2024 Workshop 2 - Using geology maps – answers and comments
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University of New South Wales *
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3203
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Geology
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Apr 3, 2024
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CVEN3203: Applied Geotechnics and Engineering Geology K.Douglas Workshop 2 – Geology Maps Page 1/6 Workshop 2 – Geology Maps – answers & comments Introduction Geotechnical Engineers need to provide information on subsurface conditions. The first thing they will do is develop a preliminary geotechnical model
– an educated guess as to what is at the site. This preliminary model becomes the basis for the site investigation
. The site investigation tries to address the uncertainty and geological challenges identified in the preliminary geotechnical model. The site investigation also provides the relevant engineering properties required for the project design. Without a preliminary geotechnical model, the job will likely run over budget and critical site issues will be missed. One of the first things that a Geotechnical Engineer will do is to check local geological maps
for indications as to the subsurface geology. These maps generally show the near surface rock
type at a particular location. Note that the rocks are likely to be overlain by soil e.g. residual (weathered rock) or fill etc. If there is a significant amount of soil overlying the bedrock (e.g. deep floodplain soils; fill; or beach sands etc.), the maps may show the soil instead. Geotechnical Engineers will use these plus cross-sections and contour maps to create their estimated subsurface profile. This workshop will require you to use the Geology maps for the Sydney region to predict the likely sub-surface materials at various locations. Copies of the geological maps can be found in the Civil and Environmental Engineering building on Level 2 in the glass cabinet (Sydney 1:100,000 and Sydney 1:250,000) and on Level 5 near Rooms 509 & 510 (Sydney 1:100,000 and Sydney Basin 1:500,000). Digital versions are also available on Moodle – see ‘Geology Resources’ section. Where possible use the most detailed map (i.e. 1:100,000). Prior to Class: (a) Using the 1:100,000 Sydney Geology Map (9130), copy the general cross-section of the geology under Sydney BY HAND. Label it with the rock units and a scale. Make sure it is 1H:1V (i.e. scale is 1 horizontal to 1 vertical). Geotechnical engineers avoid exaggerated scales as they can be very misleading. Show something hand-drawn like this (with labels and scale): Note total section height approx. 3.5km; Rh Hawkesbury Sandstone about 300m thick (b) For Q1-8, find the appropriate geology map and write down the geological material/s located at the site. (c) For Q8, walk around where you live and look for any indication of the soil and rock that may exist under your home. Can you see any evidence of sub-surface groundwater levels (natural ponds, seepage, wet grounds, plants that like a lot of moisture)? Remember that the groundwater level (or water table) is the point below which the ground is saturated. The soil above the water table can still be moist
. We discussed this at the start of lecture 1. (d) Examine your house and surrounds for any evidence that might
indicate subsurface movement. Can you see cracks in brick walls, footpaths, roads, gutters, pipes etc.? Have structures like retaining walls rotated? Are any of your doors or windows not square?
CVEN3201: Applied Geotechnics and Engineering Geology K.Douglas Engineering Geology Page 2/6 (e) Choose a city of interest from somewhere in the World (not Sydney). Find the geology map for the city and write a short summary of the geology of that city. This answer will vary depending on where you choose. The main aim was to get you looking somewhere else in the world. Show your working to your demonstrator at the start of the class. Questions: You must show your results to a demonstrator by the end of the class. The demonstrator will mark you off if you have done a reasonable job with the workshop. Note I have given extended answers that cover Q9 below 1. What type of rock is Barrangaroo sited on? From 1:100,000 Sydney Geological Series Sheet 9130: Northern end is sited on: Rh - Hawkesbury Sandstone: Medium to coarse grained quartz sandstone, very minor shale and laminate lenses. This is hundreds of m thick so ignore what is below the sandstone. As you travel south toward Darling Harbour, there is mf – manmade fill: Dredged estuarine sand and mud, demolition rubble, industrial and household waste. The depth of the fill is likely to increase as you get closer to Darling Harbour and the wharves etc. Beneath the fill there will be Hawkesbury Sandstone (note that the surface of the HS may not be horizontal and there might be some alluvial or marine soils below the fill). It is important to remember that the soils shown on the geology maps are usually independent of the underlying rock (unless it is residual soil). There is a 250 million year difference in age. The soil has been developed typically in the last 10,000 years. It is deposited in the lower eroded rock areas. 2. What type of rock/s is Chatswood founded on? From 1:100,000 Sydney Geological Series Sheet 9130: Rwa – Ashfield Shale: Black to dark grey shale and laminite (part of the Wianamatta Group). Note that you can expect that beneath the Ashfield Shale there will be Hawkesbury Sandstone (look at the cross-section you drew for Part (a) of the pre-work). It is likely that the top of the shale would be weathered to clay (residual soil) with a transition zone over several meters to fresh shale.
