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SITE CLEARANCE

Foundations

S E C T I O N

1 MODULE 1SHEET 1S E C T I O N

6

Groundworks

Before any foundation construction can begin on site a number of preparatory activitiesneed to be undertaken. These activities are generally termed groundworks.

Typical groundwork activities include: Site clearance. Site preparation. Excavation of foundation trenches. Groundwater control.

Site Clearance

It is normal practice for any above ground clearance requirements, such as removal oftrees, bushes, walls, slabs, buildings etc to be shown on a site plan. This plan may showthe position of services, both above and below ground, allowing these to be cleared ifnecessary or avoided for safety if to be left alone.

Construction sites vary a great deal, and clearance procedures will, of course, be differenton each one. A greeneld site out of town may only require soil removal, whereas abrowneld site, having been previously developed, may need extensive demolition andclearance of concrete slabs etc before any excavation can commence.

Contaminated Ground

Due to the increase in use of browneld sites, and regulations imposed by the government,many sites have to be decontaminated before structures can be built on them. This ofteninvolves the contaminated soil being totally removed from site and being replaced by cleansoil when the site is landscaped.

Protection of Trees

Trees may have a preservation order placed on them by the local authority. This protectsthe tree from displacement or removal. The removal of other trees generally is subjectto agreement with the planning authority before the work on site commences (usually atplanning stage).

When trees have to be protected, a barrier should be erected around each tree to preventaccidental damage. Care should be taken to avoid damaging branches that overhang theworking area.


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Removal of Waste

Waste material can be removed by lorry or skip to a licensed landll site, or if the waste isinert, such as brick rubble or subsoil, it can be used as ll in certain earthworks. This willgenerally reduce the cost of removal of the waste to the contractor. Burning of rubbish onbuilding sites is not normally allowed.

On browneld sites, where masonry and concrete demolition is required, it is oftencost-effective for the material to be crushed and used for hardcore in the contract (ifacceptable to the architect and building control).

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A concrete crushing machine being loaded with demolished material


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Removal of Vegetable Soil

The removal of vegetable soil, or what is commonly known as top soil, is an essential taskto be undertaken before any further work can be carried out with the foundations.

The Building Regulations Part C states that the ground to be built upon must bereasonably clear of vegetable soil and matter.

It may not be necessary to remove any vegetable soil if the work is of a redevelopment oralteration nature.

Vegetable soil is soft, easily compressed and rich in organic material. These propertiesmake it totally unsuitable for building on.

Vegetable soil is found at a variety of depths from about 150mm to 300mm, and has usuallybeen naturally building up for many years.

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This photograph shows the organic vegetable or topsoil layer on top of the lighter colouredsubsoil below.

The vegetable soil is excavated separately from the subsoil excavation, and the materialis usually stockpiled in a spoil heap. This stockpile is sited on an area of the site whichis not required for building operations. The vegetable soil is then used for landscapingpurposes when the building is completed externally, any shortfall being brought in from

external sources.

Alternatively, excess soil may be sold or simply transported to another site.


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Reduced Level Excavations

Removal of the vegetable soil often results in a level subsoil base, through which furtherexcavation work can be undertaken. It may not be necessary to remove any further soilfrom the buildings base area, other than foundation trenches.

If the area is not left reasonably level after vegetable soil removal, further excavation will benecessary. This process involves nal grading and levelling of the subsoil base to prepareit for the oor slab of the building.

Grading and levelling is carried out by the use of any of the following methods: Cut. Cut and ll. Fill.

Cut is the process of removal of high ground to produce a level work area. This methodresults in a solid and compact base.

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Cut and ll techniques are used to provide a level base for construction on sloping sites

(see gure above). This method requires compaction of lled areas of soil, in layers. The soilfor lling may be won from the cut areas. The cut area will be already sufciently compact.

Fill is used when part of the site is below the required level, and soil, or other suitablematerial, is imported to the site to raise the work area. This method requires compaction ofthe lled area in layers.

After completion of the nal grading and levelling of the site, the excavation of foundationscan commence.


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Trench Excavation

Trenches must be excavated for foundations, drainage pipes and services.

On small scale works, such as extensions to houses etc, hand tools such as pickaxes,spades, shovels and wheelbarrows are used for manual excavation of soil.

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When the depth of excavation exceeds 1.2 metre, and/or the excavation becomes larger,mechanical methods of soil removal must be employed.

Factors that inuence the choice of methods for excavation include: Volume of subsoil to be excavated. Nature of the site. Type of soil (e.g. stone requires mechanical excavation). Time constraints (how much time available for excavation). Safety of operatives.


