A geotechnical assessment report on the Wayanad twin-tunnel project site had warned of potential large-scale slope failure due to progressive weakening of the slope. The report submitted on June 14 (accessed by Onmanorama) explains in detail glaring lapses in slope management and remedial measures which need to be implemented to avoid failure.

The site inspection report is prepared monthly and submitted to the Konkan Railway Corporation Limited (KRCL), the Special Purpose Vehicle (SPV) for the project. A tripartite agreement has been signed between KRCL, PWD & KIIFB for executing the project.

Three migrant labourers died and five were missing after the slope bolstered by concrete came crashing down during heavy showers at the tunnel construction site at Kalladi in Wayanad on Tuesday. One of the experts, who has signed off on the report is a senior geologist with Dilip Buildcon Limited, the company entrusted with the tunnel work. The company officials have blamed extreme rainfall for the landslip and ruled out any structural lapses on their side.

The inspection was done to evaluate the prevailing stability condition and performance of the cut slopes at the North Portal of the Twin Tube road tunnel project under the ongoing southwest monsoon conditions. The report cites that continuous and intense rainfall has resulted in progressive saturation of the overburden mass, leading to the development of tension cracks, localised earth slumps, cavity formation and discharge of turbid seepage water.

The experts noticed fewer drain holes functioning effectively on the left-wall slope, indicating inadequate drainage performance. Based on field observations, the report cited that tension cracks may extend to considerable depths and act as preferential pathways for infiltration, seepage and internal erosion.

They backed this finding by noting down the noticeable sound of sub-surface water flow between two berms. "Under such conditions, rapid internal erosion and piping may develop within the overburden mass, resulting in progressive weakening of the slope and potentially triggering localised or large-scale slope failure," the report noted.

The team also flagged significant instability problems primarily from the ongoing heavy rainfall on the left-wall slope. Accordingly, it was suggested that the design and modelling of the slope may be reassessed.

Considering the continued development and widening of tension cracks, the expert observed that the left-wall slope exhibited a comparatively higher risk of instability than the right-wall slope. "Under the prevailing rainfall conditions, these distress signatures may culminate in sudden slope failure at any time. The distressed slope may be re-evaluated and suitably modified based on the present field conditions to mitigate the risk of progressive instability to avoid potential slope failure," the report cited.

While calling for regular maintenance of drainage systems and continuous monitoring during monsoon conditions, the experts also said in the report that until the slope is stabilised, activities beneath the vulnerable slope should be restricted during periods of intense or prolonged rainfall.

Following the incident on Tuesday, it was reported that labourers were engaged in drainage-related works close to the slope in spite of heavy showers.

Sources said that violations of the prescribed slope angle were also detected at the project site. Onmanorama couldn't independently verify if the design was changed and remedial measures were incorporated by the KRCL based on the report. Onmanorama has sent a query to the office of Deputy Chief Engineer, KRCL, for a response. The article will be updated once a reply is received.