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TRENT

GLOBAL

UNIT 6: SITE TEMPORARY WORKS

• UNIT 6: SITE TEMPORARY WORKS

• 6.1 INTRODUCTION

• 6.2 COMMON SCAFFOLDS

• 6.3 MOBILE ELEVATING PLATFORMS

• 6.4 MAST CLIMBING PLATFORMS

• 6.5 FORMWORKS

• 6.6 CATEGORIES OF FORMWORK

• 6.7 EXCAVATION AND TRENCHING

TRENT UNIT 6: SITE TEMPORARY WORKS GLOBAL


• 6.1 Introduction

• Temporary works are generally non-permanent facilities installed on site to help fulfill

execution of the actual or permanent works. These will be removed upon fulfilment of

their respective purposes.

• Temporary works cover four main site functions: access, support, protection and

groundworks (including excavations, ground supports etc).

• Sometimes temporary may have multiple roles like providing support as well as access to

the required work location.

• 6.2 Common Scaffolds

• Scaffolds may be of timber or metal (steel or aluminium) construction; but timber is

gradually phased out in many countries including S’pore.

• Despite being temporary, design of scaffolds follows the principles laid down for

permanent structures.

• Employers are required to provide safe access for employees erecting or dismantling

supported scaffolds where it is feasible. Hook on or attachable ladders shall be installed

as soon as scaffold erection is safe.


• In the process of erection or dismantling, scaffolds end frames may be used for climbing

purposes provided that:

> horizontal members are parallel, level and not more than 22 inches apart.

> erected in a manner that creates a usable ladder and provides good hand hold and foot

space.

> cross braces on scaffolds are not allowed to be climbed.

• Upon completion of installation, a competent person is needed to certify that it is “safe

for use”.


• Two types of common scaffolds in the industry are:

• > Putlog Scaffolds – basically continuous segments of direct access and working platforms

erected around the perimeter. This system relies on the vertical support from the structure

against which they are built. There are through/ wall ties that anchor the scaffolds to the

supporting walls, thus eliminating the need for rear support members.

• > Birdcage Scaffolds – commonly used for access to soffit or ceiling, as well as provide heavy

duty and sturdy support for horizontal slab casting. As it is modular and adjustable, the entire

mass of support components can be easily shaped to provide horizontal support to massive

areas and uniformly distribute the loading.


• Scaffolds are sometimes covered with nylon sheets, netting or other forms

of covering in varying degree of permeability so as to provide internal

protection against external weather, to contain dust internally and a visual

concealment from external environment. In some ways it also prevents

falling objects for people below.


• 6.3 Mobile Elevating Platforms

• Mobile elevating platforms are used as alternative to scaffolds or

suspended cradles (gondolas). They are particularly suitable for short

duration tasks requiring high mobility of the access structures, especially

so for retrofitting projects.

• There are generally two categories of mobile elevating platforms: self

propelled and vehicle mounted.


• Self-propelled Platforms – usually comes in diesel or electric powered

models. They can be easily manoeuvred into position by onboard controls.

Several common types are: scissor, telescopic and articulated.

• Vehicle-mounted Platforms – come in various sizes and capacities, from

smaller trailer mounted platforms to large truck-mounted types. The

smaller platforms are commonly used in highway and other road-related

maintenance (eg: pruning of tresss or servicing of street lamps).


• All elevating platforms used to carry personnel above ground are deemed to be “aerial

lifts”, whether or not they are capable of rotating about a substantially vertical axis and

some common points to note are as follows:

• > unless otherwise confirmed by the manufacturers, mobile elevating platforms shall be

designed and constructed in conformance with the applicable requirements of the local

authorities.

> aerial lifts may be field modified for uses other than those intended by the

manufacturer, provided the modification is certified in writing by the manufacturer, an

equivalent entity or an accredited test laboratory.


• > Prior to vehicular movement , aerial ladders shall be secured in the lower travelling

position by the locking device.

> Lift controls shall be tested each day prior to use to determine that such controls are

in a safe working condition.

> Only trained persons shall operate an aerial lift.

> Use outriggers where fitted.

> Be observant so as to never come into contact with energized overhead cables and

other overhead structures.

> Belting off (ie securing fall-arrest equipment) to an adjacent pole, structure or

equipment while working from an aerial lift shall not be permitted.


• > Employees shall always stand firm on the floor of the basket and shall not sit or

climb on edge of the basket or use planks, ladders or other devices for a working

position.

