Management of Geotechnical Risk - engineersireland.ie

10/12/2011

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Engineers Ireland South‐east RegionKilbride Theatre, Ormonde Hotel, Kilkenny

Management of Geotechnical Risk

Dr Paul Jennings

Applied Ground Engineering Consultants (AGEC) Ltd

www.agec.ie

Thursday 1 December 2011

• Introduction to AGEC

• Types of Geotechnical Risk (Typical)

Contents

• Reasons for Management of Geotechnical Risk ‐ General and Statutory

• Typical Examples of Geotechnical Risk

• Management of Geotechnical Risk: Risk Register

• Questions and Answers.

• AGEC is an independent Irish company formed in April 2001 to provide geotechnical consultancy services

• AGEC provides specialist geotechnical engineering advice to

Introduction to AGEC

• AGEC provides specialist geotechnical engineering advice to consultants, contractors, private clients and local authorities, particularly for infrastructure and renewable energy projects

• Main areas of geotechnical expertise include:• Geotechnical Due Diligence & Management of ground investigations • Foundation design • Design of retaining structures • Earthworks design • Stability assessment of soil & rock slopes • Procurement and management • Site supervision• Specialist technical advice• Contractual advice• Expert witness advice.

• Slope Instability

• Settlement

Typical Geotechnical Risks

Settlement

• Subsidence (sub‐surface voids)

• Groundwater

• Chemically reactive ground

• Contamination

f d di i ( )• Unforeseen ground conditions (extent, type)

• Inadequate geotechnical investigation

• Inappropriate design.

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Reasons for Management of Geotechnical Risk ‐ General

• Ground related risks affect and influence many parts of a project

‐ Health and safety, environment, quality, programme and cost

• Ground conditions are always uncertain

• Compliance with standards reduces ground risk but does not eliminate ground risk

• Ground related risk generally have a significant impact

‐ Highly variable nature of the ground

‐ Soil/rock mechanics engineering is good but accuracy can be poor

‐ Multiple hazards (shear failure, deformation, groundwater, chemically reactive, obstruction, etc)

‐ Unforeseen ground conditions in early stages of works (delays following works)

80

100

n Co

st (%

)

delay

Reasons for Management of Geotechnical Risk

20

40

60

Total Increase in Con

struction

ncrease project cost and

d

Typical upper bound

00 1 2 3 4 5 6 7 8 9 10 11 12

Ground Investigation Cost/Tender Construction Cost (%)

In

Reduced cost on geotechnical studies

Source: UK Highways Agency projects (1994)

• Under Safety, Health and Welfare Regulations (1) there is a requirement to appoint a Project Supervisor Design Process (PSDP)

D ti f PSDP ith t t i k/h d t

Reasons for Management of Geotechnical Risk –Statutory Framework (PSDP)

• Duties of PSDP with respect to risk/hazard management:– Identify hazards arising from the design/technical/organisational/planning/time related

aspects of the project– Where possible eliminate the hazards or reduce the risk– Communicate necessary control measures, design assumptions, or remaining risks to

the PSCS (so they can be dealt with in the S&H Plan)– Prepare S&H plan for any project where there is a Particular Risk(2) (or construction takes

>500 person days or 30 working days)– Role extends beyond design stage in temporary works during construction

• Other duties of PSDP:– Organise co‐operation between designers– Prepare a safety file for the completed structure and give it to the client– Notify the Authority and client of non‐compliance with any written directions issued– Issue directions to designers or contractors or others

(1) Safety, Health and Welfare at Work (Construction) Regulations 2006 (S.I. No. 504 of 2006)

(2) For example with respect to the ground: Schedule 1, Item 1(b) and (c) state particular risk due to (b) burial under earthfall where the risk is particularly aggravated by the nature of the work or processes used or by the environment at the place of work

• Under Safety, Health and Welfare Regulations (1) the designer has defined dutiesD i f d i i h i k/h d

Reasons for Management of Geotechnical Risk ‐Statutory Framework (Designer)

• Duties of designer with respect to risk/hazard management:– Identify any hazards that your design may present during construction and subsequent

maintenance– Where possible eliminate the hazards or reduce the risk – Communicate necessary control measures, design assumptions or remaining risks to the

