CBR testing

CBR Testing helps in the examination of the soil properties before embarking on a construction project is essential. It helps in determining and preparing for how the project and the surrounding soil will interact. It enables the involved parties in ascertaining the viability of the project on the surface in question. The results will help show the necessary precautions needed to avoid complications in the future or may give reason to call off the project. It makes CBR testing very important.

What is CBR Testing?

The California Bearing Ratio (CBR) test, formed by the California State Highway Department, is a penetration test performed in both dry and wet soil areas used in evaluating the ratio of the force required per unit area to penetrate the soil. From the investigations, it is easier to determine the density of supplies needed for the intended development.

How is CBR Testing Conducted?

Instruments Employed

When carrying out the inspection, high-quality equipment is adopted to guarantee no errors occur when measuring. The apparatus used ensures penetration and precise calibration of the CBR ratio.

The instruments used include;

  • Dial gauges
  • Loading Machine
  • Penetration plunger
  • Spacer Disc
  • Surcharge Weight

How Does a CBR Test Works

The surface on which the test is carried is levelled at the formation layer ideal for testing. The plunger, circular with a 1935mm2 cover area on the face surface, is pushed into the earth at a uniform velocity. Normally, the instruments are carried in a 4WD that avails the reaction load required to drive the CBR press.

In regular intervals, the measurements of the weight and penetration are collected. A curve is plotted with the CBR bearing value plotted as a percentage against standard soil. In other instances, the dynamic cone CBR is applied for quick evaluation of CBR values. It applies light movable equipment to give a result of an empirical test on the soil. It is used in giving a constant account of the penetration resistance of each layer. The resistance gives data that is utilized in a formula to calculate CBR value.

The CBR value is greater in denser surfaces than soft surfaces. Soft areas such as clay have a value of 2% while high-quality sub-base will have a value of above 80%. The biggest particle size used in doing CBR tests is 20mm. The tests are done on the surface, or at depths of between 500- 1000mm. It is done in 20-30m interims along the suggested building centerline.

In summary, the values are arrived at by;

  • Getting the pressure demanded to penetrate a soil specimen with a standard area plunger.
  • Getting the pressure required to attain the same penetration on a standard crushed rock material.
  • The pressure from the soil sample is divided by the pressure read from the standard crushed rock material.

As much as the process seems to be an easy activity, a slight error in the process would be a catalyst for impeding problems during or after construction. Having the right knowledge equipment or even help from professionals will help you have a successful CBR test.

When to Use CBR Testing

CBR testing is required in various situations. Whenever you want to know the endurance of the subsoil and the base material in a certain location.

  • Pavement Design. When designing roads and highways, CBR testing shows the stability of the soil. It helps in deciding the most effective pavement thickness dependent on the expected traffic flow. It also helps in taking precautionary measures for frost heave.
  • Foundation Design. Foundations are designed to hold onto the load of the structure and distributing it to the ground. They are of different types like strips, pads and rafts. The foundation is designed in line with the stability of the ground or settlement. Seasonal moisture, ground stability, frost, groundwater elevation, and river erosion influence the foundation depth.

CBR testing is critical in ensuring the structure being built is secure and doesn’t run the risk of damage. Having in-depth knowledge of the process will help you determine the best materials for your project. It is a way of mitigating risk and ensuring the investment is safe.


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