CHALLENGING SITES & HYDRO TUNNELLING CHOICE OF TECHNOLOGIES AND CONTRACT IMPLICATIONS

Transcription

1 CHALLENGING SITES & HYDRO TUNNELLING CHOICE OF TECHNOLOGIES AND CONTRACT IMPLICATIONS Harald Wagner, Consulting Engineer & ITA EXCO Expert, Bangkok/Vienna Key Words: Innovative Flexible Contracting, Standards and Guidelines, Specifications, Advanced Project Management, Geotechnical Baseline Report, Adverse Geologic Conditions, Geotechnical Monitoring, Risk Management Plan, Pre- and In Construction Investigation. 1. EVOLUTION IN UNDERGROUND CONSTRUCTION Tunnelling is the Art of dealing with Geologic Uncertainties. Hydro tunnel designers decide the future project success at an early project stage. Designers are the managers of tunnel construction. Designers choice of design and contract model is managing construction. Contract & Risk Management of Underground Works for both conventional and mechanized tunnelling shall follow state of the art of contracting. Fundamental requirements for design and construction contract of hydro tunnels are Evaluation/classification of ground conditions and particular project requirements Design with subdivisions in stages, following the project and construction development Framework for a range of potential applications Construction Methods suitable for standard plant, equipment and material Risk Management Plan (RMP) in context with Geotechnical Baseline Report (GBR) Flexible Contractual Models for fair remuneration. Demand for proper designed and contracted Underground Works is continuously growing. Conventional and TBM Tunnelling are both well suited for tunnelling in difficult, complex and rapidly changing geological and geotechnical conditions. It is paramount to maximise safety and to minimise construction cost by selecting optimum driving cross section, driving methods and lining during the design and construction phases. During tunnel excavation simultaneous and joint decision by the contract partners must be realized within predetermined limits to manage flexible response to changing ground and ground behaviour conditions. Tunnelling needs trained, qualified and experienced consultants, designers and contractors, it needs qualified and experienced engineers on site, and qualified and experienced workforce. It needs a suitable design as well as a suitable contract model. This goes hand in hand with a contract system which had to be developed for being able to cope with changes in ground behaviour based on GBR and RMP together with the character and scope of the works in a fair and objective way. 375

2 ICHPSD-2015 The most important feature of a tunnel project is the contract model applied. All contract models are based and do depend on the professional tradition of the country where the project gets implemented. Information exchange happens increasingly fast to the benefit of projects. 2. CHALLENGING DESIGN & CONSTRUCTION APPROACH Tunnel designers are managers of the project up to and beyond construction. Design is a management tool both in design and construction. As tunnel failure analysis show, financial conditions are major factors impacting tunnel development. Tunnelling at great depth through high mountainous terrain poses difficult challenges not just for Tunnel Boring Machine Drives, but also for application of traditional Drill & Blast methods. Dealing with adverse geology at any depth can be problematic and can lead to significant tunnelling delays if not adequately foreseen in contract documents. Geological problems which might be soluble at shallow depth whether with TBM or with Conventional Tunnelling when encountered at significant depth can prove disastrous depending on stress state, rock competence and prevailing ground water ingress. Discussions have been ongoing for years on the applicability of TBM s for tunnelling through the characteristically complicated ground conditions e.g. of limestone found beneath high mountainous regions. Tunnelling until recently under the Alps in Europe has developed towards TBM use. The perceived inflexibility as a result of the likelihood of them getting trapped by squeezing ground and/or by adverse faults or irreparable damage by rock bursts or heavy water ingress, all of which are causing problems, has successfully overcome. Difficult geologic conditions pose as many challenges for Conventional D&B/NATM methods as for a modern TBM drive. When similar conditions are met and encountered in D&B headings sometimes it can take as long or longer to negotiate the problem zone than it might have taken with a properly designed and well operated TBM. Until recently with a few exceptions TBM s around the world have mainly been utilized on tunnel projects where the risks associated with deep tunnelling were considered controllable. 3. CHOICE OF TUNNELLING TECHNOLOGY The underground should be properly treated in order to use it as part of the tunnel support. It is essential to know the ground characteristics plus the ground behavior during and after excavation and installation of support. To keep the mechanical properties (strength) of the ground mass, its disintegration should be prevented. To take additional stresses resulting from excavation/support installation, the ground itself should be used as far as possible. Deformations should be controlled below critical level by support after excavation. Under any ground conditions, ring closure behind the tunnel face should be achieved. To observe stabilization and to allow for adjustments, system behavior needs to be observed. 376

