Underground Space – the 4th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds)© 2007 Taylor & Francis Group, London, ISBN 978-0-415-40807-3

Dilatometer tests in deep boreholes in investigation for Brenner base tunnel

M. Zalesky & Ch. BühlerSolexperts AG, Mönchaltorf, Switzerland

U. BurgerBBT SE, Innsbruck/Bolzano, Austria/Italy

M. JohnJohn Tunnel Consultant, Innsbruck, Austria

ABSTRACT: Results from more than one-year of in-situ deep borehole dilatometer testing and data evaluationfor the Brenner base tunnel geotechnical investigation project are presented. The principle of dilatometer mea-surements is explained. Dilatometer measurements describe deformation properties of rock massif. Deformationmodulus and modulus of elasticity are determined in relationship to applied pressure. Equipment for testing indeep boreholes with diameters ranging from 96 to 158 mm is described. Information needed prior to measure-ments, testing procedures as well as evaluation of recorded data are discussed. Examples of in-situ dilatometermeasurements are presented. Range of use of gained results is discussed and conclusions are drawn.

1 INTRODUCTION

The Brenner base tunnel will be a significant part ofEuropean railway axis Berlin – Napoli – Palermo. The55.6 km long tunnel route will be built from Aus-trian Innsbruck to Italian Franzensfeste (Fortezza).Numerous deep boreholes up to 1351 m depth weresimultaneously drilled during the second phase ofgeological exploration in 2004–2005. Depth of theboreholes required drilling in several sections withdecreasing diameter. Other reason for various boreholediameters was that six different drilling companies par-ticipated in the project. To comply with the proposedprocedure of “drilling – logging – measuring”, Sol-experts AG was obliged to perform tests in boreholeswith diameters ranging from 96 mm to 158 mm.

Several types of borehole measurements were per-formed: geophysical logging, packer tests, stress meas-urements using hydraulic fracturing and dilatometermeasurements. In this paper we focus on in-situ deter-mination of deformation properties of rock massifusing Solexperts dilatometer.

2 SOLEXPERTS DILATOMETER SYSTEM

The dilatometer probe developed by SolexpertsAG hasbeen successfully used for more than 30 years (Thut

1977). Determination of the mechanical properties ofthe rock mass is based on real-time measurement ofthe applied pressure and the resulting deformation –the change of the borehole diameter.

The dilatometer probe consists of a metal core,which holds three potentiometeric displacement trans-ducers placed in the centre of the core, Figure 1.The axes of the transducers are oriented 120◦ to eachother and vertical distance between the transducers is75 mm. The borehole wall is loaded by means of a re-inforced packer sleeve fixed to the core of the probeat its both ends. The packer can be expanded bycompressed nitrogen or air via a high-pressure hoseconnected to either nitrogen bottles or a compressor.The dilatation of the borehole wall is directly measuredby the transducers, which contact the borehole wallthrough steel pins with spherical heads. The appliedpressure is measured at the surface using a pressuresensor. The electronic signals of the displacementtransducers are transmitted to the dilatometer loggerthrough a coaxial cable within the high pressure-hose.The probe can be additionally equipped with a compassfor oriented strain measurement or with two pressuresensors: one measuring the formation pressure under-neath the probe and the other measuring the pressureinside the packer. To simplify installation and mini-mize risk of damage when retrieving the probe fromthe borehole a steel cone is attached to the lower end

283

Figure 1. Scheme of the Solexperts dilatometer.

Table 1. Diameters of Solexperts dilatometer.

Diameter

Unexpanded probe Borehole – minimummm mm Sign

92 96 HQ95 101

118 122 PQ142 146148 152

of the probe and an open type bailer, called sedimenta-tion tube, to its upper end in order to prevent blockingthe probe by loose material falling down the drill hole.

