NOTES WHEN CONSTRUCTING BORED PILES

1. Application range of bored piles

Bored piles are reinforced concrete piles poured in place in holes by drilling. This method uses specialized machinery to take soil from the location where the pile needs to be drilled, then injects a solution that can create a membrane to hold the wall of the newly created hole into the hole. The solution commonly used in the process of keeping the borehole wall stable before pouring concrete is Bentonite.

This solution is a mixture of clay mineral powder and water, which has the effect of preventing water from underground aquifers from flowing into the newly drilled pile hole, while ensuring the stability of the borehole wall, so when constructing bored piles, it is necessary to consider comprehensively. After use, this solution is recovered and can be used for the next boreholes.

Reinforced concrete bored pile technology is widely used to treat foundations on weak soil for civil and industrial works, and is one of the foundation solutions widely applied in high-rise building construction in the world and in Vietnam.

They are often designed to carry large loads, so the quality of the pile is always the most important issue. The most important step to determine the quality of bored piles is the construction stage, which includes the technique, equipment, capacity of the construction unit, and strict implementation of the technological process.

When constructing bored piles in terms of scope of application, bored piles are used to reinforce the ground and connect with the foundation to stabilize the construction. This is an advanced method that can support large constructions on weak ground.

Over the past ten years, bored pile technology has been strongly applied in construction works in our country. Currently, it is estimated that we annually carry out about 50 ÷ 70 thousand meters of bored piles with a diameter of 0.8 ÷ 2.5m, with a cost of about 1,300 ÷ 1,400 billion VND.

2. Bored pile construction methods

There are many equipment and technologies for bored pile construction in the world, but there are 2 principles used in all construction methods:

2.1. Notes when using bored piles using casings.

This type is often used when constructing piles located close to existing structures or due to special geological conditions. Bored piles using steel casings are very convenient for construction because there is no need to worry about the collapse of the borehole wall, the structure is less dirty because it does not have to use Bentonite solution, and the pile quality is very high. The disadvantage of this method is that the construction machine is large and cumbersome, when the machine is working, it causes vibration and noise, and it is also very difficult to construct piles with a length of over 30m.

2.2. When using bored piles without casings

This is a very popular drilling technology. The advantages of this method are fast construction, ensuring environmental hygiene and having little impact on surrounding structures. This method is suitable for soft, semi-hard, semi-soft clay, fine sand, coarse sand or gravel with grain size from 20 - 100mm.

There are 2 methods of using bored piles without using casing:

Drilling and washing method (counter-circulation):

• The excavator uses a spiral to break the soil, the Bentonite solution is pumped down the hole to hold the wall of the excavation. When the bored pile is washed, the drilling cuttings and solution are pushed up by the pump and air compressor from the borehole into the settling tank to filter and separate the Bentonite solution for reuse. The work of placing the reinforcement is carried out normally.
• Advantages: This method has cheap equipment, simple construction, low cost.
• Disadvantages: Slow drilling speed, low quality and reliability.

Bucket drilling method:

• According to this drilling technology, the drilling bucket is often in the form of a vortex bucket that cuts the soil and brings it out. The drilling bucket is in the form of an antenna, usually 3 transmission segments that rotate from the excavator to the bucket thanks to the groove system.
• When constructing bored piles, the bucket is the borehole wall that is kept stable thanks to the Bentonite solution. The hole making process is performed in the Bentonite solution. During the drilling process, different buckets can be replaced to suit the excavated ground and to overcome underground defects.
• Advantages: Fast construction, easy and convenient quality control, ensuring environmental hygiene and little impact on neighboring works.
• Disadvantages: Must use expensive specialized equipment, high pile cost. This method requires a very strict technological process, technical staff and workers must be proficient, have a high sense of discipline.

3. Bored pile construction steps (construction process)

Step 1. Arrange the drilling location diagram

• Each drilling machine is arranged in a certain area to avoid entanglement during construction.
• Arrange drilling in order from the inside out to avoid the situation where the drilling vehicle runs on the newly poured concrete pile head
• The next center is only drilled next to the previous center when the concrete of the previous center is greater than 24 hours old.

