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This Paper was presented at "Sprayed Concrete Technology for the 21st Century” held at Edinburgh University from 10th to 11th September 1996, organised jointly by the American Concrete Institute and the Sprayed Concrete Associationon.
Contractual Aspects of Testing Shotcrete and Rockbolts
1 Introduction
The majority of shotcrete is placed to support tunnels and cut slopes of hillsides, and it is generally used in conjunction with rockbolts. It is therefore appropriate to give some thought to these as well. The authors each have experience of many tunnelling projects in various parts of the world. They are currently based in Turkey, supervising the driving of nearly five route km of twin three-lane motorway tunnels, with eight portals, each with a large cut face. Work is still in progress, and involves these estimated total theoretical quantities:
Tunnels: 300, 000 m² of shotcrete and 300,000 rockbolts;
Portals: 21,600 m² of shotcrete and 30,000 rockbolts.
This paper is based upon their experience, and the current project is used only as an example. The views expressed herein are solely those of the authors. One such is ‘When you are up to your neck in alligators, still try to remember that you came to drain the swamp.’ It is always relevant in any construction job to bear in mind the main purpose. Here, it is to enable traffic to flow. Testing should be done towards that aim. The current project includes several viaducts, some of steel, most of pre-cast prestressed concrete. Steel production, whether for fabrication or rock-bolts, is a highly technical and tightly controlled process. Concrete for the pre-stressed beams is produced in sophisticated batching plants, with ingredients accurately dispensed, and concrete is placed under closely controlled conditions. The production of shotcrete, however, is less scientific, as will be further discussed below. The viaducts are designed using well-known principles, applying a system of loading in a straight-forward application of well-proven formulae. Assumptions are made of the loading to be imposed on the ground; and tests will be carried out to establish its load-bearing capacity. The design of shotcrete and of rockbolts, however, is quite different. In tunnelling, and in the creation of cut slopes, the concern is with what loads the ground will exert when the natural support is removed. This problem does not lend itself to easy prior resolution. Hence tunnelling is considered an art, not a science; and especially so when using a support system based on the use of shotcrete and rockbolts. The main element of support is the ground itself; the shotcrete and the rockbolts are only required in order to enable the ground to be self-supporting. Consequently, it is not possible to forecast accurately the actual loading on the support. The load-bearing capacity of the support as a whole is not calculable, due to the varying thickness of the applied shotcrete, due to the strength increase in relation to the increasing load applied by the ground, and due to the gain in strength of the rockbolt grout, which allows transfer of load from ground to rockbolt. Too rigid a support at too early a stage may be no better than too flexible a support, since the support must then carry the entire loading, without mobilising the load-bearing capacity of the ground. The engineer must decide from time to time what support to apply. Only as the load comes on does it become possible for him to judge whether the support is adequate, and likely to remain so. By then, it is too late to install different primary support, and emergency measures have to be taken; a process rather like judging a recipe by the amount left on the diners’ plates. The success of the design, on the other hand, is judged solely by visual inspection and by measuring deformation. Nothing will reveal that the support is too strong, and there is therefore little incentive on most contracts even to try to reduce the support.
2 Shotcrete in general
Shotcrete provides support for the rock, to replace partly the support which has been removed, and to create a smooth continuous load path. It also holds chunks of rock in place, thus enabling them still to serve a structural purpose. A minimum thickness of shell is thus necessary, and this acts in both shear and in compression. The thickness of a layer of shotcrete required to hold loose rock in place is actually quite small - a few millimetres. When shotcrete was first introduced for this purpose, a 25 mm layer thickness was quite usual, but layers are now about five times thicker. In every contract, the quality and testing of the materials will be closely specified. In the kitchen, it is difficult to produce an unacceptable dish if the ingredients include cream, chocolate and rum. But with shotcrete, starting with good materials may or may not result in a good end product; but a good end product cannot be achieved with poor materials. The quality of the basic ingredients must comply with the specification. Our concern here is with other factors, the most significant of which is the nozzleman. Upon him rests the responsibility for the application and for the water/cement ratio, the most significant factor affecting the strength of the shotcrete. Here, we consider principally the dry shotcrete process, whereby the water is added to the mix as it leaves the nozzle.
