Dynamic CBR

Overview

Dynamic CBR Test with Light Weight Deflectometer (LWD)

  • Measures California Bearing Ratio (CBR) directly in the field
  • Penetration test
  • Evaluate the mechanical strength of subgrades and base courses
  • Immediate repeatable results so that on-site decisions can be made straight away

Using extensions to the 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. It can be used in confined spaces and difficult to access locations, with no support equipment required.

Two CBR test options are available:

Insitu CBR Extension           Portable Laboratory CBR Extension

 

California Bearing Ratio (CBR) is a simple penetration test developed to measuring the load-bearing capacity of soils for road subgrade and base courses used for building roads, airport runways or railway track beds. The dynamic CBR test, in particular the in-situ field test, is a material test on undisturbed samples, with which one can judge what level of compaction can be achieved on the tested soil.

The Dynamic CBR Extensions together with the ZFG 3000 Light Weight Deflectometer can be used instead of the static CBR test and is used primarily because of the ability of get immediate results on site.

Advantages

Advantages of the CBR field extension include:

  • Measurement takes only three minutes per test location.
  • All readings and analysis can be done in the field and the results are immediately available at the construction site.
  • The equipment is portable and no counter weight or truck mounting is required.
  • Measurements are possible in narrow situations including ditches and foundation trenches, back fill, core holes or other areas that are difficult to reach.
  • Adherence to TP BF – StB Part B 7.1, removing the need for a load frame and the static load device (compression test). In its place, a loading device according to TP BF – StB Part B 8.3 is used.

Light Weight Deflectometer (LWD) Calibration, Service and Spare Parts

Insitutek are proud to represent Zorn Instruments Light Weight Deflectometers (LWD’s) in Australia, New Zealand and the Pacific Islands and provide a very high level of client support.

We offer a complete spectrum of services including after-sale technical support, servicing, repairs, and calibrations. Our clients have exclusive access to the only Light Weight Deflectometer calibration and service cwntre in the Southern Hemisphere. Run by Zorn Instruments trained technicians, our service centre is also well stocked with spare parts. Our Australian based Zorn Instruments Calibration Stand and Service Centre is exclusive to Zorn clients and saves you significant down time compared to calibration and repairs overseas.

To find out more, Contact Us.

 

 

How it Works

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:

CBRd = 24.26 * p / s0.59%
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:

CBRd = 87.3 / s0.59%

The recommended range of operation for CBRd 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:

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

– CBRdw: is determined after soaking the samples in water

– CBRdft: 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.

Videos

Portable Laboratory Dynamic CBR Test using Light Weight Deflectometer (LWD)

In situ Dynamic Field CBR Test using Light Weight Deflectometer (LWD)

Documents

Category

Topic

Publication Date

Richtlinie über die Aufbereitung und für die Güteüberwachung von. Haldenbergen aus dem Kupferschieferbergbau im Straßenbau – DA- 9:2006-224

Der Dynamische CBR-Versuch – eine neue Qualitätskontrolle für den Erdbau – Geotechnik; Ausgabe 4, Seite 271 – 274 (2000) – Professor Dr.-Ing. Hans-Henning Schmidt und Dipl.-Ing. Jürgen Volm

Author: Professor Dr.-Ing. Hans-Henning Schmidt and Dipl.-Ing. Jurgen Volm

Instruction for the determination of the dynamic CBR value using the Light Weight Deflectometer (LWD) – Appendix 3 “Notes on the use of steel slag in road construction” Research Association for Roads and Transportation, Cologne 1996 (“Merkblatt zur Verwendung von Stahlwerksschlacken im Straßenbau” – Forschungsgesellschaft für Straßen- und Verkehrswesen, Köln 1996)

Author: Forschungsgesellschaft für Straßen- und Verkehrswesen, Köln. FGSV Köln

Dynamic CBR as a method of embankment compaction assessment – Katarzyna Zabielska-Adamska – Head of Geotechnics, Faculty of Civil & Environmental Engineering – Politechnika Białostocka (Bialystok University of Technology – Poland)

Author: Zabielska-Adamska K., Sulewska M.J0

In engineering practice, earth construction requires suitable soil compaction, usually relating to the Proctor methods. Materials of the built-in embankment and the subgrade have their own specifications, dependant on the kind of earth structure and soil plasticity characteristics. Care should be taken not to use compaction degree (% of maximum compaction) as the only parameter to assess soil compaction. This applies to both cohesive soil and to fly ash whose permeability and mechanical properties dependent on moisture content at compaction. Therefore, for these types of soils California Bearing Ratio could be used as a method of compaction assessment being an indicator of soil bearing capacity. The CBR research was done for both static (classic) and dynamic methods on fly ash samples without soaking them to replicate field conditions. The dynamic CBR tests were conducted by using Light Weight Deflectometer consisting of a falling weight to produce a defined load pulse of the CBR piston. The CBR test could be used for running compaction control during embankment erection, which specially refers to dynamic CBR test due to the speed of research execution.

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