unit 7 - site temporary works - trent global – the ... · unit 7 - site temporary works ......

School of the Built Environment

Unit 7 - Site Temporary Works

Learning Outcomes

How access to heights is gained and how structures are temporarily

supported in the construction process

Different styles of formwork for moulding concrete

Excavation and trenching and other groundwork techniques



•Temporary i.e. non-permanent works

•Temporary Works are installed on site to help fulfil the execution of the actual (permanent) contract works

•Temporary Works will be dismantled/removed from site upon fulfilment of their respective purposes.

•This Unit looks four types temporary works:

•Access, support, protection and groundworks (including

excavations, ground supports and hydro-geological controls).



•Frequently, temporary works play multiple roles but their selection is based on the primary project requirements.

•This Unit details typical examples of the intertwined functions of temporary works to be expected on modern sites

•It also describes the indispensable roles that they typically function in every construction project

•These are roles that usually account for a significant percentage of the project’s running costs


An example showcasing the extensive and combined utilities of temporary support and access structures.


Easily available, ladders are essential as both the main and back-up tools of access.


Ladders

•Ladders should be a means of access, not a working platform •Main selection considerations will reside on suitability of work, safety/durability, cost and portability. Portable, hook-on, and attachable ladders: •Must be positioned so as not to tip the scaffold. •Hook-on and attachable ladders must be specifically designed for use with the type of scaffold on which they are used. •Have rest platforms provided at a max. of 10m vertical intervals. Stairway-type ladders: •Have slip-resistant treads on all steps and landings. •Steps and rungs of ladders and stairway-type ladders shall line up vertically with each other between rest platforms.


Ladders Step ladders

•Conventional step-ladders have rectangular stiles and flat treads that are arranged to be horizontal when in use, and are restrained in position by means of stays, chains or cords.

•Some variants use flat-topped rungs, while others could be of tubular construction.


Ladders Stair ladders


Scaffolding


Common scaffolds •Despite being temporary structures, the design of scaffolds follows the principles laid down for permanent structures.

•There must be no deviation from sound structural principles.

•There is generally enough variety in standard scaffold equipment to erect a platform to suit most work requirements.

•Scaffolds are sometimes used for purposes other than access e.g. as falsework supporting a formwork system (to be discussed later) during concreting processes


Common scaffolds Framed


Common scaffolds Tubular


Common scaffolds Putlog


Common scaffolds Bird-cage


Common scaffolds Birdcage •Birdcage scaffolds are commonly used for access to soffit or ceiling, as well as to provide heavy-duty and sturdy falsework support for horizontal slab casting.

•Due to its modular assembly and adjustable members, the entire mass of support components can be easily shaped to provide horizontal support to massive areas.

•It also assists in providing a more uniformly distributed loading pattern from the structure to the ground


Common scaffolds


Hoists •Hoists are used to transport personnel and materials to different working levels.

•Modern hoists operate on the rack and pinion system. •The hoist unit have its drive motor fitted on top of the car, along with brake and gear.

•This technology enables the car to climb up and down the mast at a controlled speed.


Hoist Details


Hoist


Mobile Platform Hoists •Mobile elevating platforms are used as an alternative to scaffolds and suspended cradles (described later in the Unit)

•They are particularly suitable for short duration tasks requiring high mobility of the access structures, especially so for retrofitting projects where clients are seeking short durations.

•The market offers a wide variety of these platforms, and guidance on specific applications should be sought from the manufacturers.

•There are generally two categories of mobile elevating platforms: Self-propelled and Vehicle-mounted.


Self-propelled Platform Hoists


Vehicle-mounted platform Hoists


Self-propelled platform hoists

•These can easily be manoeuvred into position by onboard controls.

•They are mainly available with the following boom types, namely:

scissor, telescopic and articulated

Vehicle-mounted platform hoists

•These platforms come in various sizes and capacities, from small

trailer-mounted platforms to large truck-mounted types.

•Smaller platforms are commonly used in highway and other road-

related maintenance (e.g. pruning trees, servicing street-lamps etc.)

•Larger versions are used where access by other methods are

deemed either too expensive or time-consuming.


Mast climbing platform hoists



•Mast climbing platforms allow access to a localised area of a project.

•The rack and pinion drive gives an adjustable working platform that can be positioned exactly to suit the task in hand.

•Tools and materials can be carried up to the work site, together with the operatives.

• Mast climbing platforms are for work and access purposes only and must not be used for transporting men and materials between levels.

•The three main components are; Mast(s) or tower(s); a platform capable of supporting persons & equipment and a chassis supporting

the tower/mast.


