Building Technology and Management

7/31/2019 Building Technology and Management

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BSc (Hons) Building Technology and

Management

Year 2

Building Technology Notes

Building Process

Site Planning

Construction Plant I

Compaction Plant

Ground Conditions and Site Excavation

Cladding

Complex Foundations

Piling Systems I

Piling Systems II

Building Stabilization

Soil Stabilization

Building Demolition

Building ProcessBy Dr. Heng Li [[email protected]] Tel: 2766 5879
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1. Learning Objectives

"AN ANALYSIS OF A BUILDING IN TERMS OF THE PROCESS REQUIRED ANDTHE BUILDING TEAM WHICH IMPLEMENT THEM."2. Activities

Erection of a building is a complex process Activities involve:-

o Initial decision to buildo Securing of financial resourceso Selection of appropriate locationo Appointment and briefing of suitable member to be

involved with design and construction operationso Definition of precise functional requirementso Design process and decision functional requirementso Design process and decision on how to buildo Implementation of erectiono Operations necessary to maintain building in the state of

continuous performance for which it was intendedo Operations necessary to adapt building to new functions

All these activities may be affected by approvals, controls,checks and cross checks which involve the entire building teamand outside bodies in varying administrative, technical, financialand fiscal capacities

The creation of a building was formerly a leisurely occupationultimately dependent upon craft-based skills in the past, the

whole process is now greatly influenced by the desire to achieveprofit on financial investments as soon as possible and theexploitation of machine technology

Although most buildings of today are far more complex andsometimes much larger, the time scale to build is far shorter

3. Plan of Work

The Plan of Work has subsequently become widely knownamong other professions concerned with the design andconstruction of buildings as is if capable of being used in a

variety of ways.

It can assist the planning of projects and be adapted to form thebasis for control of organizational procedures.

Works are grouped into 12 stages:-


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Inception Stage

Feasibility Stage

Outline Proposals

Scheme Design

Production Information

Bills of Quantities

Tender Action

Project Planning

Operations on Site

Completion

Feedback

Building processes include:-

Planning the work and setting timetables

Carrying out work

Making proposals

Making decisions

Setting out objectives for next stages

4. The Building Team

Building is a group activity and its success depends on a good

understanding and operation between a large number of people.

The participants involved can be conveniently arranged into groups orteams according to their particular interest and /or involvement asfollows:-

4.1 Client Team


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The client or the building owner has the responsibility for defining thebuilding to suit needs, establishing and providing the necessaryfinances, agreeing design and construction phases, timetabling, and,

of course, fulfilling the management and running of the completedproject.

A potential client must establish whether to build or not to build. Havingdecided that a new building is necessary to provide additional oralternative space, it is important that consideration is then given towhen the space will be needed. Various problems are needed to besolved, such as: land acquisition, establishment of rights, developmentpermits, planning permission, building approval, contractor selectionand subsequent erection.

Most building is undertaken from money made available in the form ofa loan therefore, interest rates are important. In this respect, thegovernment has direct influence and can use the building industry as aregulator for the economy of the country.

Once the money becomes available for a building, the client willrequire speedy action for its design, construction and subsequent useso that the lost interest, which would have been gained throughalternative financial investments, may be speedily recouped. The totalcost of a building must include the professional fees of the DesignTeam which the client appoints.

4.2 User Team

User Team forms a vital link between design concepts and built reality.An example of User Team is the advisory organization formed by thetenants of public housing.

4.3 Design Team

There are a great many people in a Design Team who concerned withsupplying the design expertise which will make a building possible.

Principal Designersgenerally include architects, interior designers,and building surveyors. They are responsible for the overall design ofthe project.

Architects design and prepare the production information formost building projects. They will also inspect the constructionwork on site.


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Interior Designers can also prepare design and productioninformation for a building, and provide supervision of work, but,they may be specifically concerned with the interior of a buildingand need additional advisers in order to deal with all the designand construction processes involved in total building.

Building Surveyors are sometimes responsible for the designand supervision of certain building work although they are moreusually carry out surveys of structural soundness, condition ofdilapidation or repair, alterations/extensions to existing buildingsand market value of existing buildings.

Specialist Designersinclude civil and structural engineers, servicesengineers, and those concerned with specific aspects of architecture,including landscape, interiors, office planning, etc. They provideexpertise concerning certain aspects of a building and whoserequirements are often coordinated by the Principal Designer. Forexample:

HVAC

Communications

Drainage and plumbing

Electrical

Fire services

Security systems

Civil and Structural Engineers are employed to assistPrincipal Designers on building projects which containappreciable quantities of structural work, such asreinforced concrete, complex steel or timber work, orfoundations which are either complex or abnormal.

Services Engineers work with other designers and areconcerned with environmental control lighting, heating,air conditioning, and sound modulation; electricalinstallations, plumbing and waste-disposal systems; andmechanical services, such as lift installations andelectrical conductors.

Quantity Surveyorsprovide the cost control and financial advice toclient, principal designers and specialist designers. They are


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responsible for preparing Bills of Quantities. Also, during the actualconstruction period for a project, he must measure and value the workcarried out at regular (monthly) intervals and submit details to theoverall financial administrator (usually the principal designer) forpayments from client to contractor. They also advises on the use ofsums of money listed in the Bill of Quantities for contingency orprovisional items, the cost of making variations in areas originallydescribed in the Bills or indicated on the drawings, and settlement ofthe final account for the finished project.

Depending on the precise nature of a project, the combined cost ofthese professional fees will vary from between 12% and 20% of thefinal construction costs.

4.3 Research Team

Researchers are those making understanding and development ofcurrent construction methods (materials and technical ability). The aimof the research is to discover facts by means of scientific study and, inmatters concerning building, covers a very wide area of knowledgerequiring controlled programming of critical investigation of chosensubjects.

4.4 Legislative Team

They negotiate with the relevant authorities to clarify certain legalrequirements. Building Ordinance Office, Planning Department, FireServices Department, Highways Department, Urban Council, etc.

On site management level, a builder has to ensure that the buildingsite maintains safe and healthy conditions for employees, and that thegeneral public should be adequately protected from dangers resultingfrom site operations.

In UK, a new regulation, the Construction (Design and Management)Regulations was implemented in 1995 which share the health andsafety liabilities among all parties concerned.

4.5 Manufacturing Team

This team supplies the materials, components and equipment whichare used during the construction processes of a building, and,therefore, incorporate many organizations and interests. With the needto economize in labour and reduce costs, building procedures became


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more rationalized and mechanized.

However, the continual advancement of technology, and increases incomplexity and size of buildings today results in even more complex

construction processes. Manufacturers must extend their services fromthe supply of single components, to the supply of much larger parts ofa building (elements) and indeed whole buildings. Site operations arereduced to a minimum using mechanical plant, and methods ofbuilding become largely concerned with the organization of thesystematic supply and assembly of pre-fabricated items, i.e. SystemBuilding.

Closed System Building: some produced items whichwill not normally fit with the components of othermanufacturers.

Open System Building: when component design iscoordinated between the manufacturers of differentproducts so that they can be used together withoutalterations or become interchangeable.

4.6 Construction Team

The erection of a building depends on an industry where total relianceis placed on the diverse attitudes, abilities, and adaptability of itsworkers.

Today, most specialist trades are employed as nominated sub-contractorsby the client or principal designer on behalf of the client; arelatively few day tradesmen being employed directly by the maincontractor as domestic sub-contractors.

The main contractor is responsible for managing and directing al workson a site, and coordinating the work of each trade. The nominated anddomestic sub-contractors are responsible for managing andcoordinating their individual works. They are required to design andprovide specialist elements within a building from a statement ofperformance requirements, but the main contractor is still entirelyresponsible for the satisfactory completion of the work involved.

The client or the principal designer would employ nominatedsuppliersfor certain specialist materials, components or equipmentwhich are to be used or fixed into position by the main contractor.


