Skip to content 
When you receive a soil report, the borehole logs show what was found in the field, but it is the laboratory tests that put numbers on how the soil will behave. Among dozens of possible tests, three appear again and again in building, road, and earthworks projects in Kenya: Atterberg limits, the California Bearing Ratio (CBR), and compaction tests.
This article explains each test in plain language: what it measures, how it is done, how to interpret results, and why it matters on real projects.
Why Laboratory Testing Matters
Field observation gives a first impression, but soil can look similar and behave very differently. Two clays may both be brown and sticky, yet one swells violently while the other is stable. Laboratory tests classify the soil, measure its strength and behaviour under controlled conditions, and provide reliable inputs for design and quality control. Most tests in Kenya follow BS 1377 or equivalent ASTM/AASHTO methods, which describe standard procedures so that results are comparable.
Part One: Atterberg Limits
What they are. Atterberg limits, named after the Swedish scientist Albert Atterberg, describe how the consistency of fine-grained soil changes with water content. A clay can behave as a solid, a plastic material that can be moulded, or a liquid, depending on how much water it contains. The boundaries between these states are the limits.
Liquid limit (LL). The water content at which soil changes from plastic to liquid behaviour. It is measured using the Casagrande cup method, where a groove in the soil is closed by a specified number of blows, or by the cone penetrometer method, which is common in British practice.
Plastic limit (PL). The water content at which soil changes from semi-solid to plastic. It is found by rolling a soil thread to about 3 mm diameter, and the limit is the water content at which the thread just begins to crumble.
Plasticity index (PI). The difference between the liquid limit and the plastic limit (PI = LL minus PL). It indicates the range of water content over which the soil is plastic.
Shrinkage limit and linear shrinkage. The shrinkage limit is the water content below which further drying produces no more volume decrease. In practice, linear shrinkage is often measured by drying a soil bar in a mould and measuring the length reduction.
Why it matters. A high PI and high LL indicate a highly plastic clay, often with high swell and shrink potential, such as black cotton soil. Low plasticity indicates silts and sandy clays with lower volume change. Atterberg limits are used to classify soils (for example, using the Unified Soil Classification System and plasticity chart), estimate expansiveness, assess suitability as fill or subgrade, and choose stabilisation methods. For road materials, specifications often cap the plasticity index of base and sub-base layers, because high plasticity causes strength loss when wet.
Part Two: California Bearing Ratio (CBR)
What it is. CBR measures the strength of soil or granular material, particularly for roads, pavements, and floor slabs. It was developed in California in the 1920s and is now used worldwide.
How it is done. A soil sample is compacted into a steel mould at a specified moisture content and density, often at the optimum from a compaction test. It may be tested unsoaked or soaked, typically by immersion in water for four days, with surcharge weights placed on top to simulate pavement load, while swell is measured. A standard piston (about 50 mm diameter) is then pushed into the sample at a constant rate (1.25 mm per minute), and the load needed to achieve penetrations of 2.5 mm and 5.0 mm is recorded.
Calculating CBR. The measured load is expressed as a percentage of the standard load required to achieve the same penetration in well-graded crushed stone (about 13.2 kN at 2.5 mm and 20 kN at 5 mm). CBR is usually reported at 2.5 mm penetration, unless the 5.0 mm value is higher, in which case the test is repeated and the higher value adopted according to standard procedure. A CBR of 100 percent corresponds to the standard crushed stone, while soft clays can have CBR values of only a few percent.
Why it matters. CBR is a key input in flexible pavement design. Higher subgrade CBR means a thinner, more economical pavement, while low CBR demands thicker layers or subgrade improvement. Specifications for road bases, sub-bases, and capping layers set minimum CBR values. Soaked CBR is especially important in Kenya because rain and poor drainage can saturate subgrades, and expansive soils may lose strength dramatically. The test also records swell, helping identify problem soils.
Limitations. CBR is an empirical test and does not directly measure fundamental soil properties. It is best used with experience and combined with other data, and field checks such as DCP tests help confirm performance.
Part Three: Compaction Tests
What compaction is. Compaction increases soil density by mechanically reducing air voids, giving higher strength, lower compressibility, and lower permeability. Compacted fill beneath floors, embankments, and pavements performs far better than loose fill.