3. What type of rock/s is Long Reef point founded on? From 1:100,000 Sydney Geological Series Sheet 9130: Qpd – Medium to fine grained marine sand; overlying: Rnn – Newport Formation: Interbedded laminite, shale and quartz to lithic sandstone; overlying: Rnbh – Bald Hill Claystone: Red shale and fine to medium sandstone; overlying: Rnbu – Bulgo Sandstone: Fine to medium grained quartz-lithic sandstone and shale interbeds There is also a dyke cutting across the eastern rock platform. Note that this site is a bit confusing. If you watch the Long Reef fieldtrip video you will see that there are some folds in the bedding. For example, the Bulgo Sandstone found on the eastern end of the site dips to the east. It then folds (an anticline just to the east of the dyke) so that the bedding then dips to the west and thus, the Bulgo Sandstone beds go under the Bald Hill Claystone.
CVEN3201: Applied Geotechnics and Engineering Geology K.Douglas Engineering Geology Page 3/6 4. What type of material is the Nepean River at Penrith surrounded by (see Penrith 9035 Geology Map)? From 1:500,000 Sydney Basin geological Sheet (on wall level 5) or From 1:100,000 Penrith Geological Series Sheet 9035: Qa – Quaternary soils: Gravel, sand, silt and clay. These are mostly deep but will lie above either Rwb Bringelly Shale , Rwm Minchinbury Sandstone or likely Rwa Ashfield Shale
5. What type of material is Sydney airport founded on? From 1:100,000 Sydney Geological Series Sheet 9130: mf – manmade fill: Dredged estuarine sand and mud, demolition rubble, industrial and household waste. I would expect fill used for the airport to be dredged estuarine sand. The poorer quality materials being removed first. The mf is likely to be over some Qhd: Medium to fine grained ‘marine’ sand with podsols (old beach sand dunes) or estuarine sands and muds (the same material that was dredged to make the Port). There is also possibly some Qhs below the other soils: Peat, sandy peat and mud (from the old river channels that were filled in). Beneath the soils there will be Hawkesbury Sandstone (note that the surface of the HS would be undulating due to river erosion). The Ashfield shale only sits on high points in Sydney above the Rh Hawkesbury Sandstone. It would have been eroded away long ago in this location.
6. What type of material is Manly founded on? From 1:100,000 Sydney Geological Series Sheet 9130: Qhf – Medium to fine grained ‘marine’ sand. At the location of the beaches, this will be overlain by: Qhb – Coarse quartz sand, varying amounts of shell fragments. The soils would then overly: Rh - Hawkesbury Sandstone: Medium to coarse grained quartz sandstone, very minor shale and laminate lenses. Note that Manly sits on top of a deep paleo (ancient) valley. Before the sea level rose to its current level, the site was a river valley. 20,000 years ago during the middle of the last ice age, the sea-level was about 120m lower than today. As the sea level rose to its current level, marine sands were deposited that filled the valley. 7. What type of material can be found just to the east of Prospect Reservoir (see Penrith 9035 Geology Map)? From the 1:100,000 Penrith Geological Series Sheet 9035: A large intrusion (igneous material pushing up into the sedimentary rock from below) of Jp – Prospect Picrite (picrite, dolerite, minor basalt). Rwb (Bringely Shale) surrounds the intrusion Some Rwa (Ashfield Shale) and Rwm (Minchinbury Sandstone) can be found on top of some parts of the intrusion (the rest being eroded away). The intrusion originally formed a laccolith that uplifted the shale beds above. You could expect that some of the shale around the edge of the Jp has been thermally metamorphosed.
