Abstract

The effect of weathering on Alpine rock instability

M. Jaboyedoff1,2, F. Baillifard2,3, E. Bardou4 & F. Girod5

1Quanterra, Ch. Tour-Grise 28, 1007 Lausanne, Switzerland (e-mail: michel.jaboyedoff@quanterra.org)2CREALP (Research Centre on Alpine Environment), Rue de l’Industre 45, 1951 Sion, Switzerland

3Institute of Geology and Palaeontology, University of Lausanne, no. 15, Lausanne, Switzerland4Div. Natural Hazard–WSL (Federal Research Institute on Forest, Snow and Landscape), Rue de l’Industre 45,

1951 Sion, Switzerland5ARL, Thermo Electron S.A., 1024 Ecublens, Switzerland

Weathering affects joint or fault gougematerial in the Swiss Alps, leading to theformation of smectite, which changes themechanical properties of fault gouge and

degrades slope performance. Analysis of recent rock-slides in gneissic rocks in Switzerland indicates thataccelerations of movements are linked to precipitation,without the development of excess pore water pressure.Processes such as weathering and crushing induce soil-like behaviour of the infilling material and explain rock-slide movements induced by water seepage in both faultand joint gouges.

Keywords: weathering, fault gouge, rock instability,soil mechanics

Estimating the hazard associated with unstable rockmasses is very difficult. The usual methods use a relativescale that rarely gives information concerning thetime scale of the processes (Cancelli & Crosta 1993;Mazzoccola & Hudson 1996; Rouiller et al. 1998). Othermethods of estimating hazard are based on inventories(Pierson et al. 1990; Hungr et al. 1999). However, theprocesses leading to failure in rock slopes are stilldifficult to understand. The change of the safety factorwith time is dependent on both fatigue and weatheringof the rock mass. As a consequence, improvements inestimating the variables governing the change of thestability of a rock mass with time will improve hazardassessment. The present paper discusses the impact ofweathering and crushing processes that cause the gougematerial to behave more like soil (Leroueil 2001).

In rock mechanics, the role of excess groundwaterpressure is often proposed as a triggering factor forrockslides (Terzaghi 1962; Hoek & Bray 1984). Al-though it is a frequent process, it does not explain thechange of the peak strength of material with time.Weathering is often recognized as one of the processesdestabilizing slopes (Piteau & Peckover 1978), or takeninto account in geomechanical classifications (Romana1993). However, most of the studies of jointed andfaulted rocks do not take chemical weathering intoaccount as a factor of slope evolution (Rossmanith1998).

Gerber & Scheidegger (1969) postulated that frac-tured rock cropping out in steep slopes is submitted tomore intense weathering caused by pre-existing stresses,which favours the degradation of rock strength. Piteau& Peckover (1978) reported that the occurrence ofmontmorillonites led to rockslides in some cases. In theSwiss Alps, Girod (1999) indicated that the gneissic rockmass before the Randa rockfall was strongly affected bychemical weathering (Sartori et al. 2004). This obser-vation is supported by the fact that chemical weatheringoccurring in crystalline rock in sub-arctic climate con-ditions can produce smectite by alteration of feldspars(Hermance 1989).

The effects of chemical weathering and developmentof montmorillonite are more generally known in land-slides such as the debris-flows of Calabria (Calcaterraet al. 1998).

Recent observations from three rockslides inSwitzerland, described in the case studies below, indicatethat rock mass movements are controlled by rainfall orsnowmelt. For the two rockslides with the smallestvolume, the development of excess pore water pressurewas not possible, because the rock mass was completelycut by largely open fractures, precluding the rise of thewater table. Thus, the acceleration of movements withrainfall has to be explained by a soil-like behaviour ofthe basal slip surface, with chemical weathering andcrushing as the main processes for strength decrease. Incontrast, excess pore water pressure is certainly thetriggering factor for the largest rockslide, the Randarockfall. However, the increase in pore water pressurewas promoted by the decrease in permeability caused byweathering and crushing since the last glaciation.

Case studies

Three case studies involving rock instabilities aredescribed, located within the Mattertal (Wallis,Switzerland), 20 km north of Zermatt. They occurwithin the ‘Randa orthogneiss’, a massive augengneiss(Fig. 1). The orthogneiss is an intrusion of Permian agethat underwent Alpine deformation and the develop-ment of a light schistosity. This lithology is competent incomparison with the surrounding lithologies.

