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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
Because the pore pressure is not always measured at the top of the cone, but
is sometimes
measured either on the face, or on the shoulder, a factor must be applied to the measured pore
pressure. This factor (β) is based upon pore pressure distributions calculated using the
strain path
method. Thus:
qt = q c +(1- )(u 0 +ß∆u)
(3.6)
where β = ratio between the calculated excess pore pressure at the top of the cone and at the point
of measurement, u 0 = hydrostatic
pore pressure, and ∆u = excess pore pressure caused by cone
penetration. Pore pressure distributions measured and calculated around piezocones
are shown in
Fig. 3.8.
Figure 3.8
Distribution
of Excess Pore Pressure over the Cone
(Coutts 1986).
In soft cohesive soils, at depth, much of the cone resistance may be derived from
the effect of
overburden, rather than the strength of the soil. In these circumstances the ‘net cone resistance’
may be calculated:
qn = q c -σ v
(3.7)
3-10
March 2009