BUILDING SOIL INVESTIGATION

Site Investigation ~ this is an all embracing term covering every aspect of the site under investigation.

Soil Investigation ~ specifically related to the subsoil beneath the site under investigation and could be part of or separate from the site investigation.

Purpose of Soil Investigation ~

1. Determine the suitability of the site for the proposed project.
2. Determine an adequate and economic foundation design.
3. Determine the difficulties which may arise during the construction process and period.
4. Determine the occurrence and/or cause of all changes in subsoil conditions.

The above purposes can usually be assessed by establishing the physical, chemical and general characteristics of the subsoil by obtaining subsoil samples which should be taken from positions on the site which are truly representative of the area but are not taken from the actual position of the proposed foundations. A series of samples extracted at the intersection points of a 20„000 square grid pattern should be adequate for most cases.

Soil Samples ~ these can be obtained as disturbed or as undisturbed samples.

Disturbed Soil Samples ~ these are soil samples obtained from bore holes and trial pits. The method of extraction disturbs the natural structure of the subsoil but such samples are suitable for visual grading, establishing the moisture content and some laboratory tests. Disturbed soil samples should be stored in labelled airtight jars.

Undisturbed Soil Samples ~ these are soil samples obtained using coring tools which preserve the natural structure and properties of the subsoil. The extracted undisturbed soil samples are labelled and laid in wooden boxes for dispatch to a laboratory for testing. This method of obtaining soil samples is suitable for rock and clay subsoils but difficulties can be experienced in trying to obtain undisturbed soil samples in other types of subsoil.

The test results of soil samples are usually shown on a drawing which gives the location of each sample and the test results in the form of a hatched legend or section.

Depth of Soil Investigation ~ before determining the actual method of obtaining the required subsoil samples the depth to which the soil investigation should be carried out must be established. This is usually based on the following factors †

1. Proposed foundation type.
2. Pressure bulb of proposed foundation.
3. Relationship of proposed foundation to other foundations.

Pressure bulbs of less than 20% of original loading at foundation level can be ignored † this applies to all foundation types.

Soil Investigation Methods ~ method chosen will depend on several factors †

1 . Size of contract.
2. Type of proposed foundation.
3. Type of sample required.
4. Type of subsoils which may be encountered.

As a general guide the most suitable methods in terms of investigation depth are †

1 . Foundations up to 3 000 deep † trial pits.
2. Foundations up to 30 000 deep † borings.
3. Foundations over 30 000 deep † deep borings and in-situ examinations from tunnels and/or deep pits

Typical Trail Pit Details ~

Boring Methods to Obtain Disturbed Soil Samples ~
 
1 . Hand or Mechanical Auger † suitable for depths up to 3 000 using a 150 or 200mm diameter flight auger.
2. Mechanical Auger † suitable for depths over 3 000 using a flight or Cheshire auger † a liner or casing is required for most granular soils and may be required for other types of subsoil.
3. Sampling Shells † suitable for shallow to medium depth borings in all subsoils except rock.

Typical details ~

 

Wash Boring ~ this is a method of removing loosened soil from a bore hole using a strong jet of water or bentonite which is a controlled mixture of fullers earth and water. The jetting tube is worked up and down inside the bore hole, the jetting liquid disintegrates the subsoil which is carried in suspension up the annular space to a settling tank. The settled subsoil particles can be dried for testing and classification. This method has the advantage of producing subsoil samples which have not been disturbed by the impact of sampling shells however it is not suitable for large gravel subsoils or subsoils which contain boulders.

Typical Wash Boring Arrangement  ~

Mud-rotary Drilling ~ this is a method which can be used for rock investigations where bentonite is pumped in a continuous flow down hollow drilling rods to a rotating bit. The cutting bit is kept in contact with the bore face and the debris is carried up the annular space by the circulating fluid. Core samples can be obtained using coring tools.

Core Drilling ~ water or compressed air is jetted down the bore hole through a hollow tube and returns via the annular space. Coring tools extract continuous cores of rock samples which are sent in wooden boxes for laboratory testing.