What Is Under-Reamed Pile Foundation?
Under reamed pile primly developed for an area where black cotton soil could cause structural instability. Many times, soils undergo volumetric changes due to moisture variation underneath the ground surface. The black cotton soil excess shrinkage and expansion property can cause distress which is very dangerous and critical as far as the bearing of the foundation is concerned.
- This Pile (Under reamed piles) type developed by C.B.R.I.( Central Building Research Institute).
- The under-reamed piles are made from RCC.
- This pile type special feature is an enlarged portion are given at bottom or at midpoint which is known as ‘under reamed ‘or ‘bulb’.
- If the pile has one bulb, it is called single reamed, if the pile has two bulbs then it is called double reamed, and if there are more than two bulbs, it is called multiple under reamed piles.
It is a fact that after the introduction of under reamed pile for it is considered as the safest and economical foundation for such black cotton soils or expansive soils.
Reamed is a cutting procedure for which a cutting process creates a very precise hole size.
Definition of Under-Reamed Pile
It is a cast-in-situ pile with such an enlarged bulb at the bottom, bulb shape created either through cutting or scooping out all the soil or through other appropriate methods, is called as the Under Reamed Pile. They are often referred to as bored cast-in-situ concrete piles.
These types of piles are most constructed in the field built to transfer the load of the foundation to the soil by resistance produced as in its tip along the top, or even both, is referred to as bearing piles.
When under reamed pile offers resistance at the point or base of the pile, they also work as the “End-Bearing Pile” and even if the load is primly carried by friction along its top, these piles also work as the Friction Pile. When this is only constructed to avoid uplifting or pulling, it’s considered an “Anchor Pile.”
IS Code For Under Reamed Piles
IS: 2911 ( Part III ) – 1980 – CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF PILE FOUNDATIONS
Under Reamed Piles Foundation Details
- Diameter of the pile = 20 to 50cm
- Bulb Diameter = 2 to 3 times of dia. of the pile
- Pile Length = 3 to 8m
- Space between 2 piles = 2 to 4m
- Spacing between 2 bulbs = 1.25 to 1.5 times of bulb dia.
- The load-carrying capacity = 20 to 40 ton
- The volume of under reamed pile depends on the diameter of the pile and the size of the bulbs.
These are bored cast-in-situ concrete piles having one or more bulbs formed towards the bottom by enlarging the borehole of the pile stem.
The enlargements help in providing substantial bearing or anchorage. 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. Under-reamed piles are considered useful in black cotton soil.
Under Reamed Pile Design
The concrete used during under-ream piles should have a slump value minimum of 100-150mm for easy dumping in the hopper. In the case of concrete formwork, the concrete used must have a slump of 150-200 mm with efficient outcomes. For concrete grades M-20 and M-25, minimal cement content of 350 or 400 kg/m3 c is essential.
In dense layers of expansive soils, the minimum length of under-ream piles should be to be 3.5 m below the surface level.
The diameter of the substrata should be at least 2.5 times the diameter of the stem.
The overall vertical distance between under-reams should be 1.5 times the depth of the under-reams for piles having dia 300 mm. For bigger diameter stacks, the spacing can be decreased up to 1.25 times the diameter of the stem.
The maximum bulb should not be bigger than twice the diameter of the bulb. In the case of black cotton soils, this depth under reamed bulb should never be below 1.75 m ground level.
Generally, the amount of bulbs used in single should not exceed 2.
The minimum spacing between two under-ream piles in a group should be 1.5 times the diameter of the under-ream and is generally maintained at double the under-ream diameter.
In the case of a pile group having pile spacing of 2 Du, the size of the group can be considered equal to the amount of the load-carrying strength of the specific piles in the category. In the case of a pile group with such a 1.5 Du pile spacing, the safer load designed per pile should really be decreased by 10%.
Under Reamed Piles Bulb Calculation
The diameter of the under reamed bulbs may be of the order of 2 to 3 times the stem diameter. The spacing of the bulbs is 1.25 to 1.5 times the stem diameter. The topmost bulbs should be at a minimum depth of 2 times the bulb diameter.
Boring for the piles is carried out in the usual way. Thereafter the bulbs are formed by means of an under reamer rotated by the drill rod. The buckets are used to remove excavated soil. The reinforcement cage is then lowered and the pile concreted. The cement quantity and concrete slump shall be as recommended bored cast-in-situ piles.
Increasing Load-Carrying Capacity of Under Reamed Piles
1. By putting more bulbs
2. By increasing the dia. of the bulb
3. By increasing the length of the pile
4. By providing proper reinforcement
Under Reamed Piles Construction
The following is construction method for under reamed pile foundation with diagram.
- The machinery for under reamed piles is developed by C.B.R.I. Rurki.
- The machine is easy to use and it is light weighted.
It consists of three tools
1. Spiral Auger
2. Under Reamer
3. Boring Guide
- The hole or bore for under reamed pile is done by using a spiral auger.
- The spiral auger consists of a special cutter at the bottom to dig the bore.
- The soil that is dug is taken out by the bucket.
