Top 10 Issues for Improving Microtunneling Design

 The Northern California Pipe User’s Group
18th Annual Sharing Technologies Seminar
Berkeley, CA
February
Top 10 www.norcalpug.com
Issues for Improving Microtunneling Design Practice 18, 2010
TOP 10 RECOMMENDATIONS FOR IMPROVING MICROTUNNELING DESIGN PRACTICE
Author: Dave Bennett, PhD, PE1
1
Bennett Trenchless Engineers
ABSTRACT: Since the first US microtunneling project in Atlanta in 1984, microtunneling has come a
long way and is today a better understood and more widely accepted trenchless installation method. As
increasingly challenging projects are undertaken and new advances are made, the limits of the
technology are constantly being tested. The long-standing 1,560 LF single drive distance record was
recently shattered in Portland with a 3,055 LF drive. Unfortunately, not all projects are completed without
incident; change orders and costly claims continue to drive up project delivery costs. Contingency added
by contractors to bids on projects with ambiguous design documents and sketchy geotechnical
investigations add unknown additional costs. Burdensome specifications with provisions that are
impractical, cannot be measured, and are unenforceable, turn off some potential bidders, and can result
in wide bid spreads because bidders don’t understand design intent.
Construction problems, though undesirable, can help everyone involved in the next project – owner,
designer, and contractor –but only if we dissect the lessons and learn from them. To foster better
practice, it is imperative that design engineers be involved in construction, and establish dialogue with
and listen to the contractors’ perspectives on how to improve design documents and avoid repeating
errors. The notion of soliciting feedback from contractors is not new; many design engineers solicit
informal contractor reviews during design, and the practice is appropriate whether for microtunneling,
HDD, pump stations, pipelines, or treatment plants.
In this keynote address, I have elected to focus on microtunneling and sift through wide-ranging
discussions with contractors to glean some of the most promising ideas. The presentation organizes the
results of frank discussions with experienced microtunneling and pipejacking contractors about common
problems that persist in microtunneling designs. Through this collaborative process, we hope to highlight
elements that can improve design and reduce risks and uncertainty. Better design documents can help all
parties evaluate the potential risks involved and, consequently, the real cost of construction. By clarifying
the design intent and reducing ambiguity in the specifications and drawings, owners will be more
confident that they will receive responsive, competitive bids from well-informed contractors.
1.
INTRODUCTION
All too often, the roles of design professional and contractor are viewed as completely separate and even
adversarial. Design practice cannot evolve to achieve cost-effective project construction without feedback
from the contractors who actually build our designs. This paper attempts to harvest the reflections of
some of the most experienced and successful microtunneling and pipejacking contractors to build a set of
suggestions for improving design practice. The reflections and suggestions of these construction
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professionals have been divided into categories, because Engineers like lists and categories, to help us
compartmentalize our thoughts. The categories are:
1. Geotechnical
2. Drawings
3. Specifications
4. Shafts
5. Obstructions
6. Dispute Resolution
7. Surveying
8. Guidance and Steering
9. Annular Space Grouting
10. Intermediate Jacking Stations
2.
DISCUSSION OF TOP 10
2.1
GEOTECHNICAL
The foundation of any new trenchless construction work is a thorough understanding of the subsurface
conditions. Without accurate knowledge of the conditions to be encountered, an engineer cannot
properly prepare a design that will minimize construction risk and potential construction claims. Further,
insufficient or inaccurate geotechnical information hinders contractors’ abilities to prepare responsible,
competitive bids. A contractor must consider risk due to unknown or difficult conditions when preparing a
bid. If inadequate geotechnical information is available, the contractor is forced to make the difficult
decision of either adding significant additional cost to his bid (likely making his bid uncompetitive) 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 or undiscovered geotechnical
conditions by disavowing or not sharing boring data obtained during design, and instead requiring that the
contractor procure their own geotechnical information. This practice rarely, if ever, accomplishes the
intended goal, and is an unfair burden to the contractor. It is unreasonable to expect a contractor bidding
on a project to procure a driller, access agreements, and any necessary permits to perform a
geotechnical investigation within the 30 to 60 days typically allotted for a bid. Additionally, in the
competitive bid atmosphere, a responsible contractor who spends the money to perform their own
investigation is likely to be underbid by a contractor who chooses to make an optimistic bid without
obtaining any geotechnical data. Finally, most courts have upheld the contractor’s right to rely on
geotechnical data obtained during design.
