Drilling at Casuarina Crescent, Berriedale
15
Introduction Casuarina Crescent is located on a small flat-topped peninsula on the western shore of the River Derwent, in the Municipality of Glenorchy in southern Tasmania. The area is between approximate Australian Metric Grid co-ordinates 520 700 mE, 5 260 800 mN and 521 000 mE, 5 260 800 mN. The potential for land instability in part of the peninsula has been recognised for a number of years (Stevenson, 1976). Baynes Geologic was commissioned in 2000 by the Glenorchy City Council (GCC) to undertake a further investigation and review of land instability at Casuarina Crescent. A recommendation of this investigation was that a number of boreholes should be drilled and piezometers installed within them to allow long-term monitoring of groundwater levels near the southern boundary of the area subject to land instability. Mineral Resources Tasmania (MRT) carried out a drilling programme in 2001 to emplace piezometers in suitable locations close to the boundary (hydraulically up-gradient) of the area identified in the report of the investigation carried out for GCC. Two holes were drilled with a Casagrande-type standpipe piezometer being installed in each hole. This report contains a factual account of the work carried out. Previous Investigations The land instability problems at Casuarina Crescent were investigated by Stevenson (1976), who identified active landslide conditions in an area of land on the northern side of the peninsula, extending between the foreshore and the access road to properties in Casuarina Crescent. Stevenson also recognised the importance of elevated groundwater levels in influencing landslide activity. Subsurface investigation work (Donaldson, 1976) in the low lying ground close to the foreshore proved the existence of a sequence of sandy clays overlying sandstone. Groundwater in this area was encountered at various levels and piezometric pressures, which may be a result of subsurface movement. The report of an investigation by Baynes Geologic (Baynes, 2000) indicated the presence of a zone in which the ground was under tensional stress, extending upslope from the landslide headscarp. The report also recommended the drilling of boreholes and installation of piezometers at certain locations, to allow the long-term monitoring of groundwater levels in the area as an aid to management of the local land instability problem. Current investigation Two borehole locations were selected jointly by MRT, Dr Fred Baynes and Glenorchy City Council. Siting was also determined by the need for access for plant and the drilling techniques to be employed. The locations are shown on Figure 1. Drilling work was carried out in June 2001. The objectives of this work were to recover core samples of the strata encountered and to emplace piezometers. Two boreholes (2001/1 and 2001/2) were drilled at the locations shown on Figure 1. Both holes were commenced with a lined hollow stem auger, to obtain samples of the near-surface material with minimal disturbance. Use of this method was discontinued upon encountering hard material. Drilling of the remainder of each hole was continued by rotary coring, using HQ size wireline drilling equipment to recover 76 mm nominal diameter core, except in the case of borehole 2001/1 where rotary percussion drilling was used between depths of 1.0 m and 6.0 m below ground level. On completion of drilling, both boreholes were surveyed by total station and levelled. Each borehole encountered a soil horizon of approximately one metre thickness. In borehole 2001/2, this was underlain by 2.41 m of extremely low strength sandstone containing gravel. In borehole Tasmanian Geological Survey Record 2001/11 1 Drilling at Casuarina Crescent, Berriedale A. Waite Tasmania Tasmanian Geological Survey Record 2001/11
Transcript of Drilling at Casuarina Crescent, Berriedale
Introduction
Casuarina Crescent is located on a small flat-toppedpeninsula on the western shore of the River Derwent,in the Municipality of Glenorchy in southernTasmania. The area is between approximateAustralian Metric Grid co-ordinates 520 700 mE,5 260 800 mN and 521 000 mE, 5 260 800 mN. Thepotential for land instability in part of the peninsulahas been recognised for a number of years (Stevenson,1976).
Baynes Geologic was commissioned in 2000 by theGlenorchy City Council (GCC) to undertake a furtherinvestigation and review of land instability atCasuarina Crescent. A recommendation of thisinvestigation was that a number of boreholes shouldbe drilled and piezometers installed within them toallow long-term monitoring of groundwater levelsnear the southern boundary of the area subject to landinstability.
Mineral Resources Tasmania (MRT) carried out adrilling programme in 2001 to emplace piezometers insuitable locations close to the boundary (hydraulicallyup-gradient) of the area identified in the report of theinvestigation carried out for GCC. Two holes weredrilled with a Casagrande-type standpipe piezometerbeing installed in each hole.
This report contains a factual account of the workcarried out.
Previous Investigations
The land instability problems at Casuarina Crescentwere investigated by Stevenson (1976), who identifiedactive landslide conditions in an area of land on thenorthern side of the peninsula, extending between theforeshore and the access road to properties inCasuarina Crescent. Stevenson also recognised theimportance of elevated groundwater levels ininfluencing landslide activity.
Subsurface investigation work (Donaldson, 1976) inthe low lying ground close to the foreshore proved the
existence of a sequence of sandy clays overlyingsandstone. Groundwater in this area was encounteredat various levels and piezometric pressures, whichmay be a result of subsurface movement.
The report of an investigation by Baynes Geologic(Baynes, 2000) indicated the presence of a zone inwhich the ground was under tensional stress,extending upslope from the landslide headscarp. Thereport also recommended the drilling of boreholes andinstallation of piezometers at certain locations, to allowthe long-term monitoring of groundwater levels in thearea as an aid to management of the local landinstability problem.
