Compaction and Soil Mechanics Aspects of Highway Design - Elements of Soil Mechanics - page 448

With its collar, the mould into which the soil is placed has a diameter of 152 mm and a depth of 177 mm.
The soil is broken down, passed through a 20 mm sieve and adjusted to have the appropriate moisture
content. The final compacted sample has dimensions 152 mm diameter and 127 mm height.
(ii) Static compaction : sufficient wet soil to fill the mould when compacted is weighed out and placed in
the mould. The soil is now compressed into the mould in a compression machine to the required height
dimension. This is the most satisfactory method.
(ii) Dynamic compaction : the weighed wet soil is compacted into the mould in five layers using either the
BS 2.5 kg rammer or the 4.5 kg rammer. The number of blows for each layer is determined by experi-
ence, several trial runs being necessary before the amount of compactive effort required to leave the
soil less than 6 mm proud of the mould top is determined. The soil is now trimmed and the mould
weighed so that the density can be determined.
After compaction the plunger is first seated into the top of the sample under a specific load: 50 N for
CBR values up to 30% and 250 N for soils with CBR values above 30%.
The plunger is made to penetrate into the soil at the rate of 1.0 mm/minute and the plunger load is
recorded for each 0.25 mm penetration up to a maximum of 7.5 mm.
Test results are plotted in the form of a load-penetration diagram by drawing a curve through the
experimental points. Usually the curve will be convex upwards (Curve Test 1 in Fig. 14.11) , but sometimes
the initial part of the curve is concave upwards and, over this section, a correction becomes necessary.
The correction consists of drawing a tangent to the curve at its steepest slope and producing it back to
cut the penetration axis. This point is regarded as the origin of the penetration scale for the corrected
curve.
The plunger resistance at 2.5 mm is expressed as a percentage of 13.2 kN and the plunger resistance
at 5.00 mm is expressed as a percentage of 20.0 kN. The higher of these two percentages is taken as the
CBR value of the soil tested.
Fig. 14.11 Typical CBR results.