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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
Cone resistance, q c (normally in MPa) can be calculated from:
(3.2)
where F 10cm 2 c = force required to push the cone
.
into the ground, and A c
plan area of
the cone, i. .e.
Local side friction, f s (normally in MPa), can be calculated from:
(3.3)
where F s shear force on the friction
sleeve, and A s = area of the friction sleeve, i.e. 150 cm 2 .
Friction ratio, R f (in %), can be calculated from:
(3.4)
Because of the geometry of the electric cone,
where pore water pressure acts downwards on the
back of the cone end
(Fig. 3.7), the cone resistance will be under- recorded. When
used in deep
water, for example, for offshore investigations,
the force exerted by groundwater will be significant,
and if pore pressuress are measured (with the piezocone), cone resistance can be corrected for this
effect. The corrected, ‘total’, cone resistance, q t is:
q t
where α
typically
t = q c +(1- )u
= ratio of the area of the shaft above the cone end to the area of the c
0.15 to 0.3, and u = pore pressure at the top of the
cone.
(3.5)
2 ),
cone (10 cm 2 3-9
Figure 3.7 Definition of Cone Area Ratio, Α
March 2009
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