Current investigation
Two borehole locations were selected jointly by MRT,Dr Fred Baynes and Glenorchy City Council. Sitingwas also determined by the need for access for plantand the drilling techniques to be employed. Thelocations are shown on Figure 1.
Drilling work was carried out in June 2001. Theobjectives of this work were to recover core samples ofthe strata encountered and to emplace piezometers.Two boreholes (2001/1 and 2001/2) were drilled at thelocations shown on Figure 1. Both holes werecommenced with a lined hollow stem auger, to obtainsamples of the near-surface material with minimaldisturbance. Use of this method was discontinuedupon encountering hard material. Drilling of theremainder of each hole was continued by rotarycoring, using HQ size wireline drilling equipment torecover 76 mm nominal diameter core, except in thecase of borehole 2001/1 where rotary percussiondrilling was used between depths of 1.0 m and 6.0 mbelow ground level.
On completion of drilling, both boreholes weresurveyed by total station and levelled.
Each borehole encountered a soil horizon ofapproximately one metre thickness. In borehole2001/2, this was underlain by 2.41 m of extremely lowstrength sandstone containing gravel. In borehole
Tasmanian Geological Survey Record 2001/11 1
Drilling at Casuarina Crescent,Berriedale
A. Waite
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2001/1, the arisings recovered over the same depthrange using a different drilling technique may alsorepresent the same material, although this cannot beconfirmed.
These near-surface materials were underlain by asequence of gravel, cobbles and boulders, generally ofdolerite and sandstone. This sequence also containedtraces of an orange-brown clay in the recoveredmaterial, possibly representing the matrix of thecoarser material, when present in situ. The depositsimmediately underlying the soil horizon mayrepresent either channel or flow deposits of possibleTertiary age, with a maximum thickness of 9.6 mpresent in borehole 2001/2. A mudstone/siltstonesequence was present below these deposits, and bothholes were terminated in sandstone of probableTriassic age.
Jointing and fissures in the materials underlying thedebris flow deposits tended to dip at high angles(generally 50–85°) and indicated few obvious signs ofshear. Polishing or striation occurred in borehole2001/2 while in borehole 2001/1, joints tended to havea clay infill, with dip angles being towards the higherend of this range, and were in many cases bothpolished and striated. Striations were orientatedapproximately normal to the joint dip directions.Borehole 2001/1 is significantly closer to the zone oftension identified by Baynes than hole 2001/2.
The borehole logs are shown in Appendix 1.
Groundwater
The drilling techniques used in drilling the boreholesdid not allow detection of any water strikesencountered during drilling. A standpipe piezometer
to monitor groundwater levels was installed in eachborehole. Details of construction of these instrumentsare shown in Appendix 1.
Water levels in each instrument will be recorded on aregular basis.
Conclusions
Further information on the subsurface inland geologyat Casuarina Crescent has been obtained, andinstrumentation to allow long-term monitoring ofgroundwater levels in the vicinity of the landslideinstalled, as recommended by Baynes.
Recommendations
Regular monitoring of groundwater at CasuarinaCrescent should be continued, and the resultsconsidered in determining a management strategy forthe landslide. Should No. 16 Casuarina Crescent bedemolished, it may be appropriate to consider thesuitability of drilling a borehole to monitorgroundwater within the zone of tension referred to byBaynes, as part of the management strategy.
References
BAYNES, F. B. 2000. Investigation of the landslide at CasuarinaCrescent. Baynes Geologic.
DONALDSON, R. C. 1976. Drilling at Casuarina Crescent,Glenorchy. Unpublished Report Department of MinesTasmania 1976/42.
STEVENSON, P. C. 1976. Ground movements at CasuarinaCrescent, Glenorchy. Unpublished Report Department ofMines Tasmania 1976/16.
[5 November 2001]
APPENDIX 1
Borehole Logs
Tasmanian Geological Survey Record 2001/11 3
Tasmanian Geological Survey Record 2001/11 4
Tasmanian Geological Survey Record 2001/11 5
Tasmanian Geological Survey Record 2001/11 6
Tasmanian Geological Survey Record 2001/11 7
Tasmanian Geological Survey Record 2001/11 8
Tasmanian Geological Survey Record 2001/11 9
Appendix 2
Core photographs
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Tasmanian Geological Survey Record 2001/11 11
Sample 1, 1.00 – 2.00 m
Sample 2, 2.00 – 3.00 m
Sample 3, 3.00 – 4.00 m
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Sample 4, 4.00 – 5.00 m
Sample 5, 5.00 – 6.00 m
Hole 2001/0100.0 – 9.70 m
Tasmanian Geological Survey Record 2001/11 13
Hole 2001/019.70 – 13.45 m
Hole 2001/0113.45 – 17.95 m
Hole 2001/0117.95 – 20.00 m
Tasmanian Geological Survey Record 2001/11 14
Hole 2001/020.00 – 7.75 m
Hole 2001/027.75 – 13.75 m
Hole 2001/0213.75 – 18.25 m
Tasmanian Geological Survey Record 2001/11 15
Hole 2001/0218.25 – 20.00 m