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Mechanical Excavators

The most common machine used for the excavation of trenches is the backactor. Thereare many types of machine in a variety of sizes. The backactor has a clawing action thatscoops soil from an excavation and deposits this, either in a temporary spoil heap, or into adumper/lorry for disposal elsewhere.

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The wheeled backactor (shown above) is very versatile and can move about a site quickly.

The tracked excavator with backactor can turn through 360, and is usually larger allowinggreater volumes of soil to be excavated in a given time period.

The backactor can be tted with various size buckets. The choice of which will dependupon the width of trench required.

Modern backactor machine (wheeled) the photo on the left shows the backactor bucket clearly

Large tracked backactor excavator


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6SITE CLEARANCE

Groundwater Control

Care must be taken when excavating soil as it is possible that the water table level may bereached. This is the point at which the water level stands naturally in the ground.

The amount of water (and therefore the water table level) in a given soil depends on theseason, the composition of the soil and the nature of the land. Low lying areas are likely tohave a high water table, whilst higher ground may typically have a lower water table.

Water in soil can be classied by its position in the ground:

Water can be classied as either: Surface water. Ground water. Subsoil water.

Water table

Upper levelof waterheld in thesubsoil

Surface water

run off from thesurface

Ground water water held in thesubsoil

Subsoil water water held in theground belowthe water table

Rain, snow etcon the surface


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Problems Caused by Subsoil Water

The following are potential problems caused by the presence of water in the subsoil ofa construction site. Subsoil water can cause problems during excavation works by its natural tendency

to ll in excavations due to hydrostatic pressure i.e. it will ow from the ground whereit is under greater pressure into open space.

A high water table may cause excessive ooding of the site (and the nished works)during wet periods.

Water can cause an increase in humidity around completed substructures of buildings. Water can be a serious hazard in excavations. Construction in waterlogged trenches is difcult and may result in weaknesses

in the substructure.

The contractor must, therefore, resist the ingress of water into excavations as much aspossible. This is typically done by employing pumps.

A vacuum piston pump being used to reduce ground water from an excavation


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Denition of the Term Foundation

A foundation is that part of a building or structure which is in direct contact with theground and which transfers the loads imposed upon it to the subsoil beneath.

The foundation that is constructed as part of the building or structure is termedthe articial foundation, whilst the ground that supports this is termed thenatural foundation.

TYPES OF FOUNDATIONS

Purpose of a Foundation

The purpose of a foundation is to spread the load, from the structure above, over anadequate bearing area of the subsoil, and to provide a stable, level base on which to build.

Although foundations are normally out of sight, the effect of any movement of a foundationis likely to be very noticeable in the structure above.

The foundation is sandwiched between the forces from the structure and those from theground thus subjecting it to compressive forces, so the foundation must be constructed froma material that is strong enough to resist any forces being imposed upon it.

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Foundations

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6TYPES OF FOUNDATIONS

Foundations are required to prevent undue settlement and consequent fracture of thebuilding due to unequal loads and stresses at various points and to distribute these loadsevenly over an area.

The foundation then, requires both strength and stability to full its role.

The strength is related to the materials used to construct the foundations, but the stabilitydepends on the way in which the foundation transmits the load to the subsoil, and the wayin which the subsoil reacts.

The strength of a material is a measure of its power to resist forces placed upon it. Somematerials will resist both tensile and compressive forces; others will resist compressive butnot tensile forces.

It is therefore very important to know both the nature of the forces acting on a material andhow the material reacts to such forces.

The forces which act on a foundation are the weight of the building acting downwards, andthe supporting resistance of the ground underneath which acts upwards (see gure above).

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Spread of Loads on Foundations

The manufactured material which is used to carry the weight of the building is usuallyconcrete. Concrete is stronger than the natural soil which has to support it, so that theforces acting through the foundation may have to be distributed over a wider area of subsoilto support the weight.

It is usually assumed that this spreading of the forces through the foundation happenswithin an angle of 45 from the vertical.

As long as the foundation does not extend beyond this 45 line at its base, the forces within

it can be assumed to be compressive only.

To maintain this, however, when the wall becomes wider, the foundation should alsobecome wider and deeper.

Load

Wall

Foundation

Spread of compressive forces in the natural foundation

45


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Spread of Loads on Foundations

As stated previously the effect of spreading the load through 45 will affect the width andthickness of foundation as the load being carried becomes wider.

The effects of spreading of loads on foundationsThis widening and deepening of the foundation is unlikely to be economical though.

The amount of excavation and concrete required would result in a very heavy andexpensive structure.

Solving the problemThe solution is to make the foundation wider without making it thicker. This will ensure the

load is spread over an adequate area of ground to support it safely.

