Support for Foundation trenches.

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The trenches which have to be dug for the foundations of walls may be excavated by hand for single small buildings but where, for example, several houses are being built at the same time it is often economical to use mechanical trench diggers.

If the trenches are of any depth it may be necessary to fix temporary timber supports to stop the sides of the trench from falling in. The nature of the soil being excavated mainly determines the depth of trench for which timber supports to the sides should be used.

Fig. 39 Struts and poling boards.

Fig. 40 Structs, waling and poling boards.

 

Fig. 41 Struts, poling boards and sheeting.

Soft granular soils readily crumble and the sides of trenches in such soil may have to be supported for the full depth of the trench. The sides of trenches in clay soil do not usually require support for some depth, say up to 1.5 m, particularly in dry weather. In rainy weather, if the bottom of the trench in clay soil gets filled with water, the water may wash out the clay from the sides at the bottom of the trench and the whole of the sides above may cave in.

The purpose of temporary timbering supports to trenches is to uphold the sides of the excavation as necessary to avoid collapse of the sides, which may endanger the lives of those working in the trench, and to avoid the wasteful labour of constantly clearing falling earth from the trench bottoms.

The material most used for temporary support for the sides of excavations for strip foundations is rough sawn timber. The timbers used are square section struts, across the width of the trench, supporting open poling boards, close poling boards and walings or poling boards and sheeting.

Whichever system of timbering is used there should be as few struts, that is horizontal members, fixed across the width of the trench as possible as these obstruct ease of working in the trench. Struts should be cut to fit tightly between poling or waling boards and secured in position so that they are not easily knocked out of place.

For excavations more than I 5 m deep in compact clay soils it is generally sufficient to use a comparatively open timbering system as the sides of clay will not readily fall in unless very wet or supporting heavy nearby loads. A system of struts between poling boards spaced at about 1.8 m intervals as illustrated in Fig. 39 will usually suffice.

Where the soil is soft, such as soft clay or sand, it will be necessary to use more closely spaced poling boards to prevent the sides of the trench between the struts from falling in. To support the poling boards horizontal walings are strutted across the trench, as illustrated in Fig. 40.

For trenches in dry granular soil it may be necessary to use sheeting to the whole of the sides of trenches. Rough timber sheeting boards are fixed along the length and up the sides of the trench to which poling boards are strutted, as illustrated in Fig. 41.

The three basic arrangements of timber supports for trenches are indicative of some common system used and the sizes given are those that might be used.

Foundations on sloping sites.

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The natural surface of ground is rarely level to the extent that there may be an appreciable slope either across or along or both across and along the site of most buildings.

On sloping sites an initial decision to be made is whether the ground floor is to be above ground at the highest point or partly sunk below ground as illustrated in Fig. 15.

Where the ground floor is to be at or just above ground level at the highest point, it is necessary to import some dry fill material such as broken brick or concrete hardcore to raise the level of the oversite concrete and floor. This fill will be placed, spread and consolidated up to the external wall once it has been built. 

Fig. 15 Fill an cut and fill.

The consolidated fill will impose some horizontal pressure on the wall. To make sure that the stability of the wall is adequate to withstand this lateral pressure it is recommended practice that the thickness of the wall should be at least a quarter of the height of the fill bearing on it as illustrated in Fig. 16. The thickness of a cavity wall is taken as the combined thickness of the two leaves unless the cavity is filed with concrete when the overall thickness is taken.

To reduce the amount of fill necessary under solid floors on sloping sites a system of cut and fill may be used as illustrated in Fig. 15. The disadvantage of this arrangement is that the ground floor is below ground level at the highest point and it is necessary to form an excavated dry area to collect and drain surface water that would otherwise run up to the wall and cause problems of dampness.

To economise in excavation and foundation walling on sloping sites where the subsoil, such as gravel and sand, is compact it is practice to use a stepped foundation as illustrated in Fig. 17, which contrasts diagrammatically the reduction in excavation and foundation walling of a level and a stepped foundation.

