After identification of a particular soil, the following are the attributes that are required of a soil to be deemed suitable for use in a pavement.

1) Stability:  This may be defined as the ability of a material to resist defamation and chemical and physical change.  Strength is an important component of stability and may be described as a resistance to stress induced defamation. 

2) Resistance to air:  a material must possess a resistance to air compatible to its position in the pavement.  Materials which are exposed to traffic or weather such as an unsealed base or shoulders, require higher resistance to abrasion and erosion than do lower levels of the pavement. 

3) Permeability:  When a material is exposed to rain permeability becomes a significant factor for consideration.  In such situations a relatively impermeable material is required to protect the underlying material, by limiting the entry of moisture and associated loss of strength.

4) Workability:  For soil to be suitable for use its physical characteristics must be such that it can be acquired easily, spread and brought to the desired moisture content and compacted to the degree required for it to perform its function satisfactorily.  In addition for a base which is to be sealed a smooth dense even surface to which bitumen must adhere must be achievable.  It is with these parameters in mind that the following testing systems have been devised over time in an effort to produce empirical standard evidence which will allow engineers and field personnel to evaluate the ability of any given soil to perform. 

PARTICLE SIZE DISTRIBUTION
The particle size distribution of a soil, often referred to as the grading, is determined by passing a sample of the soil through a series of sieves and weighing the portions retained.  Particle Size Distribution is usually described in terms of the accumulative percentage mass of particles passing each sieve used in the analysis and may be plotted in the form of a graph below.  Screened sieve sizes are consistent with giving information on the soil composition by grain as discussed previously.

The Particle Size Distribution curve  gives assorted samples of varying particle size distributions which are deemed suitable for the different load bearing capacities that a particular pavement would be subjected to.  In essence, the objective of Endurazyme is to shift some of these curves so that higher clay contents can be used to carry the same loads.  This is what users over a number of years are beginning to prove for themselves.  However, this chart is useful to give a greater appreciation of the importance of ascertaining the physical properties of a soil and the importance of blending the strength of the higher aggregate with the binding ability of the fines. 

ATTERBERG LIMITS
Atterberg Limits are a series of tests which are used to give empirical information on the soils reaction to water.  This information is of a qualitative nature and tells us the plastic limit, the liquid limit, the plasticity  index and linear shrinkage of the materials.  The Atterberg limits relate to the moisture contents of cohesive soils corresponding to empirical defined boundaries between states of consistency (liquid, plastic, solids) of the fraction of soil passing the 425 micron sieve.  These boundaries and the soil phases they define are accurate.

The relationship between the plasticity index and the liquid limit of a soil gives us what is commonly referred to as the plasticity chart.  It is linked back to soil classifications and is useful for linking the subjective classification system with the empirical evidence of the Atterberg tests. 

For Plastic Index Chart click here

Internal Training Document