Negative Skin Friction

‘Negative skin friction’ or ‘down drag’ is a phenomenon which occurs when a soil layer

surrounding a portion of the pile shaft settles more than the pile. This condition can develop

where a soft or loose soil stratum located anywhere above the pile tip is subjected to new

compressive loading. If a soft or loose layer settles after the pile has been installed, the skin-

friction-adhesion developing in this zone is in the direction of the soil movement, pulling the

pile do Negative skin friction may also occur by the lowering of ground water which

increases.

Negative skin friction may also occur by the lowering of ground water which increases the

effective stress inducing consolidation and consequent settlement of the soil surrounding the

pile.

LOAD TEST ON PILE

Load test on a pile is one of the best methods of determining the load-carrying capacity of a

pile. It may be conducted on a driven pile or cast-in-situ pile, on a working pile or a test pile,

and on a single pile or a group of piles. A working pile is one which forms part of the

foundation, while a test pile is one which is used primarily to check estimated capacities (as

predetermined by other methods).

Procedure

The most common procedure is the test in which the load is maintained slowly. About five to

eight equal increments are used until the load reaches about double the design value. Time-

settlement data are recorded for each load increment. Each increment is maintained until the

rate of settlement becomes a value less than 0.25 mm per hour. The final load is maintained

for 24 hours.

Often the definition of ‘failureload’ is arbitrary. It may be taken when a predetermined

amount of settlement has occurred or where the load-settlement plot is no longer a straight

line. If the ultimate load could be found, a suitable factor of safety—2 to 3—may be used to

determine the allowable load.

The allowable load on a single pile may be obtained as one of the following [I.S: 2911 (Part

I)-1974]:

1. 50% of the ultimate load at which the total settlement is equal to one-tenth the

diameter of the pile.

2. Two-thirds of the load which causes a total settlement of 12 mm.

3. Two-thirds of the load which causes a net (plastic) settlement of 6 mm (total

settlement minus elastic settlement).