Evaluating Differing Site Conditions For A Microtunneling Project (Part 4)

By Gary Brierley, Todd Kilduff and Becky Brock

So far in Parts 1, 2, and 3 we have discussed the Owner’s responsibilities for preparing a contract document for a microtunneling project, the Contractor’s responsibility for preparing its bid based on a “reasonable and prudent” interpretation of that document, and what happens when the Contractor for a microtunneling project encounters what it believes to be a Differing Site Condition (DSC) during construction. Part 4 will provide a compendium of discussions about the various types of DSC claims for microtunneling projects based on the author’s combined experience of more than 100 years of dealing with this specific issue.

There are lots of reasons why microtunneling projects run into trouble during construction that can be related in one way or another to a DSC such as the following:

  1. Obstructions/Cobbles and Boulders (strength, size, nesting, steering issues)
  2. Sticky and/or Swelling Clays (muck removal/clogging/push pressures)
  3. Highly Abrasive Granular Soils (cutter changes)
  4. Cemented Soils/Caliche (steering issues/advance rates)
  5. Harder ground (push pressures/advance rates)
  6. Excessive Jacking Forces (pipe failures)
  7. Mixed Face (steering issues/settlement)
  8. Soft Soils Below the Invert (diving)
  9. Third Party Impacts (settlement/fracout)
  10. Contamination (hazardous/materials handling)

Many of these conditions can be identified ahead of time, with a “good” subsurface investigation, and with prior knowledge the Contractor can often modify its means and methods for a successful microtunnel project, but if any of these conditions are unknown at the time of bid then a DSC will likely ensue. Encountering a DSC on an MTBM project can cause damage, downtime, and often even require rescue operations. “Obstructions” come in all shapes and sizes and are composed of all sorts of different materials but, in general, obstructions are not a good thing for a microtunneling project. Individual boulders and/or nested cobbles and boulders can cause lots of damage to an unprepared MTBM, and cobbles and boulders in soft ground are generally not an MTBM option. MTBMs with enough power and with a well-equipped face configuration can penetrate cobbles and boulders but that usually requires an MTBM that is at least five feet in diameter. Smaller MTBMs can be stopped dead in their tracks by a boulder. Sometimes you can drill a hole from the ground surface in the soil next to a boulder and push it out of the way and/or break it up and remove it from the face. Other types of “obstructions” include trees, tiebacks, well casings, abandoned utilities, old foundations, and debris within fill deposits such as within embankments for highways or railroads.

Sometimes, the possibility for an obstruction will be indicated by the geologic setting such as in glacial till, in a weathered rock profile, or by obstructions discovered in adjacent completed tunneling projects. In any case, a detailed analysis of proposed construction activities along the tunnel alignment is required and the tunnel may be placed deeper into the ground depending on what is discovered, if possible. And, finally, pay items or allowances could be included in the contract document in order to compensate the Contractor for obstruction removal.

Sticky, swelling and/or squeezing deposits of clay can also cause problems for a MTBM if not identified ahead of time. Frequently, these problems can be compounded when a clay is both sticky and swelling and/or swelling and squeezing. Two types of problems are caused by these clay deposits; problems with muck removal at the face and problems with adhesion along the pipe being pushed into the ground. Problematic clay deposits at the face must be “conditioned” or “water jetted” in order to reduce clogging for a slurry MTBM. If sticky, swelling, and/or squeezing deposits of clay come into contact with the pipe, then the thrust forces required to advance the MTBM will increase dramatically; possibly causing the pipe and/or the push frame to fail. Obviously, a good bentonite injection system is essential to minimize this problem but, sometimes, Mother Earth simply refuses to cooperate. If this type of problem is anticipated along the tunnel alignment then is possible to increase the tunnel overcut in order to minimize the impact and, possibly, avoid a DSC.

Highly abrasive soils must be sampled and tested during the subsurface investigation in order to warn about this potential problem in the contract document. Cutter changes for an MTBM are time consuming and expensive and must be accounted for during the bidding process. Alternate cutter materials and adequate soil conditioning can be used to minimize this problem but soil conditioning, in and of itself, is also expensive. Highly abrasive soils can also cause many other types of problems for an MTBM such as the erosion of steel surfaces at the face and throughout the shield and throughout all of the muck handling components that are difficult and costly to repair once the MTBM is surrounded by soil.

