Shaft friction — also called skin friction — is the friction force acting along the shaft of a pile in the ground. In calculation it is the total friction force on the shaft divided by the shaft area. Together with the end bearing under the pile toe it determines the capacity of a piled foundation. The Dutch term is kleef.
Positive and negative shaft friction
- Positive shaft friction acts upwards and therefore carries load: the soil grips the pile. A pile that draws almost all of its capacity from it is called a friction pile, used where no competent sand layer lies within reach
- Negative shaft friction acts downwards and is an extra load: when the soil layers around the pile settle, they effectively hang on it. This mainly affects peat and clay layers, typically after a sand fill has been placed or where the groundwater level has been lowered
Negative shaft friction is not a detail. Where a fill is placed on soft ground, the downward force on a pile can be considerable — and it has to be in the calculation.
What it depends on
- Soil type — stiff clay gives relatively high shaft resistance, loose sand considerably less
- Pile type and surface — a rough, cast-in-place pile grips better than a smooth precast element
- Installation — driving, boring or vibrating changes the soil around the pile, and with it the friction
- Time — immediately after installation friction is often lower and then recovers as soil stresses re-establish
Points to watch
- The soil investigation decides — shaft friction and end bearing are derived from cone penetration tests (CPT); without ground investigation any assumption is guesswork
- Site preparation counts — a sand fill placed before piling sets settlement, and therefore negative shaft friction, in motion
- Protection against negative shaft friction — a bitumen coating or a sleeve over the upper part of the pile reduces it substantially
- Have an engineer calculate — extensions and added loads on a piled foundation are specialist work
Related terms
- Driven pile
- Pile bearing capacity
- Piled foundation
- Cone penetration test
- Settlement
- Soil mechanics
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