H&S Construction Site Hazards - Excavation & Underpinning

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Slide 1

Excavation and underpinning presentation by D Clements and C Smith.

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This presentation sets out to introduce the viewer to the potential hazards and legislation involved in excavation and underpinning works. Following this presentation the viewer will have an improved understanding of the risks and responsibilities involved with these work activities. The presentation will cover the following subjects: Excavation statistics Case studies Codes of Practice, HSE Guidance & Regulations for excavation & underpinning Hazards in underpinning Hazards in excavation Good practice examples Risk assessments

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Excavating occurs in nearly all new build projects and a vast amount of remedial works, not to mention civil engineering projects The dangers of excavating are numerous and not just focused on the digging itself Between 1991 and 1996 there were 7 fatalities and nearly 1000 injuries sustained in the construction industry from works relating to excavations

Slide 4

"Digging foundations and trenches for drains is one of the first jobs carried out on construction sites. For too many workers, it is also the last job they do” "To most people, being buried alive is the stuff of horror films, yet every day workers either knowingly take that risk or are placed at risk by their employers”

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The following case studies are taken from the HSE prosecution database with further details sourced from independent reports and national press sources. Although the full details of the case are detailed online the names of the companies and individuals involved are omitted from this presentation

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This case study examines a fatal accident during specialist excavation and underpinning works to an existing structure. The incident was subsequently investigated and prosecuted by the Health & Safety Executive. The case study will examine the events leading to the incident and the subsequent prosecutions under the relevant sections of Health & Safety law. Records show that the incident was completely avoidable with just simple measures and tragically led to the needless death of a 22 year old construction worker. The incident occurred during refurbishment works to an existing church structure and involved specialist underpinning works to the corner of the building. The refurbishment work had included the lowering the crypt floor, which required the foundations to be reformed below their existing level using underpinning. This involved excavating beneath the existing foundations in short sections and casting concrete “pins” underneath the wall.

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Risk assessments, prepared by the structural engineer, did not address the specific activity of underpinning. When the work started, voids containing rotten wood were discovered in the foundations. The main contractor’s foreman brought this to the attention of the principle contractor. An inspection was held the following day from which the structural engineer advised flooding the voids with concrete. The subcontractor agreed to produce a method statement for the works but this was never done. A few days later the 22 year old construction worker was working within the underpinning excavation when a new pin was being excavated when a 1.5 tonne section of the foundation collapsed and crushed him. The brickwork was interspersed with what HSE later described as "a matrix of rotting wood". There was no propping or other support to prevent the foundation from collapsing and fatally injuring the worker.

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Engineer fined £45,000 with costs of £30,000 Main contractor fined £25,000 with costs of £15,000. Sub-contractor fined £25,000 with costs of £15,000 The structural consultant engineer was found guilty of breaching Section 3(2) of the Health and Safety at Work etc. Act 1974 (HSW Act) in that he failed to ensure that persons not in his employment were not exposed to risks to their health and safety The principal contractor was found guilty of charges of breaching: Section 3(1) of the HSW Act, Section 16(1)(a) of the Construction (Design and Management) Regulations 1994 and Regulation 9(1) of the Construction (Health Safety and Welfare) Regulations 1996 The subcontractor was found guilty of the charges of breaching Section 2(1) of HSW Act; Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 and Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996 Following sentencing the HSE Inspector, who investigated the case, said: “Whilst welcoming the verdict handed down by the court today, the fact remains that a young man died in an incident that could have easily been avoided, had appropriate and straight forward safety measures been in place. “The untimely death came about through the failure to take appropriate action in relation to a potential risk in the underpinning work that had been brought to the attention of both the structural engineer and the contractors. The possible risks should have been addressed by uncomplicated measures including a detailed structural investigation, suitable and sufficient risk assessments and adequate protective measures, such as propping of the foundations.”

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Case two involved a housing contractor working in Falkirk Scotland. The incident occurred during excavation of part of site to construct lowered level road and car parking. The excavation was approximately 2 metres deep and sloped at an angle of approx 70 degrees and involved the construction of an insufficient retaining structure to secure the excavation and a 33kV substation at top of the bank.

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The Inadequate retaining structure that had been constructed collapsed a few minutes after backfilling. The face of the excavation continued collapsing and parts of the substation compound began to collapse above it. Emergency measures were taken by Scottish Power to secure excavation until a permanent structure built to replace the inadequate structure. The result of the poor construction in this instance could have led to a risk of explosion, electric shock and burn injuries and entrapment in the excavation.

