1.4. Load Transfer in Foundations

As everyone knows, the load of any structure is transferred to the ground to the foundation. So, this foundation can be a shallow foundation or deep foundation. Piles are deep foundation they are examples for deep foundation. So, they are capable of transferring the load from the structure through a weak stratum and it can carry till the depth where a hard bearing stratum is available.

Every structure must transfer its weight, known as load, to the ground. This transfer occurs through a foundation, which can be either shallow or deep. Shallow foundations are typically used when the soil near the surface is strong enough to hold the weight, while deep foundations like piles are needed when the surface soil is weak. Piles are specifically designed to reach down to deeper, stronger soil layers (hard bearing strata) that can support the structure's load effectively.

Imagine a large tree. The trunk represents the building, while the roots symbolize the foundation. In soft soil, only shallow roots (shallow foundation) may not be enough to support the heavy trunk, and the tree could topple. Therefore, the tree sends deep roots (piles) into the ground where the soil is firm (hard bearing strata) to stabilize itself.

The load transfer can be either on the basis of end bearing or basis of friction, that we are going to discuss in the upcoming slides. So, basically your pile foundation you consider it will act like a column, pile is like a column driven into the soil.

There are two main mechanisms by which piles transfer load to the ground: end bearing and friction. In the end bearing mechanism, the pile transmits its load directly to the solid soil layer at the bottom, functioning like a column pushing down. In contrast, friction piles rely on the resistance between the soil and the sides of the pile to hold the load. This means the friction along the pile's surface supports the weight rather than resting on a solid base.

Think of a pencil: if you press it down onto a table, the pencil's tip represents a pile using end bearing because it pushes directly into a hard surface. Now, if you hold the pencil against a wall and push, the friction along its length keeps it from falling—similar to how friction piles work.

So, when do we go for the spell foundation? Basically, when we have weak soils or soil with poor bearing capacity, we need to transfer the load till we reach a depth where you confined a hard load bearing strata. So, till that depth we need to transfer the load, so for that we need a deep foundation like pile.

Deep foundations like piles are used in situations where the surface soil is not strong enough to bear the weight of a structure. When engineers encounter weak or unstable soil conditions, they will design the foundation to extend deeper until they reach more stable soil layers that can safely support the structure.

Consider a person trying to stand on loose sand at the beach—it's likely they will sink. However, if they dig down to firmer ground underneath the sand, they can stand without sinking. This is similar to how deep foundations work—they ensure the structure 'stands firm' by reaching down to stronger, stable soil.

Based on the application, so commonly, there are different types of uses of piles. So, I am just going to discuss few common applications, so one is a sheet pile and other one is your end bearing pile and friction pile.

Piles can be classified based on their application. For example, sheet piles are primarily used as barriers to retain earth and water during excavations. End bearing piles are specifically designed to transfer loads directly to hard strata, while friction piles depend on lateral soil friction for load support. Each type of pile serves specific purposes in construction depending on the soil conditions and structural requirements.

Think of building a dam: sheet piles act like a wall holding back water, much like a solid fence blocks wind. End bearing piles can be likened to a bridge's support columns resting firmly on the bedrock. Meanwhile, friction piles are like climbing a steep wall, where your grip against the wall provides the support you need to stay upright.

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