3 Conventional (NATM) Tunnelling The theoretical basis of conventional (NATM) tunnelling is to view the ground around and on top of the tunnel not only as a load, but also as a load-bearing element of support. The ground reactions in form of progress, lining deformations and lining pressures are measured and the stability of the excavation confirmed by frequent monitoring. Depending on the project conditions (e.g. tunnels in limestone, shallow soft ground tunnel, deep rock tunnel) and the results of geotechnical measurements, the conventional tunnelling requirement for rapid rigid support or slim deformable support is identified, while other criteria apply for TBM tunneling. Contractual arrangements shall allow the most economical type and amount of support installation in the tunnel. The original approach to conventional tunnel construction is based on rock classification system related to stand-up time of an unsupported section of the tunnel. The applicable rock class is always agreed between the Contractor and the Engineer at the excavation face. It shall be based on experience and on contractual frame-work. TBM (mechanized) Tunnelling In contrary to NATM tunneling, shielded TBM tunneling is less adaptable to changing geologic conditions, while providing better progress rates. It is mostly more related to longer tunnels, e.g. longer than approx m, and has a fixed diameter. While the client should specify a generic design of both TBM and lining, the TBM- and segment designer should join early to fix the concept before start of design work. Responsibilities should be specified at interface of TBM and lining. Both designers should show more than 10 years of professional experience. 4. INNOVATIVE CONTRACTING Regardless the technology applied, any tunnel contract of today is benefitting from most carefully performed ground investigations resulting in the Geotechnical Baseline Report together with the Risk Management Plan (RMP) accepted and supported by all project stakeholders. The contract model has to provide a factual review of the Geotechnical Baselines and has to address five main contract components Technological Reasons for performance and risk of TBM s and CTM s, Establishing reasons justifying sole use of TBM s or combined use of TBM/CTM, Proposing a RMP with Mitigation Measures for improving use of TBM s and/or CTM s, Presenting Suggestions for Contractual Provisions to aid actual TBM & DBM Tunnelling, Presenting Suggestions for Technological Provisions to aid future TBM/CTM Tunnelling. 377

4 ICHPSD SCHEDULE & BUDGET STRATEGIES Tunnel Construction is facing conditions of distributed teams and requires around the clock collaboration and coordination as well as the ability to successfully navigate throughout the life cycle of the project. Today s projects and program managers need tools that deliver the necessary project schedule and budget commitments. They need the ability to integrate project and program resources with other departments and at other locations to ensure project success. Project efficiency needs permanent improvement to reduce project risk in major construction projects. They need to be enabled to implement sound risk management principles reducing the risks associated with the project, such as delays in delivery and resources needs, timelines, funding and cash flow, impact of cost errors and effect of contingencies on cost and timing. Projects have to enhance project planning methodologies, and adherence to schedules, and improve precision in managing the project to deliver projects faster, with higher quality and at lower cost. There is a need in every project to improve workflow, streamline processes, enhance project related communication, and expand project visibility. Benefits are eliminating unnecessary spread sheets, and improving the accuracy of information, improving project visibility, productivity and on time project delivery, enabling better project planning and projection, raising performance grades and enabling additional funding. Tracking project and program cash flow, plus monitor performance versus plan is important. With tools to accurately forecast project completion dates and costs against estimates, strategic adjustments can be made throughout the project lifecycle and confidently make and keep commitments to the customers. With real time key performance indicators, metrics, and financial data, deep analytical capabilities and advance warnings are received to reduce the risk of cost and schedule overruns and any resulting contract penalties. In order to perform design review, document control, cost estimating, and scheduling for completion of the project on time and within budget, project management with the help of respective suitable software programs must be able to track costs and gain inside into change orders and forecasts, identify common scheduling pitfalls and quickly remedy them, maintain And adhere to detailed project schedules, and resolve changes, make payments on time, and avoid claims in the very early stage. Suggestions for efficient risk and safety management, payment and contract penalty clause are aimed at ensuring data collection and interpretation gets done on a realistic useful time scale, so as to constantly update understanding of ground response during actual construction and thereby plan against the potential for unexpectedly intersecting problem ground conditions ahead of the tunnel drive. 378

