Working Platforms for Tracked Plant: Good Practice Guide to the Design, Installation, Maintenance and Repair of Ground-supported Working Platforms (BR 470)

53 Pages·2004·2.272 MB·English

by  Building Research Establishment

BRE
2004
Garston, Watford, WD25 9XX
Working platforms
for tracked plant:
good practice guide to the design,
installation, maintenance and repair of
ground-supported working platforms
constructing the future
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BR 470
© Copyright BRE 2004
First published 2004
ISBN 1 86081 700 9
iii
Contents
Foreword iv
1 General 1
1.1 Scope of guidance 1
1.2 Objectives of guidance 1
1.3 Causes of failure 2
1.4 Responsibilities 3
1.5 Safety legislation 3
1.6 Management procedures 3
2 Site conditions 4
2.1 Site investigation 4
2.2 Pre-construction site assessment 5
2.3 Site preparation 5
3 Design 6
3.1 General features 6
3.2 Quality of subgrade 7
3.3 Quality of platform material 7
3.4 Strengthening materials 8
3.5 Loading 8
3.6 Design calculations 10
3.7 Design life 10
4 Installation 11
5 Operation, maintenance and repair 12
Photographs 14
Appendix A Design calculations 17
A1 General approach to routine design calculations 17
A2 Routine design calculations for cohesive subgrade 24
A3 Routine design calculations for granular subgrade 32
Appendix B Determination of characteristic values of soil and
geosynthetic parameters 38
B1 Subgrade 38
B2 Platform material 39
B3 Geosynthetic reinforcement 42
Notation and abbreviations 43
References 45
iv
Foreword
Although serious incidents involving working platforms are relatively rare,
Steering group
fatalities have occurred. This guide is intended to promote safety in the
Viv Troughton Stent Foundations
design, installation and operation of these platforms.
(Chairman)
David Baker Balfour Beatty Major The Federation of Piling Specialists (FPS) has initiated a drive to improve
Projects practices related to the use of piling and associated specialist plant; the guide
Alan Bell Keller
has been prepared at the instigation of the FPS following the recognition of
David Corke Bachy Soletanche
the need for safety initiatives to improve the approach to the provision of
Ken Cromwell Rock & Alluvium
Rab Fernie Cementation working platforms. In using the guide, it should be recognised that these
Foundations Skanska platforms are subject to complex loading conditions and are difficult to design;
Abed Haimoni Keller
specialist advice is needed. This guide is not intended to replace or reduce the
Bob Handley Aarsleff
designer’s input, but rather to promote the implementation of minimum
Chris Jenner Tensar International
Malcolm Puller Consulting engineer design, installation, maintenance and repair standards.
Tony Suckling Cementation The guide was prepared under a contract let by FPS to Building Research
Foundations Skanska
Establishment Ltd (BRE). A co-ordinated approach to the problem was
Nick Wharmby Bachy Soletanche
required involving the whole construction supply chain. BRE is pleased to
BRE project team acknowledge those who have helped in the preparation of the guide.
Andrew Charles The guide has been prepared by a BRE project team working under the
Richard Driscoll
direction of a Steering Group appointed by FPS.
Hilary Skinner
Sponsors and funders
Members of the Federation of Piling
Specialists
Bauer Spezialtiefbau
Casagrande UK
Liebherr Werk Nenzing
Junttan Oy
Soilmec UK
Institution of Civil Engineers The Health and Safety Executive (HSE) has worked closely with the FPS
Tensar International
and supports the principle of reducing accidents by the use of properly
Galliford Try
designed, prepared and maintained working platforms.
1
1 General
1.1 Scope of guidance 1.2 Objectives of guidance
● Definitions ● Principal objective is to promote safety
● Nature, applicability and status of guidance ● Secondary objectives include economy and
● Limitations of good practice guidance sustainability.
