We roll a standard dynamic cone penetrometer rig onto site, usually mounted on a tracked carrier that handles the steep cobbled lanes of Edinburgh’s Old Town without trouble. The test rod is driven continuously while the number of blows per 100 mm is recorded, giving a direct CBR profile from the surface down to 1.5 m. Edinburgh’s subgrade is complex—glacial till overlays fractured sandstone in many streets—so we run at least three test points per 100 m of alignment to capture variability. For thin pavement designs we also extract bulk samples and check compaction curves with the ensayo proctor to link density to bearing capacity.
Edinburgh’s glacial till can give CBR values from 2% to 15% within 50 metres, demanding a testing density that blanket averages cannot replace.
Scope of work in Edinburgh
- Driving a 20 mm diameter cone with a 9 kg hammer falling 510 mm
- Recording blow counts at 100 mm intervals to 1.5 m depth
- Calculating CBR = 0.0028 x blows per 100 mm (the correlation curve from TRL)
- Correcting for moisture content when samples are saturated
Working video
Typical technical challenges in Edinburgh
A new housing estate near the Water of Leith had a design CBR of 5% from a desktop study, but our in-situ tests returned 2.5% after a wet spring. The contractor had already ordered 250 mm of Type 1 sub-base. We re-ran the CBR study on a drier day and the values climbed to 4%, allowing a 180 mm sub-base instead of 300 mm. That single correction saved £45,000 on aggregate alone. Relying on published correlations without local verification is a gamble that Edinburgh’s variable drift geology will punish.
Our services
We tailor the CBR study to the pavement type and traffic loading, from housing estate access roads to HGV distribution centres.
In-Situ DCP Testing
Dynamic cone penetrometer tests run at 50 m intervals along the proposed road centreline, with blow counts converted to CBR using the TRL correlation. Suitable for flexible and rigid pavement designs up to 10 million standard axles.
Laboratory Soaked CBR
Bulk samples compacted to Proctor optimum density and soaked for 96 hours before penetration testing. This gives the worst-case design CBR for areas with high water table, such as the Meadows or Holyrood Park margins.
CBR Profile for Subgrade Improvement
Continuous profiling to 1.5 m depth to identify weak layers or buried topsoil that require capping or stabilisation. We provide cut-off CBR maps and recommend lime or cement treatment where values fall below 2.5%.
FAQ
What is the difference between in-situ DCP CBR and laboratory soaked CBR?
In-situ DCP CBR gives the bearing capacity at the time of testing under field moisture conditions. Laboratory soaked CBR simulates worst-case saturation after 96 hours of soaking. For Edinburgh’s clayey tills, the soaked value can be 30-50% lower than the in-situ value.
How many CBR tests are needed for a 500 m residential road in Edinburgh?
We recommend a minimum of 10 test points: one every 50 m along the centreline, plus two at each end to capture entry/exit zones. If the ground crosses different drift deposits (e.g. till over alluvium near the Braid Burn), we increase density to one every 25 m.
Does the CBR value change with depth in Edinburgh’s glacial till?
Yes. The top 300 mm often has lower CBR (2-4%) due to root action and weathering, while the deeper compact till can reach 10-15%. We report a weighted average for the top 1 m, which is the layer that controls subgrade modulus for pavement design.
How much does a CBR study for road design cost in Edinburgh?
A standard study for a 500 m road, including 10 DCP tests and a laboratory soaked CBR on one bulk sample, costs between £150 and £260. The exact figure depends on site access, number of test points, and whether a full laboratory compaction series is needed.