ROAD UPGRADE THROUGH CLUNY SANDHILL
Chris Kiernan
BSc (Env. Studies), Grad Dip (Land Management & Ecological Processes)
Abstract
The Diamantina Developmental Road from Windorah to
Bedourie in south-west Queensland traverses two large sand
dunes, the larger of which is known as Cluny Sandhill. It is
believed that these sand dunes were formed during the Late
Pleistocene period approx 80,000 years ago. To prevent
erosion and instability of these fragile dune structures due
to road upgrading works, the road design had to take into
account the dune composition and wind effects. In this
sensitive area, various design options were considered with
1 in 3 batter slopes being adopted.
Introduction
The Diamantina Developmental Road from Windorah
to Bedourie in south-west Queensland traverses gently
undulating ironstone and silcrete covered plains within
which are two longitudinal sand dunes. Main Roads and
Diamantina Shire propose to upgrade and seal a section
of Diamantina Developmental Road through these two
longitudinal dunes (Figure 1). The largest dune is known
as "Cluny Sandhill". The existing road is unsealed and the
geometry through these dunes offers poor visibility and
lateral manoeuvrability.
Improvements to the vertical alignment will mean
deeper cuts and potentially steeper batters. Main Roads
has previously constructed roads through longitudinal
dunes in the Windorah, Birdsville, Bedourie and Boulia
regions. The long term stability of the road needs
careful consideration especially as road construction
through longitudinal sand dunes in Australia is not
well documented. How a road's horizontal and vertical
alignment affects dune stability and the dune forming
processes has not been extensively investigated. The
construction of a sealed road through longitudinal sand
dunes involves understanding the interaction of dune
forming processes, past and prevailing environmental
conditions, the effect of the road on dune forming process
and engineering attributes of dune soils.
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QUEENSLAND ROADS Edition No 6 September 2008
Figure 1. Gravel road cutting through Cluny Sandhill
Cluny Sandhill — dune formation processes
Cluny Sandhill and the adjacent dune are widely spaced
longitudinal dunes with few junctions and separated by
stony plains. These dunes have a NNW alignment and are
part of the longitudinal dune system on the eastern margin
of the Simpson Desert. Cluny Sandhill is approximately
12m high and approximately 381m wide at the base where
the road cuts through the dune. Desert dunes are comprised
of ¿ne grained source sediments (1). Based on studies
of longitudinal dunes in the Simpson Desert (2), the core
of Cluny Sandhill is assumed to comprise older material
possibly of Àuvial origin (late Pleistocene, approx 80,000
years ago).
Dune morphology studies suggest that longitudinal dunes
were formed by bi-direction helicoidal secondary winds.
During periods of signi¿cant sand transporting winds
> 9.8m/s these helicoidal winds shift sand obliquely along
the Àanks of dune (1, 3). These wind processes give rise to
a herring-bone structure of the longitudinal dune see
¿gure 2.
Cluny Sandhill and the adjacent dune to the east were
possibly formed by one or more of the following processes
operating singly or currently (4):
•
•
Wind rift extension - where a dune could advance in
response to sand being eroded and deposited only in
the lee of the dune's advancing nose
•
Wind-rift vertical accretion - where the dune grows
vertically and fairly uniformally along its length.
The internal structure of a typical longitudinal dune is
illustrated in Figure 3. The cross-strati¿cation provides
some stability to the dune by the interlocking of sand
grains in congruent layers.
Road design
The parallel con¿guration of these dunes presents road
designers and associated engineering earthworks with
fewer complexities than dunes which have a larger
number of junctions (tuning fork pattern). Apart from the
complexities involved with attaining the vertical geometry
for safe stopping distance and drainage, the other major
consideration is the interaction of the road location and
horizontal alignment with bi-directional winds.