Figure 18 is an illustration of the stepped foundation for a small building on a sloping site where the subsoil is reasonably compact near the surface and will not be affected by volume changes. The foundation is stepped up the slope to minimise excavation and walling below ground. The foundation is stepped so that each step is no higher than the thickness of the concrete foundation and the foundation at the higher level overlaps the lower foundation by at least 300 mm. 

Fig. 16 Solid filling.

The load bearing walls are raised and the foundation trenches around the walls backfilled with selected soil from the excavation. The concrete oversite and solid ground floor may be cast on granular fill no more than 600 mm deep or cast or placed as a suspended reinforced concrete slab. The drains shown at the back of the trench fill are laid to collect and drain water to the sides of the building. 

Fig. 17 Foundation on sloping site.

Fig. 18 Stepped foundation.

Raft foundation on sloping site.

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On sites where the slope of the ground is such that there is an appreciable fall in the surface across the width or length of a building, and a raft foundation is to be used, because of the poor bearing capacity of subsoil, it is necessary either to cut into the surface or provide additional fill under the building or a combination of both to provide a level base for the raft.

It is advisable to minimise the extent of disturbance of the soft or uncertain subsoil. Where the slope is shallow and the design and use of the building allows, a stepped raft may be used down the slope, as illustrated in Fig.14.

A stepped, wide toe, reinforced concrete raft is formed with the step or steps made at the point of a load bearing internal wall or at a division wall between compartments or occupations. The drains under the raft are to relieve and discharge surface water running down the slope that might otherwise be trapped against steps and promote dampness in the building.

The level raft illustrated in Fig. 14 is cast on imported granular fill that is spread, consolidated and levelled as a base for the raft. The disadvantage of this is the cost of the additional granular fill and the advantage a level bed of uniform consistency under the raft.

As an alternative the system of cut and fill may be used to reduce the volume of imported fill.

Raft foundations are usually formed on ground of soft subsoil or made up ground where the bearing capacity is low or uncertain, to minimise settlement. There is some possibility of there being some slight movement of the ground under the building which would fracture drains and other service pipes entering the building through the raft. Service pipes rising through the raft should run through collars, cast in the concrete, which will allow some movement of the raft without fracturing service pipes. 

Fig. 14 Raft on sloping site.

Raft foundation.

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A raft foundation consists of a raft of reinforced concrete under the whole of a building. This type of foundation is described as a raft in the sense that the concrete raft is cast on the surface of the ground which supports it, as water does a raft, and the foundation is not fixed by foundations carried down into the subsoil.

Raft foundations may be used for buildings on compressible ground such as very soft clay, alluvial deposits and compressible fill material where strip, pad or pile foundations would not provide a stable foundation without excessive excavation. The reinforced concrete raft is designed to transmit the whole load of the building from the raft to the ground where the small spread loads will cause little if any appreciable settlement.
The two types of raft foundation commonly used are the flat raft and the wide toe raft.

The flat slab raft is of uniform thickness under the whole of the building and reinforced to spread the loads from the walls uniformly over the under surface to the ground. This type of raft may be used under small buildings such as bungalows and two storey houses where the comparatively small loads on foundations can be spread safely and economically under the rafts.

The concrete raft is reinforced top and bottom against both upward and downward bending. Vegetable top soil is removed and a blinding layer of concrete 50 mm thick is spread and levelled to provide a base on which to cast the concrete raft. A waterproof membrane is laid, on the dry concrete blinding, against moisture rising into the raft. The top and bottom reinforcement is supported and spaced preparatory to placing the concrete which is spread, consolidated and finished level.

When the reinforced concrete raft has dried and developed sufficient strength the walls are raised as illustrated in Fig. 12. The concrete raft is usually at least 150 mm thick. 

Fig. 12 Flat slab raft.

The concrete raft may be at ground level or finished just below the surface for appearance sake. Where floor finishes are to be laid on the raft a 50 mm thick layer of concrete is spread over the raft, between the walls, to raise the level and provide a level, smooth finish for floor coverings. As an alternative a raised floor may be constructed on top of the raft to raise the floor above ground.