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The contractor was fined £6000 by the HSE and was found guilty of the charges of breaching Section 2(1) of HSW Act; Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 and Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996.

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Definitions of Regulations involved Both the case studies described breached the Health and Safety at Work Act 1974 Section 2 subsection 1 which states that ‘It shall be the duty of every employer to ensure, so as is reasonably practicable, the health, safety and welfare at work of all his employees.’ and Section 3(1) of HSW Act which states: ‘It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety.’ 5. Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 states: ‘Every employer shall make a suitable and sufficient assessment of the risks to the health and safety of his employees to which they are exposed whilst they are at work; and the risks to the health and safety of persons not in his employment arising out of our connection with the conduct by him of his undertaken. Case study 1 also involved the following breaches of the Health & Safety at work act, Construction (Design and Management) Regulations 1994 and Construction (Health Safety and Welfare) Regulations 1996. Section 3(2) of HSW Act states: ‘It shall be the duty of every self-employed person to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that he and other persons (not being his employees) who may be affected thereby are not thereby exposed to risks to their health or safety.’

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Regulation 16(1)(a) of the Construction (Design and Management) Regulations 1994 states: ‘The principal contractor appointed for any project shall take reasonable steps to ensure co-operation between all contractors (whether they are sharing the construction site for the purposes of regulation 11 of the Management of Health and Safety at Work Regulations 1999 or otherwise) so far as is necessary to enable each of those contractors to comply with the requirements and prohibitions imposed on him by or under the relevant statutory provisions relating to the construction work.’ The defendants in both cases were also found guilty of Regulation 9(1) of the Construction (Health Safety and Welfare) Regulations 1996 states ‘All practicable steps shall be taken, where necessary to prevent danger to any person, to ensure that any new or existing structure or any part of such structure which may become unstable or in a temporary state of weakness or instability due to the carrying out of construction work (including any excavation work) does not collapse accidentally.’

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The following are Codes of Practice, HSE Guidance & Regulations ascertaining to works involving excavation and underpinning. CDM 2007 BSI – Code of Practice for Foundations HSE – Safety in Excavations HSE – Be Safe, Be Shore HSE – Safer Foundations by Design For more information interested persons should visit the HSE website at www.hse.gov.uk.

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Building Control Guidance for underpinning works states that; ‘Before underpinning is commenced, an investigation should be carried out by an experienced and competent person, to determine whether an underpinning procedure will achieve the object intended, i.e. to transfer the load carried on a foundation from its existing bearing level to a new level at a suitable lower depth.’ All work should be suitably supported during excavations.

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The following are some of the hazards associated with underpinning works with which personnel should be familiar. Structural instability of the building to be stabilised, or surrounding structures Collapse of temporary works equipment Instability of excavations, slopes or spoil mounds Operation of machinery for excavation, piling or jacking Work in confined spaces, e.g. basements Working in contaminated ground Working in the vicinity of services Falls from height and into excavations are also occasional hazards

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Structures, which are particularly vulnerable to premature collapse include: Buildings with random stone walls Buildings that have been altered, because load paths may have been changed Badly dilapidated buildings, e.g. Buildings with bulging walls, deteriorated fabric, decayed brickwork or timber

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The following are some of the hazards associated with excavation with which persons should also be familiar. Full or partial collapse Contact with underground services Being struck by machinery Workers/vehicles falling into the excavation Objects falling into the excavation Undermining nearby structures Fumes within the confined space

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Other hazards to be aware of are: Exhaust fumes from generators and other site equipment. Radon gas – accumulations of which are rare but can be dangerous.

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The following are examples of good practice which should be followed for excavation and underpinning work: Gather information prior to excavation and underpinning works including: Existing services: Water, drainage, gas, electric, telecoms... Locate on plan then recheck on site Use cable scanners and if needed hand dig to test Ground conditions: Type of strata – clay, sand, made ground... Level of water table Possible contamination in ground Both effect type of excavation and methods

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Good examples of stabilising open excavations are: Battering to the angle of response of friction Benching Angle of repose anything from 5° to 45° depending on the type of strata (stiff clay being the best in dry ground – saturated silt being the worst) These are simple and effective methods of preventing excavation collapse, easily obtainable and relatively inexpensive in comparison to more technical solutions and suitable were space and adjacent buildings are not an issue of concern.