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Asphalter - roof, floor and wall (basement) finishes

Bricklayer - laying brickwork

Carpenter - structural and carcassing timber work

Concretor - placing concrete

Drainlayer - providing below ground drainage

Electrician - electrical installation

Excavation - levelling site and digging drain/foundation trenches

Floor tiler - internal floor finishes

Gas-fitter - gas installation

Glazier - fixing glass

Joiner - timber work to finished components

Metal worker - sheet metal applications (roofing)

Painter and decorator - finishing components

Paver - external paths/roads finishes

Plasterer - plastering walls/ceilings, screeding and rendering

Plumber - plumbing installation, flashing and gas pipes (interior)

Scaffolder - erecting scaffolding and working platforms

Steel erector - erecting steel columns and beams

Steel fixer - cutting, shaping and positioning steel reinforcement

Tiler and slater - roof finishes

Basic list of trades which would be employed for erection of asimple building


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Types of Building Organizations

Main contractors can be divided into three basic groups:

General Builders

General Contractors

Design and Construction Companies

Package Deal Contractors

4.7 Maintenance Team

The chosen design and construction method of a building must take

into account the effects which time will have on their performance. Theprecise methods adopted for subsequent maintenance and cleaningwill also be influenced by the attitude of the Client Team towards therunning costs of a building.

On completion of a project, the Client Team must be presented with aMaintenance Manual complied by the Design Team which incorporatesthe advice of the consultative Maintenance Team.

Maintenance Manual describes how a building can be expected toperform, what measures have been taken to ensure it does, and whataction must be taken in the future.

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Site Planning

By Dr. Heng Li [[email protected]] Tel: 2766 5879


"An analysis of the factors affecting the decisions during contract planningstage."

2. Contract Planning and Control

This involves working out a plan of campaign or a programme for thecontract as a whole and assembling the necessary data. Programme is
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required to promote the satisfactory organization and flow of the variousbuilding operations during the course of erection, by planning in advance thetimes and sequences of all operations and the requirements in labour,materials and equipment.

BRE Digest 91 states a well planned programme should:-

show the quickest and cheapest method of carrying out the work ensure continuous productive work for all operatives employed and

reduce unproductive time to a minimum provide an assessment of the level of productivity determine attendance dates and periods for all subcontractors work provide information on material quantities and essential delivery

dates, the quantity and capacity of the plant and the periods it will beon site

provide a simple and rapid method of measuring progress

2.1 Gantt Chart or Bar Chart

simple and easily read plan of operations

plot all site personnel against actual performances

only takes into account one of the resources, time

does not inform on the critical relationships between the

various activities


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analyse the difference between a production problem against linearor parallel linear methods lies in the identification of the dependencybetween operations

this approach leads to interrelated networks through which certainsequences can be seen to be "critical" to the anticipated outcome

often convenient for the network to be set up initially using non-scaledlinear arrows to represent the sequence interrelationship of activitiesand then plot the network aginst a linear time scale to examine thedistribution of resources and to the preparation of a bar chart form ofpresentation.

2.3 Master Programme

On acceptance of the tender, contract planning commences and a

working, or overall, programme is prepared. This is a guide for site activities, for detailed planning, for the buying

and delivery of materials, for the co-ordination of sub-contractors andmain contractors work and for assessing job progress.

The programme shows the major operations and phasing of the job,but detailed short-term planning at regular intervals on the site isnecessary to ensure the satisfactory allocation of labour andmaterials to each individual operation as the work proceeds.

The preparation of the overall programme consists broadly of:-

breaking the job down into a series of basic operations involving onlyone trade

establishing the quantities of work in each operation and the timecontent

arranging the operations in a sequence and balancing the size ofgangs to give a maximum continuity of work and minimum delay

breaking down a large job into phases so that several operations mayproceed simultaneously.

3. Planning Consideration

3.1 Site Conditions and Access

Site conditions will limit the type of plant that may be used.


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Wet sites: need to use tracked machines in the case of excavatorsand mobile cranes, and dumpers for transport

Sloping sites: make the use of rail mounted cranes unsuitable oruneconomical

Confined sites: insufficient room for a mixer or mixing plant and may

need to use truck mixed concrete Site closely surrounded by tall adjoining buildings: indicate the use of

a derricking jib crane rather than a horizontal jib crane in order to beable to rise and clear the buildings

Limitation of access

3.2 Nature of Job

Different type of structure and form, size and detailing of thebuilding will have effect upon the way the contract is planned.

Influence the decision of equipment and materials.

3.3 Plant

General considerations: capabilities, limitations and outputs ofdifferent types of plants

Excavation plants:

The type of excvation to be carried out

The nature of the soil to be excavated

The volume of soil to be excavated

The length of haul to tip and the terrain overwhich the machinery has to dig and travel.

Handling:Quantity and nature of materials to be handled

Degree of tower cranes utilization

Avoid "Double Handling:

Sitting of hoisting plant, materials dumps andmixing plant in relation to the building and toeach other


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Mixing:Quality and quantity of concrete required

Site conditions

Costs comparisons among various methods

Type of Plant:Nature of job

Sequence of work

Method of construction

Amount of work

Cost of equipment and running cost

3.5 Design Factors

Simplicity of construction and detailing

Separation of trades

Phasing of works

Continuity of works

4. Site Organization

4.1 Site Planning

Period Planning usually done monthly

Weekly Planning prepare towards the end of each week progress and toplan the next week progress

Progress Control a regular review of the progress of all operations andcomparison with the programme or plan

4.2 Site Layout

Site layout is divided into Administrative Area and Construction Area.


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Administrative Area

Includes stores, offices, sub-contracts huts, canteen and similaraccommodations

Locate with quick access to the site area for control reasons Ease of connectivity in telephone facilities for communication,

electricity for power, compressed air for equipment, as well as lightingand heating facilities for office huts

Careful initial site planning is required to minimize to moving ofadministrative area during the contract period

Construction Area

Includes the actual site of the buildings being constructed, materials

and equipment Positioned to minimize the time of handling and movement Areas properly identified beforehand to avoid the possibility of

dumping materials in the wrong position

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Construction Plant I


1. Reference

Jack Stroud Foster & Raymond Harington (1994), Structure & Fabric Part 2, 5thEdition, London: Longman


"An ability to select the appropriate construction plant for specific building operation."

3. Mechanization of Building Operations

Mechanization is one aspect of industrialization aiming at increasing

productivity Factors affecting efficient employment of mechanical plant:-

Nature of job and site

Mechanization is better on larger contracts rather than small contracts.A large contract normally has sufficient work of various types to justifythe introduction of specialized machines and to enable them to beused economically. Site considerations must be suitable for its safe,efficient and economic use.

Relationship between operations of plant and of men

The number of men working on any operation should be correctlyrelated to the output of the mechanical plant serving them. This is toprevent the plant being idle from time to time while the men use thematerial already delivered to them.

Careful planning and programming of the contract as a whole

To ensure the expensive plant to hire or to purchase and maintain, isoccupied to the maximum extent while on the site. Ideally, the

sequence of all operations throughout the job, whether mechanized ormanual, should be so arranged that no plant on the site is ever idle.

Suitability of the design of the building

The use of mechanical plant must be considered at the design stagecarefully. The architect should be aware of the advantages of


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bucket slides on rollers directly along the jib and thus has a more restrictedmovement. It is used for surface excavation and levelling in conjunction withtransport to haul away the excavated material.

DraglineIt is usually fitted with a long slender boom or jib and the bucket, which inoperation faces towards the machine and has no door, is supported by cable only ason a crane. It works from the side of the excavation at normal ground level and isused for excavating large open excavations such as basements when the depth isbeyond the limit of the boom of a backacter.

Crand and GrabIt consists of two hinged half-buckets or jaws pivoted to a framewhich is suspended by cable from a long jib of an excavator. The grab is used fordeep excavations of limited area on all types of soil except rock.

