The rock slopes of the Thopal-Malekhu River areas, Lesser Himalaya, were characterized applying various systems of rock mass classification, such as Rock mass Rating (RMR) and Geological Strength Index (GSI), because the study area comprises well exposed rock formations of the Nawakot and Kathmandu Complexes, across the Thopal-Malekhu River areas. In RMR system, mainly five parameters viz. Uniaxial Compressive Strength (UCS) of rock, Rock Quality Designation (RQD), spacing of discontinuity, condition of discontinuity, and groundwater condition were considered. The new GSI charts, which were suitable for schistose and much disintegrated rock masses, were used to characterize rock slopes based on quantitative analysis of the rock mass structure and surface condition of discontinuities. RMR ranged from 36 to 82 (poor to very good rock mass) and GSI from 13.5±3 to 58±3 (poor to good rock mass). Slates (of the Benighat Slate) are poor rock masses with low strength, very poor RQD, and close to very close spacing of discontinuity, and dolomites (Dhading Dolomite) are fair rocks with disintegrated, poorly interlocked, and heavily broken rock masses yielding very low RMR and GSI values. Phyllites (Dandagaun Phyllite), schist (Robang Formation) and quartzite (Fagfog Quartzite, Robang Formation and Chisapani Quartzite), dolomite (Malekhu Limestone), and metasandstone (Tistung Formation) are fair rock masses with moderate GSI and RMR values, whereas quartzose schist and gneiss (Kulekhani Formation) are very good rock masses having comparatively higher RMR and GSI. The relationship between GSI and RMR shows positive and good degree of correlation. DOI: http://dx.doi.org/10.3126/bdg.v16i0.8882   Bulletin of the Department of Geology Vol. 16, 2013, pp. 29-42

A huge landslide was identified on the right bank of the Malekhu River at about 1 km upstream from the Malekhu Bridge of the Prithvi Highway, and was named Malekhu Landslide. The landslide area consists of quartzite with sericite partings, chlorite schist and amphibolite of the Robang Formation of the Kathmandu Complex. The landslide extends for about 200 m along the river bank slope and its crown and toe have elevation respectively of 429 m and 361 m a.s.l. The major portion of the hillslope has been altered by landsliding as evidenced from the different geometry of slopes consisting of very steep scarp slope, steep to moderate slope with toppled and wedge failure debris, and several wedges and gullies. The right-side up layers have an average dip direction of 167° and a plunge of 56-90°, whereas the overturned layers have an average dip direction and plunge of 316°/32°. The direction towards which the toppling had occurred is 167°. The overturned layers had rotated to 36°, during which the layer dip direction had rotated counterclockwise to an amount of 31° from the major direction of toppling, and this rotation should have occurred during sliding of the toppled block. The Malekhu Landslide is a complex landslide experiencing more than one mode of failure of which the most prominent one is the toppling. It is a kind of flexural toppling of passive mode and had already occurred and now is suspended. The second mode of failure is the wedge sliding. The kinematic analysis of discontinuities on the slope for evaluating possibility of landsliding has indicated that the wedge failure is potential due to the presence of a line of intersection produced by the discontinuity parallel to the foliation of the right-side up layers and the overturned layers. This suggests that the toppled blocks may further slide. It is required to protect the slope from landsliding as there have been extended two unpaved roads one along the crown and the other along the toe of the landslide, and due to existence of the Malekhu Bridge in the downstream stretch of the river. DOI: http://dx.doi.org/10.3126/bdg.v16i0.8881 Bulletin of the Department of Geology Vol. 16, 2013, pp. 21-28

The Malekhu River is one of the major tributaries of the Trishuli Ganga River fl owing from the south in Malekhu region, central Nepal. Riverbank slope stability is a topic of concern as rock mass condition and slope stability of riverbank slopes are important parameters for riverbank erodibility. Fourteen sites in the Malekhu River were selected for rock mass rating (RMR) and then slope mass rating (SMR) by using a graphic method. The potentially vulnerable sites were identifi ed after conducting field study in different slopes. The results indicate that there occur modes of failures ranging from stable (good rock mass) to partially stable (normal rock mass) in all the study sites. The unstable (bad rock mass) and completely unstable (very bad rock mass) slopes are, however, distributed only in some slopes. The unstable slope of plane failure mode is Ka1, whereas the completely unstable slopes of plane failure mode are Rb2, Ml1 Slope 1 and Ml2. The unstable slope of toppling failure mode is Ml2. When wedge failure mode is considered, the slopes at Ti1 and Ka1 are unstable while the slopes at Kh1, Ka1, Ml1 Slope 1 are completely unstable. The rock slopes with unstable to completely unstable states are considered bad (SMR Class IV: 21–40) to very bad (SMR Class V: 0–20) rock mass with fair to poor rock mass rating, respectively. These bad to very bad rock mass slopes are vulnerable to slope movements and river erosion, and they require mitigative measures.