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The Lanta Khola is a major landslide on the North Sikkim Highway in the Indian state of Sikkim. The abnormally low width-to-length
ratio and slope instability in spite of the gentle surface slope (24°) make this slide unique. Geological, geophysical, and
geotechnical studies reveal that a major Himalayan discontinuity daylights within the slide. At the contact, the schist is
weathered to fine sand and silt with lower shear strength and permeability. The overlying gneiss is less weathered and exposed
at the contact. Surface runoff enters the contact zone through crevices in the overlying gneiss, and debris material is extruded
laterally from within this zone rendering instability, with blocks collapsing and eventually rolling down the slope after
cloud bursts. Numerical modeling of the slide confirms this mechanism of instability. Diversion of runoff, plugging of crevices,
and construction of pipe piles and horizontal drains are suggested as remedial measures. 相似文献
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Rainfall thresholds for the initiation of landslide at Lanta Khola in north Sikkim,India 总被引：6，自引：3，他引：3
In the Indian Himalaya, a 15 km stretch of the North Sikkim Highway that is exceptionally susceptible to landsliding is characterized
by fine-grained, low permeability debris material. Lanta Khola is one of the major debris slides in this stretch and is active
every year during the monsoons. Although the relationship between rainfall and landsliding in the area is obvious, there is
no previous study of precipitation thresholds for landslide initiation. Review of available rainfall and landslide activity
data for the area between 1998 and 2006 suggests that sliding cannot be modeled by typical exponential relationships between
cumulative rainfall (E) and rainfall duration (D). An alternative rainfall threshold has been proposed that predicts sliding if normalized cumulative rainfall for more than
15 days exceeds 250 mm. It is suggested that when this cumulative rainfall threshold is exceeded, the debris zone in the affected
stretch becomes saturated and fails, causing landsliding. 相似文献
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Landslides are very common in high-altitude Himalayan terrains. Major roads in the Himalayas are frequently blocked due to
heavy landslides and remain closed for long periods of time. Permanent mitigatory solutions to these landslides are required
to keep the highways open. Lanta Khola, located 71.2 km north of Gangtok (capital of the Indian state of Sikkim), is one of
the oldest landslides on the North Sikkim Highway and is active since 1975. The rock types on either side of the landslide
are different (augen gneiss in the east and metapelitic schist in the west), and it is believed that the Main Central Thrust
passes through the slide zone. Since the slide is invariably activated in the aftermath of heavy rainfall, it is important
to identify the subsurface structures that channel water below the landslide surface in order to understand the triggers of
slide activity. This can only be accomplished by geophysical survey; however, an appropriate geophysical technique that can
be applied in such terrains must be identified. Very low-frequency (VLF) electromagnetic survey was performed over the Lanta
Khola landside in order to delineate subsurface structures. Although a very limited number of VLF transmitters are available
worldwide, it was possible to pick up VLF signals from a number of VLF stations even in this high-altitude mountainous terrain.
VLF measurements along five profiles perpendicular to the geological strike were recorded, and a high conducting zone was
delineated from the VLF observations. This conducting zone correlates with the low resistive zone identified from gradient
resistivity profiling. The anomalies confirm that there is a water-saturated zone (soggy zone) even in the subsurface of the
slide parallel to the geological gneiss–schist contact within the Lanta Khola slide. This indicates that the conductive feature
correlates with a weak water-saturated debris layer that lies along the slide and is parallel to the geological contact. Resistive
structures on either side of the landslide zone can thus be correlated with the stable ground. It is necessary to drain out
water from the soggy zone to minimize slide activity since this zone appears to penetrate into the body of the slide. 相似文献
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