PSDP so they can be dealt with in the S&H Plan

• Other duties of designer:– Co‐operate with other designers and the PSDP or PSCS– Take account of any existing safety and health plan or safety filey g y p y– Comply with directions issued by the PSDP or PSCS– Where no PSDP has been appointed, inform the client that a PSDP must be appointed

(1) Safety, Health and Welfare at Work (Construction ) Regulations 2006 (S.I. No. 504 of 2006)

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Peat

Slope Instability ‐ Landslides

Rock with thin soil cover

Till (sandstone)

Till (metamorphic)

Slope Instability ‐ Landslides and Run‐out

Slope Instability – Peat Failures Slope Instability – Peat Failures

Ballincollig failure (2008)

Multiple tension cracks

Partially detached peat raft

Main debris track

200m

Area of worked blanket peat

Temporary road construction in peat

General slope inclination 4 degrees

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Glenglassera failure (1986)R314 road

Slope Instability – Peat Failures

1550mLarge partially detached peat raft

Main debris track

General slope inclination 5 degrees

Deposition area/debris track

Tension cracks

Headward extension into forestry

Slope Instability – Predicting Peat Failure

Road layout

Hydrological/environmental bufferWind turbine location

Slope InstabilityPreliminary Stage

• Identify general geotechnical conditions (eg initial walkover survey)

• Certain soil types more prone to instability (eg peat)Certain soil types more prone to instability (eg peat)

• Examine available landslide databases

• Areas of previous failures indicate possible future failure (eg. anecdotal information)

• Avoid steep ground (but peat will fail at 4 degrees!)

• Avoid run‐out tracks of landslide debris (several kilometres!)

Detailed Design Stageg g

• Detailed walk‐over survey

• Detailed ground investigation

• Detailed analysis and stability assessment

• Appropriate design, mitigation, zonation

Slope Instability – Anecdotal Information

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Settlement and Subsidence ‐ Karst

E i ti d f

Settlement and Subsidence – Karst Example

Existing ground surface

Soil arch eventually collapses causing subsidence at surface

Drop out sinkholes form as a cavity migrates up from rockhead with time as material is carried from the soil into the limestone rock by movement of groundwater

Soil

Rockhead

LimestoneVoid in

limestone rock

Settlement and Subsidence – Karst Example Settlement and Subsidence – Mine Collapse Example

SUBSURFACE LASER SCANNING LTD.Plan of aqueduct

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Settlement and SubsidencePreliminary Stage

• Examine available databases (GSI karst, EPA soils maps, mining records, anecdotal information)

• Identify general karst/mining/soft ground conditions (eg walkover y g / g/ g ( gsurvey)

• Certain soil/rock types more prone to settlement/subsidence

• Areas of previous subsidence indicate possible future problems

• Avoid areas of high potential settlement/subsidence (may not be practical)

• Associated risks – flooding, radon

Detailed Design Stage

• Detailed walk‐over survey

• Detailed ground investigation, geophysics, sub‐surface surveying

• Detailed analysis and assessment of settlement/subsidence

• Appropriate foundation design, mitigation and zonation

Chemically reactive ground can be either:• Brown field sites (eg industrial sites, landfills)• Green field sites (eg naturally occurring)

Chemically Reactive Ground

Green field sites (eg naturally occurring)

Common potential issues:• Acidity (pH value)• Chlorides• Sulphates and sulphides• Industrial/domestic contaminationIndustrial/domestic contamination

Sulfides• Well known sulfide is Iron Sulfide (FeS2) or “pyrite”

• Pyrite has received much press in recent years because the


mineral has been found in the sub‐floor fill of damaged houses in North County Dublin

• Most limestone and calcareous mudstone rocks in Dublin and neighbouring counties contain pyrite

• Oxidisation of pyrite can have the following negative consequences

G d b ll d i h lf i id• Groundwater can be polluted with sulfuric acid

• Sulfuric acid can attack concrete and steel

• Concrete may be subject to sulfate attack

• Growth of expansive crystals (gypsums) heave


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Chemically Reactive Ground Chemically Reactive Ground

