4.1. Cast in Situ Concrete Piles

The cast in situ concrete piles are concrete piles that are created by pouring concrete into holes drilled or driven into the ground. These can be created either through the driven method or the boring method.

Cast in situ concrete piles are constructed directly at the designated location by inserting formwork into the ground and pouring in concrete. This type of pile is beneficial because it can be shaped and adjusted based on the specific needs of the site. There are two main methods for creating these piles: the displacement method, where the formwork is driven into the soil displacing it, and the non-displacement method, where the soil is removed without creating significant disturbance around the hole. Both methods serve unique purposes depending on the soil conditions and the requirements of the construction.

Think of cast in situ concrete piles like pouring concrete into a mold directly where you want your sculpture to be made. Imagine if the sculpture needs to fit perfectly within the soil; by pouring concrete at the site, you ensure it takes exactly the shape needed to anchor into the ground.

In the displacement method, a steel casing is driven into the ground, which displaces surrounding soil, whereas in the non-displacement method, an auger is used to remove soil without disturbing the surrounding area.

In the displacement method, a steel casing is hammered into the ground to create a space for the concrete. This method is useful for creating piles in certain soil conditions, but it can cause soil displacement, leading to potential issues like increased pore water pressure. The non-displacement method, on the other hand, involves drilling with an auger, which creates a hole without disturbing the surrounding soil too much. This stability is important, especially in sensitive areas like clay soils, where excessive disturbance can lead to structural problems post-installation.

Imagine trying to plant a flower in a garden. In the displacement method, you’re digging out a big hole with a shovel and pushing the soil to the side, maybe disturbing the roots of nearby plants. In the non-displacement method, you’re using a small drill that carefully removes just the dirt, leaving the surrounding plants unaffected and stable while you plant your flower.

The auger boring method involves using a spiral or helical drilling tool to bore a hole into the ground, while simultaneously placing a casing to prevent collapse.

The auger boring method is a technique where a specialized drill, with a spiral design, is rotated into the ground. As it drills down, it pulls the soil up and out of the hole, and simultaneously, a casing is inserted to hold the hole open. Once the desired depth is reached, reinforcement is added, and concrete is poured. This method is particularly effective in clay soil, preventing instability that could arise from soil displacement.

Think of this method like using a corkscrew to open a bottle of wine. Just as the corkscrew twists down into the cork while also pulling it up and out, the auger rotates downwards into the ground while bringing soil upwards, ensuring that the surrounding area remains stable by using a casing.

Concrete is placed using a tremie pipe to ensure it reaches the bottom of the borehole effectively without causing disturbance.

After the reinforcement is set in place, concrete is poured into the hole using a tremie pipe, which allows the concrete to flow down to the bottom of the pile without disrupting the surrounding soil. This method is particularly important because it minimizes potential heaving of the surrounding soils and allows for the concrete to cure evenly in the pile structure. The tremie pipe aids in preventing air pockets in the concrete, which could weaken the integrity of the pile.

Imagine using a straw to drink a thick shake. The straw helps pull the shake smoothly without causing splashes, just as the tremie pipe helps pour concrete without displacing the surrounding soil. This way, you ensure you get the thick shake smoothly at the bottom without messing up your drink – or in this case, your pile.

Depending on the design, after concreting, the casing may be removed for reuse or left in place to enhance pile strength.

Once the concrete has cured, the construction process may involve either extracting the casing for future use or leaving it in position as part of the pile. If left in place, it acts as additional reinforcement and can contribute to the overall strength of the pile. This decision is influenced by various factors such as the design requirements, soil conditions, and general engineering considerations.

Think of a baking dish used to make a cake. If you take out the dish once the cake has cooled, you can reuse it for the next baking. However, if the dish is made of ceramic and needed to keep the cake upright, you might choose to leave it in. Similarly, engineers decide whether to keep the casing based on the structural demands of the pile and the surrounding conditions.

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