Tunnelling and Underground Space Technology 17(2002) 287–293

0886-7798/02/$ - see front matter� 2002 Elsevier Science Ltd. All rights reserved.PII: S0886-7798Ž02.00024-X

Grouting field experiment at the Aspo Hard Rock Laboratory¨ ¨

Magnus Eriksson

Division of Soil and Rock Mechanics, Royal Institute of Technology, Stockholm, Sweden

Received 12 March 2002; received in revised form 20 March 2002; accepted 17 April 2002

Abstract

This paper presents a field experiment in grouting a fracture at the Aspo Hard Rock Laboratory in Sweden. The fracture was¨ ¨investigated and described in terms of transmissivity, from which a prediction of grout spread was made, based on calculationsthat included a model for the penetrability of cement-based grouts. In the practical part of the experiment, the fracture wasgrouted and separated from the rock so that visual and chemical investigations could be performed. The grout spread evaluatedfrom the separated fracture was found to agree with the prediction. It was verified that the grout spread was halted by the limitedpenetrability of the grout, which was in accordance with the prediction.� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Grouting field experiment; Hard rock; Penetrability; Filtration; Rock fractures

1. Introduction

In tunnelling works today, the sealing of a tunnelagainst the inflow of water has become an importantand expensive activity. The most common method ofsealing underground facilities is to use pre-grouting, inwhich the grouting material, most often a cement-basedgrout, is injected into boreholes in the face of the tunnel.One task that often causes problems is calculation ofthe amount of grouting work needed to achieve asatisfactory result. This problem in turn causes difficul-ties in planning the whole production chain.

This paper presents a field experiment in grouting afracture. The experiment included predicting the resultin order to test a calculation method that uses a networkmodel to simulate the fracture geometry, in which waterflow and grout spread are calculated. The method alsouses a model for including the limited penetrabilityfound in cement-based grouts.

In the context of flow in through-fractures, severalauthors emphasise that two important parameters are thevariability of the aperture and the contact areas(e.g.Chen et al., 1989; Tsang and Tsang, 1989; Zimmermanet al., 1991; Zhao, 1998). Based on the principles forgrout spread presented by Hassler et al.(1992), the¨

E-mail address: mageri@ce.kth.se(M. Eriksson).

effect of variability of aperture on grout spread wasdiscussed by Eriksson et al.(2000b). For grout spread,the limited penetrability found in cement-based groutsis also an important issue. This limited penetrability wasextensively investigated by Schwarz(1997), who con-cluded that it depended on both mechanical and physi-ochemical causes. To include this effect in thedescription of grout spread, a model was presented byEriksson et al.(2000b) and verified with laboratorytests(Eriksson, 2001).

In order to test how reliably a calculation of groutspread in a real fracture can be made, a field experimentwas performed at the Aspo Hard Rock Laboratory and¨ ¨compared with a prediction of the result. The site hasbeen characterised and described by Fransson(2001).Based on this, a suitable technique for grouting wasproposed and a prediction of the grouting result wasmade. The actual field test included performing thegrouting, taking samples for analysis and describing theresult. The objectives of the experiment were several:

● To check how well a prediction for grout spread in asingle fracture matches grout spread in the field;

● To determine whether filtration of the grout due thelimited penetrability occurs in a way that is similarto laboratory tests;

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Fig. 1. Plan view of the test area(modified after Fransson, 2001).The pillar where the experiment was performed is situated betweenthe access tunnel and a side tunnel at the Aspo Hard Rock Laboratory.¨ ¨The two boreholes used for characterisation are marked.

● To study whether any chemical change in the pene-trated grout can be found; and

● To study and observe the spreading pattern of groutin the fracture.

The first point includes the usefulness of the resultsof the characterisation for a prediction of grout spread.The characterisation obtained prior to the test is outlinedbelow. A more detailed description can be found inFransson(2001), where complementary testing has beenincluded.

Two boreholes were core-drilled at a 1-m distancebetween centres in the pillar between the access tunneland a side tunnel at the Aspo Hard Rock Laboratory¨ ¨(see Fig. 1). The boreholes were used for hydraulicmeasurements and geological mapping.

The results showed that the right borehole(KB0007B01) was more permeable than the left andthe permeability was found mainly in the section at2.0–2.5 m. The evaluation by Fransson(2001) resultedin a transmissivity of approximately 6=10 m ys fory10 2

the left borehole and approximately 4=10 m ys fory8 2

the right. For the section at 2.0–2.5 m in the rightborehole, a transmissivity of 4.3=10 m ys wasy8 2

evaluated.Based on the core mapping and BIPS analysis, three

fractures were discovered in the section at 2.0–2.5 m.On the assumption(hypothesis) that only one fractureis conductive, the resulting hydraulic aperture was ini-tially estimated to be 40–50mm. The somewhat largeraperture that was found when analysing the specificcapacity may indicate(Fransson, 2001) a larger aperturein the vicinity of the borehole compared to the average.

