Underreamed Piles

These are bored cast in situ concrete piles having one or more bulbs formed towards the bottom by enlarging the bore hole of the pile stem. The enlargements help in providing substantial bearing or archaic. Such piles have been found to be useful in expansive soils like black cotton soil as the bulbs provide anchorage against uplift due to swelling pressures. The diameter of the underreamed bulbs may be of the order of 2 to 3 the stem diameter. The spacing of the bulbs is 1.25 to 1.5 times the stem diameter. The top most bulb should be at a minimum depth of 2 times the bulb diameter. 

Boring for the piles are carried out in the usual way. Thereafter the bulbs are formed by means of an underreamer rotated by the drill rod. The excavated soil is removed by means of buckets. The reinforcement cage is then lowered and the pile concreted. The cement content and slumps shall be as indicated for bored cast in situ piles. 

Bored compaction pile is a modified form of underreamed pile where, after the concrete is poured, the reinforcement assembly, with a cone welded at the bottom is driven through the fresh concrete with the help of a driving pipe, thereby compacting the concrete. 

Underreamed Piles

Spacing of Piles 

Spacing of piles has to be decided taking into consideration the practical aspects of installing the pile and the type of load transfer from the pile to the soil. For end bearing piles the minimum spacing is kept as 2.5 times the diameter of the shaft and if it is resting on rock it can be 2 times the diameter. The spacing between friction piles has to be such that the zones of soil from which the piles derive their support do not overlap and thereby reduce their bearing values. Generally, in such cases the minimum spacing is three times the diameter of the shaft. 

Alignment of Piles 

Piles shall be installed as accurately as possible. Generally, for vertical piles the permissible deviation is  1.5% and for raker piles 4%. The deviation from the designed position for a single pile should not be more than 50 mm (100 mm if diameter is more than 600 mm) and for each pile in a group not more than 75 mm or one tenth the diameter of the pile, whichever is more. 

Load Testing of Piles 

Load testing of piles consists of two types. Initial load test is done to determine the ultimate load carrying capacity and to arrive at the safe design load on the pile. It also helps to fix guide lines for routine tests, assess the suitability of the piling system, and to study the effect on existing adjacent structures etc. The second type of test is the routine test to check whether the pile is capable of carrying the designed load. Such tests are usually carried out on 112 to 2% of the total number of piles at the site, for a test load of one and a half times the working load, maximum settlement shall not exceeding 12 mm. 

Piles are loaded by jacking against a kentledge placed on a platform supported clear of the test pile or against a beam restrained by anchor piles. Settlements of the pile are recorded by dial gauges carried by supports clear of the pile and resting on arms fixed rigidly to the pile head. 

There are two methods of applying test loads. In the Constant Rate of Penetration (CRP) test, the load is adjusted to give a constant rate of downward movement. In the maintained load (ML) method the load is applied in increments and deflections recorded. The CRP test is suitable for determining the ultimate load, while the ML method,can be used for both initial test and routine tests. 

Pile Caps 

The depth of the pile cap should be sufficient for anchoring the column reinforcement as well as that of the pile. The pile should project 50 mm into the concrete of the cap. The cap should be rigid enough to distribute the load to the pile and to take care of differential settlements, if any. The overhang of the pile cap beyond the outer pile is of the order of 100 to 150 mm. The cap is generally cast over a 75 mm thick bed of levelling concrete. A clear cover of 60 mm is provided for the main reinforcement.