Quarterly Journal of Engineering Geology and Hydrogeology, 37, 95–103 1470-9236/04 $15.00 � 2004 Geological Society of London

The Randa rockslide

The main rockfall of this area occurred in 1991 in Randa(30 � 106 m3) (Sartori et al. 2004). Its mechanism wasmainly controlled by the sliding of a rock mass along afault located at the base of the cliff. The triggering factorwas certainly the development of excess pore waterpressure caused by a hot wind inducing snowmelt.However, fatigue and the obstruction of the per-meability of the fracture caused by chemical weatheringwithin the gouges (i.e. the cementation and build-up of afine-grained material deposit (Girod & Thélin 1998;Girod 1999)) made excess pore water pressure build-uppossible. Subsequently, a gallery used to deviate the riverwas built below the Randa scar, to prevent floods causedby the damming created by a new rockslide.

The Mattsand rockslide

The second case study described is the rock instability ofMattsand. It is a 400 m3 column (Cruden & Varnes1996) (Fig. 2) situated above a 300 m3 block that toppledon 28 October 1998 (Amatruda et al. 2002). This un-stable mass is also sliding on its basal surface. The rockmass is completely separated from the massif by openfractures. The displacements measured by extensometersindicate a strong increase of velocities during and afterrain (Fig. 3). No excess pore water pressure is possiblefor such a geometry (Fig. 4). No movement can beattributed to freeze and thaw cycles. This rock block wasblasted down for safety reasons in April 2001.

The Medji rockslide

The third case study described is the rockslide of Medji(c. 100 000 m3), which occurred above the town ofSt-Niklaus on 22 November 2002. The rockslide wascomposed of a completely fractured spur that slid on acomplex basal surface along which it was not possible todevelop excess pore water pressure. Detailed infor-mation about this event is still confidential, but it is nowknown that the displacement velocity increased afterrain, and remained constant after the rain stopped. Thegroundwater level (monitored using a borehole) waslocated below the instability. No significant rise wasobserved during or after the rain.

Chemical weathering of the Randagneiss

Observation

A detailed mineralogical analysis on the Randa orthog-neiss, the fault gouge and the percolating groundwaterwas carried out to determine the chemical weatheringmechanisms (Girod 1999).

The mineralogical composition of the Randa orthog-neiss is classical: K-feldspar (KF), plagioclase, albite,microcline, quartz, white micas, phengite, biotite, andaccessory minerals such as pyrite are present.

The chemical analysis of the water indicates that thehigher the salinity, the higher the elevation above thegallery. This means that the longer the groundwaterpath, the longer the time available for mineral dissol-ution. Furthermore, thermodynamic calculations indi-cate that those waters are undersaturated in albite andpyrite (Girod 1999), demonstrating that albite and pyriteare potentially dissolved along the entire groundwaterpath.

The fault gouge material is up to 40 cm thick and istypically fractured material with large rock elements(up to few centimetres) separated by a paste of finermaterial. The grain-size distribution is as follows: 3%clay and silts; 54% sand; 43% gravel.

Fig. 1. Location of the three instabilities within the Randaorthogneiss (modified after Steck et al. 1999).

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Scanning Electron Microscopy (SEM) shows dissol-ution figures of minerals such as pyrite and feldspars (i.ealbite and KF) (Fig. 5). Smectite (more precisely truemontmorillonite) is identified by scanning electron

micrographs and electron-dispersive X-ray spectrometryanalysis, and also by X-ray diffraction analysis of the<2 µm size fraction. Mineral precipitation of ironhydroxide is also reported.

Fig. 2. Picture of the instability of Mattsand. M, locations of the extensometers (data from these are displayed in Fig. 3).

Fig. 3. Cumulative rain, temperature and displacements recorded by the extensometers. The effect of the rain on the displacementsand the delay before stabilization should be noted.

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Process

Girod (1999) interpreted the dissolution of pyriteas a source of water acidification, which allows thedissolution of albite. Thus, assuming that Mg2+ is pro-vided by chlorite dissolution, smectite can be interpretedto be the weathering product of albite dissolution(Berner 1971):

Mg2 ++ + 3 Na Al Si3 O8(albite) + 4 H2O/

2 Na0.5Al1.5Mg0.5Si4O10(OH)2(montmorillonite) +

2Na + + H4SiO4aq.

The oxidation of pyrite can also lead to further albitedissolution (Berner & Berner 1996). The crystallizationof smectite requires slow water flow rates, becausesaturation in montmorillonite must be reached. Thiscondition is found within the gouges. In open fractures,water simply dissolves the walls without material pre-cipitation. It removes the cations from the system, as isdemonstrated by the correlation between the length ofpercolation path and the water salinity.

To sum up, the fractured rock mass system can ideallybe divided into two subsystems: (1) a highly permeablefracture network, which simply leads to rapid dissol-ution of minerals (Sausse et al. 1998); (2) a less per-meable one, which makes mineralogical changes, such asprecipitation and dissolution of minerals, possible (Gi-rod 1999) because saturation is reached. The secondsystem leads to changes in mechanical properties of the

gouge and the rock, whereas the first opens pores slowlyand increases the permeability by dissolution.

