Laboratory CBR Testing in Slough for Subgrade and Pavement Design

We see it often in Slough. A contractor submits a pavement design based on assumed CBR values, only for the subgrade to fail under the first heavy rains. The Thames Valley has pockets of soft alluvium and made ground that don't behave like textbook soils. A laboratory CBR test on a remoulded sample at the correct density and moisture content gives you a real number you can trust. Not a guess. We run the test to BS 1377:1990, soaking the sample for 96 hours to simulate worst-case field conditions after prolonged wet weather. If your borehole logs show variable ground, pairing the CBR with an in-situ permeability test helps you understand drainage behaviour before you finalise the pavement thickness. That combination saves costly overdesign or premature failure.

A soaked CBR value below 2% in the Thames Valley means you need either a capping layer or a complete rethink of your pavement foundation strategy.

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Scope of work

One thing we've learned from Slough sites: the moisture content at compaction makes or breaks your CBR value. We often receive samples taken from the Langley Silt Member or from reworked London Clay that look firm in the field but lose over 60% of their strength after soaking. The laboratory CBR test quantifies that sensitivity. We compact the specimen in a standard mould using a 2.5 kg or 4.5 kg rammer, depending on whether you need light or heavy compaction effort. Then we measure penetration resistance with a 50 mm plunger at 1.27 mm/min. The force readings at 2.5 mm and 5.0 mm penetration are compared against standard reference values, and the higher ratio becomes your CBR. We always report both soaked and unsoaked results so you can assess seasonal variation. For projects where the formation level sits near the water table, we suggest complementing this with a plate load test to verify in-situ stiffness at the actual moisture condition.

Laboratory CBR Testing in Slough for Subgrade and Pavement Design — Technical reference — Slough

Local geotechnical context

Slough sits on a mix of Taplow Gravel over London Clay, with extensive areas of made ground from past brickworks and industrial use. The superficial gravel can be thin or absent, and where the London Clay is weathered near surface, it becomes highly plastic and moisture-sensitive. A CBR test on a sample taken at natural moisture content tells an incomplete story. The 96-hour soak in the lab simulates groundwater rise during winter, which is the dominant failure mechanism we observe in local roads and car parks. If the soaked CBR falls below 2.5%, the pavement will rut within the first two years unless you design a proper capping layer. For deep formations where the clay extends several metres, a CPT test provides continuous profiling to identify the exact depth where bearing capacity improves, so you can set the formation level intelligently rather than arbitrarily.

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Relevant standards

BS 1377-4:1990 — Methods of test for soils for civil engineering purposes: Compaction-related tests, BS EN 1997-2:2007 — Eurocode 7: Geotechnical design. Ground investigation and testing, BS EN 13286-47:2012 — Unbound and hydraulically bound mixtures. Test method for the determination of California bearing ratio, Manual of Contract Documents for Highway Works (MCHW), Series 600 — Earthworks, UKAS ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories

Reference parameters

Parameter	Typical value
Normative reference	BS 1377-4:1990, BS EN 1997-2:2007
Sample preparation	Static or dynamic compaction to target dry density
Soaking period	96 hours (4-day soak) under 4.5 kg surcharge
Penetration rate	1.27 mm/min constant
Plunger diameter	49.6 mm (standard penetration piston)
CBR reporting	At 2.5 mm and 5.0 mm penetration
Surcharge during test	Equivalent to pavement weight (typically 2 x 4.5 kg weights)
Accreditation	UKAS to ISO/IEC 17025:2017

Common questions

How much does a laboratory CBR test cost in Slough?

A single-point CBR test, including compaction, soaking, and penetration, typically ranges from £100 to £190 depending on the number of specimens and whether you need both soaked and unsoaked values. Multi-point tests for moisture sensitivity curves are priced per additional point.

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

We require seven working days as standard. The 96-hour soak alone takes four days, and we allocate the remaining time for sample preparation, compaction, penetration testing, and report drafting. Expedited turnaround is available on request.

What sample size do you need for the CBR test?

We need approximately 6 kg of disturbed soil passing the 20 mm sieve. If the material contains particles larger than 20 mm, we can test the fraction passing 20 mm and note the oversize percentage on the report, but the result may be conservative for the in-situ material.

Can you run a CBR test on site instead of in the lab?

A field CBR test is possible but does not include the controlled soaking phase, which is critical for moisture-sensitive soils like London Clay. We recommend the laboratory CBR for pavement design because the 96-hour soak simulates long-term saturation. For quick in-situ verification during construction, we offer the dynamic cone penetrometer (DCP) as a correlation tool.

Location and service area

We serve projects in Slough and surrounding areas.

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