In this case, there is a tendency for the foundation to bend across its width and this in turnresults in tensile forces in the bottom of the foundation.

In this case, a material which is strong in tension and compression must be used toreinforce the foundation.

The material most commonly used is steel reinforcement. Steel reinforcement can be eithersteel bars or steel mesh which is encased within the concrete.


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MODULE 2SHEET 13

Bearing Pressure

The bearing capacity of a foundation is the force per unit area imposed by the foundationon the subsoil beneath it.

The bearing pressure of the subsoil is the force per unit area that the subsoil is capableof supporting.

Factor of Safety

Subsoil is a mixture of solid particles, air and water. As with any other material it has to be acertain strength to resist compressive forces, but it will fail if it is overloaded.

In order to provide a factor of safety, it is usual to design a foundation so that thebearing pressure on the ground, imposed by the structure through the foundation, is aboutone third of the bearing pressure of the subsoil.

Since on any particular building site it is necessary to take the subsoil as it is found, it is thefoundation which has to be designed to provide a safe bearing pressure.

This can be carried out in one of two ways:1. By spreading the load over a wide area so that the force per unit area imposed upon the

soil is reduced.2. By taking the load to a greater depth where the subsoil is stronger and the force per unit

area that the subsoil can carry is greater.


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MODULE 2SHEET 14

Settlement and Heave

The pressure which the foundation exerts on the soil will cause it to compress andmove downwards, taking the foundation with it. This downwards movement is known assettlement, and one of the functions of a foundation is to ensure that uneven settlementacross a building does not occur.

Very little of the settlement of the soil under the weight of the building is due to thecompression of solid particles in the soil. Almost all of the settlement is due to the waterbetween the particles being forced out, enabling the solid particles to pack closer together.

Therefore anything else which changes the amount of water in the subsoil will causesoil movement.

When water freezes, it expands; in very cold weather, the water in the subsoil may freezeand cause the soil to heave, i.e. move upwards. When this occurs beneath a foundation,the pressure exerted could be sufcient to lift the foundation.

Wall tends to tiltwhen ground moves,and cracks can occur

Soil exposedto elements

Depth of foundationbase 600mm or lessbelow ground level

Roof and oor ofthe building protectsthe ground below fromthe elements

Foundation will tendto tilt since the groundmovement is greaterat the outer edge


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Further Factors Affecting the Depth Required for Foundations After a long dry spell of weather, cracks sometimes appear in buildings which have stoodfor many years, indicating that the foundation has moved.In this case, the cause is likely to be that the water in the subsoil has dried out and the soilitself has shrunk.

Tree roots tend to accelerate this process by absorbing water in the soil and consequentlydrying it out. The roots of certain trees can often extend out in all directions greater thanthe tree.

When a site has been cleared of trees, the reverse can happen. The roots no longer dry outthe subsoil which in turn becomes wet and swells, resulting in heave, and possible crackingof the structure.

Cracks in external brickwork are often stepped and diagonal. The walls tend to be drawnoutwards and downwards by the movement in the ground. Care must be taken whenbuilding near trees.


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The Solution to Movement of the Structure Due to Inadequate Depth

To avoid settlement or heave due to the effects of frost (or to extreme changes in watercontent, more likely in clay subsoils) the foundation must be taken down to a greaterdepth. For some soils this may need to be 600mm deep, and for clay the depth must be aminimum of 1 metre.

Benets of ensuring the foundation is at an adequate depth

Wall remains stableunder mostconditions

At least1.00m

Roof protectsthe ground oorfrom elements

Note! No groundmovement shouldoccur if thefoundation is ata depth of morethan 1.80m

Depth at least 1.00m below ground level will produceonly very slight movement which will not affect foundation


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Typical Failure of Foundations

Strip foundations can fail due to cracking at their base induced by bending of thefoundation, or due to shear cracking when it becomes overloaded.

Reinforcing the foundation to resist the forces that cause bending and cracking

The foundation can be reinforced by inserting steel reinforcement bars or mesh into theconcrete. This will reduce the tendency of the foundation to crack or bend.

Foundation reinforced with steel

Load

Tension cracks appear whenstress exceeds the concretesown resistance. Failure willoccur through bending

Load

Strip foundationfails throughshear force

Un-reinforced strip


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Foundation Types

Foundations are generally described by their shapes.

Pad foundationFor single loads which are transmitted down a brick pier, concrete column or steelstanchion, the most common foundation is a square or rectangular block of concrete ofuniform thickness known as a pad foundation.

In order to spread the load over a greater area it is necessary either to make the pad thickeror use reinforced concrete.

