Civil Engineering Reference
In-Depth Information
14.2.5 Correction for gravel content
As noted, the standard compaction test is carried out on soil that has passed the 20 mm sieve. Any coarse
gravel or cobbles in the soil is removed.
Provided that the percentage of excluded material, X, does not exceed 5% there will be little effect on
the derived maximum dry density and optimum moisture content.
If X is much greater than 5% then the test values will be affected and may be quite different to those
pertaining to the natural soil. There is no generally accepted method to allow for this problem but for X
up to 30% some form of correction to allow for the removal of the gravel can be applied as set out below.
The percentage of material retained on the 20 mm sieve, X, can be obtained either by weighing the
amount on the sieve (if the soil is oven dried) or else from the particle size distribution curve.
In the compacted soil this percentage of coarse gravel and cobbles has been replaced by an equal mass
of soil of a smaller size. Generally the particle specific gravity of the gravel will be greater than that of
the soil of smaller size so the volume of the gravel excluded would occupy a smaller volume than that of
the soil which replaced it.
If
ρ
d
=
maximum dry density obtained from the test then:
X
Mass of gravel excluded
=
100
ρ
d
Considering unit volume, the volume of soil that replaced the gravel
=
X, so:
X
ρ
d
Volume of gravel omitted
=
100
ρ
G
w s
where G
s
=
particle specific gravity of excluded gravel.
X
X
ρ
X
ρ
d
d
⇒
Difference in volume
=
−
=
100
1
−
100
100
ρ
G
ρ
G
w s
w s
ρ
d
Corrected maximum dry density
=
X
ρ
d
w s
1
−
100
1
ρ
−
ρ
G
d
=
X
ρ
+
d
1
−
1
100
ρ
G
s
w
Similarly:
100
100
−
X
w
Corrected optimum moisture content
=
where w
=
the optimum moisture content obtained from the test.
Even when a gravel correction is applied, compaction test results are not representative for a soil with
X more than 30%. BS 1377 suggests that in such cases the CBR mould (a mould 152 mm in diameter and
127 mm high) can be used. The compaction procedure is similar to that for the standard test except that
the soil is compacted in the bigger mould in three equal layers with the same 2.5 kg rammer falling 300 mm
but the number of blows per layer is increased to 62.
Correction for the excluded gravel (i.e. the particles greater than 37.5 mm) can be carried out in the
manner proposed for the 20 mm size.
