1.0 Foundations

1.1 FunctionThe foundations of houses must carry the dead loads (weight of the structure) of the

walls; roof and floors etc., together with the imposed loads of occupants and furniture, and

transmit them safely into the ground. They must be designed so that settlement is sufficiently

controlled to keep any distortion (and possibly cracking) to within acceptable limits.

1.2 Introduction to foundation typesThe three paragraphs below provide a brief introduction to the main foundation types in the

UK. The strip foundation is by far the most common and, in most cases, the cheapest. It has

been used, in one form or another, for hundreds of years and, for low rise housing, is suitable

for the majority of ground conditions likely to be found. The other two foundation types are

more likely to be used where sites and ground conditions are more complex.

The strip foundation is basically a strip, or ribbon, of insitu

concrete running under all the loadbearing walls. This will normally

include all the external walls and possibly some, or all, of the internal

walls. The depth and width of the strip depends on the building load

and the nature of the ground. In many cases these foundations do not

need specialist design, the foundation size can be determined by

referring to the Building Regulations.

Piled foundations can be of various types. They can be used to

transmit the loads from the foundations through weak,

compressible, or unstable strata, to firmer ground beneath (end

bearing piles). In clay and other cohesive soils piles can be used

to distribute the loads into the ground through the friction

forces along the length of the pile sides. Piles are usually made

from insitu or precast concrete but can also be steel and timber.

In housing built from load bearing brickwork, a reinforced

concrete beam bridges the piles and directly supports the

1

building.

Rafts are an expensive form of construction, probably

the most expensive of the three, and are used where only a very

low load can be applied, for example, on soft or variable

ground. They are also used where differential settlement is

likely or where there is a risk of subsidence (they are common

in mining areas). The raft is a rigid slab of concrete, reinforced

with steel, which spreads the building load over the whole

ground floor area.

2

2.0 Site InvestigationBefore foundation design can begin there are a number of preliminary stages. These,

separate stages, are generally referred to as Site Investigation.

Site Investigation normally involves three basic stages:

A desk study which takes into account existing information about the site. This

information will

Come from a variety of sources and will include such diverse matters as the history of

the site, its

Topography, geology, vegetation etc.

A walk-over survey which is a direct inspection of the site giving the

engineer/designer the

Opportunity to identify the nature of the ground and the nature of any hazardous

features.

A physical exploration and inspection, of the ground by means of boreholes or trial

pits. This third

Stage is sometimes called the ground investigation.

The desk study is the first stage in the site investigation. Essentially, it comprises the

collection and analysis of existing information about the site. The information will come

from a variety of sources and, and, once analysed, will form the basis for the second stage,

the walk-over survey. The desk study has two main objectives:

To determine the nature, past use, and condition of the site.

To determine whether this has any implications for the proposed building and its

foundations.

A sensible starting point is to consult large scale maps of the proposed site and check site

boundaries, building lines, existing buildings and other man-made, or natural, features which

will affect the future buildings. A comparison with older maps may give some clues to

determine former use and, therefore, potential hazards. Geological maps, other written

records, and local knowledge will help identify the likely nature of the subsoil and determine

the extent of difficult ground conditions. Most subsoil, including firm and stiff clays,

compact sands, gravels and rocks will easily support the relatively low loads of two and three

3

storey housing using simple strip foundations. However, soft cohesive soils, peaty soils, and

of course, fill, pose problems. A site that has been mined also needs treating with caution -

foundation solutions can be costly. Large scale historical maps, often held at city and county

libraries, show the extent of former mining. Thousands of old shafts and tunnels still exist.

Other items which should come to light during the desk study include the likelihood of:

filled or contaminated ground quarrying or mining rights of way ponds, watercourses, ground water levels and the risk of flooding utility services (drains, electricity, gas, telephone, optical cables etc - see left-hand

plan) previous vegetation (ie large felled trees) landslip naturally occurring aggressive chemicals (eg sulfates), harmful gases (radon) and

landfill gases (Methane and CO2). A walk-over survey is the second stage in the site investigation. It's a detailed site

inspection which:

enables much of the material discovered in the desk study to be confirmed or further investigated

identifies other potential hazards enables the surveyor to collect photographic records gives the surveyor/engineer the opportunity to make detailed drawings of all those

items (trees, existing buildings, watercourses, etc) which will have implications for the building

design A direct ground investigation is the third stage in the site investigation. As far as low rise

housing is concerned its main objective is to determine whether strip foundations will be

suitable and, assuming they are, whether they can be designed in accordance with the simple