Pad foundation


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Strip FoundationUnder a wall, where the loading on the foundation is continuous, the support is commonly acontinuous strip of concrete known as a strip foundation.

The width of the strip will depend on the load to be carried and the strength of the ground.

The traditional or shallow strip foundation consists of a continuous strip of un-reinforcedmass concrete under the walls. This type of foundation is suitable for most subsoils andstructural loadings associated with low to medium rise domestic and industrial buildings.

In order to reduce failure through bending and shear, the foundation concrete has to bea minimum 150mm thick, and the projection from the face of the wall to the edge of thefoundation concrete has to be at least equivalent to the concrete thickness. The diagrambelow shows the dimensions which will satisfy the current Building Regulations.

The depth D must be 150mm minimum and must be equal to, or greater than, the projectionP and the distance P must be equal on either side of the wall.

Strip foundation

Depth to suitground conditions


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Wide strip foundationsIf the use of traditional strip foundations is likely to overstress the bearing strata (the layer ofground on which the foundation sits), a wide strip foundation can be employed.

The wide strip foundation would be designed to transmit the foundation loads across thefull width of the strip.

This might necessitate the foundation thickness being increased to overcome the bendingand shear stress.

In some cases, the thickness of concrete could become uneconomic and so reinforcementis used. Both longitudinal and transverse reinforcement are added to ensure the appliedloads are supported without failure of the strip.

Longitudinalreinforcement

Transverse reinforcement


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Stepped strip foundationOn sloping sites it is usual to step the foundation to follow the line of the ground andtherefore reduce the amount of excavation required.

A second benet of using stepped foundations is that the foundation is more likely toremains seated in the same strata of ground, avoiding potential differential settlement.

When constructing stepped foundations, the construction starts at the lowest level ofthe site.

Building regulations relating to stepped foundations states:

The minimum overlap L should be equal to twice the height of the step S or the foundationthickness T or a width of 300mm whichever is the greatest.

Height of step = threecourses 225mm

Height of step = twocourses 150mm

Steps should be constructedto gauge of brickwork

or blockwork to avoidunnecessary cutting


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In some cases, the working space required to build on top of a concrete strip foundationwould make the strip wider than it would need to be to carry the load.

In these circumstances, an economical alternative is the narrow strip foundation(or trench ll foundation as it is sometimes known).

A narrow strip is excavated by the mechanical excavator and backlled with mass concrete.

The bricklayer can then build off the foundation from ground level.

A high standard of accuracy in constructing such a foundation is required, particularly ifdrains are to be let through the foundation.

It is normal practice to box out drainage holes across the trench when pouring the concrete;this allows any drainage pipe or other services to pass through the foundation without riskof fracture at a later date.

This foundation is often used where tree roots exist, the face of the trench being lined withpolythene sheeting before pouring the concrete. This ensures that the concrete has a moresmooth and dense surface. This tends to divert roots and avoids any potential root ingressinto any holes in the foundation.

Narrow strip or trench ll foundation


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Short bored pilesIt is sometimes necessary to avoid constructing foundations on soil close to the surface.

This is usually due to poor bearing capacity of the soil. If this so then one alternativemethod is to transfer the load on the foundation to a greater depth. In this situation aneconomical solution is the use of a short bored pile foundation.

Short bored piles are formed by boring circular holes 300mm diameter to a depth of about3m by means of an auger.

The holes are then lled with concrete and reinforced with steel bars which are leftprojecting from the top.

The piles are placed at the corners of the building and at intermediate positions alongthe walls.

The piles support reinforced concrete ring beams which are cast in place in the ground ontop of the piles. It is on to these ring beams that the brickwork is built.

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Raft foundationsRaft foundations are often used on poor subsoils for lightly loaded buildings and aredesigned to be capable of accommodating small settlements of the subsoil.

A raft foundation covers the whole of the oor and wall area of the structure. The reinforcedoor slab is generally thickened out substantially under all the walls, and the loads from thewalls are subsequently transferred into the whole of the slab.

The simplest and cheapest form of raft is the thick reinforced concrete raft. Its rigidityenables it to minimise the effects of differential settlement.

Thick raft foundation

Raft foundation with thickened edges used in poor soil conditions

No thickened toeon edge requiredfor lightweightbuildings

Reinforcedconcrete raft

No thickened toeon edge requiredfor lightweightbuildings


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6TRANSFERRING LEVELS

MODULE 3SHEET 25

Reasons for Transferring Levels

There are a number or reasons for the transfer of levels in substructure work. These include: To establish the level of the ground for excavation (both oversite and trenches). To ensure materials such as concrete and hardcore are laid to an accurate level. To establish datum for bricklaying, e.g. DPC level. To establish a level for concrete formwork, e.g. when constructing a concrete

raft foundation.

