Tests on Soil Samples
Laboratory testing is carried out on retrieved disturbed and undisturbed soil samples during fieldwork. The following section enlists and gives details of relevant tests carried out on samples as required for determining the subsurface conditions and correlating with the information obtained from field testing and sampling.
Grain Size Analysis
A soil consists of an assemblage of discrete particles of various sizes and shapes. The object of grainsize analysis is to group these particles into separate range of sizes and to determine the relative proportions by dry weight of each size range. Grainsize analysis is conducted in two stages. Particles size distribution of coarse grained soils is performed by sieve analysis while hydrometer test is conducted to establish distribution of fine-grained soil (clayey or silty soil).
The liquid and plastic limits of soil are parameters that define the state of the soil at different water content levels. The liquid limit is the water content above which the soil goes from solid phase to liquid phase and the plastic limit indicates the water content below which the soil mass makes the transition from a plastic, remouldable solid to a brittle mass which cannot be remoulded any more. The difference in the water contents at Liquid and Plastic limits is termed as the plasticity index and it is a measure of the plasticity of the soil under consideration. The samples used for determining the limits are finer than the “#40 US sieve”.
Density and Moisture Content
The weight per unit volume of the solid portion of soil is called particle (dry) density. Whereas, the oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk (wet) density. The bulk density of a soil is always smaller than its particle density.
Natural moisture content is the quantity of water contained in a soil or rock sample. It is the ratio of the weight of water to the weight of solids in a given volume of soil or rock sample.
The specific gravity of a given material is defined as the ratio of the weight of a given volume of material to the weight of an equal volume of distilled water at standard temperature. It is denoted by Gs. This quantity is used for calculation of void ratio, in hydrometer test, degree of saturation and also weight-volume relationship.
California Bearing Ratio
California Bearing Ratio (CBR) gives the relative penetration resistance of soil samples which have been compacted in moulds. It is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min to that required for the corresponding penetration of a standard material. The California Bearing Ratio Test (CBR Test) is a penetration test developed by California State Highway Department (U.S.A.) for evaluating the bearing capacity of soil for design of flexible pavement. CBR tests are carried out on natural or compacted soils in water soaked or un-soaked conditions.
Sulphate in groundwater or soil can attack concrete placed in the ground or on surface. A reaction takes place between the Sulphate and the Aluminate compounds present in the cement, causing crystallization of complex compounds. The expansion, which accompanies crystallization, induces stresses in the concrete, which results in mechanical disintegration. The following chemical tests are carried out on soil and groundwater samples:
- Total dissolved solids
- Chloride content
- Sulphate content
Unconfined Compression Test
Unconfined compressive strength test involves axially loading a undisturbed or remoulded cylindrical cohesive soil sample to failure. The term unconfined is used because the lateral force on the sample is zero.
Direct Shear Test
The direct shear test involves, placing a test sample in a rectangular box having two portions, the top portion can move while the lower portion is fixed. The sample is consolidated under a normal load. Also, double drainage is ensured by placing porous stones on the top and bottom of the sample. Hence, the test may be carried out under drained conditions. The shear strength of a remoulded sample is measured by applying a horizontal force which pushes the top part of rectangular box, the sample is sheared to failure and the normal and shear stresses at the point of failure are noted down.
Organic matter of samples is determined by submerging the samples passing through #40 sieve in H2O2 (Hydrogen peroxide). The samples is then oven dried. The loss in weight of the sample helps in determining organic content.
Consolidation is the characteristic of a soil that is associated with the following effects of imposed loading:
- Dissipation of excess porewater pressure
The consolidation characteristic of clays is determined in the Oedometer cell. Double drainage path is provided to the sample in the form of porous stones at the top and bottom of the sample. The sample used for the consolidation test is completely saturated and remains submerged in the water throughout the duration of the test. The double drainage allows fast dissipation of excess porewater pressure generated as a result of the imposition of static normal load on the sample.
It is used to determine the relationship between water content and dry unit weight of soil for a specified compactive effort. The compactive effort is the amount of mechanical energy that is applied to the soil mass. The test is used to determine Maximum Dry Density (MDD) and Optimum Moisture Content (OMC).