'rule of thumb' approach contained in the Building Regulations. The ground investigation will

provide detailed information on:

nature and thickness of made up ground/top soil above the subsoil nature, thickness and stratum depth of subsoil an assessment of allowable bearing pressure Ground water levels, chemicals in the ground etc. existing structures or hazards in the ground

2.1 Trial pitsFor low rise housing, on green-field sites, machine-dug trial pits are probably the

most common method of ground investigation. The pits do not normally need to be deeper

than 4-5 metres unless specific problems are encountered. Trial pits should be excavated

close to the proposed foundation, but not so close as to affect its actual construction. The

4

number of pits is usually a matter for Judgment and will depend on the size of the proposed

development, the nature of the site, and the consistency of the soil across the site.

2.2 ConcreteHouse foundations are invariably formed in concrete. It is available in a range of

strengths and is usually brought onto site ready-mixed as, and when, required.

What is concrete?

The word concrete is derived from the latin word concretus, meaning grown together.

It is a mixture of several constituents which behaves as a single material. In its simplest form

concrete comprises cement, aggregate and water. The major constituent by weight in

concrete is aggregate - stone with a range of particle size from 40mm down to 0.1mm. The

aggregate is a mixture of:

Coarse aggregate - naturally occurring gravel or crushed rock

Fine aggregate - sand or crushed rock.

The aggregate is bound together by cement paste, a mixture of cement and water.

Properties

The properties of the cement paste are extremely important and largely determine the

properties of the concrete:it must be fluid enough for some time after mixing to allow the concrete to be placed and Compacted into its final shape

it must then set and gain strength so that it binds the aggregates together to make a strong Material.

The mechanism by which cement sets and hardens depends on the type of cement, usually due To a chemical reaction between the cement and the mixing water (eg Portland cement)

2.3 UsesThe great advantage of concrete as a construction material is that after mixing it is a

fluid (plastic) material which can be compacted into any shaped mould or formwork. This

may be done on site (in situ concrete), or for very high quality finishes, under factory

conditions (precast concrete). When the cement paste solidifies due to the hydration reaction

between cement and water it becomes a structural material.

Concrete is very strong in compression. Its compressive strength makes concrete an

ideal material for foundations and floor slabs and other structural elements that are mainly

loaded in compression. However, the tensile strength of concrete is relatively low, about one

tenth of the compressive strength. Therefore in structural elements such as beams, which, 5

when loaded, are in compression at the top and tension at the bottom, it is necessary to use

reinforced concrete.

Reinforced concrete contains steel reinforcing rods, usually 20-

30mm in diameter. These rods are positioned where the principal

tensile stresses will occur in the structure, and then the concrete is

poured and compacted around the reinforcement. Reinforced concrete

is therefore a composite material, where the concrete takes the

compressive forces and the reinforcing steel takes the tensile forces.

3.0 Strip FoundationsIn the latter part of the 19th century it was common to find the external walls of

houses built directly on the ground. Legislation towards the end of the 19th century required

concrete foundations under the walls. Then, as now, the depth of the foundation would

depend on local conditions.

A typical house from the

1900s or so would have a thin

strip of concrete under all the

loadbearing walls. Prior to this

walls were often built directly

onto the levelled ground;

sometimes there might be a bed

of stone or ashes to provide an

even surface.

Nowadays, the design of foundations is controlled by national Building Regulations. Strip

foundations, the most common form, can either be 'traditional' or trench-fill (see below).

They are usually 500 to 700mm wide and as deep as necessary for the type of ground. In

clays for example they are usually at least 1 metre deep to avoid problems of ground

movement caused by seasonable change in moisture content. In very dry conditions, for

example, clays will shrink slightly as the clay loses water. In very wet conditions the clay

will swell. In weaker ground the foundation has to be wider than 700mm to spread the

building load over an adequate area of ground.

6

Foundations must conform to certain standards explicit in the Regulations, or be designed by

someone competent in structural calculation. Either way, the design requires approval before

work can commence.

