Tunnel Business Magazine

OCT 2018

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paring a GBR will be provided in the next section of this paper. Very often, persons associated with tunneling projects will tell you that the GBR is the most important geotechnical report asso- ciated with a tunneling project, but that is not true. The GDR is the most important report. Without good data about the existing ground condition, it is difficult to identify the major risks associat- ed with a tunneling project and, therefore, difficult to accomplish an appropriate interpretive effort. For instance, let us assume for the moment that the existing ground contains a major fault, but that fault is not discovered as a result of the subsurface investi- gation. Obviously, when the tunnel encounters this fault during construction the project will be subjected to potentially massive delays and cost increases that will be difficult to manage and which could very well result in litigation; i.e. not a good outcome. When it comes to the geotechnical scope of work established for a tunneling project, one should not be thinking about cutting corners and/or reducing costs. The subsurface investigation is primarily responsible for identifying most of the risks associated with a tunneling project and a significant level of effort must be expended to accomplish that task. In the final analysis, and as stated above, almost all of the major decisions about how one goes about designing and constructing a tunneling project are based upon how well one understands the ground conditions that are actually present along the proposed tunnel alignment. Risk Minimization/Ground Behavior Having obtained the subsurface data, it is necessary to de- cide what that data means with respect to tunnel design, tunnel construction, and the potential for impact to third parties locat- ed along the proposed tunnel alignment. Risk minimization in- volves decisions about what is required to excavate the ground, to control the ground during the process of excavation, and to support that ground as the tunnel advances; each of these deci- sions are closely related to one's knowledge of ground condi- tions and one's expectations about how that ground will behave and/or react to the process of tunneling. When brainstorming those decisions, it is also necessary to evaluate various methods for modifying ground behavior (i.e. improving the ground) by dewatering, grouting, or freezing. Almost without exception the subsurface interpretive effort be- gins by plotting the subsurface profile along the proposed tunnel alignment as revealed by the subsurface investigation. Each test boring is plotted on the profile and an evaluation is then made about the types of ground that will be encountered by the tunnel including different deposits of soil and rock, the soil/rock inter- face, and, most importantly, the groundwater regime. Once the borings are plotted it is then necessary to decide how the various layers can be connected to show a profile and this depends largely on one's knowledge of the geological processes that produced the soil and rock deposits dating back many thousands or even mil- lions of years and one of the most important contributors to the subsurface profile process is a well-trained and highly experi- enced engineering geologist. Engineering geologists are trained not only to identify vari- ous depositional environments but also to apply that knowl- edge to the preparation of an accurate description of the sub- surface profile. The engineering geologist must also be aware of how the project will be designed and constructed in order to provide useful information about anticipated ground behav- iors. Sometimes, a good engineering geologist is able to antici- pate subsurface features that were not even indicated by the subsurface explorations such as faults or weathering features based on geologic maps, aerial photographs, or information ob- tained from similar geological environments. Having developed the subsurface profile, it is now necessary to plot the results of all of the laboratory tests on the profile in order to obtain a complete picture of possible ground behaviors. It is all well and good to know that your tunnel will be excavated in a deposit of clay but is that clay soft or stiff, will it squeeze or swell, or is it highly plastic and able to "stick" to the tunneling equipment. Sandy soils can be loose or dense, highly abrasive, and have highly variable values of permeability; all of which must be taken into account during your brainstorming effort. Rock deposits are equally complex and are described by a suite of laboratory test results that have been developed almost exclusively for tunneling projects. Laboratory testing for tunnels in rock is based upon exacting American Society for Testing and t must be applied with great precision to the testing procedures. Inaccurate rock test results can have highly detrimental impacts on how anticipated ground conditions are described in the contract document. Lastly, it is necessary to describe how the groundwater regime will interact with the tunneling operation. Groundwater has three potentially important and independent impacts on a tunneling project; i.e. how much water is expected to enter the tunnel, at what pressure, and is that water "contaminated" in some manner. Large water inflows into a tunnel can disrupt tunneling procedures and greatly slow tunnel progress. High groundwater pressures can cause the ground to flow or collapse, resulting in extra measures to control and/or to support the ground. Groundwater contamination can create unsafe conditions for the workers and even naturally occurring elements in the groundwater can make it difficult to dispose of at the ground surface without expensive treatment facilities. Tunnels can also encounter manmade contaminates such as gasoline or cleaning fluid and other problems such as combustible gases. As a result of the above, the geotechnical engineer must cre- ate some model, some description, and/or some characterization of the ground that can be carried forward into design. Reasonable and appropriate assumptions must be made about soil stratigra- phy, soil and rock properties, and groundwater characteristics such that a good representation of the existing ground is possible. At its best, this work is the output of a process involving judgment, knowledge, experience and maybe a little luck, which results in a good working model for, but probably not an absolutely perfect description of, the ground. TUNNELINGONLINE.COM 2 7 TBM: TUNNEL BUSINESS MAGAZINE // OCTOBER 2018 F E AT U R E S T O R Y

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