Suspended cradle (gondola)

•Two-point adjustable suspension scaffolds, also known as ‘swing- stage scaffolds’ or ‘gondolas’, are perhaps the most common type of suspended scaffold.

•Hung by ropes or cables connected to stirrups at each end of the platform, they are commonly seen to be used by window cleaners on skyscrapers, and play a prominent role in high-rise construction


Suspended cradle (gondola)


Abseiling

•Abseiling (or industrial roped access) can provide a safe and cost-effective method of access for light work commonly of maintenance or inspection nature.

•Modern roped access equipment and techniques allow fully trained specialist operatives to reach highly inaccessible locations, some of extreme conditions (e.g. from narrow mineshafts to overhanging external details under apexes of skyscrapers).


Abseiling

It is a highly feasible method to consider abseiling

when

•Ground conditions are either unknown or unsuitable

to support any vertical access (e.g. crane, scaffold &

the like).

•In situations when the heights of work locations

cannot be reached from the ground (e.g. due to safety

or physical constraints) and no strong anchorage

points are available for gondolas.

•Time and cost efficient for light tasks (e.g.

maintenance of the main glass pyramid at le Louvre,

France)


Formwork

BS 6100, Section 6.5,[10] defines formwork as ‘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 until it is able to support itself. It consists, primarily, of the face contact material and the bearers that directly support the face material.’ The term ‘formwork’ is commonly confused and associated with another, namely ‘falsework’, the latter being a term used to described temporary support systems such as those scaffold supports as previously covered. BS 5975[12] defines falsework as ‘Any temporary structure used to support a permanent structure during its erection and until it

becomes self supporting.’


Formwork

•Selection of the formwork system is a key factor that governs the success of a project in terms of time, cost, quality and safety

•For high-rise buildings, the most effective plan is for the works to achieve a very short floor cycle

•The key to achieving this is to exploit an efficient and appropriately designed formwork system.

•Modern buildings are generally complex in terms of scale and size so the design and use of the right formwork system, will contribute substantially to the overall success of a project.


Categories of Formwork


Categories of Formwork •Materials used for formwork are traditionally limited due to the

dilemma between cost and performance.

•Timber in general is still the most popular formwork material, due to its low initial cost and adaptability.

•Steel in either hot-rolled or cold-formed sections and in combination with other sheeting materials, is another popular choice of formwork material.

•In recent years, full aluminium formwork system has been used but the performance is being questioned by many, particularly with regards to additional costs and the need for specialised workmen.


Categories of Formwork “Tuckerbilt’s” vertical stair form with adjustable length platforms, fillers supplied to make width adjustable, and easy rollback movable side eliminates crane requirement to remove and replace side form Traditional timber formwork is widely preferred due to its low costs, adaptive nature and relative ease of

use.


Formwork

Most timber and aluminium forms can be assembled manually, due to their weight, design and construction.

•It is labour intensive, and used in simpler jobs; or occasionally used in very large or complex buildings to attain the benefit in flexibility

•Some systems are equipped with a degree of mobility to ease the erection and striking processes,

•These formworks are generally categorised as either the crane-lifted types or the mechanised slip-form systems.

•In the crane-lifted category large panels are fabricated either in steel sections and sheeting, or using plywood sheeting and stiffened by metal studs and soldiers.

•These large panels can be positioned either on a solid slab or fixed onto brackets (e.g. should they be used for external walls or shafts).


Formwork


Formwork

•Slip-form formwork systems use hydraulic or screw-jack systems (either automated or manual), and these systems allow for continuous casting till the end of a typical section is reached.

•Slip-form system derived its name from the fact that the formwork itself actually ‘slip off’ a previously cast structure

•It moves when the structure has taken physical shape with both its cementitious properties and composite bond with the reinforcements being set (i.e. harden) to a safe and acceptable level for the absence of the physical form support.

•The process is continuous and encourages a sense of urgency in the steel-fixers and casting crew to adhere to appropriately timed and scheduled activities in order to compliment the continuum.

•For automated systems, there is always a manual over-ride to halt the slow but continuous movement of the form.


Slip-form formwork


Climbing-form formwork


Custom formwork


Formwork: Construction-related factors

Repetitive sequence of work:

•High-rise block structures usually create highly repetitive cycles of work and may be suitable for certain kinds of formwork.

•However, for horizontally spanned buildings, the level of repetitiveness will be limited

Physical site constraints:

• Sites with numerous physical and contractual restrictions (e.g. sloped grounds, minimal site access or manoeuvre space, close proximity to sensitive structures), will increase difficulties from the mobilisation stage (i.e. getting the formwork onto site and storing them) to subsequent erection.