The loads from two storey houses are fairly modest and in most ground types and conditions

traditional strip foundations or trench fill foundations are more than adequate. In principle

they are very similar; they differ in that the concrete in trench fill foundations is deeper, with

the result that there are savings in brickwork and block work.

Reasons for choosing traditional strip foundations:

Proven method, most builders are familiar with traditional strip foundations

Mistakes (eg, setting out) are not too expensive to rectify once concrete is poured

Builder may want work to keep bricklayer occupied

Services will mostly cross the wall above the concrete - so not an immediate problem

Cheaper than trench fill for wider foundations

3.1 BUT Working space for bricklayers required

Walls easily damaged during backfill

Deep trenches require planking and strutting AND CAN BE DANGEROUS

Reasons for trench fill foundations

Foundations are completed fairly quickly

Clay soils less likely to swell or shrink because trenches can be completed speedily

Reduced need for planking and strutting - considerable cost savings

No need for people to work at base of trench - much safer, especially in deep

foundations

No risk of trenches collapsing (after concrete is placed) and damaging block work

Will bridge minor soft spots in base of trench 7

Service entry ducts need to be carefully placed

Good access for concrete lorry required; (or concrete pump needed)

Expensive if foundations have to be wide (or become wide)

The photos above show a trenchfill foundation for a new two storey house. The trenches are

just over 1 metre deep and about 500mm wide. They are filled with concrete to a level about

300mm or so below finished ground level. The drawing below shows a typical foundation

plan. The foundation runs under all the external walls and under the internal loadbearing

walls.

8

3.2

Require

ments for

strip

foundatio

nsThe

Building

Regulation

s set out a

number of

requiremen

ts for strip

foundation

s. Their

width is

determined

by a table

in the

9

Regulation

s which

takes into

account

building

load and

the nature

of the

ground.

Their

depth

depends on

site

conditions

and the

nature of

the soil –

at least

1000mm is

normal in

clays. The

Regulation

s also

contain

requiremen

ts

regarding

thickness

of the

concrete,

position of

the wall

relative to

the

foundation, 10

minimum

depths near

drains and

so on.

In some

situations

strip

foundation

s are not

suitable, or

are not

cost

effective.

These

include for

example:

wh

ere

larg

e

tree

s

are

pre

sen

t in

cla11

y

soil

s

wh

ere

tree

s in

cla

y

soil

s

hav

e

rec

entl

y

bee

n

rem

ove

d

in

ver

y

we

ak

or

uns

tabl

e

soil

s

wh

ere

stri12

p

fou

nda

tion

s

wo

uld

hav

e to

be

ver

y

dee

p to

rea

ch

fir

m

gro

und

wh

ere

sub

sid

enc

e is

like

ly

(i.e.

in

min

ing

are

as)

13

4.0

Piling4.1

Definition

of piling?

Pile

s can be

made from

steel or

timber

although in

most

housing

work piles

are made

from insitu

or pre-cast

reinforced

concrete.

They are

used either

to transmit

loads from

the

building

through

soft or

compressib

le ground

to firmer

strata

below (end

bearing

pile), or to

14

distribute

loads into

the subsoil

along the

length of

the pile

(friction

pile). In

housing, a

concrete

beam

across the

top of the

piles

distributes

the load

from the

loadbearin

g

brickwork

into the

piles

themselves

.

The

re are a

number of

different

piling

systems.

Some,

(replaceme

nt piles),

bore out

the ground 15

and then

replace the

void with

concrete.

A

reinforcem

ent cage is

lowered

into the

wet

concrete to

resist any

lateral

forces in

the ground

which

might

fracture the

pile, and to

provide a

connection

for the

ground

beam

which will

support the

walls.

Others,

(displacem

ent piles)

are forced

into the

ground,

pushing it

out of the 16

way as the

piles are

driven

home.

W

h

e

n

t

h

e

p

i

l

e

s

a

r

e

i

n

p

o17

s

i

t

i

o

n

a

r

e

i

n

f

o

r

c

e

d

g

r

o

u

n

d

b

e

a

m

(

i

n18

s

i

t

u

c

o

n

c

r

e

t

e

o

r

p

r

e

-

c

a

s

t

)

i

s

p

o

s

i

t19

i

o

n

e

d

o

v

e

r

t

h

e

t

o

p

.

