TBM在复杂地层掘进(论文).pdf

TBM TUNNELLING IN DIFFICULT GROUND CONDITIONS Giovanni Barla 1 and Sebastiano Pelizza 2 ABSTRACT This paper is to discuss TBM tunnelling in difficult ground conditions when problems are met which may reduce dramatically the average progress rates and practical consequences may be such as to pose seri ous questions on the use of mechanised TBM tunnelling versus drill and blast and other so called traditional excavation methods Following a few remarks on rock TBM tunnelling in relation to the selection and di mensioning of the machine the attention is posed on the limiting geological conditions which may be envis aged with respect to the use of TBM tunnelling and on the importance of geological and geotechnical inves tigations in order to derive an appropriate understanding of the rock mass conditions along the line of the tunnel The discussion is centered upon the relatively more important or difficult ground conditions includ ing borability limits instability of excavation walls instability of excavation face fault zones and squeezing Whenever available to the authors and based on project experience the point of view is illustrated by case examples which give the opportunity to underline specific difficulties encountered and recommendations INTRODUCTION TBM excavation represents a big investment in an unflexible but potentially very fast method of exca vating and supporting a rock tunnel Barton 1996 When unfavourable conditions are encountered without warning time schedule and practical consequences are often far greater in a TBM driven tunnel than in a drill and blast tunnel The unfavourable conditions can be produced by either a rock mass of very poor quality causing instabil ity of the tunnel or a rock mass of very good quality i e strong and massive rock mass determining very low penetration rates However it is to be observed that when using the full face mechanized excavation method the influence of the rock mass quality on the machine performance has not an absolute value the in fluence is in fact to be referred to both the TBM type used and the tunnel diameter Right from the beginning of its earliest applications the use of full face mechanised excavation was to overcome the limits imposed by the local geology the economic challenges and schedule competitions of the drill and blast method and other so called traditional excavation methods A prominent example is given by the recent from 1995 to 2000 construction of the one tube 24 5 km long Laerdal Tunnel in Norway the world s longest road tunnel This 100 m2 cross section tunnel is being excavated in a precambrian gneiss a very good and stable rock mass the supports are on average only 7 8 rock bolts plus a 7 cm thick shotcrete lining per meter of tunnel The excavation is carried out by the drill and blast method which has been evaluated to be less expensive and more reliable than the use of a large diameter TBM The average progress rate is 4 8 5 0 km per year with two faces against the 2 3 4 8 km per year estimated for a large diameter TBM Kovari et al 1993 With this background in mind this paper is intended to address the problem of TBM tunnelling in diffi cult ground conditions Based on a few selected case examples the discussion is centered upon the relatively more important or difficult ground conditions which can be listed as follows borability limits instability of excavation walls instability of excavation face fault zones squeezing 1 Politecnico di Torino Dipartimento di Ingegneria Strutturale e Geotecnica corso Duca degli Abruzzi 24 10129 Torino gbar la polroc polito it 2 Politecnico di Torino Dipartimento di Georisorse e Territorio corso Duca degli Abruzzi 24 10129 Torino spelizza polito it ROCK TBM TUNNELLING The practically infinite number of combinations of rock soil and environmental conditions which may be encountered during tunnel excavation has determined a great difference in the types and characteristics of the available TBM s There are many different schemes for the classification of tunnelling machines For ex ample the AITES ITA Working Group No 14 Mechanisation of Excavation is currently working on the definition of an internationally acceptable classification of TBM s with the purpose of establishing terminol ogy and guidelines for the optimum choice of the machine Table 1 Table 1 General classification scheme for tunnelling machines AITES ITA Working Group No 14 SupportExcavationMachine System Location CavityFace MethodTool Reaction Force CategoryType None Partial Face Excavating Machines PFM VariousNone or Grippers Special Rock Tunnelling Machines Mobile Miner Continuous Miner Other Cutting diskGrippers Unshielded TBM Special Unshielded TBM Cutting disk Cutting bits Cutting knives Bruland 1998 there are technological limits for the maximum dimensions of some major TBM components for exam ple the bearing and the head the intensities of both the instability phenomena and the induced convergence also increase with increas ing diameter of excavation Tseng et al 1998 Barla G and Barla M 1998 There already exists a positive and consolidated experience in the use of TBM s in rocks of different qualities and strengths for excavation diameters up to 12 12 5 m Beyond 13 5 14 m excavation diameter the present technology is probably not up to the level of guaranteeing a good performance of TBM s in hard rock Designers should take into account these limits during the tunnel design phase making use whenever possible of the advantages offered by the reduced sections of the tunnel or even by considering the possibil ity of having other tunnels running in parallel This is particularly true for motorway tunnels where in some cases it is preferable to make triple tunnels each with two lanes for the traffic flow rather than twin tunnels each with three lanes In the case of railway tunnels it is better to have two relatively small single track tunnels rather than a large double track tunnel A great help in the use