Planning for a Tunnel Project
Underground Projects Require Careful Scrutiny Compared to Surface Structures
By Kenneth Sorensen, P.E., G.E., John Struzziery, P.E., and Gary S. Brierley, Ph.D., P.E.

In any water or wastewater system, tunnels provide a critical link between the treatment facility and the distribution/collection system that reaches the end-user. However, despite their importance, many system owners are unfamiliar with how to oversee a tunnel construction project since the need to build a tunnel is often infrequent.

Tunnel procurement is radically different when compared to above-ground construction. As a result, there are a number of considerations a system owner needs to weigh and address should they become involved with the planning, design and construction of a tunnel. Doing so will ensure that an underground construction project has a successful conclusion.

A Unique Set of Circumstances

Tunnel construction comes with a unique set of conditions. For starters, any unplanned event that occurs during the construction process could potentially affect cost and scheduling throughout the entirety of the project, as well as any facilities or structures located near or above the tunnel.

A second consideration for system owners is the equipment necessary for a tunneling project. Tunnels are constructed from the inside out and from a single portal in a deep shaft, with the critical path schedule being a straight line. Therefore, tunneling contractors employ highly specialized and expensive pieces of equipment adapted to specific ground conditions and logistical requirements.

There’s also cost to consider. In order to build a tunnel, one must create the space in which the structure will be constructed. Doing so in a safe and stable environment often requires the use of various temporary opening and/or ventilation shafts. Anywhere from two-thirds to three-fourths of the cost of a tunnel is associated with the construction means and methods needed to create the underground space. Therefore, a detailed understanding of ground conditions is necessary to evaluate appropriate construction means and methods. This leads to the next condition a system owner must consider: the ground itself. Because every aspect of a tunnel’s design and construction is tailored to how the ground will react to particular activities, all associated decisions must be based on a thorough understanding of subsurface soil, rock and groundwater conditions. This includes possible environmental constraints such as contamination, combustible gas and spoil disposal.

Finally, there is a tendency to think tunnels are out-of-sight/out-of-mind structures and that tunnel construction will go relatively unnoticed by the public. Although a tunneling project’s direct impacts to a community are less intensive when compared to cut-and-cover construction, the indirect impacts can still be significant and must be controlled. The locations of launching and access shafts along a tunnel alignment are primary considerations in the planning and design stages. Issues such as ground surface settlement, sinkholes and the like may be attributed to over-mining of the tunnel heading and other factors. These potential risks must be understood and mitigated during design and construction.

Tunnel Ground Conditions

Since tunnels typically go below the depths of building foundations and will encounter ground conditions that are not easily understood, a system owner must gather accurate data on how the ground will behave during the course of an underground construction project. One-hundred percent of a tunnel comes in contact with the ground, which greatly increases the probability of claims for differing site conditions.

A thorough examination of the tunnel ground conditions by experienced professionals is vital. Boring spacing needs to be much closer than for an open-cut trench project (300-ft spacing is typical for a trench, whereas closer spacing is normally needed for a tunnel). The idea with tunneling is to reduce risk by providing as much subsurface information as is reasonable for the geologic conditions being explored. The geology will often dictate the boring type and spacing.
A proper geologic reconnaissance and mapping program performed prior to drilling the borings enables the borings to be located in the various geologic units and in the areas of special interest. This is the key to a good technical approach and cost-effective subsurface exploration program. A system owner may want to consider a preliminary geologic review and mapping phase prior to subsurface studies to get an idea of feasible tunneling methods. Then, the borings can efficiently target the information needed to properly design the tunnel.

Ensuring a Successful Outcome

Ultimately, the system owner is responsible for the successful outcome of a project. Ensuring this success means assembling a team of experienced designers and consultants who have extensive local knowledge of the geotechnical site conditions and tunneling methods. They also must be experienced in how to design and construct tunnels given various project constraints.
The most important members of this team will be the geotechnical engineer, engineering geologist and tunnel engineer who will collectively evaluate the type and extent of field explorations and laboratory tests necessary for the project. Selecting a project’s geotechnical engineering and tunnel engineering team should be based on best value and not just the lowest cost, since the overall cost of the project will depend greatly on the success of the contractor. The cost of a major change order due to unforeseen ground or differing site conditions can often outweigh the cost of geotechnical studies. It is best not to cut corners when it comes to subsurface investigations for tunneling. Getting input from tunneling contractors who have worked in similar geologic units can provide valuable insights that should be considered in project planning and design.

System owners must also be sure they properly sell the project to the community. This can be accomplished through outreach programs that convey the potential impacts the tunneling project will have upon a community, including how those impacts will be minimized, what utilities and structures will be protected during construction, how the project is being built, the times of day when work will be ongoing, and the possibility of limitations and restrictions.

Contract Documents

When it comes time to produce a project’s contract documents, it’s important they are specifically tailored to tunneling, and include subsurface information obtained during the planning and design phases of the project. The information in the contract documents should include plans, specifications, geotechnical data and baseline reports.

The contract documents should also include a section on construction means and methods. A system owner needs to specify a minimal, acceptable level of contractor performance as measured by ground behavior (for example: surface settlement). This is to grant protection in the event the ground is not adequately supported during a project and behaves poorly during construction, resulting in negative implications for both the tunneling operation, and nearby utilities and structures.

Another aspect of the contract to consider is contractor pre-qualifications. System owners should always select experienced tunneling contractors, by obtaining information about the number of years a contractor has worked, the number of projects they’ve completed of similar size and complexity, and their propensity for construction claims.

Finally, a tunneling project that is halted because it encounters “differing site conditions” can severely impact working relationships, schedules and overall cost. Therefore, having a contract that provides a method for the efficient resolution for differing site conditions is ideal. Use of Dispute Review Boards (DRBs) established jointly by the contractor and the owner prior to construction, partnering methods, and similar approaches can be used to resolve construction issues in a timely manner and minimize the risk of costly litigation and project delays.

Design-Bid-Build vs. Design-Build

There is much debate in the construction industry over whether design-bid-build or design-build practices are more appropriate for a tunneling project.

Design-bid-build offers system owners the opportunity to investigate all environmental and community requirements, as well as any third-party impacts for the proposed project. Based on this knowledge, the system owner is able to produce a contract with detailed instructions for the contractor. However, when utilizing design-bid-build practices, make certain the contract provides maximum flexibility for dealing with construction means and methods, and the design of any temporary structures without endangering third-party interests.

Design-build is advantageous with respect to schedule compression, and to the selection of the most efficient and cost-effective means and methods of construction. It also provides maximum flexibility when dealing with environmental, community and third-party interactions. When taking a design-build approach, a system owner must provide sufficient subsurface information, which allows for accurate estimates for work, as well as accurate data on design criteria for the finished facility.

In general, some combination of design-bid-build and design-build practices makes the most sense for a tunneling project.

Finishing On Time and Within Budget

Tunnel building demands that all parties involved be flexible and open to reasonable deviations during the planning, design, and construction phases. While there is no guarantee of success, considering the information detailed here will serve to improve the odds a tunnel project is finished on time and within budget. A well-designed and well-constructed finished structure is a water and wastewater system owner’s top priority.

Kenneth G. Sorensen, P.E., G.E., is an associate and senior geotechnical engineer at Kleinfelder, Sacramento, Calif. John J. Struzziery, P.E., is a principal engineer with Kleinfelder/S E A Consultants, Cambridge, Mass. Gary S. Brierley, Ph.D., P.E., is president of Brierley Associates, Denver.