Appendix B - Geotechnical Investigation Report

INVESTIGATION METHODS                                                     Mud Stabilised Drilling: Either Wash Boring or Continuous Core
                                                                          Drilling can use drilling mud as a circulating fluid to stabilise the
The following is a brief summary of investigation methods currently
                                                                          borehole. The term ‘mud’ encompasses a range of products ranging
adopted by the Company and some comments on their use and
                                                                          from bentonite to polymers. The mud tends to mask the cuttings and
application. All methods except test pits, hand auger drilling and
                                                                          reliable identification is only possible from intermittent intact
portable Dynamic Cone Penetrometers require the use of a
                                                                          sampling (eg. from SPT and U50 samples) or from rock coring, etc.
mechanical rig which is commonly mounted on a truck chassis or
track base.
                                                                          Continuous Core Drilling: A continuous core sample is obtained
                                                                          using a diamond tipped core barrel. Provided full core recovery is
Test Pits: These are normally excavated with a backhoe or a tracked
                                                                          achieved (which is not always possible in very low strength rocks and
excavator, allowing close examination of the insitu soils and ‘weaker’
                                                                          granular soils), this technique provides a very reliable (but relatively
bedrock if it is safe to descend into the pit. The depth of penetration
                                                                          expensive) method of investigation. In rocks, NMLC or HQ triple tube
is limited to about 3m for a backhoe and up to 6m for a large
                                                                          core barrels, which give a core of about 50mm and 61mm diameter,
excavator. Limitations of test pits are the problems associated with
                                                                          respectively, is usually used with water flush. The length of core
disturbance and difficulty of reinstatement and the consequent
                                                                          recovered is compared to the length drilled and any length not
effects on close-by structures. Care must be taken if construction is
                                                                          recovered is shown as NO CORE. The location of NO CORE recovery
to be carried out near test pit locations to either properly recompact
                                                                          is determined on site by the supervising engineer; where the location
the backfill during construction or to design and construct the
                                                                          is uncertain, the loss is placed at the bottom of the drill run.
structure so as not to be adversely affected by poorly compacted
backfill at the test pit location.
                                                                          Standard Penetration Tests: Standard Penetration Tests (SPT) are
                                                                          used mainly in non-cohesive soils, but can also be used in cohesive
Hand Auger Drilling: A borehole of 50mm to 100mm diameter is
                                                                          soils, as a means of indicating density or strength and also of
advanced by manually operated equipment. Refusal of the hand
                                                                          obtaining a relatively undisturbed sample. The test procedure is
auger can occur on a variety of materials such as obstructions within
                                                                          described in Australian Standard 1289.6.3.1–2004 (R2016) ‘Methods
any fill, tree roots, hard clay, gravel or ironstone, cobbles and
                                                                          of Testing Soils for Engineering Purposes, Soil Strength and
boulders, and does not necessarily indicate rock level.
                                                                          Consolidation Tests – Determination of the Penetration Resistance of
                                                                          a Soil – Standard Penetration Test (SPT)’.
Continuous Spiral Flight Augers: The borehole is advanced using
75mm to 115mm diameter continuous spiral flight augers, which are         The test is carried out in a borehole by driving a 50mm diameter split
withdrawn at intervals to allow sampling and insitu testing. This is a    sample tube with a tapered shoe, under the impact of a 63.5kg
relatively economical means of drilling in clays and in sands above       hammer with a free fall of 760mm. It is normal for the tube to be
the water table. Samples are returned to the surface by the flights or    driven in three successive 150mm increments and the ‘N’ value is
may be collected after withdrawal of the auger flights, but they can      taken as the number of blows for the last 300mm. In dense sands,
be very disturbed and layers may become mixed. Information from           very hard clays or weak rock, the full 450mm penetration may not be
the auger sampling (as distinct from specific sampling by SPTs or         practicable and the test is discontinued.
undisturbed samples) is of limited reliability due to mixing or
                                                                          The test results are reported in the following form:
softening of samples by groundwater, or uncertainties as to the
original depth of the samples. Augering below the groundwater table          In the case where full penetration is obtained with successive
is of even lesser reliability than augering above the water table.            blow counts for each 150mm of, say, 4, 6 and 7 blows, as

Rock Augering: Use can be made of a Tungsten Carbide (TC) bit for                             N = 13
auger drilling into rock to indicate rock quality and continuity by                           4, 6, 7
variation in drilling resistance and from examination of recovered           In a case where the test is discontinued short of full penetration,
rock cuttings. This method of investigation is quick and relatively           say after 15 blows for the first 150mm and 30 blows for the next
inexpensive but provides only an indication of the likely rock strength       40mm, as
and predicted values may be in error by a strength order. Where rock                          N > 30
strengths may have a significant impact on construction feasibility or                        15, 30/40mm
costs, then further investigation by means of cored boreholes may
be warranted.                                                             The results of the test can be related empirically to the engineering
                                                                          properties of the soil.
Wash Boring: The borehole is usually advanced by a rotary bit, with       A modification to the SPT is where the same driving system is used
water being pumped down the drill rods and returned up the                with a solid 60 tipped steel cone of the same diameter as the SPT
annulus, carrying the drill cuttings. Only major changes in               hollow sampler. The solid cone can be continuously driven for some
stratification can be assessed from the cuttings, together with some      distance in soft clays or loose sands, or may be used where damage
information from “feel” and rate of penetration.                          would otherwise occur to the SPT. The results of this Solid Cone
                                                                          Penetration Test (SCPT) are shown as ‘Nc’ on the borehole logs,
                                                                          together with the number of blows per 150mm penetration.




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