Engineering Geology 31 (1991) 185-195 Elsevier ScienceP ublishers 8.V.. Amsterdam Geotechnicapl roblemsc ausedb y glaciolacustrinec laysi n the FrenchA lps A. Giraud", P. Antoineu,T .W.J. Van Aschba nd J.D. Nieuwenhuisb'" ^lnstitut de RecherchesI nterdisciplinairesd e Gëologiee t de Mécanique,8.P.53x, 38041 - Grenoble Cedex. France oDepartmento f Physical Geography, Utrecht State (Jniversity,P .O. Box 80115, 3508 TC Utrecht. Netherlands 'Delft Geotechnics, P.O. Box 69, 2600 AB Delft, Netherlands (Received February 14, 1990; accepteda fter revision November 14, 1990) ABSTRACT Giraud,A ., Antoine,P ., Van Asch, T.W.J.a nd NieuwenhuisJ,. D., 1991.G eotechnicapl roblemsc aused by glaciolacustrincel aysi n the FrenchA lps. Eng. Geol.,3 l: 185-195. After moret han 10y earso f observationasn d monitoringo f severalla ndslideisn glaciolacustrincel ays in the FrenchA lps, the authorsg ive an accounto f the physicala nd mechanicacl haracteristicosf these formations.T he physicala nd geomechanicaplr opertieso f the claysa re analysedin relationt o the main typeso f instabilityo bservedo n the naturals lopeso f the areas tudiedT. he low plasticityi ndexeso f these clayse xplaint he rapid transformationo f the claysi nto a liquid statel eadingt o innummerables urface flows.T he clayss howa stronga nisotropya t peaks trengthc onditionsth at disappearast residuasl trength (c':o; $':16 18'). Natural slopesb ecomeu nstableif the gradiente xceed8s -10'. Threet ypeso f movementc an be distinguisheda t threed ifferentd epths( 0-5m; 5-l0m and greatert han l0m). The displacemenrta tesv ary from I cm/yr for the deep-seatesdl idest o as much as I m/yr for the surficial slides.D isplacemenrta tesi n theses lidesa re limited by the viscousp roperties.o f the laminatedc lays. However,e xtremer ainfall rnay causec atastrophicfa ilure. In conclusion,s omer ecommendationasr e givenf rom an engineeringp oint ol view. INTRODUCTION Glaciolacustrine clays are commonly encounteredi n the valleys of the Alps. These Quaternary clays, which were deposited in glacially dammed lakes, are very different - in their structure, mineralogy and mechanical characteristics - from the North American clays (sensitive clays and varved clays) which were mainly deposited under marine conditions (Parsons, 19761'Tavenase t al., 1979). In the Alps bf the Dauphiné, the Trièves region about 40 km south of Grenoble is of particular interest (Fig. 1). The clays in this sector,w hich outcrop to a maximum altitude of 750 m, were deposited in a glacially dammed lake covering an area of about 300 km2, impounded by the Isère glacier during the Wùrm maximum episode. Becauseo f the very uneven relief of the substratum in the basin where the sedimenta- 0013-7952/91/$03.50 @) l99l ElsevierS cienceP ublishersB .V. All rightsr eserved 186 A. GIRAUD ET AL 2 , V -:[u, . !at llrêvls - lakeb asins a Monestier-du-Percsye ctor til'l.l torrentiadle posits Ponsonnass ector I Sinard sector Fig.l. Ertent of the glaciersa t the Wùrm stagem aximum and location of' the glacial barrier of Trièves (accordingt o Monjuvent, 1973). tion took place, the thicknesso f thesec lays can vary greatly over short distances, from zero to a maximum of 200m (Monjuvent, 1973;A ntoine et al., 1981). In the Trièves region, which was studied in particular detail, the fact that the stream network has cut deeply into the formations means that thesec lays now form the greater part of the valley slopes, and their geotechnicalc haracteristicsr esult in numerouss ignso f instability. IDENTIFICATIONA ND PHYSICALP ROPERTIEOSF THE CLAY MATERIAL The clays. sometimesr eferredt o as varved or laminated clays, have a silty clay texture and are characterisedb y a marked anisotropy, i.e. finely laminated partings; with light coloured (silty) beds alternating with dark coloured (clayey) beds. The thickness of these laminae is variable (l mm-10 cm), but they are generally hori- zoîIal. "varved" The term commonly refers to a strictly seasonals equenceo f deposits. According to observations of what is occurring at present, many layers could be deposited within a single year. For example, complete drainage of the Chambon GEOTECHNICALP ROBLEMSO F GLACIOLACUSTRINEC LAYS 187 reservoir in 1980 provided the opportunity to study the sediments deposited in the lake during the previous 45 years (Sikirdji, 1982). This study revealed facies which "varved" were similar to the Trièves clays, suggestingt hat the term should be used without any suggestion as to the (as yet unknown) rhythm of formation of lhe deposits. Mineralogy Material investigations with respect to geotechnical and physical characteristics were carried out in three different areas (Fig.l). These three areas differ, especially in their mineral content (Table I), which is related to the differencesi n lithology of the surrounding