Dealing with adverse geology can be problematic, leading to significant tunnelling delays if not adequately foreseen. In mountainous regions, adverse conditions can prove disastrous depending on stress state, rock competence and groundwater inflows. Mitigating delays associated with bad ground at significant depth requires foresight and advanced planning.
By Trevor G. Carter
From the tunnelling perspective, the Himalayas arguably pose the most challenging ground conditions almost anywhere in the world. One of the prime reasons is that they are the youngest of the mountain chains and are demonstrably rising faster than anywhere else. Based on their "active" stress state alone, similar-length deep tunnel excavations under the Himalayas likely will pose significantly more challenges than an equal-length, equal-cover drive almost anywhere else in the world. These difficulties of tunnelling at depth through high mountainous terrain pose major challenges not just for tunnel boring machines (TBM) but also for use of drill and blast (D&B) and New Austrian Tunneling Method (NATM) approaches.
The more challenging the ground, the greater the pre-planning that is required before tunnelling. This challenge is not just one of tackling adverse ground by modifying excavation and support processes to deal with the specific problem zone, stress state and/or groundwater conditions. It is also often about logistics, as for deep tunnels in mountainous regions problem geologic zones often are at significant distance from the nearest portal and at such significant depth that surface pre-treatment is generally impractical.
Traversing faulted and disturbed ground at significant depth requires that tunnelling procedures are able to cope with a huge range of difficult geological conditions. Investigating, evaluating and assessing anticipated geology ahead of tunnelling and dealing with encountered difficult ground conditions requires that better understanding be gained of the interaction between complex geology and stress conditions when excavating at such depths.
Extremes of ground conditions present major contrasts to tunnelling, so much so that they inevitably demand use of flexible rock engineering solutions for the tunnel to progress. The fact that conditions within the Himalayas can be expected to be as bad as has ever been encountered elsewhere means there has to be the ability while tunnelling to allow changes in excavation procedures and in pre- and post-excavation support approaches. This need to adopt flexible solutions is often seen as being at variance with the constraints imposed by the rigidity of design elements incorporated into the fabrication of a typical TBM.
As a result, traditionally there has been a reluctance to use machines in these conditions, mainly due to the perceived extremely adverse consequences of entrapping or damaging the TBM. In some part this is due to the perception that there is more difficulty dealing with adverse ground conditions in the confined working area of a TBM, in comparison to dealing with the same problem in the larger working space of a D&B/NATM heading. Machine designers are attempting to combat some of these problems by making machines more robust and at the same time flexible enough to be capable of safely and successfully excavating through extremely bad ground.
Improving tunnelling effectiveness
Two issues essentially control our ability to improve tunnelling effectiveness for traversing through the characteristically complicated ground conditions found beneath the mountainous regions of the world. First is the influence of adverse geotechnics, i.e., dealing with difficult ground conditions and, second is the limitations of current tunnelling technology.
|Inflow of high-pressure groundwater can occur during construction of tunnels in areas with faults or tectonized zones where unusually low stress states can exist.|
Tunnelling in adverse ground is significantly less forgiving of the limitations of the tunnelling approach than tunnelling in good ground. Generally, the more difficult the ground, the more flexibility is also needed. Tunnelling in the Himalayas, the Andes and until recently the Alps has shied away from TBM use due to perceived inflexibility and the likelihood of the machines getting trapped by adverse ground conditions, either as a result of squeezing or spalling/bursting conditions or because of ground collapses associated with rockfalls or with running or flowing ground within faults. Any of these situations can lead to problematic tunnelling at best and collapses and abandonment at worst.
Dealing with such problems is always challenging but is many times worse when the tunnel heading is, say, 10 km from the nearest portal, as is the case in many TBM drives. The fact that such conditions pose almost as many challenges for conventional D&B/NATM methods as for a modern machine drive often gets ignored. 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 configured, well-operated machine with an experienced, well-trained crew.
Three main geotechnical elements control our ability to execute trouble-free tunnels at significant depth — stress state, groundwater conditions and the rock itself. Adverse characteristics of any of these three elements can, on its own, compromise D&B or TBM tunnelling, but it usually takes a combination of all three being adverse to trap a machine or halt a D&B drive to the extent that a bypass becomes necessary.
Detailed analyses are usually not warranted at early project stages but may be necessary if significant segment lengths of the tunnel are of concern and a TBM is being contemplated. Most of these approaches rely on numerical modelling, and estimating appropriate parameters may be difficult, given the usually limited laboratory data at early project stages.
While vast strides have been made with numerical analyses to better understand the behavior of difficult rock masses at the two ends of the rock competency scale, application of these methods as a predictive tool rather than for back-analysis of existing or ongoing tunnelling conditions is generally not justified at this stage, unless work has been previously done on the site or on similar materials. This is because, typically, current analytical and numerical assessment capability far outweighs early project ability to properly define input parameters.
