Desk Study and Site Investigation

1. Desk Study and Site Investigation EOSC316 Engineering Geoscience

2. Sequence of stages • Initial stage: Desk Study • Desk study of available data • Site visit and visual assessment • Preliminary report and fieldwork plan • Main stage: Site Investigation • Fieldwork • Geological mapping if necessary • Trial pits, trenches and boreholes • Geophysical survey if appropriate • Lab testing, mainly of soils • Geophysical survey • Final Report • Review stage • Monitoring during excavation and construction

3. Aims and benefits of a desk study: • To collect, understand and interpret data • To limit costs • Aid in the Design Process • Highlights problems early • Low cost & cost effective • Provides information which would otherwise be difficult to obtain

4. Sources of information • Maps • Geological maps & Memoirs • Current OS Maps • Old Maps / Aerial Photos • Archive • Historical Geotechnical Info. • Engineering Drawings • Construction Records • Libraries • Specialist Surveys • Enviro Check, Landfill etc. • Mining Records • Ecological Survey • Observational • Site Visit / Walkover survey • People (UU Ops, Construction Staff, Local Residents)

5. Walkover survey • Checklist • Ground truth air photos • Land use • Physical features • Geology • Drift • Groundwater conditions

6. Desk study • Collect the information • Highlight potential problems • Design site investigation

7. Site investigation • SITE INVESTIGATION is the exercise of undertaking a planned sequence of exploratory holes, with associated field and laboratory testing, in order to bring our understanding of the ground at a site, to an acceptable level of confidence for a particular project. • SCOPE of investigation is developed from the Desk Study review.

8. Site investigation boreholes • Percussion drilling • soils/soft clay rocks • core recovery • Rotary coring • soil or rock >100m deep • core recovery • Rock probing • rotary percussion rig • soil or rock • no core recovery

9. Cable (percussion) rig

10. Shell

11. One-way clack

12. Clay cutter

13. Rotary rig

14. Core bit

15. Core drilling

16. Rock core

17. Auger rotary drilling

18. Engineering drill log

19. How many boreholes, how deep? • Spacing: • buildings 10 – 30 m apart • road lines 30 – 300 m apart • landslides at least 5 in line for profile • Depth: • 1.5 x foundation width + 10 m control hole • 3 m below rockhead

20. Trial pits/trenches • Services - hand dug to 1.2m • Geotechnical - machine dug to 3.0m • Penetrometer test • Photographs

21. Field testing • SPT : Standard Penetration Test • no. of blows/300mm penetration • Permeability Test • Vane tests • Cone Penetration Tests ( Dutch Cone Soundings ) • In-Situ Density Test

22. Piezometer

23. Geophysical surveys • Ground probing radar • Seismic surveys • Magnetic surveys • Gravity surveys • Electromagnetic surveys

24. Geophysical surveys • Ground probing radar • uses electromagnetic microwave radar signals to map ground contrasts • main restriction: depth • 10-20 m in dry sand • 1 – 3 m in wet clay

25. Geophysical surveys • Seismic surveys • greater penetration, typically 20 m (with sledge hammer) • rockhead ususally easy to detect owing to velocity contrast • Typical seismic velocities (Vp):

26. Geophysical surveys • Magnetic surveys • Utilizes dipole anomalies arising from vertical linear features • Particularly useful for identifying buried mine shafts

27. Geophysical surveys • Gravity surveys • underground voids or low density rock/soil show up as negative anomalies • usual to drill all negative anomalies

28. Geophysical surveys - summary • Search for cavities • GPR < 10 m; gravity if size > depth • Mineshaft location • magnetic survey • Rockhead profiling • seismic survey, GPR • Rock quality ahead of tunnel • seismic survey