Almost every geotechnical decision starts with one question: what kind of soil is this? Whether a material can be used as fill, how a subgrade will behave when it gets wet, and which strength and settlement tests are worth running next all depend on knowing if you are dealing with a clean sand, a silt, or a high-plasticity clay. Soil classification tests answer that question. They are relatively quick and inexpensive, and they are the foundation that more advanced testing is built on.
In Malaysian practice these tests are commonly carried out to BS 1377-2, the classification tests part of the British Standard for soil testing (revised in 2022).
1. Moisture Content
The simplest test and the most frequently run. A sample is weighed, oven-dried, and weighed again. The moisture content is the mass of water lost expressed as a percentage of the dry soil mass.
On its own, moisture content says little. Its value comes from comparing it with the soil's Atterberg limits and with the optimum moisture content from compaction testing. A clay whose natural moisture content sits close to its liquid limit, for example, is likely to be soft and highly compressible.
2. Atterberg Limits
Fine-grained soils change behaviour dramatically as their water content changes, from a hard solid, to a crumbly semi-solid, to a mouldable plastic material, to something that flows like a liquid. The Atterberg limits mark those boundaries.
- Liquid limit (LL): the moisture content at which the soil passes from the plastic to the liquid state. BS 1377-2 gives the cone penetrometer method as the preferred method, in which a standard cone is allowed to sink into the soil paste. The traditional Casagrande cup method is an alternative.
- Plastic limit (PL): the lowest moisture content at which the soil can be rolled into a thread about 3 mm in diameter without crumbling.
- Plasticity index (PI): the difference between the two (PI = LL − PL), the range of moisture content over which the soil behaves plastically.
The Plasticity Chart
Plotting PI against LL on a plasticity chart separates fine soils into groups. Soils plotting above the chart's "A-line" are classified as clays and those below it as silts. The liquid limit then indicates low, intermediate, or high plasticity. High-plasticity clays tend to shrink and swell significantly with changes in moisture, which matters for pavements, slabs on grade, and light structures.
3. Particle Size Distribution
Particle size distribution (PSD), or grading, measures the proportions of gravel, sand, silt, and clay in a soil.
- Sieving separates the coarser fraction by passing the soil through a stack of sieves of decreasing aperture. Wet sieving is used when fines would otherwise cling to larger particles.
- Sedimentation (the hydrometer or pipette method) is used for the silt and clay fraction, which is too fine to sieve, by measuring how quickly particles settle out of suspension.
The result is a grading curve. It shows whether a soil is well graded (a good spread of sizes that compacts densely) or uniformly graded, and whether its fines content is high enough to make it moisture-sensitive.
How the Results Are Used
- Material acceptance: checking whether a borrow source meets the specification for fill, subgrade, or sub-base.
- Planning further testing: classification decides which strength, compaction, and consolidation tests are appropriate.
- Correlations: many design correlations, including those used to interpret field tests such as the Mackintosh probe, only apply to particular soil types.
- Problem soils: soft clays, and organic soils and peat, which are common in low-lying parts of Sarawak, need to be identified early because they behave very differently from inorganic mineral soils.
Note: Classification results describe the samples tested. How representative they are of the site depends on the sampling and investigation plan, which should be designed by a qualified geotechnical engineer.