Step 2. Pile drilling

• When moving the machine into position, align it correctly with the previously positioned center mark.
• Support the machine to ensure that it is secure and does not tilt when the machine is operating.
• Check the verticality of the tower with 2 standard bubbles attached to both sides of the drilling tower body, continue to monitor these two bubbles during the drilling process.
• When constructing bored piles, we need to know that after adjusting the machine, use a drill bit to drill a section about 2m deep and lower the casing (the casing is 2m long) to prevent landslides and water loss during drilling.
• Drilling with a drill bit to the design elevation of the concrete pile
• When drilling, monitor the geology and record, if there is a big difference compared to the geological survey documents, immediately notify the investor and design consultant to adjust the pile depth.
• While drilling, it is necessary to check the appropriate amount of bentonite.
• The bentonite solution has the effect of bringing the drilling cuttings from the bottom of the borehole to the solution pit and has the effect of keeping the borehole wall from collapsing, so in all cases of stopping construction due to weather or stopping overnight due to the end of working hours... it is still necessary to ensure that the borehole is always filled with solution.
• When constructing bored piles to ensure safety, during the drilling process, if you see rapid water loss through a large permeable layer, you must quickly add bentonite to the solution to prevent seepage.
• When constructing in a high groundwater level, it is important to not drill two pile centers close together to avoid water rushing from one pile to the other, leading to wall collapse.
• After drilling the first time, lower the drill bit B to pull up all the remaining mud and soil. This step can be done 1 to 2 times.
• When lowering the drill bit B, still operate as when drilling the breaker, but when pulling up, do not rotate the drill bit to avoid soil falling back into the borehole.

Bước 3. Borehole depth testing.

• Use a tape measure with a plumb bob hanging down the borehole; or measure the length of the drill rod or concrete pouring pipe.
• While drilling, some of the drilling cuttings remain in the borehole, so it is impossible to plumb to check, so at this time the borehole elevation will be checked based on the length and number of drill rods to calculate, the length of each drill rod is 3.05m.
• One of the notes when constructing bored piles is that after dropping the steel cage and concrete pouring pipe, plumb the borehole elevation again to determine the thickness of the sediment layer
• After cleaning the borehole, plumb the borehole elevation again to determine the sediment layer must be < 10cm.
• If after cleaning, concrete is not poured immediately, it must be checked again before pouring concrete to ensure that the sediment layer is within the allowable limit.

4. Some common problems that occur when constructing bored piles

- Cannot pull out the drill head

This problem often occurs during the process of drilling bored piles using casings. In some cases, there is a power outage or a crane malfunction while drilling. The rock and soil on the side of the newly created borehole will collapse while the casing has not been lowered down in time. The drill head is now filled with rock and soil, and is also stuck in the casing wall, making it impossible to pull it up.

In this case, the solution is to pull the casing up about 15 - 20 cm; then pull the drill head up and lower the casing immediately after pulling it up. When constructing bored piles, especially for casings that have been lowered deep; when pulling up, it is difficult due to the high friction with the borehole wall; it is necessary to perform water jetting with high pressure to wash away the rock and soil that filled the drill head. At the same time, make the drill head slide down to the bottom of the hole in a straight direction and then pull the drill head. It is necessary to pay attention to balancing the amount of water in the borehole by both pumping water jets and sucking water out of the hole.

- Borehole wall collapsed

This problem can occur for many reasons:

• The speed of creating a membrane to hold the borehole wall of the bentonite solution cannot keep up with the speed of creating holes.
• The wall-holding solution is not mixed in the right ratio, leading to the density; and the concentration not meeting the requirements.
• Because the groundwater has high pressure
• The casing is not placed vertically; it is bent; while adjusting it will cause the soil on the side of the hole to burst and collapse.
• One of the notes when constructing bored piles; is that the concrete pouring time exceeds 24 hours, causing the wall-holding solution to separate water; not meeting the requirements of stabilizing the borehole wall, leading to wall collapse.

To prevent this problem, it is necessary to have a good solution from the beginning:

• Closely monitor the process of preparing the retaining solution; especially with the counter-circulation bored pile drilling method. Regularly check the quality of the solution when pouring concrete to promptly fix it; if there is a situation where the solution separates water.
• If a high-pressure underground water source is detected; the casing should be dropped through the underground water to prevent the wall from collapsing.
• Maintain a reasonable drilling speed to create time for the retaining solution to form a membrane.
• When dropping the casing, be careful not to hit the borehole wall strongly, drop it vertically.

