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Inspection, maintenance and repair of tunnels: International lessons and practice
MAINTENANCE AND REPAIR OF TUNNELS
Inspection, Maintenance and Repair of Tunnels: International Lessons and Practice J. A. Richards Abetract-As the tunnels internationally andparticularly in Europe become older, the matter of inspection, maintenance and repair adopts an ever increasing degree of importance. This paper o!eecribes come of the experience being gained in this aspect abmad and the importance of learning from this and applying this knowledge to tunnel design, construction and opemtion and maintenance in South Africa. 0 1999 Pibliahed by Eleevier Science Ltd. All rights reserved.
1.
Introduction
T
he maintenance and repair of underground structures has become an increasingly important topic in the tunneling field over the last decade. Nearly twenty-five years ago, the International TunnelingAssociation (ITA) established a working group to study this subject and a number of the member nations of the ITA, including South Africa, have established their own working groups on the subject. The reasons for this interest are fourfold: ?? Cost of Repl.acement / Maintenance. Tunnels are expensive. In South Africa today an approximate value foa a 5-m-diameter tunnel is close to $6000 per metre, or $6 million per kilometre. Not only are replacement values for tunnels high, but also are the costs of major rehabilitation works taking into account the disruption to the service provided by the tunnel. ?? Age. Many of the tunnels in Europe are over 100 years old and the linings of some of these tunnels are now requiring major repair or total replacement. ?? DesignUnder-capacity. As populations and their densities and their mobility grow, as industry and its structure develops requiring the increased movement of goods and raw materials, and as modes of transport increase in efficiency in terms of size, frequency and,speed, tunnels are performing tasks for which they were not originally designed. ?? New Technologies. There have been significant advances in the design of efficient tunnel linings. New materials are being produced for the construction and repair of tunnels. The durability of these %ewn materials requires contlrmation.
These aspects are each considered in more detail in the following sections. The aspects of age, designand undercapacity are closely interrelated in that the design undercapacity is almost exclusively associated with old tunnels, and the under-capacity exacerbates deterioration and degradation of the tunnel lining. These particular aspects are accordingly considered together.
2. Primary Reasons for Well-Planned Maintenance of Tunnels 2.1 Cost of Replacement/Maintenance
of Tunnels
h mentioned above, the capital costs for constructing a new S-m-diameter tunnel in South Africa is of the order of $US6 million per kilometre, dependent upon a number of factors including location, depth, geology, length and general difficulty of construction. In Europe and America, the costs can be two to three times higher. In a paper published in 1996 by the German railways authority (Deutsche Balm AG 19961, the following figures were cited: ?? Replacement value of tunnels = DM39 million/km ?? Partial renewal of tunnels = DM20-30 million/km ?? Complete renewal of tunnels = up to DMSO million/ km under difficult circumstances The additional costs for safety precautions, alterations to the superstructure and the overhead lines are generally the same as the above-mentioned costs, and can also exceed these to a certain degree. Operational problems, especially if trains have to be rerouted as a result of essential periods of closure, can cause additional high costs, depending on the extent to which the line is utilized, the possibilities for rerouting and the length of time during which repair works take place. The time cost of disruption to the railways as well as for tunnels providing other types of services such as water,
Presentaddress:J. A. Rkbards, Consult4 (Pty)Ltd,P.O.Box73536 Fairland 2030, Johannesburg, South Africa. This paper is modified fern a presentation given at TUNCON ‘97, and is reproduced with the permission of theSouth African National
Committeeon Tunnelling.
!hmelling and Underground Space Technology. Vol. 13, No. 4. pp. 399-375, 1998 Oaa67798/99/S--see front matter C 1999 Published by Elsevier Science Ltd. AU rights reerved. Pm3oaa9-779~9a)00079-0
Woete were originally expressedin the South African currency Rands (R). F&O0 equals approximately $USl; DM1,60 equals
approximatelyUS$l.
Pergamon
hydropower, roads, etc., is difficult to calculate accurately because of the many consequential costs. One example ofcosts of disruption in South Africa is the Drakensburg Pumped Storage Scheme. The direct cost in terms of loss of electricity generation, pumping of water and synchronous condenser mode as a result of shutting down one of the two pressure tunnels, each of which serves two of the 250-MW turbines, is about US2800 per hour. When one considers that it takes approximately 2 days to de-water and 3 to 6 days to water up again, one is looking at a minimum cost for shutting down in the order of $US600,000. It is often the case that there are pressures put on the engineers to construct a tunnel within tight budget constraints. Ifthese constraints are unrealistic, this can lead to uneconomical solutions involving a lowering of technical standards, an increase in maintenance/service disruption costs, and a shortening of the life of the tunnel before major rehabilitation works are required. It is essential that at the inception of any tunnel project there is a comprehensive economic appraisal covering the capital costs, the maintenance and disruption costs, and the eventual replacement costs. As evidenced by the high costs of carrying out repairs, taking into consideration the disruption costs, in certain cases, the building of a new tunnel will be more economical than undertaking maior repairs. Notwithstanding this, a well-planned inspection and preventative maintenance programme can prolong the working life of a tunnel considerably, at the same time delaying or avoiding altogether the need for major rehabilitation works. 2. Age and Design Under-capacity of Tunnels
Aa an example of the problem of aging of tunnels in the German railway system, there are currently 746 tunnels with a total length of approximately 407 km and a replacement value of DM16 billion. Of these, 491 were built between 1840 and 1940 and alone have a replacement value of DM7 billion. Similarly, impressive figures apply to France, Italy, the United Kingdom, and many other countries of Europe encompassing road, as well as railway tunnels. However, dealing with aged tunnels is not confined to Europe. More than 200 railroad and highway tunnels in North America have been the subject ofrehabilitation and upgrading works during the last decade. Many of the railway tunnels were constructed between 1880 and 1930 and varied in length from 30 m to over 13 km. The costs, as indicated above and given in Section 2.1, are massive, and the importance of that financial investment and the asset itself continues to grow as more and more demands are placed on tunnels. The growth in populations and population density, particularly in urban areas, advances in technology and industrial development have, among a number of other factors, placed an ever-increasing demand on infrastructure. Although engineers are sometimes credited with foresight, it cannot be expected that the tunnel engineer in 1840, who designed a single-track brick-lined railway tunnel, could have foreseen that the tunnel would still be in operation some 130 years later and, furthermore, that it would be serving high-capacity railway locomotives and wagons moving at speeds that were beyond his wildest dreams. Notwithstanding the fact that the tunnels were probably over-designed in the first place, the need for close inspection ofthe tunnel condition, planned and systematic maintenance and, in many cases, the upgrading of the tunnel to safely meet the demands now being placed on it are obvious. The situation is particularly serious in the case of older road tunnels, where the exponential growth of trtic has led to increased danger of collision and the need to upgrade the ventilation and monitoring systems to cope with ex-
370 TUNNELLING ANDUNDERGROUND SPACETECHNOI&GY
haust fumes, and, in the worst-case scenario, to cope with collision-induced fires. 2.3 New Technologies The early tunnels were constructed with brick, masonry and mortar linings, sometimes with timber arched support. From this has developed over time the use of cast iron segments, cast-in-situ unreinforced and reinforced concrete, reinforced concrete segments, mesh and fibre reinforced shotcrete and localised rock support in largely unlined tunnels. The technology for excavation of tunnels, support of the excavation and lining of the tunnels is developing at an ever increasing rate. These developments are inevitably interrelated, as methods of excavation improve and become more efficient, the need for increased speed of installation of primary rock support grows to allow full advantage to be taken of the efficiency in excavation. If the primary support of the rock can contribute to the permanent support and final lining of the tunnel, this leads to further efficiencies in construction and reduction in costs. Whereas, the first tunnels used technologies in construction materials which had been known for decades and in some cases even centuries, this is increasingly not the case. The high capital costs of a tunnel and the immediate requirements for improved infrastructure services put demands on the design of the tunnel not only in terms of unit costs ofconstruction materials used, but also in terms of speed of construction. Materials and methods of construction may no longer be fully proven; there is little or no data on long-term performance. “Competitive” construction practices also play their part; when “short cuts” are taken, there is lack of attention to detail, or the construction methodology demands the over-utilisation of repair crews following on behind the main construction work. In appreciation of reduced standards in workmanship which has largely accompanied the increased speed of construction, there has been a significant growth in the market for convenient and efficient repair materials. The technological developments which have been made on plastics and resins are considerable. In most cases, the case history data associated with the use of these repair materials are either short or nonexistent. The impact of these new construction and repair materials on the requirement for maintenance of the tunnel can only be fully assessed with the passage of time. However, it can and should be assessed as part of the overall cost of the project in terms of an economic risk analysis.