of TBM s could possibly be achieved through the standardization of the section types for road motorway and railway tunnels This could favour the re use of TBM s by obtaining at the same time a constancy in the typology and quality of homogeneous construction works in addition to gai n ing considerable advantages in construction times and costs As well known the value of the TBM in terms of direct project costs is relatively insignificant Failure to achieve the desired results and maintain the time schedule however significantly affects the project From the outset it is therefore important to adopt the ap proach of utilizing the best possible equipment as far as all the aspects pertaining to the TBM and the sup porting services are concerned Generally speaking the most reliable machines are the simple ones as they have the least amount of equipment that can break down Foster 1997 In fact the TBM that is designed to cover all eventualities has too frequently in the past tended to be problematic in service and produced per formances below expectations On the basis of the above considerations and recent and past experience it is possible to establish the following points regarding the selection of the type of TBM the shielded TBM s have a wider range of application than the open TBM s this difference in the range of application increases with increasing diameter of excavation open TBM s with a double system of grippers are more sensitive to unstable ground than shields with only a single system of grippers the wider or narrower range of application of a single shielded TBM with respect to the double shielded one depends on the design and dimensioning of the TBM and on the type of limiting situations to be faced rather than the TBM type the choice between a single shielded and a double shielded TBM depends on the design of the tunnel sec tion and whether it is necessary to install a precast lining along the entire length of the tunnel to be con structed It should be recalled that in a tunnelling project there are other problems which need to be solved in addi tion to the technical ones i e one should be reminded that today there are TBM s which can reduce significantly the number of geo logical situations which cause important problems to face advance assuming that these TBM s are cor rectly designed and utilized there still exist some limiting situations which can only be overcome by special interventions with un avoidable consequences on the construction time and cost of the project the use of mixed shields is not a solution for overcoming the limits of TBM application in rock except for some very special cases in this area there is however a great possibility for further development in the design of TBM s for rock and in the definition of special interventions for application to a given situation the importance of the contractor s expertise and above all the personnel director and technical staff on site is often not given the right attention while in reality it plays a primary role in the functioning of a TBM particularly under limiting conditions the time and cost for overcoming limiting conditions in a tunnelling project acquired through a competi tive bidding should be supported by the client He is to account for adequate margins in programming and budgeting whenever a tunnel is to be excavated in difficult ground conditions according to a risk as sessment evaluation this point is valid also for tunnelling projects to be constructed by the drill and blast method Given that a TBM capable of advancing under whatever geological condition does not exist it is also true that the overall result of a project depends on the type of the TBM used and the design and special construction characteristics of the TBM adopted In fact it is not sufficient to just order from a qualified manufacturer a particular type of TBM instead a continuous collaboration and control of all design and construction details are essential by its intended user the contractor This is particularly true as far as there are still no Accepted Standards for the design and construction of a type of TBM and each TBM to be constructed is to be considered as a prototype one dif ferent from another one given that the design and manufacturing of TBM s is a continuous technologically innovative process each tunnelling project has its own characteristics and each specialised contractor has his own traditions and opinions LIMITING GEOLOGICAL CONDITIONS FOR TBM s APPLICATION A limiting condition for the use of TBM excavation can be defined where the geological conditions are such that the same TBM cannot work in the execution modes for which it was designed and manufactured For this reason the advance of the TBM is significantly slowed down or even obstructed A geological condition is intended to be a limiting one only in relation to the type of TBM used its design and special characteristics and eventually any operating errors A particular geological condition becomes a limiting one only when it is beyond a certain importance or when the associated problems are beyond a cer tain level of severity or else due to combination of events each being by itself not critical The Importance of Geological and Geotechnical Investigations Despite the excellent performance of TBM s in favourable ground conditions as reported in recent years e g more than 1 km advancement per month for some hydraulic tunnels in many cases the actual a d vancement rates have been below expectations and certainly less than claimed by TBM manufacturers It would therefore be legitimate to think that besides the unforeseen events such as breakdown or failure of the TBM components the rock mechanics problems are often under evaluated or neglected It should be noted that the purpose of construction is to achieve the objective of the design and that the work must be manlike as defined in design according to the specified safety factors and the expected time and cost The design has