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. Mineralogical influence on ring shear strength of landslide materials from Lesser Himalaya and Siwalik zones in Central Nepal Influence mineralogique sur la force de cisaillement par anneau de glissement de terrain à partir de matériaux des zones du Lesser Himalaya et Siwalik du centre du Népal

N. P. Bhandary Graduate School of Science and Engineering, Ehime University, Japan R. Yatabe Graduate School of Science and Engineering, Ehime University, Japan

ABSTRACT Various studies in the past have revealed that the percentage of landslides and related failures occuring in low to middle class mountain ranges of Nepal in Central Himalaya is far greater than that occurring in any other parts of the country. Especially, the landslides and slope failures including debris flows that frequently damage road infrastructure in Central Nepal draw significant attention in terms of national concern over economic loss and public suffering. The most important national road network in Nepal connecting the capital area to rest of the business centers and dense settlement areas in southern plains frequently suffers from this problem. However, the efforts to study these landslides and their engineering properties to go for appropriate preventive techniques are insiginificant. With an aim to understand shear characteristics of clay material of landslides along this road network, this paper addresses land sliding mechanism in terms of material shear behavior and mineralogical influence. A total of 31 locations of landslides and failure sites were investigated for field verification and soil sampling. Due to field-related difficulties and limited resources, however, soil sampling was possible at about 15 locations only. The collected samples were then tested in ring shear apparatus for peak and residual shear strength parameters and in x-ray diffractometer for minerological composition to reveal the influence of chlorite and mica like weak minerals on the shear strength of landslide clays. RÉSUMÉ Plusieurs études dans le passé ont demontre révélé que le pourcentage des glissements de terrain et les fissures qui en resultent se produisant dans les chaines de montagnes de basse et moyenne altitude du centre de l'Himalaya au Nepal sont bien supérieurs à ceux qui se produisent dans d'autres parties du pays. En particulier, les glissements de terrain et la pente des fissures ainsi que les coulées de boues qui souvent endommagent des infrastructures routières dans le centre du Népal ont suscite une attention particuliere et s' est donc revele comme etant un probleme d'interet national au vue de pertes economiques et souffrances enregistrées. Le plus important réseau routier national du Népal reliant la région de la capitale au reste de centres d'affaires et des zones densement peuplées de plaines du sud souffre souvent de ce probleme. Cependant, les efforts d'etudier ces glissements de terrain ainsi que les mecanismes sous-jacents afin d' élaborer des techniques de prévention appropriées sont insiginificants. Dans le but de comprendre les caractéristiques de cisaillement de l'argile de glissements de terrain le long de ce réseau routier, le present étude traite le mechanismes de glissement de terrain en terme du comportement du material de cisaillement et de l influence mineralogique. Un total de 31 lieux de glissements de terrain et fissures ont été étudiés pour vérification de terrain et echantillonage du sol. En raison de difficultés recontrées sur le terrain et des ressources limitées, l'échantillonnage des sols a été possible qu à environ 15 endroits seulement. Apres prelevements, le pic et la force residuelle de cisaillement ont ensuite été analysés par un appareil de cisaillement par anneau et la composition mineralogique par un diffractometre a rayon-x pour demontrer l'influence des chlorites et faible mineraux mica-like dans la force de cisaillement de l'argile de glissement de terrain. Keywords : landslides, ring shear test, mineralogy, Nepal

1 INTRODUCTION government as well as concerned people and organization to study these landslides and their engineering properties so as to Situated almost in middle of the Himalayan Arc, the mountains go for appropriate preventive techniques. in Nepal (Figure 1) consist of a low class erosion-prone hills of immature soft rocks on the south to high altitude steep rocky mountains on the north. Geological investigations and field verifications on various occasions have revealed that the percentage of landslides and similar massive slope failures occurring in low to middle class mountain ranges including the valleys between these mountains is far greater than that occurring in any other parts of the country (shaded zone in Figure 1). Roadside landslides in the past have extended a massive economic loss to the nation mainly in the form of days- to weeks-long transport blockade. Economically most important national road network connecting the capital area of to the rest of business centers and densely populated areas in southern plains (refer Figure 1) can be regarded as the most landslide-hit roadway in Nepal. Despite almost every year’s of Figure 1. Map of Nepal indicating the landslide-prone road network suffering from landslide-related damage in this road network, (Kathmandu--Narayanghat). however, there are insignificant efforts in the part of

Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering 237 M. Hamza et al. (Eds.) © 2009 IOS Press. doi:10.3233/978-1-60750-031-5-237 238 N.P. Bhandary and R. Yatabe / Landslide Materials from Lesser Himalaya and Siwalik Zones in Central Nepal Present trend of disaster prioritization in Nepal indicates that A study conducted by Brabb, as mentioned in Bhattarai et al. landslide events along the arterial roads receive a significant (2002), revealed that the landslide density in Nepal ranges from attention mainly due to heavy economic losses and large 0.2 per linear kilometer on stable lands to 2.8 per kilometer on number of people affected. Previous studies (such as very susceptible areas exposed fully to human intervention. LRA/DoLIDAR/Scott Wilson 2002) also indicate that there are Likewise, Yagi and Nakamura (1995) upon analyzing landslides many landslides and slope failures along the aforementioned in an 85x75km area peripheral to mainly on national roads, namely Prithvi Highway (Kathmandu-Pokhara, the west in the Lesser Himalayan Zone revealed the effect of Figure 1) and (Kathmandu-Hetauda, Figure relief and rock types on the process of landslide occurrence. 1) as most parts of these highways pass over geologically They calculated the mean landslide area ratio to be 6.61% of the unstable slopes of the Lesser Himalaya. A typical landslide site total area mapped with a greater values over phyllitic and in Prithvi Highway, named Krishnabhir Landslide that remained faulted quartzite zones and smaller values over granitic and active for about eight consecutive years is shown in Figure 2. quartzite zones. Similarly, a landslide inventory study in the Lesser Himalayas conducted in a period between 1985 and 1990 by the Department of Mines and Geology of Nepal indicates that slopes ranging from 21 to 30 degrees and 31 to 40 degrees consist of greater number of landslides (Karmacharya et al. 1995). This study also revealed that there is domination of translational type of slides in central Nepal and that the highest number of landslide incidences has been found in geological formations consisting of schist and gneiss. Likewise, Higaki et al. (2000) have studied the landslide distribution in Bhotekoshi area, and Dhital (2000) has discussed an overview of landslide hazard mapping practices in Nepal. Moreover, many other reports are available on different case studies of landslide hazard mapping in various mountain locations of Nepal. Little or no work is found, however, in the field of roadside landslides and their geotechnical aspect that have frequently damaged national infrastructures for the last several years and are a great challenge to Nepalese landslide professionals. Yatabe et al. (2005) have prepared a landslide map of Prithvi Highway (Kathmandu-Pokhara), Tribhuvan Highway (Kathmandu- Hetauda), and Narayanghat-Mugling Highway, as indicated Krishnabhir Landslide (2003) partly in Figure 3, and they conclude that there is dense distribution of large-scale landslides in Malekhu-Mugling Figure 2. A typical roadside landslide in Kathmandu-Mugling section of section of Prithvi Highway corridor and Mugling-Jugedibazar Prithvi Highway (Krishnabhir Landslide, 1997-2005). section of Narayanghat-Mugling Highway corridor.

Figure 3. Road corridor landslide distribution in the study area (Yatabe et al. 2005) and the sampling locations.