3. Primary Causes for Repair of Tunnels For the ITA conference held in Brazil in 1998, there was a session devoted to the repair, maintenance and upgrading of underground structures. Of eighteen papers submitted for discussion at this session, nine were related to dealing with water leakage, four were related to seismic activity, one was concerned with the dealing with an aggressive operating environment, and the remainder referred to the implementation of maintenance measures and upgrading of tunnels to achieve higher capacity. From this it is clear that water is the principal reason for damage to and degradation of tunnels and is dealt with in some detail in the following section. The matter of seismic activity is not considered in this paper because it is not a significant factor in the design of tunnels in South Africa, which is a relatively stable region seismicalv. The matter of aggressive operating environments is a specialist subject in its own right and specific in its relevance to particular environments. It is therefore only briefly discussed in terms of its general implications with regard to the maintenance of underground structures. -----d.
Volume 13, Number 4,1998
Other factors which are considered are the types of tunnel lining and methods of construction. 3.1 Wafer Leakage inI Tunnels Water is a problem in any tunneller’s life, its seriousness changing only lby degrees. Water causes problems during excavation and support of the ground, can give rise to more expensive linings and as evidenced by the number of papers on the subject at the 1998 ITA tunnelling conference, water leakage is the most common maintenance hazard, causing problems during the working life of the tunnel, not only to the tunnel lining, but also to the fittings within the tunnel. Problems are commonly encountered with incoming water in tunnels in conjunction with frost, giving rise to: . reduction of the size of the tunnel opening by the formation of ice barriers; . icing of the pavement in road tunnels; . obstruction of ventilation and other service ducts and shafts; ?? hazards from icicles formina in the tunnel roof. Notwithstanding the fact that we do not experience such problems in South Africa, water leakage damages the tunnel lining, the type of damage varying with the type of lining and also the method / integrity of the construction. Two main types of lining are reviewed: (1) Brick and masonry tunnels and (211concrete tunnels. 3.1.1 Brickwork and ‘masonry tunnels In many of the older tunnels, the lining is made up of 5 or more courses of brickwork. In areas where ground support was required during construction of the lining, this was provided by wooden beams and uprights which were often left in place. The inevitable voids between the lining and the excavated ground were filled by packed tunnel debris, generally comprising a high proportion of stones and boulders. The latter provided a good drainage medium behind the lining. Bricks are sensitive to water, even if it is not aggressive, depending on their firing temperature and chemical nature. Water leakage lencouraged by the drainage medium inadvertently formed. behind the lining leads over a period of time (decades) to decomposition or rotting of the timber supports, washing out of the ground and backfill material behind the lining causing large voids or hollow areas to form, and weathering and deterioration of the brickwork and the mortar of the lining. The effects of water on mortars, especially those made with lime, are well known: the mortar loses its strength and becomes brittle. 1.nsome cases, chemical reaction from sulphates in the seeping water causes swelling. The forming of voids can lead to increased water leakage and collapsing of ground which, together with water filled voids, may cause structural distress to the lining. 3.1.2 Concrete Tunnels These take the form of tunnels with unreinforced and reinforced cast-insitu concrete linings or tunnels with segmental concrete linings. Water leakage is most commonly the cause of giving rise to transportation of fines, the formation of voids, settlement of ground, and eventually eccentric loading and distress of the lining, as described previously for the brickwork and masonry tunnels. Inadequate sealing of the joints between concrete segments and poorly constructed joints in cast in situ concrete linings can lead to water leakage. Other aggravating circumstances are posed by rising groundwater tables. In many of the industrial capitals arcund the world, where the extraction of groundwater for
Volume 13, Number 4,1998
water supply has been replaced by alternative surface water sources and waterproofing of basement structures has improved, the water table has risen. Where tunnels were constructed above the water table and no provision was made for waterproofing joints, rising groundwater may now enter the tunnel quite freely. Water seepage through porous or cracked concrete will result in loss of cement, making the lining more pervious and less strong. Aggressive water may cause decomposition of the aggregate as well as the cement. The corrosion of reinforcement in both reinforced castinsitu and segmented concrete linings is a principal concern. Corrosion is caused by pervious concrete or inadequate concrete cover and will inevitably lead to cracking and spalling of the concrete and loss of structural capacity. The presence of soft water or chlorides in the water will accelerate the effects. In rail tunnels, where stray currents from traction supply typically occur, the electrolytic action of the moisture may be particularly severe, with the rapid migration of the chloride irons to the steelwork attracting the current. 3.2 Aggressive OpersUng Envimnmenfs Aggressive operating environments for tunnels can prevail either on the inside or the outside ofthe tunnel or both. With respect to the external environment, this is generally associatedwithcontaminatedgroundwatercontainingchlorides, sulphates or other chemical pollutants coming into contact with the lining. Particularly aggressive external environments prevail in under sea or immersed tube tunnels. The damage process in all these cases is largely as described in section 3.1. The internal environment of a tunnel can vary significantly both in terms of level and type of aggressiveness. Sewer tunnels are a typical example where the tunnel lining is subject to bacteriological and chemical attack in a moist environment. It has been necessary in a number of circumstances to replace the lining or install an inner chemically inert lining within a few years of commissioning. In such circumstances it is critically important to undertake comprehensive and detailed testing programmes of lining materials with due appreciation of the standards of construction, which can be realistically achieved. 3.3 Construct/on Methods A tunnel requires a high capital investment and demands a high standard of construction to give as high a return as possible These standards can be severely compromised by the highly competitive environment we find ourselves in and the budgetary limits imposed by the tunnel owner. These, together with a lack of craftsmanship, attention to detail and poor design can all play their part in reducing the useful life of the tunnel and increasing operating costs due to additional maintenance requirements. Typical construction faults in concrete lined tunnels which compromise the durability of tunnel linings are: ?? inadequate backtllling or grouting of voids between the lining and the excavation; ?? porous concrete; . segregation; . cracking due to thermal shrinkage; . inadequate cover to reinforcement; ?? poor joint construction between cast-in-situ concrete pours or concrete segments. The situation can be further exacerbated by poor design or unrealistic specifications which cannot be practically fulfilled. The advantages of using experienced and wellqualified designers and competent contractors cannot be overstated.
