INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Adoption of triaxial testing for the study of swelling behaviour in tunnels

Utilisation de l’appareil triaxial pour l’étude du comportement gonflant des tunnels

M.Baria & G.Baria — Dept, of Structural and Geotechnical Engineering, Politecnico di Torino, Italy

ABSTRACT: In this paper an experimental procedure performed in a new advanced triaxial equipment, deve loped to determine the swelling behaviour o f an Italian hard soil (a stiff Eocene clay), is described. The proposed procedure allows one to carefully apply to the sample the typical stress history taking place around a tunnel during excavation. This is done by considering the three-dimensional conditions at the tunnel heading, thus overcoming the limitations which characterize the oedometer test. Moreover, by introducing a distinction between the drained phase (during tunne l excavation) and the undrained phase (when the excavation is completed), it is shown how one can gain significant information on the swelling behaviour of the ground, with important consequences for the pre­ diction of tunnel performance.

RESUME: Dans le text suivant décrit une procédure e xpérimentale mise à point grâce à un nouvel équipem ent triaxial aux caractéris­ tiques avancées, ique fin d’evaluer le comportement gonflant de un terrain solide (compact) italien (une argile éocènique). La pro­ cédure proposée permet d ’imposer à l’éprouvette le stress typique qui se dévéloppe sur le contour d’un tunnel pendant l’excavation. Il est ainsi possible de tenir compte des conditions tridimensionnelles au front du tunnel, en dépassant les lim ites qui caractérisent l’essai œ dométrique. En outre, en introduisant la distinction entre la phase drainée (pendant l’excavation du tunnel) et la phase non drainée (au terme de l’excavation), on peut montrer comment il est possible de tirer d’importantes information s sur le comportement regon­ flant du terrain, avec des conséquences significatives sur la prévision du comportement du tunnel.

1 INTRODUCTION excavation, which can be adopted as appropriate input to labo­ ratory testing. The time-dependent behaviour, which is induced by the excava­ A numerical study on the stress paths that develop around a tion of a tunnel in a swelling ground, is shown to be of great tunnel during excavation, considering both 2D and 3 D condi­ relevance in geotechnical engineering. In some cases this may tions, has been performed by the junior author and described in lead to significant difficulties during excavation and even pose previous publications (Barla M. 1999, Barla M. 2000 ). If we problems for the use o f the tunnel in the long term. point our attention to an element of ground at the sidewall on a In order to be able to make appropriate predictions of this be­ certain section along the tunnel axis and consider the tunnel to be haviour, both at the design and construction stage, the tunnel en­ excavated in an isotropic, linearly elastic medium (ILE) with a gineer needs to use tools that allow him to quantify, timely and stress ratio o f K ^= l, the stress history calculated from the analy­ correctly, the swelling properties of the ground. T o this end, dif­ sis is exemplified in Figure 1. ferent testing techniques, “at laboratory scale”, have been pro­ Figures la to Id show the sequence of the face advancement posed in the past. and its consequence on the stress path o f the eleme nt of ground It appears that the oedometer test, as originally introduced by at the sidewall. This stress path is described by considering both Huder and Amberg (1970) and later modified by the ISRM two dimensional (2D) and three dimensional (3D) conditions and Commission on Swelling Rock (ISRM 1989, Madsen 1999), is the resulting behaviour is compared on the [r, s] plane where: mostly used. However, more recently, different authors have been studying this problem by triaxial testing (Pre gl et al. 1980, Bellwald 1990, Aristorenas 1992). t - mean deviatoric stress = ------; 2 ° V+C!h s = mean normal stress = ------; 2 2D AND 3D LABORATORY TESTING 2 ov = vertical stress; The aim of the different laboratory techniques deve loped so far is to quantify the ground swelling properties in order to provide a* = horizontal stress. the tunnel engineer the necessary input data for appropriate pre­ dictions of ground behaviour. An important aspect in laboratory It is easy to appreciate that prior to excavation (Figure la) the testing is the need to reproduce as close as possible the real be­ element of ground is subjected to the initial state of stress char­ haviour o f the ground on site. This is very importa nt since the re­ acterised by t equal to 0 and s equal to the geostatic stress. sults obtained show a tendency to be influenced fro m the testing As soon as the face of the excavation approaches the section conditions adopted. where the stress path is computed (Figure lb), t starts to in­ During the excavation of a tunnel, one of the major factors crease. This increase of t continues until the end of the excava­ that influence the swelling phenomenon is the stress history at tion. points in the tunnel surround as face advancement takes place. It is now important to underline the differences be tween the This can be well described by the use of the stress path method, stress path in 2D conditions (j always constant) and in 3D con­ as proposed by Lambe (1967) for a number of applica tions in ditions, where the mean normal stress initially increases, to de­ geotechnical engineering. Therefore it is of interest to show, in crease as soon as the face of the excavation overpasses the sec- the present paper, typical stress paths as created during tunnel

