«Key words: Segmental lining, durability requirements, repair classification, categories of damage, quality system, segment production, mechanical ...»
SELECTION OF SEGMENTAL LINING SYSTEM AND
Jodl Hans Georg*, Kolić Davorin**
* O.Univ.Prof. Dipl.-Ing. Dr.techn. Hans Georg Jodl
Institute for Construction Process and Construction Management
University of Technology Vienna, Karlsplatz 13/234, A-1041 Wien, Austria
** Davorin Kolić M.Sc. PE
Neuron Consult ZT, Pelikanstraße 18, A-4061 Pasching, Austria E-mail: firstname.lastname@example.org Key words: Segmental lining, durability requirements, repair classification, categories of damage, quality system, segment production, mechanical tunnel drive.
Abstract: This document provides information for selecting an appropriate segmental lining system for mechanically driven tunnels with special focus on life span durability of the concrete structures. Important part of sustainable design for precast segmental lining are suitable concepts for handling damaged segments and damaged segmental lining. A catalogue of measures for easy identification and contractual stipulated classification of damages as well as acceptable rehabilitation is proposed. Clearly indicated contractual arrangements and quality control concepts are exemplified and should be generally stated in the tender documents but finalized chiefly in the contract.
1. INTRODUCTION Design and construction of segmental tunnel lining are crucial steps for building a mechanical driven tunnel with a live span of 100 years or more. Since client, designer and contractor have different roles on a project and basically different points of view to the execution preferably clear and fair contractual arrangements should be worked out. In order to avoid conflicts prevention to avoid damages should be aspired. Since damages are hardly avoidable it is necessary to dispose of practicable contractual instruments for proper damage classification and acceptable rehabilitation measures. The specifications for the quality of the precast segments and adequate proposals for striking the goal of durability in spite of unavoidable damages are major tasks in this article addressing clients and contractors as well.
2. SEGMENTAL LINING SYSTEMSThe new age of segmental lining systems starts approximately 15 years ago with several new products on international market and at the same time with the new approach to shield machine design. Basically new products have been new types of connectors and new sorts of waterproofing gaskets. The y have asked for new details of segmental lining but also for new machines having erectors able for fine tuning, easy installation and high carrying capacity. In cross section direction they are two basic systems: one-pass (Figure 1) and two-pass (Figure 2) segmental lining.
10,20 6.95 10 15 1.20 2.20 1.00 2,74
These two systems are coming basically from different approaches to durability of lining systems, from different water tightening concepts and from different understanding of underground behaviour. Requirements on design of segmental lining are therefore also different and therefore design details as well . In longitudinal section of segmental tunnel lining we may differ among different ring division systems and different types of segment shapes: there are universal or parallel rings in use, segment shape differs from trapezoidal to rhomboidal or rectangular shape.
All mentioned types come from a need to provide easy installation of segments connected into full tunnel lining. Even though there are several types of linings that are more or less usual for distinct tunnel cross section sizes main requirement on all linings is to have easy and fast installation with no cracks. Therefore each tunnel will be a new challenge with a new machine and new erector and new details on segmental lining adopted to chosen connectors, gaskets, guiding devoices, drive and installation speed and accuracy.
3. DESIGN PRINCIPLES AND TRENDSBeside all specific requirements of any new project we may depict few specific points that have seriously influenced approach to segmental lining details. One of these is for sure chamfer details on corners (Figure 3) that have been used during last 15 years on both longitudinal and ring joints.
Chamfer detail enabled easier installation of segment into one ring and uses at the same time new type of gaskets that will provide watertight joint closure in the range of tolerances prescribed with improper installation procedure. They enable further imperfections of connectors what is one important moment because different types of connectors are today in use: some of them are longer than 40 years in use and others like different types of dowels are no longer than 5-10 years in application.
Next design principle or better to say trend in design uses nowadays more often hexagonal or so called honeycomb segmental lining or very big rectangular segments. Both systems are representing trends of segment application for best particular tunnel type. Hexagonal systems are nowadays in predominant use for long water supply systems in predominantly good rock conditions. It is anyhow the best solution for almost straight tunnels where very high advance speed has been expected. They are as well good solution for two-pass linings where hexagonal system acts as outer lining toward ground and may not be always high quality and capacity lining part (Figure 4).
Further trend comes from growing capacity of shield machines: they are getting bigger and bigger and recent biggest diameter has reached 15.0 meters of excavation diameter on example of Groene Hart tunnel in Holland passing through soft geological formations saturated with water . The results of big-diameters tunnel trend are segmental lining that have very wide ring widths, at the moment beyond 2.0 meters. Specific point in design is that due to the tunnel size there is plenty of space to install each segment and therefore the shape of segments has returned to rectangular form, as was usual 30 years ago. Details of segmental linings are not so important any more but other details and requirements are put on segmental lining: there are plenty of new details regarding safety requirements that have asked for safety and ventilation chambers on side, numerous crossadits, vault details or in bottom areas. However trends are showing continuous change in development of lining details and application of high quality concrete classes. Experience with sophisticated projects as e.g. SOCATOP project in France will for sure define new levels of tunnel usage and as well new requirements on segmental linings.
