Civil engineers, infrastructure contractors and geotechnical professionals know that any construction project starts with understanding the ground you’re standing on. One of the most important tools in this process is the CBR testing procedure.
From designing roads to building pavements or parking areas, understanding soil strength is key to avoiding costly mistakes and longevity. Here’s what CBR testing is, why it matters and how it works.
What Is CBR Testing?
The California Bearing Ratio (CBR) test is a standardised penetration test to determine the load bearing capacity and strength of soil. Originally developed by the California State Highway Department, this test is now a global standard in road and pavement construction, referenced in BS1377.
This test uses the incremental plate load test to determine the penetration resistance of the soil related to the CBR value, which measures how well the ground can support the weight of infrastructure, such as roads and pavements.
The California Bearing Ratio test measures subgrade soil strength by comparing its penetration resistance by a standard plunger to a benchmark material, usually crushed rock, at the same penetration. The harder the surface of the test area, the higher the CBR value. High-quality subgrade soil will have a CBR value of 80-100%
Why is CBR Testing Done?
The California bearing ratio test procedure is crucial for construction purposes, including road construction and civil engineering purposes. The California Bearing Ratio test is widely accepted as the standard reference, alongside the incremental plate load test values.
- Ensure Stability: The California Bearing Ratio test evaluates the soil’s ability to support a construction project without deformation.
- Optimise Designs: Provides valuable information for designing road thickness, pavements and temporary foundations. During the foundation design stage, it can help planners with material selection. Equivalent CBR values are important because they provide a standard method for engineers when designing infrastructure.
- Avoid Delays: Identifies potential soil strength and issues with subbase materials before construction starts, minimising problems down the line. Provisional results are usually available on the day, with a full report available within 2 or 3 weeks.
- Prevent Overengineering: Helps engineers avoid unnecessary costs by recommending cost-effective solutions based on accurate soil strength data.
CBR Testing Procedure
The test is performed on prepared soil samples in a lab or as a field test. It involves forcing a standard penetration piston into the soil at a controlled rate and measuring the surface stiffness and force required to penetrate the material. This is compared to the corresponding penetration of the standard material by a standard circular piston. Here’s how it works:
Step 1: Specimen Preparation
The process starts with getting a representative soil sample. Sieve the soil through a 19mm sieve, then compact it uniformly in a mould. Replace any oversized particles with finer material. Conditioning the soil to the optimal moisture content is critical as it affects the test’s accuracy.
Key Equipment:
- Mould (152mm diameter, 127mm height)
- Spacer disc
- Weighing equipment
Lab Tip: Compact the soil in layers using a hammer or mechanical compactor to simulate real ground conditions of compacted soil.
Step 2: Apply Surcharge Weights
Before testing, place surcharge weights on the soil surface. These weights simulate overlying material, such as pavement layers, to provide realistic conditions. For most tests, the weights are between 2.5kg and 5kg.
Step 3: Penetration Test
Using a CBR testing machine, a 50mm diameter standard cylindrical plunger is pushed into the soil at a rate of 1.25mm per minute. The force is recorded continuously, measuring the pressure required to achieve penetrations at 0.25mm intervals up to 5mm penetration. You can opt for either a standard plunger test or a dynamic cone penetrometer test (DCP test), which uses a metal cone and sliding hammer to penetrate the soil mass deeper; up to 3 metres.
Lab Tip: Make sure all equipment is calibrated for accuracy. Equipment includes dial gauges, loading machines and penetration plungers.
Step 4: Calculate the CBR Value
The CBR value is expressed as a percentage by comparing the measured pressure needed to penetrate the sample to the pressure required for equal penetration on standard crushed rock material.
- A higher CBR value means stronger soil that can support heavy loads, such as roads or parking decks.
- For example, clay, a weaker material, has a CBR of 2% while well-graded crushed stone has a CBR of over 100%.
This simple CBR value calculation helps to determine the pavement thickness required for the desired performance of the infrastructure.
Applications
CBR testing is used in many construction applications, including but not limited to:
- Foundation design: A CBR value can determine the design and material selection for foundations.
- Road Construction: Test subgrades and design road layer thickness.
- Pavement Design: Ensure pavements can withstand a standard load of traffic.
- Car Parks: Check bearing capacity to design strong and durable parking structures.
- Temporary Foundations: Test ground conditions for portable or temporary structures.
When Should CBR Testing Be Performed?
CBR testing should be undertaken before initiating any construction project that requires a stable foundation. This is particularly essential when designing roads, pavements, or temporary industrial structures. Key stages to conduct CBR tests include:
- Before the finalising of project designs
- During pre-construction planning phases
- When assessing soil in challenging or unknown conditions
By testing early, you ensure that your project stays on time and budget.
Why Choose Site Testing Services Ltd. for Your CBR Testing?
At Site Testing Services Ltd., we bring over 20 years of experience in providing accurate and reliable CBR testing services to the construction and geotechnical industries.
Our commitment to quality ensures that your projects are based on trustworthy data, resulting in long-lasting, effective designs. Here’s why we are the partner of choice for contractors and engineers across the UK:
- Expertise
We follow the highest standards, such as BS1377, for laboratory and in-situ CBR testing. Our experienced technicians ensure every test is conducted with precision.
- Advanced Equipment
Our industry-leading facilities utilise cutting-edge apparatus, from calibrated penetration plungers and compact loading machines to moisture-controlled environments.
- Comprehensive Services
Alongside CBR testing, we also provide geotechnical site investigations, soil analysis, and chemical testing services, offering a one-stop solution for civil engineers.
- Quick Turnarounds
We understand the importance of timelines in the construction industry. Expect reliable results with speedy delivery.
Building from the Ground Up
The CBR testing process gives you the answers to ground strength and stability so you can make informed decisions. Whether designing highways, car parks or layered pavements, CBR testing is the foundation of stable, cost-effective and durable construction projects.
Visit cbrtesting.com today for a free quote.