1375 INITIAL STATE ' OF STRESS

•4 ,

(a) (b) (c) (d) Figure 1. Stress paths in 2D and 3D conditions for a point at the sidewall of a circular tunnel, (a) before excavation, (b) when excavation has reached the section where the stress paths are computed, (c) when the face of the excavation is 1 m ahead, (d) when excavation is completed.

jo»s1,69 oh=0,3 j|

( 7 ) [ CNV8-9 |

(cNV2-3-4)^^ xp) |ov=1 Oh=1 |

1 0.2 0.4 0.6 0.8 1.4 1 © — 2D stress path X 6710 — 3D linearization s , [CNV - - ) SI |ove0,3o be1,69||

s [MPa] Figure 2. Stress paths for the sidewall and crown/invert for 2D and 3D numerical analysis compared with the Huder-Amber oedometer test. Figure 3. Different stress paths performed in the triaxial apparatus. tion of interest. An increase of s again occurs when the excava­ sample tested), undrained conditions are assumed to hold true at tion is going to be completed. least for the time duration required effectively fo r a ground ele­ If the attention is posed on the element of ground at the ment at the tunnel periphery to experience the stress path. crown or invert arch of the tunnel, the behaviour on the t-s plane During the shearing phase the sample is subjected to one of is as for the sidewall element, with opposite r, as shown in Fig­ the stress paths shown in Figure 3. Stress paths 1 and 2 pertain to ure 2. the element of ground at the sidewall for 2D and 3D conditions. Both the sidewall and crown/invert behaviours are compared Stress paths 3 and 4 pertain to the crown/invert position, again in the same figure with the stress path of a modified Huder and for 2D and 3D conditions. Amberg oedometer test. I f the stress path is followed up to a certain t value lower than It is obvious that the stress history experienced in the near vi­ failure (for example to a mobilised factor of / = 0 .8), it is then cinity of the tunnel is not well reproduced in by the modified possible to simulate a new phase. W ith the stress level constant Huder-Amberg oedometer test. In particular, near to the face of versus time and the creep deformations completed, the drainage the excavation, the ground behaviour can be well de scribed only valve can be opened and water can flow in or out from the sam­ simulating three dimensional conditions in a triaxial apparatus. ple, depending on the value of the pore overpressure reached during the undrained phase. This new drained phase corresponds to that experienced by an element of ground at a ce rtain distance 3 THE PROPOSED TEST PROCEDURE from the tunnel contour or during a standstill and can be adopted to study the swelling behaviour of the ground versus time, as In order to overcome the above limitations of the oedometer test, w ill be shown in the following. an experimental procedure to be applied in a triaxial apparatus was developed. The current procedure has been defined as con­ sisting of six phases: specimen preparation and set-up, flushing, 4 TESTING EQUIPMENT saturation, consolidation, undrained stress path phase and swel­ ling/consolidation. To perform the triaxial tests described in this paper, two spe­ The set-up of the sample is done with the dry setting method cially devised triaxial apparatuses (GDS and SRTA), developed (Lo Presti et al. 1999). Up to the consolidation phase, the proce­ at the Politecnico di Torino, were used. For a deta iled descrip­ dure adopted is that of a typical triaxial test with the precaution tion of them refer to Lo Presti et al. (1995), for the GDS, and to to inhibit swelling when the sample gets in contact with water Lo Presti et al. 1998, Barla M . 1999, Barla M . et al. 1999, for the (i.e. during the flushing phase). The stress path o r shearing phase SRTA. is carried out in undrained conditions, given the intention to Both equipments have a very stiff cell structure and consist of simulate “at laboratory scale” the stress conditions in the near two end platens connected by three tie rods located inside a per­ vicinity of the tunnel, during face advancement. spex pressure cell. The triaxial cells are equipped with local It is accepted that the issue of whether undrained or drained measurements devices for axial and radial strain, pressure trans­ conditions are more applicable to the tunnel proble m during face ducers for the cell pressure and the pore pressure, a load cell lo­ advancement depends primarily on the permeability o f the cated inside the pressure chamber and a volume variation indi­ ground, the rate o f excavation and the size o f the tunnel (M air & cator. Lateral and axial pressure control is obtained by means of Taylor 1997). If consideration is given to hard soils and argilla­ digital controllers with a resolution of 0.5 kPa. A multichannel ceous rocks with permeability lower than 10'7 m/s (as for the conditioning system is used for data acquisition. T he data are