T

h

i

s

t

a

k

e

s

t

h

e

20

l

o

a

d

f

r

o

m

t

h

e

w

a

l

l

s

a

n

d

d

i

s

t

r

i

b

u

t

e

s21

i

t

i

n

t

o

t

h

e

p

i

l

e

s

.

A

t

y

p

i

c

a

l

h

o

u

s

e

22

m

i

g

h

t

b

e

s

u

p

p

o

r

t

e

d

o

n

1

0

-

2

0

p

i

l

e

s

.

23

These are

pre-cast

piles which

are driven

into the

ground to a

depth

determined

by

engineers.

A ‘set’ is

reached

when a

specified

number of

‘hammer

blows’

provide a

specified

amount of

downward

movement.

24

In clay

soils

augered

piles are

more likely

to be

found.

These are

replaceme

nt piles.

25

Why is piling becoming more common?

20 or 30 years ago piling was comparatively rare for housing (other than medium and high rise flats). Since

then, several factors have led to an increase in the use of piled foundations. These include:

the increased pressure to re-develop 'brownfield' sites, where strip foundations may not always be

appropriate

increased costs of 'carting away' and tipping surplus excavation from foundation trenches

(particularly in cities)

the development and easy availability of smaller piling rigs and piling systems which are,

nowadays, cost effective for house foundations

greater understanding of piling in general (partly through better building education).

4.2 Factors affecting pilingThere are literally dozens of piling companies in the UK each offering a number of different piling

systems. In many cases more than one piling system will suit a particular set of circumstances. However,

when choosing a piling system there is four main criteria to consider:

building load

the nature of the ground (ie, the subsoil)

local environmental or physical constraints (noise restrictions, height restrictions)

cost

4.3 RaftsIn the 1940s and 1950s raft foundations were quite common, particularly beneath the thousands of

prefabricated pre-cast concrete or steel buildings erected during the years following the Second World War.

Most of these houses were built on good quality farm land where the soil was generally of modest to high

bearing capacity. Rafts (or foundation slabs as they were sometimes called) were often used because they

were relatively cheap, easy

to construct and did not require extensive excavation (trenches were often

dug by hand). In 1965 national Building Regulations were introduced for the first time (London

still had its own building controls), but these did not contain any 'deemed to satisfy' provisions for

raft foundations (as they did for strip foundations) - consequently each had to be engineer

designed. As a result they quickly fell out of favour. In modern construction rafts tend to be used:

Where the soil has low load bearing capacity and varying compressibility. This might include,

loose sand, soft clays, fill, and alluvial soils (soils comprising particles suspended in water and

26

Deposited over a flood plain or river bed).

Where pad or strip foundations would cover more than 50% of the ground area below the

Building.

Where differential movements are expected.

Where subsidence due to mining is a possibility.

5.0Foundation Failures

5.1 Introduction Architects, surveyors and structural engineers are all called upon at some time to examine defective

foundations and submit reports with recommendations for remedial action. Whilst we can all learn the

technicalities that form the basic knowledge of the various building professions, there is always one element

where the professional has to rely on skill to diagnose the significance of any symptoms: that is ‘experience’.

This applies especially to the analysis of foundation problems and their cause.

5.2 Causes of failure

Foundations can move as a result of loads applied causing a downward movement known as settlement.

Settlement can be tolerated by the structure provided the loads do not exceed the ‘allowable bearing 27

pressures’ stated in BS 8004. Other pos-sible causes of foundation movement known as subsidence, brought

about by activity in the ground, are:

Soil erosion caused by flowing water.

Changes in ground water level.

Buildings on made up ground.

Movement associated with mining activities or ‘swallow holes’ found in chalk.

Movement due to shrinkage or swelling of clay soils. This is the most common cause of foundation

movement.

Uneven bearing capacities of differing subsoils.

Heave is the upward movement of the ground. It is the result of an increase in moisture content in excess of

that which existed when the building was erected. It can occur when trees are removed, but it can be caused

by the interruption of a natural water course through climate change. Heave is also caused by the removal of

loads on the foundation. A rather uncommon form of heave is when the ground

Expands when frozen. The problem is usually confined to soils consisting of fine sand and chalk. Where the

water table is high and there are prolonged periods of freezing, ice layers can cause the foundations to lift,

but this only occurs during very severe winter conditions.

