Trial Pits and Hand Auger Holes

Purpose ~ primarily to obtain subsoil samples for identification, classification and ascertaining the subsoil's characteristics and

properties.

 

Trial pits and augered holes may also be used to establish the presence of any geological faults and the upper or lower limits of the water table.

Use of Trial Pits ~ subsoil can be visually examined in-situ / both disturbed and undisturbed samples can be obtained. to expose and/or locate underground services.

General use ~ liner tubes could be used if required to extract subsoil samples at a depth beyond the economic limit of trial holes

Advantages ~ generally a cheaper and simpler method of obtaining subsoil samples than the trial pit method.

 

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.

 

3

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.

 

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

 

Disturbed Soil Samples ~ these are soil samples obtained from boreholes 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. Types 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 m deep - trial pits.


2.  Foundations up to 30m deep - borings.


3.      Foundations over 30m deep - deep borings and in-situ examinations from tunnels and/or deep pits.

 

 

Boring Methods to Obtain Disturbed Soil Samples:

 

1.     Hand or Mechanical Auger - suitable for depths up to 3m using a or 200mm diameter flight auger.

2.     Mechanical Auger - suitable for depths over 3m 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 sub soils except rock.

 

Wash Boring ~ this is a method of removing loosened soil from a borehole 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 borehole, 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.

 

 

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 borehole 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.

 

Borehole Data ~ the information obtained from trial pits or boreholes can be recorded on a pro forma sheet or on a drawing showing the position and data from each trial pit or borehole thus: 

Boreholes can be taken on a 15m to 20m grid covering the whole site or in isolated positions relevant to the proposed foundation(s).