1376 Table 1. Triaxial tests performed*. 300

Name Depth W n Type of B. P. B Au ♦ Caneva clay H test [kPa| [kPa] [kP¡l [kPa] [kPa] a Varicolori clay shales 250 A CNV1 37.51 14.49 CIU 718 243 0.94 270 558 -286 o Terravecchia claystone CNV2 37,28 13.36 CIU-2D 650 350 0.87 452 1000 -349 200 t CNV3 37.06 11.47 CIU-2D 670 310 0.90 245 728 -60 a CNV4 36.91 14.10 CID 657 320 0.95 135 661 0 ♦

S ♦ CNV5 51.19 14.26 CIU 200 0 0.93 470 669 - ♦ CNV6 51.35 13.05 EIU-2D 815 200 0.99 -317 632 175 ISP ISP [kPa] o CNV7 51.49 12.28 EIU-2D 750 380 0.90 -318 595 164 100 A ♦ CNV8 36.90 11.17 CIU-3D 635 350 0.90 234 656 100 ♦ ♦ O A CNV9 45.17 9.39 CIU-3D 1150 553 0.77 317 735 -43 50 ■ O CNV10 44.87 20.52 EIU-2D 695 405 0.88 -132 479 124 ♦ ♦ ♦ Legend: w „= natural water content, o'c = consolida tion effective stress, 0 • B.P. = back pressure, B = Skempton’s parameter, t™ , s'™, = values at 0 10 20 30 40 50 60 70 80 the end o f the test, Au = excess pore pressure. Depth [m]

automatically transferred via HPIB connection from the condi­ Figure 4. Swelling pressure for Caneva clay and oth er soils. tioning system to a PC so that one can control the whole test procedure by PC. The GDS apparatus can reach maximum val­ ues of 2.5 MPa for the axial stress and 1 MPa for the confining pressure while the SRTA has a maximum capacity of 50 kN for the vertical load and 2 M Pa for the pressure ce ll.