Contractors we spoke to were vocal regarding boring spacing and locations. They felt that at a minimum
borings should be spaced at 500 feet or less. Additionally, borings should always be performed at
planned shaft locations. Such borings not only allow for a good understanding of the conditions to be
encountered when building the shaft, but the findings of the borings can be compared to the conditions
encountered in the larger excavation during shaft construction. This allows the contractor to better
prepare for conditions that may not have been possible to capture in a small-diameter boring.
Additionally, a suggestion was made related to sampling procedures for large-grained soils. The
contractors suggested that standard practice should call for alternative sampling methods such as largediameter borings or test pits if small-diameter methods indicate that cobbles and boulders may be
present, so that accurate grain-size distributions can be determined. A large auger-boring rig is shown in
Figure 1. In addition, contractors remarked that borings should always extend at least 10 feet below the
proposed pipeline invert, to confirm conditions below the pipeline. This approach allows informed choices
about whether more favorable soils can be expected below the design grade, and also helps the shaft
designer in evaluating foundation conditions.
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Figure 1. Large-diameter soil sampling being utilized to characterize cobble and boulder soils.
Finally, contractors expressed the importance of obtaining borings even in hard to access locations.
Many times trenchless construction is used to avoid disturbance to surface features such as highways,
railways, or waterways and it is tempting to avoid borings in these locations to further minimize disruption.
However, doing so may create a significant data gap. This is especially true of long river crossings.
Borings located only near the 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
can be expensive (compared to borings in more accessible locations), the costs involved in most differing
site condition claims, or the costs of recovering an MTBM that becomes stuck beneath a river can easily
be far higher. The overall message is that obtaining complete, high quality geotechnical information is
invariably less expensive and is money well spent. The first point is,
“You pay for borings whether you drill them or not! Money saved on borings not drilled
will be spent on risk contingency and claims defense.”
When asked about soil properties and laboratory testing data that contractors felt were important in
planning a microtunneling project, they indicated that blow count, grain-size distribution, and plasticity of
cohesive soils were the most important parameters. Stabilized (24-hour) groundwater levels are another
important piece of data for bore planning. On certain projects, some less common kinds of testing can be
useful such as swelling potential of high plasticity or overconsolidated cohesive soils or quartz content of
potentially abrasive granular soils. When microtunneling in rock, unconfined compressive strength,
jointing/fracturing (RQD), weathering, and mineralogy were reported as the most important parameters.
Slake durability of shales and other clay/silt/mudstones can be useful in selecting slurry separation plant,
and Cerchar abrasivity of certain rock formations can also be useful in selecting cutting tool types and
predicting tool life.
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A pet peeve of contractors (and the authors) is borings that are not surveyed after completion and
consequently boring logs that do not include surface elevation. Without accurate documentation of the
surface elevation of a boring, the conditions to be expected in the tunnel horizon can be significantly
misrepresented. While certain project settings may offer relatively flat ground where topographic
contours from base maps could be used to estimate boring elevations, many projects have significant
variations in surface topography. Too often boring locations are added to drawings using rough
approximations of distances to landmarks. On projects where roadways are elevated on embankments,
where levees are present, or where the ground undulates, such approximations can dramatically change
the assumed subsurface profile if the borings are located incorrectly. Figure 2 is an example of a good
quality boring log.
Figure 2. A good example of a boring log showing key information for microtunneling projects.
“Borings should be surveyed in all three dimensions to a precision of at least one foot and
the elevations should be recorded on the final boring logs. The surveyed locations of the
borings should be shown on the plan view of the contract drawings.”
