Borings are the main procedure used for soil exploration on site. There are different types of borings. They can generally be grouped into 3 types, they are:

• Trial pits of up to 3m deep
• Borings of up to 30m deep
• Headings and Shafts

Trial pits

Trial pits are pits of about 1.2m by 1.2 m. They can be dug with the use of labour or excavating machines. Trial pits should be dug  at distances of 20m apart and clear of the positions of the building’s foundation. Trial pits are economical up to a depth of 3m. Trial pits are suitable for small-scale works. The main advantage of this method is that soil and rock samples can easily be exposed and examined.

Deep boring.

This is the process by which hand or mechanical/machine boring tools are used to drill holes into the soil for investigative purposes. Deep borings are suitable for medium to  large scale works such as high rise buildings.

Hand or mechanical auger borings are cheap methods of boring soils which can stand unsupported. Holes can be sunk up to 3m. For soils that cannot stand on their own such as loose soils, steel casing can be dug surrounding the bored area.

A steel casing called shell is used to collect soil samples for testing.

It is done at regular intervals as hole is being dug.

For deeper borings of up to 30m, machine operated drilling must be employed. They are of many types. There is the percussion boring in which the soil formation is broken up by repeated blows after which water is added to the hole as the work proceeds. The water helps to wash out the soil debris by pressure washing or by shell auger. The samples collected by this method are disturbed samples because of the repeated blows to the soil formation.

Wash boring method uses only a strong jet of water or drilling mud to bore deep holes in the soil. This method produces undisturbed samples. However, it is suitable if the soil does not contain boulders or large gravels.

Rotary drilling is a method used for boring of soil with rock formations. The drilling bit is fitted with industrial diamond to break the rock formations. Drilling mud, water or compressed air is jetted into the drilled hole to flush out the rock debris.

Headings and Shafts.

Headings and Shafts are employed to explore steeply dipping strata. It is best suited for very large scale works such as dams, power stations etc. The method is used to create exploration tunnels. The advantage they have over pits drills is that they can be easily drained of water and also easy removal of soil debris. Shafts are bored using large power driven augers. The sides of the shafts must be supported to protect the work personnel engaged in soil inspection.

Choice of soil exploration methods.

The choice of different exploration methods will depend on the following factors:

• Topography.
• Nature of ground.
• Cost.

The construction of buildings, dams, reservoirs, docks and other types of construction works, soil investigation will require deep and closely spaced borings.

Borings will reveal the types of soil layers beneath the proposed foundation, and whether or not it will be suitable to carry the proposed structure safely.

Borings should be dug in a regular pattern in such a way that it gives a clear picture of all significant variations of the soil within the site.

Investigative boreholes should be at least 1.5m deep. Three main factors will govern the depth of exploration:

Depth to which the foundation load will act on beneath the base of the foundation.

Depth to which weathering will affect the soil.

Depth at which impervious strata occur.

Some investigative borings  can be up to 6m deep.

The standard depth for building structures is up to 1.5 times the breadth of the foundation.

How soil is stressed under foundation loading

Load of the structure is transferred to the foundation and then to the surrounding soil beneath the foundation. The soil stress beneath the foundation is reduced with depth.

The diagram above shows the soil stresses below a pad foundation. Notice that at 1.5b below the foundation level, the bulb stress is 0.2 of the original stress just under the foundation. This value is higher for a strip foundation as shown below.

The reason for this is because strip foundation is continuous while pad foundation is isolated.

Before spending money on practical on-site soil test/ investigation, it is good to  find out if these tests/investigations have been carried out in the past by Government agencies. checks may reveal records of previous, close-by soil investigations. Geological data, aerial photographs, historical information and local knowledge of the site area will help a great deal.

Obtaining preliminary data is best gotten from the relevant government agency or establishment. This will ensure that you are using standardized and government recognized data for your preliminary soil investigation.

Preliminary data may not provide all the necessary information you need concerning the  soil situation of your site, but it saves you the cost of starting an investigation from the scratch and at the same time providing recognized data for you to work with.