Pile Driving and DrillingIt is an excavator equipped with either hanging leaderswhich guide the pile and the hammer during driving, or a turntable through which the

square drilling rod or kelly bar passes for drilling piles.

b. Tractor-based Equipment

It is designed either as attachments to normal tracked or wheeled tractors or asmachines in whih the earth-moving attachments and the tractor are designed as asingle integrated unit.

A tractor which is hydraulically operated can be rigged as:-

Tractor ShovelThis consists of a tipping bucket at the front attached by

strong pivoted arms or booms to the frame of the machine. It is used forstripping top soil, excavating against a face, bulldozing and for loading spoilor loose materials.

Trench DiggerIt operates on the same principle as a backacter excavatorexcept that the bucket is controlled by hydraulic rams instead of cables andpulleys.

ScraperIt is a large box or bowl with an open front and bottom cutting cuttingedge, supported on a frame between two pairs of wheels and attached to atractor from which the bowl is raised and lowered by cable or hydraulic power.

It is used for surface excavation over large areas where the spoil can bedisposed on the site and for bulk excavation over small areas.

Bulldozer and Angle-dozerThe bulldozer consists of a rectangular steelblade with renewable cutting edge set at right-angles or about 30 degrees tothe direction of travel and attached by steel arms to the sideframes of acrawler tractor. It may be used for excavating natural soil or for moving loose


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soil or debris which it pushes forward as the ractor forces it ahead.

4.2 Hoisting

This concerns with the handling of materials and components. Most buildingmaterials used in the erection of buildings are handled several times during thecourse of construction. The plant used for hoisting, primarily performs verticalmovement.

a. Mobile Cranes

These may be either self-propelled or truck-mounted. They are suitable whereon-site or between site mobility is a primary requirement or where theduration of job is short. It is widely used for the erection of low-rise buildingswhere a long reach is not essential and the machine can approach near to thebuilding, and for the erection of low framed structures where the crane being

able to move between the columns of the structures.

For example: Self-propelled cranes, truck- or lorry-mounted cranes,telescopic jib cranes

b. Stationary Cranes

They are fixed firmly to some form of base at their working position. It canhandle its maximum loads over a greater range of radius than mobile crane.

For example: Derrick cranes, guy derrick, scotch derrick, monotower derrick,

tower crane, climbing crane, rail-mounted or travelling crane, transportabletower crane, fixed jib slewing crane, portal crane

c. Hoists

A hoist consists of a horizontal platform which is moved up and down verticalguides by a powered winch and is usually termed a platform hoist. It is usedfor materials lifting and passenger carrying which is useful for high-risebuilding construction.

For example: Platform hoists and mobile platform hoists.

d. Elevators

These consist of a series of buckets fixed to a rotating belt or chain and areused for raising aggregates into the bins of weightbatchers. Elevators canwork vertically but are usually set at an angle, according to the height of lift.


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4.3 Transporting

This implies horizontal movement primarily but it can involve some verticalmovement too.

a. Dumpers

These are vehicles designed for the transport of materials whichpreviously were usually carried out by wheelbarrows, such asexcavated spoil, hardcore and concrete. It is faster and moreeconomical than hand barrow and consists of a shallow tipping hopperor skip mounted on a wheeled chassis.

For example: Power barrow, dumper, multi-skip dumpers, highdischarge dumpers, dump truck

b. Forklift Trucks

It is essentially a powered mobile chassis on the front of which is avertical frame or mast on which a pair of "forks", that is a pair ofprojecting tines, may be raised and lowered. It is basically used forhandling unit or packaged loads which is a large individual componentor smaller components packaged into suitable units.

c. Monorail Transporter

This is a powered wagon or skip running on a single easily laid rail and

is intended primarily to carry concrete from the mixer to the point ofplacing. It is generally used in otherwise inaccessible situations.

d. Conveyors

It is used for handling small materials such as excavated spoil from thepoint of excavation to the boundaries of the site for loading intotransport, for concrete placing or for filling up aggregate bins in weigh-batchers.

e. Concrete Pumps and Placers

This consists of a pump which is mechanically operated by ram and placerspneumatically operated by compressed air. For example, mobile concrete pump.

4.4 Mixing

A large amount of material must still be mixed with water, mainly concrete, mortar


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and plaster. The advantages of mechanical mixing over hand mixing, except for verysmall quantities, are greater economy, reduce the loss of cement and more accurategauging of the water content.

a. Concrete Mixers

They are made in various types and sizes and are broadly classified as (i) batchmixers and (ii) continuous mixers. There are five types of batch mixer:

Tilting drum

Non-tilting drum

Reversing drum, a form of non-tilting mixer

Split drum

Paddle mixers (a) pan (b) turbo (c) trough

b. Weighbatchers

The batching of materials for concrete may be by volume or by weight. For example:weighbatcher incorporated with mixer, independent weighbatcher, mobile and semi-mobile weighbatcher

c. Central Mixing Plant

The concentration of batching and mixing operations in a single static plant insteadof by a number of mobile mixers is often used on large, extensive sites.

The main advantages of central mixing are:-

An increased output by fewer machines and men is possible. In suitable circumstances it is more economicl than a number of

separate mixers. Better quality control is possible

The essential components of a central mixing plant are:-

Adequate storage of aggregates at ground level Overhead aggregate storage bins to hold not less than an hours

supply at maximum output Some means of storing and weighing the cement Elevated water storage together with some means of metering the

water supplied to the mixer


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A mix or mixers Storage hoppers to contain the mixed concrete until fed into transport

or concrete pump.

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Compaction Plant



"Introduces the difference types of eartth compaction plant and methods ofcompacting."

2. Earth Compaction Plants

2.1 Static Weight Compactors

They rely on their dead weight to compact the soil. E.g. Smooth Wheeled RollersThey are used in roadworks and have limited application in earthworks. They aim tocreate a smooth surface on top of embankments so as to prevent the collection ofwater.

2.2 Kneading Compactors

They compact material by a kneading action.E.g. Pneumatic Tyred Rollers, SheepsFoot Rollers

Pneumatic Tyred RollerIt has ability to control the ground contact pressure. Theyare generally used for rolling base courses, black top on roads, and fill for largeearthworks in loamy soils. It is most suitable for compacting granular materials,particularly soils which have a low moisture content.

Sheeps Foot Rollers They may be static or vibratory. Although it performs in asimilar way to the pneumatic tyred roller, it is not as effective. In practice, it is difficulto ensure complete and uniform coverage of the fill when using a sheeps foot roller.It is generally used for rolling the fill on large earthworks in cohesive soils.

2.3 Vibrating Compactors

Vibrating compactors apply both pressure and vibration to the ground being


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compacted.E.g. Vibrating Rollers and Vibrating Plates

Vibrating RollersThey are suitable for compacting granular fill such as well gradedsand or gravel, uniform sand and hard rock. However, they are not suitable forcohesive soils.

Vibrating PlatesThey are generally used for compacting granular soils in confinedspace, compacting backfill material in pipe trenches and shallow foundation, but notsuitable for heavy compaction.

2.4 Impact Compactors

They apply an impact force to the soil which compacts the soil.

E.g. Power Rammers and Free Fall Hammers

Power RammersThis machine is hand operate. They are suitable for compactingsoil in narrow trenches and around small foundations.

Free Fall HammersUsing the free fall hammer, it is possible to compact layers ofsoil several metres deep. They are suitable for compacting most types of soilsexcept uniform granular material. This method is rather slow.

