Building Basements Safely

An overview of HSE’s approach to regulating basement excavations

James Hickman

HM INSPECTOR (CONSTRUCTION)

ROSE COURT

Building Basements Safely

My Background

• Geotechnical Engineer

• HM Inspector (Construction)

• Basement and Tunnel Inspector

• Basement Initiatives

Building Basements Safely

• Overview

• Introduction to basements

• Site findings

• What are HSE doing

Overview - The sales pitch for subterranean living

Overview - How is it achieved…?

Overview - When it goes wrong

Overview - The problems with basement construction

• Technically challenging

• Contractors often new to the industry

• Poorly planned

• Poorly supervised/managed

• Lack of competence in those planning and managing – technical/health and safety

• Complacency (60+ basements no shoring!)

Overview - Key Issues

• Collapse of excavations

• Undermining of adjacent structures

• Material falling into an excavation

• People, plant and equipment falling into excavation - WAH

• Temporary works / Permanent works

• Electrics

• Welfare

• Manual handling

Overview – Key issues cont.

• Excavation often unsafe – complacency

• Belief that vertical clay face will stand unsupported

• TW designers often absent – no calcs/drawings

• Simultaneous refurbishment with structural alteration

• Work subcontracted – main contractor absent

• Beams can weigh several hundred kg

• Vulnerable workers

Overview - Legislation

• Reg 19 CDM 2015 – Stability of Structures

• Reg 22 CDM 2015 – Excavations

• Schedule 2 CDM 2015 – Welfare

• Reg 6(3) WAH 2005 – Avoidance of risks from work at height

• Reg 6 EAW 1989 – Adverse or Hazardous Environments

Overview - Information & Guidance

• Guidance concerned with Groundworks / Excavations

• Nothing specific with regard to basement construction

• HSE Busy builder leaflet

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-Group

• Specialist Domestic: Underpinning & Subsidence Repair Techniques/Engineer Foundation Solutions /Retrofit Basement - Association of Subsidence & Underpinning Contractors (ASUC) www.asuc.org.uk

Introduction to basements

You probably all know this…..

……..but

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

• Just like being at the beach!

Angle of repose

• Starts to dry out, crack and crumble – water doesn’t help.

Angle of repose

• In time it will return to its natural angle of repose.

Angle of repose

Temporary safe slopes of soils

Table from CIRIA Report 97 - Trenching Practice

Temporary safe slopes of soils

Excavations – basic principles

• No 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).

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

Stiff Clays and soils

Open or hit and miss shoring

Soft Clays, Gravels & Sands

Close boarding or interlocked sheets

clay

Sand/ gravels

Hit and miss or close boarding required

Stiff clay Sandy silty clay

Geotechnical Report – Starting point

• Ground 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)

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.

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

Stable material

Wedge of soil at angle of repose

Earth face – no support

Foundation exerts vertical load on soil below

Wall

Excavation

Unstable Material

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

Stable material

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

Wedge of soil at angle of repose

Stable material

Presence of water

• Water table (geotechnical report)

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

• Affects stability of excavated face

• Washes out fines in granular soils

• Softens clay soils• Thames is tidal!

Planning the work – design phase

• 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 – BIA’s

• Method of construction and temporary support of excavation – sequencing

• Access/ egress and edge protection

• Plant/ equipment to be used

• Buried services • Adjacent structures• Supervision of work

Permanent v Temporary Works

• Permanent works engineer (PWE)– Responsible for designing the permanent or finished works

• Temporary works engineer (TWE)– Responsible for designing temporary works to enable

permanent works to be constructed

• Usually different people but can be the same person

• Poor 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 Website

New Build Basement Construction

• Easier if new build• Still need to batter back

or provide support

Don’t forget adjacent structures

Don’t forget adjacent structures

Unsupported clay face and risk of undermining adjacent structure

Boundary wall can slip or overturn

Unsupported clay face

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

Underpinning 1

Technique devised to treat subsidence now also used to form basements

Excavate belowexisting foundation

Form new pin or foundation

Heel can be designed out

Underpinning 2 – Schedule

Typical underpinning schedule drawn up by competent person

Contiguous Piles & Diaphragm Walls

Technique associated with larger projects and/or where there is space

Support for face of underpin section?

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

Unshored underpin excavations

• Common to find un-shored pits on site up to 4m deep

Shoring of pits is a must

• Need to plan support scheme and access issues• Can use timber or steel sheet piles

Deeper basements

3.5m-4.5m deep pins – risk of unsupported face collapsing?

Deeper basements

• The Gucci standard!

Low risk?

Middle mass

Innovative support systems emerging

Work in progress

Site findings

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

Supporting existing structure above basement

Prohibition Notice served – evacuate site and property above - temp wks engineer came back with props at 200mm centres below main walls of house

Safe access/ egress

Lightwell could have been sheet piled/ timbered first to provide large access “pit”

Protection for the public

Safe place of work

Fail to plan – plan to fail

End of terrace house – refurb + basement

EOT starts to lean towards road

Emergency works by LA contractor

Putney collapse

Welfare

Welfare

Access

Access

Access

Access

Surcharging sides of excavation

Surcharging sides of excavation?

Water ingress

Conveyor belts

Work at height?

Electrics

Temporary props

What are HSE doing about it?

• Inspections – lots of them

• Including annual 2 day concentrated inspection initiatives – most recent11/12 March 2015

• Kensington & Chelsea, Hammersmith & Fulham and Westminster – expanding.

• Intelligence gathering done by VO’s

• Recent proactive prosecutions

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 poor

• However…

Summary

• Some good news…!

• Evidence indicates that DH’s that HSE have visited in the past are showing signs of improvement

• Some of these noticeably so

• HSE working with industry on new guidance - ASUCplus

• Formation of industry associations

• DH’s met welcomed HSE intervention – want an even playing field.

Summary

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

• Need 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 site

• Competent site supervision

Excavator for sale - collection only!

Building Basements Safely

Open Forum and Q&A