A three-metre-deep sinkhole appeared on Brigade Road on Thursday afternoon, injuring a motorcyclist and causing massive traffic jams in central Bengaluru.
The sinkhole appeared two days after an under-construction metro pier fell on a family of four travelling on a motorcycle along the Outer Ring Road in HBR Layout, killing a woman techie and her toddler son.
While a 2.762-km metro line is being constructed under Brigade Road, it remains unclear what exactly caused the sinkhole.
As the motorcycle slipped into the sinkhole, the rider was quickly dragged away by people around. He escaped with minor injuries.
A well-placed source in the BMRCL suggested that the sinkhole was caused by water leakage from an underground pipeline, and not by metro tunnelling. "The sinkhole appeared because of the loss of soil and kept widening. We suspect that an old cast iron water pipeline, which was neither threaded nor welded, has burst because of ageing or excessive pressure from a drain. Continuous water leakage from the pipeline has resulted in this cavity," the source explained.
According to the source, the asphalt is approximately one metre thick, which won't be damaged by a small cavity. The metro line is being tunnelled at a depth of 35 feet. If a cavity occurs at such a depth, there won't be any water, he added.
The sinkhole is now being filled with concrete, and metro authorities will launch an investigation. As part of the assessment, a steel cone will be sent down into the rock. If the soil is loose or there's a cavity at the bottom, the cone will plunge suddenly. But if the soil is rocky, it will go slowly.
The 2.762-km underground metro line is being constructed from Rashtriya Military School (Vellara Junction) to Shivajinagar as part of the 21.386-km corridor between Kalena Agrahara on Bannerghatta Road in southwestern Bengaluru to Nagawara in the city's northeast. The corridor has the city's longest underground tunnel at 13.885 km.
L&T Limited, the contractor, has deployed two Tunnel Boring Machines (TBMs) for the tunnelling work. TBM Avni has finished Drive 1 & 2 from MG Road to Vellara Junction. TBM Lavi has completed Drive-1 and is currently tunnelling from MG Road to Vellara Junction. As of November 30, 2022, it had tunnelled 599 metres, or 52.80 per cent, of the 1,134 metres it is supposed to complete.
Past incidents
In October 2020, a part of Sultanji Gunta Road near Bamboo Bazaar caved in due to metro tunnelling. In October 2021, a portion of Tannery Road collapsed for the same reason. In December 2021, a large sinkhole appeared on JC Road because of leakage from a water pipeline.