TUNNJZLLING ANDUNDERGROUND SPACETECHNOLOGY 3’71
4. Inspection and Testing of Tunnels Notwithstanding how well a tunnel is constructed, it will require preventative maintenance to preserve its integrity and to prolong its useful life. Maintenance will necessarily require inspection and testing to determine the condition of the installation and to establish appropriate repair and maintenance measures, if any. However, by its very nature, only the intrados of the tunnel is open to visual inspection and while such an inspection, undertaken in sufficient detail, can reveal very useful data, it cannot provide the whole story. Furthermore, as explained in Section 2.1 (in the case ofDrakensburg Pumped Storage Schemes), tunnel outages are expensive in terms of loss of revenue and disruption to industry and people. It is therefore important that inspections be as short as possible and at the same time serve to provide all the necessary information. The inspection and testing of tunnel linings is an exhaustive subject on its own, and it is possible to touch only briefly on some of the international practices and the more important findings in this paper. Because of the aging oftunnel structures, the increasing demands put upon them, the increasing cost ofreplacement and the severe disruption caused by closure of tunnels, there has been considerable importance placed by tunnel owners worldwide on methods of inspection and testing. In a number of countries, there are now well-structured procedures in place. Typical components are discussed in the following sections. 4.1 Establishing a Diagnostic File on the Facility This would typically include all criteria important to the integrity of the structure: geology, hydrology, construction methods, materials, incidents, repair work, general characteristics, operating conditions and design criteria. This file should remain active and be updated on every event, visit and inspection.
and analysis. This specialist would typically be a qualified engineer or technologist with significant experience in the design and construction of underground facilities. The inspection can again take the form of a number of stages progressing from surface observations and measurements, through non-destructive testing of the lining constituents and voids/backfill behind the lining, to destructive testing involving drilling and cutting out of lining sections. In summary, the types of inspection of the intrados may include: Visual inspection: this can include video and visible light surveying, crack width measurements . Geometrical surveying: photographs are taken of a light beam on the surface. The light beam is a laser beam reflected perpendicularly to the tunnel axis. Shots are taken by a wide-angled camera travelling on a wagon in a referenced position in the tunnel cross-section. Typically, photographs are taken at 3m intervals on a vehicle travelling at 4 km per hour. Accuracy is ti cm for a lo-m-diameter tunnel. Convergence measurement: using specially manufactured devices (e.g. invar wire). Accuracy is 0.1 mm. Sonar profile: used for profiling ducts and tunnels filled with liquid. The reflection of acoustic waves on the tunnel walls is measured. Through high-definition TV images and computer processing, accuracies of less than 1 mm can be achieved. Surface thermography: this is discussedfurther under in Section 4.4 (Nondestructive testing), below. Using an infra-red radiometre, thermal radiation emitted by the lining is measured, thereby detecting humid or damp patches, and lining irregularities, as well as differences in its constituents. Measurement devices: these devices can all be adapted for remote station monitoring. Trends in the measurements would be reviewed and analysed.
4.2 Routine Monitoring and Measurement It is important that there are established procedures for the manager of the facility and operating staff to undertake routine inspections. The frequency of inspections will be determined by the type of facility, changes in the operating environment and alterations in the state of the facility. Appropriate instrumentation and measurement installations, and the updating and review ofthe diagnostic file play an important role in this regard. Typical measurements would include: ?? crack width measurements taken with a vernier gauge or similar instrument; ?? measurement of piezometres, using electric probes; ?? flow measurement in drainage channels; ?? photographic records; ?? vibrating wire strain gauge; and . extensometres. The routine monitoring would be carried out using comprehensive identification sheets on which observations and measurements can be conveniently recorded. The routine inspections will serve to determine the weak points, the damaged areas, the urgent actions to be taken and the areas to be checked by a specialist. For this purpose, the manager’s personnel must be specially trained to be in a position to objectively assess the condition of the facility. 4.3 Surveying of the lntrados by a Specialist In cases where the routine inspections reveal a change in environment and/or conditions ofthe facility, a specialist usuallywould be consulted to undertake further inspections
372 -G
ANDUNDERGROUND SPACE!L’ECHNOLOGY
4.4 Non-destructive Testing The fill range of testing as described above is not carried out in the vast majority of the operating tunnels worldwide. In many cases, the testing is limited to close visual inspection of the tunnel intrados by experts. However, in recognition of the lack of information provided by such inspections with respect to the constituents of the tunnel lining and the surrounding ground, there have been a number of developments with respect to nondestructive testing. With the exception of minor drilling and wring to calibrate themeasurement devices, this testing allows an analysis of the lining and ground to determine abnormalities, defects and/or voids without damaging the lining. Interest in these techniques has grown, particularly in France and Germany, which is understandable because of the large number of tunnels in these countries and their high asset value. Three of these techniques are briefly described and compared below. 4.4.1 Geo-radar Testing with geo-radar utilises electromagnetic waves emitted from an antenna on or near the surface The wave frequencies are between 80 and 1000 MHz. The diffusion of the electromagnetic waves through the lining medium is principally influenced by two properties of the material. . conductivity, the ability of the material to conduct electricity; and ?? diclcctricity, a phenomenon that governs the diffusion speed of the electromagnetic waves in different materials.
Volume 13, Number 4,1998
Table 1. Dklectricity
constant and diffusion speed of electromagnetic
waves in various materials.
Relative Dielectric Constant (ER)
Diffusion Spesd (m/s),
1
3 x loa
Water (pure)
81
3,3 x 10’
Salt water
81
3,3 x 10’
IU?
4
1,5 x 10’
Concrete (dry)
6
1,2 x 10”
1
Concrete (moist)
12
0,86 x 10’
5
Granite (dry)
5
1,3 x 10’
< 0,001
Granite (moist)
7
1,l x 10’
1
Limestone (dry)
7
1,l x 10”
< 0,001
Limestone (moist)
8
1,06 x 10’
25
Sandstone (moist)
6
2,2 x 10’
40
Basalt (moist)
8
1,06 x 10 ’
10
Material Air
Conductivity (m/s)
app~x
0 O,l-
0,3
400 O,l-
0,3
Concrete, sandy (dry)
4
1,5 x 10 *
OS
Concrete, sandy (saturated)
30
5,5 x 10’
7
Clay (saturated)
10
0,95 x 10”
30
1
3 x lo*
> lo-!