5 RESULTS OF THE TESTING PROGRAM

A total of 10 triaxial tests were performed, as shown in Table 1, with the aim to simulate, “at laboratory scale”, the tunnel be­ haviour in the undrained and drained phase. The soil tested is a stiff clay (Caneva clay) from North Italy (Barla M . 1999). Tests CNV1 to CNV4, CNV8 and CNV9 were performed to reproduce the behaviour at the sidewalls of a circu lar tunnel during excavation. As indicated in Figure 3, the s = constant Figure 5. Total and effective stress paths for the tests intended to sim u­ “compression” stress path was imposed to tests CNV2 , CNV3 late the behaviour at the sidewall o f a circular tu nnel (p oint S) and at the and CNV4. W hile the CNV2 test was carried out up to failure, crown/invert (point C and I). O nly undrained tests are shown. the C N V3 stress path was interrupted at a value of the mobilised deviatoric strength factor/ = 0.5. At this point the drainage valve ever, with / nearly equal to 200 kPa, a negative pore pressure de­ was opened and the swelling deformations measured. The CNV4 velops, subsequently to increase in relation to the increase of t. It test was carried out up to/ = 0.33 in drained conditions. is clearly shown that the excess pore pressure Au, negative at the The three dimensional conditions were introduced fo r test sidewall of the tunnel, is instead positive at the invert/crown. CNV8 and CN V9 that followed the simplified three dimensional The final value of Au attained in each case at the end of the test stress path. At a value of the mobilised fa cto r/= 0 .5, in order to is directly related to the stress level t. compare the results with test CNV3, the CNV8 stress path was If the attention is now posed on the CNV8 and CNV9 tests, interrupted and the drainage valve opened, while the CNV9 test which were carried out specifically to simulate three dimensional was taken up to failure. conditions during face advancement, the results obtained for the Tests CNV6, CNV7 and CNV10 were performed to investi­ excess pore pressure show a significantly different response. gate the behaviour at the crown/invert. Also in this case, when a D uring the first segment o f the stress path both the axial and the two dimensional condition is applied, the stress path is vertical ( i confining pressures in the triaxial cell are increa sing. This results = costant), however in “extension”. A ll the tests o f this group in a positive excess pore pressure for low t values. When the were undrained and interrupted at a mobilised factor / = 0.5, stress path changes direction (i.e. when the tunnel passes the opening the drainage valve. cross section o f interest) the confining pressure decreases rapidly The C N V5 specimen was sheared under a conventional com­ with a notable effect on the pore pressure. For the CNV8 speci­ pression loading stress path. men at the end of the test the excess pore pressure is positive with a value of 100 kPa. On the contrary, for the C NV9 speci­ men, which was taken up to failure, the negative excess pore 5.1 Swelling stresses from the flushing phase pressure is -43 kPa. Figure 4 compares the vertical stress at the end of flushing with As can be seen, while for the two dimensional conditions the the data available for other soils which exhibit a different degree negative excess pore pressure develops at a I value of 200 kPa, of swelling potential (Barla G. et al. 1990). These data were ob­ when the influence of the advancing face is taken into account tained from oedometer tests giving the vertical pressure which (i.e. in three dimensional conditions) a greater va lue of t is nec­ prevents swelling (ISP). Even though the data cannot be directly essary to induce a negative excess pore pressure. compared, since the testing procedures are quite different, the If this negative excess pore pressure is connected to the Caneva clay is shown to exhibit a moderate to high swelling po­ amount of swelling that is expected, in the near vicinity of the tential. sidewalls of the tunnel, the areas where swelling is likely to oc­ cur would be smaller when predicted with a three dimensional analysis instead of a two dimensional one. Moreover, at failure, 5.2 Undrained shearing phase for both cases, as a negative excess pore pressure around the tunnel results in a water inflow towards it, swelling is likely to The comparison of total and effective stress paths in Figure 5 allows one to clearly appreciate the excess pore pressure change occur as an inverse consolidation due to the intera ction between water and swelling minerals when present in the ground. To in­ which occurs during each test. It is noted that in two dimensional vestigate this behaviour for the CNV3 and CNV8 tests, the conditions the excess pore pressure Au is almost ne gligible in the first part of the test, as long as the t value remains small. How- drainage valve was opened at the constant final state o f stress.

1377 stress. On the contrary, for specimens simulating the tunnel crown/invert response, as a consequence of developm ent of positive excess pore pressure during the undrained phase, consolidation is shown to take place when the drainage valve is opened.

7 REFERENCES

Axistorenas, G. V. 1992. Time-dependent behaviour o f tunnels excavated in shale. PhD Thesis. Massachusetts Institute o f Te chnology. Boston, USA. Barla, G., Forlati, F. & Zaninetti, A. 1990. Prove di laboratorio su rocce tenere: problematiche ed esempi. Conferenze di Mecc anica e Ingegneria delle Rocce, M ir ’90. Pp 4.1-4.47. Politecnico di Torino. Torino, Italy. Figure 6. Volum etrie deformation and pore pressure excess for CNV3 Barla, M ., Barla, G., Lo Presti, D.C.F., Pallara, O . & Vandenbussche, N. and C N V7 tests after drainage opening. 1999. Stiffness o f soft rocks from laboratory tests . Proceedings o f the IS Torino ‘99, Second International Symposium on Pr e-failure de­ form ation characteristics o f geomatrials. Torino, I taly. 5.3 Drained phase Barla, M . 1999. Tunnels in Swelling Ground - Simula tion o f 3D stress paths by triaxial laboratory testing. PhD Thesis in Geotechnical En­ W ith a released state of stress and drainage occurring, deforma­ gineering. Politecnico di Torino. November 1999. Pp . 180. tion due to water adsorption and chemical reactions with the Barla, M . 2000. Stress paths around a circular tunn el - Percorsi di sol­ mineralogical constituents can take place. For this reason the lecitazione attorno ad una galleria circolare. Work shop on Squeezing axial, radial and volumetric deformations were measured, all the Rock Conditions in Tunnelling, Stoccolma, 10 giugno 1998, pub­ other conditions holding true. lished on Rivista Italiana di Geotecnica (RIG ) 1/20 00. Figure 6 illustrates for the CNV3 and CNV7 tests a plot of Bellwald, P. 1990. A contribution to the design o f tunnels in argillaceous the volumetric deformation (evoi)> obtained by either direct rock. PhD Thesis. Massachusetts Institute of techno logy. Boston, USA. measurement of volume change in the specimen (i.e. volume of Huder, J. & Amberg, G. 1970. Quellung in Mergel, Op alinuston und water entering-positive or exiting-negative the spe cimen) or anydrit. Schweizerische Bauzeitung. Vol. 88, No. 43 , pp. 975-980. computation of the first invariant of strain in terms of e„ and e,. ISRM 1989. Suggested methods for laboratory testing o f argillaceous Also shown is the plot of the excess pore pressure versus time swelling rocks. Commission on Swelling Rock. Co-ord inator: H. Ein­ which dissipates, under a constant state o f stress, in a few hours. stein. Int. J. Rock Mech. M in. Sci. & Goemech. Abstr. Vol. 26, No. 5, pp. 415-426. Lambe, T.W . 1967. The stress path method. JSMFD, AS CE, Nov., pp. 6 CONCLUSIONS 309-331. Lo Presti, D.C.F., Pallara, O. & Puci, I. 1995. A m odified commercial