The following pages deal with the transfer of levels for work involving strip foundations andraft foundations.


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Transferring Levels from the Datum to the Foundation (Strip)

During the process of setting out, as the proles are erected to mark out the excavationlines and wall lines, a datum peg is positioned to mark a predetermined level, such as DPCor oor level. This is the level from which the bricklayer or ground worker will transfer levels,to control the depth of the foundations and brick footings.

To set the level of the top of a strip foundation, a spirit level and straightedge are placedon top of the datum peg and held steady whilst adjusting it to the horizontal (level). A tapemeasure or gauge lath is held vertically at the side of the straightedge, and a peg is driveninto the base of the foundation trench.

The aim is to set the peg to be at least the minimum specied thickness of concrete abovethe base of the trench, whilst working a distance that is gauge from the datum peg. Thisensures that the distance is a multiple of brick courses and that any later adjustment forgauge is minimal.

Transferring levels from datum to foundation strip

Datum peg

Multiple ofbrick courses

Concretethickness

Concretethicknesspeg

Gauge rod

Spirit level andstraightedge


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MODULE 3SHEET 27

Having set the rst peg to gauge from the datum, the level must be transferred all aroundthe buildings foundation to provide a guide for the top of the concrete when poured.

Optical levels are most commonly used to do this (see the levelling section in the SettingOut section). However, at times the, bricklayer will nd it more convenient to simply transferthe levels around the trench by the use of a straightedge and spirit level.

The straightedge and spirit level are used as shown in the gure below. It is essentialto REVERSE the level and straightedge at each peg reading. This ensures that anydiscrepancy in the straightedge or level is not repeated at every peg, but cancelled out atevery other reading.

Transferring levels around the foundation strip

Datum pegDepth to be a multipleof brick courses

Spirit level andstraightedge


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MODULE 3SHEET 28

Another method of transferring levels around a trench is by the use of boning rods.

Boning rods are made from timber and are usually painted so that they are easily seen.

Boning rods are typically used in threes, the middle one being known as the traveller.

Two known level pegs are required to be placed at either end of the trench. Boning rods areplaced on both pegs and held upright. The other pegs can then be determined by sightingin between the two xed boning rods onto the traveller as shown below.

When all of the level pegs have been positioned, the bottom of the trench can be trimmedto give the required depth of concrete, e.g. 225mm.Soil must not be used to backll any depressions.Extra concrete should be used in this case; so for economy, great care should be taken totrim the trench base as accurately as possible.

Use of boning rods within a trench

Line of sight

Levelling pegs

Boning rods

Traveller


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MODULE 3SHEET 29

Transferring Levels from the Datum to the Foundation (Raft)

A simple raft foundation covers the whole of the base area of the building, with a substantialthickening of the reinforced concrete slab at the edges, to support the walls. Other designtypes exist but the principle of transferring levels for these remains the same.

A typical excavation for a raft foundation at the edge may look like the drawing below.

As the edge of the slab must be supported above the ground, metal or timber formworkmust be xed temporarily at the side of the foundation trench to the height of the oor slabwhich will be at datum level.

As the formwork stands proud of the surrounding ground it is generally necessary tosupport it by strutting. This is to prevent the formwork moving when the concrete is poured.

Boning rods or sight rails (permanent horizontal rails often xed above the proles atcorners of the building when setting out) are set up, and the traveller is placed on top of theformwork when xing into place to determine the top level.

A typical excavation for a raft foundation at the edge of the foundation


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6 MODULE 4SHEET 30PLACING CONCRETE IN FOUNDATIONS

Types of Concrete Used for Foundations

Conventional concrete used in building projects such as foundations requires a certainamount of compaction to achieve strength and durability by turning it into a homogeneousmass. The typical method of compaction for reinforced concrete is to use a mechanicalpoker to vibrate it, whilst a simple strip foundation can compacted by method of handtamping. This method can prove to be a costly process and can lead to delays inplacing the concrete.

The concrete should be placed into the prepared foundation with care. The main objectivewhen placing the concrete is to place it as near as possible to its nal position, quickly andefciently, to avoid segregation or separation of the materials.

The size of the foundation and the amount of concrete required will often dictate whetherthe concrete is mixed on site or purchased from a company who produce ready-mixconcrete and imported to the site.

On sites where the amount of concrete is small, the concrete is mixed using a smallconcrete mixer and placed using wheelbarrows.

