\ Avoiding Differing Site Conditions - Part 1

Avoiding Differing Site Conditions for a Microtunneling Project – Part 1

The primary purpose of this paper is to discuss how the Differing Site Conditions (DSC) clause affects all aspects of design and construction for a design/bid/build microtunneling project. According to the North American Society for Tunneling Technology (NASTT) microtunneling is defined as a construction method that utilizes a remote-controlled, laser-guided MTBM which is propelled by jacking pipe into the ground from a jacking frame installed at the tunnel entrance. Personnel entry is not required for routine operation and the face of excavation is generally pressurized in order to balance the earth and groundwater pressures encountered during construction. Hence, this paper will focus on tunneling methods where face access is limited especially for smaller diameter tunnels.

In order to address the purpose of this paper as stated above the following four topics will be discussed:

Part 1.0 – The Owners Responsibilities for Preparing the Contract Document – What does the Owner and its design consultant need to accomplish in order to avoid or to minimize the possibility of encountering a DSC during construction of a microtunnel? Stated differently, what are the necessary components of the contract document for construction of a microtunnel that are needed in order to both establish the basis for a DSC and then to provide the contract “indications” needed to serve as the basis for determining whether a DSC was encountered during construction or not.

Part 2.0 – The Contractor’s Responsibilities for Preparing Its Bid – Once the Owner’s contract document is released for bidding purposes then the Contractor becomes responsible for preparing its bid based on a “reasonable and prudent” interpretation of the work necessary to construct the project as defined by that contract. In general, that responsibility involves taking into account all of the requirements presented in the General and Special Provisions, all of the details shown on the project Plans and Specifications, and all of the ground conditions as described in the geotechnical documents. In essence, how much will it cost and how long will it take in order to construct the underground openings that will become part of the Owner’s finished facility?

Part 3.0 – What Happens if an Alleged DSC Is Encountered – What happens during the construction of a microtunneling project if the Contractor encounters what it believes to be a DSC? This doesn’t mean that somebody did something wrong. Subsurface conditions can be complex and it is not unusual to encounter something “unanticipated” during tunnel construction. If all of the professional services as described in Parts 1.0 and 2.0 of this paper have been accomplished in a proper manner, then the Owner has already saved a considerable amount of money during bidding by avoiding huge contingencies for unknown ground conditions and the Contractor will only be paid for the work required to deal with the DSC. Such is the bargain that is made for all tunneling contracts that contain the DSC clause.

Part 4.0 – Case Histories – Based on the above and based on the 100 years of combined experience of the authors of this paper, Part 4.0 of this paper will provide a discussion of the various reasons why a microtunneling project might encounter a DSC and the various ways for a Contractor to deal with those claims. In general, the “concept” of a DSC clause is valid, but things don’t always work out as intended based on a concept.

Anyone involved with tunneling is well aware of the importance of the DSC clause as it applies to tunneling projects. In general, this clause is intended to protect the Owner from paying enormous contingency allowances in the bid for “unknown” ground conditions and to protect the Contractor by providing reliable “indications” of ground conditions in the contract document and anyone reading this paper should be well aware of the basic tenets of the DSC clause as outlined below:

  • A Type I DSC is defined as a subsurface condition that differs materially from those subsurface conditions indicated in the contract document,
  • A Type II DSC is defined as a subsurface condition of an unusual nature that differs materially from those conditions ordinarily encountered or generally recognized as being inherent in the work provided for in the contract document,
  • Upon encountering an alleged DSC, the Contractor is responsible to promptly provide a written notice to the Contracting Officer describing the alleged DSC and, finally,
  • Upon such notification, the Contracting Officer is responsible for promptly investigating the alleged DSC in order to determine if it does, in fact, differ materially from contract indications.

All of the above reads as reasonable and appropriate until it is applied to a tunnel being constructed utilizing an MTBM. For instance, it is not possible to observe the ground being encountered at the face of excavation; only the disturbed material being removed from the tunnel. In addition, it is not possible to observe the soil or rock exposed along the tunnel alignment since that ground is surrounded by the pipe being used to propel the MTBM. How, then, is it possible for either the Contractor to accurately document what subsurface conditions are being encountered during construction and/or for the Contracting Officer to investigate those conditions?