> A body belt or approved fall-arrest system shall be worn and a lanyard

attached to the boom or basket when working from an aerial lift.

> boom and basket load limits specified by manufacturer shall not be exceeded.

>The brakes and outriggers shall be set when in use; ensure that they are on

pads or solid surfaces. Wheel chocks or wedges shall be used when using an

aerial lift on an incline.


• > The platform may not be moved when boom is elevated in a working position with

men, except for equipment which is specifically designed for this type of operation.

> Articulate boom and extensible (telescopic) boom platforms primarily designed as

personnel carriers, shall have both platform (upper) and lower controls. Upper controls

shall be in or beside the platform, within reach of the operator. Lower controls are

provided for over-riding the upper controls in case of emergency.

> The insulated portion of an aerial lift shall not be altered in any manner that might

reduce its insulating value.

> Before moving an aerial lift, the boom shall be inspected to see that it is properly

cradled and outriggers are in a stowed position.


• 6.4 Mast Climbing Platforms

• Mast climbing platforms allow access to a localized area. The rack and pinion drive gives

an adjustable platform that can be positioned exactly to suit the required task. These

platforms are for work and access purposes only and must not be used for transporting

men and materials between levels. (Instead they should a passenger or material hoist).

• Mast climbing platforms consist of three main components:

> Mast(s) or tower that support a platform or cage.

> A platform capable of supporting persons, equipment and material at specified load.


• > A chassis supporting the tower/mast structure. The chassis may provide stability for the

machine, with or without outriggers, up to a specified free standing height, above which

the mast must be tied back to the building.

• They are available in various configurations:

• Single or twin masts;

Mobile or static chassis;

Freestanding or tied in masts;

Variable length working platforms;

Single or twin storey working platforms.


• Some common points to note for Mast climbing platforms:

• > All components should be erected by killed and competent persons and only to be

used by trained operatives.

> All components should be inspected and tested daily prior to use.

> All components should be regularly inspected by a competent person and records of

inspection should be kept.

>The platforms should be clearly marked with safe working load and permitted number

of persons allowed on the platform at each configuration.


• > The manufacturer’s instructions on inspection, maintenance and servicing must be

followed at all times.

> The operator should not carry out repairs and adjustments unless qualified and

authorized to do so.

> Safe means of access to the platform should be provided at the lowest level. This may

be via a short fixed ladder, a raised platform, or by control arrangements that allow the

platform to be brought down to ground level.

> Ground conditions must be checked for adequate bearing.

> A safety barrier must be erected to exclude personnel from area below the platform.


• 6.5 Formworks

• Formwork is a structure, usually temporary but in some cases wholly or partly

permanent, used to contain poured concrete to mould it to the required dimensions and

support it till it is able to self support.

• Formwork is commonly confused with falsework which is a term to describe temporary

support system such as scaffold supports.

• The appropriate selection of formwork is one key factor that governs he success of the

project in terms of time, cost, quality and the overall safety of the works.


• For high rise buildings, the most effective way to expedite works is to

achieve a very short floor cycle. The key is to exploit a set of efficient and

appropriately designed formwork system.

• Concern for a punctual and timely project handover quite often

compromises quality. Problems such as misalignment, misplacement and

deflective concrete will commonly result and these will lead to abortive or

reworks.


• 6.5.1 Categories of Formwork

• Formwork can be classified ranging from size, location of use, materials of construction,

nature of operation or by proprietary systems in the market.

• Size: Classifications according to size is straightforward. Any size designed for that need

not engage the help of heavy machinery, is usually considered small in nature. Small size

panels of timber or aluminium can usually be handled by a single worker. At the opposite

end are formwork systems designed so as to reduce the number of physical joints and

number of lifts which will give shorter production times and quality consistency.


• However additional stiffening is required. These large formwork systems

may require additional shores, ties or studs. Above all, there will be a need

to provide for lifting.

• Location of use: Different structure elements of a building have different

design and performance requirements in the use of formwork. A number

of formwork systems are specially designed to construct internal or

external walls, vertical shafts, columns, beams and floor slabs. By modern

standards even complex structures such as staircases could easily be

customized on site by proprietary formwork systems.


• Materials of Construction: Materials used are traditionally limited due to the dilemma

between cost and performance. Timber is the most popular due to low initial cost and

adaptability. Steel either in hot-rolled or cold formed sections, together with other

sheeting materials is another popular choice of formwork material. More recently full

aluminium formwork system has been used but the performance is being questioned

with regards to the additional costs and the need for specialized workmen.

