By Gary Brierley
I don’t know about you but I am getting a little tired reading about all of the “risks” associated with designing and constructing tunneling projects. Having been involved with the tunneling business for more than fifty years I have always been of the mind that the best way to minimize your risk for a tunneling project is to do that which is necessary to make your project successful. Each year various organizations give awards for the design and construction of highly successful tunneling projects and it is fascinating for me, at least, to read what those owners, designers, construction managers, and contractors did in order to deserve those awards. With the above in mind it is the intention of this paper to outline five steps that one must accomplish in order to produce a successful tunneling project.
Step 1 – Project Layout
Project layout for a tunneling project in a densely populated urban environment is extremely difficult and requires input from highly trained and experienced tunneling professionals. Each project must be made compatible with the spaces available at the ground surface and with the ground conditions expected to be encountered along the proposed tunnel alignment. Although the primary objective of project layout is to provide the project owner with a long lasting and reliable finished facility, project layout must also take into account all of the constructability issues associated with creating the subsurface space inside of which the finished facility will be erected. It is obviously not possible to go into all aspects of project layout in this brief paper but exceedingly thoughtful and appropriate project layout is one of the keys to success for a tunneling project.
Step Two – Subsurface Investigations
The author of this paper is of the opinion that a well conceived and well implemented subsurface investigation is the singlemost important element of a successful tunnel project. My definition of a successful tunneling project is as follows:
Create a finished facility for the project owner in no more time and for no more money than is necessary for the Existing Ground Condition.
and this definition obviously begs the question; How well do you know the Existing Ground Condition? to which question you must be able to answer very well indeed. I have said many times in my lectures about tunnel design and construction that the singlemost important contract document, by far, is the Geotechnical Data Report. All issues that need to be resolved for a tunneling project are related in one way or another to how well informed you are about the ground conditions that will be encountered along the proposed tunnel alignment.
Step 3 – Ground Behavior/Ground Control
Having been provided with your project layout and the results of your subsurface investigation it is now necessary for you to “interpret” how that ground will react (i.e. behave) when you attempt to create the underground openings inside of which the finished facility will be erected and in order to perform that interpretive analysis you must provide answers to the following three questions:
- What is the best way to excavate the ground?
- What is the best way to control the ground during the process of excavation? and
- What is the best way to support the ground as the tunnel is advanced?
And, in order to provide answers to those three questions you must assemble a group of highly experienced tunneling and engineering geologist professionals who are thoroughly familiar with all forms of tunnel construction and ground improvement procedures. No other form of civil design and construction has such a complex interface between structure design and construction procedures because 100% of the structure must be erected inside the ground and this interpretive effort results is the creation of three more questions that must be answered:
- What is the best way to design your underground facility?
- What is the best way to construct your underground facility?; and
- What is the best way to protect nearby existing third party facilities during the process of construction?
Step 4 – Contract Document
At this point in project procurement you must create a contract document that accurately characterizes all of the issues as discussed above; i.e. project layout, subsurface investigations, project design, project construction, and third party impacts. When you put your project out to bid it should be assumed that if the project contractor successfully accomplishes all of the requirements as specified in your contract document then everyone will be pleased with the final outcome. If this is true, then it is possible to state that the singlemost important output of project design is the contract document.
Step 5 – Construction Management
Construction management for a tunneling project involves the utilization of highly experienced tunneling professionals in the field to observe and document all aspects of tunnel construction. One of the keys to a successful tunneling project is for both the Owner and the Contractor to have accurate and continuous records of what happened during construction both in order to show how the work was performed in accordance with contract requirements and to document how the ground was reacting to various construction procedures especially as related to possible claims for Differing Site Conditions. It is not unusual for the ground to create, shall we say, “road blocks” to tunnel construction and it is only by studying the construction documents that the cause and effect relationship for those difficulties can be resolved.
In conclusion, it is the opinion of the author of this article that it is more important for you to create a success register for your project rather than a risk register. What will it take for your project to be successful? Emphasize the positive and concentrate on what you must do in order for all parties associated with your project; i.e. the owner, the designer, the third parties, and the contractor to be pleased with the final outcome. The bottom line for all of the above is the proposition that the best way for you to minimize the risks associated with your tunneling project is for that project to be successful.
Gary Brierley is president of Dr. Mole Inc. Dr. Brierley has decades of experience with both the technical and nontechnical aspects of underground engineering and construction management. Beginning as a project manager in 1970, Dr. Brierley has worked on literally hundreds of projects involving all types of underground design and construction including soil and rock engineering, building foundations, braced excavations, ground improvement and slope stability.