290 Pile groups under compressive loading
5.26SCHMERTMANN, J. H., HARTMAN, J. P., and BROWN, P. R. Improved strain influence diagrams,
Proceedings of the American Society of Civil Engineers, Vol. GT8, 1978, pp. 1131–5.
5.27CHANDLER, F. W. and DAVIS, A. G. Further work on the engineering properties of Keuper Marl
Construction Industry Research and Information Association (CIRIA), Report 47, 1973.
5.28HAGERTY, A. and PECK, R. B. Heave and lateral movements due to pile driving, Journal of the Soil
Mechanics and Foundation Division, American Society of Civil Engineers, No. SM11,
November 1971, pp. 1513–32.
5.29CHOW, Y. K. and TEH, C. I. A theoretical study of pile heave, Geotechnique, Vol. 40, No. 1, 1990, pp. 1–14.
5.30BJERRUM, L. Engineering geology of normally-consolidated marine clays as related to the
settlement of buildings, Geotechnique, Vol. 17, No. 2, 1967, pp. 83–117.
5.31ADAMS, J. I. and HANNA, T. H. Ground movements due to pile driving, Proceedings of the
Conference on the Behaviour of Piles, Institution of Civil Engineers, London, 1970, pp. 127–33.
5.32BRZEZINSKI, L. S., SHECTOR, L., MACPHIE, H. L., and VAN DER NOOT, H. J. An experience with heave
of cast-in-situ expanded base piles, Canadian Geotechnical Journal, Vol. 10, No. 2, May 1973,
pp. 246–60.
5.33COLE, K. W. Uplift of piles due to driving displacement, Civil Engineering and Public Works
Review, March 1972, pp. 263–9.
5.34FLEMING, W. G. K. andPOWDERHAM, A. J. Soil down-drag and heave on piles, Institution of Civil
Engineers, Ground Engineering Group, notes for meeting on 25/10/89.
5.35HOOPER, J. A. Observations on the behaviour of a piled-raft foundation on London Clay,
Proceedings of the Institution of Civil Engineers, Vol. 55, No. 2, December 1973, pp. 855–77.
5.36COOKE, R. W., BRYDEN SMITH, D. W., GOOCH, M. N., and SILLETT, D. F. Some observations on the
foundation loading and settlement of a multi-storey building on a piled raft foundation in London
Clay, Proceedings of the Institution of Civil Engineers, Vol. 7, No. 1, 1981, pp. 433–60.
5.37PADFIELD, C. J. andSHARROCK, M. J. Settlement of structures on clay soils, Construction Industry
Research and Information Association (CIRIA), Special Publication 27, 1983.
5.12 Worked examples
Example 5.1
Bored piles 500 mm in diameter drilled to a depth of 13.9 m below ground level into a firm
to stiff clay are arranged in a group consisting of 10 rows each of seven piles, each carrying
a dead load of 250 kN and an imposed load of 110 kN. From the results of tests on samples
from three boreholes, the characteristic undrained shear strength of the clay increases from
60 kN/m^2 at 1.5 m below ground surface to 110 kN/m^2 at the base of the pile group. The fis-
sured strength of the clay at pile toe level is 80 kN/m^2. Profiles of the undrained deforma-
tion modulus Euand the coefficient of compressibility mvare shown in Figure 5.43.
Determine the overall stability and settlement of the pile group.
The first step is to calculate the factor of safety of the individual pile under the combined
dead and imposed loads, from equations 4.4 and 4.7:
Ultimate bearing capacity 9 80 /40.5^2 0.45(60110)/20.512.4
141 745 886 kNFactor of safety 886/360 2.5 which is satisfactory, and because of the increasing
strength of the clay below toe level, block failure of the group should not occur. However,
for the purpose of comparison with the recommendation in EC7 Clause 7.6.2.1 to assume
that the pile group acts as a single large-diameter pile, the stability of the group will be
checked for compliance with the EC7 rules.