Effect of rock weathering, clay mineralogy, and geological structures in the formation of large landslide, a case study from Dumre Besei landslide, Lesser Himalaya Nepal

The Dumre Besi landslide is one of the largest and most problematic failures on the Mugling-Narayanghat Highway in central Nepal. Though it was triggered by the monsoon rain of 2003, geological structures and rock weathering have played a key role in its initiation and further aggravation. The slide is also controlled to some extent by the groundwater and rugged topography with high slope angles. The landslide zone comprises thinly laminated light grey siltstone with numerous crosscutting quartz veins, grey metasandstone (quartzite), bluish grey to white phyllite, black carbonaceous slate, and dolomite. A thrust fault passes through the centre of the landslide, creating a thick deposit of loose, weathered rock material, and the fault has developed a very thick shattered zone where weathering is very intense. Using field and laboratory analyses, the rocks in the landslide zone can be divided into five zones based on the severity of weathering: none, slight, moderate, severe, and complete. Laboratory analyses showed that the chemically weathered rocks are significantly rich in smectite and vermiculite. Out of these, smectite is the most critical one, as it swells when wet. The formation mechanism of the clay minerals was analysed by various techniques, including X-ray diffraction, X-ray fluorescence, and thin-section analysis, and it was found that most of them were derived from weathering of rock. The clay minerals significantly reduced the rock strength and facilitated the extensive failure of Dumre Besi. The wide fault zone with deeply weathered and clay-rich debris is also responsible for the formation of debris flows in the monsoon season.ArticleLANDSLIDES. 10(1):1-13 (2013)journal articl

Lithostratigraphy and structures of the Siwaliks rocks in the southern part of Dang and its surrounding area, Southwestern Nepal

Late Cainozoic fluvial sedimentary sequences of the Siwalik Group is extensively accumulated in the southern frontal part of the Himalaya and well exposed along southern part of Dang and in all sides of the Deukahari Valley of west-central Nepal Himalaya.Lithostratigraphy of the succession consists of the Bankas Formation, Chor Khola Formation, Suria Khola Formation, Dobata Formation,and the Dhan Khola Formation in an ascending order.The Bankas Formation is represented by an interbedding of red-purple mudstones, shales, and fine-to very fine-grained sandstones. The Chor Khola Formation shows a gradual increase of sandstone grain size as well as thickness of beds. The sandstones are calcareous and rich in plant fossils. The mudstones are variegated in the lower part and gray-green in the upper part. The Surai Khola Formation is mainly represented by multi-storied, coarse-to very coarse-grained ‘salt and pepper’sandstones.The beds yield a great amount of petrified wood in the form of steams, branches, and roots. The Dobata Formation is predominated by mudstones with a minor amount of sandstones and conglomerates.The Dhan Khola Formation comprises compact and hard boulder-and pebble-bearing conglomerates with yellow mudstones in the lower part and not well cemented conglomerates with yellow mudstones in the upper part.The sandstones of the study area mostly belong to the litharenite and subordinately to the sub-litharenite.The rocks show coarsening-upward succession in general but many fining-upward fluvial successions on a scale from several to tens of meters is established in each lithological unit. The Siwalik rocks show a coarsening-upwards sequence, evidencing a continuous uplift of the Himalayas. However, the individual depositional units show a detailed lithostratigraphic study and geological mapping of the area show that the rocks of the present study area are lithostratigraphically similar to those of the Surai Khola area located in the east of the study area. The rocks distributed in the study area are highly deformed. The persistence of shortening between Indian Plate and Asian Plate involves their deformation. This deformation is expressed in the form of faults and folds which succeed one another in both space and time.The study area is subjected to more than one phase of deformational process.Series of faults such as the Bheri Thrust, Mali Khola Thrust, Babai Back Thrust, Tui Khola Back Thrust, and Rapti Back Thrust and also a number of folds as Baijapur Anticline, Bhaisahi Anticline, Khairi Khola Anticline, and Malai Khola Anticline, Agaiya Syncline, Satbariya Syncline, and Masot Khola Syncline delineate the study area. The pattern of thrusting here is related to the thin-skin tectonic model. The thrusts show the ramp-flat geometry and the thrusting is related to the piggy-back propagation pattern. Due to this thrusting the rocks here are repeated more than five times.The Dang Valley is located in a triangle zone bounded by the thrusts, which seems to be generated by retardation of faulting, thrusting and folding.ArticleJournal of the Faculty of Science Shinshu University 43: 1-42(2011)departmental bulletin pape

Continental weathering in the Early Triassic in Himalayan Tethys, central Nepal: Implications for abrupt environmental change on the northern margin of Gondwanaland