5 6. Risk Management Practice A comprehensive risk matrix is to be prepared and to be included in the Tender documents. In any project, various risks have to be grouped into several risk categories Design risks Geological risks Technological risks General Risks The risks have to be allotted appropriately either to the Contractor or to the Employer so that the same can be suitably taken into account by the Contractor at the bidding stage. Allocation of various risks during the design & execution of the project is given in the risk register, which should be a part of the contract document. For addressing geological risks, Geotechnical Baseline Report (GBR) is the solution. 7. GEOTECHNICAL BASELINE REPORT (GBR) The GBR describes the conditions expected to be encountered during tunnel construction and other rock excavations. It represents the Employer s best judgment of geotechnical conditions anticipated in the excavations. Actual geotechnical conditions encountered during the execution may differ from those specified in GBR. Besides the GBR, the factual geotechnical information gathered during the project investigations shall be assembled into a Geotechnical Data Report (GDR). The GDR shall be provided only for the information of the Contractor and shall not form part of Contract documents. GBR, the single source document, is charged with portraying a realistic interpretation of the subsurface conditions that are anticipated in the proposed construction. They include not only the mean conditions of ground behavior and groundwater conditions anticipated during construction, but also address the range of variances that is expected in these relevant geological characteristics. The purpose of a GBR is to establish a realistic, common basis for all contractors to use in preparing their bids and subsequently a basis for evaluating any contractor s claims for differing site conditions that develop during construction. And therefore should be included with the Tender Documents. It also helps in preparing Risk Register on the basis of the specified baselines for possible risks that may occur during execution. Each and every risk is suitably allocated between the client and the contractor. The GBR is the basis for equitable contractual risk sharing and risk 379

6 ICHPSD-2015 allocation between the project owner and their selected contractor. The contractual statements are referred to as BASELINES. Fig. 1: Approach to Risk Reduction Achievement reached through increased investigation rather than through refined design as there is an interaction between design, investigation and risk. The inclusion of GBR in contract document appears to be a simple and effective method of allocating project risks within construction contracts. They are an attractive idea for Owners as they provide the ability to FIX the level of Geotechnical Risk at a desired level. There are well established Guidelines for the preparation of GBRs and considerable international experience of their use. Typical Risks during Construction of HEP Excavations are Inaccuracy of geological investigations leading to wrong interpretation of GBR. Occurrence of hard rock strata than that specified in GBR. Occurrence of Sudden loose fall. Squeezing Ground Conditions as mentioned in GBR. Chimney formation in tunnels as per GBR. Heading inflows beyond limits described in GBR. Maximum steady state water inflows beyond limits prescribed in GBR. Occurrences of hot water springs beyond limits prescribed in GBR. Emission of harmful Gases. Occurrence of Accidents. 8. Contracting & Risk Control This risk aversion attitude has generally resulted from the perceived extremely adverse 380

