Civil Engineering Reference
In-Depth Information
32
30
Casagrande
subgroup CH
28
Casagrande
subgroup CL
26
24
22
20
18
16
14
12
10
8
6
4
2
0
10
20
30
40
50 60 70
Undrained cohesion (kN/m
2
)
80 90 100
200
300
400 500600700 800 1000
Figure 2.6
Relationship between undrained cohesion and moisture content for matrix-dominant till.
(Illustration courtesy of CIRIA, London.)
are sometimes used and the resulting
N
values employed in estimating pile capacity.
However the use of the static cone penetrometer test (CPT) is generally advantageous
in thick deposits of silts or fine sands to give a more reliable ground profile. The
method also overcomes difficulties which are commonly met in these soils where water
pressures cause piping into the base of open boreholes giving rise to low
N
values,
although the SPT drive can be extended in such soils and the penetration recorded for
each 75 mm. Values of
N
derived in this way are not however strictly comparable to
those obtained in the normal way. For a given relative density, the
N
value is dependent
on the effective overburden pressure, the over-consolidation ratio of the deposit, and,
for derived parameters such as angles of internal resistance, the grading and shape of
the particles. Of these potential influences, correction to the
N
value for the effective
overburden pressure has been given most attention by a number of researchers. This
is discussed further in section 2.5.4 in connection with medium- and coarse-grained
granular deposits.
The modern electric cone penetrometer has overcome the problem of rod friction,
which was present with the mechanical type, and no correction is required for
cumulative rod weight. The cone provides a measure of end resistance,
q
c
, and sleeve
friction,
f
s
, which can be used for the design of driven piles. Where the static cone is
