The Benefits of Automated CBR Testing

The California Bearing Ratio (CBR Test) is a penetration test for evaluating the load-bearing capacity of soils used for building roads and other hardened surfaces. The traditional method of for a CBR Test involved penetrating a plunger of fixed diameter into a soil sample using a Load Frame (usually controlled by some mechanical device) with a Load Ring for measuring Vertical Load and the Displacement measured using Mechanical Dial Gauges. This required the laboratory technician to be on hand with a clipboard and datasheet(s) to record the data when required throughout the Test.

Semi-Automation

The VJ Tech Triplex Load Frame took the CBR Test to a new level. An inbuilt microprocessor enabled a stepper motor to accurately control the movement with an easy to use control panel and large LCD display. Adjustable contrast on the Load Frame enabled the laboratory technician to set the fixed CBR speeds and rates of travel required (strain rates of 0.00001 - 50.8000mm/min), with large keys for UP, DOWN, STOP, FAST UP and FAST DOWN control platen adjustment speed if required.

Additionally, new hardware was developed that assisted the laboratory technician in many ways. A Load Cell replaced the Load Ring (for Horizontal Load) and LSCT or Potentiometric Transducers replaced Mechanical Dial Gauges for the Vertical Displacement measurement.

The advances in computing power meant that software could be developed to control the different stages of the CBR test, representing a huge step forward. VJ Tech Electronic Data Loggers (such as the Mini Scanner and MPX3000) enabled Vertical Load and Displacement measurements to be recorded in real time and then sent to the computer via an RS232 connection.

The renowned Clisp Studio csCBR software module running on the computer was able to:

• Conform to all major international geotechnical standards. (BS1377-4:1990, D1883-07 & ASTM 1289)
• Set up individual tests based on the Client, Job, Borehole and Sample.
• Set up and Control the Instruments and Transducers used in the CBR Test.
• Calibrate the Transducers when required.
• Input Specimen details and automatically calculate related Dimension and Moisture details.
• Set default starting conditions, data logging parameters and stopping conditions for both the Soaking and Test stages.
• Display live views of sensor readings and status.
• Display live data views, graphs and tables as the test progresses.
• Provide complete results data which can be exported to external data processing packages for further manipulation if required.
• Export, Import and Clone entire tests.

Although the CBR test was automated, the test still required an additional Data Logger.

Complete Automation

When VJ Tech introduced the Triplex II Advanced to the market, it obviated the need for a Data Logger because the Instrument has 4 analogue input channels on the rear panel. This enabled Load and Displacement data to be logged (with 2 additional channels for alternative Load capacities to be set up should they be required), and then forwarded directly to the computer. It also let the User select fixed speeds for either BS (1.2 mm/min) or ASTM (1.27 mm/min) and comprised an on screen display of the 2 transducer channels and a Peak hold display.