A flat slab recommended for building in areas subject to mining subsidence is similar to the flat slab, but cast on a bed of fine granular material 150 mm thick so that the raft is not keyed to the ground and is therefore unaffected by horizontal ground strains.

Where the ground has poor compressibility and the loads on the foundations would require a thick, uneconomic flat slab, it is usual to cast the raft as a wide toe raft foundation. The raft is cast with a reinforced concrete, stiffening edge beam from which a reinforced concrete toe extends as a base for the external leaf of a cavity wall as shown in Fig. 13. The slab is thickened under internal load bearing walls.

Vegetable top soil is removed and the exposed surface is cut away to roughly form the profile of the underside of the slab. As necessary 100 mm of hardcore or concrete is spread under the area of the raft and a 50 mm layer of blinding concrete is spread, shaped and levelled as a base for the raft and toes. A waterproof membrane is laid on the dried concrete blinding and the steel reinforcement fixed in position and supported preparatory to placing, compacting and levelling the concrete raft.

The external cavity and internal solid walls are raised off the concrete raft once it has developed sufficient strength. The extended toe of the edge beam is shaped so that the external brick outer leaf of the cavity wall is finished below ground for appearance sake. A floor finish is laid on 50 mm concrete finish or a raised floor constructed. 

Fig. 13 Edge beam raft.

Pad foundations.

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On made up ground and ground with poor bearing capacity where a firm, natural bed of, for example, gravel or sand is some few metres below the surface, it may be economic to excavate for isolated piers of brick or concrete to support the load of buildings of some four storeys in height. The piers will be built at the angles, intersection of walls and under the more heavily loaded wall such as that between windows up the height of the building.

Pits are excavated down to the necessary level, the sides of the excavation temporarily supported and isolated pads of concrete are cast in the bottom of the pits. Brick piers or reinforced concrete piers are built or cast on the pad foundations up to the underside of the reinforced concrete beams that support walls as illustrated in Fig. 11. The ground beams or foundation beams may be just below or at ground level, the walls being raised off the beams.

The advantage of this system of foundation is that pockets of tipped stone or brick and concrete rubble that would obstruct bored piling may be removed as the pits are excavated and that the nature of the subsoil may be examined as the pits are dug to select a level of sound subsoil. This advantage may well be justification for this labour intensive and costly form of construction. 

Fig. 11 Pad foundation.

Short bored pile foundation.

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Where the subsoil is of firm, shrinkable clay which is subject to volume change due to deep rooted vegetation for some depth below surface and where the subsoil is of soft or uncertain bearing capacity for some few metres below surface, it may be economic and satisfactory to use a system of short bored piles as a foundation.

Piles are concrete columns which are either precast and driven (hammered) into the ground or cast in holes that are augered (drilled) into the ground down to a level of a firm, stable stratum of subsoil.

The piles that are used as a foundation down to a level of some 4 m below the surface for small buildings are termed short bore, which refers to the comparatively short length of the piles as compared to the much longer piles used for larger buildings. Short bored piles are generally from 2 to 4 m long and from 250 to 350 mm diameter.

Holes are augered in the ground by hand or machine. An auger is a form of drill comprising a rotating shaft with cutting blades that cuts into the ground and is then withdrawn, with the excavated soil on the blades that are cleared of soil. The auger is again lowered into the ground and withdrawn, cleared of soil and the process repeated until the required depth is reached.

The advantage of this system of augered holes is that samples of the subsoil are withdrawn, from which the bearing capacity of the subsoil may be assessed. The piles may be formed of concrete by itself or, more usually, a light, steel cage of reinforcement is lowered into the hole and concrete poured or pumped into the hole and compacted to form a pile foundation.

The piles are cast below angles and intersection of load bearing walls and at intervals between to reduce the span and depth of the reinforced ground beam they are to support. A reinforced concrete ground beam is then cast over the piles as illustrated in Fig. 10. The ground beam is cast in a shallow trench on a 50 mm bed of ash with the reinforcement in the piles linked to that in the beams for continuity. The spacing of the piles depends on the loads to be supported and on economic sections of ground beam. 