Formwork: Construction-related factors cont’d

Speed of work: •Work on low rise construction sites can be accelerated by the introduction of additional sets of formwork to create more independent work sites.

•This increases costs and should be considered only when time is of the essence e.g. when the risk of imposed delay penalties exceeds the costs of having additional systems

•For high-rise buildings, the mere increase of formwork input cannot often fulfil the need for speed in construction, as the critical path depends on individual floor-cycles times. Therefore the selected formwork design needs to support minimal floor-cycle times.



Recycling of formwork:

•The number of times timber formwork can be reused is usually limited to its durability after every striking process (i.e. the removal of falsework, struts and wedges, followed by plywood sheetings).

•Oil-based coatings are applied to contact surfaces of the plywood sheetings and left to dry, prior to the erection process.

•Timber form may usually be used for up to ten casts, thus making it economically viable as the main option for formwork

•Though reusability of metal form is greatly superior, its high initial and maintenance costs will often discourage its choice of use.

•Careful balance between cost, speed, performance and quality of outputs should be properly maintained when making the selection.



Construction planning and management:

•Planning i.e. phasing or sectioning arrangements, integration of the structures, site-layout and setting up arrangements, and the hoisting and concrete placing facilities, etc., are influential factors in the selection and use of formwork.

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 and specific scale of work •Usually volume of concrete ranging from 50m3 (non-continuous pour from approx. 10 safely-laden ready-mixed concrete trucks) to 200m3 (continuous pour involving (e.g.) elephant concrete pumps from approx. 40 trucks of the same) per pour can be comfortably planned for most site environments.



Continuity of structures and construction joints:

•Introducing a large number of construction joints in a large structure subdivides the works into effective and workable sizes,

•Being the weak physical links of any structure, construction-joints are inevitable in all forms of building.

•Design engineers conceptualise form systems and site staff exercise common sense, in conjunction with strict adherence to design specifications, to ensure the rigidity of a structure.

•In order to ensure rigidity of the overall system during the casting process, form-ties are incorporated into the formwork design

•These accessories once (partially or wholly) removed after casting, have their locations patched with high strength grout and should not affect the overall structural integrity of their structures.



Involvement of other construction techniques: •The applications of tensioning and prefabrication techniques are often involved in the construction of modern high-rise buildings, especially so in the Far East.

•This may impede the casting schedules and dictate the selection and use of formwork, especially where pre-cast elements are to be incorporated during the casting process.

•Additional provisions of temporary supports, slot spaces and boxed-out positions in the formwork for the pre-cast elements, or additional working spaces for the placing of stressing tendons and the onward jacking process, should be allowed in such cases.


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 (in relation to its length) made below the surface of the ground.

•In general, the depth of a trench is greater than its width, and the width (measured at the bottom) is not greater than 5m.

•If a form or other structure installed or constructed in an excavation reduces the distance between the form and the side of the excavation to 5m or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.


Groundworks

Safety introduction:

•Excavating is recognized as one of the most hazardous construction operations and this Unit will highlight various trenching methods, hazards and their preventions. Some common terms and definitions are as follow:

•Accepted engineering practices are procedures compatible with the standards of practice required of a registered professional engineer.

•Adjacent structure stability refers to the stability of the foundation(s) of adjacent structures, which locations may create surcharges, changes in soil conditions, or other disruptions that have the potential to extend into the failure zone of the excavation or trench.


Groundworks

Some common terms and definitions cont’d:

•Competent person is an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to employees, and who has authorisation to take prompt corrective measures to eliminate or control these hazards and conditions.

•Confined space is a space that, by design and/or configuration, has limited openings for entry and exit, unfavourable natural ventilation, may contain or produce hazardous substances, and is not intended for continuous work occupancy.

•Hazardous atmosphere is an atmosphere that by reason of being explosive, flammable, poisonous, corrosive, oxidising, irritating, oxygen-deficient, toxic, or otherwise harmful may cause death, illness, or injury to persons exposed to it.


Groundworks


•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.

•Protection system refers to a method of protecting employees from cave-ins, from material that could fall or roll from an excavation face or into an excavation, and from the collapse of adjacent structures. Protective systems include support systems, sloping and benching systems, shield systems, and other systems that provide the necessary protection.

•Registered professional engineer is a person who is registered as a professional engineer where the work is to be performed. He has the authority to conceptualise and/or approve on designs of all temporary protective/support systems.


Groundworks


•Support system refers to structures such as underpinning, bracing, and shoring that provide support to an adjacent structure or underground installation or to the sides of an excavation or trench.