• However as emphasis is now on the environmental concerns and green construction, the

use of timber is being challenged. So alternative products are sought to primarily achieve:


• > Cutting down on purchase, installation and maintenance costs;

• > Speedy installation times;

• > Minimal requirement of specialist installation skills;

• > Flexible and adaptable to create most physical forms;

• > Strong and light to meet / exceed modern expectations.


• Nature of Operation : Most timber and aluminium forms can be assembled manually,

without mechanical assistance, due to their weight, design and construction. However it is

labour intensive and more appropriate for simpler jobs or in the extreme used in very

large size and horizontally spanned buildings with complicated layout design, so as to

attain the benefit in flexibility.

• Some system are equipped with a degree of mobility to ease the erection and striking

processes, or may allow two dimensional mechanized movements using rollers, rails or

tracks. These formworks are generally categorized as either the crane lifted types or

mechanized slip form systems.


• Crane-lifted form systems: This category is usually in the form of large panels (sometimes

known as the gang-form). It is usually fabricated either in steel sections and sheeting or

using plywood sheeting and stiffened by metal studs and soldiers.

• Slip-form systems – with the help of hydraulic or screw-jack systems (either automated or

manually adjusted, these systems allow for continuous casting till the end of a typical

section is reached. Slip form system derived its name from the fact the form actually “slip-

off” a previously cast structure just when the section of the said structure has taken

physical shape. This process is continuous so that scheduled activities of all trades are

fixed and need to be adhered to.


• Proprietary Systems: Several patented formwork systems have been hughly successful in

the industry with each system having their own specialized product. Specialist companies

will commonly name expeditious site coordination, quality assurance, and time saving

factors as reasons for them to provide a wider range of services to the main contractor.

But these will add on to the financial burden of the contractor.

• With innovative methods gaining more attention, well established formwork

manufacturers will indulge in more research and development into even more advance

formwork systems.


• 6.5.2 Technical considerations in selection

• A number of factors have to be considered in order for a formwork to fulfill its

performance targets. The major factors are as follows:

• 1) Design-related factors

• > Shape of the building – Simple block shaped buildings are no doubt the easier than

curved, inclined, stepped etc As a general rule, the use of traditional, labour intensive

formwork systems are easier for awkward shaped buildings.


• > Design of external walls – some buildings may have a lot of exterior architectural

features like fins, rib, sunshades, planter boxes, bay windows etc. As a result the choice of

formwork system is limited due to casting difficulties. In general external walls are the

easiest and fastest structures to cast using non-traditional methods, with a very wide

range of proprietary systems available.

• > Internal layout – Most commercial buildings may have very simple layout with few or

no in-situ walls. The internal walls or partitions may be added later by the use of

lightweight precast panels, partition boards or blockworks.


• > Structural forms – will affect the options of formwork. Some structural elements like

lift cores or walls will limit the use of other formwork systems other than those of self-

climbing nature. Buildings with a flat slab design will similarly leave table or flying forms as

the most optimistic choices.

• > Consistency of building dimensions – the design of some buildings may be of non-

standardized dimensions to fulfill the architectural design and layout. As such some

formwork systems like slip forms or steel form may not be feasible as frequent

adjustments to the form to meet the change in dimensions will incur extra cost and time.


• > Building span – Generally buildings with large spans will have larger

beam sizes as heavier reinforcements are required. These additional

structural accommodations will further complicate the formwork design

and erection process.


• 2) Construction-related factors

• > Repetitive sequence of work - High rise block structure usually create highly repetitive

cycles of work and may be suitable for certain kinds of formwork,

• > Physical site constraints – The size of the site has a huge decision impact on the

selection of the type of formwork. Exceptionally small or large sites, sloped ground,

minimal site access or manoeuvre space, close proximity to sensitive structures, sites

with other concurrent activities or contractual restrictions will increase difficulties from

mobilization to subsequent erection.


• > Speed of work – Large construction sites with a horizontally spanned nature can be further

expedited by the introduction of separate sets of formwork. This will of course increase the

cost of production but should be considered if time is essence. For high rise buildings, the

mere increase of formwork cannot often fulfill the need for speed in construction as the

critical path depends highly on individual floor cycles.

• > Recycling of formwork – number of times that traditional formwork can be used is limited

due to its durability after every hacking off process (removal). Additional oil based coatings

are usually applied to contact surfaces of the plywood sheetings to enhance its durability.

