2024 Workshop 2 - Using geology maps – answers and comments

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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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