Square piles VS Round piles

We know how a round pile bored, then how to bore a square hole for a square  pile?

Step 1 - Casing burial: The casing is buried by open excavation. First, a rectangular trench of a certain depth is excavated, and then the steel casing is placed in the trench by an excavator. The cross section of the steel casing is square and the size is the same as the pile diameter. Then the plane position of the four corners is re-measured and checked to ensure that the plane position of the casing is accurate and the top surface is horizontal. Finally, the casing is backfilled and compacted with plain soil around it to avoid disturbance of the casing during construction. At the same time, a safety fence is welded around the casing.

Step 2 - Center drilling: Use a conventional rotary drilling circular drill bit to drill and extract soil to a certain depth in the middle of the square pile hole. In order to ensure the stability of the hole wall, it is not advisable to drill to the design elevation at one time. It is advisable to drill to a certain depth (such as 10m) each time. The drilling slag is promptly transported to the designated location by the excavator to keep the hole position clean.

Step 3 - Four corners trimming: Use the rotary drilling rig control system to adjust the verticality of the drill rod, and then use a new drill bit with corner and edge cutters to cut and trim the four corners and four sides of the square pile from top to bottom with the steel casing as the guide. The trimming speed needs to be steady and uniform to ensure verticality and hole diameter.

Repeated operation: Repeat the above drilling and soil removal process until the designed hole depth is reached.

Step 4 - Hole cleaning: Use the side drill bit to scrape the loose soil at the bottom of the hole to the center of the pile, and then use the rotary drilling slag drill bit to remove it to ensure that the hole is thoroughly cleaned and the bottom is flat.

But different from round pile, general square piles are prefabricated piles. There are 3 main method to install the prefabricated piles:

- Hammering method: also known as pile driving method, is to use the impact energy generated by the falling of the pile hammer to overcome the resistance of the soil to the pile and sink the pile into the soil. The hammering method has the advantages of fast construction speed, high degree of mechanization, and wide adaptability. It is the most commonly used pile driving method for reinforced concrete precast piles. However, this method is very easy to produce soil squeezing, noise and vibration during construction, which should be restricted. The equipment used in the hammering method mainly includes pile hammers, pile frames and power devices. Among them, pile hammers mainly include drop hammers, single-acting air hammers, double-acting air hammers, diesel pile hammers, etc., which are used to apply impact force to the piles and drive the piles into the soil; pile frames mainly include rolling, track, walking, crawler, etc., which are used to support the pile body and pile hammer, lift the pile to the pile driving position, and guide the direction of the pile during the driving process to ensure that the pile hammer impacts in the required direction.

- Vibration method: The vibration force generated by the vibrating pile sinker fixed on the pile head is used to reduce the friction between the pile and the soil particles, so that the pile sinks into the soil under the action of its own weight and mechanical force. The vibrating pile driver consists of an electric motor, a spring support, an eccentric vibrating block and a pile cap. The eccentric vibrating blocks in the vibrating machine are divided into two groups that are symmetrical on the left and right, with the same rotation speed and opposite directions. Therefore, when working, the horizontal components of the centrifugal force of the two groups of eccentric blocks offset each other, while the vertical components are superimposed to form a vertical vibration force. The vibration method is mainly suitable for sandstone, loess, soft soil, and sub-clay foundations, and the effect is more significant in water-bearing sand layers. However, when the vibration pile driving method is used in the gravel layer, the construction is more difficult and it is also necessary to use the water flushing pile driving method.

- Static pressure method: On a soft soil foundation, a pile driving process that uses a static pile driver or a hydraulic pile driver to press the prefabricated piles into the soil with a vibration-free static pressure. The static pile driving method can eliminate the pollution of noise and vibration. There is no vibration, noise, or pollution during construction, and it has little impact on the surrounding environment. Static pile driving is usually carried out in sections. After each section of the pile is driven into the soil, when its upper end is at a certain height from the ground (e.g. about 2m), the next section of the pile is connected, and the process is repeated. The construction process of static pile driving includes site cleaning, measurement and positioning, sharp pile placement (including centering and straightening), pile driving, pile connection, pile driving again, and pile cutting.

