Piles are the most common type of deep foundation, that safely transfer loads from the super structure to the subsurface strata by means of friction, end bearing or a combination of both. Piles can be broadly classified based on the following parameters.
1. Mechanism of load transfer – Friction, End bearing, or a combination of both.
2. Method of installation of piles – Driven (displacement) piles or bored (replacement) piles.
3. Type of material used for piling – Concrete, steel, timber, sand or composite piles.
Among these, concrete piles can be classified further as pre-cast and cast in situ concrete piles. Though there are a variety of piling options, the most common type of piles used for majority of construction works in India are bored cast in situ concrete piles.
Bored cast in situ concrete piles.
The word in situ is a Latin word which literally means ‘in position’. Cast in situ concrete piles are those which are cast at site. Design and construction of bored cast in situ piles are done in accordance with IS 2911 (part-1/Sec-2) – 2010. These types of piles are very common in India. Some of the main advantages of this type of pile include the relatively less expense for installation, causing fewer disturbances to the neighborhood and adaptability for casting piles of diameter up to 5m. However, the disadvantage is their practical difficulty in installation, when the diameter of the pile is 400mm or less.
The work is basically making a borehole of designed diameter at the designated locations to the required depth, after providing temporary or permanent casing and stabilizing with bentonite slurry; and filling the borehole with concrete after placing reinforcement cage. The boreholes are drilled using rotary or percussion type drilling rigs, using direct mud circulation (DMC) or reverse mud circulation (RMC) methods. Drilling can also be done using rope operated or hydraulically operated grabbing tools. Concreting is usually done using tremie pipes.
Direct mud circulation method is the less expensive method for doing the boring activity. In this method bentonite slurry is constantly circulated into the bore hole during the boring activity through the bailer pipes, and overflow is collected and redirected to the bentonite slurry tank. The density of bentonite slurry should be around 1.12. Bentonite has a property of becoming a jelly like substance once left undisturbed for a long time. This jelly acts as plastering on the walls of the bore holes, and thereby preventing the boreholes from caving.
Direct mud circulation method is commonly used while boring with bailer and chisel, and reverse mud circulation is adopted for boring using auger. In reverse mud circulation method, the bentonite is directed into the borehole directly, and the return is collected through the drill pipe.
Next, let us see the stages involved for installing a bored cast in situ pile using DMC method.