Friday, December 13, 2013


Soil Assessment ~ prior to designing the foundations for a building or structure the properties of the subsoil(s) must be assessed.

These processes can also be carried out to confirm the suitability of the proposed foundations. Soil assessment can include classification, grading, tests to establish shear strength and consolidation. The full range of methods for testing soils is given in BS 1377: Methods of test for soils for civil engineering purposes.

Classification ~ soils may be classified in many ways such as geological origin, physical properties, chemical composition and particle size. It has been found that the particle size and physical properties of a soil are closely linked and are therefore of particular importance and interest to a designer.

Particle Size Distribution ~ this is the percentages of the various particle sizes present in a soil sample as determined by sieving or sedimentation. BS 1377 divides particle sizes into groups as follows:-

Gravel particles - over 2mm
Sand particles - between 2mm and 0.06mm
Silt particles - between 0.06mm and 0.002mm
Clay particles - less than 0.002mm

The sand and silt classifications can be further divided thus:-

The results of a sieve analysis can be plotted as a grading curve thus:-

The results of a sieve analysis can be plotted as a grading curve thus:-

Triangular Chart ~ this provides a general classification of soils composed predominantly from clay, sand and silt. Each side of the triangle represents a percentage of material component. Following laboratory analysis, a sample's properties can be graphically plotted on the chart and classed accordingly. e.g. Sand † 70%. Clay † 10% and Silt † 20% = Sandy Loam.


Silt is very fine particles of sand, easily suspended in water. Loam is very fine particles of clay, easily dissolved in water.

Site Soil Tests ~ these tests are designed to evaluate the density or shear strength of soils and are very valuable since they do not disturb the soil under test. Three such tests are the standard penetration test, the vane test and the unconfined compression test all of which are fully described in BS 1377; Methods of test for soils for civil engineering purposes.

Standard Penetration Test ~ this test measures the resistance of a soil to the penetration of a split spoon or split barrel sampler driven into the bottom of a bore hole. The sampler is driven into the soil to a depth of 150mm by a falling standard weight of 65kg falling through a distance of 760mm. The sampler is then driven into the soil a further 300mm and the number of blows counted up to a maximum of 50 blows. This test establishes the relative density of the soil.

The results of this test in terms of number of blows and amounts of penetration will need expert interpretation.

Vane Test ~ this test measures the shear strength of soft cohesive  soils.  The  steel  vane  is  pushed  into  the  soft  clay  soil  and  rotated  by  hand at a constant rate. The amount  of  torque  necessary  for  rotation  is  measured and the soil shear strength  calculated as shown below.

This test can be carried out within a  lined  bore  hole  where  the  vane  is  pushed  into  the  soil  below  the  base  of the bore hole for a distance equal  to  three  times  the  vane  diameter  before  rotation  commences.

Alternatively the vane can be driven  or jacked to the required depth, the  vane being protected within a special  protection  shoe,  the  vane  is  then  driven  or  jacked  a  further  500mm  before rotation commences.

Unconfined Compression Test ~ this test can be used to establish the shear strength of a non-fissured cohesive soil sample using portable apparatus either on site or in a laboratory. The 75mm long 38mm diameter soil sample is placed in the apparatus and loaded in compression until failure occurs by shearing or lateral bulging. For accurate reading of the trace on the recording chart a transparent viewfoil is placed over the trace on the chart.

Typical Apparatus Details ~

NB. The shear strength of clay soils is only half of the compression strength values given above.

Laboratory Testing ~ tests for identifying and classifying soils with regard to moisture content, liquid limit, plastic limit, particle size distribution and bulk density are given in BS 1377.

Bulk Density ~ this is the mass per unit volume which includes mass of air or water in the voids and is essential information required for the design of retaining structures where the weight of the retained earth is an important factor.

Shear Strength ~ this soil property can be used to establish its bearing capacity and also the pressure being exerted on the supports in an excavation. The most popular method to establish the shear strength of cohesive soils is the Triaxial Compression Test. In principle this test consists of subjecting a cylindrical sample of undisturbed soil (75mm long 38mm diameter) to a lateral hydraulic pressure in addition to a vertical load. Three tests are carried out on three samples (all cut from the same large sample) each being subjected to a higher hydraulic pressure before axial loading is applied. The results are plotted in the form of Mohr's circles.

Shear Strength ~ this can be defined as the resistance offered by a soil to the sliding of one particle over another. A simple method of establishing this property is the Shear Box Test in which the apparatus consists of two bottomless boxes which are filled with the soil sample to be tested. A horizontal shearing force (S) is applied against a vertical load (W) causing the soil sample to shear along a line between the two boxes.

Consolidation of Soil ~ this property is very important in calculating the movement of a soil under a foundation. The laboratory testing apparatus is called an Oedometer.

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