The expression working platform is restricted to Working platforms are critical for plant
ground-supported working platforms, for stability, and safety is a vital issue1. Most
tracked plant, constructed of granular material. working platforms perform well, but
No other type of working platform is overturning of rigs has occurred more
considered. The working platform is taken as frequently than it should. The principal
including not only the platform itself but also objective of this guide is to facilitate the design,
the associated ramps and accesses. specification, installation, operation,
The guidance has sought to avoid being maintenance and repair of working platforms
over-prescriptive as this might limit the scope so that an acceptable level of safety is achieved.
for innovation and the development of cost- While the principal objective of the guide is
effective solutions. The guide is an enabling safety, a secondary objective is that safety
document and does not form a code of should be achieved without unnecessary or
practice. The guidance does not in any way excessive expenditure. Although this is a
limit the responsibilities of those parties secondary issue from the standpoint of this
involved in the design, specification, guidance, it is certainly not a minor issue since
installation, operation, maintenance and repair in many applications the cost of the working
of a working platform. platform is a significant proportion of the total
While the guide describes good practice in cost of the process, such as piling or ground
general terms it cannot deal with every treatment, for which the platform has been
eventuality and site condition (see Further installed. In many cases a good-quality
research on page 13). A formulation of good platform also improves the performance of the
practice can be of value only where it is applied construction processes using it.
with careful supervision, control and For the construction of working platforms,
monitoring of the platform on site under the tenets of sustainable construction require
appropriate contractual arrangements.All the excavation of natural materials to be
parties have to exercise their own knowledge, minimised and the use of waste materials to be
experience and judgement. maximised. These objectives can be achieved
in a number of ways. Working platforms should
not be larger than they need to be and material
specifications should not prevent the use of
those recycled or secondary materials which
will perform satisfactorily. Many working
platforms are used in subsequent construction
phases and the design should be checked to
ensure that it is appropriate for these uses and
conditions.
1 See also Crane stability on
site. CIRIA C703 second
edition. Lloyd D (ed) (2003).
2 1 General
1.3 Causes of failure
Failure can be caused ● Identification of hazards
by variability in the ● Localised problems
subgrade ● Maintenance and repair
● Identify and remove ● Plant operation
weak areas of
subgrade The objective of enhancing safety without
● Break out and excessive expenditure can be achieved only by
backfill hard spots identifying the significant hazards and
● Properly backfill any determining where the primary risks lie.
excavations Otherwise there will be much unnecessary
expenditure improving matters that are already
satisfactory. Identification of the most
significant hazards is a crucial first step in the
Poorly backfilled excavations
design procedure (Figure 1).
Pileshafts Weak zones Old substructures Experience has shown that it is far more
likely that rigs will overturn owing to localised
problems rather than to a generally inadequate
platform thickness across the whole site.
Localised weaknesses can be associated with
Steep slopes can the existence of ‘soft spots’ (see Box 1) in the
cause plant instability subgrade under the platform or with weak
● Slopes should be areas within the platform formed by
<1/10 and even then inadequate backfilling of holes that have been
some rigs are unstable excavated by other contractors working on the
● Includes ramps and site. Similarly, ‘hard spots’ caused by old
accesses as well as foundations or basements can cause difficulties.
platform surface Thus the identification of weaknesses in the
● Thicken bases of subgrade and the prevention of weaknesses
ramps by 50% being formed in the platform are likely to do
more to improve safety than refinement of
design calculations. Other hazards such as
open excavations and the edges of the platform
and access ramps should be identified and
clearly marked.
The problems experienced with rig
instability are often associated with failure
Platform edges can be
adequately to maintain and, where necessary,
dangerous
repair a working platform rather than with
● Mark platform
defects in the design or installation of the
edges and areas plant
platform.
should not go
Inappropriate operation of rigs can lead to
serious incidents involving rig instability.
Figure 1 Possible causes of rig instability
Box 1 Soft spot and weaker zone
The term ‘soft spot’ is often used to denote any small localised part of a site where
the subgrade is weaker than the surrounding ground. The soil may not necessarily
be ‘soft’ in the sense that a geotechnical engineer would normally understand the
term; that is to say a soft spot is not necessarily formed of a cohesive soil with a
low undrained shear strength. The significant feature is that, in the soft spot,
the ground is weaker than allowed for in the design calculations, which
have been based on a cautious assessment of the ground conditions
generally encountered across the site. Consequently, in this guide the term
‘weaker zone’ is used.