The three possible conducting fractures had differentstrike and dip angles, but were described as planar andrough.

The above information was known in advance of theexperiment. After grouting, complementary testing withadditional holes was performed(Fransson, 2001).

2. Prediction of results

Before a prediction could be made, it was necessaryto perform several analyses and tests. The first step wasto interpret the geometry of the fracture based on theinformation obtained from the characterisation. The nextwas to suggest a grout and to obtain its behaviourparameters. After this, a decision on the choice oftechnique and prediction of the grouting result wasmade.

2.1. Interpretation of geometry and simulation of waterconductivity

The interpretation of the geometry of the fracture wasbased on appreciating the hydraulic aperture based onthe cubic law and the transmissivity evaluated. Thehydraulic aperture based on this is calculated to be 39mm at 158C. This hydraulic aperture can be geometri-cally represented in several different ways while main-taining the value of 4=10 mys evaluated for they8 2

transmissivity. One way is to ignore all likely aperturevariations and interpret the hydraulic aperture as aphysical aperture. Another way is to state that there isa variation in the aperture and that this variation needsto be recognised.

The interpretation was made on the assumption of anaperture variation with a standard deviation of half thearithmetic mean value of the aperture. Two values forthe mean aperture were studied. The first assumes thatthe hydraulic aperture is the same as the arithmeticmean value, i.e. 39mm. The second uses the relation ofZimmerman et al.(1991), which states that the physicalaperture(b) is larger than the hydraulic aperture(b )hyd

when there is a variation in aperture. With a standarddeviation of half the arithmetic mean value, this resultsin a mean aperture of 44mm.

3 y0.56Øbysb yb s1ye (1)Ž .hyd

Apart from the above, three cases with differentcontact areas, 30, 50 and 70%, were simulated. Theeffect of contact area within the fracture surface hasbeen studied by Chen et al.(1989), who found that adecrease in permeability is obtained if contact is present.This means that if contact is present, the flow channelsmay have a larger physical aperture compared to thehydraulic aperture, which justifies the inclusion of con-tact area.

The dip and strike of the fracture were not knownexactly, but the three fractures were steep and sub-perpendicular to the borehole. For this reason, a dipangle of 908 and a strike of 458 in relation to theborehole were used.

2.2. Grout behaviour

To obtain a suitable grout with ‘known’ behaviour,several series of tests were performed prior to the

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Table 1Grout properties measured during the experiment

After

10 min 20 min 30 min

Density wkgym x3 1210Separationw%x 20Penetrability

b wmxcritical 104 104 121b wmxmin 46 46 42

RheologyYield value wPax 0.3 0.4 1.4Viscosity wPa sx 0.02 0.02 0.04

experiment. The limited aperture of the fracture requireda grout optimised in terms of penetrability, which led tothe choice of a micro cement.

The grout properties were characterised with respectto rheology, penetrability and bleed. The rheology,described with the Bingham(1922) model, was meas-ured with a rotational viscometer, which evaluates theyield value and viscosity against time. The penetrabilityof the grout was measured with a specially developeddevice termed a penetrability meter, which, after evalu-ation, characterised the grout in two parameters, aminimum (b ) and a critical aperture(b ). In themin critical

calculations, no grout enters the channel if the apertureis less than the minimum value. If the aperture is largerthan the critical value, the grout can pass throughunaffected. If the aperture of the channel is larger thanthe minimum but less than the critical, the grout will befiltered and only a limited amount of grout can enter. Ifthe grout is filtered, a filter cake is formed that eventu-ally blocks the flow. The properties of the filter cakeare described in terms of density and length. Furtherinformation on the model and the measurements can befound in Eriksson et al.(2000a) and Eriksson(2001).

The bleed is expressed as a percentage and refers tothe separation of solid particles and water in the suspen-sion. One way of measuring the bleed is to use thestandardised test with a cylinder of 30 cm in height.This method has been developed for concrete works. InEriksson et al.(2000a), the effect of size on bleed wasdiscussed. The measurement of bleed in a large cylinderis probably not representative for the bleed that occursin a fracture of small aperture, and a small(1 cm inheight) cylinder was therefore recommended whenmeasuring the bleed. This method of evaluation excludessome of the effects of size, such as consolidation.