Although other possible causes of damage must be considered during the investigation, settlement,

subsidence and heave account for damage to most structural elements including floors in low-rise buildings

(Dickinson & Thornton, 2004). Typical symptoms are:

Cracks in external or internal walls. The cracks may be hairline or much wider.

Walls bulging or leaning out of vertical.

Floors slanting out of level.

28

Drains or service pipes blocked or malfunctioning.

Paving’s or drives cracking.

When foundation defects are to be investigated a thorough examination is imperative. The primary object of

this type of examination is to obtain an accu-rate diagnosis as a basis for a report. It is therefore extremely

important that all available evidence is collected together and carefully examined before decisions are

reached as to the method of repair to be adopted. This inspection may take some considerable time, but it is

essential that extensive defects are properly investigated. It is always advisable to bear in mind when making

a diagnosis that more than one cause may be responsible for a defect, although it is neces-sary to investigate

the primary cause. For example, foundation movements may have been responsible for a fractured external

wall, but rainwater could pene-trate the fracture causing dampness on the internal face of the wall. Although

the two defects will have to be remedied it does not necessarily mean that the crack has caused rainwater to

penetrate the wall. The wall may have been damp before the movement took place perhaps due to faulty

construction or porous brickwork.

Foundation repairs to existing buildings are generally the most difficult and costly to effect, which is still a

good reason why a thorough investigation should be carried out. The object of the investigation will be to

determine the nature and strength of the subsoil under load. A visual observation below ground can only be

carried out by digging trial holes at intervals along the length of the wall adjacent to the suspected position

of the foundation failure. The holes should be of a size to accommodate an adult.

When the underside of the foundation is exposed, the details of the subsoil, together with the condition of the

foundation and base of the wall should be recorded. Tests can be carried out by driving an iron bar into the

subsoil. A more detailed test for moisture and bearing capacity can be carried out by removing samples of

soil with a spade and submitting them to a laboratory for examination.

The surveyor should always bear in mind that the initial examination will only reveal conditions

as they are, and they will need to be studied over a period of time before a decision can be made.

Prior to carrying out an inspection of the founda-tion defects it is advisable to have a precise

knowledge of the soils present on site. Land used for building varies considerably from hard rock

to loose sand. Between these extremes are soft rock, firm earth, firm clay, soft clay, gravel, sand

and fill.

Soils may be divided into two categories, non-cohesive and cohesive:

Non-cohesive soils are the gravels and sands which tend to lack cohesion and have no

plasticity.

Cohesive soils are the various types of clay and silt and possess cohesion and plasticity.

Below are the principal causes of foundation failures that are considered to be most

common in the UK.

29

5.3 Differential movement Excavating the ground and placing substantial loads on it is sufficient to cause a slight movement as

the ground below is compressed to resist the load. Provided the settlement is uniform over the building area

the movement does little damage. Alternatively, there may be differential movement where part of the

foundation remains stable while the remainder moves (Richardson, 2000). A typical example of differential

movement is shown in Figure 5.1 where set-tlement occurred in the end walls with the centre portion stable.

The cracks are usually vertical or diagonal and are often interrupted by window or door openings. In such

cases a gap is formed between the frame and brickwork.

Roof level

First floor level

Ground floor level

Figure shows Example of differential settlement. Settlement in end walls with centre portion stable. Cracks

increase in width with height, and appear to be interrupted by the windows, but in fact extend round the

openings forming a gap between window frame and brickwork. The movement is at its maximum at roof

level and could cause loss of bearing in the roof slab

Stanchion or pier

Settlement Floor

Figure : Differential settlements under the point load of a stanchion

Under concentrated loads such an overloaded pier or stanchion on shallow foundations will often show

movement cracks between floor and foundation where the foundation has been ‘punched’ downwards

sometimes several centime-tres (see Figure

30

The average domestic building, however, is unlikely to weigh more than 45 kg per metre length of wall with

a nominal concrete base width of, say, 680 mm. The naturally occurring subsoils found in the UK are usually

able to sustain this type of loading, provided the foundation is deep enough not to be disturbed by the effects

of atmospheric action.

