What are methods of Bored Piling?

 BORED PILE METHOD

Bored piles are the concrete structures installed in mudrocks to transmit the heavy structural loads onto the rock head Strata.

Pre-requisites of bored pile construction.

Determining soil parameters and preparing a soil report is the very first step of the process. The different soil parameters for selecting the appropriate method for the bored piles- dry pile, wet pile, or cased pile method.

The selection of pile construction method primarily depends on-

Soil conditions: The soil condition plays a vital role in the stabilization of boreholes. The contractors need to check whether the soil is denser or loosen to decide the method of the bored piling. The denser rock is not easy to penetrate and thus needs a rigorous method of bored piling. The loosen soil requires extra effort, they involve the use of slurry, made up of different polymers to stabilize the soil condition. 

Evacuation of soil needs to be prevented, don’t let the soil flow into the pile sleeves.

Groundwater elevations: The groundwater level is an important factor that impacts bored piling. Water does not allow the concrete and casing to settle down easily. Therefore, we need to make proper arrangements to fix the groundwater for bored piling.

Site conditions: Site conditions are another crucial factor to be checked before bored piling. If the site is near the waterbed or the river, there are chances of scouring of water and hence it is crucial to make a report to check out various conditions in order to proceed successfully in bored piling.

Borehole casing-

Casings are the support structures of bored piles. Sometimes the soil conditions are not perfectly fit for bored piling and hence casings act as the savior.

Uncased boreholes

Uncased boreholes fit for the stable soil conditions where soil layers do not require any support. The bentonite and polymer slurries can be used as stabilizing fluids if excavations run through unstable soil layers. Contractors should review the soil report thoroughly before starting the projects because this method works only with a stable-soil bed. 

Partially cased boreholes

Partially cased boreholes fit for the deep and the partially unstable-soil layer. The partially-unstable soil poses a few problems, but you can stabilize it with bentonite or polymer slurry.

Cased boreholes

Cased boreholes fit for the most unstable soil layers, which need to be stabilized with the slurry fluids. It is not possible to stabilize the boreholes without such slurry agents. It is the most complex soil conditions, and you need to do a lot of hard work to work on such soil.

Steps to bore piling-

Bored piling not only requires the step-by-step activity but it also includes a quality check report and spoils removal activity to install a perfect bored pile. The various steps for perfect bored piling are the following-

1- Drilling-

It starts with the excavation of soil using the drilling tools. For drilling the bored pile, we can use various drilling machines like- Rotary drilling tools or Hydraulic bored piling rig. Chisels and grabs can also be used in combination when the soil conditions dictate. Drilling can be complex in hard soil as vibration disturbs the adjacent soil and build structures.

The process of drilling bored pipes includes the repetition process of –

• Initially, the rotary or hydraulic machines lowered in the soil with the tool aligned at the axis of the pile for doing hole.
• Rotation thrust penetrates the soil.
• The drilling tool is brought to the surface back and forth.
• That drilling rig rotates to remove the muds and rocks that collect in the bucket.
• Tools are realigned again to the pile access.

Drilling can be done through various methods. Some of those methods are-

Dry drilling method-

This process is applied where the soil is stable, and it will not slump or cave when we drilled the hole to its decided depth. The process works well where the rock bed is above the water level. The soil must be uniform in nature and stiff clay. 

Wet drilling methods-

Drilling through loose soil or soft clay below the water table requires special fluids to stabilize the bore wall holes. Bentonite and chemical polymers slurry have a greater specific weight and a capacity of waterproofing the walls of the borehole so these slurries prevent the wall from collapsing. A continuous supply of slurry is essential.

Once Bored piles are drilled for the required and decided depth, we can further move to the next step.

2- Steel reinforcement-

When it reached the desired depth, the borehole depth should be checked and verified on daily pile data. The reinforcement must conform to the mill certificate approved by any consultant.

Match the above-stated requirements, take the auger out and position the steel cage on the server crane. Position the steel cage in the borehole and rest it 2.0 m above the casing top level.   

The reinforcement steel cage is lower in the laydown section. Contractors shall manage the length and size of the piles. 

3-Concreting-

After inserting the steel cage, fill the borehole with concrete. And the concreting process is done with a tremie pipe. For this, lower the steel pipe of diameter 250 mm in the borehole and positions at the center of the shaft. According to the general rule, the string is made up of 2 or 3-meter long sections connected until they reach the bottom of the borehole.

Tremie is the most commonly used method for underwater concreting. Tremie is fitted with a hopper or funnel at the top and the concrete mixture is poured into it. The other end of the tremie pipe is kept inside the concrete for about 2m to avoid mixing of the concrete and groundwater. It is constantly being lifted upwards.

Concrete of higher slump (175mm+25mm) otherwise stated, must be used in the process.

Concrete leveling should be done every time after each truck discharges its capacity.

You can use various plug materialsin the tremie pipe to create the density difference and avoid mixing of concrete with slurry.

When the concreting is reached to the pre-established levels pouring stops and the strings are removed. But it is important to maintain the quantity of the concrete while the casing is being extracted after concreting.

Once the concreting has elapsed the excavation process is carried out on the foundation where pile heads are trimmed to detach the reinforcement bars only.

During the concreting process, it is important to take the concrete samples for testing.

For continuous assurance of the quality and integrity of concrete, it should be poured 0.6m above the theoretical cut-off(otherwise stated).

4- Spoil removal-

Spoil from piles must be cleared from the site by an excavator to the disposal area directed by the local authorities. Site-in-charge must check out the piling records provided by the foreman before submitting it to the Engineer on the bore pile record sheet.

The record should have the following data-

Pile number

Pile date of completion

Pile diameter

Concrete top level

Working platform level

Pile bored length

Bentonite tests

Concreting time and details

The theoretical volume of the concrete

The actual volume of the concrete

Drilling fluid

5- Clearing the site-

Site clearance is crucial before grouting, and it is done with the help of a pressure pump. First, the water is pumped with pressure to remove sand mud sediments to clean the piles. After the water pumping, high-pressure air pumping is done.

6- Grouting-

Grouting is aimed at reinforcing the bore pile structure to provide additional strength by introducing force between the soil and pile base. It reduces the pile settlement and enhances its bearing capacity. Also, the quality of the soil- pile combined working is improved.