Proposed explanation

Argument for unsaturated soil behaviour

From observations of movements caused by rain with-out the build-up of excess pore water pressure, it must beassumed that the material infilling fracture interfaces orgouges behaves like a soil. Such behaviour cannot beattributed to other types of materials, because one of thefew possibilities is to change the mechanical propertiesby water seepage, which is similar to the behaviour ofunsaturated soil (Fredlund et al. 1978, 1995; Leroueil2001; Leroueil & Hight 2003).

Shear strength is dependent on water saturation. Thesuction or the pressure that links grains decreases withsaturation. Thus, the shear strength is at a maximum foran intermediate saturation value depending on the soiltype (Fredlund et al. 1995). In a �–�n plot (shearstress–normal stress), a rock instability sliding on adiscontinuity is a fixed point (Fig. 6), if no excess porewater pressure is considered. The suction can be consid-ered as a contribution to an apparent cohesion. Forsimplification, let c$(uw) be the cohesion, including theeffect of suction depending on the saturation or waterpressure (uw). Thus, the strength is given by� = c$(uw) � �ntan �# (where �# is the friction angle).The distance of the discontinuity stress state from the

Fig. 4. Scheme of an instability with a water seepage, but with no possibility of excess pore water pressure.

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strength limit varies with the fluctuation of saturation,leading to fatigue (Leroueil 2001). The suction does notchange �#, and has a limited effect: it alone cannot leadto failure. However, the gouge is subjected to cycles thatcan lead to the decrease of the mechanical strength byfatigue (Demers et al. 1999). If small movements occurwithin the gouge, the shear band can modify shearstrength and permeability, by slowly changing the grainlayout. The process leading to failure is then the pro-gressive decrease of �# by chemical weathering andalteration.

Origin of the soil-like behaviour

The discontinuities have an ancient tectonic origin. Thematerial in the discontinuities has suffered displacementsand crushing, leading to the formation of gougematerial, or simply of a small amount of fine-grainedmaterial, between the discontinuity walls. The gougematerial can be created by displacement as small as10 cm (Girod 1999).

The crushing of the material during fault formationincreases the surface area of the crushed material, result-ing in an increase in permeability and facilitating chemi-cal weathering. The weathering rate increases with bothwater content and circulation (Matsukura & Hirose1999). Both dissolution and precipitation of mineralsoccur in the gouge. The soil-like behaviour is consistentwith the properties of the small grain size of precipitatedminerals and with the relatively low permeability that isnecessary for the precipitation of smectite.

The occurrence of montmorillonite in the fault gougesof the Randa orthogneiss has resulted from the effect ofweathering on alpine rocks (Girod 1999) and implieslarge changes in grain superficial tension. This has led toan increase of the suction (uw), and thus an increase inshear strength if the gouge is unsaturated, or converselya decrease if the gouge is saturated (Fredlund et al. 1995;Nishimura & Toyota 2002; Leroueil & Hight 2003).

Discussion

The effects of chemical weathering on the gouge materialhave been compared in the three case studies presentedabove. The existence of faults with thick gouges hasplayed a very important role in the destabilization of the1991 Randa rockslide (Sartori et al. 2004). The action ofweathering decreases stability progressively by slowlydecreasing �# and producing small variations of c$.Numerical analysis of the Randa rockslide by Eberhardt

Fig. 5. Scanning electron micrograph of the weathered min-erals and of the neoformed smectite. (a) Weathered pyrite(dissolution pits); (b) weathered etched albite (dissolution pits);(c) neoformed smectite.

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et al. (2002) is based on a decrease of cohesion attributedto the progressive failure of the rock bridges (fresh rockbetween discontinuities). Weathering and water seepagecycles certainly assisted this progressive rock bridgefailure by inducing small movements within the gouge.Because of the size of the Randa rockslide, the action ofweathering and seepage has been slow, as the percola-tion takes more time and the time of response to seepagevariations is longer. If the residence time of water withinthe gouge is long (because of the size of the instability)there is a greater chance of the formation of montmo-rillonite. Thus, both small movements and chemicalweathering products are the sources of decrease inpermeability that produced the excess pore water press-ure considered as the triggering factor for the Randarockslide (Sartori et al. 2004). The effects of smectite onmechanical behaviour are significant. We may recall thata pressure equivalent to more than 20 km of rock pile atsurface temperature is necessary to expel the bondedH2O in the montmorillonite interlayer (Van Olphen1963). This underlines the role of the water adsorbed atthe surface of fine-grained material, whether newlyformed or not, which must have a significant effect onlocal strength within joint gouges. This will lead to smalldisplacements favouring rock mass destabilization.