Unfortunately, it is always early in a project that decisions about TBM usage are needed. Almost always there is also inadequate definition of stress state, rock competence and groundwater for most of the tunnel, so estimating conditions in the zones geologically considered most problematic becomes the focus for minimizing risk and maximizing objectivity for decision-making.
Decisions on whether to utilize a TBM remain therefore a matter of judgment. The key issues include: How much of the tunnel length is problematic, and how much of this problematic length is of critical concern? Alone, no amount of analysis can yield the necessary answers. It requires a combination of information — yielded by the best possible geological assessment of likely conditions along a planned alignment, coupled with application of numerical and analytical techniques to back-analyze similar conditions and assess applicability. Such analyses need to be credible and done in sufficient detail that reasonable estimates can be made of critical yield extent and probable closure magnitudes. Only by such definition can difficult decisions be made on TBM applicability and the suitability of different design types.
Once some appreciation of the extent of problem sections is gained, estimates of cover and rock type can be made for typical zones within these segments and then numerical modelling can be undertaken of representative critical sections. This in turn helps to identify controlling indicators that are diagnostic for evaluation of problem conditions and aids prioritization of which parameters need to be assessed or better still measured for each anticipated problem zone along any deep tunnel route.
In mountainous terrain, when considering a decision on whether or not to use a TBM, and which type of TBM to use for a deep tunnel, it must be appreciated that, historically, three types of ground conditions have proved the most problematic from the viewpoint of halting tunnel advance. In order of severity, case records suggest bad faults, heavy water and major stress, individually and/or in combination, constitute the most problematic ground conditions. These are almost irrespective of tunnelling method.
|Squeezing ground conditions have been cited as the prime reason for the failure of tunnel boring machines used in certain areas of the world.|
For deep mountain tunnels, with few exceptions, major disturbance zones associated with faulting have posed the most problems to tunnelling advance, often historically requiring bypass drifts and significant ground treatment before being able to be traversed. While squeezing conditions associated with the weak phyllites of the Yacambu drive in Venezuela are frequently cited as the prime reason for the failure of the TBMs used for mining along this tunnel, it should be appreciated that one of the prime reasons the phyllites encountered were so contorted and stressed is that they occur within the 2 km-wide Bocono Fault zone, one of the main plate margin faults of the Andes.
Similarly, several of the faults on the Nathpa Jhakri scheme in India, including the Sungra Fault, where extremely poor ground associated with bursting and mudrush events was encountered, when viewed on a continent-wide scale can be seen to constitute a sliver off the Main Central Thrust (MCT), which slices across the southern boundary of the Himalayas. It is therefore to be expected that stress states locally to these features, when encountered in a deep tunnel, might be anomalous, with magnitudes and directions totally at variance to conditions expected to be "normal" for that depth.
The TBM inundation experience at Parbati, which also comes into close proximity with the MCT, and the D&B drivage experience at Nathpa Jhakri with respect to the Sungra Fault are diagnostic of an extreme stress riser situation adjacent to a zone of low to non-existent stress. Tunnelling behaviour in both cases was almost identical — a zone of heavy spalling and bursting being encountered just preceding a zone of major mudflow inrush.
This points to the need to carefully look not just at the basic geotechnics of deep tunnel alignments but also at regional structural geology domains. In particular, three key geological factors need consideration over and above straightforward definition of rock mass quality, cover depth and groundwater conditions. Although these three geotechnical control indicators give an initial clue to degree of adversity, alone they do not provide the extra insight needed to assess the possible degree of adversity posed by different types of faults likely to be encountered at depth along deep mountain tunnels. The three additional factors to be considered are structural geological regime, current regional tectonic state and likely palaeostress history.
In mountain zones, understanding these factors can help route planning and alignment definition, as they provide clues to probable stress regime variability associated with specific styles of geological faulting.
Improving decision making
The lack of foresight of where adverse conditions can occur is central to many of the problems encountered in deep tunnel execution. It frequently clouds understanding to the extent that errors and unnecessary uncertainties are introduced into the decision-making process related to D&B versus TBM selection and even more so related to selection of machine type, if a machine option is favored.
Further complications in the decision-making process relate to the timing when making this key decision, as it needs to be made 12-18 months in advance of actually starting tunnelling, so that sufficient lead time is available for building the machine. However, often detailed project site investigations are incomplete, still ongoing or not even started when this key decision is to be made. Furthermore, once the contract is awarded to the contractor, generally after a long and arduous tendering process, almost always insufficient time and/or funds have been allocated to allow the contractor any opportunity for additional customized exploration to support his own excavation technology selection procedures before initiating equipment procurement.
In nearly all projects, the tender exploration data, which can be exceedingly variable in quality, is the only basis on which to make equipment selection.