Preliminary Stage

• Examine available databases (GSI, EPA soils maps, historical records)

• Identify general areas of reactive ground conditions (eg initial walkover survey)


survey)

• Certain soil/rock types prone to producing reactive ground conditions

• Anecdotal information

• Areas of previous problems indicate possible future problems

• Avoid areas of high potential (may not be practical)

Detailed Design StageDetailed Design Stage

• Detailed walk‐over survey and mapping of exposures

• Detailed ground investigation including petrographic analysis

• Appropriate foundation design, specification and mitigation

Inadequate Geotechnical Investigation

Large ‐scale peat slide

Original: 731 GI locations (mostly probes) in 345ha site = 2 per haFinal: 5808 GI locations in 345ha site = 17 per ha

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Inadequate Geotechnical Investigation

• Aim of geotechnical investigation: “to establish the soil, rock and groundwater conditions, to determine the properties of the soil and rock, and to gather additional relevant knowledge about the site.”

S i d d th f d i ti tiSource Quotation/Reference

IS EN 1997‐2: 1997 (Eurocode 7, Part 2, Annex B3)

“ for high‐rise and industrial structures, a grid pattern with points at 15 m to 40 m distance”

“for linear structures (roads, railways, channels, pipelines, dikes, tunnels, retaining walls), a spacing of 20 m to 200 m”

BS5930: 1999, Code of Practice for Site Investigations, clause 12.6 Spacing

“Although no hard and fast rules can be laid down, a relatively close spacing between points of exploration e.g. 10m to 30m is often appropriate for structures.”

Highways Agency (1997), clause 2.30

“No hard and fast rules can be given for the depth and spacing of exploratory holes. This will depend on the specific circumstances of each site and the amount and

• Spacing and depth of ground investigation:

quality of existing information. Some guidance is given in BS 5930. The selection criteria for the depth and spacing of exploratory holes should always be to ensure that sufficient information is obtained to enable the proposed works to be adequately designed.”

• Type of geotechnical investigation‐ see for example IS EN 1997‐2: 1997 (Eurocode 7, Part 2)

Inappropriate Design ‐ Pit Collapse

• Employee killed when trial pit collapsed

• Lone working in unsupported pit

• Prosecution of Employer under 2007 Corporate Manslaughter Act (UK) p y p g ( )

Management of Risk: Geotechnical Risk Register (GRR)

• GRR comprises a list of geotechnical hazards and risk control measures attached to various elements of the proposed route at any project stage (eg route selection/pre planning planningany project stage (eg route selection/pre‐planning, planning, design, construction and operation)

• A subjective qualitative scale is used to define risk

• Hazards are rated in terms of how likely they are to occur (the Probability) and to what degree they would affect the project (the Impact)

• The degree of risk is determined by combining the probability and impact assessments:impact assessments:

Risk = Probability × Impact R = P × I

Clayton, CRI (2001). Managing Geotechnical Risk – Improving Productivity in UK Building and Construction. Prepared under the DETR Partners in Technology programme for the Institution of Civil Engineers. Thomas Telford, London.

No. Hazard Cause Potential Impact Risk Rating Risk Control Measure (RCM) Risk Rating after RCM Contingency Measures

Category Specific P I R Design Control Construction / Operations Control P I R

1 Liquefaction of base of excavation

Sand / subsoil mixing with water and forming liquid material.

1. High/rising tidal waters causing liquefaction (quick conditions) of sand in excavation base2. Inadequate site investigation information3 I d t d i

1. Health & Safety2. Programme3. Cost

1. Risk of death or injury by drowning2. Damage to plant3. Loss and cessation of works

2 3 6 1. Review of previous excavations in the locality and in similar ground2. Detailed site investigation to include trial trenches to expose soils3. Boreholes and trial pits to be taken below base of excavation4. Specify method statement from the Contractor for working in tidal sands and h k th t h i f ll i t ith d

1. Site supervision staff to inspect excavation daily2. No/limited access into excavation3. Areas of unexpectedly deep and weak ground to be reported and await further instruction4. Works sequenced to tides and weathered conditions 5 S i i t t ti

1 3 3 1. Stop works2. Ballast stone to be placed in base of excavation3. Ensure localised area made stable

