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Frequently Asked Questions

What is the CBR Test, British Standard?

The British Standard for Methods of Test for Soils for civil engineering purposes that governs the CBR (California Bearing Ratio) test is BS1377-4:1990. Part 4 of BS1377 is dedicated to Compaction-related Tests and includes specifications and detailed procedures for CBR tests.

This standard describes how to prepare samples, the exact weights and dimensions of the equipment, as well as the method of penetration and the calculation of CBR values. This standard ensures accuracy, consistency and reliability in the test procedure, which is crucial for engineering assessments.

Why is it Called the California Bearing Ratio?

The California State Highway Department developed the CBR test, which was first used in the 1930s. Originally, it was designed to ensure that military airfields were properly designed during World War II.

The CBR test is used today to measure the strength of base courses and subgrade soils. It helps in the design and construction of roads and pavements by comparing a soil’s bearing capacity with that of standard-graded crushed stone.

What is the 3-point CBR Test?

The three-point California Bearing Ratio (CBR) is a method that can be used to determine the strength of soil. It uses three cylindrical probes which apply vertical forces simultaneously to a sample.

This method is designed to give a comprehensive evaluation of soil behaviour by measuring the penetration at three points with constant rates, until either the soil fails or reaches the specified depth. These three probes provide the data that is used to calculate CBR.

What is the Difference Between 1-point and 3-point CBR Tests?

The main difference between the three-point California Bearing Ratio (CBR) test and the one-point CBR test is the number of loads that are applied to the soil. In the one-point CBR test, a cylindrical probe is used to apply a vertical force at a constant speed until soil failure or a specified depth of penetration has been reached. This single measurement of penetration depth and load is used to calculate the CBR value.

The three-point CBR test, on the other hand, uses three-cylinder probes that apply vertical forces simultaneously. Each probe applies a constant load to the soil until it fails or reaches a predetermined depth of penetration. CBR is calculated from the loads and penetration depths of these three probes.

The three-point CBR test is deemed more accurate than a one-point CBR test because it gives a more realistic representation of soil behaviour when loaded, simulating more complex conditions in real life. This method is more expensive than the one-point CBR test because it’s time-consuming and complex. It also requires specialised equipment.

What is a Good CBR Value?

The CBR (California Bearing Ratio) value that is considered “good” depends on the project requirements and the intended use of soil. CBR values indicate the strength of the soil and its suitability to be used as a subgrade for roads and pavements. This is a general guideline for interpreting CBR values.

• A CBR value above 80% is excellent – These soils are usually sand, gravel, or crushed stones and can support heavy loads. These soils are best used under heavy traffic surfaces, like highways and major roads.
• CBR value between 30% and 80% – This range of values is typical and good for stable soils. These soils are suitable for road construction of all types and can support heavy traffic.
• CBR value between 10% and 30% – These values are indicative of fair to poor soil strengths and are typically associated with finer sands and silts. This range of soils may require stabilisation, or thicker pavement layers to maintain structural integrity.
• CBR value below 10% – This indicates very weak soil. It is often found in organic or clayed soils. These soils require substantial improvements, such as stabilisation and replacement, to support structural loads.

The CBR value that is required for a particular project depends on several factors, including the type of vehicle to be used, the local building codes and engineering standards. These values are used by engineers, in conjunction with other soil characteristics to design pavement layers thick and strong enough to distribute loads and to prevent excessive deformation.

How Much Does a CBR Test Cost?

Costs for a CBR test can vary greatly depending on several factors. These include the geographical location, type of test required (laboratory or in the field), and service provider. Here are some guidelines for pricing:

1. Types of Test
   • Lab Tests: These are usually less expensive than field testing because they take place in a controlled setting and don’t require the mobilisation of as much equipment.
   • Field Testing: This is generally more costly due to the requirement to transport equipment to the location and possibly deal with more complex conditions on the site.
2. Number of Samples: The number of samples required can be a factor. Multiple tests ordered at the same time can reduce costs per test because of efficiencies in handling and processing samples.
3. Additional Services: Costs can also rise if you need to conduct preliminary site investigations, collect soil samples, or meet specific reporting requirements.
4. Location: Prices can vary depending on the region, due to variations in labour rates, distances travelled for field tests and local business environments.

When it comes to CBR tests, we offer one fixed rate regardless of the location. As one of the leading CBR test services in the UK, we have multiple locations throughout the UK. This allows us to maintain a fixed rate across the board.