The geochemistry of Triassic mudstones in the Himalayan Tethys sequence, central Nepal, was studied with respect to changes in sedimentary facies, grain size, and source rocks. The Triassic sedimentary facies of mudstone and carbonates show deposition in offshore to hemiplegic environments. The rare earth element (REE) pattern of the Permian and Triassic mudstones suggests uniformity correlatable to average shale. The major element geochemistry of the Early Triassic Griesbachian-early Smithian mudstones indicates a sediment supply from strongly weathered sources with the chemical index of alteration (CIA) values of 76–81. However, the mudstones in the late Smithian show weakly weathered sources with CIA values of 68–74. The lower part of the Middle Triassic Anisian mudstones return to Early Triassic paleoweathering levels. There are no significant relationships among lithofacies, the grain size of the sediments, and CIA values. Thus, the abrupt change of the degree of paleoweathering in the Early Triassic, late Smithian time, suggests a dramatic decrease in continental weathering, which is related to a predominantly arid climate in the northern marginal area of Gondwana.ArticleJOURNAL OF ASIAN EARTH SCIENCES.79, Part A:288-301(2014)journal articl

Landslide susceptibility mapping using certainty factor, index of entropy and logistic regression models in GIS and their comparison at Mugling-Narayanghat road section in Nepal Himalaya

Landslide susceptibility maps are vital for disaster management and for planning development activities in the mountainous country like Nepal. In the present study, landslide susceptibility assessment of Mugling-Narayanghat road and its surrounding area is made using bivariate (certainty factor and index of entropy) and multivariate (logistic regression) models. At first, a landslide inventory map was prepared using earlier reports and aerial photographs as well as by carrying out field survey. As a result, 321 landslides were mapped and out of which 241 (75 %) were randomly selected for building landslide susceptibility models, while the remaining 80 (25 %) were used for validating the models. The effectiveness of landslide susceptibility assessment using GIS and statistics is based on appropriate selection of the factors which play a dominant role in slope stability. In this case study, the following landslide conditioning factors were evaluated: slope gradient; slope aspect; altitude; plan curvature; lithology; land use; distance from faults, rivers and roads; topographic wetness index; stream power index; and sediment transport index. These factors were prepared from topographic map, drainage map, road map, and the geological map. Finally, the validation of landslide susceptibility map was carried out using receiver operating characteristic (ROC) curves. The ROC plot estimation results showed that the susceptibility map using index of entropy model with AUC value of 0.9016 has highest prediction accuracy of 90.16 %. Similarly, the susceptibility maps produced using logistic regression model and certainty factor model showed 86.29 and 83.57 % of prediction accuracy, respectively. Furthermore, the ROC plot showed that the success rate of all the three models performed more than 80 % accuracy (i.e. 89.15 % for IOE model, 89.10 % for LR model and 87.21 % for CF model). Hence, it is concluded that all the models employed in this study showed reasonably good accuracy in predicting the landslide susceptibility of Mugling-Narayanghat road section. These landslide susceptibility maps can be used for preliminary land use planning and hazard mitigation purpose.ArticleNATURAL HAZARDS. 65(1):135-165 (2013)journal articl

Landslide susceptibility mapping using certainty factor, index of entropy and logistic regression models in GIS and their comparison at Mugling-Narayanghat road section in Nepal Himalaya.

Landslide susceptibility maps are vital for disaster management and for planning development activities in the mountainous country like Nepal. In the present study, landslide susceptibility assessment of Mugling–Narayanghat road and its surrounding area is made using bivariate (certainty factor and index of entropy) and multivariate (logistic regression) models. At first, a landslide inventory map was prepared using earlier reports and aerial photographs as well as by carrying out field survey. As a result, 321 landslides were mapped and out of which 241 (75 %) were randomly selected for building landslide susceptibility models, while the remaining 80 (25 %) were used for validating the models. The effectiveness of landslide susceptibility assessment using GIS and statistics is based on appropriate selection of the factors which play a dominant role in slope stability. In this case study, the following landslide conditioning factors were evaluated: slope gradient; slope aspect; altitude; plan curvature; lithology; land use; distance from faults, rivers and roads; topographic wetness index; stream power index; and sediment transport index. These factors were prepared from topographic map, drainage map, road map, and the geological map. Finally, the validation of landslide susceptibility map was carried out using receiver operating characteristic (ROC) curves. The ROC plot estimation results showed that the susceptibility map using index of entropy model with AUC value of 0.9016 has highest prediction accuracy of 90.16 %. Similarly, the susceptibility maps produced using logistic regression model and certainty factor model showed 86.29 and 83.57 % of prediction accuracy, respectively. Furthermore, the ROC plot showed that the success rate of all the three models performed more than 80 % accuracy (i.e. 89.15 % for IOE model, 89.10 % for LR model and 87.21 % for CF model). Hence, it is concluded that all the models employed in this study showed reasonably good accuracy in predicting the landslide susceptibility of Mugling–Narayanghat road section. These landslide susceptibility maps can be used for preliminary land use planning and hazard mitigation purpose

Weathering and mineralogical variation in gneissic rocks and their effect in Sangrumba Landslide, East Nepal