4. DURABILITY REQUIREMENTSSegments for segmental lining systems of mechanically driven tunnels are usually produced in special job-site plant or in an existing pre-casting plant. The production process and the subsequent logistic chain may cause quite a number of different cases of segments transfer. The ready produced concrete elements have to be removed and handled several times before being erected to their final position as part of the final segmental lining, the so called Tübbing ring. The durability of any segmental lining system can not be measured directly but it has to meet functional, technologic, environmental and operational requirements. Most countries dispose of guidelines for precast concrete parts although the guidelines regularise mainly the segment production alone but do not cover the whole logistic chain from the mould to the segmental lining. Lack of defaults with rules for the contractual handling of damages is also to be indicated. Most tenders do not enclose any provisions of a contract regulation and a manageable procedure for defects in quality, repair provisions and their counterpart with deduction of price or similar .
Figure 5: Segment production chain from mould to Figure 6: Segment handling in production segmental lining hall This contribution may give some ideas for detecting the possible genesis of defects during the production cycle, handling and placing of elements as well as proposals for the specification of defects which may be corrected or not. Finally a frame of quality management references are offered to be taken into consideration with public contracts for tunnel projects with segmental lining. To meet the advertised requirements and tolerances the segments have to be produced with high accuracy. The assembling of segmental lining to a closed ring of segments without stress concentration and constant width of the joints needs precise design and preparations. The single element has to be brought to that target with a minimum of risk and a maximum of care.
Production and assembling tolerances should be defined project-specific by the producer and contractor. Samples of point of references for dimensions and tolerances are recommended .
The number of possibilities for damages on elements may be demonstrated with some figures showing the complicated application flow depending mainly on the type of segment production and of the tunnel boring machine (Figure 5). For this reason the whole production chain may be divided in two main groups of segment handling. Each production step should be monitored with great carefulness. To assure the best quality an appropriate quality management system should be installed. The main measures of quality checks are pointed out parallel to the production steps (Figure 6). This article is not concerned with the first group material manipulation because it has no effect on the state of the ready casted segments. The second group segment production and the third group segment handling remain within the segment production plant either in the precast production hall or in the storage ground which is usually located near to the hall. Production of precast concrete parts for segmental tunnel lining must be carried out with high accuracy and precision. The segment moulds and the concrete segments have to follow strictly defined tolerances which have to correspond with each other .
Moulds and segments have to be measured carefully. Tolerances have to be specified in tender documents and contract. The tolerance values are usually based on results of deformations caused by temperature influence, deformations due to quality of the casting and shrinkage calculation of the concrete body. Major tasks are the steady-going quality of the concrete mixture, skill and training of the workmanship and permanent quality checks. Key production steps are preparation of moulds before concreting, vibration of moulds filled with young concrete and stripping of the fresh segments. All the crucial procedures are setting the quality of the segment and the segmental lining. In consideration of this facts test rings should be made and carefully evaluated according to imperfections and deviations before starting the tunnel drive. With the process step stripping of the segments they have to be turned over for intermediate storage in the precasting hall to avoid possible damage caused by high flexural tensile stress until outside storage. After surface checks and measuring application of the sealing is made. Finally the segments are prepared for intermediate transport to the outside storage yard. The fourth group segment storage gives attention to numerous segment transportation cycles from the production hall to intermediate storage yard. The segments get piled ready for ring erection at the outside storage yard. The fifth group shows different intermediate transport cycles to the tunnel portal area where the segments may be optionally stored again and through the tunnel to the place of segment erection within the shield tail of the TBM.
5. DAMAGE CLASSIFICATION Tunnelling is a tough job since there are numerous possibilities for partial damage on each segment on its way from casting in the mould to placing in the tunnel. When using segmental lining in tunnelling with TBM and Shield Machines it is necessary to detect the red light of external or partial damage as soon as possible. After stripping a first optical check of the segment has to be made. Depending on compliance with the defined requirements each segment is to be distinguished between good, repairable or eliminable. Well directed measures may achieve appropriate improvement at ongoing production, storage, transport and placing of segments. The objective of this damage classification is to allow client and contractor a well disposed damage assessment balanced on relevant project conditions under defined ancillary conditions without the need to discuss on site every damaged segment whether it can be repaired or must be sorted out.
5.1 Rehabilitation of segments Segments damaged on the way from production to erection must not being assembled in that status. Slightly or partially damaged segments can be renovated and repaired. Heavily damaged segments must be sorted out. For contractual approach according to the rules quantity and quality of acceptable damage should be defined in the contract papers respectively as well as proper rehabilitation measures permitted. The client should commit the contractor to submit a statement for proper rehabilitation for acceptance. For easy and quickly addressing of possible defects a table of damage categories may be proposed (Figure 7). These general categories should be rendered by the contractor more precisely with defined values and specified repair measures. Modern allpurpose repair mortars are available in very high quality which brings the decision for repair to a primarily economic question whether a segment should be repaired or built new.
An assortment of common damages on segments and repair measures may be discussed :
• Voids: the porosity of the surface is conditioned by number and size of voids (determining diameter of a single void) per rated surface area. By adherence to defined maximum values no repair is required. Measure: closure of voids by stopping with cement-bound mortar and final surface smoothing.
• Cracks: distinction between micro-cracks or damage-cracks. Dominant characteristic is the measured crack width. Micro-cracks (in general smaller 0,2 mm) within the groove need no repair since being filled by glue. Damage cracks within the groove may be penetrated with epoxy resin of low viscosity.