Geotechnical Stability Assessment · MCU19/0008

required for some test procedures such as CBR          A Tungsten Carbide (TC) bit for auger drilling
testing.                                               into rock can be used to indicate rock strength
                                                       and continuity by variation in drilling resistance
Undisturbed samples are taken by pushing a             and from examination of recovered rock
thin-walled sample tube, usually 50mm                  fragments but provides only an indication of
diameter (known as a U50), into the soil and           the likely rock strength. Where rock strengths
collecting a sample of the soil contained in a         may have a significant impact on construction
relatively undisturbed state. Such samples yield       feasibility or costs, then further investigation by
information on structure and strength, and are         means of cored boreholes may be warranted.
necessary for laboratory determination of shear
strength and compressibility.       Undisturbed        Wash Boring: The borehole is advanced by a
sampling is generally effective only in cohesive       bit attached to the end of a hollow rod string,
soils.                                                 with water being pumped down the drill rods
                                                       and returned up the annulus of the borehole,
Details of the type and method of sampling             carrying the drill cuttings.     Changes in
used are given on the attached logs.                   stratification can be determined from the
                                                       return, together with information from “feel”
Investigation Methods
                                                       and rate of penetration.
Test Pits: These are typically undertaken with a
                                                       The borehole can be stabilised through the use
backhoe or a tracked excavator, allowing
                                                       of drilling mud as a circulating fluid. The term
examination of the insitu soils. Limitations of test
                                                       ‘mud’ encompasses a range of products
pits are the problems associated with collapse
                                                       ranging from bentonite to polymers such as
of the pits, disturbance and difficulty of
                                                       Revert or Biogel.
reinstatement and the consequent effects on
close-by structures. Care must be taken if             Continuous Core Drilling: A continuous core
construction is to be carried out near test pit        sample is obtained using a diamond tipped
locations to either properly recompact the             core barrel. This technique provides a reliable
backfill during construction or to design and          (but    relatively   expensive)    method     of
construct the structure so as not to be                investigation. In rocks, an NMLC triple tube
adversely affected by poorly compacted                 core barrel is used, which gives a core of about
backfill at the test pit location.                     50mm diameter. The length of core recovered
                                                       is compared to the length drilled and any
Hand Auger Drilling: A borehole of typical
                                                       length not recovered is shown as CORE LOSS.
diameter of between 50mm to 75mm advance
manually operated equipment.         Premature         Standard       Penetration    Tests:     Standard
refusal of the hand augers can occur on a              Penetration Tests (SPT) are used mainly in non-
variety of materials such as fill, gravel, hard        cohesive soils, but can also be used in cohesive
clays and collapse of the borehole (typically in       soils as a means of indicating density or
non-cohesive soil).                                    strength and also of obtaining a disturbed
                                                       sample. The test procedure is described in
Continuous Spiral flight Augers: The borehole is
                                                       Australian Standard 1289, “Methods of Testing
advanced using 65mm to 100mm diameter
                                                       Soils for Engineering Purposed”, Test 6.3.1.
continuous spiral flight augers, which are
withdrawn at intervals to allow sampling and           The test is carried out in a borehole by driving a
insitu testing.  Augers of up to 300mm in              50mm diameter split sample tube with a
diameter are used to recover larger volumes of         tapered shoe, under the impact of a 63kg
sample. Samples are returned to the surface            hammer, with a free fall of 760mm. The sample
by the flights or may be collected after               is driven in three successive 150mm increments
withdrawal of the auger flights. Samples can           and the ‘N’ value is taken as the number of
be disturbed and layers may become mixed.              blows for the last 300mm. In dense soils, hard
Augering below the groundwater table can be            clays or weak rock, the full 450mm penetration
less reliable than augering above the water            may not be practicable and the test is
table.                                                 discontinued.




October 2017                                                                                        pg. 2