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Additional measures for securing an excavation to prevent collapse can be: Shoring, as shown in the corresponding diagrams Soil nailing Grouting Trench box Anchored walls Ground freezing For example: Ground freezing occurred during redevelopment of Chicago transport infrastructure. Certain areas of light rail track were unsupportable at the sides, instead of shutting down the line and removing the track the ground below is frozen using a system of pipes filled with liquid nitrogen to freeze the earth solid in massively increase its cohesive strength.

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In order to protect workers, pedestrians and vehicles from open excavations and workers within excavations from falling objects and machinery it is important to use access and ingress barriers and warnings, examples of these are: Fencing Traffic Barriers Warning Signs For further good practice examples the HSE publishes a range of different information ascertaining to excavation and other work areas.

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A simple risk assessment before commencement of works can be enough to save lives. Many of the elements of a risk assessment can be interchangeable from one job to another but remember a risk assessment should always be Site Specific What follows are risk assessment examples based on the previous case studies which if followed could have led to a different and more desirable outcome.

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A risk assessment requires the individual or individuals to establish the potential hazards involved in work prior to that work being carried out. Although often the content for a particular activity may be transferable from assessment to assessment each individual analysis should be created on a site specific basis only. These ascertained risks are taken one by one and applied to the persons or structures that they could effect systematically identifying all the dangers involved with a particular operation. Using a rating of 1 – 6 (or possibly A to F) with the most and 1 the least probable we assign a rating against each of the risks established to establish the likelihood of the event occurring during works. Using a similar criteria and with the same 1-6 numbering were 1 is treated as a minor injury and 6 resulting in death to apply a figure for the severity of the incident relating to the hazards. By multiplying the likelihood figure by the severity figure for each of the hazards we are given a figure between 1 and 36 for the degree of risk without control measures from low to high and critical. In order to reduce the risk for each of the hazards controls are put in place which identify methods of working to improve safety. Following the application of controls the rating procedure is repeated to give an adjusted risk calculation for each of the hazards identified.

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The following is a risk assessment based on case study one underpinning work: The following hazards are identified: Potential instability of random stone wall during essential underpinning works Risks associated with movement on site with open excavation Working in confined spaces Stability of excavation Risk of structural damage to existing building The persons affected/damage anticipated for each of the hazards: Ground workers and site operatives at risk of injury if wall were to collapse during underpinning works Ground workers at risk of excavation collapse Site operatives at risk of falling into excavation or machinery or objects falling onto operatives during works Risk to operatives working with heavy machinery within confined spaces Risk of extensive structural damage to existing property The likelihood of hazard A. occurring is judged to be highly probable with a severity risk of death B. Is likely with a risk of major injury, C. Probable with the risk of a reportable injury, D. Likely with the risk of a reportable injury and E. Probable with a risk of major injury. These figures give an overall degree of risk without control measures of mainly high with one medium priority for hazard D and a critical rating for hazard A.

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In order to reduce the risk during the assigned works we assign the following controls to each of the risks: Carry out extensive structural survey and ensure propping schedule drawn up by structural engineer and strictly adhered to on site. Ensure that no site operatives are working within the building during underpinning works. Only qualified and experienced underpinning and ground workers to operate within excavation. Carry out ground condition survey and follow method statement for suitable excavation technique including protective measures. Produce and adhere to method statement for pedestrian and vehicle barriers in vicinity of works. Produce method statement for manual handling requirements. Ensure measures outlined for risk A. are carried out and ensure additional shoring and propping works are carried out to protect the existing structure. By applying these measures we can reduce the risk during works to predominently medium priority with just hazard A. remaining high demonstrating that the required controls are in place in order to protect the workforce and property.

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The following is a risk assessment based on case study two excavation work: The following hazards are identified: Stability of excavation Risks associated with movement on site with open excavation Movement of heavy machinery Risk of structural damage to substation Risk from existing buried utilities The persons affected/damage anticipated for each of the hazards: Ground workers at risk of excavation collapse Site operatives at risk of falling into excavation or machinery or objects falling onto operatives during works Risks to operatives of working around heavy machinery Risk of extensive structural damage or potential undermining of existing substation Risk to operatives of striking buried services with heavy or small plant The likelihood of hazard A. occurring is judged to be likely with the risk of a reportable injury B. Is likely with a risk of major injury, C. Probable with the risk of a major injury, D. Likely with the risk of a possible fatality and E. Highly Probable with a risk of fatality. These figures give an overall degree of risk without control measures of mainly high with one very high for hazard D and a critical rating for hazard E.