6.0 Unequal settlement Shallow foundations on clay present bearing capacity problems and shrink and expand due to

seasonal changes, the effect being felt to a depth of 1.4 m. The bulk of these clays are situated in the

southeast of England which has the lowest rainfall (Driscoll & Crilly, 2000).

The clay soil immediately surrounding a building will shrink and crack during hot weather, but

underneath the building the subsoil will be protected from the winter rains and also from the hot sun; thus it

will expand and con-tract much less. Therefore, a differential settlement is set up causing the foundations to

the external walls to settle downwards and the walls to lean outward during the hot summer months when the

subsoil is dry, and a ten-dency to lean inwards in the winter months when the exterior wet clay has expanded

and the interior has remained stable. During the wet seasons when the clay expands the cracks tend to

recover. This movement can also cause diagonal fractures around window and door frames and a drop in the

hori-zontal joints of a string course or bed joints in brickwork. The appearance of cracks in the outer walls

can be disturbing, so that this seasonal movement can often cause considerable concern out of all proportion

to its actual impor-tance (Richardson, 1996, 2000).

6.1 Effect of tree roots

Fast growing trees close to buildings can cause unequal settlement when active tree roots dry out the soil

causing differential soil shrinkage (BRE Digest 298, 1999). Shrinking clays affect the bearing capacity and

lead to movement in the building, especially in shallow foundations. Tree roots can extend over a consider-

able distance and can extract moisture from as deep as 6 m below the surface. It is, therefore, necessary to

make an accurate survey of their position and obtain details of the type of tree, and at the same time

establish that the tree is the cause of the damage (see Figure 5.4). Poplars and elms with fast growing root

systems can be expected to cause serious seasonal movements.

One way to avoid root problems with tall trees is to maintain a ‘safe distance’ between the tree and

the building. Some species of trees are likely to cause more problems than others. Table 5.1 shows the

different types of trees known to have caused damage, ranking in descending order of threat. It also shows

their expected maximum height on clay soils. Planting a tree close to a new or existing building will

usually entail some risk of damage. It is, therefore, suggested that the recom-mendations described in Table

5.1 are followed.

Buildings can also be damaged when well established trees are removed (Bonshor & Bonshor, 1996). The

31

resultant pressures due to the removal of trees and bushes act both vertically and horizontally. In the

majority of cases it is the horizon-tal movement that produces the greatest damage, particularly in the upper

layer of clay. In such cases there is a danger of the clay expanding over a period of years as it reabsorbs

moisture causing the foundation to ‘heave’ as described in Section 5.2 above. Where window sills crack

and rise in the middle this is an indication of soil

tree roots

Figure : Unequal settlement caused by

Stepped diagonal cracks in brickwork extending from the corner of the building back to the nearest opening

in the external wall

Unequal settlement caused by tree roots close to a building; roots absorb moisture and cause clay to shrink

Table : Risk of damage by different tree species: the table shows for each tree species the distance

between tree and building within which 75% of the cases of damage occurred. Reproduced from the

Building Research Establishment Digest 298 by permission of the Controller HMSO: Crown

copyright32

Max distance Min recommended

Max tree for 75% of separation in very highly

Ranking Species height – H (m) cases (m)

and highly shrinkable

clays

1 Oak 16–23 13 1H

2 Poplar 24 15 1H

3 Lime 16–24 8 0.5H

4 Common ash 23 10 0.5H

5 Plane 25–30 7.5 0.5H

6 Willow 15 11 1H

7 Elm 20–25 12 0.5H

8 Hawthorn 10 7 0.5H

9 Maple/ 17–24 9 0.5H

sycamore

10 Cherry/plum 8 6 1H

11 Beech 20 9 0.5H

12 Birch 12–14 7 0.5H

13 White beam/ 8–12 7 1H

rowan

14 Cypress 18–25 3.5 0.5H

Heave Differential movements will take place resulting in cracks in walls and partitions. In such cases the

removal of a tree may do more harm than good.