Part 1 – The Owner’s Responsibilities

With the above in mind, it is necessary to turn our attention to what the Owner should include in its contract document for a microtunneling project. In general, the contract document for any tunneling project consists of the five parts listed below:

  • The General/Special Provisions,
  • The Plans,
  • The Specifications,
  • The Geotechnical Data Report (GDR), and
  • The Geotechnical Baseline Report (GBR).

With respect to the primary purpose of this paper, however it is only necessary to dissect the contract for those portions that relate directly to the provision of the underground openings that are needed for construction of the Owner’s finished facility. For instance, it is within the General/Special Provisions where the Owner includes its requirements for the following topics:

  • Differing Site Conditions
  • Subsurface Indications
  • Owner’s Responsibilities
  • Contractor’s Responsibilities

And given below are comments about each of the above four topics.

  • Differing Site Conditions – As stated above, every contract document for a tunnel should contain a DSC clause based largely on the Federal version of that clause as provided above. However, some states such as Massachusetts and California have their own version of the DSC clause and many underground projects are now being procured based on EJCDC (Engineers Joint Contract Documents Committee) contract stipulations. These various clauses are all acceptable but the Owner should be very careful not to “play games” with the DSC clause as any effort by the Owner to negate the intended purpose of the DSC clause will cause all competent contractors to 1) realize that the Owner is not serious about sharing the DSC risk in a fair and equitable manner, and 2) is not willing to work together with the Contractor in order to produce a successful project.
  • Subsurface Indications – It is here that the Owner makes reference to the Contract status for both the GDR and the GBR in a straight forward and positive manner without any attempt to disclaim any portion of either of those reports. In general, both court rulings and DRB decisions have indicated almost unanimously that Contractors have a right to rely on the subsurface information provided by the Owner in the contract document with or without disclaimers. As above, any attempt by the Owner to disclaim the subsurface information will create a very negative impression of the Owner’s willingness to work together with the Contractor in order to provide a successful project outcome.

Another important aspect of subsurface indications is the identification and severity of “hazardous substances”. Many geotechnical engineers do not have the in-house capabilities for the identification of this problem and it may be necessary for the Owner to include Environmental Data and Baseline Reports in the contract document if there is any possibility that the microtunnel will encounter contaminated ground during tunnel construction.

  • Owner’s Responsibilities – The Owner’s Designer is generally responsible for reviewing Contractor submittals and for making sure that the work is being performed in “conformance” with contract requirements; especially with respect to the finished facility. However, the Owner must be careful not to interfere excessively with the Contractor’s plan for constructing the tunnel as those activities are strongly related to a Contractor’s responsibility to advance the tunnel in a safe and stable manner.
  • Contractor’s Responsibilities – Given below is a typical paragraph relating to the Contractor’s responsibilities for a microtunneling project:

Contractor shall supervise, inspect, and direct the Work competently and efficiently, devoting such attention there to and applying such skills and expertise as may be necessary to perform the Work in accordance with the Contract Documents. Contractor shall be solely responsible for the means, methods, techniques, sequences, and procedures of construction.

This paragraph seems to make sense, unless the contract document actually specifies what means and methods of construction shall be utilized by the Contractor for specific portions of the project. For instance, the contract may have specific requirements for grouting or dewatering various portions of the tunnel alignment that need to be implemented by the Contractor and which place some risk for the successful outcome of those procedures on the Owner.

In addition to the above, the Owner will provide contract requirements for constructing the underground openings in the Project Plans and Specifications. For instance, some of the Project Plans will provide drawings showing the size, shape, depth, location and alignment for all of the portals, shafts, and tunnels. These drawings may also show the minimum support elements that need to be installed during construction such as the number and length of rock bolts or the thickness of shotcrete for shaft support in rock. Ground support elements or design requirements for the shaft openings in soil may also be shown especially if those openings that are located in the immediate vicinity of existing third parties. For the tunnel, reference can be made to the allowable tunneling methods, to the pipe itself, and to the backfill grouting requirements once the pipe has been installed.