Metals
As _ _through a medium, their_.intensity is _ the __ waves pass gradually weakened, ‘due to reflection due to a discontinuity and amplitude attenuation resulting from the conductivity of the material. The :amount of wave attenuation or weakening also is influenc’ed by the frequency of the waves. The signal emitted by the antenna is reflected when it meets a discontinuity, which corresponds to a change in dielectric properties (see Table 1). Therefore, three factors-fiequency, reflection and conductivitydetermine the penetration depth of the waves into the tunnel structure. The antenna is moved across the surface and as the reflection angle and transit time of the wave changes, a tl’ansit time curve is generated and plotted, thereby locating the defect or irregularity (see Fig. 1). The geo-radar is thereby able to study . changes in the nature of masonry; . structural abnormalities (voids); . thickness variations; i constituent materials of the tunnel lining (reinforcement, arches, etc); . the surrounclmg ground (interface voids between the lining andlthe ground, detection and location of voids, water t3pots, variations in constituency, abnormalities). There are the following limitations in its use: . the nature of ,the materials crossed affect the depth which can be investigated and the wave frequency required, . wave alteration increases as the presence of water and clay increases, . the detection of defecta is made more difficult by the presence of &eel reinforcement mate. Radar antennas ofhigher frequency are required (900 MHz to 1,0 GHz). However, the higher frequency or shorter wavelengths reduce the depth of penetration.
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4.4.2. Infrared thermography Infrared thermography measures the thermal radiation emitted by the tunnel’s walls. Infrared registration techniques allow visual presentation of the temperature distribution on the surface. The temperature on the surface represents the thermal flow through the surface, which in turn is influenced by the mechanical and/or hydraulic discontinuities of the structure. Consequently, thermal discontinuities on a surface reflects abnormalities within the underlying structure. For use in tunnels, an infrared camera or scanner is generally mounted on a moving vehicle. Because the method
Antenna
5 p& Defect
B F .Z 8 ITransit Time Curve Figul v 1. Principle of geo-radar investigation. Above: Measuring Geometry. Below: Transit Time Curve.
!I’UNNEIJANG ANDUNDERGROUND SPACETECHNOLOGY 373
is dependent on measurement of temperature gradients, it is best to carry out measurements in winter, when gradients are maximised. Infrared thermography is able to identify: . water circulating at a different temperature from that of the lining or surrounding rock; . changes in the geological conditions behind the lining; ?? defects in the lining; and . voids. There are the following limitations in its use: . a basic prerequisite is a stable thermal flow through the tunnel wall. This should be constant over the period of measurement in order to assess changes in conditions and the gradient between the rock and the lining surface should be at least 2 to 4”C, dependent on the accuracy of the scanner; . the tunnel lining should not have any coating or installation that will prevent thermal radiation penetration and, as a result of the insulating effect of the coating, alter the thermal flow through the material; . variations in water content over time will cause disturbance in the results; . it is less sensitive than geo-radar. It is very difficult to assess the depth and shape of an abnormality based on thermography alone. 4.4.3. Multi-spectral analysis tests Photographs of the surface of the tunnel lining are taken using six special filters each of which cover small spectral areas of the entire light spectrum. At least one shot of the same area is taken per filter. A multi-spectral projector is used for evaluating the photographic film. The black and white film seen against a
colored backdrop, allow the fine shades of grey to become visible and by superimposing the shots with different filters, it is possible to make visible the fine spectral differences, emanating from moisture and other defects on the surface. It is possible to highlight fine cracks of down to 0,3 mm as bright lines. 4.4.4 General comments All of the above methods provide relatively rapid inspection of the tunnel lining and, if possible, should be used together to complement and corroborate results. If surface defects and fine cracks are the principal area of interest, the multispectral analysis will be the preferred method of testing. Table 2 compares the three systems of testing. It is recommended that drilling/boring of the tunnel in selected areas is carried out to provide confirmation and/or calibration of the findings of the geo-radar and infrared thermography. Only if defects, abnormalities, or changes in the lining/ ground environment are detected, would further testing using destructive methods be carried out. The speed of the investigation using the above methods depends upon the tunnel conditions and, in the case of the infrared thermography and multi-spectral analysis, can be 0,5 kmlhr and faster. Other methods of non-destructive testing include the use of ultrasound or ultrasonic waves and mechanical impedance methods involving the measurement of shock waves by geophones. It can be anticipated that the development in the technology associated with non-destructive testing will be rapid, overcoming many of its present limitations. It is foreseen that in the future, it will be possible to obtain a complete condition profile ofthe tunnel structure and that the testing will replace the arduous visual inspections, the results of which are, to a large extent, superficial and can be subjective.
Table 2. Examples of defects, which can /cannot be identified by geo-radar, infrared, thermography analysis.
and multi spectral
Defect identified by
Type of Defect
Dim. of Defect (cm)
10
Cavities and moist patches
20
Concrete Cover (cm)
Gee-radar with various anennae
Thermography
Multi-spectral Analysis
5
Yes
Yes
No
10
Yes
No
No
20
Yes
No
No
40
No
No
No
5
Yes
Yes
No
10
Yes
Yes
No
20
Yes
No
No
40
Yes
No
No
5
No
Yes
Yes
10
Yes
Yes
No
20
Yes
Yes
No
40
Yes
No
No
Moist patches at surface
Yes
Yes
Yes
Dry cracks with a width of 0.3 to 3 mm
No
No
Yes
40
374 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY
Volume 13, Number 4,1998
As the number of tunnels in South Africa and elsewhere increases and inspection and maintenance requirements increase, the use of comprehensive and reliable testing and inspection devices will become more appropriate. 5. Conclusions The conclusions with respect to maintenance and repair of tunnel structures that can be drawn from experience in South Africa:, as well as abroad, tend towards the obvious. However, in stating them, they are useful for reference, not only by tunnel owners and operators, but also by designers and contractors. A holistic approach to tunnel design and construction is thereby achieved. 1. Not withstanding the part played by tunnels in improving and conserving our environment, they require a relatively high capital investment. 2. A well-structured inspection and maintenance programme can prolong the useful life of a tunnel, thereby improving the financial rate of return. 3. Inspection of tunnels can have a high associated service disruption cost; therefore, there should be a balance between frequency, length, comprehensiveness and cost of inspection, bearing in mind that inspecticm plays a critical part in preventative maintenance. 4. It is important to avoid, as far as possible, lengthy disruption of the tunnel due to major rehabilitation works. 5. There are new technologies for testing being developed which can give comprehensive reports on the surface conditions of the tunnel lining, as well as the constituents of the tunnel lining and surrounding ground. With development, the speed and compre-
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6. 7.
hensiveness of the inspections are increasing and the associated costs are decreasing. The major cause of damage to tunnels that require repair and maintenance works is water leakage. The design of tunnel linings should be such that damage from the tunnel working environment and, particularly, water leakage-is avoided as far as is practically feasible. The additional cost of construction of an improved lining design should be weighed against the cost of maintenance and associated service disruption. The use of new lining and repair materials with no case history should be avoided. If the use of new technology is significantly advantageous, a thorough testing programme should be instituted to ensure that there are no “surprises”. The design of tunnel structures should provide, as far as possible, for easy access for inspection. The design should take account of new inspection technologies, thereby reducing costs of inspections and disruptions as well as providing for more cost-effective preventative maintenance and repair.
6. References Wide use of information provided in the following references was made in the preparation ofthis paper, particularly in Section 4, Inspection and Testing of Tunnels. AFTEX(FrenchTunnellingAssociation)Working Group No. 14. Recommendations on Diagnosis Methods for Lined Tunnels. Paris: AFTES. Haack, A; Schreyer, J.; and Jackel, G. 1996. State of the Art of Non-Destructive Methods for Determining the State of a Tunnel Lining. STUVA Report. Cologne, Germany: STUVA.