Example of Geotechnical Risk Register

3. Inadequate design and understanding of ground conditions4. Improper construction

check that he is fully cognisant with ground conditions5. Check of implications of over-excavation6. Check requirement to have ballast stone available to place in base of excavation

5. Supervision to ensure construction carried out as detailed in the method statement6. Limited exposure of excavation base to avoid potential for base (liquefaction) failure

2 Escape of potential contaminated water from works into surface water channels

Possible contaminants from leakages, spills or fines/suspended solids

1. Extended periods of wet weather and under-design of temporary pumping2. Over-pumping of excavation into surface water channel.3. Potential contamination from run-off from works 4. Potential contamination from backfill material

1. Environmental 1. Risk of contamination of surface water

3 4 12 1. Walk-over survey to identify areas where greater risk of water entering works and becoming contaminated2. Detailed site investigation to include trial trenches to expose soils and monitoring of groundwater along route3. Spillages from plant - addressed in environmental impact statement4. Consider limiting the extent of works/excavation opened in sensitive areas5. Specify method statement from contractor that clearly demonstrates their awareness of this issue6. Identify any sensitive receivers along route

1. Supervision staff to be fully briefed on the ground conditions, design requirements and construction methodology2. Supervising staff aware of weather forecasts3. Appropriate pumping facilities to be put in place during the construction phase and silt traps/bunds constructed4. Temporary bunds and drains to be installed as appropriate5. Supervision to ensure construction carried out as detailed in the method statement6. Measures to prevent contamination/clean up contamination before work continues

1 4 4 1. Stop work2. Environmental Manager to be notified immediately.3. Establish if contamination includes oil / diesel4. Identify source of contamination and solve problem immediately as per EMP.5. Oil contaminated water to be treated prior to discharge (use oil interceptors if appropriate)6. Excess water to be diverted into drainage channels with filtration / sedimentation as required7. Use simple and effective filtration measures to remove particle load e.g. straw bales/terram in drainage channels8. Sedimentation tanks to be used (and cascaded if necessary)9. Use adjacent areas within temporary working area as natural filter in agreement with NPWS e.g. for high levels of suspended peat10. Reserve / additional pumping facilities to be available

3 Unexpected hard obstructions in

1. Ground conditions differing from those

1. Programme2. Cost

1. Delays to works2. Increased noise

4 2 8 1. Carry out extensive site investigation. 2. Walk-over survey of route to identify

1. Construction personnel briefed on expected ground conditions

1 2 2 1. Stop works2. Area to be assessed

excavations, e.g. boulders, rock outcrops

indicated from site investigation2. Inadequate site investigation information

levels due to additional rock breaking requirements

areas of variable ground (e.g. shallow rock, till, peat/organic clay)3. Selection of conservative design parameters to allow for variable conditions on site4. Require detailed construction method statement that clearly demonstrates understanding of the ground conditions and risks involved5. Monitoring and observation method proposed as part of construction controls

2. Reporting by site staff on change in ground conditions from that predicted3. Advance notice of change in predicted ground condition to be fed to designers4. Supervision to ensure construction carried out as detailed in the method statement

3. Use rock breakers/non-explosive pre-splitting of rock using expansive grouts or similar

4 Open excavations and holes filled with disturbed peat

1. Excavation works2. Displacements and slides3. Improper construction methods

1. Health & Safety2. Environmental3. Programme4. Cost

1. Risk of death or injury from falling into excavation2. Damage to plant

3 5 15 1. Carry out extensive site investigation2. Walkover survey of route to identify areas of variable ground (e.g. shallow rock, till, peat/organic clay)3. Detailed method statement to be prepared with respect to excavation

1. Appropriate sequencing of works2. Supervision of works by suitably qualified person3. Tool box talks to be carried out prior to works4. Ensure no excavations left open or unprotected5. Backfill to be with a suitable material6. Supervision to ensure construction carried out as detailed in the method statement

2 3 6 1. Stop work2. Backfill open excavations with stone3. Over excavate disturbed peat localised area and backfill with stone

