When the soil layer surrounding a portion of the pile shaft settles more than the pile, a downward drag occurs in pile, then the drag is known as -

Question

Prepp Team · Accepted Answer

Understanding Downward Drag in Pile Foundations

Pile foundations are essential for transferring structural loads through weak or compressible soil layers to stronger strata below. This load transfer occurs primarily through two mechanisms: end bearing at the pile tip and skin friction along the pile shaft.

Skin Friction (also known as shaft resistance) is typically an upward force acting on the pile shaft, developed due to the shear resistance between the pile surface and the surrounding soil. This upward force helps in supporting the vertical load applied to the pile.

Identifying the Downward Drag Phenomenon

The question describes a specific scenario: "When the soil layer surrounding a portion of the pile shaft settles more than the pile, a downward drag occurs in pile". This indicates a situation where there is a relative downward movement of the soil compared to the pile.

When the soil settles more than the pile, the friction developed along the pile shaft is no longer acting upwards to support the load. Instead, the downward-moving soil layer pulls down on the pile, creating a downward force on the pile shaft.

What is Negative Skin Friction?

This downward drag or frictional force acting along the pile shaft due to the surrounding soil settling more than the pile is known as Negative Skin Friction.

Negative skin friction adds an additional load to the pile foundation, which the pile must be designed to withstand in addition to the structural loads. It reduces the effective load-carrying capacity of the pile because a portion of the potential shaft resistance is acting downwards instead of upwards.

Soil-Pile Interaction and Friction
Scenario	Relative Soil Movement	Friction Direction	Name
Pile moves down more than soil (under normal load)	Upward relative soil movement (or downward pile movement)	Upward	Normal Skin Friction
Soil settles down more than pile	Downward relative soil movement	Downward	Negative Skin Friction

Analysis of Options

Negative vibration: This term is not a standard geotechnical term related to pile loading or settlement drag.
Negative skin friction: This term accurately describes the downward drag force on a pile caused by surrounding soil settlement.
Friction piles: This refers to a type of pile whose primary support comes from normal skin friction, not the downward drag force itself.
Single acting hammer: This is a piece of construction equipment used for driving piles, unrelated to the forces acting on a pile after installation due to soil settlement.

Based on the definition and the specific condition described where the soil settles more than the pile, the downward drag experienced by the pile is correctly termed Negative Skin Friction.

Revision Table: Forces on Piles

Force Type	Direction	Cause	Effect on Pile
Structural Load	Downward	Weight from structure	Must be supported by pile capacity.
End Bearing	Upward	Resistance at pile tip	Contributes to capacity.
Normal Skin Friction	Upward	Shear along shaft (soil <=gt; pile)	Contributes to capacity.
Negative Skin Friction	Downward	Soil settles more than pile	Adds load, reduces effective capacity.

Additional Information on Negative Skin Friction

Negative skin friction is particularly relevant in situations involving consolidation of compressible soils, such as clays, often triggered by new construction, placement of fill material, or groundwater level changes. It can significantly increase the axial load on a pile or pile group, potentially leading to excessive settlement or structural failure if not accounted for in the design.

Engineers use various methods to estimate the magnitude of negative skin friction and design piles strong enough to withstand this additional load. Sometimes, mitigation measures like coatings or sleeves are used to reduce the negative skin friction force.