California Bearing Ratio (CBR) is a very important test used to access the strength of soil prior to construction. Though many new pavement structural design methods such as: empirical-mechanistic and mechanistic methods that are based on stresses are being developed, CBR is still being identified as the best indicator of soil strength despite some challenges in its measurement especially with mixed fine and coarse graded materials.
Factors that Contribute to Failure of Subgrades
Although subgrade CBR value and design traffic are solely employed in most empirical design formulas to determine the thickness of the pavement layers, it is also necessary to consider certain factors which affect the pavement of the subgrade and which have often led to the failure of the pavement in a manner that is costly to correct. These factors include:
a) Depth to the water table/perched water tables.
b) Chemical contamination risk assessment.
c) Control of piping of fine grained soils.
d) Risk of encountering loose Made Ground.
e) Need for ground improvement of foundation soils (e.g. soft Alluvium, loose Made Ground etc.)
f) Risk of collapse settlement of dry engineered fill.
g) Risk of landslips.
h) Risk of underground caves, deneholes etc.
i) Impact of adjacent developments for sites on soft Alluvium.
j) Frequency and treatment of subgrade solution features in Upper Chalk and Hythe Beds.
k) Treatment of solution features below drainage runs.
l) Frequency and treatment of other subgrade soft spots.
m) Frost susceptibility of subgrade.
n) Differential settlement risks/need for ground improvement.
o) Chemistry of subgrade soils if in-situ lime/cement stabilisation is considered.
p) Shrinkage/swelling potential of over-consolidated clays. (particularly where trees removed)
q) Risk of open fissures in underlying rock.
r) Risk of soft clay layers in a granular soil.
Where any of the factors above are identified, it is necessary to take preventive measures to ensure that the pavement does not fail prematurely.
Methods for Determination of Subgrade CBR
A known method for the determination of subgrade CBR can possibly be applied to most soils but a particular method is not exclusively suitable to determine the appropriate CBR value for a given soil. For example, the plastic and liquid limits can only be performed upon cohesive soils and consequently a granular subgrade must be investigated by another method. Thus, the method to be selected for determination of CBR should be based upon the size of the scheme, the accuracy required and the likely soils to be encountered. The factors that would determine the suitable method to use can be assessed by investigation of the likely surface geology and groundwater conditions of the soil. Situations of road works/types of soil and methods of CBR suitable for each are outlined below
Reconstruction/widening of minor roads
| Soil Type | CBR Method |
| Cohesive Soils | (i) MEXE probe
(ii) Hand Vane (iii)Plastic and Liquid Limit |
| Granular Soils (Fine-grained) | (i) MEXE probe
(ii) Particle Size Distribution |
| Granular soils (Coarse-grained | (i) Grading |
Major Road Schemes
| Soil Type | CBR Method |
| Cohesive Soils | (i) MEXE Probes
(ii) Hand Vane. (iii) Plastic and Liquid Limits. (iv) Compaction/CBR relationship where it is considered that the soils are moisture sensitive |
| Granular Soils (Fine-grained) | (i) MEXE Probe
(ii) Particle Size Distribution (iii) Compaction/CBR relationship |
| Granular soils (Coarse-grained) | (i) Grading |
New Schemes in “Greenfield” Sites
| Soil Type | CBR Method |
| Cohesive Soils | (i) MEXE probe – shallow cuttings only.
(ii) Hand Vane – shallow cuttings only (iii) Plastic and Liquid Limits – shallow and deep cuttings (iv) Compaction/CBR relationship – shallow cuttings only (v) Triaxial Tests – deep cuttings only. |
| Granular Soils (Fine-grained) | (i) MEXE probe – shallow cuttings only
(ii) Particle Size Distribution – shallow and deep cuttings (iii) Compaction/CBR relationship – shallow cuttings only (iv) Compaction/Moisture Content vs CBR. |
| Granular soils (Coarse-grained | (i) Grading |
Chalk
If chalk is present and the nature is not doubtful, CBR value of 7% can be used. Otherwise, conduct chalk crushing value (CCV) test, natural moisture content (NMC), saturation moisture content (SMC) and the ratio NMC/SMC to which the performance of chalk is sensitive
Composite Soils
If the soil consists of composite materials, there should be assessment to know which of the materials dominate using the particle size analysis because most times, the particles that dominate determine the behaviour of the material. Henceforth, CBR assessment should be done based on the dominant material
Advantages and Disadvantages of the Different Methods
A. Soil Assessment Cone Penetrometer (MEXE Probe)
Advantages
(i) Quick and inexpensive to perform
Disadvantages
(i) Correlation dependent
(ii) Only provides the existing value of CBR
(iii) Insensitive to the effects of the soils macrostructure
(iv) Cannot be used in stony soils
B. CBR’s performed upon pot samples
Advantages
(i) It allows the possibility of evaluating the CBR of a soil in varying degrees of saturation (soaked/unsoaked CBR’s).
Disadvantages
(i) Appreciable disturbance is caused during the sampling procedure which significantly affects the test results.
(ii)Large trial pit required.
C. Plastic and Liquid Limits
Advantages
(i) It allows a lower bound assessment of the CBR under recompacted conditions for a variety of effective stresses (i.e. construction conditions).
Disadvantages
(i) The analytical procedure is expensive unless standard graphs are used.
(ii) The standard graphs assume a worst case.
(iii) Can only be used on soils containing cohesive material.
(iv) Can be difficult to interpret on mixtures of granular and cohesive material.
D. Measurement of Shear strength (hand vane/Triaxial tests)
Advantages
(i) The hand vane is quick and inexpensive to perform.
(ii) Triaxial measurements of the undrained shear strength takes into account the soil macrostructure.
(iii) Remoulded tests provide a lower bound for recompacted subgrades.
Disadvantages
(i) Only provides the existing value of CBR when using the hand vane.
(ii) The triaxial measurements are moderately expensive to perform, sampling and testing time can be long.
(iii) Dependent upon a correlation although this does have a theoretical basis.
(iv) Can only be used for cohesive soils.
E. In-situ CBR
Advantages
(i) Realistic measurement of the CBR.
(ii)Takes into account the macrostructure of the soil account.
(iii) Can be used to assess current value for chalk.
Disadvantages
(i) Very expensive to perform.
(ii) Only provides the existing value of CBR.
(iii) Difficult to perform below existing ground surface.
(iv) Not suitable for coarse granular soils.
F. Laboratory compaction test
Advantages
(i) Determines the CBR of remoulded soils for a variety of moisture content
Disadvantages
(i) Insensitive to the effects of the soils macrostructure.
(ii) Expensive to perform.
(iii) Requires a large sample.
(iv) Variable results with coarse granular soils.
