college of estate management_foundations of buildings

Contents

Paper 8224V3-0 © The College of Estate Management 2012

Foundations – for low-rise buildings

. Function 3 1.1 Building Regulations 4 2. Ground conditions 5 2.1 Vegetable soil 5 2.2 Subsoils 5 2.3 Presence of trees 6 2.4 Frost heave 6 3. Development of foundations 8 3.1 Early types 8 3.2 Modern types 9 3.3 Reinforced concrete 10 3.4 Piles 12 3.5 Defects 15 4. Basements 15 4.1 Function 15 4.2 Remedial work 16 4.3 Services 18 4.4 Alternative damp-proofing methods 18

Foundations – for low-rise buildings Paper 8224

1 Function

The purpose of a foundation is to spread the load from the structure in question over a safe bearing area of subsoil and to provide a stable, level base on which to build (Figures 1 and 2).

FIGURE 1 Modern foundation arrangement

FIGURE 2 Nineteenth century foundation arrangement


Foundations are crucial to the stability of the whole house, and failure is one of the most serious defects, leading to movement and cracking of walls, roof and so on. Remedial work is very costly and often does not fully restore the building (Figure 3).

1.1 Building Regulations

The Building Regulations require that foundations be:

capable of safely sustaining and transmitting to the ground the combined dead load, imposed load and wind load in such a way as to prevent damaging settlement or movement occurring in any part of the building or any adjoining buildings;

taken down to a depth and constructed in such a way as to safeguard against damage caused by the subsoil swelling, shrinking or freezing;

capable of resisting sulphate attack.

Since the foundations must be capable of safely sustaining and transmitting to the ground all the requisite loads, we shall consider the structure of the ground itself, as this often dictates the type of foundation used.

FIGURE 3 Settlement cracks


2 Ground conditions

For our purposes, the ‘ground’ can be split into two categories: vegetable soil (or topsoil) and subsoil. The ground below the foundations is known as the sub-foundations and is always subsoil.

2.1 Vegetable soil

This is the surface layer of soft soil which covers most areas. It varies in thickness but the average depth is about 150mm. When constructing houses, this layer is stripped off as it is compressible and its organic content may have a deleterious effect. This soil is usually kept and utilised for the garden, landscaping and so on.

2.2 Subsoils

Rock Provided that it is sound and solid, rock only requires levelling and any cavities or fissures filled with concrete before building can begin. However, if the rock outcrop is on a slope, a careful examination needs to be made to establish whether a slip plane is present. If one exists, then excavation below that plane will be necessary and in some cases a retaining wall will also be needed.

Chalk Chalk may deteriorate under the action of water or frost and so, once exposed, it should be protected by a layer of concrete.

Swallow holes are liable to develop in chalk or limestone. Cavities in the rock dissolve away in underground water, and the overburden above collapses into the cavity, creating a swallow hole. Wherever possible, underground watercourses should be avoided and soakaways (discussed later) should be kept a safe distance from buildings in such areas.

Gravel Gravel has high compressive-resisting qualities which are helped by having fine particles interstitially situated between the larger stones. A loss of these particles may occur in water-bearing ground and with it a loss of its bearing qualities. Foundations on gravel should therefore be kept above the water table (Figure 4).

FIGURE 4


Sand A dense, confined bed of sand makes a good sub-foundation, being only slightly compressible. Again, washing out of the fine particles is to be avoided, as this makes the sand weak and unstable.

Clay The strength and stability of clay is directly affected by water content. It will shrink and expand with drying and wetting. The rapidity and depth of drying out will be greatly increased by the presence of tree roots.

2.3 Presence of trees

In clay soils, it is recommended that a house with shallow foundations should not be closer to a tree than 1½ times the mature height of that tree. For a group of trees this factor is increased to twice the height of the group (Figure 5).

If a lot of trees are cleared from a site there is a risk of swelling, and time should be allowed for the soil to take up its normal moisture content. Sometimes this may take several years.

If this time is not available, then there are several options: to use a flexible-framed construction without brickwork or plastering; or to make the building rigid by reinforcing the foundation or brickwork; or by constructing a basement.

The problem of heave (or expansion) and shrinkage in clays became very apparent in the summer and autumn of 1976, and the Building Regulations have been amended to help overcome these problems – e.g. by recommending a minimum depth of foundation in clay soils (Figure 6).

2.4 Frost heave

Some soils, such as gravels, are free-draining whilst others, such as fine sands, are slow. In the slow-draining soils in winter time, wedges of ice known as lenses form in the sand, expand, and cause the ground to swell, or heave.

Because the soil underneath the floor of a building is protected from frost and the surrounding soil is not, this differential expansion can cause cracking. So strong foundations are needed which go down to at least 600mm. (Frost seldom penetrates more than 450mm in the UK.)