In shallow foundations two battens are often used as a chute to slide the concrete intoplace and the concrete is compacted by use of a shovel or hand tamp (heavy weight ona rod), before tamping it to level. Mechanical compaction such as a vibrating poker mayalternatively be used.

Placing by means of a wheelbarrow


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6PLACING CONCRETE IN FOUNDATIONS

Placement of Concrete in Strip Foundations If the foundation is a strip or trench then the sides of the trench are usually supported bymeans of wooden or metal formwork to keep the concrete in place. The insides of theformwork are kept apart and supported using trench struts.

Before placing concrete into the formwork, it is important to ensure that the inside ofthe formwork has been treated with a releasing agent, e.g. mould oil. This will allow theformwork to be removed after the concrete has set, without damaging the concrete.

It is also very important that the bottom of the trench be compacted to give a sound,rm base. If the base is soft it may be necessary to lay a hardcore foundation to the base.

In deep foundations it is necessary to use a chute to allow the concrete to slide down fromthe ready-mix lorry, dumper or barrow into the foundation as near as possible to itsnal position.

Trench struts Trench bottom being compacted

MODULE 4SHEET 31


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Placing, Compacting and Levelling Concrete in a Raft Foundation

If the foundation is a raft foundation without any edges being supported by earth on thesides, then the concrete is usually placed within simple formwork to prevent the concretefrom spilling. If the raft has an edge beam then the edge of the beam will be supported toprevent any earth spilling into the trench or slab.

Unlike strip foundations, which may often be deep within a trench, most raft foundations areat ground level or just below.

There should not be a danger of falling concrete being segregated, but chutes will still berequired to place the concrete into the centre of the concreted area.

Raft foundation edge supports

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Levelling the Concrete when Placed

If the width of the concrete slab is not too great, the concrete can be levelled using tampingboards across the formwork and either moved by hand or machine.

A mechanical beam vibrator/compactor can be used to tamp a larger slab. This reduces themanual work involved in the levelling of concrete.

A further benet of using a mechanical beam vibrator/compactor is that it helps toensure that the concrete is free of excess air, thereby making the concrete stronger andmore durable.

Beam compactor Mechanical beam vibrator

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Ready-mix Concrete

To ease the burden of mixing and placing and levelling concrete for foundations, mostbuilders use ready-mix concrete.

Ready-mix concrete is durable, versatile and highly economical.

Ready-mix concrete is a mixture of ne and coarse aggregate, along with water andcement and various additives. Once the ingredients are combined, a chemical reactionoccurs between the cement and the water and this paste hardens around and between theaggregates to form the hard mass known as concrete.

Different additives can be included in the mix to make the wet concrete more workable bymaking it more uid and increasing the setting time of the concrete without it losing anystrength when it is set.

This allows this type of concrete to be placed easily, usually by means of a chute or, insome cases, by pumping it into place.

New innovations in mix design have led to a series of new types of ready-mix concrete.

These new types of concrete have been designed to make the placement and compactionof foundation concrete easier and simpler by making them more uid, and in some cases,self-levelling.

Some of these new types include such names as Self-compacting concrete, Foundationow and Supao.

The advantages of these types of concrete over traditional concrete are: Speed of application. Accuracy of placement. Reduction in overall cost. Reduction in levelling and compaction time. Good surface nish.

Care must be taken when placing this type of concrete into foundation formwork as it is aheavy uid mixture that weighs approximately 2.3 tonnes per cubic metre.

MODULE 4SHEET 34


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A full load of 6 cubic metres can weigh 14 tonnes, and a fully loaded mixer truck can weighover 20 tonnes. Therefore care must be taken when bringing this type of truck onto thesite as it may get bogged down if the site is wet, or it might cause the sides of trenches tocollapse and dislodge the formwork if it is allowed to get too close to the trench.

This type of concrete needs to be laid on well compacted ground that has been coveredwith a sheet of plastic or polythene. This will prevent moisture loss into absorbent ground,thus preventing the concrete from drying out too quickly prior to levelling and nishing. Italso allows the hardened concrete to slide over the ground without undue restraint as itexpands and contracts due to climatic temperature changes.

Unless the topsoil can be compacted to give a sound base, it should always be removedand replaced with a foundation of hardcore covered with a layer of sand which should becompacted until rm.

MODULE 4SHEET 35

PLACING CONCRETE IN FOUNDATIONS


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MODULE 4SHEET 36

The Need to Cure Concrete

Concrete should be allowed sufcient time to set. The time needed for this depends uponthe type of concrete, its thickness and its position.