Fig. 10 Short bored pile foundation.

Narrow strip (trench fill) foundation.

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Stiff clay subsoils have good bearing strength and are subject to seasonal volume change. Because of seasonal changes and the withdrawal of moisture by deep rooted vegetation it is practice to adopt a foundation depth of at least 0,9 m to provide a stable foundation.

Because of the good bearing capacity of the clay the foundation may need to be little wider than the thickness of the wall to be supported. It would be laborious and uneconomic to excavate trenches wide enough for laying bricks down to the required level of a strip foundation.

Practice today is to use a mechanical excavator to take out the clay down to the required depth of at least 0.9 m below surface and immediately fill the trenches with concrete up to a level just below finished ground level, as illustrated in Fig. 9. The width of the trench is determined by the width of the excavator bucket available, which should not be less than the minimum required width of foundation.

The trench is filled with concrete as soon as possible so that the clay bed exposed does not dry out and shrink and against the possibility of the trench sides falling in, particularly in wet weather.

With the use of mechanical excavating equipment to dig the trenches and to move the excavated soil and spread it over other parts of the site or cart it from site, and the use of ready mixed concrete to fill the trenches this is the most expedient, economic and satisfactory method of making foundations on stiff, shrinkage subsoils for small buildings.

Fig. 9 Narrow trench fill foundation.

Wide strip foundation.

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Strip foundations on subsoils with poor bearing capacity, such as soft sandy clays, may need to be considerably wider than the wall they support to spread the load to a sufficient area of subsoil for stability.

The concrete strip could be as thick as the projection of the strip each side of the wall which would result in concrete of considerable uneconomic thickness to avoid the danger of failure by shear.

The alternative is to form a strip of reinforced concrete, illustrated in Fig. 8, which could be no more than 150 mm thick.

The reason for the use of reinforcement of steel in concrete is that concrete is strong in compression but weak in tension. The effect of the downward pressure of the wall above and the supporting pressure of the soil below is to make the concrete strip bend upwards at the edges, creating tensile stress in the bottom and compressive stress under the wall. These opposing pressures will tend to cause the shear cracking illustrated in Fig. 7. It is to reinforce and strengthen concrete in tension that steel reinforcing bars are cast in the lower edge because steel is strong in tension. There has to be a sufficient cover of concrete below the steel reinforcing rods to protect them from rusting and losing strength.

Fig. 8 Wide strip foundation.

Foundation Construction - Strip foundations.

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Strip foundations consist of a continuous strip, usually of concrete, formed centrally under load bearing walls. This continuous strip serves as a level base on which the wall is built and is of such a width as is necessary to spread the load on the foundations to an area of subsoil capable of supporting the load without undue compaction. Concrete is the material principally used today for foundations as it can readily be placed, spread and levelled in foundation trenches, to provide a base for walls, and it develops adequate compressive strength as it hardens to support the load on foundations. Before Portland cement was manufactured, strip foundations of brick were common, the brick foundation being built directly off firm subsoil or built on a bed of natural stones.

The width of a concrete strip foundation depends on the bearing capacity of the subsoil and the load on the foundations. The greater the bearing capacity of the subsoil the less the width of the foundation for the same load.

A table in Approved Document A to the Building Regulations sets out the recommended minimum width of concrete strip foundations related to six specified categories of subsoil and calculated total loads on foundations as a form of ready reckoner. The widths vary from 250 mm for a load of not more than 20 kN/linear metre of wall on compact gravel or sand through 450 mm for loads of 40 kN/linear metre on firm clay, to 850 mm for loads not exceeding 30 kN/linear metre on soft silt, clay or sandy clay.

The dimensions given are indicative of what might be acceptable in the conditions specified rather than absolutes to be accepted regardless of the conditions prevailing on individual sites.

The strip foundation for a cavity external wall and a solid internal, load bearing wall illustrated in Fig. 6 would be similar to the width recommended in the Advisory Document for a firm clay subsoil when the load on the foundations is no more than 50 kN/linear metre. In practice the linear load on the foundation of a house would be appreciably less than 50 kN/linear metre and the foundation may well be made wider than the minimum requirement for the convenience of filling a wider trench with concrete for the convenience of laying brick below ground. 