7.0 Shallow foundations The surveyor will often find that foundation movement in older domestic proper-ties can be caused

by light strip foundations attached to the main building sup-porting porches, bay windows and garages. 33

There is a common misconception that these lightweight structures do not need deep foundations as does the

main building. If this type of foundation is not carried down sufficiently deep to avoid movement by

atmospheric action, the junction between the light and heavy parts of the structure will often show a diagonal

crack running down from the lower corner of the rear window as shown in Figure 5.5.

Another common example is when part of a building has a basement with foundations set in deep

geological beds. Seasonal movement will often take place in the foundations close to the surface causing a

vertical fracture at the junction between the walls built off the basement wall and the ground floor. The

heavier walls to the basement area will settle relatively more causing movement cracks between the light and

heavily loaded walls as shown in Figure 5.6. This is more noticeable when the ground floor structure is built

on shallow foundations in clay subsoil

Figure : Diagonal fractures between light and heavy structure

Garage attached Cracks will occur at the to main building first point of weakness usually at door or window

openings or at the junction between the light & heavy structures Foundation movement caused by light strip

foundations attached to main building at a minimum depth of 600 mm

34

Figure 5.6 Unequal settlements between basement and ground floor walls

7.1 Building on sloping sites Buildings on sloping clay sites can often present difficulties (Instructed, 2000). The water table on

sloping sites tends to follow the topography of the surface and if the natural contours of the site change it

does not necessarily alter this line. Where the foundations have been set at a constant depth from the

stepped level surface the concentrations of water may affect the foundations at the lowest point causing

differential settlement. Figure 5.7 shows such a case where the water table is above the foundation at the

highest point and beneath a shallow foundation at the lowest

First floor

Original surface

Figure: Building on sloping clay site

If the ground is not properly drained then the penetrating water will cause saturation of the ground adjacent

to the foundations

If a building is erected at the lowest point of a sloping site, natural drainage of the water to the lower points

can cause saturation of the ground around the base of the wall and foundations thus lowering the bearing

capacity (see Figure 5.8). In both the above cases it is advisable to check that the subsoil is effectively

drained at the highest level in order to protect the building against damage by water pen-etration. The

surveyor will often find that surface water drainage has been omit-ted especially in ancient buildings.

A similar case of water penetration often occurs around the base of the external walls of old buildings

where the ground floor level is below the ground level. The

35

Brick or stone wall

RWP

Water penetration

through open joints

Fall

Floor level

Surface water channel

Line of damp penetration

Figure 5.9 Water penetrations around base of external walls where the ground floor level is below the

ground level

surveyor will probably notice that clayware or concrete channels have been inserted around the base of the

wall to collect rainwater from the roofs and a cer-tain amount of surface water from paved areas around the

building.

The channels tend to move with the seasonal changes in the subsoil causing the joints to open, thus allowing

water to percolate through to the subsoil and the base of the wall. In such cases there is a risk of uneven

movement in the subsoil leading to settle-ment cracks (see Figure 5.9).

In chalk and limestone areas, cavities in the subsoil can be formed by underground streams or watercourses

dissolving the rock. If the sandy overburden falls into the cav-ity the foundations will drop. These cavities

are known as ‘swallow holes’. In the three cases described above the foundations need protecting and the

ground water must be directed away from the foundations by a system of ground or surface water drainage.

7.2 Building on made up ground Filled or made up ground is extremely varied in form and should be treated as suspect. Experience

has shown that the majority of foundation failures on filled ground have been due to the use of poor fill, and

inadequate compaction. Unfortunately, during an inspection, detailed knowledge of the fill is usually lack-

ing. All possible information concerning the site should be obtained by discussion with the local authority

and by studying local maps of the area. Apart from dig-ging trial holes the surveyor should observe signs of

damage to any adjacent buildings. The trial holes should be deep enough to enable the surveyor to assess the

nature of the fill, its depth, composition and degree of compaction. If any remedial work is contemplated,

such as underpinning or piling, this could well involve the protection and support of the services to the

building. A particular note of this matter should be made during the site examination.

36

8.0 Diagnosis Crack monitoring may be necessary to see whether the problem is still active (Bonshor &

Bonshor, 1996). This should be done by the application of ‘tell-tales’. Tell-tales should be fixed

internally and externally if found necessary. Although tell-tales are most important from the surveyor’s

point of view, it will sometimes be difficult to explain the size and direction of the crack to the client

by way of notes and sketches. In this respect photographs will be most useful when attached to a

report. However, the tell-tales will enable the surveyor to check whether or not the movements are

progressive. Movements due to settlement of filled ground usually cause major cracking of external

walls and partitions. Bulging can also occur in external walls. Concrete ground floors are also liable to

lift and crack.

