Environmental Engineering Reference
In-Depth Information
water content is confirmed. Some specifications allow for disputes to be settled based on
the measured water content.
Other methods which can be used to allow a more rapid determination of water con-
tent and, hence, density ratio, include drying the soil by:
-
microwave oven;
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methylated spirits;
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high temperature oven;
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heating the soil on a hot plate with gas burners.
These methods are more approximate, and require calibration between them and water
contents determined in the standard oven technique, to allow for adsorbed water being
driven off by high temperature drying.
An alternative method of routine compaction control is the use of nuclear methods for
determination of density and water content. In this method gamma rays are used to deter-
mine the density and neutrons to determine the water content. This may be done by direct
transmission, backscatter or less commonly the air gap methods. The most common
method is backscatter, because of the difficulty of penetrating the source into the com-
pacted fill, particularly if the fill has gravel particles in it.
The method requires calibration against compacted materials of known density and
water content from the site. Provided this is done, and the earthfill is not too variable or
has too high a gravel content, the method can be adequate for routine control.
14.6.2
Compaction control - some common problems
Some common problems which arise in compaction control and which lead to disputation
between contractor and engineer include:
(a) Specifying too high a compaction standard, e.g. 100% density ratio, standard com-
paction or 98% density ratio, modified compaction, for clay soils, or 100% density
index for granular soils. These are virtually unobtainable, even with very heavy rolling
equipment and, as discussed in Section 14.2, is undesirable for dam construction
because it can only be achieved by compacting the clay fill dry, resulting in a brittle,
permeable fill, or by overcompacting granular filter materials, giving excessive
breakdown.
(b) Specifying unnecessarily restrictive water content range. Specification limits must be
realistic to match the available materials. If the soil in the borrow area is, say, 4% wet
of optimum in the borrow area, and the climate is wet, it is pointless requiring 98%
density ratio at OWC
1%. The specification would be more realistically 95% den-
sity ratio at optimum to optimum
4%, or specified as detailed in Section 14.6.3.
(c) Carrying out insufficient laboratory compaction tests. The density ratio is obtained by
comparing the density in place with the maximum dry density obtained in the labora-
tory. Ideally the soil for this laboratory compaction is sampled beside the density in
place test. However, it is common practice to reduce the work involved by doing one
laboratory compaction for every 2 to 4 or more density in place tests, assuming the
soils are uniform. This is almost invariably not true and a small change in the maxi-
mum dry density can make the difference between acceptance and failure. Disputes
often arise when such short cuts are used.
(d) Breakdown of materials during compaction. Soils which contain pieces of weathered
rock, or gravels which break down under compaction, will often give a higher labo-
ratory maximum dry density if the compaction test is carried out on soil dug next to
the density in place test hole, i.e. on soil already compacted and broken down by the
