Improving Microtunneling Design Practice – Contractor’s Perspective
August 30, 2011Design practice cannot evolve to achieve cost-effective project construction without feedback from the contractors who actually build our designs. This article summarizes the recommendations of some of the most experienced and successful microtunneling contractors to improve design practice. The contractors’ opinions were gleaned from interviews and conversations with the authors.
“Money saved on borings not drilled will be spent on risk contingency and claims defense.”
The foundation for success in trenchless construction is a thorough understanding of the subsurface conditions. Insufficient or inaccurate geotechnical information hinders contractors’ abilities to prepare responsible, competitive bids, and forces the contractor to make the difficult decision of either adding additional cost to his bid or making optimistic assumptions and then having to pursue claims during construction, if more challenging conditions are encountered.
Owners occasionally feel that they can avoid risks associated with unknown geotechnical conditions by not sharing boring data obtained during design, and instead requiring that the contractor procure their own geotechnical information. It is unreasonable to expect a contractor to perform a geotechnical investigation within the 30 days typically allotted for a bid. Additionally, a contractor who performs their own investigation is likely to be underbid by an optimistic bidder without geotechnical data. Finally, the courts have upheld the contractor’s right to rely on geotechnical data obtained during design.
The contractors suggested that standard practice include boring spaced not greater than 500 ft and for large-diameter borings or test pits, if cobbles and boulders are suspected. Contractors expressed the importance of obtaining borings even in hard to access locations, including waterways. Borings located only near shaft locations can fail to uncover significantly different geotechnical conditions that have been affected by the watercourse or activities of man over time. While these borings are expensive, the costs for differing site condition claims, or recovering an MTBM stuck beneath a river can easily be orders of magnitude higher. The overall message is that obtaining complete, high-quality geotechnical information is invariably less expensive and is money well spent.
“Include as much information as possible in the drawing set.”
For a microtunneling project, the drawings must include the basic design information of bore length, diameter, depth, invert elevations and slope. However, a drawing set has the opportunity to provide far more important and essential information to the contractor.
Contractors are always under tight time constraints to put together their bid packages and wading through a thick specification book for key project information is daunting. For projects with multiple bores, it is helpful to include a trenchless crossings table to convey detailed information on various parameters of the shafts, bores, pipe material, ground conditions, as well as specific notes for each shaft or bore.
“Be logical, precise and avoid redundancy. Organize specifications according to who will execute the work.”
Pipe should be covered in the pipe specification, not in the microtunneling specification. Portal stabilization/entry and exit stabilization should be covered in the shaft specification or in a separate portal stabilization specification. Typically, the microtunneling subcontractor does not construct the shafts and does not purchase the pipe. Separating out important elements helps establish clear requirements and helps define responsibilities.
“Allow only construction options that are feasible for the ground conditions and project requirements.”
We heard repeatedly that engineers should avoid prescribing means and methods, and most engineers do avoid this. In fact, many engineers avoid this so well, that they end up telling the bidders that it is totally up to them to decide how to construct the project features, and they will be solely responsible for success or failure. The problem with this approach is that projects are awarded to the lowest, “responsible” bidder. If a more elaborate and expensive option is necessary because of ground and groundwater conditions, but less expensive options are not expressly prohibited, then the low bidder will likely be the optimistic contractor who thinks that the cheaper alternative just might work.
“Promote/require coordination between the general contractor, shaft subcontractor and microtunneling subcontractor.”
Shafts are often constructed by general contractors or other subcontractors that are not familiar with the requirements of microtunneling work. The microtunneling subcontractor may show up to the project site to find a shaft that does not provide sufficient clearance below the planned invert elevation to allow for setup of the launch seal and jacking frame. The shaft may not have a watertight face that will prevent slurry or lubricant from coming back into the shaft during jacking. Struts may cross the shaft such that pipe cannot be lowered onto the jacking frame. The native soil behind the thrust block may have been heavily disturbed during shaft construction, compromising the thrust block capacity. The shaft wall, or the annular space around the shaft, may have been filled with high strength grout or reinforced concrete that cannot be excavated with the MTBM. The distance between the shafts may not coordinate with the amount of casing pipe purchased to allow for 2 to 5 ft of pipe to stick into the shaft.
All of these situations have occurred on past projects, and all could have been avoided if thorough communications had occurred between the microtunneling subcontractor and others on the project.
“Time is indeed of the essence! Jointly investigate the cause of problems and engage proactively with the contractor to find a solution.”
Dispute resolution implies that a dispute exists regarding differing site conditions, defective specifications, quantities or other issues. The point we heard emphatically from contractors was that it would be extremely helpful if owners and engineers would proactively engage in investigating the alleged problem, crafting a solution, and giving the contractor direction. Some owner agencies are better than others in engaging early and constructively with contractors to resolve small problems before they grow into large problems. What we have heard repeatedly, both in our practice and while researching this paper, was that the cost of inaction and denial is high. The advice we heard from contractors is that they appreciate when owners have given some thought to dispute resolution and have established a mechanism for resolving disputes.
“Accurate surveys are critical for success. Specify that surveyors shall be experienced in microtunneling work. ”
Far too many microtunneling projects have wandered astray because of survey errors. Microtunneling drives have been constructed at incorrect grades or reverse grades. Drives have literally missed reception shafts because of incorrect alignment established from the jacking shaft. The microtunneling contractors emphasized the importance of using surveyors that were experienced in microtunneling work to transfer line and grade from the benchmarks at the surface to the shaft bottoms.
“Evaluate required and achievable line and grade tolerances realistically and include the necessary requirements in the specifications to achieve required accuracy.”
While laser guidance systems can work effectively for large pipe and short to moderate drive lengths, smaller-diameter pipe and long drive lengths can cause difficulties for laser systems. A laser’s reference line is affected by air density and moisture content over the length of the beam. During a shift, the air inside the tunnel becomes warmer and more humid, especially near the MTBM. The differences in density can cause refraction of the laser beam. Contractors use ventilation schemes to minimize these effects, but in pipe less than 48 in. and drives longer than 700 ft the effects of temperature and moisture variations are difficult to control.
If high accuracy is required with small pipe diameter and long drive length, the design should require appropriate features for the equipment and methods. Gyroscope systems, sometimes combined with electronic water levels, can achieve one inch accuracy over long distances regardless of the environment inside the pipe string. However, gyroscope systems are expensive and not yet widespread within the industry. If a project will necessitate the use of this technology for success, then the specifications need to spell out the requirement so that all contractors are bidding on a level playing field. In some cases the significantly less expensive electronic water level can be used in conjunction with a traditional laser system to improve grade accuracy at a lower cost. Finally, contractors recommended a design feature than can allow for flexibility on long gravity sewer projects where grade is critical: the inclusion of minimum 0.1-ft drops across manholes.
Summary and Conclusions
The objective of this article was to harvest the reflections of experienced skilled microtunneling contractors and present their views regarding how engineers can improve microtunneling designs. While some of the opinions may not be easily implemented, and some may not even be acceptable, the opinions provide a valuable basis for reflection and discussion between owners and engineers.
David Bennett, Matthew Wallin and Kate Wallin are with Bennett Trenchless Engineers, Folsom, Calif. This article was excerpted from the 2010 No-Dig Proceedings, “20 Years of Perspective: Design and Construction Professional’s Views on Improving Microtunneling.” For the complete paper, go to www.nastt.org.