1.4 Responsibilities 3
1.4 Responsibilities designer of a ‘structure’ to assess the
implications of the design on the health and
● Parties involved in provision of working safety of all persons affected by the
platform construction and maintenance of the
● Responsibilities to be defined ‘structure’. The definition of a ‘structure’ is very
● Use of appropriately qualified and broad. Previously most of the responsibility for
experienced staff safety in construction resided with contractors.
The designer should endeavour to eliminate or
Several parties are likely to be involved in the minimise the risks in construction activities
design, installation, maintenance and repair of associated with their design and pass on
a working platform, including the client, the information so that residual risks can be
principal contractor, the designer of the appropriately managed.
permanent works and the contractor or There is a statutory requirement for risk
subcontractor for whose work the platform is assessments to be carried out and lodged in the
required. health and safety file. Information on the
The platform designer must specify a subgrade assessment and what underlies the
platform installation that will adequately working platform should be included.
support the stated plant and equipment on the
disclosed ground conditions for the expected 1.6 Management procedures
duration of the works. The contractor or
subcontractor should provide the platform ● Installation
designer with the plant loading and resulting ● Monitoring, maintenance and repair
pressures which will be applied to the ground, ● Operating requirements
and any other specific requirements. The
platform contractor is required to build the Installation requirements need to be specified
platform to the platform designer’s and a quality management process is required
specification and notify the designer of changes so that it can be certified that the platform has
in ground or groundwater conditions revealed been installed as specified.
during platform installation. Contractual Operation, maintenance and repair of the
arrangements should ensure that the platform platform should ensure that it performs
is adequately operated, maintained and, as according to the design over its design life.
necessary, repaired throughout its working life. Management systems should be in place to
The respective roles of the various parties ensure that:
should be clearly understood and the ● Monitoring of the platform takes place
responsibilities and liabilities of all parties regularly; this should involve visual
should be defined in the relevant contract. This assessment of the performance of the
guide does not extend or affect such platform and operating plant and drainage
responsibilities and liabilities. ● Regular maintenance is carried out to keep a
The design of working platforms for level surface
tracked plant is a geotechnical design process ● Repairs are carried out when problems are
and should be carried out by a competent identified and the platform reinstated after
person. any excavation or damage
1.5 Safety legislation Operating requirements need to be specified so
that the platform is not subjected to loads or
● CDM Regulations other conditions which were not allowed for in
● Designer the design. Procedures should be established to
● Health and safety file enforce the operating requirements.
Responsibilities need to be clearly established.
Safety regulations2apply to all aspects of
construction, including temporary works. The
CDM Regulations3are aimed at improving the
management of health and safety throughout
construction projects and place duties upon
clients, designers and contractors.
2 In the UK this refers to the
The designer of the working platform has
Health and Safety at Work
etc Act, 1974. statutory responsibilities under CDM and
3 Construction (Design and other regulations. Indeed, the main effect of
Management) Regulations,
1994. CDM Regulations is to place duties on the
4
2 Site conditions
2.1 Site investigation areas needs to be assessed and groundwater
conditions evaluated. Variability in ground
● Importance of site appraisal conditions needs to be identified.
● Ground investigation The strength and variability of the near-
● Weather dependency of near-surface ground surface ground is critical for the design and
● Identification of weaker zones and hard operation of the working platform. In
spots contrast, for most construction projects it is the
● Location of services strength at greater depths that is important
and, consequently, the near-surface strength
The importance of a thorough appraisal of the may not have been adequately characterised in
entire site at an early stage cannot be over- the site investigation for the construction
emphasised. It should include a site inspection project (see Box 2).
and it is particularly important to check for and Localised weaker zones in the subgrade,
report on the location of any weak and variable including poorly backfilled excavations, can be
areas and drainage. critical for the performance of the platform and
Appropriate and sufficient ground need to be identified. However, hard spots,
investigation is vital to ensure the provision of such as old basement walls, can be just as
an adequate working platform. hazardous as weaker zones. It is necessary that
Information pertinent to the working the whole of the platform area is examined. On
platform design should be collected during the a brownfield site, the records of the demolition
site investigation. Historical records of site use, contractor on site reinstatement should be of
demolition records and the local knowledge of assistance.
nearby residents may prove helpful and should The location of existing and abandoned
be included in the health and safety plan. Trial services should be carefully investigated.
pits facilitate examination of the near-surface After the site investigation for the main
ground, but careful reinstatement is essential to works, it may be necessary to augment
avoid creating weaker zones in the subgrade. information obtained for the construction
Strength properties of the ground are required project with further investigation specifically
for design purposes. The likelihood of weaker for the working platform.