3. Shotcrete quality and quantity
The following are fairly typical requirements of a specification:
1. Mix design -- water/cement ratio:
The water content has to be controlled by the nozzleman to suit the conditions of the shotcreting surface and location of application. An indication that the water/cement ratio is in the correct range is that the shotcrete seems to have a slightly shiny appearance immediately following application.
2. Placing of shotcrete:
Measures to establish the total thickness of shotcrete are usually required. These may include visual guides installed before shotcreting or holes drilled after completion of shotcreting.
3. Quality control tests -- in-situ compressive strength:
A usual method of testing is for test panels to be sprayed at the same time as production shotcrete is being placed, perhaps one panel for every 100 cubic metres of shotcrete applied. Several cores are then taken from the panel, for testing at various ages. As a control, it is common also occasionally to drill test cores from the in-situ shotcrete.
4. Qualification of operators:
Every operator should be trained, and tested by the spraying of test panels, to be tested and approved before he is employed to apply production shotcrete. Such qualification is necessary not only for nozzlemen, but also for the plant operators.
5. Placing of shotcrete:
The optimum distance between nozzle and surface of application is normally taken as 1.0 to 1.3 metres. The nozzle should be positioned at right angles to the surface of application. Both of these measures serve to ensure good compaction, and to reduce re-bound. Rock breaks to a ragged profile, raggedness which may be reduced by careful blasting, or, in the extreme, by using a TBM. The final shotcreted surface, however, is required to be relatively smooth. Shotcrete thickness therefore varies considerably, from the barest minimum at high spots to quite substantial thicknesses where the rock has broken back to natural joints. It is normal for a minimum thickness to be specified. However, it is not possible to measure this minimum thickness after the shotcrete has been applied, unless pins are used, fixed to the high spots before shotcreting starts, but this is a rare practice. When shotcrete is sprayed into place, some bounces off; the reuse of such rebound is generally prohibited. The proportion of rebound depends principally on the skill of the nozzleman, but may be 50% or more. It may sometimes be possible, although rarely practicable, to measure the quantity of this rebound, and thus deduce the mean thickness of applied shotcrete. In practice this is never done, and the shotcrete thickness is taken as a mean of measurements taken through drill holes. Even these, though, are not very accurate, and their location is virtually at random for establishing the true thickness of shotcrete, a figure only really of significance for payment. From a technical viewpoint, the amount of shotcrete to be applied remains a matter of judgement -- does it appear sufficient? To provide proper support, it is necessary that enough shotcrete is placed to smooth out the discontinuities of the broken rock face, and to provide shear resistance to pieces of rock which otherwise might fall and possibly lead to unravelling of the rock formation.
4. Shotcrete failure, its significance and the avoidance of failure
The only tests on the completed shotcrete in the tunnel are compressive tests and measurement of the thickness. The shotcrete may fail the strength test, or prove to be not as thick as specified. Despite such failure, the shotcrete may be perfectly adequate for its intended purpose. It is quite normal for the specification to require that, if the shotcrete fails either test, extra shotcrete be placed in the tunnel to increase the strength of the support. However, if the support is adequate, then this extra support is unnecessary. In addition, such application of extra support may subsequently have to be trimmed away to obtain the required clearances inside the tunnel. If this is likely to be the case, then it may be a requirement of the specification that the existing support be removed and replaced, even though it is structurally adequate but happens not to comply with the specification. It is in situations such as these that it is well to keep a sense of proportion, and to remember the primary purpose of the contract. As well as the above tests, deformation measurements might show that the applied loads are more than the shotcrete can take, or this may be deduced from observing cracking of the shotcrete. In such cases the shotcrete is not sufficient to resist the loading, even though it may have passed the strength tests, and be of adequate thickness. It may prove necessary to remove such support, trim the ground, and apply a stronger system of support, despite the shotcrete complying with the specification with regard to both strength and thickness. Only good materials must be used for making shotcrete, but good workmanship is even more important. Although the specification may include guidance on method, this is not often followed in practice unless the nozzle is remotely operated; but then the water content, which relies upon visual inspection, may be even less precise. The job of nozzleman is not a pleasant one sought by the best workmen. Even with the most enlightened of employers, proper protection, ventilation and lighting for the nozzleman is extremely rare. It is interesting to contrast the working conditions and remuneration, and the consequences and costs of failure of a nozzleman and of a chef. If the top man on site were to be required to be certificated as a nozzleman, then conditions might be improved to such an extent that consistently good shotcrete would always be properly applied. To be realistic, shotcrete as applied generally turns out to be more than adequate for its purpose. The contractor or designer may well consider it cheaper to make provision for extra excavation and extra thickness of shotcrete than to improve the working conditions.
