Desk Study and Site Investigation

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Desk Study and Site Investigation

• EOSC316 Engineering Geoscience

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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

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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

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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)

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Walkover survey

• Checklist
• Ground truth air photos
• Land use
• Physical features
• Geology
• Drift
• Groundwater conditions

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Desk study

• Collect the information
• Highlight potential problems
• Design site investigation

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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.

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Site investigation boreholes

• Percussion drilling
• soils/soft clay rocks
• core recovery
• Rotary coring
• soil or rock gt100m deep
• core recovery
• Rock probing
• rotary percussion rig
• soil or rock
• no core recovery

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Cable (percussion) rig
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Shell
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One-way clack
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Clay cutter
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Rotary rig
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Core bit
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Core drilling
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Rock core
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Auger rotary drilling
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Engineering drill log
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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

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Trial pits/trenches

• Services - hand dug to 1.2m
• Geotechnical - machine dug to 3.0m
• Penetrometer test
• Photographs

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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

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Piezometer
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Geophysical surveys

• Ground probing radar
• Seismic surveys
• Magnetic surveys
• Gravity surveys
• Electromagnetic surveys

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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

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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)

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Geophysical surveys

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

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Geophysical surveys

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

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Geophysical surveys - summary

• Search for cavities
• GPR lt 10 m gravity if size gt depth
• Mineshaft location
• magnetic survey
• Rockhead profiling
• seismic survey, GPR
• Rock quality ahead of tunnel
• seismic survey