Finally, we discussed geotechnical baseline reports (GBR’s) with contractors to get their impressions on
the effectiveness of GBR’s. In general, contractors appreciated the concept of the GBR and liked having
a “line in the sand” to use as a basis for identifying differing site conditions. However, they commented
on the need for specific baselines to either follow logically from the data presented on the boring logs, or
to be accompanied by a reasonable explanation for any significant deviations from the boring data. They
felt that sometimes baselines are set that are dramatically more conservative than the conditions
indicated in the borings. Unless a justifiable explanation is presented to discuss the discrepancy, a
responsible contractor will likely be underbid by a contractor who hopes to encounter the more favorable
conditions shown on the boring logs. So the next point is,
“Geotechnical baselines must be rationally derived from and related to the geotechnical
data.”
2.2
DRAWINGS
Plan and profile drawings represent the most basic and important portion of the contract document
package. For a microtunneling project the drawings must include the basic design information of bore
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length, depth, invert elevations, and slope. However, a drawing set has the opportunity to provide far
more important and essential information to the contractor, thus leading to the first point regarding
drawings.
“Include as much information as possible in the drawing set.” (Corollary No. 1 –
Contractors don’t always read specifications.)
Contractors are always under tight time constraints to put together their bid packages, and being faced
with a thick specification book to wade through looking for key project information is daunting at best.
The interests of both the contractor and the design team are best served by presenting as much key
information as possible on the plans, without creating excessive clutter. This is especially true for design
features that may differ from the norm. Some items that are always valuable to show on the drawings
include: boring locations in plan view, boring logs in the profile view, approximate shaft sizes and
locations, allowable pipe materials, design requirements such as ground improvement or contact
grouting, settlement instrumentation and monitoring, and staging or work areas. For projects with multiple
bores a useful method for providing a lot of information on the plans is to include a shaft and trenchless
crossings table. As illustrated in Figure 3, a trenchless crossings table can convey detailed information
on various parameters of the shafts, bores, casing and/or carrier pipe, ground conditions, as well as
providing a location for specific notes for each shaft or bore.
Figure 3. Example of Shaft and Trenchless Crossings Table
Vertical excavations are a requirement for almost all microtunneled crossings. However, the design of
microtunneling shafts is almost always the responsibility of the general contractor, as they are typically
temporary features. Therefore, any illustration of shafts on the drawings should be approximate and
clearly noted as such. However, while the contractor will be ultimately responsible for design and
construction of the shafts, the designer still needs to take into account the space and logistical needs. A
basic shaft outline should be included on the drawings that captures the reasonable maximum outside
dimensions of the support system that will be constructed. This is an important consideration during the
design phase to ensure that adequate space is available to construct the shaft, considering utilities, traffic
control, and easements/ rights-of-way. The outline should reflect the likely shaft shape (round vs.
rectangular), or the larger of the two if either is allowed. As previously mentioned, the drawings should
also note that while the shaft outline shown is there to reflect that a shaft can be successfully constructed,
the exact size and location is the responsibility of the contractor and needs to be such that the pipeline
can be constructed as shown on the drawings and the tunneling machine can be retrieved.
“Show approximate shaft locations that are reasonable and appropriate for the work.”
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An often overlooked consideration on microtunneling projects is adequate work areas for shaft
construction and tunneling operations. Microtunneling equipment layout is very flexible, allowing for many
varied layouts to fit available space. However, while the shape of the area can be flexible, contractors
told us that the minimum area needed is about 7,000 square feet for installing small diameter pipe of
approximately 24 to 48 inches. As the installed pipe size increases to the high end of microtunneling
capabilities at 72 to 96 inches, the minimum jacking shaft work area increases to 10,000 square feet.