The triaxial testing programme, developed with the main pur­ triaxial testing system for sm all strain measuremen ts: preliminary re­ sults on Pisa Clay. A STM Geotechnical Testing Journ al: Vol. 18, N. pose to investigate the tunnel behaviour during excavation in 1, pp. 15-31. M arch 1995. swelling ground, has been described in the present paper. On the Lo Presti, D.C.F., Pallara, O., Cavallaro, A. & Jam iolkowski, M . 1999. basis of the work performed so far, the following m ain conclu­ Influence of reconsolidation techniques and strain rate on the stiff­ sions can be drawn. ness of undisturbed clays from triaxial tests. ASTM Geotechnical The testing procedures adopted are shown to be very effec­ Testing Journal, GTJODJ, Vol. 22, No. 3, September 1999, pp. 211- tive in simulating the soil behaviour in the particular condi­ 225. tions and stress histories which are experienced by a ground Madsen, F.T. 1999. Suggested methods for laboratory testing o f swelling element in the near vicinity of a circular tunnel. rocks. Int. J. Rock Mech. M in. Sci. & Geomech. Vol. 26, No. 3, pp. 211-225. From the results of the flushing phase, the Caneva clay is M air, R.J. & Taylor, R.N. 1997. Bored tunnelling in the urban environ­ shown to exhibit a moderate to high swelling potential, as ment. Proc. 14th ICSM FE. Hamburg, Germany.Pp. 2353-2385. expected on the basis o f its mineralogical composition. Pregi, O., Fuchs, M ., M uller, H., Petschl, G., Ried m iiller, G. & Schwaig- The Caneva clay specimens, isotropically consolidated to the hofer, B. 1980. Dreiaxiale Schwellversuche an Tonge stein. Geotech- in situ state o f stress, exhibit a negative excess pore pressure nik 3, H eft 1. during the undrained “compression” stress paths, typical of the tunnel sidewall response simulation. This behaviour is shown to hold true for both two dimensional (pure shear) and 8 ACKNOWLEDGEMENTS three dimensional conditions, when taking into account the influence of the advancing face. The work described in this paper was carried out with the finan­ It is also shown that, when three dimensional conditions are cial support of the Italian M inistry for University and Techno­ simulated in the triaxial cell, a greater value of I is necessary logical Research (M.U.R.S.T.) as part of the Research Pro­ to induce negative excess pore pressure in the specimen gramme “Tunnelling in difficult conditions” (40%). since during the first portion o f the stress path (i.e. when the tunnel face is approaching the cross section of interest) a positive excess pore pressure develops due to a stress in­ crease. When the attention is taken to the tunnel crown/invert be­ haviour and the tests on the Caneva clay specimens are sim ilarly carried out under “extension” conditions, following a pure shear stress path, a positive excess pore pressure is shown to develop during the undrained phase of the test. W ith the drained phase, which follows in each case a creep stage, swelling is shown to occur for specimens sim ulating the tunnel sidewall behaviour, as the negative excess pore pressure dissipates under the imposed constant state of

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