1.1. Lecture-16: Piles and Pile Driving Equipment (Part 1)

Piles are deep foundations capable of transferring the load from a structure through a weak stratum to a hard bearing stratum at a greater depth.

Piles function as a deep foundation, transferring the load of a structure like a column. When soil conditions are weak or have low bearing capacity, piles are essential. They reach a harder layer underground that can support the weight of the building. The load can be transferred in either of two ways: by end bearing, where the load pushes down onto a solid layer beneath, or by friction, where the load is distributed along the sides of the pile to the surrounding soil.

Imagine trying to plant a large umbrella in soft sand at the beach. If the ground is too weak to support it upright, you would need to push the pole deeper until it hits firmer ground. Similarly, if the sand is too soft, you might also resort to surrounding the pole with concrete to reinforce it, much like using piles in weak soil.

Types of piles include sheet piles, end bearing piles, and friction piles based on their applications. Sheet piles act like a barrier during trenching or excavation, while end bearing piles transfer the load through to hard strata and friction piles distribute load using friction from the soil.

Sheet piles are crucial for containing soil and preventing collapse, especially during excavation. They create a barricade against lateral pressures. End bearing piles penetrate through weak soil until they reach a solid base, transferring loads directly downwards. In contrast, friction piles rely on the resistance along their sides against the surrounding soil to carry loads. Pile types are selected based on soil conditions and structural requirements.

Think of a trampoline: for safety, if you stand on the edge, it flexes and bends. Now, if you jump on the center, your weight is directly supported by the poles underneath—this is like end bearing piles working with strong soil. Alternatively, if you were to squeeze the sides of the trampoline, it would hold up even more weight because of the friction along its sides, similar to how friction piles work.

Timber piles are the oldest type of piles, known for being easy to work with. However, they require treatment to prevent them from rotting and being attacked by insects. The bark must be stripped to reduce the potential for slippage when used as friction piles. Naturally treated with saltwater or modern preservatives, untreated timber piles might only last a few years when above the waterline.

Timber piles are favored for their availability and cost-effectiveness. However, their organic composition makes them vulnerable to decay and insect damage when exposed to fluctuating water levels. By treating the wood with preservatives, we can significantly extend its lifespan. For good performance, timber should be submerged to avoid rotting from air and moisture, but careful assessment of timber quality is necessary to identify defects before use.

Consider wooden boats that have been treated with special oils to resist water damage. Just like these boats can last longer on the water, if timber piles are treated properly, they can also endure longer and remain structurally sound when placed below the waterline.

Concrete piles can be classified as either cast in situ or precast, each with its own methods of installation. Cast in situ piles can be made using either the displacement method, which disturbs surrounding soil, or the non-displacement method, which minimizes disturbance.

Concrete piles offer enhanced strength and resistance to decay compared to timber. Cast in situ piles are poured directly into the ground, which can be done in two ways: displacement method causes significant soil disturbance as concrete is driven into place, while non-displacement method excavates soil without causing much disruption. This distinction is critical when considering soil types and site conditions.

Imagine building a sandcastle. If you push your hand into loose sand to create a tower (displacement), the sand around it shifts. But if you dig out a small hole first (non-displacement) and then pour glue into it, the surrounding sand stays in place, just like non-displacement methods for installing concrete piles.

The auger boring method is a non-displacement method used for installing cast in situ concrete piles by removing soil as the auger rotates, allowing for clean drilling without disturbing the soil too much.

In the auger boring method, a helical auger drills into the ground and removes soil, while simultaneously providing casing to prevent collapse of the bored hole. This installation allows for better control and less pore water pressure buildup compared to the displacement method, making it ideal for clay soils. Once the hole is prepared and reinforced, concrete is poured in using a tremie pipe.

Think of a corkscrew that digs into a cork to remove it from a bottle. As the screw turns, it pulls the cork out without altering the shape of the bottle. Similarly, the auger operates to clear a space from within the soil while keeping the surrounding structure intact.

The displacement method in cast in place concrete piles involves driving a steel casing into the ground, with a mandrel inside to protect it from damage while concrete is poured.

This method is called 'displacement' because driving the casing pushes soil away. It uses a mandrel to minimize damage to the casing, enabling reuse. After the casing reaches the required depth, reinforcement is placed, followed by concrete. Leaving the casing results in a cased pile, while removing it creates an uncased pile—each with its own structural implications.

Imagine using a straw to sip a thick milkshake. You need to push a bit to get it through. If you had a protective tube around the straw, you could push harder without bending it. In construction, this protective casing helps to ensure the structural integrity of the pile when being installed.

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