Geology PAG 9: Site investigations

Suggested Activity 1: Geotechnical desk study

Instructions for teachers & techniciansThis practical activity is composed of two parts; a teacher/technician section and the learner activity which can be found on page 8. This practical activity supports OCR AS/A Level Geology.

When distributing the activity section to the students either as a printed copy or as a Word file you will need to remove the teacher instructions section.

This is a suggested practical activity that can be used as part of teaching the OCR AS and A Level Geology specifications helping to fulfil the requirements of the Practical Endorsement.

These are not required activities, nor are they coursework tasks.

You may modify these activities to suit your students and centre. Alternative activities are available from, for example, ESTA, Earth Learning Idea, CLEAPSS and publishing companies. Support for mapping activities to the requirements of the Practical Endorsement is available from OCR – see www.ocr.org.uk/positiveaboutpractical or email us at PASS@ocr.org.uk.

Students can collaborate during the activities but each student must individually demonstrate competence in each of the practical skills being assessed (see Practical Skills below).

It is possible for a student to achieve some but not all of the practical skills involved in an activity (and this can be recorded as individual skills in the OCR PAG Tracker).

Further details are available in the specifications (Practical Skills Topics).

OCR recommendations:

Before carrying out any experiment or demonstration based on this guidance, it is the responsibility of teachers to ensure that they have undertaken a risk assessment in accordance with their employer’s requirements, making use of up-to-date information and taking account of their own particular circumstances. Any local rules or restrictions issued by the employer must always be followed.

CLEAPSS resources are useful for carrying out risk-assessments: (http://science.cleapss.org.uk).

Centres should trial experiments in advance of giving them to students. Centres may choose to make adaptations to this practical activity, but should be aware that this may affect the Apparatus and Techniques covered by the learner.

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 1 © OCR 2019

DISCLAIMER

This resource was designed using the most up to date information from the specification at the time it was published. Specifications are updated over time, which means there may be contradictions between the resource and the specification, therefore please use the information on the latest specification at all times. If you do notice a discrepancy please contact us on the following email address: resources.feedback@ocr.org.uk

IntroductionStudents will be carrying out a desk study to evaluate the foundation characteristics for a proposed 2 MW wind turbine. Based on a site visit and their interrogation of BGS iGeology or GeoIndex they will propose a suitable foundation design for the chosen site. Prior to any civil engineering project geologists will carry out a geotechnical desk study of the geology to identify any potential issues which could be mitigated for or avoided at the design stage.

Students will have to retrieve existing geological and geotechnical data using one of two BGS map viewer, Geology of Britain or GeoIndex (Onshore) – see PAG 10.2. This is intended as an activity that is suitable for students who are studying 6.2.1 or 6.2.2.

There is a substantial body of research on the benefits of GIS in helping high school students to develop higher level thinking skills in both science and geography. However this can only be developed through the students’ direct experience of GIS; learning with GIS rather than learning about GIS. Students who do not directly experience GIS tend to perceive it in a very static way, such as a way to access bus times, and fail to recognise its wider current application (see Healy & Walshe, 2018, School students’ perceptions of the value and nature of GIS, www.geography.org.uk/Previous-conference-materials#13).

Aims to use a GIS to access georeferenced geological data sets to model the suitability of different wind turbine foundations for the ground conditions to communicate their recommendations in a report for the use of a decision maker

Intended research and report writing time 2 hours

Practical Skills – competence assessed by the teacher1.2.1 (f) present information and data in a scientific way1.2.1 (g) use appropriate software and tools to process data, carry out research and report

findings1.2.1 (h) use online and offline research skills including websites, textbooks and other printed

scientific sources of information1.2.1 (i) correctly cite sources of information1.2.2 (m) use of ICT to: collect, process and model geological data.

CPAC(1) follows written procedures(5) researches, references and reports

Links to Specifications6.2.1 (a)(ii) the measurement of rock strength under compression and under shear; to include a qualitative understanding of peak strength and residual strength6.2.1 (b) how the strength of rocks and sediments is changed by weathering, fracture density and geological structures6.2.1 (c) how the strength of rocks and sediments is changed by hydrostatic pressure (pore water); to include the resolution of forces6.2.1 (d) how existing data sets and ground investigations are integrated in a geotechnical site assessment; to include existing BGS mapping

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 2 © OCR 2019

Mathematical Skills – learning opportunity within activity Mathematical skills must be applied in the recording of the data and calculations, and in

analysing the data. These steps require the appropriate application of the following mathematical skills:o M1.1 Recognise and make use of appropriate units in calculationso M1.2 Recognise and use expressions in decimal and standard formo M1.3 Use an appropriate number of significant figureso M1.6 Estimate resultso M3.3 Substitute numerical values into algebraic equations using appropriate units for

physical quantitieso M3.7 Translate information between graphical, numerical and algebraic formso M4.1 Calculate the circumferences, surface areas and volumes of regular shapes

EquipmentEach student will require: computer or tablet with internet access

Health and Safety Health and safety should always be considered by a centre before undertaking any practical

work. A full risk assessment of any activity should be undertaken including checking the CLEAPSS website (http://www.cleapss.org.uk).