3. Factors Influence The Choice of Compaction Plant

The choice of compaction plant depends on the following factors:

Material Factors Spatial Factors Plant Factors Legal or Contractual Factors

3.1 Material Factors

Materiaal factors influencing the choice of compaction plant include:

the type of soil or rock;

the grading (particle size) of the soil, and the moisture content of the soil

3.2 Spatial Factors

Large compactors have a higher output than small compactors because their rollersare wider and because they are able to compact thicker layers of fill with fewer


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passes. Factors need to be considered include:

the area to be filled and compacted and its topography; the volume of each type of material to be compacted; and the length of time for which the compaction plant is required

3.3 Plant Factors

The choice of plant or combination of plant will be determined by:

the output rate of the plat; the cost of compaction, and the availability of the plant.

3.4 Legal or Contractual Factors

They are the specifications of the contract.

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Ground Condition and Site Excavation


1.Reference

R. Chudley, Construction Technology, Longman


On completion of this lecture, you should be able to:

list the difference types of ground conditions; and

describe, with illustration, the different types of methods

used to control ground water

3.Introduction

There are various processes for improving soil properties insitu eithertemporarily or permanently.The methods are fall into the following


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categories:

Ground water exclusion Ground water lowering

Soil stabilization including improvement of load bearingcapacity

4.Sources of Ground Water

Ground water in a pervious soil stratum may be replenished directly fromrain falling on the ground surface or be percolation from run offs, streams ornearby rivers.

The water pressures in pervious layers will usually change as a result invariations in seasonal rainfall or rivers and sea level in cases where there is

direct connection between the pervious stratum and these sources.

5.Problems caused by ground water

The problems created when surface and ground water enter excavationsare:

Erosion or collapse of the sides of the excavation

Instability of base of excavation Reduction in the angle of repose of the embankment

Settlement of adjacent structures due to erosion ofground

Collapse of temporary support to excavation

Waterlogging of ground Need for special concreting procedures

6.Ground water control

In some cases, the ground water conditions found during site investigationmay change before or during site investigation. Such changes may be due

to the construction of basements nearby, natural flooding or artificialcauses, such as a burst water main.

The methods of ground water control may be divided into three broadgroups:

1. pumping,


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2. cut-off walling, and3. special methods.

The choice of method depends mainly on site conditions and on the soil

characteristics. These include:-

size and location; thickness and type of soil strata; magnitude of water pressures in various strata; proposed permanent structure relative to soil strata; length of time for which the excavation must be open; prevention of damage to adjacent structures; relationship between the proposed dewatering method and

the construction sequence.

7.Dewatering Methods

Pumping

Pumping systems utilize:

1. sumps2. wells3. well-points

7.1 Pumping from sumps

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 Instability at formation level in timbered excavations owing to

upward movement of water

The general solution is to dig sump at corner of excavation below formationlevel.

7.1.1 Open Sump

The sump is usually formed away from the construction area in acorner of the excavation. The water is led into the sump, either bysloping the ground towards it or by using shallow garland drainswhich feed into the sump. Pumping from open sumps is limited to amaximum depth of about 8m.


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7.1.2 Jetted Sump

In this method, a hole is formed in the ground by jetting metal tube. A

disposable intake strainer connected to a disposable flexible suctionpipe is then lowered into the hole, and the void filled with sand filtermedia. This suction pipe is connected to a pump which pumps outthe ground water.

7.2 Pumping from wells

For depths 9m, use other methods. Use wells where wellpoints are notsuitable.

7.2.1 Tube Well

1. Sink lined borehole, diameter 300 600 mm to depth required(i.e. below impermeable stratum as a rule)

2. Place smaller tubeinner well lining inside, having portionperforated at level to be dewatered. Lower end acts as sump.

3. Use plunger to surge initial flow and wash out unwantedfines.

4. Connect pump to lining operate submersible type is used.5. Disconnect and withdraw both linings as annular space is

filled, or in stages, or on completion.6. Depth of well depends on depth of impermeable stratum. If far

below excavation formation level wells can be spaced wellapart to create draw-down curve just below formation level.

7.2.2 Horizontal Wells

Formation level at or slightly within impermeable stratum where verticalwells impracticable.

A. Method 1:

1. Sink vertical well outside excavation area to below proposed

formation level.2. Make horizontal borings in radial pattern from vertical well. 3. Results: water drains from upper surface of impermeable

stratum into large well. Pump (submersible) operates there.

B. Method 2:


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1. Lay 80mm PVC suction pipe (perforated) up to 6m deptharound excavation area. Suction pipe is covered with nylonfilter sleeve which prevents particles of soil from entering pipe(Laying is be special horizontal well-point placing machine: it

digs trench, lays pipe, backfills in one operation at up to 160m/hr).

2. Connect suction pipe to pump (Length of pipe per pumpdepends on soil conditions and pump capacity). Water flowsto drainage channel formed in laying pipe.

7.2.3 Pumping from Well Points

It is used for non-cohesive soils, minimum grain size 0.1 mm. The wellpointconsists of a slotted or perforated pipe which is covered with a screenmesh. At the foot of this pipe is an orifice which permits jetting of the pipe

into the ground during installation. A simple ball valve above the orificeprevents the entry of soil particles through the orifice when water is suckedin during the pumping operation.

The construction steps in the wellpoint system are:

1. a wellpoint is jetted into the ground;2. the annular void is filled with filter media;3. the wellpoints are connected to a header pipe by means of a

riser;4. the header pipe is connected to two suction pumps for

pumping.

A. Multi-Stage Wellpoint Installations

It consists of the installation of wellpoints at two or more levels. It is also beused in shallow well pumping systems.

B. Shallow Well Systems

This system can handle greater volume of water compared with Wellpointsystems. The construction steps are:-

1. a cased well is bored,2. a filter tube is placed in the borehole,3. filter media are placed in the annular void and the casing

withdrawn,4. the filter if flushed and a suction pipe is lowered into the filter

tube,


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5. the suction pipe is connected to the header main, and 6. the header main is connected to a self-priming pump.

C. Deep Well System

The method of forming a deep well is similar to that of a shallow well,except that in a deep well, a submersible pump is used to pump out thewater.

D. Vacuum Wells

In vacuum wells, a vacuum artificially increases the water flow towards thewells or wellpoints. All the pumping systems described so far are effectiveonly in gravels and sands. If more than 10% silt is present, it is necessary toincrease the water flow towards the wells.

Method:-

Jet and sand in; Seal top 1m of hole with clay vacuum is created in

sand filter around wellpoint; Only free water is removed; Sudden shock can cause pronounced disturbance; Limitations 6m maximum lift. Over 5m use multi-stage

wellpoints; Preparatory work from platform for each tier of

wellpoints around excavation of platforms on whichheader pipe is situated.

E. Electro-Osmosis

It is used in cohesive soils, where vacuum pumping isineffective. It is because soil particles carry negative electricalcharge, attracting + positively charged (hydrogen) ends ofH2O water molecules.

Electro-osmosis has been found to be most successful inuniform beds of fine silts. However, the method can be very

expensive and is therefore not commonly used.

8.Dewatering Water Exclusion Techniques

8.1 Freezing Methods

The principle of ground freezing is to change the water in the soil into a


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solid wall of ice. This wall of ice is completely impermeable.

Method:

Steel freeze pipes are inserted into soil at approximately 1m centres aroundsite to be excavated. Pipes above ground level are insulated.Brine ispumped through system at15 to25C, using calcium chloride ormagnesium chloride cooled by refrigeration plant nearby usually trailer-mounted.

Applications:

Competitive for depths of 7m+, becoming relatively cheaperfor greater depths.

Soil moisture content of 8%+ is sufficient.

Deep shafts, tunnels, large excavations.

Problems:

Expansion of soil up to 2% in clays and silts may affectadjacent structures. No appreciable expansion of sands andgravels

Thermal insulation required for exposed excavations and wellsurfaces e.g. white polythene, or glass fibre blankets,between 2 polythene sheets.

8.2 Compressed Air

This method has been extensively used in the construction of the MTR inHong Kong. It is used in caisson sinking and tunnel driving in waterloggedground. Air pressure up to 350 KN/m2 is possible which allows working atdepths up to 35m below water table, using 1m

3of fresh air per person in

working chamber.

