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THE KARUAH BYPASS: A CASE-STUDY IN THE ENGINEERING
GEOLOGY OF THE SOUTHERN NEW ENGLAND FOLD BELT.
Stephen Fityus1, John Gibson1, David Bax2 and Tim Rannard3
1
The University of Newcastle, 2Theiss Contractors, 3SMEC
ABSTRACT
The recent construction of the Karuah Bypass provided an excellent opportunity to improve the state of knowledge of a
poorly documented geological sequence in the southern New England Fold Belt and to evaluate some of its engineering
characteristics. The 2500 m thick, fault bounded, sequence of rocks transected by the Bypass has been previously
consigned to the western Myall Block of the Tamworth Belt. This work has established that it spans an interval that
begins with the Johnson’s Creek Conglomerate to the west, includes the McInness and Booral Formations and
terminates with the Karuah Formation, which is probably truncated by a fault. These predominantly terrestrial
formations are dominated by thickly bedded sandstones and conglomerates, with a variable tuffaceous component
(mostly siliceous) and minor shales and rare coal seams. They contain several significant volcanic units, which despite
having considerable thickness, were generally not encountered in excavations along the selected road alignment.
Residual soils derived from these formations are almost exclusively clays, ranging broadly from low to high plasticity.
Where encountered, rocks were mostly of high to very high strength, with some units retaining very high strengths in
close proximity to the surface. Due to the presence of extensive localised and regional faulting, outcrop in some areas
of the alignment was very poor and the depth of mottled, residual clay rock was considerable. Anomalous conditions
encountered along the Bypass include pyrite-bearing dacitic volcanics with acid sulphate potential, layers of pedogenic
silcrete that impeded pile driving, numerous deeply weathered basaltic dykes and bedding-parallel thrust faults that
show small displacement.
1
INTRODUCTION
The major New South Wales population centres of Newcastle, Gosford, Sydney and Wollongong are all situated in the
Permo-Triassic Sydney Basin. As a consequence, the engineering geology of the Sydney basin has been the subject of
extensive investigation and research, in many cases driven by the widespread residential, commercial and industrial
development that has proliferated throughout this region over the past 200 years. The so-called New England Fold Belt
(NEFB) (in which the Bypass is constructed) is a region of older and more structurally complex Cambrian to
Carboniferous rocks that extends from the margin of the Sydney basin, to the Queensland Border Ranges. At its
southern margin, it comprises mostly Devonian-Carboniferous rocks, covering a wide area, roughly bounded by the
Hunter River, the Barrington Tops, the Liverpool Ranges and the coast. This area is less populated, less developed and
consequently the engineering geology has received less attention from researchers and consultants.
The stratigraphy, structure, sedimentology, petrology and palaeontology of the southern NEFB have been studied by a
variety of researchers from universities and the Department of Mineral Resources (see review in the following Section).
With a few notable exceptions such as Chichester Dam, recent Pacific Highway reconstructions at Raymond Terrace
and Bulahdelah and construction of the Balickera Canal (Rattigan, 1966) there have been few major development
projects that have required detailed assessment of engineering geology in this region.
The construction of the Karuah Bypass provided an excellent opportunity to undertake a detailed study of the geological
and engineering characteristics of a significant section of the geological succession. Fortuitously, the alignment of the
Bypass transects a region that has long been recorded as ‘undifferentiated’ on available geological maps and allowed
clarification of the stratigraphy of the rocks in this region.
This paper describes the geology exposed during construction of the Karuah Bypass. It provides an interpretation of the
geological sequence encountered in the context of the current geological understanding of the western Myall block of
the southern New England Fold Belt. It also considers some of the rocks encountered during construction and their
significance to the construction project.
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FITYUS et al.
BACKGROUND
2.1
THE KARUAH BYPASS
The Karuah Bypass was constructed as part of a major upgrade of the Pacific Highway between Newcastle and the
Queensland Border. Construction commenced in 2002 and was completed in September 2004. It included the
construction of 9.8 km of new divided carriageway, 2 major twin bridges (to cross the Karuah river and adjacent
wetlands, respectively), 9 smaller bridges, 2 interchanges, and 10 major cuts and 10 fills, the largest cutting being 27 m
deep. It included the production of 100,000 m3 of Portland cement concrete (batched on-site) and 800,000 m3 of
earthworks of which 600,000 m3 required drilling and blasting.