always been carried out by using a deterministic approach Reality of construction how ever has never been so This is due to the large number of uncertainties that cannot be avoided at the design stage geological geotechnical hydrogeological uncertainties different types of machines available new or used and different construction techniques Pelizza 1998 Hence at the design stage it is impossible to know every aspect of the geological profile It is therefore necessary to decide whether to optimize the choice of the construction method or the selection of the m a chine for a given tunnel on the basis of the understanding of site geology and geotechnical conditions or of the level of prediction about these conditions up to which point are these predictions optimistic or pessimis tic On the other hand the problem of global optimization is very complex given the large number of geo logical technological environmental and economic financial variables involved At the present time it is becoming possible to manage in probabilistic terms the decision strategies for tunnelling under uncertainty conditions involving various levels of variability Einstein 1996 Xu et al 1996 The fundamental problem is always determined by the physical and geotechnical heterogeneity of the rock mass in which the tunnel is to be excavated For a full face mechanised excavation which is a rather rigid system the strength heterogeneity of the material to be excavated is even more important be it a rock or soil Prior understanding obtained in a correct manner of the geological and geotechnical conditions of the site is fundamental for the development of underground works Up to now too little money has in general been spent on preliminary investigations It has in fact been demonstrated that money spent on such investi gations is greatly compensated by the savings made in terms of construction cost and time Forward probing from a TBM driven pilot tunnel or a main tunnel is not an alternative to an adequate pre investigation A considerable example of the positive effects on the performance of a TBM purposely constructed by accounting for a good understanding of geology obtained on the basis of a detailed and preventive geological investigation is offered by the recent 1995 1997 construction in Spain Trasvase Guadiaro Majaceite of a 12 185 kilometer long water conveyance tunnel 4 2 m inside diameter Castello et al 1999 The tunnel runs under a maximum overburden of 500 m through heterogeneous and complex ground composed of sedimentary and tectonized rocks from hard limestone marbly limestone and soft and swelling argillo arenaceous and clayey flysch The preliminary investigations comprised 29 probe holes for a total length of approximately 6000 m 0 5 m of investigation hole per meter of tunnel Significant convergences with a squeezing behaviour were observed to occur in flysch along a 3940 m length in the northern section of the tunnel where a lining formed by precast segments was installed In the remaining tunnel length in a predominantly hard limestone where stable rock conditions were expected shotcrete and rock bolts were installed followed by a cast in situ concrete lining On the basis of a detailed understanding of both the geological conditions and the construction require ments the TBM used has been constructed with a continuous interaction between the TBM manufacturer and the contractor The TBM had to be designed to bore in both the soft argillaceous flysch and in the hard limestone with the purpose to avoid it from being trapped especially in the clayey sections under a thick overburden The important characteristics of the TBM are the following double shielded TBM with four cylindrical shells of 4794 mm diameter so as to allow the lining installa tion in the argillaceous sections and to advance with grippers in the hard rock mass section cutter head diameter of 4 88 m with the opportunity to overcut up to 50 mm and to be used in the clayey sections with squeezing behaviour variable speed rotation of the cutter head from 0 to 9 rpm versus the ground type with a maximum torque of 3900 kNm the standard upward thrust necessary to the TBM was about 3300 ton and up to 3700 ton when the TBM staggered and set off again The recommended value of the total upward thrust for the design of the TBM was 4000 ton in squeezing ground Lombardi and Panciera 1997 This 10 margin has been very use ful to prevent blocking and has revealed to be insufficient only in very few situations The TBM exhibited a good performance in both hard and soft ground The overall average penetration of 1 82 cm min calculated for all the tunnel sections corresponds to a TBM average daily advance of 26 m The best performances of the TBM have been 78 m day 342 m week 1335 m month The tunnel has been completed in 17 months with an average monthly progress of 747 73 m TYPICAL CASES In the following the relatively more important or frequent difficult ground conditions which can affect TBM performance will be considered borability limits instability of the excavation walls instability of the excavation face fault zones squeezing It should be pointed out that tunnel excavation by a TBM may en counter other difficult ground conditions due to the presence of clayey soil soft ground resulting in settle ment of the TBM strong inflow of groundwater and gas rock bursting rock and water at high temperature and karstic cavities Borability Limits A rock is said to be not borable if the TBM cannot penetrate the face to a sufficient rate and or the wear of the cutting tools exceeds an acceptable limit The borability of a rock should not be established in an ab solute manner but only relative to an alternative drill and blast method comparing the cost and scheduling aspects of both the methods The main index describing the capacity of a TBM to excavate a given rock is the penetration rate per revolution of the cutter head which the TBM is able to achieve under the maximum thrust It i