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Lecture
Site Investigation
1. Introduction: definition, success factors, objectives
2. SI Stages: codes of practice, standards &
Eurocode 7
3. Investigative Methods: desk study, invasive
investigation & in situ testing
4. Results: presentation & avoiding surprises
Defining site investigation
The processes whereby all relevant
environmental and ground conditions data
concerning the site of a proposed
development and its surrounding area is
gathered
Ground investigation is the sub-set of
operations that involves acquisition of data
on the ground conditions using exploratory
techniques (drilling, pitting, geophysics)
Critical success factors
Site Investigation:
“Prerequisite of all successful and economic
design of engineering structures and earthworks”
Identification of ground hazards
Management of ground risk
Providing value for money for the client
Provision of relevant, reliable information
Key performance indicators Preparation – desk study and walk-over
SI Design
Procurement (equipment, staff)
Management – project, risk and quality
Supervision (during SI)
Reporting – factual, interpretative, ground
model
Outcome – client satisfaction, project
review, and user feedback
Site investigation objectives Assess the general suitability of the site and its
environs
Enable economic and safe design
Plan best method of construction
Design any remedial measures (site clean up)
Explore sources of construction materials
Establish sites for waste disposal
Check existing structures
Check for environmental changes arising from
the works (wildlife surveys, piezometers)
Continues during and even after construction
Professional codes for site investigation and
design
Site investigation stages according to Eurocode 7
STRATEGY FOR DESIGN Proposed structure Geology Existing information & experience
Strategy for geotechnical design
GEOTECHNICAL INVESTIGATIONS DESIGNProposed structure
Planning and re-evaluation of the investigation
programme
Evaluation of geotechnical
Geology Geological model parameters and coefficients Geotechnical model
Field investigations including field tests and
sampling to required standards Test results
Design: Geotechnical design report
Laboratory testing and investigations Test results geotechnical
structural Design report (final project)
Reporting
Ground
investigation
report
Specifications
Program for inspection
supervision & monitoring
Call for bids based on a project
EXECUTION OF THE WORKS
Basic Principle
Any site investigation has to be continued
until the ground conditions are known and
understood well enough for the civil
engineering work to proceed safely
Even doubling the cost of the SI will
generally add <1% to the project cost
After an inadequate SI unforeseen ground
conditions can raise project costs by 10%+
Basic Principle
Any site investigation has to be continued
until the ground conditions are known and
understood well enough for the civil
engineering work to proceed safely
Even doubling the cost of the SI will
generally add <1% to the project cost
After an inadequate SI unforeseen ground
conditions can raise project costs by 10%+
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 1
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 1
• Planning and reconnaissance stage of project
• Make an initial assessment of ground conditions; identify geotechnical
problems
• Project feasibility & site suitability
• Desk Study
Historical records, literature, mapping (geological: solid & superficial, ordnance
survey, hazard, UXO), imagery, remote sensing…
• Site Walkover Survey
Nature, basic soil & rock distribution, topography, access & obstructions, drainage,
previous land uses i.e. workings, buildings
Check the outcome of the desk study: Ground Truth
Site Inspection
Initial Terrain model Potential Dam site:
Product of walk over
survey:
• Surface features
• Deposits
• Water courses
• Topography
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS Phase 1
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS Phase 1
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 2
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 2
Ground Investigation
Trial Pitting or Backhoe
Trial Pit Sections
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 2
Light cable percussion rig
Dynamic percussion
drilling rig
Typical land rotary coring drill rig
Offshore drilling rig
Doghouse
Drill string
Drill hydraulic
head unit
Geotechnical
soils lab
Derrick
Drill string
(unused)
Mud store
Offshore drilling rig
Sampling methods & sample types
Engineering Soils Undisturbed Disturbed
Piston Sampler
• Sample tube U100
• Shelby tube
• Length: 45-100 cm
• Extrude sample
• Core log
• Quarting: 25 cm, tin
foil & wax
• Clays , loose sands
& silts
Hammer Sampler
• Split barrel sampler
• Two parts; splits along
length
• 63 kg weight repeatedly
falls 76 cm on to top of
sampler (similar Dando)
• Count number of blows to
penetrate 30 cm
• Measure of resistance to
penetration
• (Standard Penetration Test)
• Samples double bagged
• Dense sands and weak
rocks
Sampling methods & sample types
Rocks Rotary Coring
Rotary Open Holing
Rotary Coring preferred to Open Holing
• Intact rock samples preferred to cuttings
• Coring produces undisturbed samples
• Laboratory testing: strength, particle
size analysis, moisture content
• Logging easier, Stratigraphic order
• Open holing: quicker, less skill, useful,
accurate rock desc. less important
Example of a
drilling log
Cone penetration testing (CPT) • In situ down hole tool
• Resistance to penetration of a cone
• Mechanical information about lithologies
• Proxy for piles design
How many boreholes and how deep? Spacing: buildings 10-30m apart
roads/railways 30-300m apart
landslides – at least 5 in a row
extra holes at all structures
Depends on site complexity
To a depth that may be significantly affected by
construction loading
Depth: 1.5 x (foundation width) below the foundation
depth plus at least one deeper hole to 10m below
foundation level unless rockhead reached.
