642
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
or indicate the load-carrying capacity. It should be noted here
that
CBR
values in pavement design do not reflect the shear
stresses that are generated due to repeated traffic loading. The
shear stress depends on many factors; none of them is fully
controlled or modelled in CBR test (Rico Rodriquez et al. 1988,
Brown 1996).
In laboratory,
CBR
penetration test is performed on material
compacted in a specified mould and placed in loading machine
equipped with a movable base that rises at uniform rate used in
forcing the penetration piston into the specimen. Tested
specimens are penetrated directly after compaction or are to be
previously soaked.
CBR
test
in-situ
is carried out with a
mechanical screw jack for continuous increase of the applied
load to the penetration piston. A reaction forcing the penetration
piston into the soil is provided by a lorry equipped with a metal
beam and attachments under its rear.
The dynamic
CBR
,
CBR
d
, test can be performed both in
laboratory and
in situ
. The test can be conducted as an
alternative to the static
CBR
test, especially due to the short
period of time required.
CBR
d
advantage, compared with the
classic
CBR
, is the elimination of a loading frame necessary in
static loading. The
CBR
d
test is carried out with the use of Light
Weight Deflectometer
,
where a falling weight is used to
generate a defined load pulse on the
CBR
piston.
CBR
d
is
calculated on the basis empirical formula (Zorn 2002) as:
(2)
where 87.3
is the number standing as a value of dynamic
loading including empirical coefficient, and
s
is the settlement
in millimetres.
CBR
d
is recommended to specify when it is
greater or equalled 20% and is equalled or lower than 150%.
Turnbull and Foster (1956) carried out broad studies on
CBR
for compacted mineral soils. They determined penetration
resistance of unsoaked samples of lean clay, compacted by
means of four different energy values and at different moisture
contents. It was proved that the
CBR
value for compacted clay
is a function for both water content as well as dry density.
Compacted samples reached higher
CBR
values when higher
energy values were applied. Moisture increase of compacted
samples decreased
CBR
value and in cases of compacted
samples with moisture contents greater than optimum water
content, penetration resistance was close to zero. Soaking of
samples caused the decrease of
CBR
value, quite significant in
compacted samples – dry of optimum, less significant at
optimum water content. The smallest decrease was observed in
samples compacted at wet of optimum. Rodriguez et al. (1988)
described
CBR
dependence on compaction parameters–
moisture contents and dry densities, as well as on conditions of
compaction– energy and methodology of compaction. The
authors point to the fact that the
CBR
value of the soil
compacted with higher energy value may be lower than that
resulting from the compaction with lower energy value.
CBR
dependence on moisture in the process of compaction was
confirmed in the course of studies conducted by Faure and
Viana Da Mata (1994). The authors straightforwardly claim that
dry density resulting from the compaction of a sample does not
have any impact on
CBR
value which, on the other hand is
influenced by moisture present in the process of compaction.
CBR
’s relationship with moisture content was also observed in
the case of compacted marl from Saudi Arabia (Aiban 1995),
where marl was subjected to tests at moisture optimum and
moisture on the dry and wet sides of optimum. Moisture–
density curves and
CBR
(
w
) dependency curves were said to be
similar; the highest
CBR
values were obtained at optimum
moisture. The studies of the samples tested immediately after
compaction and the soaked samples confirmed that the effect of
soaking is decreased when the samples are compacted at
moisture greater than optimum.
Zabielska-Adamska (2006 and 2011) concluded that the
highest
CBR
values for unsoaked samples of fly ash (class F)
appear in modified compaction – in case of moisture level
below optimum, and in standard compaction – in case of
moisture level within or slightly below optimum. In saturated
samples, the highest values for bearing ratio
CBR
are present in
moisture level equal optimum for both compaction energy
levels. Once optimum moisture is exceeded,
CBR
value drops
dramatically, regardless of the compaction energy and method
of preparation of samples, soaked or unsoaked. High moisture
results in the loss of contact among fly ash grains. Hence
CBR
value dependence on moisture level of fly ash is quite apparent.
CBR
of samples compacted by means of modified method for
optimum moisture is almost twice as high than in the case of
optimum compaction by standard method, which points to a
significant influence of compaction energy and dry density. It is
interesting how compaction energy influences
CBR
in samples
of the same level of moisture, compacted, however, with the use
of different energies. Ash samples with moisture value
w,
compacted by Proctor modified compaction, where
w
>
w
opt1
,
show far lower
CBR
than samples of the same moisture level
w
,
but compacted by standard method where
w
<
w
opt2
. The lowest
CBR
in the analysis of various samples of fly ash was obtained
in case of fly ash of the finest graining which influences
increase of optimum moisture and decrease of density of solid
particles. Zabielska-Adamska and Sulewska (2009) studied
relationships between
CBR
and analysed parameters of various
samples of fly ash by means of Artificial Neural Networks
(ANNs) and as a result concluded that the most relevant
variables were
d
and relation
w
/
w
opt
, which confirms the fact
that optimum water content and moisture content at compaction
are the most significant parameters in
CBR
. Dry density, as
another significant parameter, should be considered as dominant
when comparing
CBR
values for different fly ash shipments
compacted with the use of different energies.
(%
The results of the dynamic
CBR
are extremely poorly
represented in the literature, which is probably due to a low
prevalence of this method in the world. The first study of
CBR
d
,
done on the road mineral materials, were presented by
Weingard et al. (1986). A good correlation between test results
was obtained using static and dynamic method. A study
conducted by Schmidt and Volm (2000) is the only one known
to the authors of this paper which presents results of research
with
CBR
d
carried out on cohesive soil with different
compaction. The studies were conducted for silty clay with
moisture content grade from 11 to 18%, and optimum water
content established as 15.6%. As a result of laboratory studies,
the researchers obtained two curves
CBR
d
(
w
) and
CBR
(
w
),
shifted in relation to each other by approximately 5–7%. In case
of moisture content greater than optimum, the difference
between static values and dynamic values changed to approx.
9%. Higher bearing ratio was obtained in dynamic studies.
CBR
d
is recommended for control research in embankment
erection with the use fine grained soils compacted at moisture
contents lower than optimum.
2 LABORATORY TESTS
All the tests were conducted on the basis of fly ash from hard
coal burning in Bialystok Thermal-Electric Power Plant, stored
at a dry storage yard. The fly ash shipment corresponded in
graining to sandy silt. Physical parameters are shown in Tab. 1.
The laboratory
CBR
tests were carried out to establish
relationship between bearing ratio and fly ash compaction. The
tested samples were compacted by two methods: the Standard
Proctor and the Modified Proctor at moisture contents within
the range of
w
opt
±5% for each compaction method. The fly ash
samples were saturated 24 hrs prior to the test so that their
moisture content could increase by approx. 2.5%. After that,
they were deposited in sealed containers. Each compaction
curve point was designated on a separate sample. During the
compaction tests, individual samples of fly ash were used only
) 3
9
CBR
.87
5.0
s
