Pile Design and Construction Practice, Fifth edition

From the above table the pile head deflection is satisfactory and the moment of resistance
of the pile is not exceeded. Because the piles are spaced at 125/46.7 2.67 diameters
the group will act as a single unit equivalent to a block foundation having a width of
5 1.25 m 6.25 m and a depth below the ground surface of 9 m. The ultimate passive
resistance to the horizontal thrust from a block foundation is

For a safety factor of 1.5 on the passive resistance, the total horizontal load on the pile group
must be limited to 11 912/1.5 7941 kN. Thus the load on each pile must be limited to
7 941/36 220 kN, which provides a safety factor of 2 on the value estimated for the single pile.
It is also necessary to calculate the horizontal deflection of the pile group under the
working load of 220 kN per pile. The values of PFabove show that the horizontal load is
effectively distributed over a depth of 4 m below the ground surface. Thus the load on the
group can be simulated by a block foundation having a ‘width’Bof 4 m, a ‘length’of 6.25 m
and a depth of 6.25 m. The elastic modulus of a medium-dense sand can be taken as
20 MN/m^2. From equation 5.22, with H/B 1000, L/B 6.25/4 1.55, D/B 6.25/4 1.55,
 1 0.85, and  0 0.91 as Figure 5.20:

elastic settlement

This is within safe limits and, as would be expected, it is more than 10 times greater than the
deflection of the single pile.

Example 6.6

In the previous example the horizontal wind load of 36 220 7 920 kN is applied at
the level of the centroid of a 7.25 m square by 2.0 m deep pile cap. If a centrally applied
vertical load of 8 000 kN also acts at the centroid, find the resultant vertical component of
the load on each pile of the group due to the combined horizontal and vertical loading.
Weight of pile cap 7.25^2 2.09.812.4 2 475 kN. Thus the total vertical
load 8 0002 475 10 475 kN. From Figure 6.45 the resultant of the horizontal and
vertical loads cuts the underside of the pile cap at a point 0.76 m from the centroid. From
equation 6.51.

i

0.85  0.91  220 ^36

4  6.25

 4  1000

20  1000

49 mm

Pu 12 BD^2 Kp 12  6.25  1.3  9.81  92 ^ 3.69 11 912 kN.

366 Piles to resist uplift and lateral loading

x(m) 0 0.5 1.0 1.5 2.0 2.5 3.0 4.0 5.0

0 0.27 0.53 0.80 1.06 1.33 1.60 2.13 2.66

Fy 0.92 0.90 0.82 0.71 0.61 0.50 0.37 0.18 0.04
yF(mm) 12.6 12.4 11.3 9.7 8.4 6.9 5.1 2.5 —
Fm 0.91 0.65 0.40 0.18 0.03 0.10 0.19 0.25 0.21
MF(kN/m)  411  293  180  81  14 4.5  86  113  95
Fp 0 0.25 0.45 0.57 0.62 0.62 0.57 0.38 0.13
PF(kN/m) 0 32.0 57.6 73.0 79.4 79.4 73.0 48.6 16.6

z x
T