Correct Answer - Option 2 : Soft soil with filled up ground
Concept:
Under reamed piles are bored cast-in-situ concrete piles having one or more number of bulbs formed by enlarging the pile stem.
These piles are best suited in soils where considerable ground movements occur due to seasonal variations, filled up grounds or in soft soil strata.
Provision of under reamed bulbs has the advantage of increasing the bearing and uplift capacities. It also provides better anchorage at greater depths.
These piles are efficiently used in machine foundations, over bridges, electrical transmission tower foundation sand water tanks.
Indian Standard IS 2911 (Part III) - 1980 covers the design and construction of under reamed piles having one or more bulbs.
According to the code the diameter of under reamed bulbs may vary from 2 to 3 times the stem diameter depending upon the feasibility of construction and design requirements.
The code suggests a spacing of 1.25 to 1.5 times the bulb diameter for the bulbs. An angle of 45° with horizontal is recommended for all under reamed bulbs.