The major disadvantage of using compressed air is the health risk toworkers working in such chambers.

8.3 Grouting Methods

It is used where pumping is likely to be uneconomic e.g. permeable soils,or variable ground (especially harder rocks) where boring of wells andwellpoints very costly. It is grouted by injecting rock or soil through pipes orholes in ground, using fluids which seal or reduce permeability of ground on


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setting.

The major problem is the vulnerability of existing u/ground structures; -cracks may be penetrated. The choice of media depends of soil particle

sizes or sizes of fissures in rock.

8.3.1 Cement Grouting

It is suitable for very permeable coarse materials.

Method:

Holes bored around excavation. Cement grout injected, starting thin and increasing viscosity

by reducing water-cement ratio e.g. neat cement and water;

4 parts sand, 1 cement; or PFA 1, cement 1, water 2, by wt. Secondary holes bored and injected mid-way between original

holes to ensure complete grouting.

8.3.2 Bentonite Grouting

Bentonite adds very little strength to the soil. This system is used where soilparticles are too small for cement grouting, especially alluvial soils beneathdam structures to create permanently impermeable layer.

When bentonite coagulates, it forms an impermeable gel. Bentonite may

also be mixed with Portland cement or soluble silicates to form a permanentbarrier.

8.3.3 Chemical Groutings

It is used in sandy soils of medium to coarse grading. Liquids are usedwhich gel by reaction between base substance and hardener.

One Shot ProcessIn this process, two chemicals are mixed together prior to injection. Onechemical is the base and the other the hardener or catalyst. When the two

chemicals are mixed, a reaction takes place and a gel or solid is formed. Inthis process, the gel time should be sufficiently delayed to allow for the fullpenetration of the chemicals before gelling occurs. The time may beaccurately controlled by varying the proportions of the two chemicals.

Two Shot ProcessIn this process, the first chemical, normally sodium silicate, is injected into


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the ground. The second chemical, normally calcium chloride, is theninjected. An immediate reaction occurs resulting in the formation of a tough,insoluble silica gel.

Chemical grouting strengthens the soil and reduces its permeability.

Advantages over other methods:

Stricter control of gel time (e.g. seconds to many hours)Fewer holes to boreGreater penetration of groutGreater flexibility in grouting time

8.3.4 Resin Grouting

Resin grouts have a low viscosity and are formed by adding a catalyst orhardener to a base solution. It is used in sandy soils of fine grading, andsilts. The choice of materials depends on the chemical content of groundwater.

8.3.5 Bituminous Grouting

It is used in fine sandy soils e.g. cut-off walls beneath dams, etc. whereno strengthening is required. It is not suitable for under-pinning.

Back Next Home

Cladding


1. References

Edward Allen, 1990, Fundamental of Building Construction Materials andMethods, 2nd Edition, John Wily & Sons, USA


"An understanding of the basic concept of claddings."


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3. Functions of Cladding

3.1 Primary Functions

The major purpose of cladding is to separate the indoor environment of a buildingfrom the outdoors in such a way that the indoor environment can be maintainedat levels suitable for the intended use.

Keeping out of water

Cladding must prevent the entry of water, e.g. rain, snow and iceinto a building, especially when water on the face of a building isoften driven by wind at high velocities and high air pressures, inevery direction. The water must be drained away from a wind-wardbuilding face during a heavy rainstorm, and the water pushed by

wind, will readily penetrate the smallest crack or hole and enter thebuilding.

Preventing air leakage

The cladding must prevent the unintended passage of air betweenindoors and outdoors. Smaller air leaks are harmful because theywaste conditioned air, carry water through thewall, allow moisturevapour to condense inside the wall, and allow noise to penetratethe building.

Controlling light

The cladding must control the passage of light, especially sunlight.Sunlight is visible light, useful for illumination but bothersome if itcauses glare. Sunlight includes destructive ultraviolet wavelengthsthat must be kept off human skin and away from interior materialthat will fade or disintegrate.

Controlling the radiation of heat

The role of cladding is to regulate the flow of radiant heat from sun,

it should present interior surfaces that are at temperatures that willnot cause radiant discomfort. (not too cold and not too hot)

Controlling the conduction of heat

It must resist to the required degree of the conduction of heat intoand out of the building. It should avoid thermal bridges, wall


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component such as metal framing members that are highlyconductive of heat and therefore likely to cause localizedcondensation on interior surfaces.

Controlling water vapour

It must retard the passage of water vapour. Vapour moving througha wall assembly is likely to condense inside the assembly in coldweather and cause problems of staining, lost insulating value,corrosion and freeze-thaw deterioration.

Controlling sound

It should isolate the interior of a building from noise outside, or viceversa. Noise isolation is best achieved by walls that are airtight,

massive and resilient.

3.2 Secondary Functions

Resisting wind forces

The cladding of a building must be adequately strong and stiff tosustain the pressure and suctions that will placed upon it by wind.The upper reaches of taller buildings are buffeted by mush fasterwinds, and wind directions and velocities are greater.

Adjusting to movement

Several kinds of forces are always at work throughout a building,tugging and pushing both the frame and the cladding: thermalexpansion and contraction, moisture expansion and contraction,structural deflections.

Thermal expansion and contractionIndoor/outdoor temperature differences can causewarping of cladding panels due to differentialexpansion and contraction of their inside and outside

faces. The building frame itself will expand andcontract to some extent, especially between the timethe cladding is installed and the building is firstoccupied.

Moisture expansion and contraction Bricks andstone expand slightly after theyre installed. Concrete


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blocks shrink slightly as its curing is completed andexcess moisture is given off.

Structural movements Building foundations may

settle unevenly, causing distortion of the frame.

Resisting Fire


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According to the building regulations.

Weathering gracefully

To maintain the visual quality of a building.

4. Conceptual Approaches to Watertightness in Cladding

Keep water completely away from the wall, however, this is impossibleas it requires a very broad overhang.

Eliminate openings from a wall, sealing every seam in the wall.

Eliminate or neutralize all the forces that can move water through thewall.

4.1 Forces that can move water through the wall

a. Gravity

It is a factor in pulling water through a wall only if the wall contains an inclinedplane that slopes into the building.

a. Momentum

It is the horizontal component of the energy of a raindrop falling at an

angle toward the face of a building. Momentum is easily neutralized byapplying a cover to each joint in the wall.

b. Surface tension

It causes water to adhere to the underside of a cladding component, it canallow water to be drawn into the building. The provision of a simple dripon any underside surface to which water might adhere will eliminate theproblem.

c. Capillary action

It is the surface tension effect that pulls water through any opening thatcan be bridged by a water drop. This action can be eliminated by providinga concealed capillary break somewhere inside the opening.

d. Wind currents


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The generic solution to the wind current problem is to let wind pressuredifferences between the outside and inside of the cladding neutralizethemselves through a concept know as the rainscreen principle.


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5. Sealant Joints in Cladding

All cladding systems require sealant joints. The role of a sealant is to fill the jointsbetween cladding components, preventing the flow of air and/or water, while still

allowing reasonable dimensional tolerances for assembly and reasonableamounts of subsequent movement between the components.

Sealants are typically used to seal joints between panels of stone or precastconcrete in a curtain wall, to seal the joint beneath the shelf angle in a brickcurtain wall, and to seal joints between dissimilar materials, such as where ametal and glass curtain wall ends against a masonry wall.

5.1 Sealant Joint Design

The time of year when the sealant is to be installed must be taken into account

when specifying the size of the joint and the type of sealant.

In cold weather - sealant will have to stretch very little during its lifetimebut will have to compress a great deal in summer.

In hot weather - as the materials around it expand and crowd together.Sealant will have to compress very little but will be greatly stretched inwinter.


