TECHNICAL EXPERIENCE PAGES
NEGATIVE SKIN COATING
Supply and drive 1900Piling points, between 36m and 45m deep, and each pile point having 24m length of 6mm thick bituminous slip coating, from the ground level. All piles were driven to set.
Due to the nature of the soil at
Due to the high daily temperature in
In order to obtain a thickness of 6mm on all four surfaces each piles had to be dipped 3 times and rotated.
Finally a pull out test was performed to confirm the
adhesion of the bitumen to the pile surface.
MINIMIZING NEGATIVE SKIN FRICTION (Extract from Pile design and Construction Practices by MJ Tomlinson)
The effect of drag-down can be minimized by employing slender piles (e.g. H-sections or prestressed concrete piles, but more positive measures may be desirable to reduce the magnitude of the drag-down forces. Negative skin friction forces on pre-cast concrete or steel tubular piles can be reduced by coating the portion of the shaft within the fill with soft bitumen.
Claessen and Horvat describe the coating of 380X450mm pre-cast concrete piles with a 10mm thick layer of bitumen having a penetration of 40 to 50mm. The skin friction on the 24m piles was reduced to 750kN compared with 1600 to 1700 kN for the uncoated piles.
Shell Composites Ltd markets it's Bitumen Compound SL for coating bearing piles. The material has the following characteristics.
Penetration at 25 degrees C 53 to
The bitumen is heated to 180 degrees C (maximum) and
sprayed or poured onto the pile to obtain a coating thickness of 10mm.
Before coating, the pile surface should be cleaned and primed with Shell
Composites Bitumen Solvent Primer applied by brush or spray at a rate of
about 2kg/10sq.m. Alternatively the SL compound can be fluxed with 29%
white spirit to provide the primer. The bitumen layers should not be
applied over the length of the shaft which receives support from skin
friction, and Claesson and Horvat
recommend that a length at the lower end of ten times the diameter or width
of the pile should remain uncoated if the full end bearing resistance is to
References can be made to publications by Shell International Petroleum Company Limited on Shell Bitumen Slip Layer Compound.