The maximum measurement range of the displace-ment transducers is 25 mm, also due to their highaccuracy of 0.001 mm. The packer is designed for thisrange of strain too. To perform tests in boreholes withdiameters ranging from 96 mm to 158 mm there had tobe either a single probe for each diameter or one typeof probe, where the packer element and transducerextension pins could be easily changed. Solexpertshas developed a dilatometer probe body (core), whichcan be equipped by packer elements of different diam-eters. A set of corresponding sedimentation tubes andcalibration tubes have also been provided, Table 1. Theprobe is installed with so called hydro-frac tubing of1′′ using a drilling rig.

The high-pressure hose containing signal cable iswound on a winch on the surface. The hose/cable wasmanufactured in sections of 200 m with the appropriateconnectors. Connecting multiple sections, made it

possible to perform measurements up to 1400 m indepth. One or more 200 m sections of the hose/cablecan be replaced, if needed.

The inflation pressure is manually regulated andautomatically measured at the control panel witha pressure sensor. Additionally it can be observedtogether with the pressure in the system and therest pressure in the nitrogen bottle (or compres-sor) with mechanical pressure gauge in the controlpanel. For special long-term dilatometer experimentsa fully automated monitoring system (the SolexpertsGeoMonitor II) can be used. In this case however, thedilatometer system is pressurized with silicon oil.

The pressure sensor and the signal cable fromthe probe are connected to special dilatometer dataacquisition system, which is controlled by a com-puter via the software program DilatoII developed bySolexperts. This program provides data acquisition,testing control, graphical and numerical real-time datapresentations, as well as test analysis and reporting.

3 DILATOMETER TESTS

3.1 Planning and preliminary information

To comply with a “drilling – logging – measuring”procedure requires a significant planning, decisionmaking and preparation is necessary prior to executionof each series of tests.

In-situ testing i.e. dilatometer measurements orhydraulic tests using a double packer system can bedone during the drilling phase or after the entire lengthof the borehole has been drilled. Performing measure-ments according to the state of drilling progress maybe difficult to manage especially when several bore-holes are being drilled simultaneously, as it was for theBrenner exploration project.

In most cases however, measurement campaigns arecarried out after the drilling has been successfully fin-ished. The geologist in charge usually overviews theentire borehole and specifies the borehole sections ofinterest. Results of tests might be affected by “boreholehistory effect” (especially affecting the hydraulic testsresults). Another problem to be considered for testingafter drilling is the stability of the borehole. Especially,investigation of faults and shear zones is of interest forthe design of planned excavation. In cases of poor bore-hole stability only the “measurement-while-drilling”approach can be applied.

The approach chosen for the geotechnical andhydrogeological investigation for the Brenner basetunnel was a compromise of the presented possibili-ties: a series of test measurements was done after a firstsection of the borehole with one diameter was drilled.

Borehole logging and geophysical measurementswere carried out at the beginning of a testing campaign.

284

The logs were used to decide about the subsequent test-ing strategy and number of tests. For the dilatometertests the most important information was:

– nominal borehole diameter,– test depths,– type of rock / inspection of borehole cores,– dipping of shistosity,– calliper log for actual borehole diameter or– borehole scan for geologic structure and– ground water table / hydrostatic pressure at each test

depth.

The calliper log and/or the borehole scan providedinformation about the true borehole diameter as well asthe possible existence of caverns, discontinuities, largecracks and fractures of the massif. Uneven boreholewalls result in higher risk of packer damage and, in theextreme case, the possible blockage of the borehole byan irretrievable probe. Additionally, to high artesianwater pressure (determined by the hydrogeologicaltests) can cause difficulties during the tests.

3.2 Test procedure

The dilatometer probe must be calibrated prior to test-ing. The calibration is done by inflating the probeinside a steel tube of known diameter. The DilatoIIsoftware then records the data from each displacementtransducer and calculates offsets so that a “zero read-ing” corresponds to the known inner diameter of thetube. The actual borehole diameter can then be pre-cisely determined. It is needed for control of the probeexpansion state during the test and for test analysis.