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In order to reduce the risk during the assigned works we assign the following controls to each of the risks: Produce method statement for manual handling requirements. Carry out ground condition survey and follow method statement for suitable excavation technique including protective measures. Produce and adhere to method statement for pedestrian and vehicle barriers in vicinity of works and set up one-way system. If vehicles should be accompanied by a banksman while working around excavations. Consult geotechnical engineer for ground stability around existing substation. If excavation is within zone of influence provide suitable ground support. Ensure information of indicative locations of utilities is gathered off service providers and market on site. Specific location of utilities should be located on site using scanners, hand excavated and supported where necessary. By applying these measures we can reduce the risk during works to predominently medium priority with just hazards A and C remaining at high risk.

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That concludes this presentation on excavation and underpinning, for further information and guidance visit the HSE website at www.hse.gov.uk or obtain literature from any HSE bookshop.

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EXCAVATION AND UNDERPINNING Dan Clements Christopher J Smith

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Introduction This presentation sets out to introduce the viewer to the potential hazards and legislation involved in excavation and underpinning works. Following this presentation, the viewer will have an improved understanding of the risks and responsibilities involved with these work activities. The presentation will cover the following subjects: Excavation statistics Case studies Codes of Practice, HSE Guidance & Regulations for excavation & underpinning Hazards in underpinning Hazards in excavation Good practice examples Risk assessments

Slide 3

Excavation occurs in nearly all new build projects and a vast amount of remedial works, as well as civil engineering projects Excavation Statistics Source: HSE The dangers of excavation are numerous and not just focused on the digging itself Between 1991 and 1996, there were 7 fatalities and nearly 1000 injuries sustained in the construction industry from works relating to excavations

Slide 4

"Digging foundations and trenches for drains is one of the first jobs carried out on construction sites. For too many workers, it is also the last job they do.” "To most people, being buried alive is the stuff of horror films, yet every day workers either knowingly take that risk or are placed at risk by their employers.” Source: HSE ‘Be Safe, Be Shore’ initiative Excavation Statistics

Slide 5

Case Studies The following case studies are taken from the HSE prosecution database, with further details sourced from independent reports and national press sources. Although the full details of the cases are detailed online, the names of the companies and individuals involved are omitted from this presentation.

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Case One: London, England Church refurbishment involving lowering crypt floor Unsupported underpinning works Author’s own diagrams

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1.5 tonne section of foundations collapsed onto 22 year old worker below, fatally injuring him Case One: London, England Voids containing rotten wood discovered in foundations Sub-contractor and engineer failed to produce method statements Author’s own diagrams

Slide 8

Case One: London, England Engineer fined £45,000 with costs of £30,000 Main contractor fined £25,000 with costs of £15,000. Sub-contractor fined £25,000 with costs of £15,000 The principal contractor was found guilty of charges of breaching: Section 3(1) of the HSW Act, Section 16(1)(a) of the Construction (Design and Management) Regulations 1994 and Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996 The structural consultant engineer was found guilty of breaching Section 3(2) of the Health and Safety at Work, etc. Act 1974 (HSW Act), in that he failed to ensure that persons not in his employment were not exposed to risks to their health and safety The subcontractor was found guilty of the charges of breaching Section 2(1) of HSW Act; Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 and Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996

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Case Two: Falkirk, Scotland Housing contractor Excavation 2m deep unsupported 33kV substation on top of excavation Author’s own diagrams Inadequate retaining structure constructed to secure bank

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Case Two: Falkirk, Scotland Face of excavation started to collapse, as did the substation compound itself Emergency measures were taken to prevent collapse, which could have resulted in an explosion or electric shock Inadequate retaining structure collapsed Author’s own diagrams

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Case Two: Falkirk, Scotland Contractor fined £6000 The contractor was found guilty of the charges of breaching Section 2(1) of HSW Act; Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 and Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996

Slide 12

Definitions of Regulations Involved Section 2(1) of HSW Act states: ‘It shall be the duty of every employer to ensure, so far as is reasonably practicable, the health, safety and welfare at work of all his employees.’ Section 3(1) of HSW Act states: ‘It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety.’ Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 states: ‘Every employer shall make a suitable and sufficient assessment of the risks to the health and safety of his employees to which they are exposed whilst they are at work; and the risks to the health and safety of persons not in his employment arising out of our connection with the conduct by him of his undertaken. Section 3(2) of HSW Act states: ‘It shall be the duty of every self-employed person to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that he and other persons (not being his employees) who may be affected thereby are not thereby exposed to risks to their health or safety.’