Furthermore, unlike many civil geotechnical tunnel works, for subways and such like, because of the depth and length of many deep mountain tunnels in mountainous terrain, such as the Himalayas, investigating tunnel alignment is a challenge all its own. In urban areas, tunnel investigations frequently end up with boreholes on 50 meter centers or closer along the alignment. This is impractical if not cost prohibitive for deep mountain tunnels. As a result, heavy reliance needs to be placed on gaining as best as possible an appreciation of geological conditions at depth and along the alignment.
On the scale of typical project risk reduction, even for the most heavily investigated of deep tunnels, probably only 20-30% understanding of what was finally known was available at the time decisions were already to be made about D&B versus TBM and with respect to TBM type. While this possible 70% lack of understanding arguably led to many of the delays and cost overruns ultimately experienced in Himalayan tunneling, it is important to note that the extent of actual ground problem zones, given the length of the tunnels, was quite minor, affecting less than 5% of the length. And, if forewarned of such zones, their treatment would only amount to a small fraction of the total cost of the project.
Identifying the likely location and character of adverse geological structure is of paramount importance to early decision-making. The importance of good focused site investigation cannot be over-emphasized as it is upon the data acquired from early investigations that the decision must be made between D&B and TBM or about what type of TBM. It is clear that excavation of deep rock tunnels poses several unique challenges that can be daunting, but all must be addressed in the best possible manner when considering potential TBM applications.
The key risks include:
— High rock stress leading to spalling or squeezing. Predicting whether any given tunnelling situation will result in conditions that will entrap a machine requires careful evaluation of the likely alignment geology, geotechnical properties and stress regime through which the tunnel will be driven;
— High temperature. Temperature at the tunnel level depends on the geothermal gradient at the project site. For deep tunnels in young mountain belts, temperatures of the rock and groundwater frequently can exceed 40°C;
— High-pressure groundwater. While generally deep tunnels are well below the groundwater table and thus can be at high pressure, due to the depth and high in-situ stress, most rock fractures are tight, thus inflows are generally not a problem. The exception is faults and tectonized zones where unusually low stress states can exist. Groundwater inflows from such features can yield significant water under appreciable pressure; and
— Access and logistics. Not only does this issue affect schedule and costs, it can adversely affect tunnelling effectiveness and in rare situations even stability due to lack of rock support/segments when a crucial fault zone is encountered. Where use of a TBM is being contemplated, component sizing (for transport along tortuous mountain roads) and machine assembly must be considered, Ancillary equipment supply (for conveyors, shotcreting and grouting gear) and just the continuous maintenance of material that must be routinely supplied can be significant issues. The progress of a state-of-the-art TBM has in a number of cases been reduced severely by lack of support supplies, such as segments, mesh and rockbolts.
Many case records of difficult tunnelling through complex ground conditions in mountainous areas — particularly associated with faults and often with mud and debris flows into the tunnel — have been reported through the history of mountain tunnelling. Case records dating to the turn of the 20th century indicate the extensive use of bypass tunnels to navigate around the most difficult fault zones. For example, in July 1908 in the 14.6 km-long old Lotschberg railway tunnel (now called the apex tunnel to distinguish it from the recently completed base tunnel), during construction through faulted ground a section collapsed, killing 25. The collapsed section was reportedly beyond repair, so a bypass tunnel was driven around the site of the disaster.
Few collapses these days end with fatalities, especially where TBMs are employed, but the magnitude of today's inrushes and collapses are, nonetheless, just as dramatic, and the need for bypass tunnels remains, with several recent examples evident within both D&B and TBM-driven tunnels. One recent classic case occurred on the Gibe II project in Ethiopia. In this case, significant multiple mudflow events occurred, initially burying a state-of-the-art Universal Double Shield machine until it could be extricated and refurbished.
The scope of the problem when it was encountered was completely <. More than 30 long exploratory drillholes and significant exploratory and bypass drifting was completed to develop a scheme that could successfully advance the TBM though this difficult zone. Arguably this again poses the same typical question — could the problems have been foreseen using regular probe drilling and, if so, what could have been done differently?
In more difficult ground conditions, such as those encountered in the Himalayas, with minimal investigation comes the wider risk of the TBM getting trapped by adverse ground conditions — either as a result of squeezing or spalling/bursting conditions or because of ground collapses associated with rockfalls or with running or flowing ground within faults, always in these cases complicated by heavy water inflows. To reduce these risks to an acceptable level, considerably more investment must be made in the hydropower design process in these complex mountainous regions. Significant reduction of real risk can only be achieved through more investigative effort, not through design refinement (see Figure 1). Cost and schedule analysis of past case records suggests that for complex ground conditions, some 5% of the engineer's estimate of capital expenditure is required to be expended on investigating ground conditions to push the process in the right direction (see Figure 2).
Dr. Trevor Carter is principal - Rock Engineerin g Division of Golder Associates in Toronto, Ontario, Canada.