Tunnel Business Magazine

OCT 2018

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TUNNELINGONLINE.COM 2 8 TBM: TUNNEL BUSINESS MAGAZINE // OCTOBER 2018 Based on the above interpretive ef- fort and on one's knowledge of tunneling methods and ground improvement tech- niques, it is possible to develop a plan for your tunneling project that reduces risk exposure to what is referred to in tunnel- ing terminology to "as low as reasonably possible" (ALARP). It is truly not possible to eliminate all risk for a tunneling project as ture has a perverse ability to make things difficult for tunneling con- tractors. However, the bottom line on all of the above is to assemble a contract pack- age that you sincerely believe is appropri- ate for the existing ground condition and that will allow the contractor to price the work required to create the space required for construction of the finished facility. The actual contract document that is used to summarize all of the above thoughts, evaluations, and decisions is the GBR. A complete description about how one goes about preparing a GBR is described in the Geotechnical Baseline Re- ports for Construction, Suggested Guide- lines by the American Society of Civil En- gineers, 2007, and given below are quotes from that document about the purpose and scope of a GBR: 1.2 The Geotechnical Baseline Report Projects involving subsurface excava- tion present many risks, all of which must be assumed by either owner or the con- tractor. The greatest risks are associated with the materials encountered and their behavior during excavation and installa - tion of support. 1.3 Purpose of the GBR The principal purpose of the GBR is to set clear realistic baselines for conditions anticipated to be encountered during sub - surface construction, and thereby provide all bidders with a single contractual inter- pretation that can be relied upon in pre- paring their bids. Other key objectives of the GBR include: • Presentation of the geotechnical and construction considerations that formed the basis of design. • Enhancement of the contractor's un- derstanding of the key project con- straints. • Assistance to the contractor in evalu- ating the requirements for excavating and supporting the ground; and • Guidance to the owner in administer- ing the contract and monitoring per- formance during construction. From the above it can be ascertained that the primary objective of a GBR is to help the contractor understand important ground characteristics and anticipated ground behaviors in order to improve the probability for project success and that is the essence of a good GBR. Good tunnel- ing must also be thought of as a team ef- fort whereby the owner, the designer, and the contractor work together to produce a successful project. The contractor is not the enemy and one of the most important roles of the geotechnical engineer is to make a sincere effort to provide the con- tractor with the subsurface information necessary to evaluate the best methods for excavating, controlling and supporting the ground. Risk Allocation/ Contract Document The contract document for a tunneling project consists of five parts: • General Conditions, • Plans, • Specifications, • GDR, and • GBR One of the most significant risks that must be "allocated" in the contract docu- ments is the risk of a differing site condi- tion. As defined by numerous contract forms, a differing site condition is a ground condition discovered during tunneling that differs materially from those ground conditions indicated by the contract or which differs materially from those ground conditions that would normally be anticipated by the contractor under simi- lar circumstances. For instance, if the con- tractor encountered rock where none was indicated by the test borings or encoun- tered archeological remains or abandoned utility lines when there was no provision in the contract for that possibility, then the contractor would be eligible to file a claim for a differing site condition. The other possibility for a differing site condi- tion associated with tunneling projects is related to claims by the contractor that the ground is not "behaving" as he was led to believe by contract indications, including the project specifications. Virtually all claims for differing site con- ditions are filed during construction when the contractor is attempting to excavate, control, and/or support the ground and re- volve around the answers to two questions: 1. What did the contract indicate about ground conditions? 2. What ground conditions did the con- tractor actually encounter during construction? Clearly, the vast majority of contract indications related to ground conditions have to do with work performed by the geotechnical engineer. As indicated above, all of the data about ground conditions is contained within the GDR and all of the interpretations derived from that data will be presented and explained in the GBR. Hence, and as a result, the geotechnical engineer will be front and center during the evaluation of a differing site condi- tion. Another big problem associated with a differing site condition is the possibility of causing significant project delay. As in- dicated above, tunneling projects have a serial construction schedule which means that if the face of excavation is not advanc- ing then pretty much all activities on the project are delayed and the costs associ- ated with that delay can be substantial. Clearly, it is not possible to discuss all of the possible ramifications of a claim of differing site condition in this short pa- per (i.e. numerous entire volumes of legal proceedings have been published about this topic) but it is important to note that all three steps for risk assessment as dis- cussed above involve services provided by the geotechnical engineer. Subsurface risk identification, subsurface risk minimi- zation, and subsurface risk allocation are all central to successful tunneling and are part of the package of services provided by geotechnical engineers. As a result, it is right to say that this activity is not for the faint of heart and should only be per- formed by geotechnical engineers who are well versed in the intricacies and nu- ances associated with tunneling projects. Performing subsurface investigations and preparing the geotechnical reports for inclusion in the contract document for a tunnel is not a great opportunity for on- the-job training. It is also imperative for tunnel owners to realize and to accept the importance of good quality geotechnical services when it comes to tunneling and to be prepared to make the investments that are required in order to make certain that this work is being performed in a proper manner. Risk Management/ Construction Monitoring As mentioned above, a claim of differ- ing site condition involves providing an- swers to two questions: 1. What did the contract indicate about ground conditions? 2. What ground conditions did the con- tractor actually encounter during construction? The process of tunneling is a highly dy- namic and transient activity and you either know what was happening on a day-to-day basis or y ecifications for tunneling projects require the contractor to monitor his activities on a daily basis and to prepare reports associated with those activities, but it is also absolutely essential that the owner have an independent set of reports describing those same activities F E AT U R E S T O R Y

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