After all investigations have been completed the surveyor may consider obtain-ing the services of a

specialist in this field to advise on any remedial work required.

37

9.0 FUNCTIONS OF FOUNDATIONS

The sub- structure which transmits the loads of super-structure to the underlying soil is termed as

foundation.

To distribute the load of the super- structure over a wide area. Protect differential settlement of the

structure.

Anchor the structure against the lateral forces.

38

9.1 TYPES OF FOUNDATIONS FOR RESEDENTIAL BUILDINGS.

Open strip foundation.

Isolated footing.

Combined footing.

Strip footings or strap footings.

Inverted “Tie” strip foundation.

Under reamed pile foundation.

Raft foundation.

39

10.0 OTHER FAILURES. Undermining of safe support.

Load transfer failures.

Lateral movement.

Unequal support.

Drag down and heave.

Design error.

Construction error.

Floating and water level changes.

Vibration effects.

Earth quake effects.

Undermining of safe support.

A careful study of the soil strata at the site of the proposed building along with the

adjacent existing structures is very important. Temporary and permanent supports to

the structure such as underpinning have to be installed to prevent the undermining.

40

Load transfer failures.

A rigid frame structure will tolerate foundation movements when the walls, floors

and partitions are rigidly connected by a frame, the system will adequately adjust itself to

differential foundation movement when the inter connecting rigidity fails, the load at the

point goes to the soil vertically through the support at the point

41

Lateral movement. Lateral movements are caused by either the elimination of existing

lateral

Unequal support.

Footings bearing on different soils with different and unequal soil-bearing resistances.

All the soil support deficiencies can be corrected by underpinning the weakened support.

Soil stabilization by cement or chemical injection or sub-surface enclosures-usually a

tight sheet pile. The dewatering may also be the

42

Drag down and heave.

When the footing is loaded the supporting soil reacts by yielding and compressing to

provide resistance. In plastic soils the new settlements are often accompanied by upward

movements and heave some distance away. Since the liquid in the soils cannot change

volume, every settlement must produce an equal-volume heave.

43

Design error.

Many foundations are designed with insufficient sub-surface

investigations.

Construction error.

There are two common sources of these errors. 1)

Temporary protection measures.

44

2) Foundation work itself.

45

Floating and water level changes.

A change in water content will modify the dimensions and structure of the

supporting soil whether from flooding or from dewatering. Pumping from adjacent

construction excavations also affect the stability of the existing footing. Clays heaves from

over-saturation. Water level should be monitored.

Vibration effects.

The earth masses which are not fully consolidated will change volume when exposed to

vibration impulses. The sources of vibrations can be blasting, construction equipment (esp.

pile drivers), mechanical equipment in a completed building, traffic on rough pot-holed

pavements adjacent to the site.

Earth quake effects.

Foundations at the earth quake affected zones must be designed to tolerate the

expected shock by the Nature. The quakes of short duration have less severe effect on

the foundation than on the super structure.

46

CONCLUSIONS

The practice of foundation design has relied extensively on empiricism and relatively

simple analyses (such as the bearing capacity equation). This was needed because crucial

elements in a rigorous analysis of soil mechanics problems were missing until very

recently: computation power, rigorous methods for analyzing elasto-plastic boundary-

value problems and realistic constitutive models. Our discipline is in a state of transition.

The science of soil mechanics has developed considerably in the last 20-30 years.

The progress in the science is gradually finding its way into practice, which still

overwhelmingly relies on traditional Methods.

The evaluation of case histories using modern methods of analysis is a useful way

to show the usefulness of these methods. In this paper, we have used finite-element limit

analysis (FELA) and the finite element method (FEM) to reveal features of foundation

engineering problems that would not otherwise be detectable with simple methods. We

have done so without resorting to sophisticated constitutive models, relying instead on the

well-known Tresca yield surface for clay in all three case histories and linear elasticity in

the last of three case histories examined, but in more complex problems, certainly those

involving frictional soils, more realistic soil models would be required.

47