Project design was undertaken on the basis of two geotechnical investigations: a preliminary investigation by Arup
Geotechnics (Arup, 2001) and a detailed investigation by SMEC (SMEC, 2002). In total, the investigations comprised
99 test pits and 103 drilled boreholes with most including rock coring.
2.2
GEOLOGICAL SETTING
The area considered in this study lies within a geological region which is generally referred to as the New England Fold
Belt (NEFB) ranging in age from late Devonian to Late Carboniferous (Carey and Browne, 1938). It is more correctly
described as the New England Orogen, although the more traditional term of Fold Belt will be retained here, because of
its wider familiarity. It is bounded by the Hunter River in the south, the Bowen Basin in the north and laterally by the
coast to the east and the Hunter/Mooki Fault System defining the western margin of the New England Tablelands.
Murray (1997) contends that the New England Orogen (Voisey, 1959b) is that “eastern most element in the Tasman
Orogenic Zone of eastern Australia, extending 1500 km from Newcastle in the south to Bowen in the North”. Murray
further divides the New England Orogen into three provinces, the Yarrol and Gympie Provinces in Queensland and the
New England Province in New South Wales.
The geology of the NEFB is lithologically and structurally complex. It contains a wide range of terrestrial and marine
sedimentary rock types, deposited in a variety of gradational facies, intruded by several plutonic suites and overlain by
younger volcanics. It is complicated by low to moderate deformation comprising gentle to tight folding and locally
intense faulting of many types. As a consequence, the stratigraphy and structure has been the subject of research for
over 150 years and is still being unravelled.
The earliest work within the area was undertaken by Strzelecki (1845). David (1907), Benson (1913) and Sussmilch
and David (1919) were the first to suggest a Hunter Valley Carboniferous stratigraphy. Subsequent work by Carey and
Browne (1938) and Voisey (1958) formalised the stratigraphy of Kuttung and Burindi Series, making it consistent with
the then current Code of Stratigraphic Nomenclature. Osborne (1950) and Roberts (1961, 1965) recognised that the
Burindi series was predominantly marine, whilst the Kuttung series was predominantly terrestrial and that
contemporaneous/time-equivalent stratigraphies could be recognised to relate them. Engel (1962, 1965) and Campbell
(1961), investigating eastern equivalents to those investigated by Roberts, recognised a third stratigraphic series that
was grouped under the term “Myall Facies”.
Engel (1966), revised the Carboniferous stratigraphy in his Explanatory Notes on the Newcastle 1:250,000 Geological
Series (S1/56-2) Sheet, noting the inadequacy of the ‘series’ definitions and the ambiguity resulting from blurring of
“time-rock” (chronostratigraphy) definitions, “rock” (lithostratigraphy) definitions and “facies” (depositional
environment) definitions. He advocated the use of the term “facies” organising the previously recognised Devonian and
Carboniferous geological formations of the southern NEFB into 3 parallel stratigraphies: a mostly Lower
Carboniferous, marine Burindi facies; a mostly Upper Carboniferous, terrestrial Kuttung facies and the mixed marineterrestrial Myall facies. Despite this improved interpretation, the stratigraphy of the rocks in the study area could not be
unravelled, and was simply consigned to “undifferentiated Carboniferous”.
The structure of the New England Province was redefined by Roberts et al. (1991), dividing it into a number of blocks:
most notably the Tamworth Belt (Korsch & Harrington, 1981) to the south and west (bounded by the Hunter-Mooki and
Peel-Manning faults), the Central Block through the central New England region and the north coast of NSW and the
Hastings Block of the mid-north coast of NSW. The southern extension of the Tamworth Belt, located within the
Hunter Valley, has been further subdivided into 3 blocks: the Rouchel Block, to the west of the Karrakurra fault, the
Gresford Block between the Karrakurra and Williams River Faults and the Myall Block between the Williams River
fault and the ocean (refer to Figure 1, also included as Key Figure 4). A typical stratigraphic succession has been
defined for each of these blocks, with the Myall Block being further differentiated into an eastern and a western
succession.
In a structural interpretation compiled by Scheibner (1998) and in a later compilation by the Geological Survey of New
South Wales (GSNSW, 2003), the area considered in this study is consigned to the western Myall Block. The
44
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FITYUS et al.
