Building Basements Safely - PowerPoint Presentation

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Building Basements Safely

An overview of HSE’s approach to regulating basement excavations

James

H

ickman

HM

INSPECTOR (CONSTRUCTION)

ROSE COURTSlide2

Building Basements Safely

My Background

Geotechnical Engineer

HM Inspector (Construction)

Basement and Tunnel Inspector

Basement InitiativesSlide3

Building Basements Safely

Overview

Introduction to basements

Site findings

What are HSE doingSlide4

Overview - The sales pitch for subterranean livingSlide5

Overview - How is it achieved…?Slide6

Overview - When it goes wrongSlide7

Overview - The problems with basement construction Technically challengingContractors often new to the industryPoorly plannedPoorly supervised/managedLack of competence in those planning and managing – technical/health and safetyComplacency (60+ basements no shoring!)Slide8

Overview - Key IssuesCollapse of excavations Undermining of adjacent structuresMaterial falling into an excavationPeople, plant and equipment falling into excavation - WAHTemporary works / Permanent worksElectricsWelfareManual handlingSlide9

Overview – Key issues cont.Excavation often unsafe – complacency Belief that vertical clay face will stand unsupported TW designers often absent – no calcs/drawingsSimultaneous refurbishment with structural alterationWork subcontracted – main contractor absentBeams can weigh several hundred kgVulnerable workersSlide10

Overview - LegislationReg 19 CDM 2015 – Stability of StructuresReg 22 CDM 2015 – ExcavationsSchedule 2 CDM 2015 – WelfareReg 6(3) WAH 2005 – Avoidance of risks from work at heightReg 6 EAW 1989 – Adverse or Hazardous EnvironmentsSlide11

Overview - Information & Guidance

Guidance concerned with

Groundworks /

Excavations

Nothing specific with regard to basement construction

HSE Busy builder

leafletSlide12

Overview - Information & Guidance IStructE – Subsidence of low-rise buildings (Aug 2000)CIRIA - Trenching practice. 2nd edition (2001 revision)Shoring technology interest group www.cpa.uk.net/p/Shoring-Technology-Interest-GroupSpecialist Domestic: Underpinning & Subsidence Repair Techniques/Engineer Foundation Solutions /Retrofit Basement - Association of Subsidence & Underpinning Contractors (ASUC) www.asuc.org.uk Slide13

Introduction to basementsYou probably all know this….. ……..butSlide14

Basic soil mechanicsSides relieved of lateral support from surrounding groundBecomes unstableMaterial above natural angle of repose will partially or totally collapse at any timeSlide15

Angle of reposeJust like being at the beach!Slide16

Angle of reposeStarts to dry out, crack and crumble – water doesn’t help.Slide17

Angle of reposeIn time it will return to its natural angle of repose.

Angle of reposeSlide18

Temporary safe slopes of soilsTable from CIRIA Report 97 - Trenching Practice Slide19

Temporary safe slopes of soilsSlide20

Excavations – basic principlesNo ground can be relied upon to stand unsupported.In urban areas you don’t know history of previous ground disturbance. All practicable steps must be taken to prevent danger to any person, including, where necessary, the provision of supports or battering, to ensure that no excavation or part of an excavation collapses (Reg 22 CDM 2015).Slide21

Control Measures - battering or stepping backBattering or stepped excavationsBased on safe angle of reposeHowever you need the space on site to do this.A lot of sites maximise the footprint and hence no option to batter or step excavationSlide22

Stiff Clays and soilsOpen or hit and miss shoringSlide23

Soft Clays, Gravels & SandsClose boarding or interlocked sheets

clay

Sand/ gravelsSlide24

Hit and miss or close boarding required Stiff claySandy silty claySlide25

Geotechnical Report – Starting pointGround conditions need to be known to design both the temporary and 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 – ‘BIA’.Information can also be used by contractor when devising construction method (including support of excavations) Slide26

Incidents can happen to “experts”Geotechnical surveyor killed by collapsed trench Sep 2008 - Geologist Alex Wright, 27, from Cheltenham, died when a 3.5m deep trial pit he was working in collapsed.February 2011 – His employer, Cotswold Geotechnical Holdings became the first company to be convicted of the new offence of corporate manslaughter. In convicting the company, the jury found that their system of work in digging trial pits was wholly and unnecessarily dangerous. The company ignored well-recognised industry

guidance, by requiring

junior employees to enter into and work in unsupported trial pits,

typically from

2 to 3.5 metres deep. Mr Wright was working in just such a pit when he died.

Company was fined £385,000, and it was upheld on appeal.