First, the grout based on micro cement(d -12 mm)95

with a wyc ratio of 2.5 was tested in the laboratory atthe Royal Institute of Technology. Three series of testswere carried out at 108C. The results were similar ineach series of tests and the properties were found to besuitable for the experiment.

The same recipe was then tested on site in the tunnelto validate the results. This time, 1000 ppm eosin Ywas added to the mixture to facilitate subsequent tracingof the spreading pattern. It was found that the tempera-ture in the tunnel was somewhat higher than expected,at 12 8C. The grout properties differed to some extentbetween the laboratory and the field. Two circumstanceswere identified as possible causes: the addition of 1000ppm of eosin and the somewhat higher temperature inthe tunnel than in the laboratory.

This round of testing gave a penetrability of the groutin the field that was unsatisfactory for the experiment.It was assumed that this result might have been partlydue to the additive eosin Y, and it was therefore decided

that a smaller amount should be added. Another decisionwas to increase the mixing time from 2 to 5 min.

In connection with the experiment, a test on groutbehaviour was performed. The properties obtained inthe grout injected are shown in Table 1. These valueswere used in the calculations.

2.3. Calculations

In order to test and verify the model calculations ofwater and grout flow, a number of numerical tests wereperformed prior to the experiment. The calculations inrelation to water flow were mainly performed to deter-mine whether values similar to Fransson(2001) couldbe obtained together with the aperture value interpreted.The calculations relating to grout flow were made withthe modelling approach presented in Eriksson et al.(2000b).

The calculations were performed by realisation of 100cases which fulfilled the criteria of:

● Mean aperture of(a) 39 or (b) 44 mm; and● Three cases where the contact area ratio was 30, 50

and 70%, respectively.

Of 100 realisations, the first five that fulfilled thecriteria T s4"0.5=10 m ys, i.e. matched they8 2

calculated

values measured, were used for prediction of thegrouting.

The 100 realisations that were performed gave theresults for calculated transmissivity, as shown in Fig. 2.The five realisations selected in each case are shown inFig. 3.

It was concluded that different geometries had beenfound that fulfilled the values of transmissivity andaperture. The geometries, together with the values forgrout behaviour, were used for predicting the groutingresult. Both the predictions and the grouting operationsused a grouting pressure of 400 kPa and a grouting timeof 600 s. The results of the calculations relating to grouttake are shown in Fig. 4. It is evident that no actualgrout volume penetrated the fracture in cases wherethere was a small contact area. In the cases where a

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Fig. 2. Result of 100 realisations concerning mean and variation incalculated transmissivity. The input parameters in each calculation aremean aperture, standard deviation of the aperture and contact area.

Fig. 4. Distribution of grouted volume within the five selected real-isations for mean aperture of(a) 39 and(b) 44 mm.

Fig. 3. Distribution of transmissivity within the five selected realisa-tions for mean aperture of(a) 39 and(b) 44 mm.

Fig. 5. Calculated frequency distribution of grout take in theexperiment.

contact area of 70% was simulated, a small amount ofgrout was found to penetrate. In every case, stoppagewas due to the limited penetrability, i.e. the grout formedan impermeable filter cake.

To obtain the range of possible results based on theknown and the assumed geometrical conditions, all thevalues calculated are plotted in the same diagram(Fig.5). The curve consists of the 30 values of grout takethat were calculated. It is evident that the most likely

result was that no grout would enter the fracture andthat not more than 0.015 l was predicted. Some examplesof the spreading pattern where grout entered the fractureare shown in Fig. 6.

3. Short description of field work

The physical field work was divided into two parts:grouting the fracture and collecting material for analys-ing the result.

Grout was injected into the fracture with speciallydeveloped equipment. It was recognised that in order toevaluate the experiment it was necessary to have exactcontrol over the pressure and grouted volume: therefore,a standard grouting rig was not considered suitable. Thegrouting equipment developed has a pressurised contain-er, which was weighed during the experiment in orderto record the grouted volume. In the grout hole, a doublepacker with a fixed packer distance(0.5 m) was posi-tioned over the section where water loss was observed.The procedure during the experiment was as follows:

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Fig. 6. Examples of spreading patterns obtained.(a) Mean aperture 39mm, 50% contact,Vs7 ml. (b) Mean aperture 39mm, 70% contact,Vs8 ml. (c) Mean aperture 44mm, 50% contact,Vs4 ml. (d) Mean aperture 44mm, 70% contact,Vs10 ml.

● The packers were installed in fixed positions.● The grout was mixed.● The grouting equipment was filled with grout and the

amount to fill the volume between the packers wasmeasured. Pressure was applied(400 kPa) and thetime was recorded.

● After applying pressure for 10 min, the pressure wasreduced and the hoses blocked, i.e. grouting wasstopped.