Long distant transport - where the dunes are elongating
as result of the transport of the sand along the length of
the dune coming to rest in the lee of the nose
41
Approx 1 km separation
Helicoidal wind vortices
Herringbone effect
Wind
Wind
Wind
Helicoidal wind
vortices
Figure 2. Dunes formed by bi-direction helicoidal winds
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QUEENSLAND ROADS Edition No 6 September 2008
The assessment of potential impacts on the dune system
from realignment of the vertical geometry includes:
•
assessment of the existing road alignment through the
dune
•
potential impact that the earthworks associated with the
preferred option will have on the internal structure of
the dune
•
potential for wind induced erosion (known as
'blowouts') caused by induced turbulence to occur that
will destabilise the road and the dune.
Three options have been proposed in the business case
for Cluny Sandhill realignment. The options have been
developed for improving visibility across the crest. The
options are:
Option A — Normal design standard
The existing road alignment cuts through the upper soil
pro¿les of the dune. A visual inspection of the current dune
crest and Àanks revealed that:
•
the current cutting has not destabilised the longitudinal
dune system
•
there was no visual evidence of major blow outs on the
Àanks of the dune adjacent to the road that would have
suggested that the horizontal alignment had altered the
air current to create eddies
•
the batter slopes of the existing alignment have reached
the natural angle of repose.
The cutting width and the incident angle that the cutting
makes to the dune does not appear to funnel air Àow
through the cutting causing erosion of the table drains and
batter faces. If the cutting alignment was changed to align
with the prevailing winds, the cutting would funnel the
windward helicoidal winds through the opening to converge
with the leeward helicoidal winds. This would result in
progressive erosion of the windward batter slopes in the
direction of the prevailing winds.
This proposed design conforms to the sight distance for a
car to stop for a 0.2m high object for the design speed of
the road. This option requires 2.6m of the dune existing
vertical alignment to be excavated to a design vertical curve
of radius 9500m.
Option B — Extended design domain with
reduced sight distance.
This option has a reduced crest curve radius which meets
the requirements for sight distance. A radius of 4105m
is selected to provide adequate sight distance for a car to
stop for an object of 0.2m high on the road. To meet safety
requirements, additional pavement width is required which
includes 1.5m traversable shoulders and two through lanes
of 3.5m. It is also part of the design for this option to seal
up to 0.5m of the batters.
Option C — Extended design domain with
manoeuvre sight distance
Manoeuvre sight distance is the distance required to see
and negotiate round an obstacle in an emergency situation.
The lower the sight distance allows a reduced vertical
curve with extra formation width for vehicles to manoeuvre
around an object on the road. A radius of 2675m allows a
road train 4s to negotiate round an object. At this radius,
both the truck and car lose stopping sight visibility therefore
there needs to be a minimum of 3.0m shoulder/ traversable
width and two through lanes of 3.5m. This radius requires
an extra 6m formation for 120m across the crest as a result
of the reduced sight distance. The batter slope is Àattened
to 1 in 10 for road trains. The seal would be extended to the
cut face to provide a sealed running surface for road trains
and to reduce erosion.
43
Prevailing wind
Various deposited sand layers
Gibber plain - clay base
Figure 3. Longitudinal section of a typical sand dune
48m
37m
30m
0.87m cut
1.5m cut
2.6m cut
Existing surface
1 in
3
Option C
Option B
Option A
Figure 4. Cut cross section
Existing surface
Option C
Option B
Option A
Figure 5. Longitudinal section of cut
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QUEENSLAND ROADS Edition No 6 September 2008
1 in
3
Vertical
Cut
Fill
Batter toe
Trafficable
Cut batter
1 in 3
Approx
curve
volume
volume
width
verge
slope
batter area
earthworks
radius (m)
3
3
(m )
(m )
(m)
batter slope
Option A
R9500
13044
2169
12
1 in 8
1 in 3
2198
125
Option B
R4105
8256
1314
12
1 in 6
1 in 3
1314
80
Option C
R2675
1351
161
14
1 in 10
1 in 3
865
15
m
2
cost ($'000)
Figure 6. Summary of cutting parameters
Assessment of the proposed vertical
alignment
The proposed alignment options were assessed for
safety and disturbance to cultural heritage material and
environmental factors. The proposed earthworks will cut
further into the core of the dune structure and widen the
opening in the dune (Figures 4 & 5).