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Maximising the Footprint. Beware of undermining - doesn’t mean you have to dig directly beneath the foundation

S

table

material

Wedge of soil at angle of repose

Earth

face – no support

Foundation exerts vertical load on soil below

Wall

Excavation

Unstable MaterialSlide28

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

Soil slips off stable material

Wedge of soil at angle of repose

Collapsed earth

face

Foundation and load left unsupported

Wall

S

table

materialSlide29

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

Wedge of soil at angle of repose

S

table

materialSlide30

Presence of waterWater table (geotechnical report)Ingress of water (e.g. broken water main)Affects stability of excavated faceWashes out fines in granular soilsSoftens clay soilsThames is tidal!Slide31

Planning the work – design phase Company Standards for excavation work Risk Assessment procedureConsider underground servicesTemporary works input (Calculations & Drawings)Production of Method Statements (safe system of work)Selection of right plant and equipmentCompetent site management and supervision (especially front line supervisors) Slide32

Method Statements Geotechnical information – BIA’sMethod of construction and temporary support of excavation – sequencingAccess/ egress and edge protectionPlant/ equipment to be usedBuried services Adjacent structuresSupervision of workSlide33

Permanent v Temporary WorksPermanent works engineer (PWE)Responsible for designing the permanent or finished worksTemporary works engineer (TWE)Responsible for designing temporary works to enable permanent works to be constructedUsually different people but can be the same personPoor contractors will not have made any provision for a temporary works engineer or think that the permanent works engineer does it all.The management of temporary works in the construction industry SIM (Sector Information Minute) 02/2010/04 – HSE WebsiteSlide34

New Build Basement Construction Easier if new buildStill need to batter back or provide supportSlide35

Don’t forget adjacent structuresSlide36

Don’t forget adjacent structuresUnsupported clay face and risk of undermining adjacent structure

Boundary wall can slip or overturn

Unsupported clay faceSlide37

Installing support after the event can be problematic and create new risksSlide38

Underpinning 1 Technique devised to treat subsidence now also used to form basements

Excavate below

existing foundation

Form new pin or foundation

Heel can be designed outSlide39

Underpinning 2 – ScheduleTypical underpinning schedule drawn up by competent person Slide40

Contiguous Piles & Diaphragm WallsTechnique associated with larger projects and/or where there is spaceSlide41

Support for face of underpin section?Slide42

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 failSlide43

Support of existing foundationUnderpinning – failure to survey existing wall and to prop Slide44

Unshored underpin excavationsCommon to find un-shored pits on site up to 4m deepSlide45

Shoring of pits is a mustNeed to plan support scheme and access issuesCan use timber or steel sheet pilesSlide46

Deeper basements3.5m-4.5m deep pins – risk of unsupported face collapsing?Slide47

Deeper basementsThe Gucci standard!Slide48

Low risk?Slide49

Middle massSlide50

Innovative support systems emerging

Work

in

progressSlide51

Site findingsA selection of photographs taken by inspectors during site inspections showing typical working environments within a basement excavation.Slide52

Supporting existing structure above basementProhibition Notice served – evacuate site and property above - temp wks engineer came back with props at 200mm centres below main walls of houseSlide53

Safe access/ egressLightwell could have been sheet piled/ timbered first to provide large access “pit”Slide54

Protection for the publicSlide55

Safe place of workSlide56

Fail to plan – plan to failSlide57

End of terrace house – refurb + basement Slide58

EOT starts to lean towards roadSlide59

Emergency works by LA contractorSlide60

Putney collapse Slide61

WelfareSlide62

WelfareSlide63

AccessSlide64

AccessSlide65

AccessSlide66

AccessSlide67

Surcharging sides of excavationSlide68

Surcharging sides of excavation?Slide69

Water ingressSlide70

Conveyor beltsSlide71

Work at height?Slide72

ElectricsSlide73

Temporary propsSlide74

What are HSE doing about it?Inspections – lots of themIncluding annual 2 day concentrated inspection initiatives – most recent11/12 March 2015Kensington & Chelsea, Hammersmith & Fulham and Westminster – expanding.Intelligence gathering done by VO’sRecent proactive prosecutionsSlide75

What are HSE doing about it?Basement initiative headline stats:127 sites visited over two days,136 contractors met.On average 1 in 2 sites received enforcement action (NoC, IN or PN) On average 1 in 3 sites received a Notice (IN or PN).Main areas of enforcement were WAH, Excavation and Welfare. A couple of sites were shut due to management of H&S being so poorHowever…Slide76

SummarySome good news…!Evidence indicates that DH’s that HSE have visited in the past are showing signs of improvementSome of these noticeably soHSE working with industry on new guidance - ASUCplusFormation of industry associationsDH’s met welcomed HSE intervention – want an even playing field. Slide77

SummaryHigh Risk Work – needs careful planning before work starts on siteNeed Temporary works engineering input 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 siteCompetent site supervision Slide78

Excavator for sale - collection only!Slide79

Building Basements SafelyOpen Forum and Q&A