Carboniferous succession of the western Myall Block (Roberts et al., 1991), commencing with a sequence of unnamed
basal Devonian sedimentary beds and conformably terminated by the Permian Alum Mountain Volcanics, is:
Wooton Beds - A shallow-water marine unit that is generally featureless and consisting of dark grey mudstones and
greenish grey siltstones, attaining a thickness of over 1400 m thickness.
Conger Formation -Also a shallow marine sequence that is 400 m in the type section but can be 600 m thick
elsewhere. It is conformable with the underlying Wooton beds, becoming alluvial in its upper reaches. It consists of fine
to coarse lithic sandstones with minor lensoidal bands of conglomerate and very thin rhyodacitic to andesitic
ignimbrites.
Nerong Volcanics - A sequence that is 820 m in the type section, but can be considerably thinner in other areas. It
conformably overlies the Conger Formation, and consists predominantly of rhyodacitic ignimbrites that weather to a
reddish buff colour. The ignimbrites are interbedded with tuffaceous sandstones and conglomerates.
Karuah Formation - Essentially a marine sequence that unconformably overlies the Nerong Volcanics. It is 600 m
thick in the type section, but can vary between 330 m and 400 m in other areas. The Karuah Formation is characterised
by lithic sandstones and conglomerates, the clasts being derived from the underlying Nerong Volcanics. This formation
is dominated by the characteristic Karuah sandstone, which weathers to “a distinctive purple colour” (Roberts et al.,
1991).
Booral Formation - A marine sequence of about 1125 m thick in the type section, that conformably overlies the Karuah
Formation. It consists of grey, often siliceous mudstone and siltstone, together with grey lithic often pebbly sandstone.
There are also minor banded rhyolite and rhyodacitic to dacitic ignimbrites present. This unit is essentially marine, but
there are alluvial units near the top.
McInnes Formation - The McInnes Formation consists of lithic sandstones interbedded with minor conglomerate,
mudstone, coal and minor silicic volcanics. It is essentially a terrestrial sequence, but marine fossils indicate that it can
be marine in part. In the type section it is 620 m thick but faulting indicates that the unit may be thicker. The McInnes
Formation is conformable with the underlying Booral Formation.
N
Pee
l
ul
Mo
fa
Wauchope
t
#
ok
#
i
E
S
an
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W
t
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CENTRAL
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M
M
A
T
fa
O
Pacific Ocean
R
H
T
#
Port W
Macquarie
Murrurundi
Taree
fa
un
te
aku
H
r
#
#
MYALL
BLOCK
fa u lt
Dungog
t
Gloucester
LT
GRESFORD
BLOCK
K a rr
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rr a
Muswellbrook
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#
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R iv e r
fa u lt
#
ROUCHEL
Scone
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ul
Forster
LEGEND
Geology
Singleton
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W ill ia m
SYDNEY
BASIN
Th
s
Quaternary
#
Karuah
Tertiary
#
Main Roads
K A RU A H
#
Nelson Bay
Triassic
Permian
Faults
#
Newcastle
Carboniferous
site of
Karuah Bypass
(see Figure 3)
Rivers
Devonian
Silurian
# Swansea
30
0
30
60 Kilometers
Figure 1: Geological Setting of part of the Southern New England Fold Belt. (Produced from GSNSW, 2003 and
Roberts et al., 1991)
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THE GEOLOGY OF THE KARUAH BYPASS
FITYUS et al.
Johnsons Creek Conglomerate - This unit, which conformably overlies the McInnes Formation, is dominated by
pebble to cobble conglomerates of which 80% of the clasts are silicic volcanics. There are also sandstone, cherts,
mudstones and minor volcanic and plutonic rocks in this section of about 830 m in its type area.
3
GEOLOGY OF THE KARUAH BYPASS
3.1
THE KARUAH SECTION AND STRATIGRAPHIC INTERPRETATION
As noted above, prior to revisions in the 1990s, the section considered here was recognised only as “undifferentiated
Carboniferous”. Following the studies of Scheibner (1998), the section is consigned to the western Myall Block and
considered to comprise the Booral and Karuah formations to the west of the Karuah River and the Nerong Volcanics to
the east.
