Appendix D Geotechnical report App · MCU18/0013

-2-

 

 

INVESTIGATION METHODS
The following is a brief summary of investigation
methods currently adopted by the Company and some
comments on their use and application.
Test Pits - These are normally excavated with a
backhoe or a_ tracked excavator, allowing close
examination of the insitu soils if it is safe to descend
into the pit. The depth of penetration is limited to about
3m for a backhoe and up to 6m for an excavator.
Limitations of test pits are the problems associated
with disturbance and difficulty of reinstatement and the
consequent effects on close-by structures. Care must
be taken if construction is to be carried out near test pit
locations to either properly recompact the backfill
during construction or to design and construct the
structure so as not to be adversely affected by poorly
compacted backfill at the test pit location.
Hand Auger Drilling - A borehole of 50 to 100mm
diameter is advanced by manually operated
equipment. Refusal of the augers can occur on a
variety of materials such as hard clay, gravel or rock
fragments and does not necessarily indicate rock level.
Continuous Spiral Flight Augers - The borehole is
advanced using 75 to 300 mm diameter continuous
spiral flight augers, which are withdrawn at intervals to
allow sampling or insitu testing. This is a relatively
economical means of drilling in clays and in sands
above the water table. Samples are returned to the
surface by the flights or may be collected after
withdrawal of the augers. Information from the drilling
(as distinct from specific sampling) is of relatively lower
reliability due to remoulding, inclusion of cuttings from
above or softening of samples by groundwater, or
uncertainties as to the original depth of the samples.
Augering below the groundwater table has a lower
reliability than augering above the water table. Various
drill bits are attached to the base of the augers during
the drilling. The depth of refusal of the different bit
types can provide information as to the strength of the
material encountered. Generally two different bit types
are used. The 'V' bit is a V shaped steel bit and the
/ ‘TC' bit is a tungsten carbide tipped screw type bit.
Wash Boring - The borehole is usually advanced by a
rotary bit with water or fluid pumped down the hollow
drill rods and returned up in the space between the

rods and the soil or casing, carrying the drill cuttings.
Only major changes in stratification can be determined
from the cuttings, together with some information from
“feel” and rate of penetration. More accurate
information on soil strata is gained by regular testing
and sampling using the Standard Penetration Test
(SPT) and undisturbed thin walled tube samples (U50).
Mud_ Stabilized Drilling - Either Wash Boring or
Continuous Core Drilling can use drilling mud as a
circulating fluid to stabilize the borehole. The term
"mud" encompasses a range of products ranging from
bentonite to polymers such as Revert or Biogel. The
mud’ tends to mask the cuttings and_ reliable
identification is only possible from regular intact
sampling (eg. from SPT and U50 samples) or from
rock coring, etc.

Continuous Core Drilling - A continuous core sample
is obtained using a diamond or tungsten carbide
tipped core barrel. Provided full core recovery is
achieved (which is not always possible in very weak
rocks and granular soils), this technique provides a
very reliable method of investigation. In rocks, NMLC
coring (nominal 52 mm diameter) is usually used with
water flush. The length of core recovered is compared
to the length drilled and any length not recovered is
shown as CORE LOSS. The location of losses is
determined on site by the supervisor. If the location of
the loss is uncertain, it is placed at the top end of the
run, when the core is placed in a storage tray and
recorded on the log.

Standard Penetration Tests - Standard Penetration
Tests (SPT) are used mainly in non-cohesive soils, but
can also be used in cohesive soils, as a means of
indicating density or strength. The test procedure is
described in Australian Standard 4289, "Methods of
Testing Soils for Engineering Purposes" - Test 6.3.1.
The test is carried out in a borehole by driving a 50mm
diameter split sample tube with a tapered shoe, under
the impact of a 63 kg hammer with a free fall of 760

mm. It is normal for the tube to be driven in three
'N' value is

 

successive 150 mm increments and the
taken as the number of blows for the last 300 mm, the
upper 150 mm_ being neglected due to possible
disturbance from the drilling method. in dense sands,
very hard clays or weak rock, the full 450 mm

 

 

NOTEREP.SAM

SOIL SURVEYS ENGINEERING