Excavators and dump trucks were deployed to remove soil from the collapsed stretch of National Highway 66 at Mylakkadu near Kottiyam in Kollam on Saturday. | Photo credit: C. Sureshkumar
Preliminary assessments by the National Highways Authority of India (NHAI) suggest that the cause of the collapse of the section of National Highway 66 at Mylakkadu near Kottiyam in Kollam is “a deep-seated shear/bearing capacity failure, meaning the underlying soil was too weak to support the fill”.
NHAI says that as the incident raises serious concerns about the bearing capacity and quality of the soil used in the design and construction of structures along NH-66, massive, multi-pronged initiatives will be launched to identify and rectify any potential safety issues. Following the Mylakkadu failure, NHAI has temporarily suspended the concessionaire and its promoters (M/s Shivalaya) and the independent engineer (M/s Feedback – Satra JV) from submitting bids for future projects. They were also issued notices of possible disqualification (up to three years for concessionaires and up to 2 years for IEs) along with monetary penalties.
Meanwhile, experts say that the main reason for the collapse of the NH section is the construction method without considering the terrain and ground water characteristics. They point out that the site is inherently high-risk, characterized by compressible soil and many waterways between the surrounding polders, making it highly susceptible to collapse due to water saturation. “This is a geotechnical failure because the low strength conditions were not properly addressed during the design and construction process,” says Jayakrishnan Menon, senior geotechnical engineering consultant.
While geotechnical failures carry serious financial and structural risks, including catastrophic damage, delays and cost overruns, their most critical consequence is the potential for loss of life. “The coastal and lowland areas of Kerala are geotechnically very sensitive, dominated by soft marine, alluvial and lagoonal residual deposits, which behave very differently from soils in many other Indian states where high embankments are more common. At Mylakkadu, 4 to 5 meters of embankment fill was placed in one section. This caused an additional load that could be roughly 1 to 10 substances higher than if the applied load were significantly exceed the shear strength and bearing capacity of the underlying soil, failure becomes inevitable,” explains Mr. Menon.
Complaints went unheeded
Allegedly, the contractor proceeded without an adequate geotechnical investigation, resulting in a poor understanding of the actual soil profile. Residents have repeatedly complained to the authorities, saying the earth fill method is unsuitable for the monsoon-dominated wetlands of Kerala. “We have asked them to abandon the unscientific method of earth filling and go for the pillars of the supported (viaduct) structure,” says Gireesh Babu, a resident.
Addressing geotechnical failures requires a systematic, multi-step approach focused on proper investigation, analysis, and a scientifically designed remedial plan. The exact methods implemented must always be tailored to the specific type and severity of the disorder. “Once a major soil failure occurs, it cannot simply be repaired or patched over. The disturbed section must be removed and soil conditions must be properly restored through a detailed subsurface investigation before any reconstruction can begin,” says Mr Menon. He adds that while the Malappuram fault required the construction of a bridge, Mylakkadu may be suitable for soil improvement techniques – but only after confirming the strength of the soil through proper testing. “Methods such as stone columns, which have been used successfully on many more sensitive soil conditions such as the ICTT Vallarpadam container terminal road, are potential alternatives. However, their suitability can only be confirmed after detailed investigation,” he notes.
Mr Menon also warns that fabricated, inaccurate or insufficient land survey reports pose a serious long-term risk. “Geotechnical design depends on both the immediate behavior and the long-term behavior of the soil, and in most cases the long-term behavior is neglected. Failure can occur even five or six years later, long after the contractor’s warranty period has expired. Therefore, accurate and true soil data is critical, especially in geologically sensitive areas like the Kerala coastal belt,” he says.
Published – 10 Dec 2025 21:23 IST