- Unable to pull out casing (when using bored piles using casing)

Reason:

• Due to the nature of the soil, especially in sandy soil layers; and the influence of underground water pressure, the pulling force of the crane cannot overcome the inertia of the casing jam. The friction force with the wall is large, making it difficult to pull the casing up.
• Because the casing was tilted and skewed when lowered, when pulled up; the casing could not go up vertically, the pulling force of the crane was also significantly reduced.
• Due to pouring too much concrete before pulling the casing; increasing the friction of the casing with the concrete.

Remedy:

• When constructing bored piles; it is necessary to ensure that the construction machinery and equipment are suitable for the method; the crane must be able to carry the weight of the casing.
• It is necessary to test the feasibility of removing the casing immediately after drilling the hole; and before pouring concrete (try shaking and pulling up about 15 - 20 cm).
• In case the casing is not lying vertically; it is necessary to shake gently and choose to return the casing to the most favorable direction to exert the pulling force.

5. Safety in bored pile construction

5.1. General notes before constructing bored piles

• The operator of the crate packing machine must be over 18 years old and carefully trained.
• Before packing, it is necessary to determine the location of additional tools and protect them in a safe way; it is necessary to determine to avoid additional warehouses, additional water sources; or floors that may be hazardous to the work.
• There must be a platform or pad for the necessary pillars.
• When constructing the crate, helmets, eye and ear protection if necessary.
• The upgraded machinery and equipment must be technically inspected and licensed for use. Those machines must also have the load and capacity to meet the construction requirements.
• Special attention must be paid to prevent damage to the base configuration due to falling into the hole.
• The upgraded machine to lift the workers up and down must have a handle, the infrastructure must be electrically operated. The crate to lift the workers up must be solidly designed, cannot rotate or tilt.
• The crate packing worker should request the contractor to provide sufficient explanations; which outlines the points to be noted, in relation to the packaging styles they are to do.

The consolidated worker fields must be checked or cleaned for errors.

5.2. Principles to know before going down to check or clean the borehole

• The minimum borehole diameter is 75cm;
• The borehole is also considered a narrow space; therefore, it is necessary to strictly follow the instructed measures; to create a safe space.
• Waste materials during drilling must be kept away from the borehole;
• There must be specialized equipment designed to be sturdy; and anti-rotation such as cages, chains to lower workers. The power supply for lifting equipment must always be maintained; when there are workers working under the borehole.
• When constructing bored piles to ensure safety, during the process of working under the borehole; workers must always be trained to master the emergency procedures; when working under deep boreholes. Training should be conducted regularly.
• It is recommended to arrange people to work above during the construction process under the borehole.
• Arrange enough lighting and means of communication for people working under the borehole; use low voltage electricity to ensure safety.
• In case of possibility; It is best to replace workers entering the borehole with cameras; or remote inspection equipment.

6. Introducing a reputable design & construction consulting service provider

The importance of construction units is undeniable, their reputation will ensure the quality and aesthetics of your project. Currently, there are many units providing construction design services to meet the increasing construction needs in our country. Therefore, finding a reputable unit is not easy and requires you to spend time researching. In the selection process, you need to research carefully and ensure that the accompanying unit must have high expertise, many years of experience, have a clearly signed contract, transparent costs, ... To help customers save time searching, Công ty TNHH Đầu tư Xây dựng Hưng Nghiệp Phú is confident that it will be the best choice for you.

Hung Nghiep Phu Construction Investment Co., Ltd. with the mission of providing the best solutions and services, building a prosperous community with customers, Hung Nghiep Phu is gradually affirming its brand through sincere cooperation, with a leadership team with ethical capacity, creativity, high expertise and strategic vision. Hung Nghiep Phu owns a team of skilled, highly qualified employees who will bring customers the best quality technology.

Hung Nghiep Phu Construction Investment Co., Ltd. - specializes in constructing civil and industrial works. We look forward to accompanying customers in construction projects as well as continuous procedures such as planning diagrams, applying for construction permits, completing procedures, applying for fire prevention and fighting certificates, ... If you have any questions or are in need of design, completing procedures, please contact us immediately for free consultation!

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Source: Compiled from the Internet