Drill 3 m into rockhead to prove sound rock; in limestone
probe 3-10m to locate rock cavities and integrate with
geophysics
Site investigation costs
Drilling costs are an accumulation of: supplying
rig to site; setting-up at each hole; cost per
metre of hole drilled
Offshore significantly higher: £250k, ~£500pm
Costs vary with number of in situ tests and
instruments deployed.
Process (2008 figures in £) supply set -up per m.
Cable percussion in soil >8 m deep 700 50 22
Sampling in soil or rock 700 50 7
Rotary coring in rock 800 60 75
Trial pits, 4 m deep, backfilled 750 - 8 pits (i.e. 1 day hire JCB)
DESK STUDY
LOCAL KNOWLEDGE
AND PREVIOUS
EXPERIENCE
SITE INSPECTION
SPECIFICATION OF
FIELDWORK
PRELIMINARY REPORT
FIELD WORK: MAPPING,
TRENCHES AND PITS
BOREHOLES AND IN
SITU TESTING
GEOPHYSICAL SURVEY
FACTUAL REPORT ON
GROUND
INVESTIGATION INTERPRETATIVE
REPORT (Revised terrain
and ground models)
TOPOGRAPHIC SURVEY
(as required)
COLLECT SAMPLE
LABORATORY TESTING
INITIAL TERRAIN AND
GROUND MODELS
Phase 2
Geophysics for Site Investigation
Applied methods should be sensitive to
the properties of the target
Should be suitable for the lithologies
Complement invasive investigations;
identify areas of interest in subsurface
Interpolate between boreholes
Derive geotechnical parameters; e.g.
seismic methods for RQD, Resistivity for
porosity…
Electromagnetic Surveying (EM) • Induced field interacts with subsurface and induces a second field
• Second field proportional to subsurface electrical conductivity
• Produces 1D line or map of subsurface electrical conductivity
• Sensitive to lithology and moisture content: high EC: clay, basalt, water;
whereas sand and limestone have low EC.
Ground Penetrating Radar (GPR) • Similar approach to EM, transmitted EM energy reflects off boundaries
• Produces subsurface profile based on lithology contrasts
• Poor depth penetration in clay
• Identify lithological boundaries and subsurface man-made structures
(pavement thickness)
Gravity Surveying • Measure variations in Earth’s gravity field caused by local differences in
density of subsurface material
• 1D profiles of gravity variation or contour maps
• Identifying cavities or mineshafts
• Measurement of variations in earth’s
total magnetic field
• Caused by local differences in the
magnetisation of subsurface rocks and
soils
• Magnetometer and a magnetic survey
to identify mine shafts
• Caveat; interference: use prohibited
near power lines and metal fences…
• Also used for unexploded ordnance
(UXO) and archaeological
investigations
Magnetic Surveying
Geophysical survey costs Comparisons not easy as each method is best applied to
only certain ground problems. Rough guide given by
approximate coverage for a given fee - £2000 at 2008
prices
Type of survey Area
Microgravity survey 0.25 ha on 5 m grid
Magnetic survey 1.5 ha on 1 m line spacing
Electromagnetic survey 2.0 ha on 2 m line spacing
Ground penetrating radar 1.0 ha on 1 m grid
Seismic refraction 5 soundings by 20 m deep
Seismic tomography 2-D profile to 40 m depth (between available
boreholes)
Borehole 1 cored hole 20 m deep
Developing the
ground model – the
total geology model
3D block model should show:
• Solid geology- bedrock
• Structural setting
• Superficial geology
(quaternary materials)
• Geomorphological features
(landslides, solifluction
deposits)
• Weathering state of units
• Water table
• Surface water courses
Inadequate Site Investigation
One third of construction projects are
delayed by ground problems
Unforeseen ground conditions are the
main cause of piling claims
Half of over-tender costs on road projects
are due to inadequate SI or poor
interpretation of the data
Fookes, Baynes & Hutchinson
2001
There must be a specific and determined
endeavour to understand the engineering
geology and geomorphological
environment of the site and to incorporate
that understanding into the project design
Around the world it is often the problems
related to the Quaternary geology and
geomorphological events that dominate
Fookes, Baynes & Hutchinson
2001 The preliminary stage is when the
engineering geomorphologist and
geologist can have the most significant
influence on the project by indicating
potential hazards and their consequence on
the economy of design, matters of
construction and expected performance of the
works
Summary Eurocode 7
Building the conceptual ground model
Stages of a Site Investigation: desk studies;
reconnaissance mapping; main site work stage –
pitting, boring, geophysics; laboratory testing;
reporting; on-going checks during construction
Reading: Site investigation chapters