The test sequence usually begins at the lowesttest position in the borehole. During installation intodeep boreholes the probe must be partially inflatedin order to counterbalance the acting hydrostatic pres-sure.After lowering the probe to the depth given by thegeologist, the dilatometer measurement can start. Theactual borehole diameter can be determined with thedilatometer probe in cases where the calliper log showssignificant variations in borehole diameter along theborehole section selected to be the test position, orthe borehole cores showed discontinuities, but espe-cially if no core could be retrieved.The actual diametercan be determined by inflating the probe just enoughto overcome the hydrostatic pressure and the stiffnessof the packer. Then the packer is in contact with theborehole wall. Under low inflation pressure there isno danger for the probe even if a cavern or fractureexists at this borehole section. The risk of damagingthe equipment can be minimized by selecting a testposition with an appropriate diameter after measuringthe borehole diameter at several locations along thetest zone using the described procedure.

The dilatometer tests for Brenner base tunnelproject were performed with four loading cycles. At

first, the probe was inflated to a base pressure, whichwas about 0.5 MPa higher than the hydrostatic pres-sure at the test position in order to establish a goodcontact between the packer and the borehole wall. Thefirst cycle consisted of initial loading and subsequentunloading to the base pressure. The following cyclesconsisted of reloading up to the maximum pressure ofthe previous load cycle, initial loading to a higher leveland then unloading to the base pressure level.

The maximum level of loading pressure has to bechosen with respect to the rock type, its assumedcompressibility and the aim of the test. In case of meas-urements for a future base tunnel, the applied pressureshould reach the same order as the horizontal pres-sure at rest in the mountain massif. One of the aimsof a good dilatometer test is to describe the ratio ofplastic and elastic deformations together with generalbehaviour of the rock, which can be also mostly elas-tic or plastic. The range of applied pressures should bechosen so, that the maximum of elastic deformationphase of the rock is used, but causing massive plas-tification or fracture of the rock is to be avoided ifpossible.

After the test is finished, the probe is unloaded underthe level of acting hydrostatic pressure and raised tothe next higher measurement zone.

3.3 Evaluation of dilatometer test

The moduli are calculated based on Lamé’s equationfor a tube with infinite wall thickness (1):

where �p = pressure difference; d = borehole diam-eter; �d = change of borehole diameter; ν = Poisson’sratio. In case where no results of laboratory tests areavailable for the tested rock formation, we assume thePoisson’s ratio of 0.33 with respect to rock quality.

The deformation modulus (V) is calculated basedon data of initial loading from each load cycle. Themodulus is computed using a linear regression alongthe corresponding section of the curve. The deform-ation modulus from reloading phase of each cycle iscalculated in a similar manner.

The modulus of elasticity (E1) is calculated fromthe slope of a secant connecting the first and the lastpoint of the unloading phase, which corresponds to theinitial loading range for each cycle. However in casesof very step slopes which correspond to very smallelastic deformation of the borehole wall and thus tovery high moduli, it appears to be more appropriate tocalculate E2 (Fig. 2) from the entire unloading curvefor each load cycle.

The averaged displacement measurements of the allthree transducers are mainly used for the calculation

285

Figure 2. Dilatometer test in borehole Va-B-03/04s,986.5 m depth, chlorite schists. Graphical representationfrom the DilatoII software program: average from deform-ation measurements of all three extensometers.

Figure 3. Dilatometer test in borehole Va-B-03/04s, depth986.5 m. Comparison of pressure/deformation graphs forthree extensometers.

of the moduli. Based on comparison of all three pres-sure / deformation graphs, conclusions on elastic orplastic behaviour of the rock massif can be drawn andestimates of anisotropy can be made. Figures 2 and 3show a dilatometer test in chlorite schists at 986.5 mdepth. The borehole Va-B-03/04s is inclined 30◦ fromvertical.

The method of analysis described above allowsassessing one dilatometer test at a time. The stiffnessof the examined rock can be classified according tothe values of deformation and elasticity modulus fromthe last loading cycle. For this purpose we use theclassification based on E-modulus values, which wasproposed by AFTES (1993).