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6. Curtain Wall

Curtain wall is an exterior cladding supported at each story by the steel frame,rather than bearing its own load to the foundations. The principal advantage ofthe curtain wall is that because it bears no vertical load, it can be thin andlightweight regardless of the height of the building.

The name of "curtain wall" derives from the idea that the wall is thin and "hangs"


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like a curtain on the structural frame. The walls are supported from the bottom ateach floor level.

The curtain wall must fullfil the same functional requirements as any other

system of external walling. The main problem in the design of curtain walls lies inthe framework which holds the panels and it is normally metal or timber. Curtainwall may be faced outside with any non-combustible material suitable forexposure to the weather. It can be constructed in place or prefabricated.

6.1 Connection

Their fixing must be designed accordingly. Fixings should be of stainless steel ornon-ferrous metal and so designed, that should one fail the remaining fixings arecapable of taking all the loading on the walling. This provides a margin of safetyand prevents progressive failure of a number of fixings.

Fixing devices must be capable of adjustment in any direction to provide forinaccuracies in the structural surfaces to which the framing is attached. Cast-inanchor channels are commonly used in concrete frames to provide the horizontaladjustment.

Fixing to steel frames is to plates welded to the steelwork at the required fixingpoints. Bolt holes should be slotted and packing pieces or shims used to providefor movement and adjustment. Plastic washers should be interposed betweenadjacent surfaces to allow adequate tension in the bolts combined with sufficientreduction in friction to permit differential movement.

Back Next Home

Complex Foundations


1. References

Jufri S A R and Wellman R J, 1987, Civil Engineering Construction IV,Volume 3



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After completion of this unit, students should be able to,

Define shallow foundations

List different types of shallow foundations

Describe, with illustrations, the different type of strip foundations;combined foundations and raft foundations

3. Types of Foundations

Shallow foundations are founded a depth of less than 3m below the finishedground level. Unlike piles, these foundations spread their load over a largearea of ground.

Shallow foundations are grouped into four broad types:-

Pad Foundation Strip Foundation Combined Foundation Raft Foundation

4. Pad Foundations

Pad foundations are generally used to transfer the load from a column, pieror heavy machinery to the ground. Pad foundations are usually square or

rectangular in plan; they are formed of either unreinforced concrete orreinforced concrete.

4.1 Unreinforce Concrete Pad Foundation

The main feature of unreinforced concrete pad foundations is that theconcrete does not carry any reinforcement.

4.1.1 Mass Concrete Pad Foundation

This kind of foundation usually carries very light loads. The foundation is

designed to be of such a thickness that no tensile stresses will develop onthe underside. This can be achieved by making the length of the projectionequal to the depth of the foundation, on the normal practice of a 45distribution of loading.

4.1.2 Mass Concrete Pad Foundations with Grillage Base


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These foundations carry heavily loaded structural steel columns(stanchions). They usually have a very large area and the steel grillage isdesigned to transfer the load to the ground. The function of the concrete inthis case is to protect the steel against corrosion.

The grillage may consist of two or three tiers of universal steel beamsplaced across each other. These beams are spaced such that the concreteis able to flow between and underneath them and, at the same time, be fullycompacted.

4.2 Reinforced Concrete Pad Foundations

These foundations carry reinforcement and placed at the bottom of thefoundation to resist tensile stresses. Compression and shear reinforcementare normally not provided.

4.2.1 Reinforced Concrete Plain Pad Foundations

They are usually square in plan. The plan area of the base is determined bydividing the total in-service loads of the column and the base by theallowable load-bearing pressure on the soil.

The thickness of the pad, which is designed to resist punching shear,should be checked for beam shear stress at critical sections. This is toensure that no shear reinforcement is required. The amount of tensilereinforcement in the base is designed to resist tensile stresses caused by

bending moments in the base.

4.2.2 Reinforced Concrete Pad Foundations with Sloping Top Face

They are similar to plain pad foundations. They are based on the samedesign principle as plain pad foundations, and are square in plan. The onlydifference between this foundation and the plain pad is that the top face ofthis foundation has a sloping surface. The greatest depth of a sloping facefoundation is at the column face, and the least depth is at its perimeter. Thereason for having a sloping face is to reduce the quantity of concrete used.

4.2.3 Stepped Reinforced Concrete Pad Foundation

This foundation has a stepped top surface. This reduction in depth alongthe edges of the foundation reduces the quantity of concrete used.However, although less concrete is used, the cost of formwork mayincrease.


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4.2.4 Eccentrically Loaded Reinforced Concrete Pad Foundations

They are required for peripheral columns which have to be built close toadjacent buildings. They are rectangular in plan and may have a plain or

sloping top face.

They are designed on the same principles as the other types of padfoundations except that the columns are eccentric (that is, the column iscloser to one edge of the foundation than to the other).

5.Strip Foundations

Strip foundations are used to transfer the load from a wall, or from asuccession of closely spaced piers or columns, to the ground. They consistof a continuous ribbon-shaped strip formed of either unreinforced concrete

or reinforced concrete.

5.1 Unreinforced Strip Foundations

They are constructed of either mass concrete or brickwork; they do notcarry any reinforcement.

5.1.1 Plain Mass Concrete Strip Foundations

These foundations carry very light loads. Site conditions govern theminimum size and the depth of the foundation. The trench for the foundation

must be excavated by hand, and the strip must be wide enough for abricklayer to lay the footing courses.

For general guidance, the minimum thickness of unreinforced stripfoundations for different types of walls is shown:

Type of Wall Thickness

Load-bearing walls

Not less than150mm or the

length of projectionfrom the face of thewall.


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Sleeper wallssupporting onlysuspended groundfloors.

Not less than75mm

Partition walls notmore than onestorey high.

Not less than100mm

5.1.2 Deep Mass Concrete Strip Foundations

They are excavated by mechanical excavators. The plant normally used is a

backacter with a narrow bucket.

The excavated trench should be self-supporting until the foundation isconcreted. This restricts its use in self-supporting soils such as stiff clays. Insoils such as soft clays and loose sands, which require support by closetimbering, it would be uneconomical to form deep strip foundations.

The depth of the foundation varies. The width of the foundation depends onthe thickness of the superimposed wall or on the width of the bucket of themechanical excavator.

5.1.3 Stepped Concrete Strip Foundations

Where the structure is being built on sloping ground, it may be necessary tostep the foundation along its length. This reduces the volume of excavation.

The steps should be lapped for a distance equal to the thickness of thefoundation, or twice the height of the step, whichever is the greater. Specialprecautions are necessary if the step is greater than the thickness of thefoundation or if steps are formed in deep strip foundations. Theseprecautions may be in the form of providing reinforcement at the steps toprevent cracking.

A stepped concrete strip foundation may also be stepped along its cross-section. This may reduce the quantity of concrete used. However, the totalcost of construction may be reduced.

5.2 Reinforced Concrete Strip Foundations


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These are strip foundations reinforced both in the longitudinal andtransverse directions. They are more economical than unreinforcedconcrete strip foundations, when used in shallow wide strip foundations,because less concrete and less excavation are required.

A 50 to 75 mm binding layer should be placed as soon as excavation iscompleted. The binding layer consists of lean concrete and protects thereinforcement against contamination by the soil. It also provides a cleansurface for tying the reinforcement and a hard surface for placing thespacers.

The minimum concrete cover to the bottom reinforcement is usually 40mm.

5.2.1 Rectangular Slab

They are similar to mass concrete strip foundations except that they arereinforced.

The reinforcement takes the form of mild steel or high tensile steel bars.Transverse reinforcement is designed to resist tensile stresses caused bybending moments at the base of the foundation. Longitudinal reinforcementis also desirable because it enables the foundation to bridge local hard orsoft spots in the soil at foundation level. Shear reinforcement is usually notrequired because the thickness of the foundation is designed to resistshear.

