Laboratory CBR Testing in Belfast: Geotechnical Design for Subgrade and Pavement

A laboratory CBR is performed on a sample compacted into a standard mould under controlled conditions, usually after a 4-day soak to simulate long-term saturation. A field CBR, by contrast, is measured in-situ using a plunger or a Dynamic Cone Penetrometer on the undisturbed ground. The lab test gives the design value for pavement thickness; the field test verifies that the constructed subgrade actually meets that design. In Belfast, where the groundwater rises quickly in winter, the soaked lab CBR is often the more conservative and governing value.

A single-point soaked CBR test to BS 1377-4 typically falls between £110 and £170, depending on whether it includes moisture content determination, Proctor correlation, and a full swell log. A full pavement investigation suite with three CBR points, grading and plasticity tests for a typical commercial development in Belfast runs from £450 to £700.

The 96-hour soak under a surcharge load models the worst moisture condition the subgrade will experience during its service life. In Belfast, where the water table is often less than 1.5 metres below ground level, a subgrade that tests well at its natural moisture content can lose over 60% of its bearing capacity once it reaches saturation. The soaked CBR value is the one used in the pavement design catalogue in HD 25/94.

Yes, but the method differs slightly. For coarse granular materials and crushed concrete aggregates, we follow BS EN 13286-47, which uses a larger mould and a modified compaction effort. These materials typically produce CBR values above 30% and are used as Type 1 sub-base in Belfast's heavy-duty pavements. The test confirms the material meets the specifications in the SHW Series 800 before it is placed on site.

Under the guidance of CD 225 and the regional road authority, a subgrade CBR of at least 5% is generally required for a flexible residential pavement with light traffic. If the laboratory CBR falls below 2.5%, the standard pavement design requires either a capping layer of at least 350 mm or a stabilised subgrade. In Belfast's softer alluvial areas near the Lagan, we frequently see values between 1.8% and 3.2%, which triggers a capping layer requirement in the design.