Health and Safety Executive

Health and Safety Executive

Introduction to Excavation Safety

Luke Messenger HSE Construction Division, Birmingham

WWT Event 7th June 2015

What are the key issues?

Key Issues

• Collapse of excavations

• Undermining of adjacent structures

• Material falling into an excavation

• People, plant and equipment falling into excavation

• Buried Services

When it all goes wrong

Law

CDM 2015 Regulation 22(1) • All practicable steps must be taken “where necessary” to prevent danger

to any person to ensure that - • An excavation (or part of) does not collapse • No material is dislodged or falls • No person is buried or trapped

Also • Steps to be taken to prevent persons, equipment or materials falling in

(Reg 22(2)) • Prevent excavation or adjacent ground from being overloaded (Reg

22(3)) And • No work should be carried out unless the excavation, equipment and

materials have been inspected by a competent person (Reg 22(4))

Enforcement

In the last 5 years - • Average 3 trench collapse fatalities per year • 27 RIDDOR Major Injuries • 430 Notices (Mostly Prohibition Notices) covering

• Risk of collapse of excavation onto persons • Risk of collapse that would carry

person/plant/structure into excavation • Risk of falls into unguarded excavation

• Other Notices requiring daily inspection • Significant proportion during basement excavation

Enforcement Cont.

Prosecutions • 9 brought by HSE resulting in fines • 2012 one fine of £150K + £28K costs • 2014 two fines of £15k + £2k costs (Not including FFI) • 2011 Cotswold Geotechnical - Corporate Manslaughter Death of geologist in 3.5 m deep pit in 2008 Fined £385K • 2014 Site Manager jailed for 3 years 3 months Consultant jailed for 9 months Manslaughter charges following death of labourer in Fulham in 2010 – Basement Excavation

Excavation Safety Basics

You probably all know this…..

……..but

Excavation Safety Basics

• A cubic metre of soil weighs around 1.75 Tonnes.

• Any fall of material presents a risk. • Unless the excavation can be battered to

a safe slope, the sides will need supporting to prevent collapse and provide safe conditions for persons working in or adjacent to the excavation.

• Support enables work to be carried out without interruption and protect adjacent property and /or services.

Perception

• The importance and need to provide proper support to excavations is often hard to explain to site operatives.

• Clays, to the inexperienced, often look very stable when first excavated.

• They stand up vertically, in many cases, for a surprisingly long time and a false sense of security is built up.

• Remember, it’s not if? but when? an excavation face will collapse

Basic soil mechanics

• Sides relieved of lateral support from surrounding ground

• Becomes unstable

• Material above natural angle of repose will partially or totally collapse at any time

Angle of Repose

Soil poured from a container will stand in a heap, the slope of which is known as the “Angle of Repose.”

This is a stable slope in the soil

Temporary safe slopes of soils

Table from CIRIA Report 97 - Trenching Practice

Excavation of trench

Surcharge loading

Presenter

Presentation Notes

Long tract of excavation exposed which has started to collapse The contractor now has a problem. How does he get back in to place the support and excavate the additional material?

Reaction to foundation loading

Load

Wedge of soil under foundation driven into

ground

Soil pushed out and driven

upwards

Ground heaves

Undermining doesn’t mean you have to dig directly beneath the foundation

Soil slips off stable material

Wedge of soil at angle of repose

Earth face

Foundation exerts vertical load on soil below

Wall

What will the inspector do?

Presenter

Presentation Notes

1.7m trench excavated for drainage. Worker near trench breaking concrete. Confirmed workers had entered trench to fit pipework although no means of support or battering back. Also no barriers or covers to prevent falls into excavation. Inspector will expect the work to be properly planned…

Planning the work

• Company Standards for excavation work • Risk Assessment procedure • Consider underground services • Temporary works input (Calculations &

Drawings) • Production of Method Statements (safe system of work)

• Selection of right plant and equipment • Competent site management and supervision

(especially front line supervisors)

Method Statements

• Geotechnical information

• Method of construction and support of excavation

• Access/ egress and edge protection

• Plant/ equipment to be used

• Buried services • Adjacent structures • Supervision of work

Geotechnical Report – Starting point

• Ground conditions need to be known to design the permanent or finished works.

• Information can be found from desktop study or previous experience of local area.

• More complicated jobs will need a site investigation including a geotechnical report

• Information can also be used by contractor when devising construction method (including support of excavations)

Excavation Solutions

Trench Boxes

Close Boarded Trench Support

Design of trench box - access ladder and

edge protection attached to box

Waler Frames – For Medium Sized Trenches

Hydraulic / Mechanical Struts Strutting Frame

Control Measures - battering or stepping back

• Battering or stepped excavations

• Based on safe angle of repose

• However you need the space on site to do this.