Another source of face obstruction could be a cemented soil condition such as caliche. Deposits of caliche have been encountered with a UCS strength of as much as 4,000 psi. Caliche can cause the tunnel to be diverted, can result in highly abrasive face conditions, and/or can result in the ponding of perched water in the vicinity of the tunnel. Lenses or layers of caliche can be discovered by the test borings but the lateral extent of those deposits is difficult to determine.

Another form of frictional resistance along the pipe could be loose deposits of silt or sand that are being washed into the overcut by groundwater pressures and a bentonite injection operation with adequate face pressures would be required in order to deal with this unforeseen problem. This problem is also much more difficult to control if the MBTM is forced to tunnel in a downhill orientation.

High to very high push pressures can also result in the failure of the pipe being used to advance the MTBM. Pipe failure can be catastrophic for an MTBM tunneling operation and can be caused either by high push pressures and/or by defects in the pipe itself. Pipe construction for an MBTM tunneling operation must be performed to exacting quality control standards in order to avoid the possibility of pipe defects. This is even more important if the jacked pipe is intended to serve as the final lining or if the tunnel is being excavated below a highway or a body of water.

It would seem that adequate face pressure should be sufficient to prevent loss of soil at the face of excavation but that is not always the case. Artesian water pressures can cause soils to flow into the face and mixed-face soil conditions can also be a problem. Soil profiles with glacial till at the invert and flowing sand near the crown will allow additional sand to enter the face from above with each face rotation and result in a sinkhole formation. Stiff clay at the crown with flowing silt or sand at the invert can result in loss of soil at the tunnel invert and, thereby cause the MBTM to dive. If possible, it is best to avoid mixed-face tunneling. Another good way to prevent the MTBM from diving is to perform ground improvement prior to tunnelling, i.e., to install a row of jet grout columns directly below the tunnel invert prior to the start of tunneling.

A detailed analysis of possible third party impacts along the proposed tunnel alignment must also be performed. In general, pressurized face tunneling is a good way to avoid soil loss at the face and the ground deformations resulting from those losses, but, as described above, not always. Sometimes, especially for sensitive third party impacts such a large diameter sanitary sewer, it might be best to use ground improvement such as chemical grouting in advance of tunneling to provide an extra measure of protection for those types of third parties. Detailed and continuous monitoring of soil movements above the tunnel crown is also required to make certain that the microtunneling operation is being performed in a proper manner. Or, it might be possible to make allowance in the contract document for either compensation or compaction grouting in order to prevent soil losses at tunnel depth from causing problems higher up in the soil profile.

It is also necessary to make certain that a stable mass of soil exists both at the tunnel entrance and exit locations either by locating the tunnel in competent ground or by stabilizing that soil by grouting or freezing. A pressurized face tunnel needs to penetrate a certain distance into the ground before adequate face pressures can develop and if that is not the case then sinkholes can result near the entrance and exit locations. It is also necessary to make certain that the tunnel is in almost perfect alignment with the exit collar at the exit shaft. It is highly problematic and expensive to “rescue” an MTBM that becomes stuck just outside the exit collar.

And, finally, the last item that must be given serious consideration is the possibility of encountering contaminated and/or hazardous soil or water deposits during microtunnel construction. Many geotechnical engineering companies are not qualified to evaluate this problem and an environmental consultant will need to be retained during design in order to evaluate the possibility for encountering contaminated ground during tunneling. Contaminated soil or water or the occurrence of hazardous gases such as methane or hydrogen sulfide can add significantly to the risks and the costs of a tunneling operation.