Contractors went on to discuss some important details of the work area that they felt are overlooked on
some projects. It is important that work areas have appropriate access for the type of equipment that will
be coming and going from the site, including spoil haul trucks, pipe delivery trucks, cranes, and front-end
loaders. Consideration must be given to whether vehicles can enter and exit in one direction or whether
the trucks will need to attempt to turn around to leave the site. Finally, the overhead constraints of a
particular work area are extremely important. Much of the equipment used on a microtunneling site has
high clearance requirements, or will be placed using cranes. When an otherwise appropriate work area
has overhead utility lines crossing the work space, or large trees that overhang the area, the usefulness
of the site is severely hampered. By pre-planning work areas at each shaft location and delineating these
on the plans, all parties can understand the constraints that may exist and the contractor can plan
appropriately for any special equipment or layout needs. Figure 4a shows a well- delineated work area
for a microtunneling shaft. Figure 4b demonstrates the use of off-site storage areas for situations where
adequate space may not be available immediately adjacent to the shaft.
“Plan for and show reasonable and adequate staging areas on the drawings.”
Figure 4a. A well-delineated shaft work area showing flexible use of available space. Though not
necessary for all projects, traffic control details are also shown on this sheet.
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Figure 4b. Off-site storage areas, as depicted in the photograph, for shaft materials or jacking pipe
can be effective when work areas at shafts are constrained.
2.3
SPECIFICATIONS
Far too often, engineers spend most of their time and budget on preliminary design, permit acquisition,
and drawings, and feel compelled to just grab the specifications from the firm’s last project that may have
been remotely similar and throw them in the manual; sometimes they do remember to change the project
name in the footer. Indeed, design costs for projects are routinely based on the number of sheets
multiplied times a unit price per sheet. Some owners are comfortable with this approach.
The problem is that trenchless construction project specifications need to be carefully crafted to address
the unique aspects of the project. A one-size-fits-all approach to specification preparation, such as
illustrated in Figure 5, seldom turns out well. Even if the engineer recognizes that certain unique aspects
need to be addressed in the specifications, the approach all too commonly used is to just throw it in there
in several locations, to make sure it is covered. So the first point is,
“Tailor the technical specifications to the project.”
SECTION 02669
Master Specification - All Purpose Pipe
1.0
REQUIREMENT
1.1
All pipe used on OWNER’S pipeline contracts shall meet the
requirements of “Master Specification - All-Purpose Pipe”. No substitute
specifications of “Specific” Pipe for specific projects and applications
shall be allowed without the written approval of the ENGINEER.
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2.0
PRODUCTS
2.1
MATERIALS
2.1.1
2.1.2
2.1.3
2.2
All pipe is to be made of a long hole, surrounded by metal,
concrete, or plastic centered around the hole.
The long hole in all pipes shall be hollow throughout the entire
length. Holes of a different length than the solid stuff surrounding
the hole in the pipe shall not be used.
All pipe shall be supplied with nothing in the hole for the entire
length of each pipe, so that water, sewage, steam, electricity, or
other stuff can be put inside at a later date. Water, sewage,
steam, electricity, or other stuff shall not be co-mingled within
individual pipes. Rather, separate pipes shall be used for
separate products.
DIMENSIONAL TOLERANCES
2.2.1
The inside diameter (I.D.) of all pipe shall not exceed the outside
diameter (O.D.). Pipe with ID larger than OD shall be rejected, as
the hole will be on the outside of the solid stuff. Contractor shall
remove all rejected pipe and replace with pipe with the ID
smaller than the OD, at no additional cost to the OWNER and
without schedule extension.
Figure 5. An example of the one-size-fits-all approach to technical specifications.
What we heard from contractors is that it would be very helpful to be concise, logical, and precise. Avoid
redundancy, and put things where they belong. A specific example is that 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.
The second point is,
“Be concise, precise, and avoid redundancy. Organize the specifications logically
according to which contractor is likely to execute the work.”
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 (this time). Therefore, the
third point is,
“Allow only options that are feasible for the ground conditions and project requirements.”