Slide 13

Definitions of Health & Safety Breaches Regulation 16(1)(a) of the Construction (Design and Management) Regulations 1994 states: ‘The principal contractor appointed for any project shall take reasonable steps to ensure co-operation between all contractors (whether they are sharing the construction site for the purposes of regulation 11 of the Management of Health and Safety at Work Regulations 1999 or otherwise) so far as is necessary to enable each of those contractors to comply with the requirements and prohibitions imposed on him by or under the relevant statutory provisions relating to the construction work.’ Regulation 9(1) of the Construction (Health, Safety and Welfare) Regulations 1996 states ‘All practicable steps shall be taken, where necessary to prevent danger to any person, to ensure that any new or existing structure or any part of such structure which may become unstable or in a temporary state of weakness or instability due to the carrying out of construction work (including any excavation work) does not collapse accidentally.’

Slide 14

CDM 2007 BSI – Code of Practice for Foundations HSE – Safety in Excavations HSE – Be Safe, Be Shore HSE – Safer Foundations by Design Codes of Practice, HSE Guidance & Regulations for Excavation & Underpinning Source HSE

Slide 15

‘Before underpinning is commenced, an investigation should be carried out by an experienced and competent person, to determine whether an underpinning procedure will achieve the object intended, ie, to transfer the load carried on a foundation from its existing bearing level to a new level at a suitable lower depth.’ All work should be suitably supported during excavations. Building Control Guidance Underpinning Underpinning and propping drawing courtesy of Barron Wright Partnership, Consultant Engineers, Dumfries

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Hazards Associated with Underpinning Structural instability of the building to be stabilised, or surrounding structures Instability of excavations, slopes or spoil mounds Operation of machinery for excavation, piling or jacking Collapse of temporary works equipment Work in confined spaces, eg, basements Working in contaminated ground Working in the vicinity of services Falls from height and into excavations are also occasional hazards

Slide 17

Structures, which are particularly vulnerable to premature collapse include: Hazards Associated with Underpinning Buildings with random stone walls Buildings that have been altered, because load paths may have been changed Badly dilapidated buildings, eg, buildings with bulging walls, deteriorated fabric, decayed brickwork or timber Author’s own picture

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Hazards in excavations include: Full or partial collapse Contact with underground services Being struck by machinery Workers/vehicles falling into the excavation Objects falling into the excavation Undermining nearby structures Fumes within the confined space

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Other Hazards Exhaust fumes Generators and other plant equipment Radon Gas Accumulation rare, but dangerous Author’s own diagrams

Slide 20

Good Practice Both affect type of excavation and methods. Existing services: Water, drainage, gas, electric, telecoms Locate on plan then recheck on site Use cable scanners and if needed hand dig to test Ground conditions: Type of strata – clay, sand or made ground Level of water table Possible contamination in ground Firstly, gather site information:

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Where the constraints of the site allow, it is good practice to employ the following open excavation techniques: Benching Battering to the angle of repose (or friction). Good Practice Author’s own diagrams

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When required, employ methods of excavation support such as: Shoring Soil nailing Grouting Trench box Anchored walls Good Practice Ground freezing Shoring diagrams by author

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Use access/ingress solutions: Fencing Traffic barriers Warning signs Good Practice Author’s own pictures

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Risk Assessments A simple risk assessment before commencement of works can be enough to save lives. Many of the elements of a risk assessment can be interchangeable from one job to another, but remember, a risk assessment should always be site-specific What follows are risk assessment examples based on the previous case studies which, if followed, could have led to a different and more desirable outcome.

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Risk Assessments Hazard identified Persons affected Ratings assigned Controls put in place Ratings re-assigned

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Risk Assessment Example: Underpinning

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Risk Assessment Example: Underpinning

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Risk Assessment Example: Excavation

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Risk Assessment Example: Excavation

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EXCAVATION AND UNDERPINNING For further information and guidance, visit the HSE website at www.hse.gov.uk

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