TUNNELLING ANJJ UNDERGROUND SPACE!~‘ECHNOLQGY 376
Inspection, Maintenance and Repair of Tunnels: International Lessons and Practice J. A. Richards Abetract-As the tunnels internationally andparticularly in Europe become older, the matter of inspection, maintenance and repair adopts an ever increasing degree of importance. This paper o!eecribes come of the experience being gained in this aspect abmad and the importance of learning from this and applying this knowledge to tunnel design, construction and opemtion and maintenance in South Africa. 0 1999 Pibliahed by Eleevier Science Ltd. All rights reserved.
1.
Introduction
T
he maintenance and repair of underground structures has become an increasingly important topic in the tunneling field over the last decade. Nearly twenty-five years ago, the International TunnelingAssociation (ITA) established a working group to study this subject and a number of the member nations of the ITA, including South Africa, have established their own working groups on the subject. The reasons for this interest are fourfold: ?? Cost of Repl.acement / Maintenance. Tunnels are expensive. In South Africa today an approximate value foa a 5-m-diameter tunnel is close to $6000 per metre, or $6 million per kilometre. Not only are replacement values for tunnels high, but also are the costs of major rehabilitation works taking into account the disruption to the service provided by the tunnel. ?? Age. Many of the tunnels in Europe are over 100 years old and the linings of some of these tunnels are now requiring major repair or total replacement. ?? DesignUnder-capacity. As populations and their densities and their mobility grow, as industry and its structure develops requiring the increased movement of goods and raw materials, and as modes of transport increase in efficiency in terms of size, frequency and,speed, tunnels are performing tasks for which they were not originally designed. ?? New Technologies. There have been significant advances in the design of efficient tunnel linings. New materials are being produced for the construction and repair of tunnels. The durability of these %ewn materials requires contlrmation.
These aspects are each considered in more detail in the following sections. The aspects of age, designand undercapacity are closely interrelated in that the design undercapacity is almost exclusively associated with old tunnels, and the under-capacity exacerbates deterioration and degradation of the tunnel lining. These particular aspects are accordingly considered together.
2. Primary Reasons for Well-Planned Maintenance of Tunnels 2.1 Cost of Replacement/Maintenance
of Tunnels
h mentioned above, the capital costs for constructing a new S-m-diameter tunnel in South Africa is of the order of $US6 million per kilometre, dependent upon a number of factors including location, depth, geology, length and general difficulty of construction. In Europe and America, the costs can be two to three times higher. In a paper published in 1996 by the German railways authority (Deutsche Balm AG 19961, the following figures were cited: ?? Replacement value of tunnels = DM39 million/km ?? Partial renewal of tunnels = DM20-30 million/km ?? Complete renewal of tunnels = up to DMSO million/ km under difficult circumstances The additional costs for safety precautions, alterations to the superstructure and the overhead lines are generally the same as the above-mentioned costs, and can also exceed these to a certain degree. Operational problems, especially if trains have to be rerouted as a result of essential periods of closure, can cause additional high costs, depending on the extent to which the line is utilized, the possibilities for rerouting and the length of time during which repair works take place. The time cost of disruption to the railways as well as for tunnels providing other types of services such as water,
Presentaddress:J. A. Rkbards, Consult4 (Pty)Ltd,P.O.Box73536 Fairland 2030, Johannesburg, South Africa. This paper is modified fern a presentation given at TUNCON ‘97, and is reproduced with the permission of theSouth African National
Committeeon Tunnelling.
!hmelling and Underground Space Technology. Vol. 13, No. 4. pp. 399-375, 1998 Oaa67798/99/S--see front matter C 1999 Published by Elsevier Science Ltd. AU rights reerved. Pm3oaa9-779~9a)00079-0
Woete were originally expressedin the South African currency Rands (R). F&O0 equals approximately $USl; DM1,60 equals
approximatelyUS$l.
Pergamon
hydropower, roads, etc., is difficult to calculate accurately because of the many consequential costs. One example ofcosts of disruption in South Africa is the Drakensburg Pumped Storage Scheme. The direct cost in terms of loss of electricity generation, pumping of water and synchronous condenser mode as a result of shutting down one of the two pressure tunnels, each of which serves two of the 250-MW turbines, is about US2800 per hour. When one considers that it takes approximately 2 days to de-water and 3 to 6 days to water up again, one is looking at a minimum cost for shutting down in the order of $US600,000. It is often the case that there are pressures put on the engineers to construct a tunnel within tight budget constraints. Ifthese constraints are unrealistic, this can lead to uneconomical solutions involving a lowering of technical standards, an increase in maintenance/service disruption costs, and a shortening of the life of the tunnel before major rehabilitation works are required. It is essential that at the inception of any tunnel project there is a comprehensive economic appraisal covering the capital costs, the maintenance and disruption costs, and the eventual replacement costs. As evidenced by the high costs of carrying out repairs, taking into consideration the disruption costs, in certain cases, the building of a new tunnel will be more economical than undertaking maior repairs. Notwithstanding this, a well-planned inspection and preventative maintenance programme can prolong the working life of a tunnel considerably, at the same time delaying or avoiding altogether the need for major rehabilitation works. 2. Age and Design Under-capacity of Tunnels
Aa an example of the problem of aging of tunnels in the German railway system, there are currently 746 tunnels with a total length of approximately 407 km and a replacement value of DM16 billion. Of these, 491 were built between 1840 and 1940 and alone have a replacement value of DM7 billion. Similarly, impressive figures apply to France, Italy, the United Kingdom, and many other countries of Europe encompassing road, as well as railway tunnels. However, dealing with aged tunnels is not confined to Europe. More than 200 railroad and highway tunnels in North America have been the subject ofrehabilitation and upgrading works during the last decade. Many of the railway tunnels were constructed between 1880 and 1930 and varied in length from 30 m to over 13 km. The costs, as indicated above and given in Section 2.1, are massive, and the importance of that financial investment and the asset itself continues to grow as more and more demands are placed on tunnels. The growth in populations and population density, particularly in urban areas, advances in technology and industrial development have, among a number of other factors, placed an ever-increasing demand on infrastructure. Although engineers are sometimes credited with foresight, it cannot be expected that the tunnel engineer in 1840, who designed a single-track brick-lined railway tunnel, could have foreseen that the tunnel would still be in operation some 130 years later and, furthermore, that it would be serving high-capacity railway locomotives and wagons moving at speeds that were beyond his wildest dreams. Notwithstanding the fact that the tunnels were probably over-designed in the first place, the need for close inspection ofthe tunnel condition, planned and systematic maintenance and, in many cases, the upgrading of the tunnel to safely meet the demands now being placed on it are obvious. The situation is particularly serious in the case of older road tunnels, where the exponential growth of trtic has led to increased danger of collision and the need to upgrade the ventilation and monitoring systems to cope with ex-
370 TUNNELLING ANDUNDERGROUND SPACETECHNOI&GY
haust fumes, and, in the worst-case scenario, to cope with collision-induced fires. 2.3 New Technologies The early tunnels were constructed with brick, masonry and mortar linings, sometimes with timber arched support. From this has developed over time the use of cast iron segments, cast-in-situ unreinforced and reinforced concrete, reinforced concrete segments, mesh and fibre reinforced shotcrete and localised rock support in largely unlined tunnels. The technology for excavation of tunnels, support of the excavation and lining of the tunnels is developing at an ever increasing rate. These developments are inevitably interrelated, as methods of excavation improve and become more efficient, the need for increased speed of installation of primary rock support grows to allow full advantage to be taken of the efficiency in excavation. If the primary support of the rock can contribute to the permanent support and final lining of the tunnel, this leads to further efficiencies in construction and reduction in costs. Whereas, the first tunnels used technologies in construction materials which had been known for decades and in some cases even centuries, this is increasingly not the case. The high capital costs of a tunnel and the immediate requirements for improved infrastructure services put demands on the design of the tunnel not only in terms of unit costs ofconstruction materials used, but also in terms of speed of construction. Materials and methods of construction may no longer be fully proven; there is little or no data on long-term performance. “Competitive” construction practices also play their part; when “short cuts” are taken, there is lack of attention to detail, or the construction methodology demands the over-utilisation of repair crews following on behind the main construction work. In appreciation of reduced standards in workmanship which has largely accompanied the increased speed of construction, there has been a significant growth in the market for convenient and efficient repair materials. The technological developments which have been made on plastics and resins are considerable. In most cases, the case history data associated with the use of these repair materials are either short or nonexistent. The impact of these new construction and repair materials on the requirement for maintenance of the tunnel can only be fully assessed with the passage of time. However, it can and should be assessed as part of the overall cost of the project in terms of an economic risk analysis.