5.2.2 Inverted T-beam

They are required if the soil conditions vary at foundation level, over thelength of the wall. This type of foundation may also be required if thefoundation has to bridge services such as water mains or drainage pipes.

The bottom slab is reinforced with transverse and longitudinalreinforcement. The web (the up-stand beam) is reinforced with mainlongitudinal reinforcement together with binders.

6.Combined Foundations

The base area of any foundations must be large enough to distribute theload of the foundation to the soil. In some cases, strip or pad foundationscannot provide the required base area. This situation usually arises withperimeter columns which are close to adjacent buildings.

A combined foundation is one in which the exterior foundation is joined to


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the interior foundation. Combined foundations resist the tilting tendencycaused by eccentrically loaded exterior columns. This type of foundation isalso suitable when the space between individual footings is small. If thefooting is less than the width, it would be more economical to combine the

foundations.

As all combined foundations encounter both negative bending momentsbetween the columns, adequate reinforcement has to be provided not onlyto the bottom face but also to the top face of the foundation. In some cases,shear reinforcement, in the form of binders or bent up bars, may also benecessary.

6.1 Trapezoidal Combined Foundations

These foundations are trapezoidal in shape or plan. The centre of gravity of

the columns and the foundation should coincide in order to eliminateeccentric loading.

Trapezoidal combined foundations are required in the following situations:

where the length of the foundation is restricted; or

where the outer or inner column carries the heavier load. The widerend of the foundation is located nearest to the column carrying theheaviest load.

A trapezoidal base helps to distribute the load to the soil uniformly, eventhough the columns do not carry the same load.

6.2 Rectangular Combined Foundations

They are rectangular in plan. They are much easier to construct thantrapezoidal combined foundations. The symmetrical shape of therectangular combined foundation facilitates certain tasks, such as settingout the foundation, excavating, fixing formwork, and cutting and bending thereinforcement.

If the two columns carry different loads, then the additional bearing arearequired for the heavier column can be obtained by extending the base nearthis column whilst keeping the sides of the foundation parallel.

6.3 Cantilever Combined Foundations

Cantilever combined foundations may consist of separate bases, or a slab


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base with a beam over it.

These foundations are specified where it is necessary to ensure that noadditional load is imposed on an existing adjacent foundation or services

such as drains, sewers or water mains.

6.4 Balanced Base Foundations

They consist of two pad foundations connected by a beam. The perimeter(outer) column is usually eccentric on its base. This creates a tendency forthe base to rotate. The rotational tendency can be resisted by the innercolumn base and the connecting beam.

7. Raft Foundations

A raft foundation is a continuous foundation in two directions, usuallycovering an area equal to or greater than the base area of the structurewhich it carries.

They are useful in the following cases:

for lightly loaded structures on low bearing capacity ground; for heavier structures such as multi-storey buildings; where mining subsidence is likely to occur; and where the column loads and/or soil conditions are such that

the resulting footings occupy most of the site.

7.1 Plain Slab Rafts

Plain slab rafts are suitable for lightly loaded structures such as smallhouses. They are also be used for heavier structures if the ground conditionis good and no differential settlement is expected.

A plain slab raft consists of a reinforced concrete slab, usually slightly largerthan the area of the building. Reinforcement in the form of a mesh fabric isprovided on both the top and bottom faces of the slab.

7.1.1 Plain Slab with Integral Projecting Toe

This is used as a ground floor slab. The peripheral part of the slab isstepped down to prevent ingress of water. A stiff beam is also formed withinthe stepped down portion of the raft.

7.1.2 Plain Slab with Separate Wide Strip Foundation


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The plain raft foundation rests on top of the strip foundation provided for theperipheral wall. The strip foundation also stiffens the edge of the raft, but toa lesser degree than the integral beam.

7.1.3 Plain Slab with Stiffened Edge

It has a deeper and more reinforced edge beam. The plain slab raft withstiffened edge is suitable in soils of high to very high compressibility, suchas soft peaty clays or fill material.

7.2 Slab and Beam Raft Foundations

They are used where poor soil conditions are encountered. The slab andbeam provides stiffness and prevents the distortion of the building.

7.2.1 Down-stand Beam Raft Foundations

They are suitable in stiff clays. Difficulties can arise if water bearing soilsare encountered. The main advantage of the down-stand beam raftfoundation is that it provides a level surface slab which can form the groundfloor of the building. Another advantage is the saving in excavation costs.

7.2.2 Up-stand Beam Raft Foundation

They involve extensive earthworks. The foundation not only has to beexcavated, but also has to be backfilled to form the ground floor slab.

However, the up-stand beam raft foundation provides a usable void belowthe ground floor if a suspended ground floor slab is used.

With both down-stand and up-stand beam raft foundations, the columnsmust be positioned at the intersection of the beams.

7.3 Cellular Raft Foundations

They consist of two reinforced concrete slabs linked by internal walls whichdivide the void into cells. This type of foundation is suitable where poor soilis encountered at a shallow depth, and where it would be uneconomical to

use slab and beam raft foundations.

Back Next Home


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Piling System I


1. References

R. Chudley, Construction Technology, Volume 1 to 4, 2nd Edition,Longman


Students should master the following issues after the lectures,

1. Choice factors for selecting different piling systems.2. Bored piles3. Caissons4. H-piles5. Limitations of each systems.

The material will be covered by two lectures. The first lecture covers thefollowing:

The choice factors

Bored Piles

Caissons (Part 1)

Emphasis of the study should be given on the choices factors, thelimitations of each system, and the applicable situations of each pilingsystem.

3. The Choice Factors

3.1 Reasons for Piling

Where weak strata over lies firm stratum, piles can be used to reachthe firm stratum by by-passing all the weak/stratum.

Where concentration of loading occurs, it is best dealt with by pilingbecause it is most economical to transfer a load directly from thepoint of application to the bearing stratum.


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When uplift to building may occur, friction piles may be used toovercome the uplifting force.

When the loading is so high that other foundation methods would notbe appropriate. Piles supported on rock create the greatest bearing

capacity.

When the ground floor slab has to be carried above the ground, e.g.on a sloping site.

3.2 Factors Affecting Choice of Piles

Depth to be reached.

Loading requirements.

Soil conditions regarding strength, corrosion and groundmovement, etc.

Environmental restrictions e.g. noise and air pollution control.

Access of site may limit the use of long precast pile.

Congested or open site may limit the use of piling rigs.

Headroom restriction e.g. under a bridge flyover.

Effect on adjoining buildings, if adjoining buildings areunstable, excessive vibrations is to be avoided.

Piling plant and equipment available.

Reliability of types of piles and the expertise and familiarity ofthe specialist sub-contractor.

Time available for completion of the piling contract.

Cost per unit length of pile.

4. Statutory & Q.C. for Piles

In Hong Kong, in-situ replacement piles have come into increasing use inrecent years for stability of foundations. To prevent possible pile failures,


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statutory and quality control become necessary.

4.1 Statutory Control (HK)

The statutory control of all piling work in Hong Kong is laid down clearly inthe Building (Construction) Regulations 1995, Chapter 123, Section B,Clause 35.

In general, the foundation of a building should safely sustain and transmit tothe ground all dead load, wind load and imposed load of the building withoutimpair the stability of the building and any adjoining buildings. Clause 38particularly concerned with caissons which include bored pilesexceeding 900mm in diameter. The one that we should pay particularattention is clause 38(5)which states that "where any doubt exists as to thecapacity of any caisson to sustain, adequately and without undue

settlement, the load for which it has been designed the caisson shall betested:-

i. By means of core drilling of the completed in-situ concrete.ii. By any other method to the satisfaction of the Building Authority, in

which case, the Building Authority shall determine the standard ofacceptance to be adopted.

4.2 Pile TestingThe main objective of forming a test pile is to confirm that the design andformation f the chosen pile type is adequate.

