Engineering geology of British rocks and soils Mudstones of the ...

Plasticity index (%)
Plasticity index (%)
Plasticity index (%) Plasticity index (%)
50
40
30
20
10
0
40
30
20
10
0
40
30
20
10
0
40
30
20
10
0
0
Area 1 LKM Area 1 LKSB
Area 1 MKM Area 2
Area 3 Twm Area 4
Area 5 Area 6
10 20 30 40 50 60 70 80 90
The results of liquid and plastic limit tests are shown in
Figures 7.5 and 7.6 as traditional Casagrande plasticity
plots (liquid limit, w L vs. plasticity index, I P), and in
Figure 7.7, 7.8 and 7.9 as special plasticity plots of plastic
limit, w P vs. plasticity index, I P. The latter attempts to
display the data as independent, rather than partially
dependent, relationships as is the case with the former.
This results in non-linear relationships that in some cases
show groupings according to area or formation. Also, the
whole of the plot area is available, unlike the normal
Casagrande plot where only half of the plot is available due
to the fact that plasticity index cannot exceed liquid limit.
A total of 2598 liquid limit results are contained in the
Liquid limit (%) Liquid limit (%)
38
0
10 20 30 40 50 60 70 80 90
Figure 7.6 (above and opposite) Casagrande plasticity plots — liquid limit, W L vs. plasticity index I P (subdivided by area).
database. Values for liquid limit range from 11 to 133%.
Overall medians for the ten areas range from 30 to 52%.
Areas 1 and 9 have notably higher liquid limits than the
other areas. The respective overall medians for these two
areas are 44 and 52%. Samples with liquid limits greater
than 70% (that is ‘very high’ or ‘extremely high’ plasticity)
represent less than 0.5% of all data. There is a general
trend of increasing liquid limit with increased weathering
(i.e. higher weathering zone), although this increase does
not appear to be ‘linear’. For example, whilst significant
change is seen from Zone 2 to 2/3 or from 4a/4b to 4b,
little or no change is seen between 3a and 4a. The highest
value recorded (133%) is for glacially reworked material