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