Pile is the long slender structural members of deep foundation which are driven into the weak soil strata in numbers to increase the bearing capacity so that massive superstructures can be erected over it.
Under reamed piles are such specific piles, having bulging intrusion within the pile stem at certain specific lengths. Depending upon the number of such bulging bulbs present, under reamed piles can be classified as single, double, and multi under reamed piles.
The load-bearing capacity increases as the number of bulbs increased. The basic principle of mechanism of the under reamed piles is that the bulbs provide proper Anchorage and neglect the effect during swelling or shrinking of soil.
Thus the soil between the bulbs also acts as a pile member and thus apparently the effective diameter of the pile is increased.
The diameter of the bulging bulbs in pile can be two or three times the diameter of the slender pile. And the spacing of such bulbs should be 1.5 times – 2 times the diameter of the bulb.
How under reamed pile foundation are constructed?
Generally, Under reamed piles are bored and cast insitu piles and is constructed on manual work based.
Thus the bore holes are initially drilled by help mechanical augur. And then special type of Belling bucket augur are used in order to create the bulb structure(under reamed). The blade of cutters of it are pressurize in order to create the bulb ream.
The cut soil is prevented to get inside the boreholes with help of bucket, which is frequently discharged ay surface after it has been filled.
Then the skeleton of a reinforced cage is inserted in the hole. After the insertion, the hole is compactly filled with the designated concrete grade.
This way the whole process of pile insertion over the required area can be done. The piles are inserted in alternative order of spacing 10-20m so that during the pile insertion process, the other installed pile might not get affected due to instability of soil.
Generally, the diameter of the under reamed pile is standardized to 20cm-50cm, and length to 3-8m along with the spacing of each piles 2-4m.
Why under reamed pile foundation are constructed?
Under reamed pile foundation are basically used in the soil like cotton clay soil which has excessive swelling and shrinkage property, and fluctuates on moisture variations.
So in order to decrease the effect of swelling and shrinkage due to moisture variation, over the substructure and superstructure, the soil is first stabilized with help of under reamed piles.
Moreover, the under reamed piles also increase the bearing capacity, provide proper anchorage, and neglect the effect of settlement and uplift due to shrinkage and swelling due to the presence of a bulb-like structure.
Also sometimes these piles are recommended for the sandy soil with the high groundwater table and in a structure where uplifting forces like wind pressure are relatively high.
Types of under reamed pile foundation
Generally under reamed pile foundation are classified on the basis of the number of bulbs(under ream) present in it. They are;
Single bulb under ream pile:
In this type of pile foundation, the bulb is generally present a few meters above the bottom of the pile. And the soil below the bulb acts as the single unit of the pile, giving the thick base area for load transfer
Load carrying capacity formula for Single bulb under ream pile,
Double or multiple bulbs under reamed pile:
In this type of pile foundation, two or more bulbs are present in the pile stem. The soil trapped between the bulbs acts as a part of the pile increasing the effective thickness of the pile. Thus, the bearing capacity increases as the surface area of the pile for friction increases. Also, the effective bottom area increases to transfer the load.
Load carrying capacity formula for Double or multiple bulbs under reamed pile,
However, the under reamed pile foundation can be classified on the basis of the shape of a ream(bulb) too, which are of the following types.
- Single cone
- Double cone
- Single Spherical
- Full sphere
- It increases the bearing capacity of the soil.
- It increases the load-bearing capacity of under reamed pile foundation as the number of bulbs increases.
- It makes the moist soil to bear sufficient load of the superstructure.
- It is economical than normal pile having a higher shaft diameter because it consumes less amount of concrete.
- It is more energy-consuming and time-consuming process.
- It needs high experience and skilled human power for construction and installation of under reamed pile foundation
- It required expensive equipment as well as tools for detecting the performance of under reamed pile foundation
- sometimes it is done with a manual hand without any equipment which adds the extra expense in a boring hole.
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shresth