Figure 4. Schema for computation of E and � for Schnei-der’s criterion. Graph: Test in borehole Pf-B-03/05, depth39 m, quartzphyllite. Average from deformation measure-ments of all three extensometers.

Another approach, Schneider’s criterion (1967),also uses the Lamé’s equation, but allows interpre-tation and comparison of series of dilatometer testsfrom a wide range of rocks. Development of thedeformation properties can be evaluated with respectto acting pressure. Measurements with four loadingcycles are required for the Schneider’s criterion, whichdescribes the relationship of a factor of permanentdeformation Cp (2) to a ratio of elasticity modulus Eto deformation modulus � (3):

where dp = permanent deformation after a completeloading cycle; and pmax = maximum loading pres-sure applied with this cycle. The factor of permanentdeformation is calculated for each loading cycle.

The modulus of elasticity E is defined as a slopeof a straight line, which connects points d25 av andd75 av. These points represent average deformationfrom unloading (d25 U, d75 U) and reloading (d25 L,d75 L) phase of the corresponding load cycle (seeFig. 4).Values of d25 and d75 are determined at pressurelevels of 25% and 75% from the maximum pressureapplied in the loading cycle. The E moduli are calcu-lated for the first to the third load cycle.The calculationof E-modulus from the last (fourth) cycle is based ondata from the unloading phase only.These calculations

286

Figure 5. Schneider’s criterion, after Schneider (1967).Graph: dilatometer tests in shales, borehole Va-B-03/04s.

can be done by interpolation between the data pointsrecorded during the measurement (see Figure 2) orestimated in the chart. The analysis data presented inthis article were interpolated. The deformation mod-ulus � is determined as the slope of a secant, whichconnects the first and last point of an initial loadingphase of a load cycle, Figure 4.

The resulting data is plotted in a semi-logarithmicgraph, which is also used for rock classification, afterSchneider (1967), Figure 5.All four data sets presentedin this example indicate compact rocks with mainlyelastic deformation behaviour.

Data analysis for Schneider’s criterion is not inte-grated into the analysis part of DilatoII program.Therefore the analysis was made with a spreadsheetprogram, which is rather time consuming. Further,presentation of a large number of tests in a dia-gram according to Schneider can become confusing.For these reasons we propose a modified version ofSchneider’s criterion based on the values from the last(fourth) loading cycle only. The results of a completedilatometer test are reduced to only one point in theSchneider’s graph, instead of a polygon line in theoriginal Schneider’s criterion. This method allows aquick comparison of rock properties in a large set ofdilatometer tests. This procedure does not depict thedevelopment of the moduli and the amount of plasticdeformation with respect to pressure changes duringthe test but makes evaluation of the distribution ofmeasured moduli much easier.

We use the data of V (deformation modulus cal-culated as regression) and E (elasticity modulus)computed by the DilatoII program in the modifiedSchneider’s analysis. The E2 modulus calculated fromthe entire unloading phase of the fourth loading cyclewas chosen, because it is closer to the original Schnei-der’s criterion than the E1 value determined from theupper part of the corresponding unloading phase only,Figure 2. This simplifies determination of coefficientE/V (instead of original E/�), whereas the coefficientCp remains unchanged. All results are then displayed

Figure 6. Modified Schneider’s criterion. Distribution of 70test results. The position of the points corresponds with aver-age of E/V and Cp for each presented rock class. The size ofpoints relatively depicts the number of tests.

in Schneider’s graph. To get a better overview of thedistribution of test results, the average is calculatedfor each rock class. Seventy dilatometer tests exe-cuted during the exploration campaign for Brennerbase tunnel project are shown in Figure 6.

The proposed analysis and graphic presentationgives an overview of the deformation properties alongthe whole project. Based on the modified Schnei-der’s criterion, it is obvious, that that about 95% ofthe tests indicate a compact rock, of mainly elasticbehaviour in the range of applied pressures. Only fourtests (roughly 5%) represent rock with partly plasticdeformation behaviour. In the next step, these testsshould be reviewed in detail and further investigationmight be considered if these results appear to be criticalfor the project.

