What is Deep Foundation (Pile Foundation)? Types, Uses & Load Bearing Piles Explained

📌 Introduction

Pile foundations are a type of deep foundation used to transfer building loads to deeper, stronger soil layers. When surface soil can't support a structure, piles provide the necessary stability.

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🏗️ What is Pile Foundation?

• Used to transmit load of a structure to hard strata or deeper soil.
• Constructed by driving or boring long, slender members into the ground.
• Load is transferred either through the pile tip or via friction along sides.
• Used where surface soil is weak or loose and cannot support the structure.

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⚙️ Uses of Piles

1. Heavy load and uneven soil distribution.
2. Subsoil water level rises or falls seasonally.
3. Costly pumping of subsoil water.
4. Raft or grillage foundation is expensive or impractical.
5. Hard strata found at a greater depth (e.g., 20–30 meters).
6. Difficult trenching due to collapsing soil or poor timbering.
7. Construction near:
   • Canals or drainage systems.
   • Seashores or riverbeds (risk of scouring).
8. Used as:
   • Anchors to resist uplift forces.
   • Fender piles to protect docks and piers from damage.

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📂 Types of Piles

Piles are classified into two main types:-

1. Load Bearing Piles
2. Non-Load Bearing Piles

🧱 Load Bearing Piles

These piles support the structure by transferring loads to the soil or rock below.

• Usually placed vertically.
• Batter piles are inclined to resist horizontal forces.
• They can work by:
  • End bearing – resting on hard strata.
  • Friction – resisting through surface contact with soil.

🔹 Types of Load Bearing Piles:

1️⃣ Bearing Piles

• Penetrate through soft soil and rest on a hard layer.
• Used as end-bearing piles or piers.
• Soft soil adds lateral support, improving load capacity.

2️⃣ Friction Piles

• Used when no firm stratum is available at reasonable depth.
• Carry load by frictional resistance with soil around the pile.
• Friction depends on:
  • Soil type and contact area
  • Depth of penetration

Ways to increase frictional resistance:

1. Increase the diameter of the pile.
2. Drive pile to a greater depth.
3. Make the pile surface rough.
4. Place piles closely together.
5. Group the piles.

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📌 Conclusion

Pile foundations are crucial where surface soil can't support structural loads. Whether using bearing piles or friction piles, they ensure a safe and reliable base for buildings and infrastructure.

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🔩  Materials Used in Construction of Load Bearing Piles

The materials which are used in the construction of load bearing piles are as follows:

1. Cast-iron piles
2. Cement concrete piles
3. Sand piles
4. Steel piles
5. Timber piles
6. Wrought-iron piles

Each of the above materials used in the construction of load bearing piles is described in detail below.

🛠️ Cast-Iron Piles

Cast-iron piles are generally hollow. The inside diameter of a pile is about 300 mm and the thickness is around 25 mm. The length of a pile is about 3 meters and with suitable devices, it can be extended to any desired length.

As cast iron is brittle, it is not possible to drive the piles into the ground using a hammer. Hence, special screws are provided at the bottom of the piles, which are then driven like a screw into the ground. These are called cast-iron screw piles.

✅ (1) Advantages of Cast-Iron Piles

• Useful for areas where timber piles are attacked or damaged by insects or worms.
• Suitable for heavy vertical pressure.
• If shocks or vibrations would endanger nearby structures, cast-iron piles are preferred.

⚠️ (2) Disadvantages of Cast-Iron Piles

• Cannot be subjected to shocks or vibrations.
• Unsuitable for works under sea water.

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🧱 Cement Concrete Piles

Cement concrete offers excellent compressive strength. With the development of reinforced cement concrete (R.C.C.), these piles are becoming more popular, rapidly replacing other materials.

R.C.C. piles are divided into two types:

1. Cast-in-situ concrete piles
2. Pre-cast concrete piles

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🏗️ Cast-in-Situ Concrete Piles

In this type, a bore is dug into the ground by inserting a casing. This bore is then filled with cement concrete after placing reinforcement, if needed. The casing may be left in place or removed later.

The types are:

• Cased cast-in-situ concrete piles
• Uncased cast-in-situ concrete piles

Various patented processes exist for each category. One is explained below:

🔹 (1) Cased Cast-in-Situ Concrete Piles

The casing ensures the pile remains vertical, straight, and undamaged. However, keeping the casing with the pile increases the cost. It protects freshly placed concrete from ground pressure, movement, and intrusion. The shell lengths can be adjusted on-site as needed, based on subsoil conditions.

📌 Examples of Cased Cast-in-Situ Concrete Piles

1. Raymond piles
2. Mac Arthur piles
3. Monotube piles
4. Cobi pneumatic mandrel piles
5. BSP base-driven piles
6. Swage piles
7. Button-bottom piles

🔧 (i) Raymond Piles

In 1897, A.A. Raymond developed a practical and economical method for placing cast-in-situ concrete piles using a system known as the Raymond pile system. Two types of Raymond piles are commonly used:

(a) Raymond Standard Concrete Pile

This consists of a thin corrugated steel shell closed at the bottom. The shell is driven into the ground using a steel mandrel or core. When the desired depth is reached, the mandrel is removed. The shell is then inspected, and if found damaged, it's replaced. Concrete is then poured into the shell to complete the pile.

The usual tip diameter is about 200 mm, and spirally wound wires at 80 mm pitch serve as reinforcement.

(b) Raymond Step-Taper Concrete Pile

This pile consists of shell sections of different diameters. The bottom of the first shell is closed with a flat steel plate. Diameter increases in steps of 25 mm with each shell section. The shell sections are joined using screw connections to get the required pile length.

The construction process is the same as the standard Raymond pile.

Advantages:

• Grants on-the-job flexibility of length.
• Permits internal inspection after driving.
• Steel shell left in position protects the concrete filling.

🔩 (ii) Mac Arthur Piles

In this type, a heavy steel casing with a core is driven into the ground. After reaching the desired depth, the core is withdrawn and a corrugated steel shell is placed inside.In this type, a heavy steel casing with a core is driven into the ground. When the desired depth is reached, the core is withdrawn, and a corrugated steel shell is placed inside the casing. Then, concrete is poured in and compacted. The steel casing is gradually withdrawn as the concrete is placed.

🔩 (iii) Monotube Piles

A monotube pile consists of a tapered fluted steel shell without a mandrel. The shell is driven to the required depth, then inspected. If needed, excess shell is cut off. Concrete is filled in, and the shell can be extended by welding. These piles are rigid, watertight, and suitable for many soil types from end-bearing to frictional soils.

🔩 (iv) Cobi Pneumatic Mandrel Piles

This consists of a corrugated steel shell and a Cobi mandrel, made of four curved steel segments held by internal supports around a central expandable core.

The mandrel’s diameter is about 20 mm less than the shell’s. It is inserted into the shell, then expanded by air or nitrogen (at 9 kg/cm² or 0.90 N/mm²) to tightly fit. Then both are driven to the desired depth. The pressure is released, the mandrel collapses and is withdrawn, and concrete is filled into the shell.

🔩 (v) BSP Base-Driven Piles

This consists of a helically welded steel shell. A concrete plug is placed at the bottom, and a pile hammer drives it into the ground. Once at the desired depth, the pile is filled with concrete.

🔩 (vi) Swage Piles

These piles use a pre-cast concrete plug with a slightly conical shape placed at the bottom of a steel shell. Multiple forming stages are shown in the respective figures.