3. Primary Causes for Repair of Tunnels For the ITA conference held in Brazil in 1998, there was a session devoted to the repair, maintenance and upgrading of underground structures. Of eighteen papers submitted for discussion at this session, nine were related to dealing with water leakage, four were related to seismic activity, one was concerned with the dealing with an aggressive operating environment, and the remainder referred to the implementation of maintenance measures and upgrading of tunnels to achieve higher capacity. From this it is clear that water is the principal reason for damage to and degradation of tunnels and is dealt with in some detail in the following section. The matter of seismic activity is not considered in this paper because it is not a significant factor in the design of tunnels in South Africa, which is a relatively stable region seismicalv. The matter of aggressive operating environments is a specialist subject in its own right and specific in its relevance to particular environments. It is therefore only briefly discussed in terms of its general implications with regard to the maintenance of underground structures. -----d.
Volume 13, Number 4,1998
Other factors which are considered are the types of tunnel lining and methods of construction. 3.1 Wafer Leakage inI Tunnels Water is a problem in any tunneller’s life, its seriousness changing only lby degrees. Water causes problems during excavation and support of the ground, can give rise to more expensive linings and as evidenced by the number of papers on the subject at the 1998 ITA tunnelling conference, water leakage is the most common maintenance hazard, causing problems during the working life of the tunnel, not only to the tunnel lining, but also to the fittings within the tunnel. Problems are commonly encountered with incoming water in tunnels in conjunction with frost, giving rise to: . reduction of the size of the tunnel opening by the formation of ice barriers; . icing of the pavement in road tunnels; . obstruction of ventilation and other service ducts and shafts; ?? hazards from icicles formina in the tunnel roof. Notwithstanding the fact that we do not experience such problems in South Africa, water leakage damages the tunnel lining, the type of damage varying with the type of lining and also the method / integrity of the construction. Two main types of lining are reviewed: (1) Brick and masonry tunnels and (211concrete tunnels. 3.1.1 Brickwork and ‘masonry tunnels In many of the older tunnels, the lining is made up of 5 or more courses of brickwork. In areas where ground support was required during construction of the lining, this was provided by wooden beams and uprights which were often left in place. The inevitable voids between the lining and the excavated ground were filled by packed tunnel debris, generally comprising a high proportion of stones and boulders. The latter provided a good drainage medium behind the lining. Bricks are sensitive to water, even if it is not aggressive, depending on their firing temperature and chemical nature. Water leakage lencouraged by the drainage medium inadvertently formed. behind the lining leads over a period of time (decades) to decomposition or rotting of the timber supports, washing out of the ground and backfill material behind the lining causing large voids or hollow areas to form, and weathering and deterioration of the brickwork and the mortar of the lining. The effects of water on mortars, especially those made with lime, are well known: the mortar loses its strength and becomes brittle. 1.nsome cases, chemical reaction from sulphates in the seeping water causes swelling. The forming of voids can lead to increased water leakage and collapsing of ground which, together with water filled voids, may cause structural distress to the lining. 3.1.2 Concrete Tunnels These take the form of tunnels with unreinforced and reinforced cast-insitu concrete linings or tunnels with segmental concrete linings. Water leakage is most commonly the cause of giving rise to transportation of fines, the formation of voids, settlement of ground, and eventually eccentric loading and distress of the lining, as described previously for the brickwork and masonry tunnels. Inadequate sealing of the joints between concrete segments and poorly constructed joints in cast in situ concrete linings can lead to water leakage. Other aggravating circumstances are posed by rising groundwater tables. In many of the industrial capitals arcund the world, where the extraction of groundwater for
Volume 13, Number 4,1998
water supply has been replaced by alternative surface water sources and waterproofing of basement structures has improved, the water table has risen. Where tunnels were constructed above the water table and no provision was made for waterproofing joints, rising groundwater may now enter the tunnel quite freely. Water seepage through porous or cracked concrete will result in loss of cement, making the lining more pervious and less strong. Aggressive water may cause decomposition of the aggregate as well as the cement. The corrosion of reinforcement in both reinforced castinsitu and segmented concrete linings is a principal concern. Corrosion is caused by pervious concrete or inadequate concrete cover and will inevitably lead to cracking and spalling of the concrete and loss of structural capacity. The presence of soft water or chlorides in the water will accelerate the effects. In rail tunnels, where stray currents from traction supply typically occur, the electrolytic action of the moisture may be particularly severe, with the rapid migration of the chloride irons to the steelwork attracting the current. 3.2 Aggressive OpersUng Envimnmenfs Aggressive operating environments for tunnels can prevail either on the inside or the outside ofthe tunnel or both. With respect to the external environment, this is generally associatedwithcontaminatedgroundwatercontainingchlorides, sulphates or other chemical pollutants coming into contact with the lining. Particularly aggressive external environments prevail in under sea or immersed tube tunnels. The damage process in all these cases is largely as described in section 3.1. The internal environment of a tunnel can vary significantly both in terms of level and type of aggressiveness. Sewer tunnels are a typical example where the tunnel lining is subject to bacteriological and chemical attack in a moist environment. It has been necessary in a number of circumstances to replace the lining or install an inner chemically inert lining within a few years of commissioning. In such circumstances it is critically important to undertake comprehensive and detailed testing programmes of lining materials with due appreciation of the standards of construction, which can be realistically achieved. 3.3 Construct/on Methods A tunnel requires a high capital investment and demands a high standard of construction to give as high a return as possible These standards can be severely compromised by the highly competitive environment we find ourselves in and the budgetary limits imposed by the tunnel owner. These, together with a lack of craftsmanship, attention to detail and poor design can all play their part in reducing the useful life of the tunnel and increasing operating costs due to additional maintenance requirements. Typical construction faults in concrete lined tunnels which compromise the durability of tunnel linings are: ?? inadequate backtllling or grouting of voids between the lining and the excavation; ?? porous concrete; . segregation; . cracking due to thermal shrinkage; . inadequate cover to reinforcement; ?? poor joint construction between cast-in-situ concrete pours or concrete segments. The situation can be further exacerbated by poor design or unrealistic specifications which cannot be practically fulfilled. The advantages of using experienced and wellqualified designers and competent contractors cannot be overstated.
