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Monday, 11 February 2013

Under Reamed Pile Foundation

Under reamed piles have mechanically formed enlarged bases that have been as much as 6 m in diameter.According to the research carried out at Central Building Research Institute Roorkee (INDIA) and elsewhere it is found that under reamed piles provide an ideal solution for foundation in black cotton soil.The form is that of an inverted cone and can only be formed in stable soils. In such conditions they allow very high load bearing capacities.
The diameter of the pile stem (D) varies from 20 to 50 cm. The diameter of the under-ream bulbs (Du) is normally 2.5 times the diameter of the pile stem. It may however, vary form 2 to 3 times (D) under special circumstances. In case of double or multi-under-reamed piles, the centre to centre vertical spacing between two bulbs may vary from 1 ¼ to 1 ½ times the under-reamed diameter (Du). The length of under-reamed piles varies from 3 to 8 meter and their centre to centre spacing should normally be not less than 2 times the under-reamed diameter. 
Under reamed piles are the most safe and economical foundation in Black cotton soil. Under reamed piles are bored cast in situ concrete piles having bulb shaped enlargement near base. A Pile having one bulb is called single under reamed pile. In its closed position, the under reamer fits inside the straight section of a pile shaft, and can be expanded at the base of the pile to produce the enlarged base.
    The cost advantages of under-reamed piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material.
1. Pre-cast under reamed piles
2. In-situ cast under reamed piles
1. Pre-cast under reamed piles
Pre-cast piles require specialized pile driving equipments. They are advantageous over in-situ cast pile because they don’t need holes to be bored and the pile is already cast complete with the reinforcing steel in place.
2. In-situ cast under reamed piles
This type of pile installation is used to replace soil removed by drilling rather than occupying the space of displaced soil as in driven piles and thus, it mostly relies on end-bearing capacity of the earth layer at the drilled depth.
The bearing capacity of the pile is increased by increasing the number of bulbs at the base. Such type of foundations is ideally suitable in the areas where the black cotton soil or expansive soil is beyond 2.50 meter. The basic principle of under reamed pile is to anchored the structure at a depth where ground movements are negligible due to moisture content variation or other reasons.
Simple tools are required for construction of under-reamed piles like spiral auger, under reaming tool, and boring guide. This is a well proven and established technology for construction of foundation in expansive soils. For speeding up the construction bore and under ream for large diameter and deeper pile a mechanical rig can be used. The construction and design of such foundation can be done in accordance with Indian Standard Code of Practice IS 2911-Part III.
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. 

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