Technical Report: Design Considerations for Microtunneling

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Technical Report:
Design Considerations for Microtunneling
Successful design of microtunneling projects can be
achieved by paying attention to the following:
1. Developing, understanding and defi ning project
needs and requirements,
2. Exploring and defi ning ground conditions,
3. Superimposing the project “in the ground”, and
4. Creating a project “environment” that will allow for
Contractor success.
The design process has four stages: planning, risk assessment, design and contract documents. Each stage is
discussed in detail in the rest of this paper.
Planning
Planning questions must begin with “Is this a microtunnel
project?” There are many excellent trenchless construction methods to consider in addition to microtunneling: auger
boring, pipe jacking, guided auger boring, pipe ramming,
guided pipe ramming, horizontal directional drilling and
conventional tunneling. Depending on the project length,
diameter, ground conditions, groundwater conditions, access
restrictions, allowable pipe material, permits, available
right of way and available shaft locations, microtunneling
may not be the preferred construction method. In fact, of
the alternative trenchless methods, microtunneling represents “the Cadillac” method: costly, but capable of successfully excavating within tight alignment tolerances in cohesionless soils below groundwater.

If it is determined microtunneling is the best alternative,
the question “Can a microtunnel boring machine (MTBM)
handle the project requirements and anticipated ground
conditions?” must be answered. Experienced tunnel engineers are needed to evaluate the drive length, tunnel/shaft
depth, hydrostatic head, anticipated ground conditions, potential for obstructions and contractor access and laydown
areas to determine if the project can be designed and built
using microtunneling methods. Contingencies must be
considered to handle hard obstructions, mixed face ground
conditions, hard rock or refuse fi ll if they occur.
If it is determined that an MTBM can handle project
conditions, the question “Can the Owner afford the project?” must be answered. Before the project design begins,
a planning level cost estimate must answer this question.
Project risks and the resultant “risk pricing” anticipated for contingencies, potential third-party impacts or project
parameters must be evaluated. In addition to cost estimates
for shafts and microtunnel drives, the designer
should consider the cost for contingencies including: dewatering, if needed, for shaft construction; and ground
modifi cation (chemical grouting, compaction grouting
North American Microtunneling 2011 Industry Review
Technical Report:
Design Considerations for Microtunneling
Proper Planning Needed to Ensure Successful Construction
By Tracy J. Lyman
or ground freezing) for shaft and/or microtunneling.
Additional costs to cover potential impacts to third parties such as buried utilities or structure foundations
above the microtunnel; or simply neighbors who may be
inconvenienced by the project, must also be considered.
If the answer from all of this planning is still “Yes, this is a microtunnel project,” then let the design phase begin!
Risk Assessment
An effective Project Risk Assessment is a powerful tool
to identify, assess and develop design control strategies to minimize project risks. Performed early in the design process, the Risk Assessment can engage key project participants
and encourage open communication about project
risk. Risk Assessment is well-explained in an excellent document entitled “A Code of Practice for Risk Management of
Tunnel Works” published by the International Tunneling
Insurance Group in 2006. The reader is referred to this
document for a thorough description of the Risk Management
process and a discussion of its advantages.
Basically, a Risk Management process will ask the following
questions: “What is uncertain?;” “What can change?;” “What scenarios...
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