NotesCentres are advised to trial this activity before using it with students. In particular: This is intended as an activity that familiarises students with the use of GIS. The objective is

for students to experience how a GIS can help them to access and make sense of a georeferenced database. It is not intended that students will master the use of GIS, however their use of the BGS Map Viewers should provide them with a practical experience of GIS that will help them to understand why GIS is used to analyse spatial data;

The BGS Geology of Britain/iGeology app is a map-based index of geological data, intended for the general user, that can be accessed through a web interface on a computer or a mobile device (www.bgs.ac.uk/data/mapViewers/home.html). The Geology of Britain/ iGeology viewer is quite intuitive and will only take a few minutes to learn.

The BGS GeoIndex Onshore is a more powerful map-based index of geological, geochemical and geophysical data that can be accessed through a web interface on a computer or a mobile device (www.bgs.ac.uk/data/mapViewers/home.html). The data for Northern Ireland is available from the GSNI separately (www.bgs.ac.uk/gsni/data/ ) ;

The following quick tutorial should give basic familiarity with GeoIndex Onshoreo go to http://www.bgs.ac.uk/geoindex/ and open the onshore GeoIndex. This will launch

GeoIndex in a separate window. Now tile the GeoIndex window and your main browser window so that you can see both on your screen;

o In the Enter location box put “Wales, Yorkshire” and then return to search;o Click the Data icon (stacked layers) and add data: Surfical deposits 1:50k, Bedrock

geology 1:50k, Borehole scans, Mass movement deposits, and Artificial ground; o Increase the transparency on the Bedrock geology layer so that you can see the

underlying basemap;o Zoom out until you can see both Wales and Junction 31 on the map. Click the Search

icon (magnifying glass) and choose circle. Drag the circle out until both Wales and Junction 31 are in the selected area. You will now see the data listed on the screen;

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 3 © OCR 2019

o Select the Create PDF report icon (printer) and generate a report. Now select open pdf in a new window to see the report on your main browser screen.

o In the GeoIndex window select the Borehole scans(249) tab and scroll to near the bottom of the list and double click on SK48SE361 SOUTH YORKSHIRE COA NMCS2 UPGRADE M59 to zoom to the location. Now single click on the green well icon on the map. When the pop up appears select Scan to see the borehole record.

o Zoom back out and single click cross hatched area between Wales and the M1. You can now click on More information to see the BGS record.

o Select the yellow patch to the southeast of Wales. A pop up will appear with information on surficial sediments. Click on More information for information on channel fill deposits.

To run this activity you will first outline the task to your students:o to reduce the school/college energy bills it has been proposed that a 2 MW wind turbine

should be built on the school/college grounds.o you have been asked to carry out a desk study to identify a suitable site based on the

local geology and foundation conditions.o you will use a GIS on the British Geological Survey website to find out the geology below

the school/college and identify any geotechnical hazards.o using the information on the attached resource sheet you will model the ground load of

the wind turbine tower and the type and size of foundation needed.o You will write a short summary of your findings and recommendations so that the

school/college governors can decide whether the wind turbine could be built safely and for a reasonable cost on the school/college grounds

The report needs to review the ground conditions below the school, identify any potential geotechnical problems and present recommendations. The report will show the calculations used to determine the final recommended foundation design.

You may wish to introduce Geology of Britain/GeoIndex Onshore to your students but they will need to learn how to use the GIS and do the research and report writing on their own.