5 Rockbolts
5.1 Types of rockbolt
Standard lengths:
• For tunnels: 4m (80%), 6m (17%), 8m (3%).
• For slopes: 4m (5%), 6m (15%), 8m (20%), 12m (40%), 16m (10%), 18m (5%), 22m and 24m (5%).
Standard types:
• SN (Stor Norfors), PG (post grouted), IBO (injection bore bolts)
• All bolts are fully grouted and untensioned.
• In tunnels, 90% are SN bolts, 10% are IBO, PG not used. On slopes, 50% are PG, 30% are SN and 20% are IBO.
5.2 Rockbolt proof tests
Earlier in the contract, proof tests were performed in various ways and using differing procedures. In 1994, the method proposed by the ISRM [4] was included in the new technical specification. A principal requirement of the ISRM method is that the rockbolts must be loaded progressively and the corresponding deformations measured until failure occur. Tests are required to be carried out on ‘each type’ and for each ‘different geological condition’. In addition the technical specification requires the testing of at least five rockbolts of each type. If we combine these requirements with the assumed geological conditions (Rock Classes 3, 4, 5 and 6) the tests required are:
Table 1. Rockbolt tests required
Total 19 tests x 5 bolts each = 95 tests to be performed. However, there is still some ambiguity regarding:
• ‘different types’ -- does a change of length represent a different type?
• ‘different geological conditions’ -- it is not possible to find all rock classes in a defined surface.
• may the test be performed on the main slope surface? Or must it be nearby?
• is it necessary to test types such as PG, which are less often used in practice?
During 1994, 67 rock bolts were tested, ranging from 0.50m to 24m. Although the minimum length of rockbolt to be used was 3m, it was found that, regardless of length, failure always occurred in the steel bar, at its ultimate tensile strength. These tests thus revealed little about the anchorage. It was therefore decided to reduce the length of the test bolts to ensure that the failure occurred between grout and steel and/or between grout and ground. The results of those tests have been presented in reference [3].
5.3 Proof tests at the east portal of the Ayran tunnel
One of the latest series of tests to be performed was required for the east portal slope of the Ayran tunnel, a permanent cut slope about 35 metres high. Alternative designs were considered for support of the rock debris, or tunnelling either partly in debris or all in sound rock. The second solution was chosen, requiring a heavier support for the slope itself, but increasing the safety of the tunnel drive. An agreement was made between the contractor and the designer on the one hand and the engineer on the other, about what tests should be performed. Only SN type bolts were to be tested, made from 28mm and 32mm diameter rebar, and three bolts were to be tested for each length.
Table 2. Numbers of bolts to be tested (45 in all)
Part of the permanent slope section was cut to the designed inclination and supported by shotcrete and mesh in accordance with the design. On this surface the 1.00, 1.25 and 1.50m long rockbolts were installed at the designed angle. On a different horizontal surface the 0.50 and 0.75m long rockbolts were installed vertically, to ensure the required grouted length. Because of the deep location of bed-rock, rockbolts for the rock were installed inclined on a nearby rock surface, prepared as above. The bedrock, formed by alternations of sandstone and siltstone, is covered by slope wash and scree deposit of considerable thickness. This rock debris comprises mainly sandstone and quartzite in a matrix of light brown silty sand. Sound rock consisted of interbedded sandstone and siltstone conglomerate. The theoretical bond length was calculated for each:
Table 3. Calculated theoretical rockbolt bond lengths
The surface preparation, the drill holes and installation of bolts were performed by a newly established subcontractor with only limited experience in the use of shotcrete and rockbolts. The strength of the grout was 23.3, 30.8 and 37.3 N/mm for rock debris and 21.8 and 26.0 N/mm for the rock. Specified minimum strength for grout is 20 N/mm. All holes were drilled 68 mm in diameter. Of the 45 tests which were to be performed, five are not included in the results. Two of the bolts were not tested, one SNØ28-1.50m in debris and one SNØ32-0.50m in rock. From the test results of two other bolts, SNØ32-0.25m, and SNØ32-0.75m, it is evident that they were ‘locked’, and only the rebar was tensioned. A fifth bolt, SNØ32-0.50m was not properly grouted and failed at less than 20 kN pull force. There are thus 25 test results for rock debris and 15 for rock available for further analysis. For each bolt a load-deformation curve was plotted, and a test report prepared. The results were re-analysed, partly confirming the previous conclusions. The loaddeformation curves were re-plotted to give more accurate diagrams and the loadcarrying capacity was thus more easily defined.
