Construction of Rock-Socketed Secant Piles in Vicinity of Heritage and High-Rise Buildings: Case of Mumbai Metro

Underground projects for sustainable urban infrastructure development in India witnessed a positive breakthrough in last few years. Metro rail projects executed in cities like Mumbai have left with underground excavation methods as the lone choice for connecting localities due to the presence of huge traffic on the road. Bored cast in situ secant piles are being preferred as retaining wall for underground station excavation in cities like Mumbai, which experience shallower groundwater table, sound bed rock, presence of heritage and sensitive buildings with heavy traffic movements. One of the major challenges during construction of rock-socketed piles is to ensure adequate "socketing in rock" and pile termination as per design criteria. To overcome this challenge, it is important to identify key parameters involved with pile boring and develop guidelines as inadequate rock socketing creates problems of pile toe instability and increased pile movements causing damage to nearby structures during active stages of excavation. This paper presents a pile termination criteria based on borehole data analysis followed by identification of key parameters for ensuring rock socketing during construction of Mumbai Metro. Piling rig penetration rate is one of the key parameters selected and analyzed for around 180 piles. Developed guidelines helped in successful construction of more than 5000 piles without causing any damage to structures in the vicinity. The suggested pile termination criteria and guidelines will be helpful for other projects of similar litho-stratigraphic disposition and surrounded by sensitive, high rise and heritage structures in proximity.