The type of soil and rock that comprises the earthworks can greatly affect the ease of construction, the fill placement and compaction methods, the potential environmental effects of the operation and ultimately the cost of the job.
Determining soil and rock properties requires a technical background. This link provides an overview of the detailed analysis required to assess basic properties. A very useful reference is the two-page Guideline for Field Classification and Description of Soils and Rock for Engineering Purposes, New Zealand Geotechnical Society, December 2005.
Soil and rock come in many shapes and forms. The following provides basic descriptions of the broad classifications. One relatively easy aspect on which to categorise soils is by the size of the particles:
- Boulders – large rock segments (> 200 mm). They typically must be removed during road construction, however, in most situations, they make excellent material for retaining walls
- Cobbles – larger rock segments 60-200 mm. Unless they are integrated into a deep base course, they are typically too large to integrate into the pavement. They move as trucks pass over them and create voids where water can get in. Cobbles are typically an ideal size for armouring and rip rap material
- Gravels – particles between 2-60 mm. They should make up the bulk of any improvement layer
- Sand – rock fragments of various sizes and shapes. These may be either rock fragments or single minerals
- Silt – very fine sand particles. Silt is less plastic and more permeable than clay, and displays ‘dilatant’ and ‘quick’ behaviour. Quick behaviour refers to the tendency of silt to liquefy when shaken or vibrated, while dilatancy refers to the tendency to undergo volume increase when its shape is changed (deformed)
- Clay – particles that are fundamentally different from silts. They are formed by chemical weathering and exhibit the properties of ‘cohesion’ and ‘plasticity’, which are not found in sand or gravel. Cohesion refers to the fact that the material sticks together, while plasticity is the property that allows the material to be deformed without volume change or rebound, and without cracking or crumbling. Plasticity can be estimated in the field by rolling a sample in the hand to form a clay ‘worm’ that stays connected without splitting. Clay is also what makes a pavement impermeable to protect the base course from high moisture content and subsequent failure. The strongest roads will be built with an adequate amount of clay in the pavement.
Grain size criteria
Other soil properties include:
- Organic soil – a soil that has such a high organic content that it no longer behaves like a silt or clay. Organic soil typically has little to no strength, and will degrade over time. Soils containing small to moderate amounts of organic material still retain the properties of silts or clays
- Difficult soils – New Zealand soils that do not fit easily into the above classification. For example, volcanic ash.
Unless a soil has been extensively sieved, it will be a mixture of the components above. Most in-situ soils will be primarily a mixture of the smaller components – sand, silt and clay.
Soils are categorised as being uniform if they have a narrow range of particle size, whereas ‘well graded’ soils have a broad range of particle size. Sand, gravel or cobbles will not exhibit any bonding characteristic, and soils with an overall high percentage of these components will be referred to as ‘granular soil’. Conversely, soils with higher percentages of fines (defined as silt or clays) are referred to as ‘cohesive’ soils.
The following are basic ways to estimate particle size and soil properties in the field:
- Rub a small quantity of the soil between the palms of your hands and try to shake the material off by rubbing and patting your hands together. Sand grains will fall off, leaving your palms clean, and silt and clay will stick in the fine creases, leaving a ‘dirty’ appearance.
- Next try to roll the material into a 3-4 mm wide worm by rolling a sample in the hand to see if it stays connected without splitting. Easily rolled indicates medium to high plasticity, so it has a high clay content.
- Check for quick behaviour. A simple test of ‘patting’ a saturated soil sample in the hand can be undertaken to assess these properties and distinguish silt from clay. Roll a wet sample into a 20 mm ball and flatten it. Now shake or tap the hand to see if water rises. If so, it is a silt.
Rocks fall into three broad categories: Sedimentary, metamorphic and igneous (volcanic). Engineering properties cannot be inferred from rock names. Rock strength and the degree of weathering are important considerations for the forest engineer in determining pavement properties. Rock is described on a scale from extremely strong to extremely weak. There are many methods and scales for describing the weathering of rock masses.
An additional suggested reference is the NZTA Standard Specification F/1 – Earthworks Construction.