TUNNJZLLING ANDUNDERGROUND SPACETECHNOLOGY 3’71
4. Inspection and Testing of Tunnels Notwithstanding how well a tunnel is constructed, it will require preventative maintenance to preserve its integrity and to prolong its useful life. Maintenance will necessarily require inspection and testing to determine the condition of the installation and to establish appropriate repair and maintenance measures, if any. However, by its very nature, only the intrados of the tunnel is open to visual inspection and while such an inspection, undertaken in sufficient detail, can reveal very useful data, it cannot provide the whole story. Furthermore, as explained in Section 2.1 (in the case ofDrakensburg Pumped Storage Schemes), tunnel outages are expensive in terms of loss of revenue and disruption to industry and people. It is therefore important that inspections be as short as possible and at the same time serve to provide all the necessary information. The inspection and testing of tunnel linings is an exhaustive subject on its own, and it is possible to touch only briefly on some of the international practices and the more important findings in this paper. Because of the aging oftunnel structures, the increasing demands put upon them, the increasing cost ofreplacement and the severe disruption caused by closure of tunnels, there has been considerable importance placed by tunnel owners worldwide on methods of inspection and testing. In a number of countries, there are now well-structured procedures in place. Typical components are discussed in the following sections. 4.1 Establishing a Diagnostic File on the Facility This would typically include all criteria important to the integrity of the structure: geology, hydrology, construction methods, materials, incidents, repair work, general characteristics, operating conditions and design criteria. This file should remain active and be updated on every event, visit and inspection.
and analysis. This specialist would typically be a qualified engineer or technologist with significant experience in the design and construction of underground facilities. The inspection can again take the form of a number of stages progressing from surface observations and measurements, through non-destructive testing of the lining constituents and voids/backfill behind the lining, to destructive testing involving drilling and cutting out of lining sections. In summary, the types of inspection of the intrados may include: Visual inspection: this can include video and visible light surveying, crack width measurements . Geometrical surveying: photographs are taken of a light beam on the surface. The light beam is a laser beam reflected perpendicularly to the tunnel axis. Shots are taken by a wide-angled camera travelling on a wagon in a referenced position in the tunnel cross-section. Typically, photographs are taken at 3m intervals on a vehicle travelling at 4 km per hour. Accuracy is ti cm for a lo-m-diameter tunnel. Convergence measurement: using specially manufactured devices (e.g. invar wire). Accuracy is 0.1 mm. Sonar profile: used for profiling ducts and tunnels filled with liquid. The reflection of acoustic waves on the tunnel walls is measured. Through high-definition TV images and computer processing, accuracies of less than 1 mm can be achieved. Surface thermography: this is discussedfurther under in Section 4.4 (Nondestructive testing), below. Using an infra-red radiometre, thermal radiation emitted by the lining is measured, thereby detecting humid or damp patches, and lining irregularities, as well as differences in its constituents. Measurement devices: these devices can all be adapted for remote station monitoring. Trends in the measurements would be reviewed and analysed.
4.2 Routine Monitoring and Measurement It is important that there are established procedures for the manager of the facility and operating staff to undertake routine inspections. The frequency of inspections will be determined by the type of facility, changes in the operating environment and alterations in the state of the facility. Appropriate instrumentation and measurement installations, and the updating and review ofthe diagnostic file play an important role in this regard. Typical measurements would include: ?? crack width measurements taken with a vernier gauge or similar instrument; ?? measurement of piezometres, using electric probes; ?? flow measurement in drainage channels; ?? photographic records; ?? vibrating wire strain gauge; and . extensometres. The routine monitoring would be carried out using comprehensive identification sheets on which observations and measurements can be conveniently recorded. The routine inspections will serve to determine the weak points, the damaged areas, the urgent actions to be taken and the areas to be checked by a specialist. For this purpose, the manager’s personnel must be specially trained to be in a position to objectively assess the condition of the facility. 4.3 Surveying of the lntrados by a Specialist In cases where the routine inspections reveal a change in environment and/or conditions ofthe facility, a specialist usuallywould be consulted to undertake further inspections
372 -G
ANDUNDERGROUND SPACE!L’ECHNOLOGY
4.4 Non-destructive Testing The fill range of testing as described above is not carried out in the vast majority of the operating tunnels worldwide. In many cases, the testing is limited to close visual inspection of the tunnel intrados by experts. However, in recognition of the lack of information provided by such inspections with respect to the constituents of the tunnel lining and the surrounding ground, there have been a number of developments with respect to nondestructive testing. With the exception of minor drilling and wring to calibrate themeasurement devices, this testing allows an analysis of the lining and ground to determine abnormalities, defects and/or voids without damaging the lining. Interest in these techniques has grown, particularly in France and Germany, which is understandable because of the large number of tunnels in these countries and their high asset value. Three of these techniques are briefly described and compared below. 4.4.1 Geo-radar Testing with geo-radar utilises electromagnetic waves emitted from an antenna on or near the surface The wave frequencies are between 80 and 1000 MHz. The diffusion of the electromagnetic waves through the lining medium is principally influenced by two properties of the material. . conductivity, the ability of the material to conduct electricity; and ?? diclcctricity, a phenomenon that governs the diffusion speed of the electromagnetic waves in different materials.
Volume 13, Number 4,1998
Table 1. Dklectricity
constant and diffusion speed of electromagnetic
waves in various materials.
Relative Dielectric Constant (ER)
Diffusion Spesd (m/s),
1
3 x loa
Water (pure)
81
3,3 x 10’
Salt water
81
3,3 x 10’
IU?
4
1,5 x 10’
Concrete (dry)
6
1,2 x 10”
1
Concrete (moist)
12
0,86 x 10’
5
Granite (dry)
5
1,3 x 10’
< 0,001
Granite (moist)
7
1,l x 10’
1
Limestone (dry)
7
1,l x 10”
< 0,001
Limestone (moist)
8
1,06 x 10’
25
Sandstone (moist)
6
2,2 x 10’
40
Basalt (moist)
8
1,06 x 10 ’
10
Material Air
Conductivity (m/s)
app~x
0 O,l-
0,3
400 O,l-
0,3
Concrete, sandy (dry)
4
1,5 x 10 *
OS
Concrete, sandy (saturated)
30
5,5 x 10’
7
Clay (saturated)
10
0,95 x 10”
30
1
3 x lo*
> lo-!
