What is soil?

Soil is the “skin” that covers most of the land on the earth. It is the result of erosion (weathering) of rock, or the deposition of sediments by wind, water or gravity.

Soil is composed of different combinations of minerals in small particles of three main sizes:

CLAY particles (the smallest) and SILT particles, both of these are less than 0.075mm in diameter – too small to see or feel individual particles.

SAND particles 0.075mm to about 2.0mm in diameter (sand can be fine, medium or coarse).

There are also gravels, cobbles and boulders which make up soil, but we are interested here in the three main particles: SAND, SILT and CLAY. Depending on the proportion of each of these particles, the soil is described as a CLAYEY soil, SANDY soil etc.

Why have soil testing?

For the protection of your investment:

For a small sum you can protect, possibly, the largest investment of your life. Without the right foundations, buildings can move and walls can crack. Why risk the expense for the cost of a soil test? Problems can be discovered prior to purchase or building. Being aware of any problems will enable you to determine future costs and help you to make vital decisions about the suitability of the land for building. This is a better alternative to finding problems after purchase of land or after building has commenced.

About soil testing:

A Drill Rig and other equipment are used to collect information and samples at various levels on the site. Samples are taken to the laboratory for testing. A detailed report and summary of data is prepared. This is used by the engineer to design the foundations in accordance with Australian Standards. This design will match the site conditions, give you peace of mind yet will reduce the risk of over design thus saving you money.

More about soil testing:

Soil Testing is an essential part of foundation design. Foundations are vital to the integrity of every structure being the interface with the soil or rock below. Soil has a variety of properties which can vary from place to place and from layer to layer even within the limits of the proposed structure. At times soil characteristics can change considerably within a small area. Weather, climatic changes, and site management can, in the future, affect the bearing qualities of the soil. Putting it simply, if the foundations are not designed to the carrying capacity of the soils then they will fail and so will the building.

Who needs to get a soil test?

In short – anyone starting a new building . The main reason a soil test is needed is to identify the soils that exist on a site and to predict the way these soils are going to behave. Some of the soils around Geelong for example are notoriously bad – in fact some of the worst soil and foundation problems in Victoria occur in Geelong. That is why you need an experienced consultant to correctly identify any potential problems before you start work.

How is a soil test done?

A skilled geotechnical consultant can usually gain all the information he/she needs by digging into the soil and carrying out some simple field tests. The ability to correctly identify the soil comes with training and experience. Often soil samples are collected and subjected to various laboratory tests in order to determine soils types or behaviour predictions. The soil samples can be collected in a variety of ways, including by hand auger, drill rig or backhoe.

Other equipment or tests you may see referred to include: VANE SHEAR APPARATUS – used to measure the cohesion of clay soil, hence calculate its strength and bearing capacity. PENETROMETER TEST or DCP (dynamic cone penetrometer) – used to measure the density of sandy soils so that the strength and bearing capacity can be determined. LINEAR SHRINKAGE, CORE SHRINKAGE, and LIQUID LIMIT TESTS – these are all tests to measure the properties of soil in the laboratory and help to predict its behaviour on a site.

Why is reactivity important? and what is the soil classification?

For clarity and consistency, an Australian Standard (AS 2870) has been written to help in the description of soils. All soils are CLASSIFIED depending on their behaviour.

CLAY soils are classified as being:

SLIGHTLY REACTIVE     Class S

MODERATELY REACTIVE     Class M

HIGHLY REACTIVE     Class H

EXTREMELY REACTIVE     Class E

Once the classification has been made, then a footing to suit the site can be chosen. (The footing is that part of the building that rests on the foundation – that is, the soil). The more reactive a soil, the stronger the footing that is needed to resist the shrinking and swelling movement as the clay becomes wet or dry. If the soil movement is not resisted, then building movement and damage may occur.

Why is the description of the soil important?

The three main soil types each behave differently.

CLAYS because of their very small particle size are sensitive to variations in moisture content, they absorb a huge amount of water and SWELL in the process and if the water is removed they SHRINK. This shrinking and swelling is called REACTIVITY.Different types of clays have different degrees of reactivity. The clay particles also stick together and they are said to have cohesion, this is what gives clays their strength.

SANDS do not shrink or swell and do not absorb much water, their strength is derived from friction between the particles which can be altered by the addition or removal of moisture.

SILTS have some of the properties of clay and some of sand. Pure silt is rare but it often occurs with other soils eg. silty clay or silty sand. The addition of silt changes the behaviour of sands and clays. The design of the building’s footings (and some aspects of the building itself) depends on the soil type, thus it is important to correctly identify what type of soil exists on a site where a building is proposed.