Geologists conduct investigation in landslide areas

MOKOKCHUNG — A four-member team of geologists from the Directorate of Geology and Mining (DGM) began a geological and geotechnical investigation at two landslides/ subsidence areas along NH-702 Mokokchung-Mariani road on September 12.

The team led by geologist David Lhoupenyi along with assistant geologists namely Vilzoto Valeo, Neveyo Vero and Sune M investigated the landslide areas below Assam Rifles camp at Mongsenbai ward near Sewak Gate (about 3.5 acres in area) and below Chungtia village (about 8.5 acres in area).

The objective of the investigation team was to delineate suitable remedial measures for the two landslides/ subsidence and subsequently table recommendations.

Team leader David Lhoupenyi stated that landslides/subsidence areas at Mongsenbai ward were a unique scenario and suggested that they needed to come out with a different type of preventive measure to mitigate the affected areas than the usual normal measures.

“We anticipated water which is the pre-dominant matter in most of the landslide/ subsidence areas in Nagaland but after drilling the sub-surface in both the boreholes turned out to be dry,” he said.

According to the geologist, water-surface or groundwater plays a secondary role in the slide. He added that the structurally disturbed area and landslide failures are taking place along the rock bedding plane.

According to Lhoupenyi, the geological conditions of the affected area consisted of Disang/Barail rocks made up of shale and sandstone that were moderate to highly weathered.

“More than two faults forming wedges in the area are inferred. Soil is mostly sandy clay. Also, freshly dug earth materials were transported from other places and were dumped there. Otherwise, the natural soil cover ranges about 1 foot to 4/5 feet thick,” he added.

According to Lhoupenyi, the Chungtia landslide is predominantly controlled by sub-surface water and both landslides are different.

“Timeline will depend on how quickly we prepare the report by October, 2023 but the availability of funds to execute the work depends on PWD (NH). A cost estimate will be given by PWD (NH) based on our recommendation; however the methodology should be cost-effective so that it can be applied in other parts of the state,” he said when queried about the estimated timeline and cost for addressing the problem.

Apart from borehole drilling for collecting precise sub-surface data, the team would be also collecting data on lithological and structural mapping collecting both surface and subsurface data, mapping of landslide morphology, topographical mapping using Trimble catalyst DA2 GNSS and total station drilling -Down-the-hole drilling (DTH) rig get real solutions to the problem. The team would be here till September 15 to continue their investigation.

This might be the third time drilling machinery has been used to probe landslides subsidence areas in Nagaland for collecting data.

According to executive engineer PWD (NH) division II, Mokokchung, Limatoshi, the chief engineer of PWD (NH) and superintendent engineer, Kohima, toured Mokokchung-Zunheboto road (NH 702 A) and Mokokchung-Tuli road (NH 02) erstwhile NH 61 and Mokokchung-Mariani road (NH 702 D) at the end of August 2023 after taking charge.

After the inspection, the chief engineer PWD (NH) directed the executive engineer, PWD (NH) division II to request a geologist for the investigation of landslides/subsidence areas.

The executive engineer said that along the 63 KM stretch (from Mokokchung- Tzurang Bridge) on NH 702D, six landslides/ subsidence were identified but selected only two critical locations, which they are currently investigating. 

The investigation was executed as per the request from executive engineer, PWD (NH) Division II, Mokokchung.

Also read: NH 702D: New Camp to Longtho not traffic-worthy, STMC proposal submitted