As such, up until now, most landslide studies in Nepal have the rectangle in Figure 1, which consists of about 75 km of the been focused more on geological and geomorphological above road network. The geology of this area consists mainly of features and distribution pattern. This paper therefore aims at phyllite, slate, and quartzite as the dominant rock types and evaluating ring shear strength of roadside landslide materials closely passing major tectonic thrusts, namely Main Central and the influence of constituent minerals in an area indicated by Thrust (MCT) and Main Boundary Thrust (MBT) along with N.P. Bhandary and R. Yatabe / Landslide Materials from Lesser Himalaya and Siwalik Zones in Central Nepal 239 numerous local faults (Stöcklin & Bhattarai 1982). Repeated fraction. The angles of peak internal friction when plotted landslide problems in this road network resulting in massive against the clay fractions of the samples show a trend of economic loss and human suffering urgently demand an decreasing friction angles with increased clay fraction (Figure integrated scientific approach to study landslide mechanism and 7). This trend is similar to the results presented by various their mitigation in more engineering way. previous researchers (such as Skempton, 1964; Borowicka, 1965; Binnie et al., 1967). However, the data is widely scattered, which perhaps is due to dissimilar composition of 2 MATERIAL AND METHOD constituent minerals. For example, one sample with high quartz content (sample no. 7-1) is seen to have high angles of internal Altogether 16 clayey soil samples were collected from 10 friction despite having more than 20% clay fraction. landslide sites in the study area, indicated by location number 5 to 16 in Figure 3. Due to limitations of sampling technique, however, the samples consisted of both exposed clayey soils of the landslide mass and exposed slip layer soils. All the collected samples were tested for physical properties, drained shear strength in ring shear machine (Figure 4(a)), and mineralogical composition by X-ray diffractometer. All samples tested in the ring shear machine were sieved through 425μm mesh, and shearing was conducted over remolded, saturated, normally consolidated annular specimens of 8 cm inner dia, 12 cm outer dia, and 10-12 mm thickness, as shown in Figure 4(b). The shear rate was set at 0.15 mm/min through the shear plane so as to let any developed pore-water pressures dissipate completely. Although the amount of displacement for obtaining residual state of shear differed from sample to sample, in an average all samples were sheared for about 20 cm of displacement. In X- ray diffraction (XRD) tests, on the other hand, all samples were examined by powder method (for overall constituent minerals) and oriented aggregate method (for constituent clay minerals). Figure 4 (a). Structural features of direct shear type ring shear machine employed in the laboratory tests.

3 RESULTS AND DISCUSSION

The results of laboratory tests including physical parameters, strength parameters, and main constituent minerals are summarized in Table 1, and the typical graphical interpretations of ring shear test are presented in Figures 5(a) and 5(b). The ring shear test results, as presented in Figure 6 in terms of variation of internal friction angles with plasticity index indicate that the peak angles of drained internal friction (φd) vary from 20 to 35 degrees and the residual angles of internal friction (φr) vary from 18 to 33 degrees. From this figure, it is also clearly Figure 4 (b). Dimensions of ring shear test specimen. understood that the average drop from φd to φr is 2 degrees, which is comparatively small for soils with more than 20% clay

Table 1. Summarized results of physical tests, ring shear tests, and X-ray diffraction tests. Sample Solid density Liquid Plastic Plasticity 㱢 㱢 Grain size distribution (%) Peak intensity of main constituent minerals d r Dominant rock type No. (g/cm3) Limit (%) Limit (%) Index, I p (degree) (degree) < 2μm2~75μm 75~425μm Sm Chlorite Micas Quartz Feldspar 5(2004.11) 2.74 34.10 20.69 13.41 22.04 21.00 21.00 59.70 19.30 - 300 3600 800 - Chloritic phyllite, tuffaceous 6(2004.11) 2.78 - - - 25.77 25.32 18.00 70.50 11.50 - 250 2300 800 - (2004.11) 2.72 31.70 18.26 13.44 24.00 23.06 12.00 80.20 7.80 - 950 1800 900 - 7 Slates, phyllites 7-1(2004.11) 2.68 29.42 15.61 13.81 35.69 33.87 21.50 58.20 20.30 - 150 220 1500 - 7-2(2004.11) 2.81 34.50 14.76 19.74 26.05 25.32 6.50 76.60 16.90 - 450 1320 650 - 8(2004.11) 2.83 27.90 14.14 13.76 22.39 20.96 24.00 69.40 6.60 - 2600 3350 200 - Phyllite, phyllitic quartzite, 2.73 29.60 16.81 12.79 26.88 26.63 27.00 62.70 10.30 - 500 1100 700 - 10(2004.11) conglomerate 11(2004.11) 2.73 24.57 14.55 10.02 31.15 30.34 8.50 39.10 52.40 - 200 750 800 - 12(2004.11) 2.76 34.67 20.41 14.26 22.51 20.39 16.00 76.40 7.60 - 4045 1115 260 - Phyllites, quartzite, dolomite 14(2004.11) 2.79 30.50 13.21 17.29 20.07 18.74 20.50 68.00 11.50 1620 870 720 130 Dolomite, dense stromatolitic 15-1(2004.11) 2.77 28.50 15.36 13.14 24.07 23.51 27.00 65.80 7.20 - 1000 1160 485 - 15-2(2004.11) 2.76 33.25 14.33 18.92 26.44 23.51 21.80 67.60 10.60 - 1600 950 500 - 2.79 38.30 18.67 19.63 23.63 22.58 21.00 67.20 11.80 䂾 555 340 370 590 15-I(2005.11) Slates, phyllites 15-II(2005.11) 2.80 31.00 16.50 14.50 26.67 24.42 24.80 66.50 8.70 - 610 430 495 960 15-III(2005.11) 2.76 32.15 19.62 12.53 24.96 23.83 16.00 69.70 14.30 䂾 785 465 425 490 16(2004.11) 2.86 25.50 12.72 12.78 28.91 27.91 13.00 75.90 11.10 - 1415 - - 1135

