Introduction to Excavation Safety

Transcription

1 WWT Event 7 th June 2015 Health and and Safety Executive Introduction to Excavation Safety Luke Messenger HSE Construction Division, Birmingham

2 What are the key issues?

3 Key Issues Collapse of excavations Undermining of adjacent structures Material falling into an excavation People, plant and equipment falling into excavation Buried Services

4 When it all goes wrong

5 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))

6 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

7 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

8 Excavation Safety Basics You probably all know this....but

9 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.

10 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

11 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

12 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

13 Temporary safe slopes of soils Table from CIRIA Report 97 - Trenching Practice

14 Excavation of trench Surcharge loading

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16 Reaction to foundation loading Ground heaves Load Soil pushed out and driven upwards Wedge of soil under foundation driven into ground

17 Undermining doesn t mean you have to dig directly beneath the foundation Wall Foundation exerts vertical load on soil below Soil slips off stable material Earth face Wedge of soil at angle of repose

18 What will the inspector do?

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20 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)

21 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

22 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)

23 Excavation Solutions Close Boarded Trench Support Waler Frames For Medium Sized Trenches Trench Boxes Design of trench box - access ladder and edge protection attached to box Strutting Frame Hydraulic / Mechanical Struts

24 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

25 Easier if new build Still need to batter back or provide support

26 Hit and miss or close boarding required Stiff clay Sandy silty clay

27 Stiff Clays and soils Open or hit and miss shoring

28 Soft Clays, Gravels & Sands clay Sand/ gravels Close boarding or interlocked sheets

29 Soil types layered strata Layered soil strata can be very unstable Therefore close support system with possible groundwater control required

30 Clay over Sandy/Gravel Fatality Clay layer Corner sheared off Void where gravel layer fell away

31 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

32 Use equipment properly

33 Don t forget adjacent structures Unsupported clay face and risk of undermining adjacent structure Boundary wall can slip or overturn Unsupported clay face

34 Protection for the public

35 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

36 Underpinning 2 Schedule Typical underpinning schedule drawn up by competent person

37 Should there be a requirement to support face of underpin section? Clay face starting to fail Need to consider sacrificial face support usually steel sheet piles

38 Support of existing foundation Underpinning failure to survey existing wall and to prop

39 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

40 Supporting existing structure above basement Prohibition Notice served temp wks engineer came back with props at 200mm centres below main walls of house

41 EOT starts to lean towards road

42 Emergency works by LA contractor

43 Excavator for sale - collection only!

44 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

45 Buried Services Further information HSE Website on/excavations.htm HSG47 Avoiding Danger from Underground Services Busy Builder CIS65

46 Information & Guidance Guidance concerned with Groundworks/ Excavations HSE Website Structural Stability During Excavations HSE Busy Builder Leaflets CIS64 on Excavation and CIS66 on Domestic Basement Construction HSE Website Excavation and Underground Services

47 Information & Guidance Construction Plant-Hire Association (CPA) Shoring Technology Interest Group Publications -technology-interestgroup-stig-publications/ (Guidance on Management of Shoring in Excavations, Selection of shoring equipment) Association of Specialist Underpinning Contractors (ASUC) (Guidance on Underpinning, Basement Extensions)

48 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

49 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

50 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

51 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

52 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