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

Chapter 1 PLANNING AND SCOPE
1.7.4 Exploration Pit Excavation
Exploration pits and trenches permit detailed examination of the soil and rock conditions at shallow
depths and relatively low cost. Exploration pits can be an important part of geotechnical explorations
where significant variations in soil conditions occur (vertically and horizontally), large soil and or or
non-soil materials exist (boulders, cobbles, debris) that cannot be sampled with conventional
methods, or buried features must be identified and or or measured.
Exploration pits are generally excavated with mechanical equipment (backhoe, bulldozer) rather than
by hand excavation. The depth of the exploration pit is determined by the exploration requirements,
but is typically about 2 m (6.5 ft) to 3 m (10 ft). In areas with high groundwater level, the depth of
the pit may be limited by the water table. Exploration pit excavations are generally unsafe and or or
uneconomical at depths greater than about 5 m (16 ft) depending on the soil conditions.
1.7.5 Probing
A wide range of dynamic and static penetrometers are available, with different types being used in
different countries. However, the objective of all probing is the same, namely to provide a profile of
penetration resistance with depth, in order to give an assessment of the variability of a site. Probing
is carried out rapidly, with simple equipment. It produces simple results, in terms of blows per unit
depth of penetration, which are generally plotted as blowcount or depth graphs
1.7.5.1 MacKintosh Probe
The Mackintosh prospecting tool (also commonly known as JKR probe) consists of rods which can be
threaded together with barrel connectors and which are normally fitted with a driving point at their
base, and a light hand-operated driving hammer at their top (Fig. 1.10). The tool provides a very
economical method of determining the thickness of soft deposits such as peat.
The driving point is streamlined in longitudinal section with a maximum diameter of 27mm. The drive
hammer has a total weight of about 5kg. The rods are 1.2 m long and 12mm dia. The device is often
used to provide a depth profile by driving the point and rods into the ground with equal blows of the
full drop height available from the hammer: the number of blows for each 300 mm of penetration is
recorded. When small pockets of stiff clay are to be penetrated, an auger or a core tube can be
substituted for the driving point. The rods can be rotated clockwise at ground level by using a box
spanner and tommy bar. Tools can be pushed into or pulled out of the soil using a lifting or driving
tool. Because of the light hammer weight the Mackintosh probe is limited in the depths and materials
it can penetrate.
In Malaysia, this method of investigation is usually employed during preliminary investigative works.
It involves the use of:
• 5 kg hammer weight,
• Dropped from a guided free fall height of 280mm (JKR probe), and
• Usually carried out up to a depth of 12m, or upon encountering the 400 resistance blows or 300
mm.
The test itself is relatively cheap and quick to execute, and is used to establish:
• Localised soft area or weak layer or spot or slip plane;
• The presence of hard or bearing layers or shallow bedrocks, as in the case of limestone profiling;
• Preliminary subsoil information (eg. soil consistency & undrained shear strength, c u ); and
• The interpolation between boreholes or piezocones.
1-24 March 2009