Slip Coating

Innovative Construction

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 Westport, all piling positions were driven with piles coated with a slip coating of 6mm thick. Instead of the usual spray or pour method described below, coating was done in two specially fabricated dipping tanks capable of dipping piles of up to 12 meter lengths in a single dip. All tanks were fitted with a mini furnace to maintain the necessary heat for dipping.

Due to the high daily temperature in Malaysia, care had to be taken to prevent piles from being stacked to closely together after coating to prevent adhesion of piles to one another.

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 70mm
Softening Point (R and B)        57 to 63 degrees C
Penetration Index                   Less Than +2

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 be mobilized.

REFERENCES

References can be made to publications by Shell International Petroleum Company Limited on Shell Bitumen Slip Layer Compound.