The XRD test results, on the other hand, show that the main from the XRD charts are considered to inversely affect the constituent minerals of the tested soil samples are chlorite, frictional resistance of a sample. A quick and rough estimation micas, and quartz (minerals with minor presence are excluded). of relative amount of constituent minerals can be made by Since the presence of chlorites and micas in the tested samples comparing the peak intensities, especially when the degree of is notably high, it was considered that there is significant mineral decomposition is found negligible. Figure 8 shows influence of these two minerals on the shear behavior of the influence of the ratio of sum of peak intensities of chlorites and collected soil samples. Assuming influence of other factors to micas to that of quartz and feldspar on the internal friction be little, the peak intensities of chlorites and micas as obtained angle. The trend is that the increased ratio results in reduced 240 N.P. Bhandary and R. Yatabe / Landslide Materials from Lesser Himalaya and Siwalik Zones in Central Nepal frictional resistance. This is because of lesser frictional 4 CONCLUDING REMARKS resistance of chlorites and micas against any applied stress, especially in presence of water. This paper mainly addresses features of roadside landslides in Central Nepal from shear strength and mineralogical points of view. A total of 16 soils samples collected from 10 landslide

200 ı'䋽98.1 kN/m2 locations along the arterial road corridor in Lesser Himalaya Sample # 7 '䋽196.2 kN/m2 and Siwalik zones of Central Nepal were investigated for ring

) ı 2 150 ı'䋽294.3 kN/m2 shear strength and constituent minerals. The ring shear and N/m

䌫 mineralogical tests revealed that the landslide soils in the target ( IJ 100 area have comparatively high angles of internal friction and possess high composition of chlorites and micas as main 50 constituent mineral, especially in phyllitic zones. An analysis of

Shear stress stress Shear influence of chlorite and mica content on the frictional 0 resistance of collected samples indicated that the soil strength 0 50 100 150 200 remarkably decreases with the increase in chlorite and mica Displacement (mm) composition. Particularly, the sheet-like mineral structure of Figure 5 (a). Typical graphical interpretation of ring shear tests 2 (displacement vs. shear stress). micas with specific surface in the range of 65 to 100 m /g gives them greater water absorption capacity, and illite (i.e. hydrous 200 Sample # 7 micas) particles tend to disaggregate considerably in water ) 2 (Grim 1962) causing reduced particle-to-particle attraction 150 Peak strength leading to lesser frictional resistance. Residual strength ( kN/m ( IJ 100 ACKNOWLEDGEMENT 50 㱢㪻㩷㪔㩷㪉㪋㪅㪇㰛 㱢㫉㩷㪔㩷㪉㪊㪅㪇㰛 Shear stress stress Shear 0 This study is a part of the research project entitled ‘A study on 0 50 100 150 200 250 300 the mechanism of large-scale slope soil disasters in Hindukush Effective normal stress ı' (kN/m2) Himalayan Watersheds and their preventive measures’ funded Figure 5 (b). Typical graphical interpretation of ring shear tests by the Government of Japan under ‘Grant-in-Aid for Overseas (effective normal stress vs. shear stress). Scientific Research and Investigation (2003-2006)’ program. 㪋㪇

High quartz

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