The section exposed by the Bypass has provided a wealth of new information that allows the validity of earlier
interpretations to be tested. Most notably, a sequence of thickly bedded conglomerates was exposed in cuttings and
boreholes at the western end of the section. Whilst minor conglomerates are known from the McInnes and Booral
Formations, the units encountered here are of an extent that suggests that they might be equivalent to the Johnson’s
Creek Conglomerate, which overlies the Booral Formation. Also notable is the presence of significant thick beds of
purple sandstone in the easternmost part of the section, occurring with a minor pink rhyodacitic ignimbrite. This
association creates somewhat of a dilemma in regard to stratigraphic interpretation. Whilst the presence of pink
rhyodacitic volcanics is consistent with the Nerong Volcanics, the thickly bedded purple sandstones are consistent with
the Karuah Sandstone. Just beyond the eastern end of the section, exposures of pink dacitic volcanics in the “Karuah
Red” aggregate quarry have long been correlated with the Nerong Volcanics (Morton, 1999). Taking all of this into
account, the outcome of this study differs from earlier interpretations, in recognising the massive purple beds of
sandstone as the Karuah Sandstone and thus consigning the Karuah Bypass section to span the interval from the
Johnson’s Creek conglomerate to the Karuah Formation and include the Booral and McInnes Formations.
Figure 2 shows three stratigraphic sections. The first is a compendium of recorded sections of the western Myall Block
(Roberts et al., 1991) in areas closest to Karuah. The second is a section along the Bypass showing only outcrops that
were exposed in the cuttings throughout the project. The third is an interpretative section showing all available
information (exposure, borehole and regional mapping), extrapolated to build a more complete section. Figure 3 (also
included as Key Figure 5) shows the approximate distribution of the four recognised formations and marker beds within
the study area. In summary, the formations of the Karuah bypass sections are described as follows:
3.1.1
Karuah Formation
The basal units of the Karuah Formation are unlikely to be present within the Karuah Bypass works, as the unit is
considered to be in faulted contact with the older Nerong Volcanics. Due to the inferred occurrence of another fault that
crosses the measured section within this formation (refer to Figure 3), around 100 m of section is believed to be absent
from the section measured along the Bypass. This missing portion has been estimated in the third column in Figure 2,
on the basis of regional mapping. The top of the Karuah Formation has been taken as a thin (<10 m), pink, felsic
ignimbrite. Although this unit was not exposed during construction, it caps many of the steep-sided hills and ridges east
of the river. The thick-bedded (Karuah) sandstones (up to 8 m) that comprise most of the Karuah Formation are
uniformly medium to coarse grained, very well sorted and conspicuously purple to oxide red in colour. Sedimentary
structures tend to be few, consisting mainly of shallow crossbedding. There is little variation in rock type within the
cuttings and when seen these comprise minor thin, black, bands of carbonaceous siltstones (usually less than 1 m in
thickness). Small bands of conglomerate are common in outcrop and test pits. The authors agree with Roberts et al.
(1991) that these deposits are probably beach deposits, based on their low angle crossbeds and textural maturity.
3.1.2
Booral Formation
The 800 metre thick Booral Formation is a more diverse unit than the underlying Karuah Formation. While dominated
by medium to fine grained grey feldspathic sandstones, it also contains significant units of tuff, siliceous mudstone and
siltstone, with erratics of pebbly sandstone and conglomerate, the latter usually occurring as lenses. Occasional thin
carbonaceous beds and laminations are present, ranging in thickness from centimetres to decimetres. Also present
within this formation are several volcanic units, consisting of pink and grey felsic ignimbrites. One of these, a pink
rhyolitic ignimbrite, 50 m thick, outcrops extensively on the hillslopes on the eastern bank of the river but was not
encountered during construction. Textural and mineralogical maturity of the sedimentary sequences is only moderate,
the sedimentary structures consisting of crossbedding, slumping, casts of ice crystals and drop stones. Beds within this
formation yield a wide range of plant fossils (Rhacopteris ovata, Nothorhacopteris argentinica Cyclostigma australe,
Archaeocalamites radiatus) indicating an age between 325 to 298 Ma.
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3.1.3
FITYUS et al.