This approach might be helpful for decision mak-ing in the early stages of geological and geotechnicalsurvey. The knowledge gained from the modifiedSchneider’s criterion may be used:

– as another point of view for preliminary character-istics the rock mass,

– to find areas, where further investigation may berequired,

– to obtain an overview of the deformation propertiesof the rock mass and

– to estimate suitable tunnelling techniques and thetype of primary tunnelling supports.

4 APPLICATION OF RESULTS

For the determination of the stiffness parameters ofthe rock mass at the Brenner base tunnel various for-mulas given in the literature such as Hoek et al 2002have been used. Results obtained revealed a consider-able scatter. The results of the dilatometer tests havebeen used to verify calculated results. Due to the fact

287

that dilatometer tests represent the deformability ofsections of rock mass of less than 1 m, whereas duringexcavation stress redistributions affect sections of upto 10 m results of dilatometer tests have been regardedas an upper limit taking into consideration the scaleeffect.

5 CONCLUSIONS

Results of dilatometer in-situ measurements describedeformation properties of rock mass. A large num-ber of tests were performed during the second phaseof geological exploration for the Brenner base tunnelin 2004–2005. The borehole diameters were rangingfrom 96 to 158 mm and the boreholes were reach-ing to depths of more than 1300 m. The results fromthe provided dilatometer tests were used to verifycalculated stiffness parameters of the rock mass atthe Brenner base tunnel. Based on our experiencewith test analysis and interpretation, a modificationof Schneider’s analysis of dilatometer tests has beencreated in order to allow quick overview and compari-son of numerous dilatometer tests. The most importantmodifications are: moduli calculated only from thelast loading cycle are directly used. Each dilatome-ter test is then represented by only a single point in theSchneider’s graphical classification. Even large seriesof tests covering the whole explored area of inter-est can be assessed. Evaluation of the distribution ofpoints in the resulting graph allows characterization ofthe major type of rock mass deformability. The rockclasses where few tests occur can indicate areas to bereviewed or further investigated.

ACKNOWLEDGEMENTS

The authors would like to thank BBT SE for thepermission to publish results from the 2004/2005exploration campaign.

REFERENCES

AFTES, Groupe de travaille No. 1, Géologie – Géotechnique.Texte des recommandations pour une description des mas-sifs rocheux utile à l’étude de la stabilité des ouvragessouterrains. Tunnels et ouvrages souterrains, supplémentau No117, Mai 1993, pages 12–21, Annexe 1.

Hoek, B., Carranza-Torres, C., Corkum, B. 2002. Hoek-Brown Failure Criterion – Edition 2002. Vancouver.

Schneider, B. 1967. Moyens nouveaux de reconnaissance desmassifs rocheux. In Nouredine Dinia et al, Méthodologiedes essais au dilatomètre: application à quatre grandsbarrages Marocains; Proc. intern. Symp. Essais en placevolume 2, Paris 1983.

Solexperts 2005. Brenner Basistunnel Erkundungsbohrun-gen 2004 B0008-Wipptal BohrungVa-B-03/04s Dilatome-terversuche. Final report. Mönchaltorf: Solexperts.

Solexperts 2005. Brenner Basistunnel Erkundungsbohrun-gen 2004 B0008-Wipptal Bohrung Pf-B-03/05 Dilatome-terversuche. Final report. Mönchaltorf: Solexperts.

Thut, A. 1977. Deformationsmoduln der Mergelschiefer derFlyscherie und des verlehmten Gehängeschuttes Vergle-iche zwischen Dilatometer- Lastplatten- und Druckver-suchen. Mitteilungen der Schweizerischen Gesellschaftfür Boden- und Felsmechanik No. 96, Frühjahrstagung,Luzern, 13. Mai 1977.

Zalesky, M., unpubl. 2006. Comparison of all results ofdilatometer test form Brenner base tunnel (2nd phase ofexploration) and modification of the Schneider’s criterion.

288