Answers and Guidance to Extension Activities8. Onsite testing of ground conditions is preferred over laboratory testing as it can be difficult to

recover undisturbed samples. Possible in situ testing techniques include:a. penetrometer – handheld spring device that is pushed into the soil to measure the

compressive strength of the soilb. Schmidt hammer – similar to a penetrometer but with a much stronger spring that is

used to measure in situ rock strength.c. standard penetrometer test (spt)/cone penetrometer – a borehole device with a conical

end that is pushed into the soil. Continuous readings or blows per test drive – 75 mm.d. vane test – hand held or borehole device with a crossed vane head that is twisted in the

soil and measures peak and residual shear strengthe. plate loading tests – a steel plate is placed on the ground surface and a hydraulic jack

mounted on a large vehicle pushes down on the plate to measure the settlement resulting from the applied load

f. boreholes – standard descriptions of recovered core material, similar process used for description of mechanical excavations

9. Subsidence may be caused by the compaction of the soil/rock under an imposed load (e.g. weak rock, peat or made ground) or due to subsurface failure (e.g. mines, sinkholes). Differential compaction occurs when the subsurface layers show lateral variation, such as on the edges of buried channels, and the rate of subsidence below the foundation slab varies specially which can cause the foundation to fail or the wind turbine to overturn.

10. Instant access and the ability to carry out desk studies without having to visit a number of different archives and consult paper copies. The volume of material that can be consulted has also increased. In the past a archivist or professional researcher would have been commissioned to copy information from the National Coal Board archive, for example.

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 4 © OCR 2019

RecordsAs evidence for the Practical Endorsement, students:

should not need to re-draft their work, but rather keep all of their notes as a continuing record of their practical work, dating their work clearly ;

contemporaneous notes of the research and references to the sources used; foundation design report produced to the project brief supplied by the teacher.

Extension questions help students develop their understanding of the underlying geological theory and are a preparation for the written examinations. They also help students to develop the practical science skills assessed indirectly in the written examinations. Students should be encouraged to record their data appropriately, for example showing full workings in calculations, and stating final answers to the appropriate number of significant figures.

Document updatesv1.0 June 2018 Original version.v1.1 February 2019 Minor edits for clarity.

Example resultsCambridge University Press Sports Ground. Grid Reference: TL45585637, 14 metres elevation.

The 1886 1:10,560 map shows some small gravel workings and marl pits associated with farms. Surficial geology Quaternary river terrace sands and gravels with pockets of silt and peat. Bedrock West Melbury Marly Chalk Formation, up to 15 m of soft marly chalk overly the Gault Clay. Thickness of river terrace gravels between 1 m–4 m, probably due to old river channels.

Composite log constructed from recent geotechnical boreholes (to 15 m) and older water wells using BGS GeoIndex (Onshore). Shear strength of Gault Clay from BGS report:

Rock/SoilDescription Thickness / m Depth to base

/ mGeotechnical information

(spt)

Probable Bearing capacity/ kN m-2

Made ground, brown clay gravel with broken brick 1.0 1.0 – –

Firm silty clay with some flint gravel. River Terrace Gravels 2.2 3.2 12N 200

Firm to stiff calcareous clayey silt with bands of flint – West Melbury Marly Chalk Fm

3.9 6.1 16N 400

Stiff silty clay, closely fissured with some pockets of very soft clay (19m) – Gault Clay

46.4 52.5 26N 250–150

Spread foundation on the Chalk Marl (400 kN m-2) safety factor of 4 = radius 5m = load of 98 kN m-2.Gravity foundation on the Gault Clay (150 kN m-2) safety factor of 2 = radius 9m = load of 74 kN m-2.Piled foundation in Gault Clay (Shear strength = 75 kN m-2), 5 × 10 m piles = 5 × 31.4 m2 = 157 m2. Friction on piles 11775 kN, weight of wind turbine on 3 m radius footing = 4103 kN

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 5 © OCR 2019

Typical bearing capacities of British soils and rocks

Type of Rock of SoilMaximum bearing capacity

kN m2 kg cm2

Rock – hard crystalline rock with no defects 3300 33.0

Rock – sound unweathered sedimentary rock in beds 1650 16.5

Rock – crystalline/sedimentary with joints or fractures 900 9.0

Rock – soft or weathered 450 4.5

Gravel / sand and gravel – compact 450 4.5

Coarse sand – compact and dry 450 4.5

Medium sand – compact and dry 250 2.5

Fine sand / silt – compact and dry 150 1.5

Gravel / coarse to medium sand – loose 250 2.5

Fine sand – loose and dry 100 1.0

Shale or Clay – stiff 450 4.5

Clay – firm 250 2.5

Clay with sand – soft 150 1.5

Clay – soft 100 1.0

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 6 © OCR 2019

Cambridge University Press sports ground.

TL45585637

14 metres elevation

Proposed location to the right of the cricket nets.

Clay – very soft 50 0.5

Shrinking and swelling clays, dry <50% saturation

Not suitable foundation material for wind turbinesPeat

Made up ground and land fills

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 7 © OCR 2019

Example computations which assume a densirty of 2.55 t m-3 for reinforced concrete and 2.00 t m-3 for soil and back fill. Height of the footing was 2.5 m in all cases.