The atmospheric conditions when the concrete is poured, in particular, will affect the settingtime. When concreting in hot weather or windy conditions, there is a great risk of concretenot reaching its maximum potential strength, owing to early drying.

Concrete should be allowed to set and harden at its natural rate to create maximumstrength and impermeability.

To ensure that this happens, the concrete should be cured, i.e. prevented from losingexcess water too quickly which effectively stops the chemical process of hydration. This isessential for the concrete to gain maximum strength and durability.

Curing the concreteWater can be retained in concrete a number of ways, including: Covering concrete with damp hessian and spraying to maintain its damp state. Covering it with polythene sheeting. Spraying proprietary curing compounds onto the concrete to seal the surface.

Curing time, as a general rule, should be approximately seven days for horizontal surfacesand four days for vertical surfaces.

Protection from frost In inclement weather, it is necessary to cover the concrete to protect it from frost. Hessiansheets are also used for this purpose. However, in this case, the sheets should be as dry aspossible, rather than damp.


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Brickwork up to DPC Level on Strip Foundations

After the foundations have been concreted, the bricklayer has to set out the position of themain corners of the building.

When a site is set out (refer to the section on setting out), corner proles are erected toshow where the excavation should take place, as indicated below.

Then at this stage, with the concrete foundations placed, set and ready to be built on, thecorner proles are used again to support ranging lines. The proles will have been markedwith nails or saw cuts to indicate the positions of trenches and walls.

The rst step in establishing the wall positions is to attach the ranging lines to theappropriate marks on the corner proles.

These ranging lines will cross at the corners of the building to indicate the position of eachbrickwork/blockwork quoin.

At this stage the lines must be checked for accuracy. This is essential as the proles mayhave been disturbed during the excavation/concreting stages.

A spirit level is now used to plumb down from these lines to transfer their position to theconcrete foundation.

A mortar screed should be spread along the foundation, and thin lines cut into this with thetrowel to indicate the position of the lines.

MODULE 5SHEET 37

BRICKWORK BELOW GROUND LEVEL


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6MODULE 5SHEET 38BRICKWORK BELOW GROUND LEVEL

Building Corner Brickwork

The distance from the concrete to the datum should be a multiple of brick courses. Thedatum is usually set at DPC level.

If this distance is not a multiple of brick courses, you may have to either: Grind down the bed joints. Build up the bed joints.

Split courses should be avoided as these would produce a waste of materials and take a lotof time to build. Careful planning at the outset will prevent the need for using split courses.

The bricks are laid as for normal brickwork, and the corner is erected by raking back eachcourse until the correct height is reached. It is important to dry bond the walls fully beforebuilding corners, to prevent any broken bond occurring.

Corners are laid and raked back


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6BRICKWORK BELOW GROUND LEVEL

MODULE 5SHEET 39

Problems with Working below Ground Level

After the corners have been erected, the walls can be run in.

Working below ground level is very different to working above ground. The materials haveto be set out above ground on the banks of the excavation taking care not to place toomuch pressure on the trench sides. The space in the trench may be very tight. The spaceavailable may be limited to only 150mm in some cases.

If cavity walls are being built, then the facing wall will be easier to build because there ismore room when building the rst leaf of the wall.

The inner wall may be very tight to construct. It is wise to wear waterproof trousers if thetrench sides are wet and muddy. These may also offer some protection if the trench sideshave stones in them, which can scratch the legs badly.

Sometimes the bricklayer might decide to work from the ground level and bend down intothe trench.

Cavity wall below ground level


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MODULE 5SHEET 40

Foundation Blocks

The use of foundation blocks is now very common. The blocks used are a very densestrong block that can support the brickwork and blockwork built above ground level.

The use of these, rather than bricks or concrete blocks in two separate walls, speeds upthe work considerably. This method also eliminates the need to ll the cavity with weakconcrete ll.

When building in this way it will be necessary to change to brickwork on the outer orface wall.

This usually occurs two or three brick courses below ground level.

Blockwork below ground level


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MODULE 5SHEET 41

Stepped Foundations

When building foundation brickwork on stepped foundations extra care should be taken.

Work must always start at the lowest level. The rst course is set out as for ordinary stripfoundations but care should be taken when ranging in the rst course due to the differencesin level.

The concrete foundation should be marked out as stated previously for level foundations. Itis wise to check that each step in the foundation is in multiple brick courses at this stage.

Before commencing to lay bricks it is important to set out the bond dry to avoid brokenbond or reverse bond (starting with a header rather than a stretcher or vice versa, possiblyresulting in a half bat in the wall).

This is less likely to be a problem when you are building block footings as the blocksare generally used up to the point where the walls are levelled around. The bonding ofbrickwork is then done on top of the levelled blockwork.