A pet peeve of contractors we talked to is the requirement for calculations to be stamped and signed by a
professional engineer registered in the state where the project is being built. Of course, this requirement
is necessary in some instances, such as for design of the shaft supports, carrier pipe, or other permanent
features, but may be unnecessary and costly in other cases. For example, the microtunneling contractor
is likely the most experienced and competent party to prepare jacking force calculations. Routinely,
however, the contractor must hire a PE who may have little or no direct experience, teach them what they
need to do, and pay them to submit the calculations. The fourth point is then,
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“Avoid requiring calculations be stamped by a licensed PE where unnecessary.”
Contractors appreciate Engineers who are reasonable. Engineers and owners must accept responsibility
for ensuring that ground conditions are compatible with the method specified, that adequate work areas
are available, that diameters and drive lengths are achievable, and work hours and time for completion
are compatible with the work to be done. While world records may get you a cover story in Trenchless
Technology magazine, Bernie Krzys is not going to bail you out if your attempt at longest, biggest, and
baddest doesn’t go well.
“Be reasonable.”
2.4
SHAFTS
We added shafts as a separate category because all too often, microtunneling construction problems
originate with shafts. Only methods that are compatible with the ground and groundwater conditions
anticipated should be allowed. The corollary is don’t allow shaft construction methods that are not
compatible with the conditions anticipated, and don’t leave selection of shaft construction method entirely
to the contractor, to avoid having to accept a low bid with less expensive methods that result in high risk
of failure. Rather, place all bidders on equal footing by listing only those shaft types that are compatible
with the anticipated conditions. Figure 6 illustrates various shaft types compatible with different ground
conditions. Therefore the first point regarding shafts is,
“Specify only shaft construction methods that are compatible with ground and
groundwater conditions anticipated.”
Figure 6. Photographs of Various Shaft Types Appropriate for Different Ground Conditions.
Figure 6a (above) shows an auger-drilled shaft, appropriate for high groundwater conditions.
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Figure 6b. (left) shows a soldier pile and lagging shaft for conditions with little or no external
groundwater pressure, including dewatered sites. Figure 6c. (right) shows a sheetpile shaft for
high groundwater conditions.
Microtunneling contractors would like to see portal stabilization measures specified either in a separate
specification or in the shaft specification, so the general contractor or shaft subcontractor understands
their responsibility to provide stable entry and exit portals for the microtunneling contractor. Too often, the
general contractor does not understand the microtunneling contractor’s needs for a stable, dry shaft, and
fails to ensure measures are taken to preclude excessive groundwater and soil inflows. Portal
stabilization can usually be achieved through a number of methods, as illustrated in Figure 7, from
grouting to use of double walls, so performance criteria can be effectively crafted to ensure that the
portals are adequately stabilized. For example, the performance criteria can limit the quantity of
groundwater inflows to less than 5 gpm, and limit soil inflows to less than 2 cubic feet. The specification
can also require demonstration that the performance criteria have been met by imposing an incremental
process for opening the entry and exit holes. During the incremental process, if groundwater or soil
inflows exceed specified limits, the holes can be sealed and additional stabilization measures can be
implemented to ensure stable conditions. The second point regarding shafts is,
“Specify portal stabilization criteria that ensure the microtunneling contractor can safely
execute the work, without imposing means and methods.”
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Figure 7. Examples of portal stabilization measures. Figures 7a (left) and 7b (center) illustrate
entry and exit rings, respectively, with a steel frame and rubber seal ring. Figure 7c (right)
illustrates a double-wall (guillotine wall) entry portal.
Shaft bottom stability is essential for microtunneling. Unstable shaft bottoms can result in boiling and
flooding, line and grade problems, and inability to complete the drive. Ground and groundwater conditions
must be thoroughly investigated to evaluate the risks for bottom heaving and boiling. Again, performance
criteria can be included in the shaft specification to ensure that the shaft bottoms are stable. Performance
criteria should address design groundwater elevation, allowable differential external and internal
groundwater elevations, groundwater monitoring requirements, the need for a tremie plug, and minimum
factors of safety against uplift. Detailed submittals should be required of the contractor for complying with
the performance criteria. In addition to establishing performance criteria, it is advisable to specify that a
concrete working slab shall be installed with a recessed sump pump for handling nuisance water inflows
in all jacking shafts. A concrete slab can be an option in reception shafts. The third point regarding shafts
is,
“Specify performance criteria for shaft bottom stability and specify a concrete working
slab in jacking shafts.”
