California Bearing Ratio (CBR) or Bearing Ratio, is the ratio of the force required to penetrate a circular piston of 1935mm2 cross-section into soil in a mould at a rate of 1.27 mm per minute, to that required for similar penetration into a standard sample of compacted crushed rock. CBR is determined at penetration of 2.5mm and 5.0mm; and the higher value is used (Head 1994, O‟Flaherty 2002).
……….Eqn 3.1
The CBR test is a constant rate of penetration shear test in which a standard plunger is pushed into the soil at a constant rate and the force required to maintain that rate is measured at suitable intervals. The load-penetration relationship is drawn as a graph from which the loads corresponding to standard penetrations are read off and expressed as ratios (percent) of standard loads (Head 1994, O‟Flaherty 2002). The accepted percentage is known as the California Bearing Ratio or CBR value of the soil in the condition at which it was tested. The CBR value can be regarded as an indirect measure of the shear strength of the soil, but it cannot be related directly to shear strength parameters. An assumed mechanism of failure of the soil beneath the plunger (Black 1961) is indicated in figure 3.1.
Figure 3.6: Assumed mechanism of failure beneath CBR plunger (Black 1961).
41 CBR can be carried out on most types of soil ranging from heavy clay to medium gravel soils. The test can be used on the sub-grade, sub-base and base course materials and the results obtained enable maximum utilisation to be made of low cost materials where better quality material is not available. It should also be noted that the CBR test can be used to provide a rational method of design for flexible pavements (such as macadam or asphalt), rigid (concrete) pavements, and granular base courses. Hence, CBR test data are applicable to the design of airfield runways and roads. Furthermore, CBR values enable a suitable thickness of sub-base construction to be determined to withstand the anticipated traffic conditions (vehicles or aircraft), in terms of axle loadings and traffic frequency, over the design life-span of the pavement (Head 1994).
Figure 3.7: Some terms used in pavement construction (Head 1994).
Table 3.3: Typical ranges of CBR (%) values for compacted soils (O‟Flaherty 2002)
Type of soil Plasticity Range of CBR (%) values
Clay CH 1.5 to 2.5
CI 1.5 to 3.5
Silty clay CL 2.5 to 6
Sandy clay PI = 20 2.5 to 8
PI = 10 2.5 to 8 or more
42
Silt - 1 to 2
Sand – poorly graded - 20
Sand – well graded - 40
Sandy gravel – well graded - 60
3.3.4.2 CBR laboratory experiment
CBR laboratory experiment covers the laboratory determination of the CBR of a compacted sample of soil – three point method; which is the prescribed method in the pavement and materials design manual (TMH1 1986). The table below shows the testing schedule for the CBR test program.
A series of equipment were used in the determination of CBR test like CBR metal moulds, rammer, steel rod, steel straight edge, balance, oven, filter papers, mixer, and dial gauge.
Furthermore, soaking tank, surcharge disks, cylindrical metal plunger, CBR compression machine, loading ring, and stop watch (TMH1 1986; BS1377 1990; Head 1994).
The particle sizes of soil and PET plastic waste used in this research were less than 20mm, hence there was no need to first pass the samples through a test sieve of 20mm. About 20kg of dry soil was weighed after getting its representative sample by the quartering method.
For the soil-PET plastic waste composite specimens, 3 different samples of 20kg each were weighed separately, together with their respective PET plastic waste fibres inclusions of 12.5%, 22.5% and 32.5% by dry soil weight. Furthermore, each test sample was mixed with the optimum moisture content (OMC) which had been earlier determined in the compaction test.
A set of CBR tests were carried out on sand and sand-fibre composite materials, at varying percentage fibre inclusion of 12.5%, 22.5% and 32.5%., the effects of PET plastic waste content on the CBR value of the sandy soil was the main purpose of carrying out this test. The CBR tests were performed in the Civil Engineering laboratory of Stellenbosch University.
Specimens were moulded in a steel CBR mould with an inside diameter of 152mm and internal height of 152mm. The specimens to be tested were prepared with optimum moisture contents (OMC) obtained from previously conducted compaction tests as per BS 1377: Part 4: 1990 and
43 TMH 1: Method A7: 1986 (Edinçliler & Cagatay 2013). An electric mixer was used to mix sand and sand-fibre composite specimens until a homogeneous state was attained.
There were four (4) specimens to be tested and each specimen needed three (3) CBR moulds, and compaction was done using an automated mechanical compactor. The first specimen was compacted in such a way that, in the first mould, five (5) layers of soil were subjected to 55 blows per layers using a 4.5kg rammer; second mould, five (5) layers of soil were subjected to 25 blows per layer using 4.5kg rammer; and third mould, three (3) layers of soil were subjected to 55 blows per layer using 2.5kg rammer. The subsequent specimens were compacted in the same manner as for the first sample; and as per BS 1377: Part 4: 1990 and TMH 1: Method A8: 1986.
After compaction, the specimens were soaked in a curing tank for four (4) days and swelling readings were taken at the beginning and end of the soaking period using the dial gauge.
The specimens were penetrated using a standard CBR machine with a penetration speed of 1.27mm per minute as described in BS 1377: Part 4: 1990 and TMH 1: Method A8: 1986; and procedures described in these standards were employed.
44 Figure 3.8: CBR machine setup (Head 1994).