Octagonal foundation only Wind Turbine

radius / m area / m2 volume / m3 mass / t weight/ kN

load/ kN m-2

2.00 11.314 28.284 72.1 3219 2852.25 14.319 35.797 91.3 3407 2382.50 17.678 44.194 112.7 3617 2052.75 21.390 53.475 136.4 3849 1803.00 25.456 63.640 162.3 4103 1613.50 34.648 86.621 220.9 4678 1354.00 45.255 113.137 288.5 5342 1184.50 57.276 143.189 365.1 6093 1065.00 70.711 176.777 450.8 6934 986.00 101.823 254.558 649.1 8879 877.00 138.593 346.482 883.5 11179 818.00 181.019 452.548 1154.0 13832 769.00 229.103 572.756 1460.5 16839 74

10.00 282.843 707.107 1803.1 20200 71

Narragansett Regional High School Wind Turbine – example of a school engineering project.https://coastalengineeringcompany.com/portfolio/narragansett-regional-high-school-wind-turbine

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 8 © OCR 2019

OCR Resources: the small printThis formative assessment resource has been produced as part of our free A Level teaching and learning support package. All the A Level teaching and learning resources, including delivery guides, topic exploration packs, lesson elements and more are available on the qualification webpages.

If you are looking for examination practice materials, you can find Sample Assessment Materials (SAMs) on the qualification webpages: here

OCR’s resources are provided to support the teaching of OCR qualifications, but in no way constitute an endorsed teaching method that is required by the Board, and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. © OCR 2019 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work. OCR acknowledges the use of the following content: n/a

Please get in touch if you want to discuss the accessibility of resources we offer to support delivery of our qualifications: resources.feedback@ocr.org.uk

We’d like to know your view on the resources we produce. By clicking on ‘Like’ or ‘Dislike’ you can help us to ensure that our resources work for you. When the email template pops up please add additional comments if you wish and then just click ‘Send’. Thank you.

If you do not currently offer this OCR qualification but would like to do so, please complete the Expression of Interest Form which can be found here: www.ocr.org.uk/expression-of-interest

Looking for a resource? There is now a quick and easy search tool to help find free resources for your qualification: www.ocr.org.uk/i-want-to/find-resources/

Geology PAG 9: Site investigations

Suggested Activity 1: Geotechnical desk study

Student activityIntroduction

In this practical activity you will be carrying out a desk study to evaluate the foundation characteristics for a proposed 2 MW wind turbine on the school site. Using a simple web-based GIS you will research the geology of the site and use this data to design foundations that would safely support the load from the wind turbine. A desk study is the first stage of any civil engineering project. In a desk study a geologist uses existing data to identify any potential issues so that the project manager can decide the final location and design of the structure.

You will have to familiarise yourself with the use of either Geology of Britain or the Onshore GeoIndex (www.bgs.ac.uk/data/mapViewers/home.html) in order to find any existing geological records for the proposed wind turbine site.

Aims To use a GIS to access georeferenced geological data setsTo model the suitability of different wind turbine foundations for the ground conditionsTo communicate your recommendations in a report for the use of a decision maker

Intended research and report writing time2 hours for research and evaluation + 1 hour report writing

Equipmentcomputer or tablet with internet access

Health and Safety You should follow your centre’s practice on using computers and the internet.

ProcedureBefore starting your practical work, read the information below.

1. Go to the BGS View maps webpage (www.bgs.ac.uk/data/mapViewers/home.html) and try using both the Geology of Britain/iGeology and GeoIndex (Onshore) GIS database browsers. Both browsers have mobile versions which will run on Android and iOS devices.

2. Using either Geology of Britain or GeoIndex (Onshore) research the surficial and bedrock geology at the proposed wind turbine site. Search the database of existing borehole data in the local area and summarise the expected geology at the site to 50 m below the surface.

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 9 © OCR 2019

3. Construct a geotechnical graphic log for the site based on your research. This should include any information on the strength of each subsurface layer, and any geological features which could be a geotechnical hazard (e.g. differential compaction, subsidence, slope instability).

4. Research the bearing capacities of the different soil and rock layers. Remember that to an engineering geologist all loose uncompacted material is soil. Convert the bearing capacity values to the same units (i.e. N m-2) and add this data to your graphic log.

5. Using the information on the resource sheet select an appropriate foundation design and calculate the load of the wind turbine and foundations on the ground. Modify your foundation design until you have a 50% safety contingency to allow for extreme wind loading so that the load of the wind turbine + foundations ≤ 0.67 × the bearing capacity of the ground. Use a spreadsheet to do your calculations as this will allow you to find the smallest size of foundation that could be safely used to support the load from the wind turbine.

