Pile Design and Construction Practice, Fifth edition

loading and at more than 6 to 8 diameters parallel to this direction. Piles at a closer spacing
can be considered to act as a single unit in order to calculate the ultimate resistance and
deflections under lateral loads (Figure 6.42). In soft clays and sands the effect of driving
piles in groups at close spacing is to stiffen the soil enclosed by the group thus increasing
its capability as a single unit to resist movement when carrying horizontal loading.
Calculations to determine the ultimate bearing capacity of pile groups carrying vertical
and horizontal or inclined loading can be performed using the Brinch Hansen general
equation (Section 5.2). The calculations assume that the pile group takes the form of an
equivalent block foundation. EC7 in Clause 7.6.2.1(4) states that the resistance of the group
to compression loading can be calculated by assuming that the group acts as a single large-
diameter pile. However, Clause 7.6.3.1 requires the resistance of a group subjected to ten-
sion loading to be provided by the frictional resistance of the soil enclosing a block
foundation. No guidance is given in respect of pile groups carrying transverse loading.
Clause 7.7.1(4)P merely requires group action ‘to be considered’.

6.6 References

6.1 RADHARKRISHNA, H. S. and ADAMS, J. I. Long-term uplift capacity of augered footings in fissured
clay. Canadian Geotechnical Journal, Vol. 10, No. 4, November 1973, pp. 647–52.
6.2 MEYERHOF, G. G. and ADAMS, J. I. The ultimate uplift capacity of foundations, Canadian
Geotechnical Journal, Vol. 5, No. 4, November 1968, pp. 225–44.
6.3 UK DEPARTMENT OF ENERGY. Offshore installations: Guidance on design and construction,
HMSO, London, 1984.
6.4 (a) The strength of grouted pile-sleeve connections for offshore structures: Static tests relating to
sleeve buckling.Wimpey Offshore Engineers and Constructors Ltd. HMSO Offshore Technology
Series, OTH.85.223, 1986.
(b) The strength of grouted pile-sleeve connections.A composite report for DoE. HMSO
Offshore Technology Services, OTH.86.210, 1986.
(c) A study of length, longitudinal stiffening and size effects on grouted pile-sleeve connections.
Wimpey Offshore Engineers and Constructors Ltd. HMSO Offshore Technology Series,
OTH.86.230, 1987.
6.5 LITTLEJOHN, G. S. Soil Anchors, Proceedings of the Conference on Ground Engineering,
Institution of Civil Engineers, London, 1970, pp. 41–4.
6.6 HUTCHINSON, J. N. Discussion on ref. 6.5, Conference proceedings, p. 85.
6.7 WIMPEY LABORATORIES LTD. unpublished report.

356 Piles to resist uplift and lateral loading

Spacing less than 3B

Direction of

loading

Width of

equivalent

single pile

B 9

B

Figure 6.42Piles at close spacing considered as single unit (after Prakash(6.27)).