Pile Construction

This articles is about the pile construction. It is recommended to read the following articles before read this

· Classification of piles

· Pile machines

· Pile tests

There are many kinds of pile construction such as timber piles, precast concrete piles, steel piles…etc. among those creations of piles; the cast in situ piling method was selected by the site which our group members visited. As cast in situ piles are economically suitable with current situation in the country these are become more common. The diameter of the pile could be 450-1800 mm in Sri Lanka.

Cast in situ concrete piling method is practicable in two ways.

1. Drilling/ Percussion

The cracks can be occurred due to the vibration therefore the risk is high and currently this method is not granted permission by the Environmental authorities.

2. Dry Boring

This method also can be divided into two methods based on equipments used.

Ø Mechanical method

Ø Hydraulic method

First of all the hole should be bored according to the required length. The driller machine, which is used to bore the hole, is shown in the figure

Most probably, pile goes up to bedrock and the rock is bored to make proper connection between the rock and the pile. The boring of bedrock is called as “socket-in”. Normally “socket-in” depth is equal to 1 diameter of the pile and sometimes it might be 2 diameters or up to 1 m. those piles are called as ‘end baring piles’. However, if the condition of the soil is capable with load bearing, pile does not need to go up to the bedrock. Those are friction piles.

The borehole should be cleaned with cleaning buckets but when the hole is becoming deeper it is difficult to have proper clean and for that most probably Bentonite solution is required. According to the point of view as the condition of the soil at the site was stable, no need of having Bentonite solution. Instead of that, they have used a method as described below.

After excavation, reinforcement is sent through the hole and then a steel pipe called ‘Trimmy pipe’ is sent through the reinforcement. There after by using pressure gun the pressured air injected into the bottom of the hole as shown in the figure by using ½ inches GI pipe.

Flow of pressures air makes a vacuum area in bottom of the hole and due to the that mud or soil particles which remain in the bottom are come up to the surface level with the air because of the less mud condition of the soil it has become favorable to this method.

In the most of the time, the bottom of the hole closed by the temporary plug or a loose shoe is first driven in to the ground. After 20-30 minutes cleaning, the borehole the ready mix concrete is feed to the hole by using funnel as shown in the figure. The properties of concrete which has been use to the piles as follows

· Grade -30 Concrete

· Slump height 180-200 mm

Although for testing piles they have been used Grade-40 concrete due to able to gain the strength of Grade- 30 concrete in less than 28 days.

The essential features of these method is that the concrete is cast-in-place indirect contract with the soil as the steel casing (trimmy pipe) is withdrawn there will be some released of the stress that were generated in the soil during driving and if the concrete is compressed as it is put into place restricting will result. In certain cases the steel casing is remain as it is.