FIGURE 5 Effect of trees on foundations

FIGURE 6 If trees are removed then the likelihood of heave increases as the soil returns to its natural moisture level


3 Development of foundations

3.1 Early types

The need for foundations has been known since the Middle Ages, when wooden piles were used. However, until 1855 no set rules were laid down as to the type of foundation that should be used in dwellings; indeed, there was no compulsion to use any at all!

The Metropolitan Building Act introduced in that year said that dwellings ‘should rest on solid ground’. Just after this Act three partially built houses collapsed with dire consequences. The result was the formulation of bye-laws effective from 6 October 1879, which provided for walls to have concrete foundations at least 9 inches (225mm) in thickness.

Earlier foundations were not always badly designed, as many older properties testify. Even with today’s reinforced concrete design, failure still occurs due to circumstances beyond the designer’s control.

There are many variations of the types of foundation to be found in older property:

a complete absence in the modern sense;

spreading the load with stepped brick footings (a method used to spread the load by ¼-brick offsets, the bottom course being twice the width of the wall; this practice was common but is seldom used today);

a type of strip footing of an inferior form of lime concrete, usually with stepped brickwork as well (found in later buildings) (Figure 7).

FIGURE 7 Development of foundations


3.2 Modern types

In modern practice, five types of foundation are commonly found. These are:

concrete strip; reinforced concrete; narrow strip; shortbore piles; rafts.

Strip This is the most common form of foundation. It comprises a concrete strip not less than 150mm thick with sufficient width to spread the load. This width can be calculated from tables which appear in the Building Regulations.

Where the strip is wider than the wall, the thickness of the concrete must be at least equal to that of the projection from the wall face, as shown in Figure 8.

The reason for this is that if concrete cracks under load, it will do so at 45º – the projection accommodates this without reducing the effective area over which the load is spread.

British Standard 882 (withdrawn, but cited in the Building Regulations) prescribes what concrete mix to use for strips, this being 50kg of cement to 0.1m³ of fine, and 0.2m³ of coarse aggregate. A stronger mix may be used but not a weaker one (i.e. more cement may be employed but not less).

FIGURE 8


Stepped foundations On a sloping site, foundations should be horizontal and in the form of steps. These steps should overlap each other by at least 300mm and never less than the thickness of the slabs used (Figure 9).

3.3 Reinforced concrete

Principles When a strip of concrete is placed upon the ground and a load placed on it there is a tendency for that strip to bend, as shown (exaggerated) in Figure 10.

It will be seen, say by experimenting with a soft rubber, that the bottom of the beam is stretched (tension) and the top is compressed (compression). Concrete is structurally quite strong in compression but very weak in tension, so on the stretched side of the beam some additional strengthening is required.

This strengthening is usually in the form of steel bars transversely as well as longitudinally placed in the strip. Ideally, these bars should be situated at the very outside edge of the strip. In practice this is not possible, as the bars would be too susceptible to corrosion, so it is usual to have a cover of 50–75mm of concrete over the bars.

The size and spacing of bars can be calculated, but usually the main reinforcement bars are 12mm diameter mild steel rods spaced from 150mm to 225mm apart, and the longitudinal bars some 450mm to 900mm apart.

FIGURE 9 Stepped foundations

FIGURE 10


Wide strip provides extra spread for the load of the building and should have both transverse and longitudinal reinforcing bars. The minimum cover of concrete to these bars should be at least 50mm but is more commonly 75mm (Figure 11).

With foundations on clay ground, shrinkage and swelling may cause the slab to tilt and the wall above it to crack.

The volume change varies with depth. Below 1m movement is not significant, hence the minimum depth for foundations in clay is specified as being 1m. However, near trees, severe shrinkage may occur and greater depth may be required (Figure 12).

FIGURE 11 Reinforced concrete wide strip foundation

FIGURE 12 Movement of foundations due to clay shrinkage


Narrow strip This type of foundation is suitable for two-storey construction. Only a narrow trench need be dug, normally by mechanical means, and usually 380mm wide by 1m deep; concrete is poured into this trench to fill it. This method saves bricklaying and, as a consequence, soil removal, as no working space need be provided for the bricklayers (Figure 13).

3.4 Piles

Short bore piles This method avoids the need for excessive excavation in clay soils. A shallow trench is dug; then, with an auger, or by mechanical means, short cylindrical holes are dug in the ground. These cylinders are usually 250–350mm in diameter and vary between 1,800 and 3,650mm in length, the former only being used in internal parts of the building. These holes are filled with concrete and mild steel bars 19mm in diameter are inserted to a depth of 600mm. The bars are bent over at right angles to tie into the ground beam when it is cast.

The piles are normally sited at corners, bases of chimney breasts, wall junctions, and at 900–1,800mm intervals to support the load in question adequately.

Compressible material such as ash or clinker is put in a layer beneath the ground beam to allow for any relative movement between the beam and the clay (Figures 14A and 14B).

FIGURE 13 Narrow strip foundation


Raft This is a reinforced concrete foundation slab covering the whole area of the building and even extending beyond the outer walls. The reinforcing consists of either mild steel bars at right angles to each other, or welded steel fabric. The slab is often made thicker under walls.

