Soil Testing

Laboratory tests

A wide variety of laboratory tests will be performed on soils to measure a wide variety of soil properties. Some soil properties are intrinsic to the composition of the soil matrix and are not affected by sample disturbance, while other properties depend on the structure of the soil as well as its composition, and can only be effectively tested on relatively undisturbed samples. Some soil tests measure direct properties of the soil, while others measure "index properties" which provide useful information about the soil without directly measuring the property desired.

Atterberg limits

The Atterberg limits define the boundaries of several states of consistency for plastic soils. The boundaries are defined by the amount of water a soil needs to be at one of those boundaries. The boundaries are called the plastic limit and the liquid limit, and the difference between them is called the plasticity index.

Direct shear

The direct shear test determines the consolidated, drained strength properties of a sample. A constant strain rate is applied to a single shear plane under a normal load, and the load response is measured. If this test is performed with different normal loads, the common shear strength parameters can be determined.

Particle-size analysis

This is done to determine the soil gradation. Coarser particles are separated in the sieve analysis portion, and the finer particles are analyzed with a hydrometer. The distinction between coarse and fine particles is usually made at 75 µm. The sieve analysis shakes the sample through progressively smaller meshes to determine its gradation. The hydrometer analysis uses the rate of sedimentation to determine particle gradation.

Maximum Dry Density (MDD) & Optimum Moisture Content (OMC)

These tests are used to determine the maximum unit weight and optimal water content a soil can achieve for a given compaction effort.

California bearing ratio

A test to determine the aptitude of a soil or aggregate sample as a road subgrade. A plunger is pushed into a compacted sample, and its resistance is measured.

Triaxial shear tests

This is a type of test that is used to determine the shear strength properties of a soil. It can simulate the confining pressure a soil would see deep into the ground. It can also simulate drained and un drained conditions.

Unconfined compression test

This test compresses a soil sample to measure its strength. The modifier "unconfined" contrasts this test to the triaxial shear test.

Water content

This test provides the water content of the soil, normally expressed as a percentage of the weight of water to the dry weight of the soil.