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CVEN3201: Applied Geotechnics and Engineering Geology K.Douglas Engineering Geology Page 4/6 8. What type of material is your house founded on (soils and rock)? The geology depends on where your house is. Did you find any of the following: - Cracks in the walls that open and close during different seasons (reactive clay soil??)? - Doors or windows that are not square, get stuck, are cracked (reactive clay soil??)? - Cracks in footpaths or the road pavement around your house (reactive clay soil??)? - Retaining walls that are cracked or are leaning over? - Large natural boulders that have fallen, rolled etc into place? - Natural seepage? TO BE DONE IN CLASS: 9. For questions 1-7, draw simple cross sections showing the geological materials, and their distribution, that you would expect between the ground surface and 30m below the ground surface. You will need to look at cross sections on the maps, and the proximity of your site to rock boundaries, to estimate the other (if any) rock types within 30m below your site. See my additional comments for questions 1-7 above Note that a plan for Long Reef (Q2) would also be good. Students may not have done this. Get them to do it in class where they can. Most geo maps are online (via Moodle) so they can access them from their laptops, ipads, phones etc. 10. Boral run a mine and a quarry at South Marulan. The Marulan South Mine operates in the northern end of the Sbnl
outcrop whilst the Peppertree Quarry operates just to the north in the Darg
exposure. Use the Mossvale 1:100,000 map provided in Moodle to determine the rock types Sbnl
and Darg
. How old are these rock types? How do these rock types form? Sbnl
– Limestone (Early Silurian – about 440Ma) – formed in a warm shallow sea (reef) Darg
– Granodiorite (Early Devonian – about 400Ma) – formed as a plutonic intrusion Note that the limestone is used for many products including those used in cement and steel manufacturing as well as agriculture. Even the calcium carbonate in toothpaste comes from limestone mines. The granodiorite is used by Boral as a hard rock aggregate. The exposure of Sbnl is shown as narrow and long. Why do you think this is? If you did the extended tutorial last week, you would know that the plan of an exposure of a horizontal
bed of material should follow the shape of the contour lines. You would expect a limestone bed to be at least sub-horizontal as it is formed from soil in a warm shallow sea. However, this exposure is long and narrow and cuts across the contour lines. The reason for this is that the limestone and adjacent beds must have been regionally folded. Subsequent to that, the upper part of the fold must have eroded away. This would leave sub-vertical beds of limestone. If you look carefully at the photo, you can see a dark line running up the pit wall facing you (to the right of the yellow line). This is a dyke that ran up one of the bedding planes (interface between beds – usually sub-horizontal) of the limestone. Note the limestone runs south under the photographer.
CVEN3201: Applied Geotechnics and Engineering Geology K.Douglas Engineering Geology Page 5/6 Marulan South Mine (looking north from Bungonia National Park) 11. Where could you expect to find the following rock types (as surface outcrops) near Sydney (you may need the 1:500,000 scale map for some of these)? a) Basalt (i) as a surface flow; and (ii) as a dyke. Basalt is the common material in the dykes in and around Sydney. These dykes are shown as the black lines with dots on the 1:100,000 Sydney Geological Series Sheet 9130. From 1;250,000 Sydney or 1:500,000 Sydney Basin geological Sheet: Basalt flows (Tv) can be found around Taralga, Bathurst, west of the Jenolan Caves and the Southern Highlands (Bowral, Moss Vale and Robertson) b) Granite From 1;250,000 Sydney or 1:500,000 Sydney Basin geological Sheet: Granite (Cg) can be found west of the Blue Mountains particularly at Bathurst. You can see it in the ‘pink’ road cuttings on your way to Bathurst.
c) Coal From 1;250,000 Sydney or 1:500,000 Sydney Basin geological Sheet: Coal (Pi or Pn) can be found at Wollongong, Singleton (near Newcastle) and Lithgow (at the foot of the western Blue Mountains). In fact, all these coals seams are actually linked in the shape of a basin (hence the name Sydney Basin). If you dug down several hundred meters below the Civil & Environmental building you would also find the same coal seams.
11. Further group discussion in class: Describe what you would expect the geological environments to be like at the locations found in Q11. (a) Basalt - Dykes: These are formed from magma flowing up toward the ground surface through weaknesses (defects like joints etc). The magma cools just before reaching the surface. It is still volcanic (basalt in this case) if it cools rapidly. If there is a dyke at your site, the look of it will depend on the weathering properties of the dyke and the host rock (examples below). The shape in plan will depend on the defects that the magma has flown up. Initially laid down horizontally Now inclined after uplift, tilting and erosion.
CVEN3201: Applied Geotechnics and Engineering Geology K.Douglas Engineering Geology Page 6/6 Basalt – Flow – If the basalt is in a flow, it will be in layers. These layers were once liquid (lava flows) so will be variable in thickness and will overlie whatever was on the ground surface at the time (including soil). They are likely to have cooling joints (columnar basalt) in the layers. (b) Granite – Granite forms deep in the crust. If granite is found at the surface then it will have been uplifted and then the material above it eroded. This will cause significant detressing to the rock mass. Initially the grabite when formed has almost no defects. The uplifting and destressing leads to destressing joins (long curved tensile joints) forming at the surface of the granite. This can also accelerate weathering where the defect are (weathering will be discussed in a future lecture). (c) Coal – Coal is a sedimentary rock and as a result, will normally be found in sub-horizontal beds. Coal forms from organic material (e.g. peat) and as such, comes form a ‘low energy’ environment. This means that the beds of other rocks adjacent to the coal are likely to be formed in similar low energy environments (i.e. fine grained materials). You can expect shale, claystone and siltstone beds above and below the coal bed. Sandstone beds may also be present but probably not directly adjacent to the coal. Dyke and surrounding rock weathers at same rate (or is still fresh) Surrounding rock weathers (breaks down to soil) and erodes faster than the dyke Dyke weathers (breaks down to soil) and erodes faster than the surrounding rock May still be clay if dyke has weathered but noy hasn’t eroded Possible thermal/contact metamorphism effects
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