•Subsurface encumbrances include underground utilities, foundations, streams, water tables, transformer vaults, and geological anomalies.

•Surcharge means an excessive vertical load or weight caused by spoil, overburden, vehicles, equipment, or activities that may affect trench stability.

•Tabulated data are tables and charts approved by a registered professional engineer and used to design and construct a protective system.


Groundworks


•Underground installations include, but are not limited to, utilities (sewer, telephone, fuel, electric, water, and other product lines), tunnels, shafts, vaults, foundations, and other underground fixtures or equipment that may be encountered during excavation or trenching work

•Unconfined compressive strength is the load per unit area at which soil will fail in compression. This measure can be determined by laboratory testing, or it can be estimated in the field using a pocket penetrometer, by thumb penetration tests, or by other methods.

•Angle of repose: This term has been replaced by maximum allowable slope.


Groundworks

Soil mechanics: An overview

•A number of stresses and deformations can occur in an open cut site or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation.

•The following diagrams show some of the more frequently identified causes of trench failure.


TENSION CRACKS. Tension cracks usually

form at a horizontal distance of 0.5 to 0.75

times the depth of the trench, measured from

the top of the vertical face of the trench.

Figure 9.7.1: Tension Cracks

SLIDING or sluffing may occur as a result of

tension cracks.

Figure 9.7.2: Sliding

TOPPLING. In addition to sliding, tension

cracks can cause toppling. Toppling occurs

when the trench's vertical face shears along the

tension crack line and topples into the

excavation.

Figure 9.7.3: Toppling

SUBSIDENCE AND BULGING. An

unsupported excavation can create an

unbalanced stress in the soil, which, in turn,

causes subsidence at the surface and bulging of

the vertical face of the trench. If uncorrected,

this condition can cause face failure and

entrapment of workers in the trench.

Figure 9.7.4: Subsidence and Bulging

HEAVING OR SQUEEZING. Bottom

heaving or squeezing is caused by the

downward pressure created by the weight of

adjoining soil. This pressure causes a bulge in

the bottom of the cut, as illustrated in the

drawing above. Heaving and squeezing can

occur even when shoring or shielding has been

properly installed.

Figure 9.7.5: Heaving or Squeezing

BOILING is evidenced by an upward water

flow into the bottom of the cut. A high water

table is one of the causes of boiling. Boiling

produces a "quick" condition in the bottom of

the cut, and can occur even when shoring or

trench boxes are used.

Figure 9.7.6: Boiling

Groundworks: Soil mechanics



Determination of soil type

There are generally four types of soil and rock deposits, namely: Stable Rock, Type A soils, Type B soils and Type C soils

•Stable rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. It is usually identified by a rock name e.g. granite or sandstone.

•Type A soils are cohesive soils with an unconfined compressive strength of 144 kilograms per square centimetre (kPa) or greater. Examples are: clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam.

•Type B soils are cohesive soils with an unconfined compressive strength greater than 48 kPa but less than 144 kPa. Examples are: angular gravel; silt; silt loam; previously disturbed soils unless otherwise classified as Type C;



Determination of soil type

•Type C soils are cohesive soils with an unconfined compressive strength of 48 kPa or less. Examples are granular soils such as gravel, sand and 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. Each layer may be classified individually if a more stable layer lies below a less stable layer, i.e., where a Type C soil rests on top of stable rock.



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.


Groundworks: Groundshores

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 the cut makes sloping back to the maximum allowable slope 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 supporting struts are the main components that resist the push factor from the surrounding earth. •The preferred materials for struts are timber and aluminium, the former being cheap and readily available, with the latter being strong and light


Groundworks: Groundshores •The modern trend is towards the use of hydraulic shoring, a prefabricated strut and/or wale 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


Groundworks: Groundshields


Groundworks: Groundshields •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


Groundworks: Trenching


Groundworks: Ground water control •The existence of underground water tables, rain and other forms of ingression are out with human control.

•However these factors will significantly fluctuate ground moisture content on site, thus altering its geological structure and integrity, leading inevitably to uncertainty and a likely compromise in safety.

•There are various processes for improving soil properties in-situ either temporarily or permanently. The methods fall chiefly into the following categories:

•Ground water removal •Ground water exclusion •Soil stabilization including improvements in load bearing capacity.


Groundworks: Ground water control •The existence of underground water tables, rain and other forms of ingression are out with human control.