Sangrumba landslide is one of the largest and the most active landslides in Nepal Himalaya. Geologically the landslide belongs to the Higher Himalaya and consists of Pre-Cambrian biotite–garnet and sillimanite gneiss with some quartzite. The present paper aims at describing various degrees of rock weathering and their effect in Sangrumba landslide. Field study followed by mineralogical, geochemical and geotechnical analyses of the collected rock and soil samples from the landslide zone were used in characterizing weathering degree. The gneisses are intensely weathered while quartzite is unweathered. Petrographical and X-ray diffraction analyses showed that the rocks in the landslide zone had undergone weathering process with the formation of different types of clay minerals as kaolinite, vermiculite, smectite and chlorite. This was further confirmed by the Scan Electron Microscope and Energy Dispersive X-ray analyses. These clay minerals drastically reduced the rock strength facilitating the extensive failure of the Sangrumba landslide. The major and trace element composition of the rock and soil samples was calculated from the XRF analyses. The geochemical analyses and weathering indexes of rocks showed that they are significantly weathered and had a major influence in the formation of the Sangrumba landslide. In addition, mechanical strength measurement of rock/soil showed that the strength drastically decreases as the weathering intensity increases. Rainfall followed by the rock type are the most dominant parameters influencing the weathering process which leads to the formation of large landslide as the present one. These findings can be used in other areas with similar geological and topographical conditions

Sedimentology of a Mid-Late Ordovician carbonate mud-mound complex from the Katmandu nappe in central Nepal

peer reviewedThis sedimentological study of the Godavari quarry is the first relating to the Palaeozoic Tethyan sedimentary rocks of the Katmandu nappe (Central Nepal). Sedimentological analyses led to the identification of six microfacies belonging to a large carbonate mud-mound complex, which can be divided into mound, flank and off-mound main depositional settings. Identification of two dasycladaceans (Dasyporella cf. silurica (Stolley, 1893) and Vermiporella sp.) in the mound facies gives a Mid-Late Ordovician age to this newly discovered Godavari carbonate mud-mound, which makes this mound one of the oldest ever described in the Asian continent. The mound microfacies are characterized by a high micritic content, the presence of stromatactis and the prevalence of red coloured sediments (the red pigmentation probably being related to organic precipitation of iron). The flank microfacies are characterized by a higher crinoid and argillaceous content and the presence of bio- and lithoclasts concentrated in argillaceous lenses. Finally, the off-mound microfacies show very few bioclasts and a high argillaceous content. Palaeoenvionmental interpretation of microfacies, in terms of bathymetry, leads us to infer that the Godavari mud-mound started to grow in a deep environment setting below the photic and wave action zones and that it evolved to occupy a location below the fair weather wave base. Cementation of cavities within the mound facies underlines a typical transition from a marine to a burial diagenetic environment characterized by: (1) a radiaxial non luminescent feroan calcite cement (marine) showing a bright orange luminescent band in its middle part; (2) a bright zoned orange fringe of automorphic feroan calcite (meteoric phreatic); (3) a dull orange xenomorphic feroan calcite cement in the centre of cavities (burial) and (4) a saddle dolomite within the centre of larger cavities. The faunal assemblage (diversity and relative proportion) of the Godavari mound facies is dominated by crinoids and ostracods, which makes this carbonate mud-mound comparable to the Meiklejohn Peak mounds (Nevada)

Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake

On 25 April 2015, a large earthquake of Mw 7.8 occurred along the Main Himalayan Thrust fault in central Nepal. It was caused by a collision of the Indian Plate beneath the Eurasian Plate. The epicenter was near the Gorkha region, 80 km northwest of Kathmandu, and the rupture propagated toward east from the epicentral region passing through the sediment-filled Kathmandu Valley. This event resulted in over 8000 fatalities, mostly in Kathmandu and the adjacent districts. We succeeded in observing strong ground motions at our four observation sites (one rock site and three sedimentary sites) in the Kathmandu Valley during this devastating earthquake. While the observed peak ground acceleration values were smaller than the predicted ones that were derived from the use of a ground motion prediction equation, the observed peak ground velocity values were slightly larger than the predicted ones. The ground velocities observed at the rock site (KTP) showed a simple velocity pulse, resulting in monotonic-step displacements associated with the permanent tectonic offset. The vertical ground velocities observed at the sedimentary sites had the same pulse motions that were observed at the rock site. In contrast, the horizontal ground velocities as well as accelerations observed at three sedimentary sites showed long duration with conspicuous long-period oscillations, due to the valley response. The horizontal valley response was characterized by large amplification (about 10) and prolonged oscillations. However, the predominant period and envelope shape of their oscillations differed from site to site, indicating a complicated basin structure. Finally, on the basis of the velocity response spectra, we show that the horizontal long-period oscillations on the sedimentary sites had enough destructive power to damage high-rise buildings with natural periods of 3 to 5 s