Hindustan Construction Co (HCC) has holed through on excavation of the 5.8km long Maroshi-Vakola section of a major water tunnel project in Mumbai.

Tunnelling on the 3.6m diameter bore by TBM involved driving through variable ground and in heavy groundwater flow below two runways at the airport at a depth of approximately 70m.

Significant ground treatment and tunnel protection works were called for in the variable strata, which also saw groundwater seepage increase to approximately 25,000m3 per day during the monsoon season.

Progress reached up to 40m in a day and the average daily rate of excavation was 20m. Final concrete lining works are underway.

The Maroshi-Vakola tunnel is part of a larger, 12.2km long water tunnel being built in three sections to Ruparel College. The second drive is 4.5km long from Vakola to Mahim, and the final short drive is to Ruparel College.

Three shafts of 12m diameter and 70m-80m deep will be used for the works – at Maroshi, Vakola and Mahim. The tunnel is to be commissioned by August 2012.

The contractor’s most recent water tunnel works in the city, on the almost 12km long Bhandup Complex to Charkop drive, also involved difficult geology but was completed early, it said.

Separately, HCC has won a contract that involves excavation of a 2.8km headrace tunne’ with a 7.5m wide modified horseshoe-shaped cross section, plus diversion and tailrace bores, on the Pare hydro project.

The tunnels are part of a larger package of civil works on the project in the state of Arunachal Pradesh. The client is North Eastern Electric Power Co (Neepco).