Table of Contents

4.2.2 Assessment

The suitability of a natural granular soil for use as a pavement material is usually assessed using a series of relatively simple tests. Attributes such as stability, wear resistance, permeability and workability may be inferred from the results of these routine tests, which include the following:

  • particle size distribution
  • plastic limit
  • liquid limit
  • plasticity index
  • linear shrinkage
  • maximum dry compressive strength
  • ball mill value
  • static triaxial shear tests (including Texas triaxial test)
  • repeated load triaxial (RLT) test
  • California Bearing Ratio (CBR).

In addition to these tests, the rapid methods of field assessment and classification outlined below are also used.

It is not usually necessary to use all the tests listed. Engineering judgement should be used to determine which tests should be employed, according to circumstances. The tests themselves are only discussed briefly in this Part, and standard test procedures should be consulted.

Pre-treatment

The application and interpretation of test results is based on an assumption that the samples are tested under similar condition to that those likely to be applicable in the field. As some materials may require mechanical breakdown before use, or be prone to excessive chemical or physical degradation, either during handling or subsequently, pre-treatment of material samples may be necessary before testing.

Generally pre-treatment is not required if the particles are rounded, hard and weather resistant, or if the amount of fines exceeds that required to fill the voids in the coarse particles. Soft rock that requires crushing, and other materials prone to mechanical breakdown or weathering, should be pre-treated.

The laboratory pre-treatment procedures available generally simulate mechanical breakdown, weathering or a combination of both. The form of pre-treatment depends on the nature of the particular materials. For example, materials such as conglomerates, concretionary laterites and limestones may be broken with a hammer, or subjected to a compactive effort equivalent to the crushing action expected from field construction plant. Materials such as shales, on the other hand, may be subjected to cycles of wetting and drying in an attempt to simulate any breakdown likely to occur under field weathering conditions.

Field assessment and classification

A preliminary assessment of the need for comprehensive testing of a material can be conducted using field procedures. The techniques available generally rely on visual observations of the material and its reaction to a variety of simple treatments. Experience is the most important guide.

A simple assessment of a material may be gained by wetting the material and squeezing it in the hand. For example, with well-graded materials, if a portion with particles smaller than about 5 mm is wet and then squeezed in the hand, the following characteristics may be observed:

  • the material feels extremely gritty
  • the material can be formed into definite shapes that retain their form even when dried
  • the hands may be slightly discoloured because of the adherence of clay; if more than enough material to slightly discolour the hands adheres, examination should show that it consists of both sand and clay and not clay alone
  • when the wet sample is patted into the palm of the hand it will compact into a dense cake that cannot be penetrated readily with the blunt end of a lead pencil.

The grittiness of the sample indicates the presence of sufficient granular material. Development of some strength on drying indicates the presence of a sufficient amount of binder material. Resistance to the penetration of the pencil-size stick, even when the sample is thoroughly wetted, indicates interlocking of the grains and the presence of sufficient internal friction.

The presence of too much sand will cause the sample to fall apart when dried whilst the presence of too much clay will leave the hand muddy after the wet sample is squeezed. As a result, the wet sample, after being patted, will offer little resistance to the penetration of the stick.

The Unified Soil Classification System (Appendix A) provides a more formal method of assessing a material with simple field tests that may be confirmed by subsequent laboratory procedures. This system of classification is useful because the properties of the various material groups are known, and have been tabulated in engineering use charts.

This system divides materials into two major divisions: coarse-grained and fine-grained. Highly organic materials are also described. Identification in the field is accomplished by visual assessment of the coarse grains, and by a few simple hand tests of the fine grained materials or fractions.