McInnes Formation
The base of the McInnes Formation is conformable with the Booral Formation and is taken as a distinctive 33 metre
thick chert and sandstone bed, beneath a 25 metre thick dacite unit. The top was not observed in the (highway) section,
and is believed to have been disrupted by faulting. It is taken to be a 100+ m sequence of bedded volcanics, comprising
mostly flow banded rhyolites and dacites, with some tendency to more mafic compositions in its upper part. The basal
units of the formation are dominated by indurated, blue-grey, fine to medium grained sandstones with uncommon coaly
Figure 2: Stratigraphic sections of the Karuah Bypass.
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FITYUS et al.
units. Above these units there follows a two hundred metre section of non-exposure, with test pits and regional
mapping suggesting the main lithology is sandstone and pebbly sandstone. The central section consists of interbedded
conglomerate, sandstone, shales and coal seams, up to a metre thick. Matrix-supported, boulder to cobble sized
conglomerate is common, with the clasts being predominantly of acid volcanics. The top 300 metres, only encountered
in shallow bores and test pits, consist of tuffs, acid volcanic units and shales. Regional mapping suggests that the
bedded, banded rhyolite described above is uppermost within this interval. Rhacopteris ovata is found in both cuttings
suggesting a similar age to that of the Booral Formation. Seat earths within the central section of the McInnes
Formation suggest that it is terrestrial.
3.1.4
Johnsons Creek Conglomerate
The base of a 150 metre thick section of boulder to pebble conglomerate was taken as the base of the 650 metre thick
Johnson’s Creek Conglomerate, the clasts of which are predominately volcanic. This section is intercalated with thick
beds of sandstone that are often bioturbated. The top of the formation was not observed but is supposed to coincide with
the western interchange, possibly truncated by the Tarean Fault (discussed below). The section of strata between the
Tarean Fault and the observed basal section of the Johnson’s Creek Conglomerate is within a topographically low-lying
and flat area that is deeply weathered and that was not excavated. Bedrock was observed in only a few test pits, with
shales more common toward the top and conglomerate being encountered towards the basal section.
3.2
DYKES
Thirteen mafic dykes were encountered within cuttings, all occurring to the east of the Karuah River. Ten dykes are in
Cut 6 (immediately east of the bridge), one is in Cut 8 and two in Cut 9. The dykes have two primary orientations, eight
trending north/south and five trending northeast. Within Cutting 6, the northeast trending dykes are much thicker (3-6
m) than the northerly trending dykes (0.3-1 m) and are more altered. The northerly dykes are composed of basalt with a
similar appearance. One of the thicker, northeasterly dykes is composed of fractured basalt, veined with secondary
minerals. Another of the thick northeasterly dykes is composed of dolerite and contains plagioclase and pyrite, with
“clotted” plagioclase making up about 40% of the rock.
Chemical analyses of six of the dykes within Cut 6 (Offler 2004, pers. com.) suggest that the north-south and northeast
groups have slightly different chemical compositions; the former calc-alkaline signatures, while the latter show islandarc, tholeiitic signatures.
The most significant aspect of the dykes from an engineering perspective is their deep weathering and alteration, which
was high to extreme at depths of up to 10 m or 15 m. In shallower situations, the extremely weathered dykes comprised
wide, sub-vertical zones of weak, erodable material within jointed, thick beds of more competent material. In this
context, they exhibit great potential to become eroded, leaving large rock masses unsupported and hence prone to
collapse/toppling. In most instances, they required shotcreting soon after exposure.
3.3
STRUCTURE
A detailed structural analysis has not been undertaken, but limited observation suggests the following.
3.3.1
Bedding
A contoured equal area stereographic plot of poles to 40 bedding planes, 25 along the bypass and 15 in adjacent areas,
gives a regional dip of the strata, striking 136º, dipping 25º south west. The plot is slightly scattered, suggesting that
small variations occur between regions within the study area.
3.3.2
Jointing
Jointing is well developed within the area. A limited joint study within the McInnes Formation revealed three steeply
dipping sets with strikes trending northeast/southwest, east/west and southeast/northwest, with the first two indicating
predominantly strike-slip movement. These sets are similar to those recorded in a study of joints around the Williams
River Fault by Offler (1996). However, a fourth set with a north/south orientation occurs only sporadically in the study
area. The joints within the bypass area are believed to have been influenced by movement on the adjacent Tarean Fault.
