Negative skin friction on the pile decreases the load carrying capacity of the pile. Let's understand why this happens:

Negative Skin Friction:
Negative skin friction, also known as dragdown, occurs when a pile undergoes downward settlement while the surrounding soil remains in place. This creates a drag force along the length of the pile, which opposes the pile's capacity to carry loads. Negative skin friction commonly occurs in cohesive soils, such as clay, due to consolidation or creep.

Effects on Load Carrying Capacity:
Negative skin friction has several detrimental effects on the load carrying capacity of the pile, leading to a decrease in its overall capacity:

1. Reduction in Effective Length:
Negative skin friction can reduce the effective length of the pile by causing it to settle. This reduces the embedment depth of the pile, resulting in a decrease in the pile's ultimate capacity to resist loads.

2. Increase in Pile Stresses:
The drag force generated by negative skin friction transfers the load from the pile to the surrounding soil. This increases the stress on the pile, and if it exceeds the pile's design capacity, it can lead to pile failure.

3. Reduction in Shaft Resistance:
Negative skin friction reduces the shaft resistance of the pile, which is the load-carrying capacity provided by the frictional resistance between the pile and the soil. The drag force acting on the pile reduces the effective frictional resistance, leading to a decrease in the overall load-carrying capacity.

4. Pile Settlement:
Due to the downward movement of the pile caused by negative skin friction, the pile experiences settlement. This settlement can result in additional deformations, strains, and stresses in the pile, further reducing its load-carrying capacity.

Conclusion:
In summary, negative skin friction on the pile decreases its load carrying capacity by reducing the effective length, increasing pile stresses, reducing shaft resistance, and causing pile settlement. It is crucial to consider negative skin friction effects during the design and construction of piles in order to ensure their optimal performance and load-carrying capacity.