These may be used up to ten casts, making it economically viable and the main option.


• > Construction planning and management – Planning such as phasing, zoning or

sectioning arrangements, integration of structures, site layout and setting up

arrangements, or the provision of hoisting and concrete placing facilities etc are

influential factors when considering matters related to formwork selection and

application.

• > Area or volume of cast per pour – The optimum volume of cast per pour will

be different and in accordance to the types of formwork used, elements of

structure to be placed, specific cale of work, different levels of provision of

facilities etc.


• > Continuity of structures and construction joints – It is common to introduce a large

number of construction joints in a large structure in order to subdivide the works into

effective and workable sizes. Construction joints are inevitable and are the weak link of

any structure; so it is up to the design engineers to conceptualise better form systems to

ensure the rigidity of the structure.

• > Accessibility to work – During the course of construction, physical accessibility may be

a problem due to segregation, temporary discontinuation or blocking of a layout due to a

partially completed building.


• > Involvement of other construction techniques – The applications of tensioning and

prefabrication techniques are often used. This may impede the casting schedules and

dictate the selection and use of formwork. Additional provisions of temporary supports,

slot spaces, and boxed out positions for precast elements, or additional work spaces for

the placing of stressing tendons and jacking process should be allowed.

• > Dependence of work – for formworks that have to be mobilized by plants and

equipment (eg cranes, hoists, hydraulic jacks) , selection will also be dependent on their

layout and other associated considerations like sequence and safety compliance.


• Feasibility of alternative designs – Traditionally buildings are designed without regard to

simplicity in casting, so long as the design is deemed feasible in all aspects and practical to

achieve. It is quite common nowadays for contractors to submit alternative design

proposals for clients, architects and engineers to consider without detracting from the

major design intent, in order to realise more effective formworking methods. Of course

in any proposal, balance to time, cost, quality and safety have to be given considerations.


• 6.7 Groundworks – Excavations and Trenching.

• An excavation is any man made cut, cavity, trench or depression in an

earth surface that is formed by earth removal. A trench is a narrow

excavation made below the surface of the ground and defined by the fact

that the depth is greater than its width.


• 6.7.1 Safety Introduction

• Excavation can be extremely hazardous. Some common terms and definitions are

as follows:

• > Adjacent structure stability – refers to the stability of the foundations of

adjacent structures, which locations may create surcharges, changes in soil

conditions.

> Confined space – is a space that by design has limited openings for entry/exit,

unfavourable natural ventilation, may contain or produce hazardous substances

or fumes.


• > Ingress and egress – mean entry and exit respectively. In trenching and excavation

operations, they refer to the provision of safe means for employees to enter or exit an

excavation or trench.

> Subsurface encumbrances – include underground utilities, foundations, streams, water

tables, geological anomalies.

> Surcharge – means an excessive vertical load caused by spoil, overburden vehicles or

activities that affect trench stability.

> Unconfined compressive strength – is the load per unit area at which soil will fail in

compression. This measure can be determined by laboratory test and can estimated in

the field using a pocket penetrometer.


• 6.7.2 Soil Mechanics

• A number of stresses and deformations can occur in an open cut site or

trench. Increases or decreases in moisture content can adversely affect

the stability of a trench or excavation.


• 6.7.3 Determination of soil type

• > Stable rock – is a natural mineral matter that can be excavated with vertical sides and

remain intact while exposed. It is usually identified by a rock name such as granite or

sandstone.

• > Type A soil – are cohesive soils with an unconfined compressive strength of 1.5 ton per

sq foot or 144 kg/cm2 or greater. Examples of type A cohesive soils are clay, silty clay, clay

loam etc.


• > Type B soil – are cohesive soils with an unconfined compressive strength greater than

0.5 tsf (48 kg/cm2) but less than 1.5 tsf (144 kg/cm2). Examples of type B soils are :

angular gravel, silt, silt loam, previously disturbed soil.

• > Type C soil – are cohesive soils with an unconfined compressive strength of 0.5 tsf (48

kg/cm2) or less. Other type C soils include granular soils such as gravel, sand, loamy sand,

submerged soil, soil from which water is freely seeping and submerged rock that is not

stable.

• > Layered geological strata – where soils are configured in layers , i.e where a layered

geologic structure exists, the soil must be classified by its weakest layer.


• 6.7.4 Test equipment and method for evaluating soil type.

