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

Chapter 1 PLANNING AND SCOPE
Limitations associated with this test include:
• Relatively shallow test depths (deeper depths in coarse materials give misleading results); and
• Prone to human errors: variation in drop weight or exerting force, gives rise to misleading
results, and risks of wrong counting unless mechanical counter is used.
Precautionary measures to be observed require that:
• The drop of the hammer should be free falling and consistent with each drop height; and
• The components and apparatus must be properly washed and oiled.
1.7.6 Examination In-Situ
1.7.6.1 Trial Pit
Figure 1.11 Mackintosh Probe
Trial pits provide the best method of obtaining very detailed information on strength, stratification,
pre-existing shear surfaces, and discontinuities in soil. Very high quality block samples can be taken
only from trial pits.
It is as well to note that every year many people are killed during the collapse of unsupported
trenches. Remember to be careful — do not enter trenches or pits more than 1.2m deep without
either supporting the sides or battering back the sides. Even so, if a pit is dug and remains stable
without support, a quick means of exit such as a ladder should be provided.
Trial pits may be excavated by either hand digging or machine excavation. In general, machine
excavation is used for shallow pits, whereas hand excavation is used for deep pits which must be
supported. In the limited space of a trial pit, which is often 1.5m x 3m in plan area at ground level, it
is usually impossible to place supports as machine excavation proceeds. Shallow trial pits provide a
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