Claessen and Horvat(4.63)describe the coating of 380 450 mm precast concrete piles
with a 10 mm layer of bitumen having a penetration of 40 to 50 mm at 25 C. The skin
friction on the 24 m piles was reduced to 750 kN compared with 1600 to 1700 kN for the
uncoated piles.
Shell Composites Ltd. markets its Bitumen Compound SL for coating bearing piles. The
material has the following characteristics:
Penetration at 25 C: 53 to 70 mm
Softening Point (R and B): 57 C to 63 C
Penetration Index: Less than 2The bitumen is heated to 180 C (maximum) and sprayed or poured onto the pile to obtain
a coating thickness of 10 mm. 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
2 kg/10 m^2. Alternatively, the SL Compound can be fluxed with 29% of white spirit to
provide the primer. The bitumen slip layers should not be applied over the length of the shaft
which receives supportfrom skin friction, and Claessen 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 resistanceis to be mobilized.
Negative skin friction is a most important consideration where piles are installed in groups.
The overall settlement of pile groups in fill must be analysed as described in Section 5.5.
The above measures to minimize negative skin friction can be quite costly. In most cases
it will be found more economical to increase the penetration of the pile into the bearing
stratum thereby increasing its capacity to carry the combined loading.
4.9 References
4.1TOMLINSON, M. J. The adhesion of piles driven in clay soils, Proceedings of 5th International
Conference, ISSMFE, London, Vol. 2, 1957, pp. 66–71.
4.2TOMLINSON, M. J. The adhesion of piles in stiff clay, Construction Industry Research and
Information Association, Research Report No. 26, London, 1970.
4.3BOND, A. J. and JARDINE, R. J. Effects of installing displacement piles in a high OCR clay,
Geotechnique, Vol. 41, No. 3, 1991, pp. 341–63.
4.4RANDOLPH, M. F. and WROTH, C. P. Recent developments in understanding the axial capacity of
piles in clay, Ground Engineering, Vol. 15, No. 7, 1982, pp. 17–25.
4.5SEMPLE, R. M. and RIGDEN, W. J. Shaft capacity of driven pipe piles in clay, Symposium on Analysis
and Design of Pile Foundations, American Society of Civil Engineers, San Francisco, 1984,
pp. 59–79.
4.6RIGDEN, W. J., PETTIT, J. J., ST. JOHN, H. D., and POSKITT, T. J. Developments in piling for offshore
structures, Proceedings of the Second International Conference on the Behaviour of Offshore
Structures, London, Vol. 2, 1979, pp. 276–96.
4.7WELTMAN, A. J. and HEALY, P. R. Piling in ‘boulder clay’and other glacial tills, Construction
Industry Research and Information Association (CIRIA), Report PG5, 1978.
4.8TRENTER, N. A. Engineering in glacial tills, Construction Industry Research and Information
Association (CIRIA), Report C504, 1999.
4.9MEYERHOF, G. G. and MURDOCK, L. J. An investigation of the bearing capacity of some bored and
driven piles in London Clay, Geotechnique, Vol. 3, No. 7, 1953, pp. 267–82.
4.10WHITAKER, T. and COOKE, R. W. Bored piles with enlarged bases in London Clay, Proceedings of
6th International Conference, ISSMFE, Montreal, Vol. 2, 1965, pp. 342–6.
220 Resistance of piles to compressive loads