Environmental Engineering Reference
In-Depth Information
adjust the values from
Figure 6.48
(based on Poulos, 1978). In
Figure 6.49
,
the stress level
is the ratio of actual stress to failure stress.
In reality it is often very difficult to assess the degree of over-consolidation of sands,
unless they are overlain by clays for which the OCR can be determined by laboratory
oedometer tests, or the erosion/depositional history of the site can be established. In other
cases it is usually necessary to assume the soil is normally consolidated which may give
conservatively low moduli.
The undrained Youngs Modulus E
u
for cohesive soils cannot be estimated with any
degree of accuracy directly from q
c
values. E
u
can be estimated indirectly from CPT or
CPTU, by first estimating S
u
, and then relating E
u
to S
u
by using Figure 6.50.
The Self Boring Pressuremeter (SBPM) can be used to determine the drained modulus in
sandy soils and undrained modulus in clayey soils. Details are given in Clayton et al. (1995).
The SBPM test gives the most reliable means of determining the modulus, but is rela-
tively expensive. Usually it would only be used on important projects, and then often as a
means of calibrating CPT tests.
The Menard Type Pressuremeter is inserted into a pre-drilled borehole. It is very depend-
ent on the drill hole staying open and is affected by disturbance, particularly in sands and
weak clays. More recent developments of the Menard Pressuremeter include a push-in pres-
suremeter and a self boring type. These should have a similar accuracy to the SBPM.
6.2.2.4
Laboratory testing
The use of laboratory triaxial tests to determine the modulus of a soil is discussed in Baldi
et al. (1988), Jardine et al. (1984), Atkinson and Evans (1984). They point out that con-
ventional triaxial equipment is not able to measure the modulus and, if used, will gener-
ally significantly underestimate the modulus. Generally it will be better to rely on
estimated moduli, or values determined by
in-situ
testing as described above.
1600
1400
1200
1000
800
600
I
p
30
400
30
I
p
50
200
I
p
50
0
1
1.5
2
3
OCR
456789 0
Figure 6.50.
Relationship between undrained modulus at a stress ratio of 25% of the ultimate strength
(Eu
25
) versus over-consolidation ratio (OCR), and plasticity index (PI) (Lunne et al., 1997;
after Duncan and Buchignani, 1976).
