Civil Engineering Reference
In-Depth Information
determined from traffic studies and, from these, the percentage of OGV2 vehicles in the total flow is
determined. To establish the cumulative design traffic, use is made of design charts. The charts give the
cumulative design traffic (in millions of standard axles) depending on the forecast traffic flow at opening
together with the percentage of OGV2 vehicles in the flow. Consideration is also given to wear of the
pavement through the traffic loads and the growth in traffic anticipated during the road's design life.
14.5.7 Design life of a road
An important part in the design of a road is the decision as to the number of years of life for which the
road can be economically built. The DMRB suggests a design life of 20 years for bituminous roads, because
their life may be extended by a strengthening overlay. However, during the design of streets in residential
areas, a design life of 40 years may be considered.
14.5.8 The moisture condition value, MCV
The selection of a satisfactory soil for earthworks in road construction involves the visual identification of
unsuitable soils, the classification tests described in Chapter
1
together with the use of at least one of the
three compaction tests described in this chapter. The compaction test chosen is the one that uses a
compactive effort nearest to the expected construction compactive effort and is used to determine the
optimum moisture content value, i.e. the upper value of water content beyond which the soil becomes
unworkable. The system can give good results, particularly with experienced engineers, but there are
occasions when the assessment of a particular soil's suitability for earthworks is still difficult.
The moisture condition test is an attempt to remove some of the selection difficulties and was proposed
by Parsons in 1976. It is essentially a strength test in which the compactive effort necessary to achieve
near full compaction of the test sample is determined. The moisture condition value, MCV, is a measure
of this compactive effort and is correlated with the undrained shear strength, c
u
, or the CBR value, that
the soil will attain when subjected to the same level of compaction (Parsons and Boden,
1979
).
The test procedure is given in BS177-4:1990 and details of the apparatus are shown in Fig.
14.13.
The
test consists of placing a 1.5 kg sample of soil that has passed through a 20 mm sieve into a cylindrical mould
of internal diameter 100 mm. The sample is then compacted to maximum bulk density with blows from a
7 kg rammer, 97 mm in diameter and falling 250 mm. After a selected number of blows (see Example
14.5)
,
the penetration of the rammer into the mould is measured by a vernier attachment and noted. The test is
terminated when no further significant penetration is noted or as soon as water is seen to extrude from the
base of the mould. The latter requirement is essential if the water content of the sample is not to change.
MCV tests are intended to replace the previous techniques of defining an upper limit of moisture content
for soil acceptability. Generally speaking a soil with an MCV of not less than 8.5, which is comparable to
the
in situ
compactive effort of present day plant, will be acceptable for earthworks. Although no difficulty
will be experienced with testing the majority of soils, particularly those of a cohesive nature, problems
can arise with granular soils with a low fines content, such as glacial tills.
Required calculations
The difference in penetration for a given number of blows, B, and a further three times as many blows
(i.e. 4B blows) is calculated and is plotted against the logarithm of B. The calculation is repeated for all
relevant B values so that a plot similar to Fig.
14.14
of Example
14.5
is obtained. The MCV is taken to be
10 log
10
B where B
=
the number of blows at which the change of penetration
=
5 mm. As is seen from
Fig.
14.14
the chart can be prepared so that the value of 10 log
10
B, to the nearest 0.1, can be read
directly from the plot. The value of 5 mm for the change in penetration value was arbitrarily selected as
the point at which no further significant increase in density can occur. This avoids having to extrapolate
the point at which zero penetration change occurs.
The MCV for a soil at its natural moisture content can be obtained with one test, the wet soil being
first passed through a 20 mm sieve and a 1.5 kg sample collected.