The main difference between square and round piles lies in their shape, which impacts various aspects of their performance, installation, and costs.

1. Shape and Geometry:

- Square Piles: These piles have a square cross-section. The sides are flat and perpendicular to each other.

- Round Piles: These piles have a circular cross-section, with a smooth and continuous curve.

2. Load Distribution:

- Square Piles: Square piles tend to have better surface area contact with the surrounding soil in the lateral directions (the sides). However, due to the corners, they may experience non-uniform distribution of stresses, especially when subjected to eccentric or lateral loads. This can make them more susceptible to failure under certain conditions, but they are typically stronger against lateral forces compared to round piles.

- Round Piles: Round piles tend to distribute loads more uniformly in all directions. The smooth, continuous surface leads to more even soil contact, which can enhance their overall load-bearing capacity. They are better at handling vertical loads, and their shape is often preferred for compression-heavy applications.

3. Skin Friction:

- Square Piles: Square piles generally have more skin friction due to the flat faces of the pile. The edges provide additional surface area in contact with the soil, which can be beneficial for transferring load to the surrounding soil. However, the friction may be less uniform than in round piles because of the corners.

- Round Piles: Round piles have less surface area for skin friction compared to square piles, but their smooth, continuous shape results in more uniform contact with the soil. This makes them more efficient in softer soils where uniform friction is needed for load transfer.

4. Construction and Installation:

- Square Piles: These piles are often easier to form in the field, particularly in tight spaces, since the corners provide more stability when forming the pile. Square piles are easier to align and typically do not require as precise a method for forming as round piles.

- Round Piles: Round piles, while requiring a bit more precision during the formation process due to their circular shape, have the advantage of a smoother and more predictable installation. The round shape allows for better penetration into the ground, especially in dense or cohesive soils. The smoother surface reduces resistance during installation.

5. Soil Penetration:

- Square Piles: Square piles may face more resistance when being driven or drilled into dense soils or rock layers due to the sharp edges of the pile. However, they are more efficient in resisting lateral forces during installation.

- Round Piles: Round piles typically penetrate the soil more easily during installation, especially in stiff or hard soils, because their continuous circular shape provides less resistance. However, their smooth edges may not "bite" as effectively into hard soils or rock compared to square piles.

6. Bending and Lateral Loads:

- Square Piles: Square piles are better suited to handling lateral loads and bending moments due to their shape. The sharp corners of the square cross-section provide better resistance to bending, making them more suitable for structures where lateral forces or bending moments are a concern.

- Round Piles: Round piles are less efficient at resisting lateral loads and bending moments compared to square piles because of the lack of edges. However, they tend to have better overall vertical load-bearing capacity due to their more uniform shape.

7. Cost and Construction Time:

- Square Piles: Square piles may be easier and quicker to cast in place because they require less precise forming, especially when there are tight tolerances. This may result in cost savings, but the difference is usually not significant unless large quantities of piles are involved.

- Round Piles: Round piles require more precise forming, which can increase the labor and time involved in construction. However, this additional cost may be offset if the site conditions are more favorable for round piles (e.g., in terms of easier penetration and reduced friction).

8. Aesthetic and Practical Considerations:

- Square Piles: In some cases, square piles may be preferred for aesthetic reasons, especially in projects where the pile shape is visible or interacts with other structural elements that have square or rectangular forms.

- Round Piles: Round piles are generally less visually noticeable and are often preferred when the pile will not be exposed, or when the construction involves large amounts of piling that need to be uniform.

9. Applications:

- Square Piles: Square piles are often used in situations where lateral loads, bending, or high shear forces are anticipated, such as in bridge foundations, retaining walls, or high-rise buildings.

- Round Piles: Round piles are more commonly used for applications where vertical loads dominate, such as in marine structures, tall buildings, tunnel supports, or offshore foundations. They are also preferred when high soil bearing capacity is required, and the pile shape ensures uniform load transfer.