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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
In a comparative study based upon case records, Dikran found that the ratio of calculated/observed
settlements fell in the range 0.21—2.72, for four traditional methods of calculation using the CPT.
For the SPT the variation was 0.15—10.8.
When calculating the point resistance of piles in sand based upon cone resistance, it is normal to
consider the static cone penetrometer as a model of the pile, and simply apply a reduction factor of
between two and six to give allowable bearing pressure (Van der Veen and Boersma 1957; Sanglerat
1972). Sand deposits are rarely uniform, and so an averaging procedure is used with the q c values
immediately above and below the proposed pile tip position (Schmertmann 1978). The side friction
of piles may be calculated directly from the side friction of the cone, or by correlation with cone
resistance.
In cohesive soils, the CPT is routinely used to determine both undrained shear strength and
compressibility. In a similar way to the bearing capacity of a foundation, cone resistance is a function
of both overburden pressure (σ v ) and undrained shear strength (c u ):
q c = N k C u +σ v (3.8)
so that the undrained shear strength may be calculated from:
c u = q c -σ v
N k
(3.9)
provided that N k is known, or can be estimated. The theoretical bearing capacity factor for deep
foundation failure cannot be applied in this equation because the cone shears the soil more rapidly
than other tests, and the soil is failed very much more quickly than in a field situation such as an
embankment failure.
At shallow depths, or in heavily over-consolidated soils, the vertical total stress in the soil is small, so
that:
c u q c
N k
(3.10)
Typically, in these conditions, the undrained shear strength is about 1/15th to 1/20th of the cone
resistance.
N k is not a constant, but depends upon cone type, soil type, overconsolidation ratio, degree of
cementing, and the method by which undrained shear strength has been measured (because
undrained shear strength is sample-size and test-method dependent). The N k value in an overconsolidated
clay will be higher than in the same clay when normally consolidated
Typically, N k varies from 10 to 20. Lunne and Kleven have shown that this variation is significantly
reduced, giving N k much closer on average to 15, if a correction (N k * = N k /µ) is made to allow for
rate effects, in a similar way to that proposed by Bjerrum for the vane test (see below), but this is
rarely done in practice.
3.3 FIELD VANE SHEAR TEST (VST)
The vane shear test (VST), or field vane (FV), is used to evaluate the in-place undrained shear
strength (s uv ) of soft to stiff clays & silts at regular depth intervals of 1 meter (3.28 feet). The test
consists of inserting a four-bladed vane into the clay and rotating the device about a vertical axis.
Limit equilibrium analysis is used to relate the measured peak torque to the calculated value of s u .
Both the peak and remoulded strengths can be measured; their ratio is termed the sensitivity, S t . A
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