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Content of Geotechnical Risk Register (GRR) Hazard Cause Potential Impact Risk Rating Risk Control Measure (RCM) Risk Rating

following RCMContingency Measures


HazardsHazard & Cause

Potential ImpactRisk RatingP × I = R

Risk Control Measures (RCM)• Design Controlg• Construction / Operation Control

Risk Rating following RCMP × I = R

Contingency Measures

Example List of Geotechnical Hazards

1 Slope Instability

Hazard Cause Potential Impact Risk Rating Risk Control Measure (RCM) Risk Rating following RCM



1. Slope Instability

2. Settlement

3. Subsidence (sub‐surface voids)

4. Groundwater

5. Chemically reactive ground

6. Contamination

7 Eroding/mobile ground conditions (coastal/marine)7. Eroding/mobile ground conditions (coastal/marine)

8. Unforeseen ground conditions (extent, type)

9. Inadequate geotechnical investigation

10. Inappropriate design

*Any hazard can be sub‐divided in terms of a sub‐set or geographic location

Potential ImpactHazard Cause Potential Impact Risk Rating Risk Control Measure (RCM) Risk Rating

following RCMContingency Measures


Category:

All hazards belong to one of the following categories:

H lth & S f t

Specific:

Examples of specific hazards are as follows:

• Risk of injury or death (Health & Safety)• Health & Safety• Environmental• Programme• Cost• Quality

• Change of hydrology (Environmental)• Collapse of excavation (Programme)• Damage to plant (Cost)




Risk Rating

b bili i kProbability × Impact = Risk P × I = R

PROBABILITY (P)

1 Negligible

2 Possible

IMPACT (I)

1 Very Low

2 Low

PROBABILITY (P)

1 2 3 4 5

1 1 2 3 4 5Routine2 Possible

3 Likely

4 Probable

5 Very Likely

2 Low

3 Medium

4 High

5 Very HighIMPA

CT (I)

2 2 4 6 8 10

3 3 6 9 12 15

4 4 8 12 16 20

5 5 10 15 20 25

Routine

Requires Attention

Unacceptable

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Risk Control Measures

• If the risk is deemed excessive, then Risk Control Measures (RCM) are required to lessen the risk

• These can be:Design controls:eg. Detailed site investigation, locally re‐routing, micro‐siting

Construction or operation controls:eg. Site supervision staff to inspect critical pylon locations

• Risk Control Measures mitigate the Probability and/or the Impact




Risk Rating following RCM

• Following the applications of the Risk Control Measures, the degree of risk is again assessed

• Ideally, the Risking Rating should now be tolerable





• Contingency Measures are then considered, should the hazard occur, eg.

Stop worksMonitor situation

• The risk register is generally a live document: as new hazards, risk control measures or contingency measures present themselves, these should be addedthese should be added

Control & Reporting of GRR

• The GRR is a key method of communication of risk between the various parties at different stages of the project (eg route selection pre‐planning planning design construction andselection, pre planning, planning, design, construction and operation)

• Geotechnical risk management should be integrated into the overall project management process

• Review and feedback allows continuous improvement of the risk management system

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Worked Examples ‐ Peat Slide

HazardPeat Slide

Potential Impact1. Risk of death or injury by inundation2. Environmental damage3. Damage to persons, plant, property and livestock4. Loss and cessation of works5. Adjacent land affected due to peat/ground movement

Risk Rating2 × 5 = 10

Risk Control Measures (RCM)• Design Controls (list)• Construction / Operation Controls (list)

Risk Rating following RCM

1 × 5 = 5

Contingency Measures1. Stop works2. Use sheetpiling to stop ground movements3. Monitor movements following installation of sheetpiles untilmovements have ceased

4. Reduce speed of works5. Reduce excavation lengths prior to backfilling with stone

Summary ‐Management of Geotechnical Risk

• Ground conditions are always uncertain

• Realistically, not all ground risk can be avoided or eliminated

• Compliance with standards reduces ground risk but does not eliminate ground risk

• Ground related risk can generally have a significant impact

• Management of ground risk is essential for success:

‐ allow for sufficient budget

‐ use the right managements tools

‐ employ appropriate specialists

Questions & Answers

Thank You

Documents

Management of Geotechnical Risk - engineersireland.ie