Soil index tests

SOCOTEC uses a helium pycnometer to determine the density of solid soil particles. The test results can be used, among other things, to accurately calculate the pore volume, pore number and degree of saturation.

When determining the density of solid soil particles, the sample material is first dried at 110°C. It is then finely ground and at least 10 grams is separated out and placed in a pressure chamber. Helium is then pumped into the chamber at a constant temperature. When a pressure equilibrium has been achieved between the chambers, the specific mass of solid soil particles can be determined using the general gas equation. 

The geotechnical laboratory can determine the organic matter and lime content for you using two different methods. It concerns a loss on ignition and a chemical method for both contents. Read more about these two methods for both the organic matter content and the lime content here. In both cases the material is initially dried for 16 to 24 hours at 110°C.

Indexing the organic matter and lime content is important in order to further characterise the soil, specifically also gaining more insight into the depositional environment of the soil stratum. The organic matter content also has a major influence on the geotechnical behaviour of the soil and can therefore be an important addition to a geotechnical project’s available data.

With a grading curve, SOCOTEC’s geotechnical laboratory can very accurately analyse the particle-size distribution of (very) coarse or fine soil for you. This grading curve is used to characterise the soil and can also be used, for example, to determine the indicative permeability (k-value) of the material.

The particle-size distribution is plotted on a graph and then forms a curve, the so-called grading curve. If the grading curve is steep, then mainly the same particle size is present, so that the sand easily allows water to pass through. With a gradual grading curve, also called an s-curve, different particle sizes are present, with the result that porosity is reduced. As a result, the soil forms a poorly permeable layer and can also be firmly compacted. 

To determine particle shape, a combination of the angularity, sphericity and roughness of the particles is considered. The different types of particle shapes are described according to ‘Powers’ and are also listed in NEN-EN-ISO 14688-1:2019+NEN8990:2020. At SOCOTEC we can determine this particle shape for you under a microscope and photograph it to record it.

In the case of (very) coarse soils, the angularity of the particle is very important, because this has a major impact on the angle of internal friction and the compaction of the material. The particle shape also indicates a lot about the type and length of transport that the particles have undergone. This therefore provides insight into the sedimentological processes of the sand layers.

We offer you two options for determining the consistency limits of fine soil: determining the flow stress using the Fall Cone or the Casagrande method. In addition to the fact that one of these two methods must be carried out to determine the flow stress, the plastic limits must also be determined; flow stress, plastic limit and with them the plasticity index are collectively referred to as the Atterberg limits. The difference between the Fall Cone and the Casagrande method as well how the plastic limit is determined is further explained here.

Determining the consistency limits of fine soils can be performed in order to visualise the workability of the soil and to accurately determine the soil’s classification (clay versus silt). The classification of fine soil is of great importance for schematising the stratification of the underground.