Ratio of bearing capacity of double underreamed (UR) pile to that of single-reamed pile is nearly

Explanation

According to IS 2911: Part III 1973, the ratio of bearing resistance for the double under-reamed pile to that of a single under-reamed pile is 1.5 for sandy and clayey soils including black cotton soils.

Note:-

As per IS 2911: Part III (Some other recommendations)

1. For deep deposits of expansive soils the minimum length of piles, irrespective of any other considerations shall be 3.5 m below the ground level. For recently filled-up grounds or other strata or poor bearing, the piles should pass through them and rest in good-bearing strata.

2. The diameter of under-reamed piles may vary from 2 to 3 times the stem diameter depending upon the feasibility of construction. For Bored cast in situ under-reamed piles, the bulb diameter normally be 2.5 times while for compaction piles it is 2 times.

3. For piles up to 30 cm in diameter, the spacing of bulbs should not be greater than 1.5 times the diameter of the bulb. For piles of diameter greater than 30 cm, spacing can be reduced to 1.25 times the stem diameter.

4. The topmost bulb should be at a minimum depth of 2 times the bulb diameter. In expansive soils, it should not be less than 1.75 m below ground level. The minimum clearance below the underside of the pile cap is embedded in the ground and the bulb should be a minimum of 1.5 times the bulb diameter.

5. Under-reamed piles with more than two bulbs are not advisable without ensuring their feasibility in strata needing stabilization of boreholes by drilling mud. The number of bulbs in case of bored compaction piles should not exceed tow in such strata.