Table 3. Summary of Clegg hammer results for uncon-
fined compressive strength.
Coefficient of
Mean
Standard deviation
variation
(kPa)
(kPa)
(%)
December March
December March December March
Site 1
490
228
255
110
52
48
Site 2
2117
1172
917
952
43
81
Site 3
800
428
393
186
49
43
350 psi (17222411 kPa) in December to a range
tions tested in December to avoid the influence of
between 50150 psi (3441033 kPa) in March. The
previous testing.
calculated mean strengths for the December and
Overall, site 1 (Fig. 14a) showed the greatest
March tests are 307 and 170 psi (2166 and 1172
reduction in mean CBR values from the surface to
kPa), respectively. As observed at site 1, site 2 dis-
a depth of approximately 150 mm. At 305 mm be-
plays approximately a 50% reduction in strength
low the surface, the mean CBR values show no sig-
from December to March. As shown in Table 3,
nificant change between December and March.
the coefficient of variation increases significantly
Site 3 (Fig. 14b) showed a reduction in the mean
from 43% in December to 81% in March. Even
CBR values at 150 and 305 mm below the surface.
with the disparity between the coefficient of vari-
At both depths, the individual CBR values exceed
ation, site 2 showed greater compressive strength
the minimum strength requirement. A summary
values than either sites 1 or 3, and since the values
of the statistical analysis is provided in Appendix
are well above the 30-psi limit, the variation is of
C.
little concern.
Site 3 displays the same trend with the greatest
CONCLUSIONS
number of strengths in December ranging be-
tween 75100 psi (517689 kPa) and shifting
Based on the Clegg hammer and DCP test
down to 5075 psi (344517 kPa) for March. For
results, the overall strength of the stabilized areas
site 3, the December mean was 116 psi (799 kPa)
was reduced by approximately 50% during the
and the March mean was 62 psi (427 kPa). This
freezing season of 199697. However, based on the
again displays approximately a 50% decrease in
COE/NED minimum requirement of 30-psi
the mean strength. The coefficient of variation
unconfined compressive strength, we found that
displayed a small change, 49% to 43%, from De-
approximately half of the data from site 1 from
cember to March. It should be noted that even
March fell below the 30-psi limit based on results
with the substantial range of variation in the sta-
from the Clegg hammer tests. The findings from
tistical values, all testing sites showed the general
the DCP data show that the mean strength was
trend of an overall decrease in strength of 50%
below 30 psi in approximately the top 50 mm of
the structure in the testing areas. NED/EPA should
over the freezing season.
consider the findings from this field study (as well
as minimum strength criteria, equipment limita-
DCP results
tions, and the presence of debris within the soil)
Using previously discussed eq 1 and 2, we
when determining the extent of restabilization of
determined that the unconfined compressive
the material.
strength of 30 psi is approximately equivalent to a
Based on the temperature data measured at the
CBR of 10. This finding was used to determine
site, frost penetration for the 199697 freezing sea-
the lower limit of CBR 10 for the DCP data, since
son was approximately 500 mm. Based on the
DCP values are expressed in CBR. This analysis
computer simulations run in the first phase of this
concentrated on four depths below the surface
project, the maximum predicted frost penetration
of the material, 150 mm, 230 mm, 305 mm, and
was approximately 500 mm. The predicted frost
460 mm, to coincide with the depth of frost pene-
penetration from the computer simulations corre-
tration measured at the site. For each site, a mean
lated very well with temperature measurements
CBR value was calculated at each depth. The CBR
from the field. Therefore, the design thickness of
values obtained at each sampling point were then
910-mm base cover would be sufficient to prevent
compared to the mean. The testing in March was
frost penetration into the stabilized waste fill.
performed approximately 150 mm from the loca-
10
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