The Department of Geology and Mining, Nagaland has embarked on a vital mission to unearth the reasons behind the land sinking plaguing NH-702D, the Mokokchung-Mariani road in Nagaland. In response to a request from the Executive Engineer, PWD (NH) Division II, Mokokchung, a team led by Geologist David Lhoupenyi initiated a comprehensive Geological and Geotechnical investigation, with the ultimate aim of proposing effective remedies for these persistent landslides and subsidence issues.
David Lhoupenyi, the Geologist leading the investigation, elucidated the methodology on Tuesday, 12 September 2023. He explained that their first step is to delve into the geological causes of the land subsidence. Subsequently, a report will be generated, followed by the identification of strategies to curtail these issues in the affected areas. Lhoupenyi emphasized that the traditional approach of using retaining walls alone would not suffice, highlighting the importance of applying geo-scientific methods and borehole drilling to gather precise subsurface data.
The other technical officers involved in the field work are Assistant Geologists Vilzoto Valeo, Neveyo Vero and Sune M.
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The investigation primarily focused on two locations earmarked by the PWD (NH) Mokokchung: the area below Assam Rifles camp at Mongsenbai ward near Sewak Gate and the site below Chungtia village.
While Nagaland typically grapples with water-induced subsidence, the team discovered that the sinking of the Sewak section was primarily due to structural disturbances along rock bedding planes, with water playing a secondary role.
Limatoshi, EE, PWD (NH) Division II Mokokchung (Top L), David Lhoupenyi, Geologist (Top R
Executive Engineer PWD (NH) Division II Mokokchung, Limatoshi, shared the concerning state of NH 702D, highlighting that the road from New Camp to Longtho is not in a traffic-worthy condition. However, he provided a glimmer of hope by mentioning the submission of a Short Term Maintenance Contract (STMC) proposal to the head office. This proposal, if approved and sanctioned, would address maintenance issues until the NH 702D two-laning Detail Project Report (DPR) is approved.
Limatoshi clarified that the STMC’s scope would be limited to granular sub-base work, excluding bitumen works. Additionally, the department is actively engaged in pothole repairs, jungle cutting, and landslide clearance along NH 702D.
In the context of the proposed NH 702D two-laning project, Limatoshi disclosed that the Ministry of Road Transport and Highway, Regional Office Guwahati, had entrusted Marc Technocrats Private Limited with Project Management Consultancy Services (PMCS). This includes feasibility studies, detailed project report (DPR) preparation, execution of work, and a maintenance period by the contractor.
Thsuvisie Phoji, DC Mokokchung, also conducted an inspection of the affected area, engaging with Executive Engineer PWD (NH) Division II Mokokchung, Geologist David Lhoupenyi, Limatoshi, and ward members to assess the road conditions.
In response to specific questions posed to David Lhoupenyi regarding the Sewak sinking, the geologist leading the Geological and Geotechnical probe provided detailed insights:
Reasons for Land Sinking
Lhoupenyi attributed the sinking of the road section to structural disturbances along rock bedding planes, with water playing a secondary role.
Geological Conditions
The geological conditions in the area consist of Disang/Barail rocks, primarily composed of shale and sandstone, which are moderately to highly weathered. The presence of faults forming wedges in the area was also noted, and the soil is predominantly sandy clay.
Historical Records
While historical records specific to this location were not available, local elders suggested that the issue dates back to the road’s construction around 1971.
Assessment Data
Lhoupenyi’s team utilized various data sources and instruments, including lithological and structural mapping, topographical mapping using Trimble Catalyst DA2 GNSS and Total Station, and drilling via Down-the-hole drilling (DTH) Rig. Geotechnical studies involved various soil tests in the laboratory at BPCCIPL, Kolkata.
Depth of Sinking
Subsidence ranged from 2-3 feet, with a long-term rate requiring further monitoring.
Underground Water Sources
Surprisingly, the sub-surface in boreholes was dry, indicating that failure occurred primarily along the bedding plane rather than due to sub-surface water.
Recommendations
Lhoupenyi noted the rarity of such land sinking in the region and suggested referencing standard methods suitable for addressing this type of failure.
Preventative Measures
He expressed readiness to propose solutions when funding becomes available.
Timeline and Cost
The timeline for addressing the issue is contingent on the prompt preparation of the report, expected by October 2023, while the cost estimate would be determined by PWD (NH) based on their recommendations.
Chungtia Comparison
Lhoupenyi briefly observed the Chungtia land subsidence, highlighting that it is primarily controlled by sub-surface water, distinguishing it from the Sewak area land sinking.
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Drilling History
While it was not the first time drilling machines were used for landslides or land sinking in Nagaland, such investigations have been rare. Previous instances occurred around 25 years ago in Keziekie, Nagabazaar, Kohima and five boreholes were drilled for Kohima Assam Rifle landslide studies in 2014, making this the third such endeavour.
As Nagaland grapples with these road-related challenges, the collaborative efforts of the Department of Geology and Mining and PWD (NH) Division II Mokokchung are working with an aim to pave the way for safer and more sustainable transportation infrastructure in the region.
Geological and Geotechnical investigation of the two sites along the NH-702D Mokokchung-Mariani road will end on 16 September 2023.