Fig. 6 Strip foundation.

The least thickness of a concrete strip foundation is determined in part by the size of the aggregate used in the concrete, the need for a minimum thickness of concrete so that it does not dry too quickly and lose strength and to avoid failure of the concrete by shear.

If the thickness of a concrete strip foundation were appreciably less than its projection each side of a wall the concrete might fail through the development of shear cracks by the weight of the wall causing a 45° crack as illustrated in Fig. 7. If this occurred the bearing surface of the foundation on the ground would be reduced to less than that necessary for stability.

Shear is caused by the two opposing forces of the wall and the ground acting on and tearing or shearing the concrete as scissors or shears cut or shear materials apart.

Fig. 7 Shear failure.

Functional Requirement - Strenght and stability – Foundation.

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The functional requirement of a foundation is: strength and stability.

The requirements from the Building Regulations are, as regards ‘Loading’, that ‘The building shall be so constructed that the combined, dead, imposed and wind loads are sustained and transmitted to the ground safely and without causing such deflection or deformation of any part of the building, or such movement of the ground, as will impair the stability of any part of another building’ and as regards ‘ground movement’ that ‘The building shall be so constructed that movements of the subsoil caused by swelling, shrinkage or freezing will not impair the stability of any part of the building’.

A foundation should be designed to transmit the loads of the building to the ground so that there is, at most, only a limited settlement of the building into the ground. A building whose foundation is on sound rock will suffer no measurable settlement whereas a building on soil will suffer settlement into the ground by the compression of the soil under the foundation loads.

Foundations should be designed so that settlement into the ground is limited and uniform under the whole of the building. Some settlement of a building on a soil foundation is inevitable as the increasing loads on the foundation, as the building is erected, compress the soil. This settlement should be limited to avoid damage to service pipes and drains connected to the building. Bearing capacities for various rocks and soils are assumed and these capacities should not be exceeded in the design of the foundation to limit settlement.

In theory, if the foundation soil were uniform and foundation bearing pressure were limited, the building would settle into the ground uniformly as the building was erected, and to a limited extent, and there would be no possibility of damage to the building or its connected services or drains. In practice there are various possible ground movements under the foundation of a building that may cause one part of the foundation to settle at a different rate and to a different extent than another part of the foundation.

This different or differential settlement must be limited to avoid damage to the superstructure of the building. Some structural forms can accommodate differential or relative foundation movement without damage more than others. A brick wall can accommodate limited differential movement of the foundation or the structure by slight movement of the small brick units and mortar joints, without affecting the function of the wall, whereas a rigid framed structure with rigid panels cannot to the same extent. Foundations are designed to limit differential settlement, the degree to which this limitation has to be controlled or accommodated in the structure depends on the nature of the structure supported by the foundation.

Trial Pits - make an examination of the subsoil on a building site.

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To make an examination of the subsoil on a building site, trial pits or boreholes are excavated. Trial pits are usually excavated by machine or hand to depth of 2 to 4 m and at least the anticipated depth of the foundations. The nature of the subsoil is determined by examination of the sides of the excavations. Boreholes are drilled by hand auger or by machine to withdraw samples of soil for examination. Details of the subsoil should include soil type, consistency or strength, soil structure, moisture conditions and the presence of roots at all depths. From the nature of the subsoil the bearing capacity, seasonal volume changes and other possible ground movements are assumed. To determine the nature of the subsoil below the foundation level it is either necessary to excavate trial pits some depth below the foundation or to bore in the base of the trial hole to withdraw samples. Whichever system is adopted will depend on economy and the nature of the subsoil. Trial pits or boreholes should be sufficient in number to determine the nature of the subsoil over and around the site of the building and should be at most say 30 m apart.

Ground movements that may cause settlement are:

(1) compression of the soil by the load of the building
(2) seasonal volume changes in the soil

(3) mass movement in unstable areas such as made up ground and mining areas where there may be considerable settlement
(4) ground made unstable by adjacent excavations or by dewatering, for example, due to an adjacent road cutting.