The Proctor test. In the 1930s, R.R. Proctor discovered that, for a given compaction effort, there is an optimum water content at which a soil reaches its maximum dry density. The laboratory test compacts soil in a mould in layers using a rammer dropped a set number of times, at several different moisture contents. Dry density is calculated for each, and a compaction curve is plotted.
Standard and modified efforts. The standard Proctor (or “light” compaction) uses a lighter rammer (about 2.5 kg dropped from 300 mm). The modified Proctor (or “heavy” compaction) uses a heavier rammer (about 4.5 kg dropped from 450 mm) and delivers more energy. The choice depends on the project and specification, with pavements and heavily loaded fills often using modified effort.
Key results. The peak of the curve gives the maximum dry density (MDD) and the optimum moisture content (OMC). Compacting wetter or drier than optimum gives lower density for the same effort.
Why it matters. The compaction test sets the target for site compaction. Specifications typically require field density to reach a stated percentage of MDD, such as 95 or 98 percent, with moisture within a range of OMC. Field density tests (sand replacement or nuclear gauge) verify compliance. Without a proper compaction test, there is no reliable way to know whether fill is adequate.
How the Three Tests Work Together
These tests are closely connected. Atterberg limits classify the soil and warn of expansive or weak behaviour. Compaction determines how to prepare the soil at the right moisture and density. CBR then measures the strength achieved at that condition. For a road subgrade, the workflow might be: classify soil, determine MDD and OMC, prepare CBR specimens at those values, and report soaked CBR to the pavement designer.
Practical Examples in Kenya
On a road through black cotton soil, high liquid limit and plasticity index results would trigger replacement with better material or lime stabilisation. The stabilised mix would be tested again for Atterberg limits, compaction, and CBR to confirm improvement. For a building platform, compaction tests on murram fill establish the density target, and CBR results indicate floor slab support. For a borrow pit survey, comparison of Atterberg limits and CBR among samples identifies the most suitable material.
Common Pitfalls
Pitfalls include unrepresentative sampling, drying samples too much before testing, using the wrong compaction effort, testing at the wrong moisture content, reporting unsoaked CBR where soaked conditions are relevant, and poor calibration of equipment. Reports should state the standard used, the compaction effort, soaking conditions, and sample details.
Frequently Asked Questions
Which test is most important? It depends on the project, but together they provide a basic characterisation.
Why soak CBR samples? To represent the worst moisture condition the subgrade might experience.
Can I rely on field tests alone? Field tests are valuable for quick checks, but laboratory tests under controlled conditions provide the standard for design and specification.
Other Laboratory Tests Often Requested
Beyond these three, a typical programme may include particle size distribution by sieve and hydrometer to classify the soil; specific gravity and bulk density to compute volumetric relationships; direct shear and triaxial tests to obtain strength parameters for bearing capacity and slope stability; consolidation (oedometer) tests for settlement prediction in clays; swell and free swell tests for expansive soils; permeability tests for drainage and seepage analyses; and chemical tests such as pH, sulphates, and chlorides to assess aggressiveness to concrete and steel. For aggregates, tests like Aggregate Crushing Value, Los Angeles Abrasion, and flakiness index evaluate suitability for roads and concrete.
Choosing a Reliable Laboratory
Quality results depend on a well-run laboratory. Look for calibrated equipment, trained technicians, adherence to recognised standards, clear sample handling procedures, and transparent reporting that includes test conditions. Accreditation or evidence of regular proficiency testing offers extra assurance. Ask how long samples are stored, how they are protected from moisture loss, and whether the laboratory can provide raw data sheets. A reputable laboratory welcomes such questions.
Conclusion
Atterberg limits, CBR, and compaction tests form the backbone of routine laboratory soil testing. Atterberg limits tell you how plastic and potentially troublesome a soil is, compaction tells you how to prepare it, and CBR tells you how strong it will be as a subgrade or fill. Understanding them helps clients and engineers read reports intelligently, specify materials correctly, and build durable roads, floors, and embankments across Kenya’s varied soils.
