Dynamic CBR

The dynamic CBR (California Bearing Ratio) test is a material test to enable one to determine load-bearing capacity of a soil for subgrade and base courses typically for road, airport runway or rail track bed construction. Using extensions to the ZFG 3000 Light Weight Deflectometer, dynamic CBR (California Bearing Ratio) is measured in-situ and also laboratory CBR in a cylinder can be simulated at the construction site.

The dynamic CBR test is a stamp penetration test. The 50mm CBR load stamp is pressed into the sample under a defined dynamic pulse load and the setting amplitude (deflection) of the CBR load stamp is measured and used for the calculation of the dynamic CBR – value. The dynamic load is produced with the load device of the Light Weight Deflectometer (German regulation: “Technical test regulation for soil and rock in road construction, TP BF – StB part of B 8.3″). The loading device consists of a 10 kg falling weight, which falls freely onto a set of steel springs and produces a load pulse with a peak of 7.07 kN and a pulse duration of 18 ms.

The dynamic CBR – value is a complex strength coefficient that depends on:

• the strength, grain shape, the frost resistance and the grain roughness of the single grain
• the grain composition, the content of fines, water content and compaction of the mineral mix

The CBR-value is calculated as follows:

CBR_d = 24.26 * p / s^0.59%
where s is the settlement amplitude (in mm) of the CBR stamp and p is the peak pulse load amplitude (in N/mm²). The peak pulse load is 7070 N and a diameter of the stamp is 50 mm (cross section area -1963 mm²). Hence p is 3.6N/mm². Therefore:

CBR_d = 87.3 / s^0.59%

The recommended range of operation for CBR_d is 20% ≤ CBRd ≤ 150% and the maximum grain size is 22mm.

Depending upon different pretreatment of the samples, three types of dynamic CBR – values are defined as follows:

– CBR_do : is determined immediately after compressing the samples without any pretreatment

– CBR_dw: is determined after soaking the samples in water

– CBR_dft: is determined for samples that were embedded in water and subjected to freezing and thawing cycles

CBR Values and Site Conditions

• CBR values “on site” may not bear any relationship to the CBR values employed in the road design, due to softening from wet weather and trafficking from site vehicles.
• The opposite is also true, if CBR values are taken on site after the sub-grade has been exposed and dry weather has caused the moisture content of the soil to decrease, increasing soil stiffness, the CBR value will be higher than natural moisture content, this is an incorrect value for design purposes and if accepted will cause a serious under design of the road pavement.
• Natural soil moisture content, after drainage, is the correct moisture content for determining CBR values for highway design purposes because in the course of time natural soil moisture conditions will be re-established.

This is of course true for any design method you employ if the soil conditions at the time of construction are different to the soil conditions upon which you based your design. It could be some time before the properties of the soil revert back to their original engineering condition, and by this time failure could have occurred.

Good drainage is an essential part of road construction to allow the optimum strength/CBR to be obtained, and maintained, from the soil foundation, whether it be in-situ soil or imported fill. It follows that the drainage must be kept operating efficiently during the life of the road to prevent the strength/CBR decreasing through weakening of the foundation by a rising water table.