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[The Use of Transmissivity Data from Probe Holes for Predicting Tunnel Grouting
Analyses of Data from the Access Tunnel to Aspo Hard Rock Laboratory
Abstract: Results from hydraulic tests in probe boreholes in the access tunnel to the Aspo Hard
Rock Laboratory, Sweden, were compared to the amount of performed pre-grouting of the tunnel made in order to reduce water inflow. It was found that the probe holes identified major hydraulic conductors well. Also, parts of the tunnel with very tight rock could be predicted. In the intermediate range, the precision of the grouting predictions can be improved with a probe hole patter that is better adjusted to the geometry of the conductive fractures. Data for this can be obtained by mapping of orientation and trace lengths of the water bearing fractures.
The endpoints of the curves could be analysed. For the lower end, it is clear that there are very few grouted fans for Tmax*hw<1.7*10--5m3/s, corresponding to a predicted leakage to the section of about 11/min. This is a leakage that is noticeable, but, in general, it will not cause any trouble. This makes it possible, though, to disregard 20% of the probe hole sections with the least Tmax*hw. In the upper end, we find that the curves are very close. This means simply that the Tmax*hw captures virtually all highly conductive sections where grouting was performed. Thus, the probe holes also very clearly show where the major troubles are to be expected.
Figure 3 presents a cross-plot between maximum and minimum transmissivities evaluated for probe holes drilled at the same position of the tunnel front.
As shown, there are often considerable ratios between Tmax and Tmin. The obvious interpretation of this is that the two probe holes, at the same tunnel front do not penetrate the same conductors. There is indirect evidence for this, since there is a tendency for the ratio between Tmax and Tminto be smaller at the high end of the diagram. This leads to the conclusion that, in that range, both holes of a pair penetrate the same structure and that the very conductive structures, in general, have a larger geometric extension. This may also explain why the probe holes pick up the very conductive structures well.
The second conclusion is that the distance between the probe holes is too large to pick up the more common conductive fractures. This was also confirmed when relating the different samples in Figure 2 to geology. Three or more fans have been grouted in larger fracture zones often associated with fine-grained granite, whereas one fan was mainly performed in areas of less fractured diorite (Aspo-diorite). The need for two grouting fans was, for example, found in the vicinity offractures that had an extension exceeding the tunnel area. For one of the fracture zones within the investigated area, the main fracture orientation was sub-horizontal, and complementary (~ perpendicular ) probe holes would have given valuable in formation.
The effective distance between the probe holes and the tunnel wall was analysed by Rhen et al. (1993a), and the median was found to be 6 m, corresponding to a median distance between the holes of 18 m, which again tells something about the extension of the major conductors. The distance from the boreholes to the tunnel wall, 6 m, is , however, small enough to give a predictive value for grouting.
Another data-set that could be used to analyse the size of the water-bearing fractures is the evaluated transmissivities of water pressure tests (WPT) and water flow tests (WFT)in the grouting fan boreholes. For all grouting fans, the borehole positions on the tunnel front were mapped, and simple hydraulic tests were performed between sections 0/997 m and 1/375 m. Figure 4 shows a variogram of the log transfordmation of the transmissivities found by the WFTs (Stille et al. 1993). A spherical variogram is fitted to the data, 772 pairs of values, and shows a typical correlation length of 3-4 m, which coulld be seen as a measure of the distance over which the conductive fractures are connected.
A third data-set is the mapped orientations and lengths of the fractures in the tunnel. An evaluation of the mapped water-bearing fractures for tunnel sections 0/700-1/475 is given in Rhen et al. (1993a). Leaking fractures outside the areas with shotcrete are mapped with respect to length, orientation and object type. The object type is divided into five classes with different character of inflow and with a number and total inflow according to Table for the mapped part of the tunnel.
The high number of diffuse inflows is explained by the difficulty of identifying exactly where a fracture lets in water to the tunnel. It is , however, clear that the majority of the inflows are concentrated at rather few points along the leaking fractures. This is due not only to the inhomogeneous transmissivity distributions in the fracture but also to the fact that the free boundary at the tunnel face concentrates the flow to wet channels (Larsson 1992).
On the other hand, a borehole that penetrates a conductive fracture into the rock should, on average, pick up the cross-fracture transmissivity of the fracture (Geier et al.1992). Similar results are presented in Fransson(1999), where the effective transmissivity of a fracture replica was found to be close to the median value of a number of hydraulic tests. An analysis of the orientation and length statistics is, therefore, justified.
The dominating directions of the water-bearing objects of the “Extensive” type (see Table 1) are a subvertical set striking WNW and a subhorizontal set (Rhen et al.1993a). Since the tunnel direction is N19W, the angle to the dominating subvertical set is approximately 50D. However, there is also an important water-bearing subvertical set striking N-S that is partly obscured by the tunnel direction.
The length distribution of all leaking fracture , including wet and dry sections thereof, is show in Figure 5. The median length of these is approximately 4m and should, thus, be a measure of the distance over which a direct connection generally exists.
Considering the angle between the grouting-boreholes, which are sub-parallel to the tunnel, and the water-bearing fractures, there seems to be agreement between the correlation lengths for grouting-borehole transmissivities and the fracture lengths. One can therefore assume that the effetive distance between the probe holes is, in general, too large to pick up the moderately transmissive fractures that demand grouting.
Conclusions
The analysis of the relation between transmissivity data and grouting from probe holes in the AHRL access tunnel has shown that, with the applied design, very conductive structures that require repeated grouting activities are picked up well. For areas with tight rock, where no grouting is required, the predictive value of the probe holes is also good.
For the intermediate range, one may assume that the predictive value of the probe holes will be increased considerably if the distance between them is decreased. From an analysis offracture length statistics and variograms of grouting-borehole transmissivities, it is likely that the distance should be in the range of 3-4m. Based on a simple analysis of this kind, the probe hole design could made on the basis of field data for most tunnel projects.
Another conclusion is that long investigation boreholes ahead of a tunnel can be used to predict the very conductive zones. Where extensive grouting operation must be applied, and the areas where no problems are to be expected. This is, however, in line with the fact that tunnel design is directed either to finding the potential trouble areas, so that they can be avoided; or to allowing proper measures to be taken; or to identifying the very good areas where plugs and other critical constructions can be located. The intermediate levels, where most standard grouting activities take place, are best handled on a short time basis, where data from probe holes (or grouting-boreholes) in a pattern adapted to the system of conductive fracture in the rock can be used as a good basis for grouting decisions.
Acknowledgment
The data on which analysis is based have been provided by SKB through their Aspo Hard Rock Laboratory.
References
Cooper, H.H. and Jacob, C.E.1946. A Generalized graphical method for evaluating formation constants and summarizing well field history. Am. Geophy. Union Trans. 27:526-534.
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James Campbell
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