7 consequences of entrapping and/or damaging a TBM under high stress conditions. An adequately foreseen Risk Management Plan in the project is the best contractual tool to handle such situations. Fig. 2: Flow Chart Contracting Design Risk Mitigating delay problems associated with dealing with exceptionally bad ground at depth particularly in a TBM drive requires considerable foresight and advanced planning. Such planning involves investigating, evaluating and assessing anticipated geology ahead of tunnelling. Fig. 3: Flow Chart Contracting Construction Risk 381

8 ICHPSD-2015 Only by being forewarned is it possible to be ready to implement counter measures for dealing with encountered difficult ground conditions at great depth. Project studies have to be governed by the demand for deepened Geologic and Geomechanic knowledge of the project. 9. PRECONSTRUCTION CONTRACTING During the preconstruction period, expenditure and effort needs to be spent on focussed site investigation in order to gain a better understanding of interactions between complex geology and stress conditions along a proposed alignment for the proposed tunnel depth. Problem Pending Financing Solution Approved Financing Preconstruction Opening Ceremony Design & Construction Contract Negotiation Unspecified Contract Model Preliminary Design Alignment Tender Design Cross Section Final Design Geotech. Baseline Report International Competitive Bidding Public Tendering Award of Contract Fig. 4: Contract Requirements related to Preconstruction Phase Acquisition of this data ahead of time is the only effective way to substantially reduce project risks. It is not good enough to just trust to chance while driving the tunnel. Extremes of ground conditions present major contrasts to tunnelling, so much so that they often demand use of flexible rock engineering solutions. This need is often seen as being at variance with the constraints imposed by the rigidity of design elements incorporated into a typical designed TBM and subsequent lining system. Hard rock machine designs are however moving forward to more hybrid and universal TBM concepts that encompass ability to execute e.g. full umbrella forepoling and soft rock machine face conditioning, pre grouting and ground treatment. Even though significant advances have been made in equipment design and in the capabilities of advanced TBM s, nowadays used on tunnelling projects, minor seems to have been achieved about enhancing site investigation - either in the pre-bid stage, for design or as part of contract provisions for execution as pre-investigation ahead of actual tunnelling, e.g. by 382

9 probe drilling with cored probe holes in a program of pre-planned investigations for difficult fault zone areas. 10. CONTRACTING WITH RISK REDUCTION Similar risk reduction measures have been implemented for the Swiss Alp Base Tunnels e.g. for the Piora fault zone, which actually has not been faced. Risk reduction by rerouting of alignments needs serious investigation and evaluation a longer route through less challenging ground can be cheaper and quicker than a deep straight route that requires considerable pre-treatment and drainage of major fault problems before the tunnel can even be excavated along the route. The experience from all of the tunnelling projects that have been examined in detail from project works in mountainous high cover situations, both TBM and DBM, shows clearly, that adverse conditions if encountered unexpectedly can lead to major collapse problems with delays of years sometimes being incurred. Suggestions are made that current practice be enhanced by imposing mandatory contract language to ensure that better pre-investigation is achieved for deep tunnelling projects and that ground pre-treatment will not be implemented only on an ad hoc basis, with the inevitable likely to continue to happen major collapses, delays and entrapment of TBM. 11. CONTRACT INVESTIGATIONS Despite problems in the past with stuck and abandoned TBM s and unexpected collapses in D&B tunnels, the level of investigations being implemented for even latest projects elsewhere in the world is still far from adequate and certainly nowhere near the state-of-the-art level implemented e.g. in the Alps. Problem Solution In Construction Award of Construction Fund Finding Missions Choice of Alignment Choice of Cross Section Geotechnical Investigation Hydraulic Load Investigation Risk Management Report Management of Unexpected Project Changes Detailed Design Work Preparation Baseline Construction Plan Construction Monitoring Risk Management Plan Constant Risk Assessment Fig. 5: Contract Requirements related to Construction Phase 383