The issue of treatment and disposal of nuisance groundwater inflows into shafts has become increasingly
important because of constraints on groundwater discharge permits. Again, it is unfair to expect the
contractor to put money into the bid for treatment and disposal of nuisance inflows, unless the issue is
addressed in the specifications. If constraints exist for discharge quality, quantities, and locations, the
constraints should be identified in the specifications and permits. If off-site hauling and disposal are
required, they should be specified, to place all bidders on equal footing. So,
“Identify any constraints regarding groundwater discharge quality, quantities, and
locations.”
Perhaps the most passionate input we received from microtunneling contractors regarding shaft
construction had to do with project communication. Microtunneling contractors, who are often
subcontractors that don’t construct their own shafts, reported that too often shafts are 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 (typically two to four feet). The shaft may not have a watertight face that will prevent slurry
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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 two to five feet of pipe to penetrate the shaft seals and stick
into the shaft. Other common problems include insufficient staging area around the shaft, no concrete
floor slab, and no portal stabilization as discussed in previous sections.
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.
Those interviewed asked that requirements be put in the specifications requiring that certain critical items
regarding the shaft design and construction be coordinated with the microtunneling subcontractor before
the submittals are turned in. The authors have used coordination requirement language in previous
specifications to some positive effect. However, an as yet untested suggestion by the authors is to
require that the general contractor submit an affidavit signed by him and all relevant subcontractors
indicating that the shaft design and procedures have been discussed by all parties and accepted by the
microtunneling sub as adequate for his work.
“Promote/require coordination between the general contractor, shaft subcontractor, and
microtunneling subcontractor.”
2.5
OBSTRUCTIONS
Obstructions prevent the continued advancement of the MTBM and can lead to costly delays.
Obstructions usually require that a rescue shaft be sunk to dislodge or remove the obstruction.
Obstructions are one of the most common reasons for microtunneling disputes and claims. Contractors
strongly suggested that the definition of obstructions and the contractual mechanisms for dealing with
obstructions be reconsidered by Engineers and Owners. As microtunneling practice has evolved, it has
become common to define obstructions as objects encountered partially or completely within the
microtunneling machine’s path that exceed a certain percentage of the diameter of the MTBM, such as 25
to 33% of the MTBM OD. A minimum strength criterion is also often specified, such as unconfined
compressive strength exceeding 30,000 psi. The contractors generally thought this definition was unfair
because it fails to recognize some relevant circumstances. For example, they pointed out that numerous
small cobbles can collect in the crushing chamber and cause the MTBM to stall, as illustrated in Figure 8.
They also noted that the openings in the cutter wheels of some MTBM’s would accommodate objects of
only up to about 20 to 22% of the MTBM OD. The contractors suggested that Engineers develop a more
relevant and fair definition for obstructions, but were not specific as to what the revised definition should
be. The key point is,
“Clearly and reasonably define obstructions and how obstructions will be contractually
addressed. Be reasonable in what can constitute an obstruction.”
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Figure 8. Photograph looking down a three-foot diameter borehole at an MTBM obstructed by
large quantities of gravel, cobbles, and boulders. This MTBM became stuck despite no individual
rocks larger than 30% of the MTBM OD being encountered.
2.6
DISPUTE RESOLUTION
Dispute resolution implies that a dispute exists regarding differing site conditions, defective specifications,
quantities, or other issues. Unfortunately, disputes are far too common for underground construction,
including trenchless construction. The point we heard emphatically from contractors we interviewed was
that it would be extremely helpful if Owners and Engineers would proactively engage in investigating the
alleged problem, crafting a solution, and (Owners aren’t going to agree on this point) 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. Others, not so much. 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.
They appreciate it even more when Owners and Engineers shed their cloak of plausible deniability and
engage constructively to actually solve problems.