It is to anticipate and accommodate these movements that site investigation and exploration is carried out.

Site Investigation - Select a foundation from tables, or to design a foundation.

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To select a foundation from tables, or to design a foundation, it is necessary to calculate the loads on the foundation and determine the nature of the subsoil, its bearing capacity, its likely behaviour under seasonal and ground water level changes and the possibility of ground movement. Where the nature of the subsoil is known from geological surveys, adjacent building work or trial pits or borings and the loads on foundations are small, as for single domestic buildings, it is generally sufficient to excavate for foundations and confirm, from the exposed subsoil in the trenches, that the soil is as anticipated.

Under strip and pad foundations there is a significant pressure on the subsoil below the foundations to a depth and breadth of about one-and-a-half-times the width of the foundation. If there were, in this area below the foundation, a soil with a bearing capacity less than that below the foundation, then appreciable settlement of the foundation might occur and damage the building. It is important, therefore, to know or ascertain the nature of the subsoil both at the level of the foundation and for some depth below.

Where the nature of the subsoil is uncertain or there is a possibility of ground movement or a need to confirm information on subsoils, it is wise to explore the subsoil over the whole of the site of the building.

As a first step it is usual to collect information on soil and subsoil conditions from the County and Local Authority, whose local knowledge from maps, geological surveys, aerial photography and works for buildings and services adjacent to the site may in itself give an adequate guide to subsoil conditions. In addition geological maps from the British Geological Survey, information from local geological societies, Ordnance Survey maps, mining and river and coastal information may be useful.

Foundation of a Building.

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The foundation of a building is that part of walls, piers and columns in direct contact with and transmitting loads to the ground. The building foundation is sometimes referred to as the artificial, and the ground on which it bears as the natural foundation.

Ground is the general term for the earth’s surface, which varies in composition within the two main groups, rocks and soils. Rocks include hard, strongly cemented deposits such as granite and soils the loose, uncemented deposits such as clay. Rocks suffer negligible compression and soils measurable compression under the load of buildings.

The size and depth of a foundation is determined by the structure and size of the building it supports and the nature and bearing capacity of the ground supporting it.

Foundations and Oversite Concrete History.

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Up to the latter part of the nineteenth century, when Portland cement first carne into general use for making concrete, the majority of buildings were built directly off the ground. Walls of stone or brick were built on a bed of rough stones or brick footings and timber framed buildings on a base of rough stones or brick. As walls were built their weight gradually compressed soils such as clay, sand or gravel to form a sound, adequate foundation.

Local experience of the behaviour of soils and rocks, under the load of buildings, generally provided sufficient information to choose a foundation of the required depth and spread by this method of construction.

Where a small variation of the degree of compression of soils under buildings occurred the natural arching effect of the small, bonded units of stone and brick and the flexibility of lime mortar would allow a transfer of load to the sound foundation without damage to the building. 

Fig. 1 Brick fottings.

From the beginning of the twentieth century concrete was increasingly used as a foundation base for walls. Initially concrete bases were used for the convenience of a solid, level foundation on which to lay and bond stone and brick walls. Brick walls which, prior to the use of concrete, had been laid as footings, illustrated in Fig. 1, to spread the load, were built on a concrete base wider than the footings for the convenience of bricklaying below ground. This massive and unnecessary form of construction was accepted practice for some years.

With the introduction of local and, more recently, general building regulations in this century, standard forms of concrete foundations have become accepted practice in this country along with more rigorous investigation of the nature and bearing capacity of soils and rocks.

The move from the practical, common sense approach of the nineteenth century to the closely regulated systems of today has to an extent resulted in some foundations so massive as to exceed the weight of the entire superstructure above and its anticipated loads. This tendency to over design the foundations of larger buildings has been exacerbated by the willingness of building owners to seek compensation for damage, caused by the claimed negligence of architects, engineers and builders who, in order to control the amount of premium they pay for insurance against such claims, have tended to over design as an insurance.