Once the corner bricks, have been established and laid, a line can be stretched diagonallybetween the corners, and this will help to mark the positions of the wall at each step andwill help to maintain the true line of the wall.

Brickwork below ground level laid on to stepped foundations

Height of step = threecourses 225mm

Height of step = twocourses 150mm

Steps should be constructedto gauge of brickworkor blockwork to avoidunnecessary cutting


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MODULE 5SHEET 42

Stepped Foundations

When the wall positions have been determined, the corner can be built and the wallcompleted as shown below.

When all the brickwork or blockwork on the steps has been built up to level, the remainderof the footings are built as for normal masonry foundations.

Brickwork to Raft Foundations

Depending on the design of the raft foundation, there is very little work up to DPC.

With the simplest raft foundations, which are used for lightweight structures, only twocourses are required.

Brickwork levelled off to rst course

Simple raft foundation


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MODULE 5SHEET 43

Brickwork to Raft Foundations

Some raft foundations are deeper into the ground and would therefore require morefoundation brickwork or blockwork. The number of courses would depend upon how deepthe foundation is in the ground but care must be taken when positioning the DPC so that itconforms to the building regulations as to its height above ground level.

NOTE:

It is generally simpler to build walls from a raft foundation than from a foundationwithin a trench.

Once all the work has been nished up to DPC level, and the mortar has set and achievedstrength, then the excavation will require backlling and any surplus subsoil to be removedfrom the site.

Deeper raft foundation

If foundation is deeper, more

courses of brickwork will berequired to reach DPC level


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6 MODULE 6SHEET 44SOLID GROUND FLOORS

Solid Ground Floors

Following the completion of the foundation brickwork, the ground oors of the building canbe constructed.

There are two main types of ground oors: Solid oors. Hollow oors.

Some buildings can be designed to incorporate both hollow and solid ground oors.

Concrete ground oors consist of the following components, in order: Hardcore. Blinding. DPC membrane. Site concrete. Insulation. A oor nish.

Typical solid ground oor


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6SOLID GROUND FLOORS

MODULE 6SHEET 45

Components and Sequence of Building a Solid Ground Floor

Hardcore and blindingHardcore is used to provide a rm and solid base on which to place the concrete slab.

Blinding is usually building sand spread on top of the hardcore to provide an even bed andto prevent any puncturing of the DPC membrane by sharp pieces of hardcore.

Hardcore should be laid in 100mm layers maximum, and each layer should be wellcompacted and consolidated.

Hardcore can include such materials as stone, old masonry etc which has been acceptedby the architect, and is either tipped direct into the excavation or tipped on site and cartedinto the excavation. The hardcore should not contain any water-soluble sulphates or otherharmful matter which could cause damage to any part of the oor.

This rule prevents the use of any material which will expand on becoming damp, causingpressure to form under the oor.

The damp-proof membrane sheeting should be laid on top of the blinding and should belapped onto the horizontal DPC of the brickwork to form a continuous membrane.

DPC membrane, blinding and hardcore in place


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Compaction of hardcoreHardcore must be compacted in 100mm layers maximum thickness. This should preventany unacceptable settlement beneath the nished solid oor.

The main item of small plant which is used to compact hardcore is a plate compactor.

The whole area of the foundation base has to be completely compacted and consolidated,and care must be taken not to damage or disturb the brickwork when using the compactor.

For small areas a hand tamp can be used.

Plate compactor or wacker

MODULE 6SHEET 46


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InsulationThe Building Regulations require ground oors to be insulated to prevent heat loss.There are several types of insulation available such as expanded polystyrene and rigidfoam insulation.

Insulation should always be laid above the DPC membrane preferably under the oor slab,or alternatively under the oor nish. If it is not it could cause the insulation to becomedamp which will impair its performance.

MODULE 6SHEET 47

Insulation positioned below oor nish

Insulation positioned below oor slab


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Oversite concreteThe concrete placed inbetween the ground oor walls is known as oversite concrete.The brickwork or blockwork on the internal walls acts as shuttering for the concrete.

Oversite concrete should be at least 100mm thick. (If placed on insulation it is usual to havea minimum of 150mm thickness.) The thickness of hardcore should be at least equal to theconcrete thickness.

There is no need to trowel nish the concrete unless it is the nished oor of the buildingsuch as in a garage or store. A tamped nish is preferable to allow a oor screed to properlyadhere to the oversite concrete.

Sometimes, a solid oor has a oating oor or battened oor constructed on top of it. Inthis case it is necessary to nish the oor with a smooth wood oat nish.

MODULE 6SHEET 48

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