Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 1 PLANNING AND SCOPE
monkey. The test is carried out in conjunction with shell and auger boring and rotary drilling.
(Principal features of the equipment are given in subsequent chapters of this manual, along with
notes on its use. Interpretation of the test is empirical and common correlations used to interpret
test results are covered in subsequent chapter).
Most other types of probes are used to penetrate the soil without the need for a borehole. Probes fall
into two main categories:
a. Dynamic cones, in which the probe is driven into the soil by means of a falling hammer. (Thus
the SPT is a form of dynamic probing). For deeper penetration, without the use of a borehole,
it is necessary to reduce skin friction between the soil and the rod being driven into the
ground. Various methods are used to overcome the problem of skin friction.
b. Static cones, which are jacked into the ground at a steady rate. Cone resistance and skin
friction are measured separately, usually by providing a separate sleeve and incorporating
strain gauges into the sleeve and tip. The results obtained can be correlated with bearing
capacity and settlement factors for foundations.
A small hand probe, known as the Mackintosh probe, consists simply of a standard probe head and
connecting rods. The resistance of the soil is measured by counting the number of blows of a
standard drop hammer which is required to drive it to a set distance (usually l50mm). The device is
useful in that it gives a rough indication of subsoil conditions quickly, usually during preliminary
exploration.
1.6 EXPLORATION AND SAMPLING
The site investigations should be carried out in a scientific, orderly and cost effective manner to
determine the actual ground conditions at the site and to obtain the design parameters for
engineering analysis and design.
Because the planning of ground investigation is so important, it is essential that an experienced
geotechnical specialist is consulted by the initiator of the project and his leading technical designer
very early during conceptual design.
Planning of a ground investigation can be broken down into its component parts as summarised in
Table 1.1.
The most important step in the entire process of site investigation is the appointment of a
geotechnical specialist, at the early planning stage of a construction project. At present, much site
investigation drilling and testing is carried out in a routine way, and in the absence of any significant
plan. This can result in a significant waste of money, and time, since the work is carried out without
reference to the special needs of the project.
March 2009 1-5