- Special cutters are used for making the bulb. The dia. of the bulb can be increased by applying excess pressure. The auger is set up on a tripod after the digging is done. So the digging is done in the vertical direction.
- As reaching sufficient depth auger is pulled out. The reinforcement is then inserted into the hole. Concreting is done. If more than one pile, the beam is constructed which connects the piles and make them act like a single unit. The wall is then constructed on the beam.
Uses of Under-Reamed Piles
Under-Reamed Piles are generally used to strengthen the foundation in various types of soils such as sandy soils, clay soils as well as expansive soils.
The following are the condition where under reamed piles are used,
- To control adverse effects of seasonal moisture variations in vast soils such as black cotton soil.
- To explore rough strata in the foundation
- To achieve a sufficient resistance for the forward, downward, lateral loads as well as moments.
- To transfer foundations just below scour mark
- Under reamed piles are more suitable for factory buildings, including machine foundations.
- These piles are often used in cases where friction and noise produced by pile construction are minimized.
In case soil filling is done by some professional or generally such a layer of 20 cm thick with optimal moisture content, it is willing to deal. Therefore, to balance this defect the grade slab is constructed on a set of Under Reamed Pile spaced around 3 m center to center.
In this grade, slabs are constructed as a flat slab and supported by Under Reamed Piles, where even the pile cap serves as a column pedestal as with a flat slab, but their beams are not provided to reduce the cost.
This method is performed is really well for ages. This technique already achieved in terminal buildings for Surat and Amritsar International Airports along with various public buildings in India.
Application of Under Reamed Piles
- As black cotton soil has a property that gets expand as it comes in contact with moisture or water and it gets contracted as it is dried out. Because of this reason, there may be a chance of cracks in the structure.
- To avoid damage caused by soil volume change, the under reamed pile used.
- Sometimes, when foundation soil does not have sufficient bearing capacity, under reamed piles are used to increase the capacity.
- The under reamed piles are suitable when the water level in sandy soil is high.
- It should be used. When there are lifting forces acting,
Advantages of Under-Reamed Piles
- These piles helps to decrease the differential settlement as well as the vertical settlement.
- Under reamed piles are mostly used for soil that expands and shrinks in response to fluctuations in moisture or the expansive structure of the soil.
- A provision of under-reams or bulbs does get the benefit of increasing the ability of bearings and bulbs.
- As the number of bulbs increases from one to two, the load-carrying capacity of the Under-Reamed Pile is raised.
- Bulb provided as benefit the foundation as under-reamed bulbs to prevent uplifting and being used as anchors.
- The under-reamed pile is proved to cost-effective as a smaller depth of the pile pipe, which means that a lower concrete quantity is used to replace the excavated material.
Disadvantages of Under-Reamed Piles
- The depth of under-reamed piles is restricted and depending on the climatic situation, the under-reamed piles are not proved ideal for waterlogged soil since they seize over by friction.
- Heady machinery and equipment require stringent quality control and constant monitoring during production.
- For a long period of time, the Under Reamed Piles are done by the manual hand-operated pump. During the installation of under reamed pile, It is very important to maintain the plumbing line of the pile since if they’re not in the plumbing, the entire load transfer process will shift.
Is Code for Pile Foundation
- Indian Standard IS code 2911 (Part 3) – 1980 follows the design and construction of under-reamed piles with one or more bulbs.
- As IS Code Recommendation, the bulb diameter of under reamed pile will range from 2 to 3 times the diameter of the stem based on the viability of the construction and design specifications.
- IS Code gave spacing of 1.25 to 1.5 times the lamp diameter for the lamps.
- A horizontal angle of 45 is suggested for all bulbs underneath. Indian Standard IS code 2911 specifies mathematical expressions for the calculation of bearing and lifting capacities.
Watch Video: Under Reamed Pile Construction
Q.1 What are Under Reamed Piles?
Bored cast-in-situ piles made of concrete which might have one or more bulging portions called bulbs made by the widening of piles stems are termed as Under Reamed Piles. Under reamed pile primly developed for an area where black cotton soil could cause structural instability.
Q.2 When the under-reamed pile has only one bulb it is called?
Single Under Reamed Pile is the term used for under reamed pile having only one bulb, where the bulb is the enlarged portion formed by enlarging the stem of piles. Similarly, an under-reamed pile having more than one bulb is called Multi Under Reamed Pile.
Q.3 What are bored Pile?
Bored Piles are concrete cast in situ piles, inserted into the grounds caring out various boring techniques and methods. Under Reamed Piles are sometimes referred to as Bored Piles.
Definition of Under-Reamed Pile
Under reamed piles are cast-in-situ concrete piles having one or more concrete bulbs formed by enlarging the pile stem. Under reamed pile primly developed for an area where black cotton soil could cause structural instability. Many times, soils undergo volumetric changes due to moisture variation underneath the ground surface.
IS Code for Pile Foundation
Pile Foundation IS Codes are as follows,
IS 2911: Part 1 : Sec 1 : 1979 Driven cast-in-situ concrete piles.
IS 2911 : Part 1 : Sec 2 : 1979 Bored cast-in-situ piles.
IS 2911: Part 1 : Sec 3 : 1979 Driven precast concrete pile.