“Time is indeed of the essence! Jointly investigate the cause of problems and engage
proactively with the contractor to find a solution.”
2.7
SURVEYING
Surveying is a basic requirement for all pipeline projects, and perhaps because of this, it often does not
receive the attention it deserves. Far too many microtunneling projects have wandered astray because of
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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.
They also pointed out the need to require accurate as-built surveys of the shafts, including the bottoms of
the shafts. Shaft bottom elevation is critical to allow the contractor to complete the drive at the correct
grade and elevation. Shaft bottom position is also important to account for deviations from plumb.
Therefore,
“Accurate surveys are critical for success. Specify that surveyors shall be experienced in
microtunneling work, and that accurate as-builts of the shafts shall be provided before
microtunneling commences. Specify quality control checks to ensure accurate surveys.
Measure twice; cut once is the carpenter’s axiom.”
2.8
GUIDANCE AND STEERING
One of the highly touted advantages of microtunneling is the high level of accuracy with which pipe can
be installed. A reference laser and active target system combined with a fully-articulated steering head
are typically used, and allow a microtunneled pipeline can be installed to within plus or minus one inch of
design line and grade. However, while such accuracy is achieved on many projects, contractors wanted
to discuss some of the challenges of achieving these results.
Laser guidance, illustrated in Figure 9, remains by far the most common system used for microtunneling
in the United States. While these systems can work very effectively for large pipe and short to moderate
drive lengths, smaller-diameter pipe and long drive lengths can cause difficulties for laser systems. For a
laser's reference line to remain straight, true, and focused requires consistent air density and moisture
content over the length of the beam. The environment that develops inside the jacking pipe during
microtunneling operations is not always conducive to these needs. During a shift the air inside the tunnel
becomes warmer and more humid, especially near the MTBM. The differences in density between the
cooler, drier air near the shaft and the warmer, moister air near the target can cause refraction of the
laser beam, resulting in a skewed reading of line and/or grade. Contractors can, and do, use ventilation
schemes to minimize these effects, but in smaller pipe (~48 inches and less) and long drives (~700-800
feet or longer) the effects of temperature and moisture variations are difficult to control. The first message
that we heard from contractors with regard to guidance and steering was:
“Evaluate required and achievable line and grade tolerances realistically”
Contractors felt that too often very tight tolerances were required on projects where such accuracy wasn't
actually needed. It was suggested that when the pipeline will operate under forced flow, or if an
oversized casing pipe is being installed initially, that line and grade is probably not critical and that
allowing tolerances of plus or minus two to four inches can help improve contractor productivity and
therefore bid prices. The second point made by those we interviewed was:
“Include the necessary requirements in the specifications to achieve high accuracy”
If the project does indeed require high accuracy, in conjunction with small pipe diameter and/or long drive
length, the design needs to provide appropriate features and include requirements for the equipment and
methods necessary. New guidance systems are beginning to enter the US market that allow for
significantly improved reliability for long bores where traditional lasers may have difficulties. Gyroscope
systems, sometimes combined with electronic water levels, illustrated in Figure 9, 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
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gravity sewer projects where grade is critical; the inclusion of minimum 0.1-foot drops across manholes
(between adjacent microtunnel bores). If 0.25-foot drops can be accommodated, so much the better.
Figure 9. Examples of a traditional laser guidance system (left) and a new gyro-compass and
electronic water level guidance system (right).
Another item that contractors felt was important with regard to steering of the MTBM was sufficient radial
overcut. All MTBM’s are set up to create a bore diameter that is larger than the maximum outside
diameter of the jacking pipe to be installed. The machine overcut creates an annular space around the
advancing MTBM and jacking pipe that provides several benefits including reducing skin friction, allowing
for the injection of lubricant, and providing space for the front section of the MTBM to articulate, facilitating
steering. Designers may be inclined to limit the overcut that a contractor is permitted to use due to
concerns about systematic settlements of features above the bore. However, without sufficient open
space around the MTBM, the cutting head may not be able to articulate and perform steering corrections.