• A lot of sites maximise the footprint and hence no option to batter or step excavation

• Easier if new build • Still need to batter back

or provide support

Hit and miss or close boarding required

Stiff clay Sandy silty clay

Stiff Clays and soils

Open or hit and miss shoring

Soft Clays, Gravels & Sands

Close boarding or interlocked sheets

clay

Sand/ gravels

Soil types – layered strata

Layered soil strata can be very unstable

Therefore close support system with possible groundwater control required

Clay over Sandy/Gravel Fatality

Corner sheared off Clay layer

Void where gravel layer fell away

Presence of water

• Water table (geotechnical report)

• Ingress of water (e.g. broken water main)

• Affects stability of excavated face

• Washes out granular soils

• Soften clay soils

Use equipment properly

Don’t forget adjacent structures

Unsupported clay face and risk of undermining adjacent structure

Boundary wall can slip or overturn

Unsupported clay face

Protection for the public

Underpinning

Technique devised to treat subsidence now also used to form basements

Excavate below existing foundation

Form new pin or foundation

Heel can be designed out

Underpinning 2 – Schedule

Typical underpinning schedule drawn up by competent person

Should there be a requirement to support face of underpin section?

Need to consider sacrificial face support – usually steel sheet piles

Clay face starting to fail

Support of existing foundation

Underpinning – failure to survey existing wall and to prop

Basement Construction

• High Risk Work – needs careful planning before work starts on site

• Need Temporary works engineering input - can be an engineer or an experienced person.

• Method Statement should be in place, e.g. a clear method of how work is to be undertaken including shoring of excavations and support of existing or adjacent structures.

• Shoring material/ equipment is on site • Competent site supervision

Supporting existing structure above basement

Prohibition Notice served – temp wks engineer came back with props at 200mm centres below main walls of house

EOT starts to lean towards road

Emergency works by LA contractor

Excavator for sale - collection only!

Buried Services

• Plan the Work

• Obtain cable plans – only an indication

• Use cable locating devices – essential – Locate, identify and mark – Training – Check and maintain equipment

• Safe digging practices – Trial holes, horizontal digging,

insulated tools

Buried Services

Further information

• HSE Website http://www.hse.gov.uk/electricity/information/excavations.htm

• HSG47 ‘Avoiding Danger from Underground Services’

• Busy Builder CIS65

Information & Guidance

• Guidance concerned with Groundworks/ Excavations

• HSE Website ‘Structural Stability During Excavations’ www.hse.gov.uk/construction/safetytopics/excavations

• HSE Busy Builder Leaflets CIS64 on Excavation and CIS66 on Domestic Basement Construction

• HSE Website ‘Excavation and Underground Services’

www.hse.gov.uk/electricity/information/excavations.htm

Information & Guidance

• Construction Plant-Hire Association (CPA) – Shoring Technology Interest Group Publications

www.cpa.uk.net/shoring -technology-interest-group-stig-publications/ (Guidance on Management of Shoring in Excavations, Selection of shoring equipment) • Association of Specialist Underpinning

Contractors (ASUC) www.asuc.org.uk (Guidance on Underpinning, Basement

Extensions)

So what are temporary works?

Temporary Works is defined in BS5975 : 2008 as

“parts of the works that allow or enable construction of, protect, support or provide access to the

permanent works and which might or might not remain in place at the completion of the works“

Temporary Works (BS 5975:2008)

• Anything and everything used to assist with construction of the permanent works.

• An engineered design used to assist with construction and which may be removed; or left in situ (abandoned); or may be reused to become part of the permanent works.

• Includes basic excavation support

• Procedures for management and coordination are required

Small builders get confused……..

Remember

• Temporary condition is the problem

• Temporary Works is the solution

Also

• Decisions about build sequence, time allocation, construction method, const materials – all influence scale, complexity, cost and criticality of TW needed

Procedures for dealing effectively with TW

Small or simple job (SME / Micro dutyholder)

• Developer / contractor coordinates TW – paperwork to suit type & structure of company

• Subcontractor/s given lead within their specialism

• TW engineer used for bespoke or complex areas with independent design check on complexity / risk basis

• Installation / alterations / loading / unloading / dismantling overseen by main contractor working with subcontractor

Procedures for dealing effectively with TW

Big or complex job

• Appointment of TW coordinator & fine tuning of existing company paperwork procedures

• Appointment of TW designer

• Independent design check especially where complex / high risk

• Appointment of TW contractor

• Appointment of TW supervisor/s

• Full control of implementation – installation, alterations, loading, unloading, dismantling