3.3.3
Faulting
Major regional faults have previously been recognised at each end of the bypass. The western end is bounded by the
north/south trending Targan Fault (Newcastle 1:250,000 Map) or Tarean Fault of Roberts et al., (1991). The presence of
this fault is indicated by intense weathering which produces deep mottled clays (up to 10m deep) in the region of the
western interchange. The eastern end is cut by the northwesterly trending Karuah fault zone (Morton, 1999) that
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FITYUS et al.
probably truncates the Karuah Formation. It is manifested as an intensely brecciated zone, greater than 100 metres wide,
probably occurring mostly within the Nerong Volcanics.
The faults described above bound a block (see Figure 3) in which the Bypass is mostly situated. This block is dissected
by a number of sub-parallel, east-northeast trending faults. The relationship between the bounding faults and these
dissecting faults, suggests that the dissecting faults might be secondary to the principal faulting. Their position and
displacements appearing in Figure 3 have been inferred from displacements of marker beds, as revealed by regional
mapping. The strike and dip-slip components of these faults could not be determined in this study. However, if strikeslip movement (as is evident from the jointing) is assumed, the movements are interpreted as predominantly dextral,
with displacements of around 50-100 m. There is also some suggestion that the sequence wedges out toward the south,
with a reduction in the unit thicknesses in more southerly blocks.
Figure 3: Interpreted geology of the Karuah Bypass.
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FITYUS et al.
Small displacement (<1 m), low-angle, thrust faulting was noted in several areas and extensively developed within cut
6. These fault planes usually follow bedding planes and are difficult to detect. Within the softer rocks of the McInnes
Formation it is seen as fault-propagated micro-folding as the fault steps from one bed to the next and it is highlighted by
coal seams contained within the formation. Within the more indurated rocks of the Booral Formation, shearing along
bedding planes is apparent through iron-stained, crushed zones up to 50 mm thick that frequently serve as sources of
seepage. The most notable indication is displacement of the basalt dykes eastwards by up to a metre. A deeply
weathered zone, suspected of being a fault, was also encountered at the western end of the bridge over the wetlands.
This feature comprised a zone of deep weathering trending parallel to the strike of bedding, but dipping at 45º towards
the northeast. A nearby bore encountered a high-strength fractured rock mass with evidence of fault movements.
Smaller, sub-vertical faults occur in several cuttings. One set, examined in detail in Cut 6, formed a small graben
several metres wide, with a vertical displacement of approximately 1m. The plunge of striations, observed on the
surfaces of these faults, varies between 10 and 20 degrees, indicating a major strike-slip component in their movement.
It has been speculated (informally) that the Karuah River may have exploited a major fault in its formation. The fact
that bedding orientation and regional mapping shows no displacement, together with a lack of positive borehole
indications within the river bed, suggests that the river did not make use of a major fault system.
4
ENGINEERING CHARACTERISTICS
4.1
ROCK QUALITY
A wide variety of lithologies occurs along the Karuah bypass. In general, the engineering properties of moderately
weathered to fresh rock materials encountered ranged from fair to good. Consequently, of the rock materials won on-site
during construction, there was little material deemed unsuitable for reuse in the project, while at the same time, there
was nothing that was suitable for the production of concrete making aggregates. Of the moderately weathered to fresh
materials encountered on site, strengths mostly ranged from moderate to very high (Figure 4).
Point Load Index Is(50) MPa
0.01
0.00
0.1
1
10
100
5.00
10.00
20.00
extremely
high
40.00
very
high
35.00
high
30.00
Conglomerate
Dacite
Rhyolite
Sandstone
Siltstone
Tuff
Karuah SAST
medium
25.00
low
very
low
Depth
15.00
Figure 4: Rock strength as a function of type and depth for the Karuah Bypass.
Figure 4 indicates that several of the different rock types exhibit great strength at relatively shallow depth. In general,
the clastic sedimentary rocks exhibit varying strength and these include the weakest rocks encountered, with sandstones
in some cases possessing only low to moderate strength at depths as great as 20-25 m. Tuffaceous rocks generally
possess medium to very high strength, being mostly of high to very high strength at depths greater than about 10 m.