4.2.1 Coring TestThis test is used to check the strength of the concrete and the intersectionbetween the caisson base and the rock stratum. It can be used to detectvarious defects occurring in concrete, e.g. honeycombing, segregation,voids, cracks, etc.

4.2.2 Loading TestA loading test is made usually for one or other of the following reasons:

To determine the load-settlement relationship, particularly inthe region of the anticipated working load.

To serve as a proof test to ensure that failure does not occur

before a load is reached which is a selected multiple of thechosen working load.

To determine the real ultimate bearing capacity as a check onthe value calculated from dynamic or static formulae, or toobtain information that will enable other piles to be designed


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by empirical methods.

4.2.3 Integrity TestsThey are used to check the soundness of the caisson shaft and the quality

of the concrete. They are generally very rapid, cause minimal disruption tothe site and are relatively inexpensive so that large numbers of piles on asite can be economically examined. There are two main groups of tests:Sonic Test and Vibration Test.Sonic Test

It is based on measuring the propagation time of a sonic signal between twovertical tubes cast into the pile during construction.

Vibration TestIn this test, an electro-dynamic vibrator is placed on the head of the

caisson, applying a constant sinusoidal force of 50N within the range 20-5000 Hz. The head of the pile simple moves up and down as the frequencyas the vibrator where the velocity of this movement is measured by thetransducer for analysis.

Bored PilesThere are various methods of installing bored piles. Two methods ofproviding this are:-

by using a steel casing, for example, with small diameter boredpiles; and

by using drilling mud (bentonite) to support the excavation, forexample, with large diameter bored piles.

5.1 Small Diameter Bored Piles with Temporary Casing

Small diameter bored in-situ pile range in diameter from 300 to 950 mm andis designed to carry loads of up to 1500 KN.

Crane mounted or lorry mounted rotary drilling rigs or a percussion boringtool is used to excavate these piles. The stages of installation using a tripod

or shear leg percussion boring tool are described below.

Stage 1The percussion tool, consisting of the tripod or shear leg, a winch and thecutter, are set up. A starter hole is then made by dropping the cutter fromthe raised position.


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Stage 2The first section of casing, which has a cutting edge, is placed in the starterhole. This section of casing is known as the cutter casing. The casing,which varies in length from 1 to 1.4m, can be extended by screwing on

additional lengths.

Stage 3Additional lengths of casing are screwed on to the cutter casing. A drivingcap is placed on top of the casing, and the casing is driven ahead of boring.The casing is driven by using the weighted head of the percussion toolwhich acts as a drop hammer.

Stage 4Soil is bored out from within the casing by means of the percussion toolcutter.

Stage 5Stages 3 and 4 are repeated and the boring and casing extended to therequired depth.

Stage 6A steel reinforcement cage is placed in the borehole and a high slumpconcrete is then placed into the temporary casing. The concrete should beplaced by means of a tremie pipe or by trunking.

Stage 7

The temporary casing is extracted when concreting is completed.

This type of pile with temporary support can be very long, and the lengthcan be varied to suit different site conditions. The pile can be constructed inrestricted headroom, or confined sites, and within centimetres of existingstructures and old sewers without damaging their stability.

The problems associated with small diameter piles are the limitations in thediameter of the pile (maximum diameter 600mm), the difficulty of positioningthe reinforcement cage correctly, and the possibility of causing settlement toadjacent structures.


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5.2 Large Diameter Bored Piles Supported by Bentonite

Large diameter bored in situ piles range in diameter from 1000 to 3600mm,and are designed to carry loads of up to 30,000KN.

Truck or crane mounted rotary drilling rigs or grab rigs with or withoutcasing oscillators are used to excavate these piles.

The four stages in the installation of these piles using a crane mountedrotary drilling rig are described below.

Stage 1A short length of casing is pitched in the required position. Excavation iscarried out within the casing by means of a helical auger (or grab), and thecasing is then inserted into the ground. This short length of casing preventssurface water and debris from entering the borehole. It also prevents thecollapse of the loose surface soil at the mouth of the borehole, and the lossof the bentonite through the loose surface soil.

Stage 2The borehole is filled with bentonite suspension from storage tanks. Whenmixed with the correct amount of water, bentonite exhibits thixotropicproperties. Boring proceeds through the bentonite, which is fed continuouslyinto the hole during boring.

Stage 3


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Boring stops when the desired depth is reached. The auger is thenwithdrawn. A reinforcement cage is lowered through the bentonite andconcrete is placed through a tremie pipe. The bentonite, which is displacedby the concrete, is pumped back into storage tanks. The bentonite can be

strained to remove soil particles, and then re-used.

Stage 4On completion of the concreting, the tremie pipe is removed and the shortcasing is withdrawn. Large diameter bored piles, the maximum diameter ofwhich can be 3.6m, the pile length can be varied to suit site conditions, andpiles can be as long as 60m.

However, care must be taken to ensure that the reinforcement cage iscorrectly positioned to provide adequate cover to all the reinforcement.

5.3 Equipment for Installing Bored Piles

Rotary Drilling Rigs and Augers

The drilling rig may be mounted on a mobile crane, or a truck. The drillingrig consists of an auger, mounted on a telescopic or extendable kelly bar.The kelly bar is a square hollow steel shaft which is generally about 7.5mlong, but can be extended if required.

Crane and Grab Rigs and Casing OscillatorGrabs consist of clamshell buckets which are opened and closed by means

of wire ropes, or hydraulically. They are suspended from mobile cranes.

The grab is dropped on to the soil in an open position and is then closed. Itis then raised to the surface and emptied at the side of the shaft.

The temporary casing is driven into the soil by means of an oscillator, whichis hydraulically powered. The oscillator clamps itself to the casing and, by acombination of rotating and pushing, forces the casing into the ground. Theoscillator is also used to withdraw the casing on completion of theconcreting of pile.

Topic

Pile Type

Small Diameter Bored PilesLarge Diameter BoredPiles

Noise Pollution Very little Very little


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Possibility of installing in restrictedheadroom

Yes, possible No, not possible

Possibility of ground heave No risk No risk

Possibility of necking or waisting May occur"Necking or waisting mayoccur

Possibility of forming a large diameterpile

Not applicable Yes, up to 3.6m

Ease of quality control of the pileNo, difficult tocontrol

No, difficult to control

Special equipment requiredNo specialequipmentrequired

No special equipment

required

Possibility of varying the pile length onsite

Yes, possible Yes, possible

6. Hand-Dug Caissons

In Hong Kong, Hand-dug caisson is a term generally used for what otherwould called a Hand-dug pile. A hand-dug caisson is a cylindrical shaftformed in the ground, with openings at both top and bottom. The shaft isexcavated in stages by hand and the wall of the shaft is lined with 75 to

100mm thick in-situ concrete as excavation progresses. If the water table ishigh, the thickness of lining may be increased to 150 or 200mm at depths of30m.

In Hong Kong, hand-dug caissons are often used instead of large diametercast-in-situ piles. The diameter of the caisson ranges from 800 to 3000mm.It is common practice to excavate the shaft to a depth of 45m. This mayinclude 20m or more, below the water table.

6.1 Why Hand-dug Caisson is Popular in Hong Kong

Various Soil ConditionsThe geological conditions in Hong Kong varies drastically from slopedgranolithic sites near the foot of the Victoria Mountain in Hong Kong Islandto some of the sandy-silt areas in Yuen Long. Hand-dug caissonconstruction is more flexible and it can adjust the method of excavationeasily from simply hand-digging for soils to rock drilling.


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Comparative Low Plant or Equipment CostBoth bore piling construction and pre-cast pile driven construction willrequire the use of heavy machines. However, Hand-dug caissonconstruction requires only simple equipment. Thus the plant cost for this

particular construction method is comparatively low.

Availability of Manual LabourFor each caisson, two workers are usually required. One on the groundlevel receiving the steel bucket for discharge while the other is workinginside the caisson. The cost of caisson is the cheapest among all type

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