Metals
As _ _through a medium, their_.intensity is _ the __ waves pass gradually weakened, ‘due to reflection due to a discontinuity and amplitude attenuation resulting from the conductivity of the material. The :amount of wave attenuation or weakening also is influenc’ed by the frequency of the waves. The signal emitted by the antenna is reflected when it meets a discontinuity, which corresponds to a change in dielectric properties (see Table 1). Therefore, three factors-fiequency, reflection and conductivitydetermine the penetration depth of the waves into the tunnel structure. The antenna is moved across the surface and as the reflection angle and transit time of the wave changes, a tl’ansit time curve is generated and plotted, thereby locating the defect or irregularity (see Fig. 1). The geo-radar is thereby able to study . changes in the nature of masonry; . structural abnormalities (voids); . thickness variations; i constituent materials of the tunnel lining (reinforcement, arches, etc); . the surrounclmg ground (interface voids between the lining andlthe ground, detection and location of voids, water t3pots, variations in constituency, abnormalities). There are the following limitations in its use: . the nature of ,the materials crossed affect the depth which can be investigated and the wave frequency required, . wave alteration increases as the presence of water and clay increases, . the detection of defecta is made more difficult by the presence of &eel reinforcement mate. Radar antennas ofhigher frequency are required (900 MHz to 1,0 GHz). However, the higher frequency or shorter wavelengths reduce the depth of penetration.
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4.4.2. Infrared thermography Infrared thermography measures the thermal radiation emitted by the tunnel’s walls. Infrared registration techniques allow visual presentation of the temperature distribution on the surface. The temperature on the surface represents the thermal flow through the surface, which in turn is influenced by the mechanical and/or hydraulic discontinuities of the structure. Consequently, thermal discontinuities on a surface reflects abnormalities within the underlying structure. For use in tunnels, an infrared camera or scanner is generally mounted on a moving vehicle. Because the method
Antenna
5 p& Defect
B F .Z 8 ITransit Time Curve Figul v 1. Principle of geo-radar investigation. Above: Measuring Geometry. Below: Transit Time Curve.
!I’UNNEIJANG ANDUNDERGROUND SPACETECHNOLOGY 373
is dependent on measurement of temperature gradients, it is best to carry out measurements in winter, when gradients are maximised. Infrared thermography is able to identify: . water circulating at a different temperature from that of the lining or surrounding rock; . changes in the geological conditions behind the lining; ?? defects in the lining; and . voids. There are the following limitations in its use: . a basic prerequisite is a stable thermal flow through the tunnel wall. This should be constant over the period of measurement in order to assess changes in conditions and the gradient between the rock and the lining surface should be at least 2 to 4”C, dependent on the accuracy of the scanner; . the tunnel lining should not have any coating or installation that will prevent thermal radiation penetration and, as a result of the insulating effect of the coating, alter the thermal flow through the material; . variations in water content over time will cause disturbance in the results; . it is less sensitive than geo-radar. It is very difficult to assess the depth and shape of an abnormality based on thermography alone. 4.4.3. Multi-spectral analysis tests Photographs of the surface of the tunnel lining are taken using six special filters each of which cover small spectral areas of the entire light spectrum. At least one shot of the same area is taken per filter. A multi-spectral projector is used for evaluating the photographic film. The black and white film seen against a
colored backdrop, allow the fine shades of grey to become visible and by superimposing the shots with different filters, it is possible to make visible the fine spectral differences, emanating from moisture and other defects on the surface. It is possible to highlight fine cracks of down to 0,3 mm as bright lines. 4.4.4 General comments All of the above methods provide relatively rapid inspection of the tunnel lining and, if possible, should be used together to complement and corroborate results. If surface defects and fine cracks are the principal area of interest, the multispectral analysis will be the preferred method of testing. Table 2 compares the three systems of testing. It is recommended that drilling/boring of the tunnel in selected areas is carried out to provide confirmation and/or calibration of the findings of the geo-radar and infrared thermography. Only if defects, abnormalities, or changes in the lining/ ground environment are detected, would further testing using destructive methods be carried out. The speed of the investigation using the above methods depends upon the tunnel conditions and, in the case of the infrared thermography and multi-spectral analysis, can be 0,5 kmlhr and faster. Other methods of non-destructive testing include the use of ultrasound or ultrasonic waves and mechanical impedance methods involving the measurement of shock waves by geophones. It can be anticipated that the development in the technology associated with non-destructive testing will be rapid, overcoming many of its present limitations. It is foreseen that in the future, it will be possible to obtain a complete condition profile ofthe tunnel structure and that the testing will replace the arduous visual inspections, the results of which are, to a large extent, superficial and can be subjective.
Table 2. Examples of defects, which can /cannot be identified by geo-radar, infrared, thermography analysis.
and multi spectral
Defect identified by
Type of Defect
Dim. of Defect (cm)
10
Cavities and moist patches
20
Concrete Cover (cm)
Gee-radar with various anennae
Thermography
Multi-spectral Analysis
5
Yes
Yes
No
10
Yes
No
No
20
Yes
No
No
40
No
No
No
5
Yes
Yes
No
10
Yes
Yes
No
20
Yes
No
No
40
Yes
No
No
5
No
Yes
Yes
10
Yes
Yes
No
20
Yes
Yes
No
40
Yes
No
No
Moist patches at surface
Yes
Yes
Yes
Dry cracks with a width of 0.3 to 3 mm
No
No
Yes
40
374 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY
Volume 13, Number 4,1998
As the number of tunnels in South Africa and elsewhere increases and inspection and maintenance requirements increase, the use of comprehensive and reliable testing and inspection devices will become more appropriate. 5. Conclusions The conclusions with respect to maintenance and repair of tunnel structures that can be drawn from experience in South Africa:, as well as abroad, tend towards the obvious. However, in stating them, they are useful for reference, not only by tunnel owners and operators, but also by designers and contractors. A holistic approach to tunnel design and construction is thereby achieved. 1. Not withstanding the part played by tunnels in improving and conserving our environment, they require a relatively high capital investment. 2. A well-structured inspection and maintenance programme can prolong the useful life of a tunnel, thereby improving the financial rate of return. 3. Inspection of tunnels can have a high associated service disruption cost; therefore, there should be a balance between frequency, length, comprehensiveness and cost of inspection, bearing in mind that inspecticm plays a critical part in preventative maintenance. 4. It is important to avoid, as far as possible, lengthy disruption of the tunnel due to major rehabilitation works. 5. There are new technologies for testing being developed which can give comprehensive reports on the surface conditions of the tunnel lining, as well as the constituents of the tunnel lining and surrounding ground. With development, the speed and compre-
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6. 7.
hensiveness of the inspections are increasing and the associated costs are decreasing. The major cause of damage to tunnels that require repair and maintenance works is water leakage. The design of tunnel linings should be such that damage from the tunnel working environment and, particularly, water leakage-is avoided as far as is practically feasible. The additional cost of construction of an improved lining design should be weighed against the cost of maintenance and associated service disruption. The use of new lining and repair materials with no case history should be avoided. If the use of new technology is significantly advantageous, a thorough testing programme should be instituted to ensure that there are no “surprises”. The design of tunnel structures should provide, as far as possible, for easy access for inspection. The design should take account of new inspection technologies, thereby reducing costs of inspections and disruptions as well as providing for more cost-effective preventative maintenance and repair.
6. References Wide use of information provided in the following references was made in the preparation ofthis paper, particularly in Section 4, Inspection and Testing of Tunnels. AFTEX(FrenchTunnellingAssociation)Working Group No. 14. Recommendations on Diagnosis Methods for Lined Tunnels. Paris: AFTES. Haack, A; Schreyer, J.; and Jackel, G. 1996. State of the Art of Non-Destructive Methods for Determining the State of a Tunnel Lining. STUVA Report. Cologne, Germany: STUVA.
TUNNELLING ANJJ UNDERGROUND SPACE!~‘ECHNOLQGY 376