relief, varying from crystalline to sedimentary rocks. Among the non-clayey minerals calcite and quartz predominate, together with some feldspars. The predominant clay minerals are illite and chlorite, commonly accompanied by swelling, interstratified materials in small quantities, as confirmed by the relatively low values of the plasticity indices (see below). P hy sica l identif ca tion Grain size analyses of the Trièves clays show that particles smaller than 2p make up 40o of the light-coloured beds and 60o of the dark beds, so these are classified as clayey silts (Fig.2). The dry density is of the order of l5-l6kN/m3, which correspondst o a porosity of approximately 40o/o.S pecific gravity of the particles has a mean of about 2.6. The measured values of the Atterberg limits vary between 30' and 48o/of or the liquid limit, and betweenl 0o and 25o lor the plasticity index (Fig.3). A relativelyl ow plasticityi ndex meanst hat the liquid limit is quickly reachedn ear the surface, under the natural fluctuations of moisture content. These laminated claysa re therefores ubjectt o transition from the plastic state to the liquid state as the moisture content increasesw. hich explains the innumerable solifluction flows, TABLE I Mineralogicacl ompositiono f the laminatedT rièvesc lays Mineral Monestier-du-Percy Ponsonnas Sinard composition sector sector sector Quartz (%) 10-20 l4- 16 Calcite (%) 25-50 50-70 t5-20 Feldspar (%) 5- 10 l0-20 5-10 Illite (%) l 0 - 1 5 42-47 Chlorite (%) 6-10 t4-16 Kaolinite (%) 5-6 Montmorillonite (%) < t 1 0-5 Vermiculite (%) 20-30 1 8 8 A. GIRAUD ET AL. - 1 0 0 s, \ i 9 0 b B 0 5 N ë 7 0 I \" & o o 50 \ N 40 \ N{ 30 20 .l \ ':i:i:i:ii 1 0 \ 0 I 200 100 50 20 10 5 2 1 05 0.2 0.1 0.05 002 0.010 .005 00020.001mm diameter Fig.2. The grading range of laminated clays. % 40 .æ Æ A \Q ^ D X X O X x ^ P I a - a x OA ++ x I t t 0 0 2 0 40 50 7 0 % O clayeym oraineso f Sinard û laminatedc lavso f Ponsonnas + l a m i n a l e d c l a y s o f S i n a r d X laminatedc layso f Lavars ^ laminatedc layso f Moneslierd u Percy a gravellyg rey clay of Ponsonnas * r e w o r k e d c l a y s o f S i n a r d I blockc layo f Roissard Fig.3.P lasticityd iagramo f the Trièvesc layeyf ormations( after Vuillermet,1 989).P l:plasticity index; Wr:liquid limit. GEOTECHNICAL PROBLEMSO F GLACIOLACUSTRINEC LAYS 189 observed at the surface as well as the rapid transformation of the slide front into a mud flow. In addition, the textural anisotropy explains why the permeability may be esti- mated at about 10-10 mis perpendicular to the bedding planes, and on the order of 10-8 m/s parallel to these planes (Nieuwenhuis and Van Genuchten, 1986). The anisotropy of thesec lays is also reflectedb y the differencesi n seismicv elocities, which give a 12o difference in velocity perpendicular to (greatest) and parallel to the iamination. In general, velocities measured in the laboratory and the field range between 1000 and 2000 m/s. For disturbed varved clays, velocities never exceed 1200m /s; this is ascribed to their greater porosity. Resistivity values measured in undisturbed clays vary between 10 and 70 Ohm'm; whereas in disturbed material greater ranges are measured,v arying from 20-250 Ohm'm. These velocity values are more scattered owing to greater variations in the water content of the clays. GEOMECHANICALC HARACTERISTICS The anisotropy of the laminated clays is clearly demonstrated in all types of strength measurements.T able II shows a survey of the results for the different sites. Triaxiai consolidated drained (CD) tests show peak @'v alues varying from 23-26". Somewhat lesser @' values for triaxial CD tests were found where the slip planes develop along the larninae (20-21'). The anisotropic character is especiallye xpressed by the differencesi n cohesion: l-5kPa along the laminae, and 13-23 kPa normal to them. Direct shear tests along laminae show the same cohesion measured by TABLEI I Mechanical characteristicso f the laminatcd Trièves clays (averaged over 3 different areas) Paramctcr Value Test Remarks Peak @ values( -) 23-26 CD triaxial acrossl aminae 20-21 CD triaxial along laminae 22-23 CD direct shear along laminae 20 23 Backa nalysis Residual { values (") 18-19 CD directs hear 17-19 Backa nalysis Peak C values (kPa) 13-23 CD triaxial across laminae 1- 5 CD triaxial along laminae l- 5 CD direct shear along/across laminae 29-40 Back analysis Residual C values (kPa) 0 CD direct shear across-along laminae 6- 7 Backa nalysis Undrained cohesion (kPa) 46-68 UU triaxial across laminae 30-42 UU triaxial along laminae Dynamic viscosity coefficient (kPa s) 2.5x 108 CD direct shear continuous creep 1.4x 103 CD direct shear slip plane Overconsolidation ratio - OCR (-) 13-20 Oedometer Elastic modulus (MPa) l- 5 Pressiometer disturbed 10-60 Pressiometer undisturbed 190 A.