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
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Geotechnical Engineer
HM Inspector (Construction)
Basement and Tunnel Inspector
Basement Initiatives
Building Basements Safely
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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
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Technically challenging
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Complacency (60+ basements no shoring!)
Contractors often new to the industry
Poorly planned
Poorly supervised/managed
Lack of competence in those planning and
managing – technical/health and safety
Overview - Key Issues
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Collapse of excavations
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Temporary works / Permanent works
Undermining of adjacent structures
Material falling into an excavation
People, plant and equipment falling into
excavation - WAH
Electrics
Welfare
Manual handling
Overview – Key issues cont.
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Excavation often unsafe – complacency
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TW designers often absent – no calcs/drawings
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Work subcontracted – main contractor absent
Belief that vertical clay face will stand
unsupported
Simultaneous refurbishment with structural
alteration
Beams can weigh several hundred kg
Vulnerable workers
Overview - Legislation
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Reg 19 CDM 2015 – Stability of Structures
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Reg 6 EAW 1989 – Adverse or Hazardous
Environments
Reg 22 CDM 2015 – Excavations
Schedule 2 CDM 2015 – Welfare
Reg 6(3) WAH 2005 – Avoidance of risks from
work at height
Overview - Information &
Guidance
• Guidance concerned with Groundworks / Excavations
• Nothing specific with regard to basement construction
• HSE Busy builder leaflet
Overview - Information &
Guidance
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IStructE – Subsidence of low-rise buildings (Aug
2000)
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CIRIA - Trenching practice. 2nd edition (2001
revision)
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Shoring technology interest group
www.cpa.uk.net/p/Shoring-Technology-InterestGroup
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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
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Sides relieved of
lateral support from
surrounding ground
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Becomes unstable
Material above
natural angle of
repose will partially
or totally collapse at
any time
Angle of repose
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Just like being at the beach!
Angle of repose
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Starts to dry out, crack and crumble –
water doesn’t help.
Angle of repose
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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
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No ground can be relied upon to
stand unsupported.
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In urban areas you don’t know
history of previous ground
disturbance.
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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
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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
clay
Sand/ gravels
Close boarding or interlocked sheets
Hit and miss or close boarding required
Stiff clay
Sandy silty clay
Geotechnical Report – Starting point
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Ground conditions need to be
known to design both the
temporary and permanent (or
finished) works.
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Information can be found from
desktop study or previous
experience of local area.
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More complicated jobs will need a
site investigation including a
geotechnical report – ‘BIA’.
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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
Wall
Wedge of soil
at angle of
repose
Foundation exerts
vertical load on soil
below
Unstable Material
Earth face –
no support
Stable material
Excavation
Undermining doesn’t mean you have to dig directly
beneath the foundation
Wall
Foundation and load
left unsupported
Wedge of soil
at angle of
repose
Collapsed
earth face
Stable material
Soil slips off
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
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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
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Company Standards for excavation work
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Production of Method Statements (safe system of work)
Risk Assessment procedure
Consider underground services
Temporary works input (Calculations &
Drawings)
Selection of right plant and equipment
Competent site management and supervision
(especially front line supervisors)
Method Statements
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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
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Permanent works engineer (PWE)
– Responsible for designing the permanent or finished works
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Temporary works engineer (TWE)
– Responsible for designing temporary works to enable
permanent works to be constructed
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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
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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 below
existing foundation
Heel can be designed out
Form new pin or
foundation
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?
Clay face
starting to
fail
Need to consider sacrificial face support – usually steel sheet piles
Support of existing foundation
Underpinning – failure to survey existing wall and to prop
Unshored underpin excavations
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Common to find un-shored pits on site up to 4m deep
Shoring of pits is a must
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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
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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?
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Inspections – lots of them
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Kensington & Chelsea, Hammersmith &
Fulham and Westminster – expanding.
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Intelligence gathering done by VO’s
Including annual 2 day concentrated
inspection initiatives – most recent11/12
March 2015
Recent proactive prosecutions
What are HSE doing about it?
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Basement initiative headline stats:
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On average 1 in 3 sites received a Notice (IN or PN).
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A couple of sites were shut due to management of H&S
being so poor
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However…
127 sites visited over two days,136 contractors met.
On average 1 in 2 sites received enforcement action (NoC,
IN or PN)
Main areas of enforcement were WAH, Excavation and
Welfare.
Summary
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Some good news…!
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Some of these noticeably so
Evidence indicates that DH’s that HSE have visited in the
past are showing signs of improvement
HSE working with industry on new guidance - ASUCplus
Formation of industry associations
DH’s met welcomed HSE intervention – want an even
playing field.
Summary
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High Risk Work – needs careful planning before
work starts on site
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Need Temporary works engineering input
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Shoring material/ equipment is on site
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.
Competent site supervision
Excavator for sale - collection only!
Building Basements Safely
Open Forum and Q&A