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

Chapter
5 INTERPRETATION OF SOIL PROPERTIES
Figure 5.2 Factors Affecting Cone Penetrometer Test Measurements
in Soils (Hegazy, 1998)
A general rule of thumb is that the tip stress in sands is q t > 40 atm (Note: one atmosphere ≈ 1
kg/cm 2 ≈ 1 tsf ≈ 100 kPa), while in many soft
to stiff clays
and silts, q t < 20 atm. In clean sands,
penetration porewater pressures are near hydrostatic values
(u 2 ≈ u o ≈ γ w w.z) since the permeability is
high, while in soft to stiff intact clays, measured u 2 are often 3 to l0 times u o . Notably, in fissured
clays and silts, the shoulder porewater readings can be
zero or negative (up to minus one
atmosphere, or - 100 kPa). With the sleeve friction reading (f s ), a processed value termed the friction
ratio (FR) used:
CPT Friction Ration, FR = R f = f s /q t
(5.1)
With CPT
data, soil classification can be accomplished using a combination of two readings (either
and f s or q t and u o ),
or with all three readings. For this, it is convenient to definee a normalised
porewater pressure parameter, B q , defined by:
q t
5-4
March 2009