Negative skin friction occurs when the load on a pile causes the surrounding soil to consolidate and squeeze the pile downwards. This downward movement of the pile creates frictional forces that resist the pile's movement. Negative skin friction can cause additional stresses on the pile, which can lead to structural failure.

Causes of negative skin friction:

Soil consolidation: When the load on a pile is applied, the soil around the pile consolidates and squeezes the pile downwards. This downward movement of the pile creates frictional forces that resist the pile's movement, which leads to negative skin friction.

Loose soil: Loose soil is more prone to negative skin friction because it has more voids that can be filled with water, leading to consolidation and settlement.

Water table: The position of the water table can also affect the occurrence of negative skin friction. If the water table is high, the soil around the pile can become saturated, leading to consolidation and negative skin friction.

Long term construction: Negative skin friction can also occur in the long term when a pile is subjected to loads over an extended period. This long-term load can cause the surrounding soil to consolidate and lead to negative skin friction.

Conclusion:

Negative skin friction is a common problem in pile foundations. It occurs when the load on a pile causes the surrounding soil to consolidate and squeeze the pile downwards. The causes of negative skin friction include soil consolidation, loose soil, water table, and long term construction. Understanding the causes of negative skin friction is important in designing pile foundations that can withstand this problem.