In contrast to the Randa rockslide, the triggeringfactor of the two other rockslides appears to be acoupled phenomenon of chemical weathering and un-saturated soil-like behaviour. The 1998 Mattsand rockinstability clearly demonstrates a rapid response to therains. Movements are linked to seepage and cease rap-idly, because the size of the basal sliding surface islimited. Thus, the water circulates rapidly, and theunsaturated state is quickly reached during or afterthe end of the rains, again increasing the strength.The safety factor of the instability decreases with thesaturation (Fig. 7).

The 2002 Medji rockslide is of intermediate sizebetween Randa and Mattsand. Its behaviour is slightlydifferent. After the rains ended, the movements did notcease, but continued at the same rate. This can beexplained by a longer delay before the gouge once morereached an unsaturated state. In this case, the chemicalweathering leads to instability with a safety factor veryclose to one, even in unsaturated condition. However,the period of saturated state is sufficiently long topromote movements that irreversibly damage the mech-anical properties of the gouge by crushing the grains andchanging their arrangement (Leroueil 2001).

The ‘weathered gouge’ system process in the Randaorthogneiss seems to be an important factor for rockslope stability, even in the Alpine climate (Fig. 8). Thesoil-like behaviour underlines this effect. However, thepreliminary results on the grain-size distribution of thesegouges (Girod 1999) is surprisingly very similar to thosefor the weathered crystalline rocks from Calabria (Italy)and their associate debris-flow fans (Calcaterra et al.1998; Parise & Calcaterra 2000). More surprising is thepresence of smectite in the debris-flow fans in the SwissAlps and in Calabria (Parise & Calcaterra 2000). Thisfinding is very important, because it shows that alimiting chemical weathering state may exist. This wouldbe the limit beyond which chemical weathering affectsthe mechanical properties of the rocks leading, as forinstance in gouges, to a particular soil-like behaviour.The material can then be subject to rapid changes inmechanical characteristics because of its high chemicaland mechanical reactivity.

Conclusion

(1) This study demonstrates that the chemical weather-ing of granite–gneisses in an Alpine climate plays an

Fig. 6. Effect of changes in saturation on the apparent cohesion c$(uw) and the effect of weathering on �#. Left: evolution of theshear strength with time (modified from Leroueil 2001). Right: evolution of the shear strength and of the two variables, c$ and �#with time (modified after Demers et al. (1999) and Locat et al. (2000)).

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Fig. 7. Decrease in material strength as a result of seepage in fault gouges (loss of suction). This leads to a local decrease in strength,affecting the global safety factor. This can lead to fatigue or failure (see text for symbol explanation).

Fig 8. Tentative characterization of the effect of controlling parameters on gouge strength, and interfacial discontinuity surfaces atquasi-constant load. It should be noted that the yield strength can be increased or decreased by mineral precipitation. For instance,calcite precipitation is a cement.

WEATHERING AND ALPINE ROCK INSTABILITY 101

important role in slope processes. Inspection of insta-bilities within the Randa orthogneiss demonstrates theeffect of weathering coupled with unsaturated soil-likebehaviour. In addition, the rock bridges graduallydisintegrate by the variations of the safety factor of theentire rock mass caused by saturation–unsaturationcycles and weathering.

(2) These results, inferred from the behaviour ofrock instabilities, indicate that the effect of chemicalweathering, and especially the formation of smectite(Calcaterra et al. 1998), is potentially a way to quantifythe hazard associated with rock slope processes (Parise& Calcaterra 2000) by assessing the smectite occurrenceand its amount.

By studying the weathering grade in rock slopes, away can be found to quantify the hazard of Alpine rockslope, i.e. rock slope evolution. Geochemical and min-eralogical study has to be used. As Perel’man (1972, inFortescue 1992) wrote about the Russian specialitylandscape geochemistry: ‘We can therefore say thatlandscape geochemistry is the history of atoms inthe landscape’, which means that the dependence ofthe landscape on chemistry is of primary importance.The mapping of weathering and geochemistry isprobably very significant in slope evolution.

Future research should inspect weathering and mech-anical behaviour of the fractures and fracture gougeswith respect to saturation.

Acknowledgements. The director of CREALP, J.-D. Rouiller,is thanked for providing information and for discussions. Wealso thank C. Levesque (University Laval) and M. Wood-bridge (Transport Research Laboratory; now seconded to theMinistry of Public Works, Lebanon) for the English languagecorrections. We thank D. Calcaterra (Federico II University,Naples), and M. Woodbridge for their constructive reviews,which greatly improved the manuscript. The first authoralso thanks L. Laloui (EPFL, Lausanne) and S. Leroueil(University Laval) for introducing him to non-saturated soilmechanics.

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