Option B was selected as the preferred option based on a
number of factors — formation width for safety, volume
and cost of earthworks and surface area of the dune affected
by the construction. Figure 6 summarises the information
used for selecting option B.
To meet the safety requirements of an R4105m radius
curve, for a car to avoid a 0.2m high object additional
pavement width for traversable shoulders is required. The
12m formation consists of two through lanes of 3.5m and
two 1.5m wide shoulders.
The low dry strength (internal cohesion) of loamy sand and
clayey sands limit the slope at which the cut batters were
constructed. Batters steeper than 1 in 2.5 would result
in the slipping (dry Àow) of the batter face to the slope
of repose. Under wet conditions, experienced during the
summer months, these batters could slump resulting in
the scouring the face of the batter. It is proposed that the
failure plane of dune soils is the natural slope of repose at
approximately 1 in 3 slopes. The requirement to shore the
batter faces at slopes, steeper than 1 in 2.5 was outside the
scope of works and budget of the project.
In the particularly arid area of the Cluny Sandhill, projected
foliage cover is typically 5-30% on the dune Àanks. To
stabilise the batters, hydro-mulching or revegetation were
not practical solutions. Hence 1 in 3 batters were selected
as the cut batter slope.
At cut batters > 1 in 3, but ” 1 in 2.5, it is anticipated that
the cut batters would be eroded by helicoidal and laminar
wind transport processes. While constructing 1in 3 batter
slopes will not prevent the process of detachment and
sliding of some sand grains by wind currents down the face
of the batter, the risk of accelerated erosion of cut batters
would be signi¿cantly reduced by constructing cut batters
to 1 in 3. This slope best approximates the angle of repose
of unconsolidated dune soils.
Under wet conditions, the movement of sand grains by
wind is expected to cease because the batter slopes are near
the natural slope of repose where lateral and downward
forces have reached equilibrium. It is believed that the
weight of water in the pores is not likely to cause slumping
of the batter face.
To resist wind and water erosion, the table drains are
clay lined and compacted to the same compaction as the
pavement traf¿c lanes. Erosive energy of the helicoidal
airÀow is anticipated to be signi¿cantly reduced. The
aggregate used in the pavement chip seal will increase the
frictional drag of the wind Àowing over the tangents to
the vertical alignment. The decrease in wind velocity will
result in sand being deposited on the formation (wind drift)
as the saltation velocity of the sand is decreased.
45
References
Conclusions
The impact of option B to the structural integrity of the
dunes is assessed as being low. The long term stability of
the Cluny Sandhill and the adjacent longitudinal dune to
the east is largely attributed to the following factors:
•
•
•
1. Folk R L. Longitudinal dunes of the northwestern
edge of the Simpson Desert, Northern Territory,
Australia. Sedimentology. 1971
the wind transport processes which have formed these
dunes and shift sand is a geomorphic process occurring
at a very slow rate
2. Hesse P P, Simpson R L. Variable vegetation cover
reduced depth of cut into the core of the dune
3. Selby M J. Eolian Processes and Land Forms
the herring-bone structure provides interlocking of
sand grains in cross laminating layers
and episodic sand movement on longitudinal desert
sand dunes. Geomorphology. 2006
In Earth's Changing surface. An Introduction to
Geomorphology. 1985
4. Hollands C B, Nanson G C, Jones B G, Bristow C S,
•
possible increase in clay content of sands assists in the
structural integrity of the dune and resists entrainment
by wind
•
the 1 in 3 batter slopes approximate the natural slope
of repose, thereby providing stability to the cut batters
•
low rainfall (125mm per year) and low erosive rainfall
intensities
•
most of the earthworks will be undertaken in the
winter months (dry season) signi¿cantly reducing
potential water erosion damage to this sensitive
structure.
Price D M, Pietsch T J. Aeolian-Àuvial interaction:
evidence for late quaternary channel change and
wind-rift linear dune formation in the northwestern
Simpson Desert, Australia. Quaternary Science
Reviews. 2006.
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