• Many kinds of equipment and methods are used to determine the type of soil prevailing on

the site, namely:

• > Pocket penetrometers

> Shearvane (Torvane)

> Thumb penetration test

> Dry strength test

> Plasticity or wet thread test

> Visual test

• The evaluator should also look for signs of bulging, boiling, or sluffing, as well as signs of surface

water seeping from the sides of the excavation or from the water table.


• 6.7.5 Types of ground shore

• Ground shoring is the provision of a support system for trench walls, used to prevent

movement of soil, underground utilities, roadways and foundations. Shoring or shielding is

used when the location or depth of cut makes sloping back to acceptable gradient

impractical. There are generally two types of shoring systems, each with its own sub-

categories.

• > Ground shores support soil structures usually beneath ground level and are commonly

used in conjunction with trenching and sheet-piling systems to prevent the inward

collapse of the surrounding earth.


• > Structural shores typically support either existing building structures that are deemed

too structurally dilapidated to be in self-support, or as a falsework in support of the

erection process of new structures.

• Strut shoring - Strut shoring is the most basic form

of ground shoring support. The system consists of posts,

walers, struts, and sheeting. The preferred materials for

struts are timber and aluminium, the former being

cheap and readily available, with the latter being strong

and light.


• Hydraulic shoring - The modern trend is towards the use

of hydraulic shoring, a prefabricated strut and/or waler

system manufactured of aluminium or steel.

• Hydraulic shoring provides a critical safety advantage over

traditional strut shoring as workers do not have to enter

the trench to install or remove the shoring components.

• Other advantages of most hydraulic systems are:

> light enough to be installed by one worker.

> are gauge regulated to ensure even distribution of pressure


• > can have trench walls “preloaded” to use the soil’s natural cohesion to

prevent movement and

> can be adapted easily to various trench depths and widths.

• All shoring should be installed from top down and removed from the

bottom up. Hydraulic shoring should be checked at least once per shift for

leaking hoses and/or cylinders, broken connections, cracked nipples, bent

bases and any other damaged or defective parts.


• Pneumatic shoring – works in a manner similar to hydraulic shoring. The primary

difference is that pneumatic shoring uses air pressure in place of hydraulic pressure. A

disadvantage to the use of pneumatic shoring is that an air compressor must be present.

• Screw jacks – this system differs from both hydraulic and pneumatic in that the struts

of a screw jack system must be manually adjusted. This creates a hazard because the

worker is required to be in the trench in order to adjust the strut. In addition uniform

preloading cannot be achieved with screw jacks and their weight creates handling

difficulties.


• 6.7.6 Underpinning

• This process involves stabilizing adjacent structures, foundations and other

intrusions that may have an impact on the excavation. As the term dictates,

underpinning is a procedure in which the foundation is physically

reinforced. Underpinning should be conducted onlt under the direction

and approval of a registered professional engineer.


• 6.7.7 Ground shields

• Ground shields or trench boxes are different from shoring.

Instead of shoring upwards or otherwise supporting the

trench face, they are intended primarily to protect workers

from cave-ins and similar incidents.

• The excavated area between the outside of the trench box

and the face of the trench should be as small as possible.

The space between the trench boxes and the excavation side

are backfilled to prevent lateral movement of the box.


• Shields may not be subjected to loads exceeding those that the system

was designed to withstand.

• Trench boxes are generally used in open areas, and may also be used in

combination with sloping and benching. The box should extend at least 18

in (0.45m) above the surrounding area if there are slopes inclining towards

the excavation. This can accomplished by providing a benched area

adjacent to the box.


• 6.7.8 Sloping and benching

• Sloping: Maximum allowable slopes for excavations less than 20 ft (6.09m) based on soil

type and angle to the horizontal as follows:


Soil Type Height / Depth ratio Slope Angle

Stable Rock Vertical 90 deg

Type A ¾ : 1 53 deg

Type B 1 : 1 45 deg

Type C 1 ½ : 1 34 deg

Type A (short term) ½ : 1 63 deg

For maximum excavation depth of 12 ft

• Benching: There are two basic types of benching – simple and multiple. Only types A and

B soil are suitable to be benched. The type of soil determines the horizontal to vertical

ratio of the benched side.

• Spoils: Temporary spoils must be placed no closer than 2 ft (0.61m)from the surface edge

of the excavation, measured from the nearest base of the spoil to he cut. This distance

requirement ensures that loose rock or soil will not fall onto the operatives in he trench

as well as unnecessary burden to the walls of the trench. Spoils should be placed so that

it channels rainwater away from the excavation.


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