In addition to the above, numerous other considerations must be taken into account when bidding a microtunneling project. For instance,

  • Does the contract contain a valid DSC clause? By valid, we mean a DSC clause that is comparable to the standard Federal Type I and Type II clause and, therefore, compatible with all of the judicial decisions that have been made with respect to that clause. Poorly worded and/or downright defective DSC clauses may not be enforceable in the event of a claim.
  • Is there a GBR, and is it compatible with the project plans and specifications? Is the tunnel located in the types of ground characterized by the GBR and are those ground conditions compatible with the type of tunneling being required by the project specifications? When studying the contract document the Contractor must make a conscious decision about whether or not the work being specified is doable based on the Contractor’s background and experience. If not, then the Contractor must seriously consider if the project can be accomplished without major claims.
  • What type of surface access is available along the tunnel alignment? If the tunnel is located below a body of water, a major highway or railroad, or is located in a densely populated urban area, then the ability to access the tunnel for possible repairs or even to make test borings for proof of a DSC will be extremely limited. An MTBM that runs into trouble for any reason below a body of water or a major highway may well represent a case for project termination.
  • Has the microtunneling Contractor been retained directly by the Owner or indirectly by a Prime Contractor. Prime Contractor’s can be even more onerous when dealing with their subs as compared to an Owner. Does the sub agreement contain a valid DSC clause and what other provisions of the prime agreement are being “flowed down” to the proposed subcontractor. At times, some of these “flow-down” provisions may seem innocuous but the severe imposition of those provisions can result in highly unfavorable legal interpretations. And, finally, will the Prime Contractor even allow its sub to make a DSC claim against the Owner?
  • Sometimes it may be preferable to make a claim for damages or delays under a different form of claim as compared to a DSC. For instance, if the specified MTBM is running into trouble it may be better to file a claim of defective specification or implied warranty. In essence, the project specification has at least “implied” that the specified method of tunneling is appropriate for the ground conditions baselined in the GBR and it may be that the MBTM specification is wrong and not appropriate for the ground. Claims can also be made based on the Spearin Doctrine, superior knowledge, unjust enrichment, and/or cardinal change instead of a DSC.
  • It is also necessary for a microtunneling subcontractor to realize that the cost and the time for implementing litigation against a large Owner or a well-established Prime Contractor, may not be feasible. Out-of-pocket litigation costs for attorneys and experts and the need for the in-house diversion of assets may be simply cost prohibitive for a microtunneling subcontractor. It is much better to pick and choose your projects up front in order to avoid and/or to at least minimize the possibility of a major DSC claim.
  • And, finally, the microtunneling contractor must take into account if the contract contains provisions for alternative dispute resolution. Almost any form of alternative dispute resolution is preferable as compared to litigation and indicates a tendency for the Owner to be “cooperative” if trouble does develop for a variety of different reasons.

When all is said and done, however, the microtunnel contractor must satisfy itself at the time of bid that there is a good or even a high probability for success when bidding on a microtunneling project. Contract documents with poorly written specifications, a cover-your-ass GBR, and/or both a tunnel alignment that traverses highly problematic or intensely mixed-face ground conditions should probably be avoided. It may also be a good idea to hire an outside consultant to perform a so-called “prebid” investigation of the contract document in order to obtain a totally independent and objective opinion of potential problems discussed in the project GBR. Sometimes, it might be better for your competition to “win” a “bad” project and to let them suffer the consequences; i.e. getting “caught up” in a win is not necessarily a good thing. Highly decisive evaluations about any possible negative consequences for various parts of the contract document is a major part of bringing home a successful microtunneling project.

One of the primary objectives of this four-part series of papers about differing site conditions was to help all parties involved with microtunneling projects such as Owners, Designers, and Contractors to AVOID DSCs by utilizing thoughtful and proactive contract preparation and bidding practices. As has been said many times, no one benefits from a DSC claim except, possibly, the attorneys.

Gary Brierley is President of Doctor Mole, Inc.; Todd Kilduff is President of Kilduff Underground Engineering, Inc.; and Becky Brock is President of Brock Geo-Consulting, LLC.

Avoiding Differing Site Conditions for a Microtunneling Project – Part 1

Avoiding Differing Site Conditions for a Microtunneling Project – Part 2

Avoiding Differing Site Conditions for a Microtunneling Project – Part 3

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