Acid volcanics mostly possess high to very high strengths however, despite extensive, well developed surface
expression in adjacent areas, almost no volcanics were encountered within 5 m of the surface during the initial site
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investigation. The relatively tuffaceous, characteristically purple, Karuah sandstones possess high to very high strength
at shallow levels (as shallow as 2-3 m), consistent with their strong outcropping tendencies at the eastern end of the
section.
Uses of excavated rock material included general embankment fill, select fill (produced by onsite crushing and
screening) and coarse product including cobbles and boulders for lining drainage channels and batter slopes. Although
units of volcanic rocks with the potential for quality aggregate production occur within the section, only three relatively
thin units were encountered in excavations and these were not of sufficient extent to warrant an attempt at aggregate
production. A pink rhyodacitic ignimbrite of 50 m thickness, with the potential to produce quality aggregates, outcrops
on hillslopes just east of the Karuah river but, due to the alignment being located in a saddle at this location, this unit
was not intersected during construction. On the basis of regional mapping, it is considered that a different alignment
through this region would be likely to encounter significantly more volcanic units than were encountered by the
constructed alignment.
As previously noted, the ends of the Karuah Bypass coincide closely with major fault structures. In each location, these
faults produced wide, fractured and deeply weathered zones. At the western end, excavation and replacement of 6 m to
8 m of weathered materials was undertaken to improve founding conditions for the western interchange. At the eastern
end, fractured and deeply weathered volcanic rocks were encountered over a width of at least 200 m in a cutting where
the bypass ties into the existing highway. Deep clay soils were also encountered at the western end of the bridge over
the wetlands, due to the suspected fault described in Section 3.3. As a consequence, the northbound bridge abutment is
founded on piles below the suspected fault, while the adjacent southbound bridge is founded on shallow footings. It is
interesting that this fault does not appear to have controlled the formation of the palaeo-valley between Horse Island
and the western bank of the river.
4.2
ROCK MASS CHARACTERISTICS
As noted previously, the structural characteristics of rocks in the Bypass include shallow dipping beds, persistent
jointing in several directions and localised minor faulting. These combine in various ways to affect rock mass
characteristics. Most notably, the dip of the sedimentary beds led to the potential for block sliding, where the alignment
of cuttings is close to strike. For dips of around 28º or less that occur along the Bypass, block sliding analyses indicate
that sliding on bedding planes is unlikely to occur. However, due to the relatively high strength of the fresher rocks,
drilling and blasting was required in most of the cuttings and as a consequence significant loosening of the strata
occurred in some areas. There is some evidence to suggest that some of the bedded sandstones in the Booral Formation
(Cut 6) are particularly prone to bedding plane separation upon unloading. In some cases relatively thick sandstone
units in the base of deeper cuttings are observed to separate into numerous, distinct thinner beds as they are traced updip toward the ground surface.
In Cut 6, the maximum cutting face angle was restricted to 0.5H:1V (63 degrees) to reduce the size and number of
potentially unstable blocks and to limit the amount of maintenance (such as scaling and bolting) over the life of the
road. The right hand side cut faces (facing Bulahdelah) in Cut 6 were further limited to 1H:1V (45 degrees) as a set of
joints dipping 60 degrees to the north were present. Upon completion of the excavation, loosened blocks in the left hand
side cut face, formed by the intersection of sub-vertical joints and the dipping bedding planes, required stabilisation by
rock bolting, to reduce the risk due to toppling failure.
Although low angle faulting, parallel to bedding, is recognised, there is no evidence to suggest that this has exacerbated
the loosening of strata, or had an adverse effect on excavation stability. These fault zones do, however, act as
groundwater conduits and this characteristic should be recognised and addressed if excavation-face stabilisation is to
include shotcreting.
Despite the presence of multiple joint sets, they do not appear to have a widespread adverse impact on cutting face
stability. Locally, individual pervasive joint planes produced separations of blocks of significant size that were removed
as a precaution during construction. Such blocks were not, however, widespread.
At one location in a sequence of thinly bedded siltstones and fine sandstones the intersection of a dipping bed and a
pervasive joint produced a triangular wedge, extending over the full height of an 8 m high cutting. This wedge failed
spectacularly after a significant quantity of soil was stockpiled on top of the cutting.