● The grouted volume was recorded● The grout properties were tested simultaneously with

grouting.

The next stage was to obtain material for analysis,i.e. to obtain samples of the grouted fracture in order tostudy penetration and chemical composition. For thispurpose, it was found that the best method was to takelarge core samples(200 mm) from the fractures. Apattern of core drills was chosen which provided infor-mation on the spreading pattern. As it turned out, onlyone core drill was found necessary to obtain the fullgrouted area within the fracture.

After the injected grout had reached a strength ofapproximately 1 kPa, the double packer was removed.A piece of iron was positioned over the grouted sectionas reinforcement and the borehole was filled with adifferent grout to the original. After this grout had fullyhardened, drilling of the large diameter core(200 mm)was performed. The fracture could thus be analysed‘undisturbed’ in the laboratory.

4. Analysis

To test how well the predictions agreed with theresults obtained, the core drill that was taken from thefield was analysed in two different ways. One testentailed opening the fracture and recording the groutspread pattern, while the other involved taking samplesof the injected grout for chemical analysis.

The first test revealed the spreading pattern shown inFig. 7. It was visually found that in the vicinity of theborehole(within a radius of approx. 50 mm) the groutwas spread over approximately 50% of the surface. This

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Fig. 7. A picture of the opened fracture with one surface(left part)exposed. A line has been placed in the figure showing where the maingrout was found.

Fig. 8. Frequency distribution of calculated result, with the approxi-mate grouted volume obtained marked with a straight line.

Fig. 9. Result of the chemical analysis of grout samples taken fromthe fracture surface. The chemical analysis is compared to the originalcomposition of UF12. The uppermost diagram illustrates where thesamples were taken on the two fracture surfaces and the diagrambelow shows the chemical composition at the different positions.

is interpreted as 50% contact. In some positions on theside where grout had entered, more soluble componentsof the grout were found somewhat further away, up to10 cm from the borehole.

The volume that entered the fracture was too smallto weigh, but was back-calculated from the spread areaand the aperture(40 mm). This resulted in an approxi-mate volume of 0.0005 l and is shown in Fig. 8 by astraight line.

The second test was a chemical analysis of the groutin the fracture, which was performed by the SwedishCement and Concrete Research Institute(CBI). Forfurther information on the method used, see Lagerbladand Fjallberg (1998). The chemical analysis of the¨injected grout confirmed the visual picture of the groutspread. This means that in the area surrounding theborehole in Fig. 9, the grout had a chemical compositionsimilar to the grout that was injected; however, furtheraway from the borehole, the chemistry deviated. Thismeans that the limited penetration that blocked the flowalso changed the chemical composition of the grout.Fig. 9 shows the chemical composition at variouspositions on the fracture surface.

A comparison of the samples from the various posi-tions to the composition of UF12 shows that the samplesfrom positions 1, 2 and 6 are almost the same as theoriginal. In the other samples, a clear difference wasfound. In positions 3 and 4, traces of more solublecomponents were found, whereas in positions 5 and 7only fracture minerals were found. The results show thatthe more soluble components can travel further thanother parts of the grout, which results in a chemical

separation due to mechanical filtration, as was theconclusion by Lagerblad and Fjallberg(1998).¨

5. Conclusions from the field experiment

The main objective of the field experiment was todetermine whether the method of predicting the groutingresult produced a similar result to actual tests in thefield.

Based on a description of transmissivity obtainedfrom Fransson(2001), interpretations of possible geom-etries and tests on grout material, a prediction of thegrouting result was made using numerical calculations(Eriksson et al., 2000b). It was found that the result

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was dependent on assumptions regarding the contactarea and variability in fracture aperture, but the resultobtained was within the span of the predictions.

The prediction based on this found that the span ofpossible results was between ‘zero’ grout spread andaround 200 mm, with a median value around zero grouttake. The prediction showed that regardless of thecontact area and variability, the fracture was too smallin aperture to be grouted more than to a limited extent.The result obtained agreed with the prediction and lendssupport to the model. However, the test did not revealhow significant the influence of contact area and varia-bility would be in a fracture with a larger aperture.

To produce a more detailed description of the groutspread, it is necessary to obtain a more detailed descrip-tion of fracture geometry. If, for example, the contactarea in the fracture had been known, this would havelimited the number of possible cases in the prediction.

Acknowledgments

The author wishes to express thanks to the SwedishNuclear Fuel and Waste Management Co for theirfinancial support of this work and to Prof H. Stille forvaluable comments on the manuscript. The good co-operation with Per Delin, Asa Fransson and Gunnar˚Ramqvist is highly appreciated.

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