressive strength. It is also common to install pipe in very fluid soils with blow count n-values as low as 1.
The comparison of open cut and trenchless methods for sewer installation confirms the cost, environmental and safety advantages of the trenchless methods, specifically for smaller diameter pipe installations.
The cost reduction of trenchless technologies and the cost increases for open cut methods are expected to continue. The advantages of new and continued improvements in trenchless technology are resulting in less risk, less cost and more profit for contractors.
Mike Ellis is owner of Trenchless Equipment Co. (TEC) based in Orem, Utah. For information visit trenchless.net.