4.3
SOIL CHARACTERISTICS
Topsoils and residual clays of varying thickness occurred throughout the project length. Characterisation testing of soils
throughout the section indicates that they are almost exclusively clay soils, ranging from low to high plasticity, as
shown in Figure 5. It is clear from Figure 5 that there is little correlation between plasticity and geological formation,
Australian Geomechanics Vol 40 No 2 June 2005
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THE GEOLOGY OF THE KARUAH BYPASS
FITYUS et al.
except for the soils from the western part of the section that suggest that the Booral Formation is more likely to produce
clays of high plasticity.
60
CL
CH
Plasticity Index (%)
50
40
30
MH-OH
ch1000-ch5500
Booral Formation
20
ch6200-ch6800
McInnes Formation
ch6800-ch8050
McInnes Formation
10
ch8050-ch10500
Karuah Formation
ch5500-ch6200
ML-OL
CL-ML
0
10
20
30
40
50
60
70
80
Liquid Limit (%)
Figure 5: Properties of residual clays, according to formation.
4.4
GEOLOGICAL ANOMALIES
The alignment of the Karuah Bypass encountered several geological conditions that might be considered as anomalous
by usual standards.
Acid sulphate rock was encountered in the Booral Formation in Cut 6. A pyritic unit up to 8 m thick was encountered at
the top of the cut, underlying the pink ignimbrite that caps the adjacent ridge. A second pyritic layer (a dacite not
identified during the investigation) was encountered at road level in the eastern end of Cut 6, and is exposed in the
batters either side of the alignment. After exposure, the high pyrite content became particularly evident through the
extensive formation of yellowish staining on exposed surfaces. Testing of the rock indicated that due to its significant
pyrite content it had the potential to oxidise to produce substantial amounts of acidic leachate. Hence, the presence of
pyritic material was a significant environmental constraint, as acid run-off has the potential to affect oyster leases
downstream of the alignment. The exposure of the pyritic rocks in the cut face was considered to pose only a small
environmental risk, as it was assessed that the relatively wide spacing of fractures and the soundness of the blocks so
produced, meant that oxidation would be limited to the relatively small area of pyrite exposed in the fracture surfaces
within the rock mass and the amount of the potentially oxidisable pyrite exposed in the cut would be small enough not
to constitute a serious problem. The increased surface area of the excavated material was of greater concern. This was
addressed by undertaking detailed geological mapping during construction to determine the extent of pyritic material
produced and ensuring that all pyritic material was encapsulated in the fill embankment to the east of the cut. The
significance of acid sulphate rocks may be greater for other projects in related geological environments, where the units
may be more substantial.
In another example, not related to the Carboniferous geology that is the subject of this paper, an unusual occurrence of a
‘silcrete’ led to some difficulty in the installation of prestressed concrete piles in the wetland area behind Horse Island.
The presence of this silcrete is unusual in that there are few recorded examples of such pedogenic layers in the
Quaternary sedimentary sequences of eastern Australia. Although there is no firm evidence to support such an idea it is
considered likely that this layer may predate the overlying Quaternary sediment.
The silcrete, which is best described as a silica cemented silty sand, was up to 4 m thick and whilst it usually
fragmented during core drilling it was able to resist the driving of piles. It is noteworthy that, although some piles
refused within the silcrete at the time of initial driving, it was found that if a second attempt was made to drive piles
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after some weeks, then piles that had previously refused could be driven an additional distance, in many cases, to the
design installation depth through the silcrete layer.
5
CONCLUSIONS
The upgrade of the Pacific Highway between Newcastle and Brisbane will traverse extensive tracts of the New England
Fold Belt, many of which have been poorly described and in which engineering experience is limited and largely
unrecorded. In many cases construction projects will provide the best opportunity to collect data to study and observe
the stratigraphy and engineering characteristics of these geological sequences in fresh exposure. However, due to the
need to reduce erosion and to ensure longterm stability, many of the geological exposures created may be observed for
only a short time before they are top-dressed or shotcreted.
The Karuah Bypass provided an excellent opportunity to record a freshly exposed section through four formations of
the western Myall Block of the southern New England Fold Belt and to record their geotechnical characteristics. The
study found that despite the structural complexity that typifies the southern NEFB, the Karuah Bypass section was only
mildly affected by major faulting, resulting in a reasonably complete and continuous section.
6
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