Box 2 Characterisation of subgrade
Where a working platform will be required, the specification for the site investigation
should include a requirement to investigate the upper 2 m of ground across the likely
area of the platform.
All soils should be carefully described. Strength determination could be carried out
by plate load tests, CBR tests or vane tests. Made ground can be difficult to assess
from in-situ tests and it is therefore particularly important to identify the constituents.
The near-surface strength of cohesive soils is very weather dependent. The
recorded undrained shear strength will be dependent on whether measurements of soil
properties were made after a long drought or a long period of heavy rainfall. The
operational strength of the subgrade will depend to a large extent on its condition at
the time that the platform was installed, but could be affected by subsequent climatic
conditions, particularly with a shallow thickness platform (see Appendix B1).
2.2 Pre-construction site assessment 5
2.2 Pre-construction site The presence of a surface crust and/or
vegetation may be of great benefit in providing
assessment
additional strength in the uppermost layers of
● Check site investigation findings the subgrade. When layers containing
● Current site drainage vegetation are retained for temporary
● Latest information on working platform platforms, designers of permanent works
location and level should be informed so that, where necessary,
arrangements can be made for their later
The site investigation information available removal.
before the construction stage of a project starts Natural or man-made voids such as cavities
may be inadequate for the needs of working in weak rocks, shafts, wells, culverts and
platform design and installation. Further site basements, require special consideration.
investigation may be required to establish Measures such as backfilling or grouting may
suitable parameters for the platform design that be required4. Where these are not practicable
are relevant to the proposed location and level the void should be clearly marked and isolated
of the platform. from site activity by barriers or other means,
When the site becomes available for which should be carefully maintained during
construction, existing information should be the course of the works. Voids or weaker areas
reviewed, any additional requirements can be concealed beneath surface crusts or
identified and any further specific site paved areas.
investigation carried out. Where a weak subgrade is particularly soft or
Site drainage requirements may vary loose, some form of stabilisation or ground
throughout the year and the need for any treatment may be considered to improve the
additional drainage should be assessed and properties of the ground5.
incorporated into the platform construction Deterioration of the exposed subgrade
programme. before platform installation should be
Last-minute project design changes can lead prevented by drainage or by minimising
to changes in construction, and hence working exposure to wet and frosty weather (Photo 1).
platform levels. The potential implications of The time between exposing the subgrade and
such changes, particularly when there is cut installation of the platform should be
and fill on a site, should be recognised and the minimised. Where deterioration does occur,
working platform designer informed. affected material should be removed and
replaced with suitably compacted materials
2.3 Site preparation before placement of the platform.
Major changes in site level, for example after
● Excavation of weaker zones and old cut and fill operations, can have a significant
foundations and services effect and should be checked against the
● Effect of surface crust design.
● Backfilling of voids Inspection and recording of the subgrade, by
● Ground treatment a competent person, immediately before
● Deterioration of subgrade placing the platform material is essential to
● Inspection of subgrade before placement of verify the design assumptions.
platform
During site preparation, weak or variable areas
should be identified (Box 3). It may be
necessary to excavate localised weaker zones,
old foundations and abandoned services. Such
excavations should be backfilled with suitable
material adequately compacted. While it may
be sufficient to excavate localised small areas of
weak ground, larger areas will require more
4 CIRIA report C573 detailed consideration. Such problems could
(Mitchell and Jardine, 2002) include the presence of inadequately backfilled Box 3 Variability of ground conditions
provides a helpful
introduction to ground trenches or service tunnels. It may be The design should state the characteristic strength of the
treatment methods.
appropriate to zone the site in terms of bearing subgrade (see Appendices A and B) that has been
5(C ChaIRrlIeAs r eapnodr Wt Cat5t7s,2 2002) resistance or to specify a minimum distance assumed and the acceptable variability. Weaker zones
describes the assessment from a pit or a trench at which the plant can be and hard spots should be identified on the basis of their
of engineering properties strength in relation to the design assumptions.
following ground treatment. used safely.