The contractors we spoke to indicated that they felt a minimum radial overcut of 0.75 inches was
necessary in most cases to ensure proper steering response. Conversely, too much radial overcut can
also hamper steering response, if the deflection of the trailing portion of the MTBM is not opposed by a
reaction force from the bore wall. Contractors’ opinions varied regarding maximum overcut that can be
used. Maximum recommended values of radial overcut ranged from one to two inches, with the
appropriate value increasing with increasing jacking pipe diameter and soil stiffness.
“Specify appropriate minimum and maximum radial overcuts.”
2.9
ANNULAR SPACE GROUTING
The contractors we interviewed generally expressed their beliefs that annular space grouting between the
pipe and bore wall was unnecessary and ineffective. They expressed the opinion that grout take was
almost always negligible, and that grout injected under reasonable pressures could not expel bentonite
lubricant already in the annulus. However, most also admitted that grout mixture, ground conditions and
elapsed time between completion of the bore and attempting to grout were important factors. Contractors
generally recognized that grouting of the annulus must sometimes be specified and attempted to satisfy
permit requirements for critical highway and railroad crossings. In addition, they did not strongly object to
attempting to grout the annulus, but generally believed that it was unnecessary and ineffective. The key
point is for grouting to be effective; it must be undertaken very soon after completion of the bore, before
the ground has closed in around the pipe. It is also important to specify appropriate materials to obtain a
flowable mix that can achieve good filling of the annular space. One contractor suggested that a mix of
cement and bentonite, with no coarse or fine aggregate (sand), worked well for filling the annular space.
Therefore, if grouting is necessary to satisfy permit requirements and limit settlements,
“Specify annular grouting and establish a time limit for its completion.”
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2.10
INTERMEDIATE JACKING STATIONS (IJSs)
Contractors generally agreed that IJSs, as shown in Figure 10, are good insurance for long drives and
high jacking forces, especially where there is no convenient access for a rescue shaft. They agreed that a
reasonable strategy for IJS’s is to specify that one or more fully assembled IJS’s be on-site for drives
exceeding a certain length, and their use be specified when jacking forces reach a certain level. For
example, it seems reasonable to specify that at least one fully assembled IJS be on-site for drives
exceeding 600 feet, and at least two be on-site for drives exceeding 1,000 feet. Likewise, it seems
reasonable to specify that an IJS be deployed when jacking forces reach 70% of the capacity of the
lesser of the pipe capacity, the jacking frame capacity, or the thrust block capacity. The recommendation
is,
“Establish criteria in the specifications for the use of IJS’s based on drive lengths and
jacking forces experienced.”
Figure 10. Photographs of intermediate jacking stations (IJSs).
3.
SUMMARY AND CONCLUSIONS
The objective of this paper was to harvest the reflections of experienced skilled microtunneling and
pipejacking contractors and present their views regarding how engineers can improve microtunneling
design practice. 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. Clearly, it is in everyone’s best interest to improve design practice and reduce claims and the
adversarial relationships that permeate microtunneling and other areas of construction. We hope the
audience will reflect on the issues presented, and consider implementing those that make sense for their
projects. Constructive interaction among Owners, Engineers, and contractors is essential for improving
practice.
4.
ACKNOWLEDGEMENTS
The author would like to formally acknowledge and thank the contractors that contributed directly to this
paper. Personnel from Nada Pacific Corporation of Caruthers, California and Vadnais Corporation of
Vista, California, along with Steve Christy, an independent consultant with many years experience in
operating MTBM’s, supervising microtunneling projects, and providing inspection services, all generously
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contributed their time to discuss their views on the issues presented in this paper. I also wish to
acknowledge Michels Corp and Pacific Boring personnel who have offered helpful comments and advice
over the years.
Additionally, the author wishes to thank Matthew Wallin, Mary Asperger, and Kathryn Wallin of Bennett
Trenchless Engineers for their significant contributions to the genesis and preparation of this paper.
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