Laboratory CBR Test Cambridge – BS & Eurocode 7 Compliant

The geology flips completely between north and south Cambridge. Castle Hill sits on Lower Chalk overlain by thin glacial till, while the area around Addenbrooke’s Hospital rests on Gault clay and river terrace gravels. That difference alone can swing a laboratory CBR value from 3% to over 20% on the same project. We see it constantly when samples arrive from residential extensions in Chesterton versus commercial builds near the Biomedical Campus. Our lab runs the California Bearing Ratio test under BS 1377-4 on samples you deliver or we coordinate collection of. The soaking procedure follows BS EN 1997-2 guidelines for worst-case moisture conditions. Before we even start the CBR, we often recommend a grain-size analysis to confirm the fines content that governs swelling potential during the four-day soak.

A CBR value without a documented soaking history tells you nothing about how that subgrade will behave in a Cambridge winter.

Our service areas

How we work

Cambridge sits on the driest side of Britain with barely 560 mm of annual rainfall, yet the clay-rich soils of the Gault Formation hold water stubbornly through winter. That contradiction drives our test setup. We compact specimens at optimum moisture content using the 2.5 kg or 4.5 kg rammer method from BS 1377-4, then split them into soaked and unsoaked batches. The soaked specimens sit submerged for 96 hours with a surcharge ring simulating overburden, while we log the swelling daily. The penetration stage uses a 50 kN frame running at 1.27 mm/min. We plot force versus penetration at 2.5 mm and 5.0 mm, reporting the higher ratio. For projects near the River Cam, where the water table fluctuates within a metre of the surface, we pair the CBR with Atterberg limits to verify the plasticity index before anyone specifies a pavement thickness that will fail in the first wet winter.

Laboratory CBR Test Cambridge – BS & Eurocode 7 Compliant — Technical reference — Cambridge

Local considerations

Cambridge City Council approved over 1,200 new dwellings in 2023 alone, many on infill plots over old fen-edge deposits. We have tested samples from sites where the natural CBR sat below 2% before any treatment. A designer who assumes a textbook 5% value for clay subgrade on those plots is gambling with the pavement life. The real risk shows up two years later, when the asphalt cracks in a pattern that traces the exact footprint of poor compaction beneath. Heavy and light compaction methods give different CBR values on the same soil. If the contractor uses heavy compaction for a light-traffic residential road, the result is optimistic and the pavement is under-designed. We report the compaction method clearly on every certificate to prevent that mismatch.

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Regulatory framework

BS 1377-4:1990 – Soaked and unsoaked CBR, BS EN 1997-2:2007 (Eurocode 7) – Ground investigation and testing, Manual of Contract Documents for Highway Works (MCHW), Series 600

Typical values

Parameter	Typical value
Standard followed	BS 1377-4:1990, BS EN 1997-2
Compaction effort	2.5 kg rammer (light) or 4.5 kg rammer (heavy)
Soaking period	96 hours submerged with surcharge rings
Penetration rate	1.27 mm/min ± 0.02 mm/min
Penetration measured at	2.5 mm and 5.0 mm
Specimen diameter	152 mm (CBR mould)
Swelling measurement	Daily dial gauge readings during soak

Questions and answers

What does a laboratory CBR test cost in Cambridge?

For a single-point CBR test on a sample you deliver to our Cambridge lab, expect between £90 and £190 depending on whether you need the four-day soaked procedure or just the unsoaked penetration. Two-point tests with compaction curves and classification data sit at the upper end. We issue a fixed quote before any work starts.

How long does the CBR test take from sample to report?

A soaked CBR test cannot be accelerated. The 96-hour soaking period is mandatory under BS 1377-4. Add one day for compaction and setup and one day for penetration and reporting. You will have the certificate within five to six working days of sample receipt. Unsoaked tests can be reported within 48 hours.

Do you test remoulded samples or only undisturbed cores?

Almost every CBR we run in Cambridge is on remoulded material compacted to a target density and moisture content, because that is what the pavement will be built on. If you need a CBR on an undisturbed core, we can do it, but BS 1377-4 is written around remoulded specimens for subgrade assessment.

What surcharge weight should I specify for a road project in Cambridge?

For most residential and local roads, we use a surcharge ring equivalent to 4.5 kg, which represents around 50 mm of pavement. For heavily trafficked roads or industrial yards, we move to a higher surcharge. Tell us the design traffic loading and we will set the surcharge accordingly before the soak begins.

Location and service area

We serve projects in Cambridge and surrounding areas.

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