Rafts are used where there is a likelihood of settlement or where the soil bearing capacity is low. Their design is somewhat complex and is really an expert job. Care has to be taken that the raft’s strength is not unduly reduced by holes for services, and provision for access to these services has also to be considered. This type of foundation, however, is seldom found in use for a domestic dwelling (Figure 15).

FIGURE 14A Short bore piles

FIGURE 14B Short bore piles: ‘worm’s-eye’ view


FIGURE 15 Types of raft foundations


3.5 Defects

The most serious foundation defects are due to subsoil movement and are indicated by horizontal, vertical and diagonal cracks running along, down or across external walls. There may be an overhang of brickwork at the damp-proof course level, and/or bowing of the wall itself.

Subsoil movement, including a change of water content, can cause foundation movement and hence failure (Figure 16).

4 Basements

4.1 Function

Basements were built into a lot of houses, particularly in the last century. They were often used for the domestic activities of the household and hence contained such rooms as sculleries, pantries, kitchens, coal stores and so on. By virtue of the fact that they are below ground level, basements are inherently damp.

However, some basements are constructed so as to overcome this problem by having a light well between the external wall of the house and the soil, provided that this well is adequately drained (Figure 17).

The degree of dampness can range from a small amount of efflorescence (salt crystals forming on the walls), and dampness to the touch, to water seeping through the walls.

The need for damp-proofing was appreciated in the nineteenth century and builders utilised such things as engineers’ brick (a hard impervious brick) for the outer part of the walls. Some houses had a vertical damp-proof course of thin slate plastered into the walls.

FIGURE 16 Defects in foundations


4.2 Remedial work

If remedial work is carried out to rectify these damp conditions, the basement can provide a very functional addition to a house. In order to make a basement dry, a protective, waterproof layer is required, usually of asphalt. The layer can be applied to existing properties to recover this commodious area, the method being known as asphalt tanking.

Asphalt tanking If a minimum of 600mm working space is available, then the vertical tanking can be applied externally. With internal tanking, it is necessary to build a loading wall to resist ground water pressure. Figures 18A and 18B illustrate the basic methods for both internal and external tanking. The horizontal asphalt is built up in three layers, totalling 48mm in the internal case, and the vertical is also built up in three layers to make an overall thickness of 19mm. The external tanking method shown in Figure 18A is for new properties and the internal method shown in Figure 18B is for existing ones.

FIGURE 17 Separating well between basement wall and subsoil


FIGURE 18A External asphalt tanking (for new properties)

FIGURE 18B Internal asphalt tanking (for existing properties)


4.3 Services

A pipe or service passing through the tanking creates a weak spot in the damp protection and requires careful attention. Figures 19 and 20 indicate a general approach for the case of little or no hydrostatic (ground water) pressure, and one possible solution where hydrostatic pressure exists.

Little or no hydrostatic pressure

1. Pipe thoroughly cleaned

2. Pipe painted with bituminous paint

3. Asphalt sleeve placed in position.

With hydrostatic pressure present

Asphalt tanking is the most effective method of damp-proofing an existing building, but it is very expensive and not always possible. Often less permanent but less expensive approaches are taken. The following methods are not really a complete solution, but they do provide a means of preventing damp from damaging decorative wall finishes. They all work on the same basic principles of providing a barrier between a damp wall surface and the finish itself. All these methods work only when there is no hydrostatic pressure (i.e. there is not a particularly high water table).

4.4 Alternative damp-proofing methods

Polyethylene lath This is an inherently stable, high density polyethylene 0.5mm thick, formed into a pattern of raised studs 8mm high, linked by reinforcing ribs. These studs face the wall and create air channels. Polythene mesh is thermic-welded in the manufacturing process to the surface on one side. It provides a rot-proof key for plaster and renders. The lath should be fixed with a small air gap top and bottom, to permit circulation of air.

FIGURE 19

FIGURE 20


The lath is fixed with polypropylene plugs. A variety of renders and plasters may be used.

Damp-proofing paints These rely on their adhesive powers to form a damp barrier on the wall. Various types exist; the most common are bituminous, plastic or epoxy-resin based.

Careful preparation is needed, and old paint and efflorescence (salt crystals) must be removed by thorough wire-brushing before the paints can be applied.

Bituminous paints are affected by oil-based decorative paints and will not take water-based ones. It is usual, then, to take two courses of action: either to paper over the bitumen and then decorate, or to put on three coats, the third being ‘blinded’ (covered) with sharp sand to provide a key for rendering, and then rendered. These methods can also be employed for the plastic type paints, although direct painting with water-based paints is also a possibility. The epoxy types are only painted. Generally the ‘paint-on’ types of protection are not very important as they are usually forced away from the wall surface by efflorescence or at a point of weak bondage. They do, however, provide a good, reasonably priced alternative to tanking.

FIGURE 21 Polyethylene lath showing raised studs, air channels and polythene mesh

college of estate management_foundations of buildings

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