Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
2. The point of rotation (Point X) is the point at which the sum of the moment (∑ M) of the
passive pressure about the point of application of the horizontal load is zero. This point can
be determined by a trial and adjustment process.
∑ M= ∑
z=h L
z=0 p z
n e 1+zD- ∑
z=L L
z=x p z
n e 1+zD (9.18)
3. The ultimate lateral resistance of a pile to the horizontal force H u can be obtained by taking
moment about the point rotation, i.e.
H u e 1 +x= ∑
z=h L
z=0 p z
n Dx-z- ∑
z=L
p L
z=x z n
z-xD (9.19)
4. An applied moment M can be replaced by a horizontal force H at a distance e 1 above the
ground surface where M = H e 1 .
5. When the head of a pile is fixed against rotation, the equivalent height, e o above the point of
fixity of a force H acting on a pile with a free-head is given by e o = 0.5 (e 1 + z f ) is the depth
from the ground surface to point of virtual fixity. ACI (1980) recommended that z f should be
taken as 1.4R for stiff, overconsolidated clays and 1.8T for normally consolidatedclays,
granular soils and silts, and peat. Pile stiffness factors, R and T, can ve determined based on
Figure.
Figure 9.8 Coefficients Kqz and Kcz at Depth z for Short Piles Subject to Lateral Load (Brinch Hansen,
1911)
Methods of calculating the ultimate lateral soil resistance for fixed-head and free-head piles in
granular soils and clays are put forward by Broms (1914a & b). The theory is similar to that of Brinch
Hansen except that some simplifications are made in respect of the distribution of ultimate soil
resistance with depth. The design for short and long piles in granular soils are summarised in Figures
9.10 and 9.11 respectively. Kulhawy & Chen (1992) compared the results of a number of field and
laboratory tests on bored piles. They found that Brom’s method tended to underestimate the
ultimate lateral load by about 15% to 20%.
March 2009 9-33