•The problems due to excessive surface and ground water content in excavations are:

•Erosion or collapse of the sides of the excavation •Instability of the base of excavation •Reduction in the maximum allowable angle of the embankment •Settlement of adjacent structures due to erosion of ground •Collapse of temporary supports into excavation •Water-logging of ground •Need for special concreting/construction procedures


Groundworks: Ground water control

•However these factors will significantly fluctuate ground moisture content on site, thus altering its geological structure and integrity, leading inevitably to uncertainty and a likely compromise in safety.

•There are various processes for improving soil properties in-situ either temporarily or permanently. The methods fall chiefly into the following categories:

•Ground water removal •Ground water exclusion •Soil stabilization including improvements in load bearing capacity.


Groundworks: Ground water removal

Pumping from sumps

•This technique is widely used in deep excavations for trench or basement. There are several major problems:

•Soil movement due to settlement

•Ground affected by water flow towards sump (i.e. a pit where pump is positioned)

•Instability at formation level in timber-trenched excavations owing to upward movement of water

•The general and effective solution may be to create sumps at corners of excavation below formation level, subject to allowances in the works.



Pumping from wells

•An enhanced variation is to pump water from lined wells that could

reach deeper than unlined sumps.

•Pumping from wells basically involves the creation of strategically

sited voids in the ground (i.e. internally-lined wells) and the

concentration of water removal activities via mechanical pumping at

the bases of these voids.

•Main considerations of this method shall be on the nature of existing

strata, its porosity and water retention properties, and the overall

layout and level of the site.

•Its main advantage is that site works will not be physically affected

by the dewatering process and its equipment.



Pumping from well-points

•This method lowers the surrounding water table to a position below

the formation level so as to provide a dry working area, and is better

suited for use in non-cohesive soils.

•A series of well-points are jetted into the subsoil and connected to a

common header pipe which is further connected to a vacuum pump.

•The well-point is created by a slotted or perforated pipe that serves

as a plunger-head to jet itself into the ground and then remain at the

base of the formed well-point for dewatering purpose.

•The installation of well-points at two or more vertical levels is known

as a ‘multi-stage well-point’ system.



Electro-osmosis

•Electro-osmosis is usually used in cohesive soils.

•Soil particles carry a negative electrical charge, and the dewatering

process works by attracting positively charged (hydrogen) ends of

H2O water molecules.

•Electro-osmosis has been found to be most successful in uniform

beds of fine silts.

•However, the method can be very expensive and is therefore not

commonly used.


Groundworks: Ground water exclusion

Freezing method

•Ground freezing turns sub-soil water in into a solid barrier of ice that

is impermeable to itself in liquid state.

•Steel freeze pipes are inserted into soil at approximately 1m centres

around the site to be excavated.

•Pipes above ground level are insulated. Brine is pumped through

system at –15 to –25°C, using calcium chloride or magnesium

chloride cooled by refrigeration plant nearby.

•It takes between 10 – 17 days for the ice barrier to form.

•It is suitable for deep shafts, tunnels and large excavations, and is

more cost-effective for excavations of greater depths.

•It is not a feasible method to consider for use in warm countries.



Contiguous piling method

•This forms a permanent structural wall of interlocking bored piles.

•The wall formed is solid and watertight, and is commonly used as a

structural retaining wall e.g. at the foot of hills & basement carparks

•This system is suitable for most types of subsoil and has the

advantages of being economical on small and confined inner-city

sites, capable of being formed close to existing foundations and can

be installed with minimal noise and vibration.

•To better ensure the absence of voids in between piles that may

allow water ingression, the entire external façade of the wall may be

coated with a sprayed layer of chemically enhanced cementitious

admixture.



Grouting method

•Used when the pump/removal methods are unlikely to be economic (e.g.) in cases of highly permeable soils or variable ground consisting of stable rocks, such that boring of wells/well-points will be costly.

•Rock or soil strata will be injected via pipes or voids and be filled with fluids/compounds that will seal or reduce permeability of the ground upon setting.

•A major problem would be the weakening of surrounding strata and structures, due to the grouting force required, which could lead to abnormal settlement, cracks and other defects both immediate and inherent.

•The choice of grout material is dependent on the nature of soil and

fissures in rock.



Cement grouting

•Suitable for very permeable and coarse soil type

•Holes bored around site upon clearance excavation

•Cementitious grout is injected, commencing from a thin mix, with increasing viscosity by reducing the water-cement weight ratio

Bentonite grouting

•Bentonite adds very little strength to the soil and is used where soil particles are too fine for its cement counterpart to be effective

•When bentonite coagulates, it forms an impermeable gel

•It may also be mixed with Portland cement or soluble silicates to form a permanent barrier e.g beneath dam structures to create a permanent and impermeable water barrier.

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