10 ICHPSD-2015 The experience being gained across the tunnelling world in TBM tunnelling through events occurring at project sites where TBM s are actually operating provides a wide range of difficulties that can be expected in mountainous conditions in TBM tunnelling when conditions ahead of the TBM are not pre investigated. It is difficult to investigate deep tunnels. This is often stated as the cause why there are no drillholes along most of this tunnel alignments. It is recommended, that routine cored probeholes from underground are mandated within contract provisions in order to preinvestigate conditions ahead of TBM drives. Pre-construction and In-construction investigation measures should identify and outline suggestions for mandatory contract language that might be implemented to improve early stage site investigation and implementation of results. 12. CONCLUSIONS ON CONTRACTING Suggestions for efficient Project Management, payment and contract penalty clause are aimed at ensuring data collection and interpretation gets done on a realistic useful time scale, so as to constantly update understanding of TBM response during actual construction and thereby plan against the potential for unexpectedly intersecting problem ground conditions ahead of the tunnel face. Tunnel Construction is facing conditions of distributed teams and requires around the clock collaboration and coordination as well as the ability to successfully navigate throughout the life cycle of the project. Today s projects and program managers need contractual tools that deliver the necessary project schedule and budget commitments. They need the ability to integrate project and program resources with other departments and at other locations to ensure project success. Project efficiency needs permanent improvement to reduce project risk in major construction projects. They need to be enabled to implement sound risk management principles reducing the risks associated with the project, such as delays in delivery and resources needs, timelines, funding and cash flow, impact of cost errors and effect of contingencies on cost and timing. Projects have to enhance project planning methodologies, and adherence to schedules, and improve precision in managing the project to deliver projects faster, with higher quality and at lower cost. There is a need in every project to improve workflow, streamline processes, enhance project related communication, and expand project visibility. With tools to accurately forecast project completion dates and costs against estimates, strategic adjustments can be made throughout the project lifecycle and confidently make and keep commitments to the customers. With real time key performance indicators, metrics, and financial data, deep analytical capabilities and advance warnings are received to reduce the risk of cost and schedule overruns and any resulting contract penalties. 384

11 In order to perform design review, document control, cost estimating, and scheduling for completion of the project on time and within budget, project management with the help of respective suitable software programs must be able to track costs and gain inside into change orders and forecasts, identify common scheduling pitfalls and quickly remedy them, maintain And adhere to detailed project schedules, and resolve changes, make payments on time, and avoid claims in the very early stage. In conclusion, regulations and standards are helpful tools for owners and contractors during project development in hydro tunnelling at challenging sites. Contracting, in particular the choice for contract modelling as well as for tunnelling technology, needs to be decided from the best risk coverage. Lessons learned in design and in construction show that Geotechnical Baseline Report and Risk Management Plan shall be part of any contract document. Contracting trends from practice show that project owners tend to shift the geotechnical risk of adverse geology to the contractor. The clear distinction of risk ownership shall contribute to project success. To make use of EPC (Engineering Procurement) Contracts requires more geotechnical investigations and related expenses than conventional contracts BIBLIOGRAPHY 1. Wagner H., Innovative Use of Underground Space in Transportation, Disaster Prevention and Energy. International Bridge & Tunnel Technology Congress, Shanghai,China, (08/2012). 2. Wagner, H. William Barclay Parsons WBP Lecture 2013, Tunnellling in an Urbanizing World. American Society of Civil Engineers (ASCE), NYC, USA, (4/2013). 3. Wagner, H. Choice of Tunnelling Methods. EIT Engineering Institute of Thailand, Seminar on Experiences in Rock. Bangkok, July Wagner, H. Urban Underground Space Construction Technologies. Proceedings of the 10 th Iranian Tunnelling Conference. November Wagner, H. Conventional & TBM Methods in the Himalayas Achievements and Prospects. Nepal Tunnelling Conference. Kathmandu, December Wagner, H. Contracting Tunnelling Works Basic Knowledge of Standards & Specifications. SRT Tunnelling Seminar, Design Construction & Maintenance of Railway Tunnels. Bangkok, January