Likewise, some of the Project Specifications are also related to the work required to construct the tunnel such as those listed below:

  • Tunnel Construction by MTBM
  • Shaft Construction
  • Temporary Structures
  • Shotcrete
  • Tunnel and Shaft Grouting
  • Groundwater Control and Dewatering
  • Ground Improvement Procedures

In general, these types of specifications are intended to provide “guidelines” for the above tasks with a requirement for the Contractor to prepare and provide “submittals” that explain exactly how the Contractor intends to accomplish all the above for “review and approval” by the Owner’s Project Representative. These types of project submittals are generally stamped by a professional engineer employed by or retained by the Contractor and are intended to communicate the means and methods of construction that the Contractor proposes to use in order to provide each of the activities listed above. Hence, the primary purpose of project submittals is to create a meaningful and cooperative relationship between the Owner and the Contractor that is so important for the successful completion of a microtunneling project.

Having discussed the above, we can now turn our attention to what the authors of this paper believe to be the single-most important contract document for the successful completion of a microtunneling project; the Geotechnical Data Report. Almost every decision relating to the successful completion of any tunneling project, but especially for a microtunnel, revolves around how well you know what exists in the ground along the proposed tunnel alignment and the only way to provide an answer to that question is to perform an accurate and comprehensive subsurface exploration program. It is beyond the scope of this paper to discuss all of the field and laboratory testing procedures that are available to investigate the ground, but suffice it to say that the Owner must recognize that the money spent for this activity is not a cost; it is an investment in the successful completion of the project. All portions of the proposed tunnel alignment and at each shaft location must be investigated and then accurately explained and summarized in the GDR. A “good” GDR also sets the stage for the preparation of a “good” GBR, but, and as explained below, it is literally impossible to write a good GBR without reference to the accurate and comprehensive subsurface information contained within a good GDR.

During the past few years the preparation of GBRs for microtunneling projects has become more common; and for good reasons. GBRs were first recommended for tunneling projects in 1997 and the latest (2022) ASCE Guidelines Document for GBRs reads as follows:

  • The principal purpose of the GBR is to set realistic, measurable, and observable baselines that represent the best estimate of subsurface conditions that will be encountered during construction. In doing so, the bidders are provided with a single contractual interpretation that can be relied on in preparing their bids and in the administration of the DSC clause in the contract.

Other key objectives of the GBR with respect to bidding include:

  • Presentation of the subsurface and construction considerations that formed the basis of design for the subsurface components and for specific requirements that may be included in the specifications;
  • Enhancement of the contractor’s understanding of the key project constraints and important requirements in the contract plans and specifications that need to be identified and addressed during bid preparation and construction;
  • Assistance to the contractor in evaluating the requirements for excavating and supporting the ground;
  • The GBR is more than a collection of baselines. This report is the primary contractual interpretation of subsurface conditions, and the report should discuss these conditions in enough detail to accurately communicate these conditions to the bidders. As noted in subsequent chapters, the discussions should explain the rationale for the baselines and why any baselines differ from the available data.

As explained above, the Owner is willing to accept more responsibility for “indicating” those subsurface conditions that the Contractor will encounter during construction in exchange for the use of lower contractor contingencies during bidding. There are also some very interesting and pertinent references above as to how the GBR must be read in conjunction with project requirements as expressed in the contract plans and specifications. In essence, and as explained above, the GBR together with the project plans and specifications as prepared by the Owner, must provide one clear and consistent explanation of what the Contractor is obligated to take into account during bidding and to accomplish during construction with respect to 1) designing subsurface components, to 2) understanding specific requirements included in the project plans and specifications, and to 3) evaluating the requirements for excavating and controlling the ground.

Accomplishing all of the above by an Owner and its design consultant is not easy and requires input from highly experienced and competent tunnel designers and engineering geologists. As explained above, the Owner must provide fair and equitable risk sharing provisions in the General and Special provisions, an appropriate set of project plans and specifications that clearly explain the work that is required to construct the underground openings, an accurate representation of the proposed finished facility, and, finally, an accurate and comprehensive explanation of the ground conditions and ground behaviors (i.e. ground reactions) that will be encountered during construction. These contract requirements can vary widely depending on the types of third party interactions, the complexity of ground conditions expected to be encountered, the type of contract being used for project procurement, and the tunneling equipment and methods provided for in the project specifications. When all is said and done, however, the Owner must assume that if the Contractor does everything as provided for in the contract document, then the project will be constructed for the cost and within the time frame established by the Contractor’s bid.

Part 2 of this paper will explain what the authors of this paper believe the Contractor should do when it receives a contract document for the construction of a microtunneling project.

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.

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