6. Check that there is sufficient space for your preferred foundation design on site.7. Once you have finalised your preferred design write a foundation design report (no more

than two sides of A4). Your report should summarise: the geology at the site, identify any potential geotechnical hazards and show the calculations you have used to determine your final recommended foundation design. All sources used must be referenced in your report.

Extension opportunities8. Your desk study was very persuasive and the governors have decided to go ahead with the

wind turbine project. Outline a presentation that explains what geotechnical tests should be carried out on site before the wind turbine is designed is finalised.

9. The governors have heard that subsidence and differential compaction could be a problem. Explain the difference between subsidence and differential compaction and suggest a method that could be use to identify the relative risks at the wind turbine site.

10. Before BGS developed their GIS tools (e.g. GeoIndex) the data was still available and could be accessed through paper records. To see the records for each borehole you could visit the main BGS office near Nottingham or request a photocopy of the paper record. After paying a small fee the record would arrive at your library within six weeks.What are the advantages of accessing and interrogating geological data sets through a GIS rather than paper copies of maps and documents or a microfiche database?

RecordsAs evidence for the Practical Endorsement, you need records of:

the contemporaneous notes of your research and references to the sources used; your foundation design report produced to the project brief supplied.

All work should be clearly dated.

In addition you should have considered the above questions as the answers to these questions will aid you in preparation for your written examinations.

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 10 © OCR 2019

Resource sheet2 MW Wind Turbine – design factsheet

PAG 9.1 – Designing foundations for wind turbines © OCR 2019

Nacelle – 70 t

The nacelle contains the gearbox and generator unit. It rotates on the tower to keep the rotor facing into the wind.

Rotor – 38 t

The three blades and the hub form the rotor. The pitch of the blades changes automatically to optimise efficiency at low speeds, and to reduce the force acting on the blades at high wind speed. Power is generated between 3 m s-1 to 22 m s-1. The sweep area of this rotor is 6362 m2

Tower – 148 t

The tower is a slender hollow steel tube. The tower has to support the weight of the nacelle and rotor, and resist the force of the wind on the structure which could buckle the tower. They are delivered to site in 20 m sections.

Ground Level

The ground level footprint of the wind turbine will depend on the type of foundation used.

Footing

The footing is a reinforced concrete structure that couples the tower to the foundations. A bolt cage or a steel ring is built into the footing and acts as a collar to couple the tower to the foundation. Wind turbine footings are octagonal in plan and between 1 m and 3 m high. For shallow foundations the footing is the foundation but for deep foundations the footing acts as a platform connecting the turbine tower to the friction piles.

Height = 90 m

Diameter= 90 m

2 × radius

height

area of top surface ¿2√ (2 r2)

collar

Safety factor

A safety factor of four means that the bearing capacity of the ground is four times the design load on the foundations.

How do onshore wind turbine foundations work?

PAG 9.1 – Designing foundations for wind turbines © OCR 2019

Piled foundation

Friction piles work by increasing the surface area in contact with the soil/rock. The shear strength of the soil acts over surface area of the sides of the piles by friction. In this way the strength of deeper layers is transferred up to the surface. An auger drills a 1 m diameter hole, a steel cage of reinforcing rods is inserted and the hole filled with concrete. The footing is built on top of the piles. A safety factor of two should be used.

Piling to bedrock is not suitable for wind turbines as load is then taken by the ends of the piles which can fail under the very large compression and tension loads caused by the bending moment.

the weight of the soil over this half of the foundation

resists overturning

the bending moment doubles the load on this

half of the foundation and can cause the

tower to fall over.

the bending moment caused by wind loading

wind loading is the force of the wind on the tower and rotor

weight of thewind turbine

Spread foundation

The large slab at or just below the ground surface spreads the load of the wind turbine over the ground. These are the simplest to construct. However, they are only suitable for strong rock/soil. They cannot be used if soil compaction is large or there is the possibility of differential compaction. As it is the radius of the foundation slab that resists the bending moment and prevents overturning of the tower a safety factor of four should be used.

Gravity foundation

By excavating down below the zone of weathering the foundation can be placed on stronger soil/rock (usually at least three metres below the surface). Gravity foundations work in a similar way to spread foundations. But the weight of the soil/rock over the footing also resist overturning and the radius of the footing can be reduced. A safety factor of two should be used.

Density of foundation materials: reinforced concrete = 2.55 t m3

compacted backfill = 2.00 t m3