Two aspects deserve emphasis:
• The number of rockbolts tested is insufficient to be regarded as conclusive.
• Not all test results can be regarded as accurate.
Considering previous test results, and on the assumption that longer bond length has higher carrying capacity, the results diagrams were adjusted. The calculated bond length is compared with the site results, as shown in Figure 1. The 0.25m long rockbolts show a carrying capacity less than expected. One reason for this may be the different grouted lengths (which practically are difficult to achieve with high accuracy and strict definition). Another reason may be that the grout nearest the mouth of the hole is not transferring the load to the rock, the bond here being destroyed by the high tensile force transferred by the bolt itself. Accordingly we may consider that the actual bond length was less than 0.25 m. Results obtained in the rock debris are generally less than the calculated values. This may be explained by the high tendency for the walls of the drillholes to collapse, by the difficulty of cleaning the hole and by the inexperience of the subcontractor. During the execution phase, many PG type rock bolts were replaced by IBO type bolts, which install better in these conditions. Bond length in m. 0 50 100 150 200 250 300 350 0 0.25 0.5 0.75 1 1.25 1.5 th. rock bond length 28 th. rock bond length 32 th debris bond length 28 th debris bond length 32 rock/YS 28mm rock/ UTS 28 mm rock/ YS 32 mm rock/ UTS 32 mm debris/ YS 28 mm debris/ UTS 28 mm debris/ YS 32 mm debris/ UTS 32 mm Fig. 1. Relation between bond length and carrying capacity (“th”=theoretical)
6 Conclusions
6.1 Shotcrete
Practical problems occur on site in applying the specification, which lays down methods of working. These are frequently unrealistic, and can neither be complied with, nor insisted upon. The writer of the specification needs to have an appreciation of the practical difficulties, and consideration for the workmen involved. The suitability and relevance of the prescribed tests are open to question. Whilst it remains important to ensure that the materials used are of the required quality, the actual strength of the shotcrete and its actual thickness are of less importance than the answer to the question ‘Are they adequate for the purpose?’. No test other than observation has yet been devised to establish this.
6.2 Rock bolts
The number of rock bolts to be proof-tested per type is not sufficient for statistical interpretation. To allow a meaningful conclusion to be drawn, the number of tests needs to be of the order of hundreds, which is simply not practical nor economic in the case of small projects. The tests take a significant time, which is likely to be under-estimated by the contractor. The testing procedure is not complicated, but requires a qualified experienced and professional team, unlikely to be available at the start of a new job, when shotcrete and rockbolts are most likely to be required. The proof test is actually not of very great significance. Results of such tests might influence the design, but generally this is complete well before test results become available. On such a small scale as usually obtains, the test results may only confirm the predictions, but will not be sufficient to contradict them. It is most unlikely that the results of the tests will be used to justify any reduction in the number or length of the rock bolts, even if this could have a significant influence on the cost of the project. The tests required during construction have only been mentioned in passing. They absorb much time and money; but are they worth it? As a practical result of the proof test, it may be concluded that failures will occur only in rockbolts that are not properly grouted. All that a successful test can indicate is that there is sufficient bonded anchorage length somewhere along the bolt. This may mean, for example, that a twentyfour metre long bolt passes the test, but is completely ungrouted except for a short length at the mouth of the hole. Such a bolt would serve no useful tensile function.
7 References
1. Austrian Concrete Society (1990)Guideline on Shotcrete, ACS, Vienna
2. Turkish Standard TS 1246
3. Takacs, A.P. (1994) Testul de verificare prin întindere a ancorelor la tunelul Kîzlaç A din Turcia Constructii Subterane, (ed. Romanian Tunnelling Association), Issue No. 5-6, pp. 5-7.
4. Brown, E.T. (Editor) (1981)Rock Characterization, Testing and Monitoring -- ISRM suggested Methods: Document No. 2, Suggested methods for rockbolt testing, International Society for Rock Mechanics, Pergamon Press, Oxford.
