Foundation Engineering

7/17/2019 Foundation Engineering

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Budapest University of Technology and Economics

DepartmentofGeotechnics

FOUNDATION ENGINEERING

Dr. Farkas JzsefJzsa Vendel

Dr. Szendefy Jnos

2014. May


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Foundation Engineering Department of Geotechnics

2

CONTENTS

Preface 4

1

INTRODUCTION ........................................................................................................................................... 5

1.1

The role and function of the foundation ...................................................................................... 5

1.2

The properties of the foundations ................................................................................................ 5

2

GROUND INVESTIGATION ...................................................................................................................... 6

2.1

Direct processes ............................................................................................................................. 7

2.2 Exploration of the groundwater ................................................................................................. 17

2.3

The necessary extent of ground investigation .......................................................................... 19

3

SHALLOW FOUNDATIONS .................................................................................................................... 22

3.1

Types and structures of shallow foundations ........................................................................... 22

3.1.1

Stripe foundations ................................................................................................................. 22

3.1.2

Pad (point) foundations......................................................................................................... 23

3.1.3 Raft slab foundations ............................................................................................................ 24

3.1.4

Continuous footing (beam grid foundation) ....................................................................... 25

3.1.5

Slab foundations .................................................................................................................... 25

3.1.6

Box foundations .................................................................................................................... 26

3.1.7

Shell foundations ................................................................................................................... 26

3.2

Steps of shallow foundation design ........................................................................................... 26

3.2.1

Taking up the foundation level ............................................................................................ 26

3.2.2

Areal dimensioning on the bases of load bearing capacity ............................................... 27

3.2.3

Calculation of load bearing capacity according to MSZ EN 1997-1:2006 ..................... 29

3.2.4

Vertical dimensioning of shallow foundations .................................................................. 30

3.2.5

Dimensioning of raft slab foundations ................................................................................ 32

3.3

Settlements ................................................................................................................................... 33

3.3.1 Components and timescale of settlements .......................................................................... 33

3.3.2

Settlement calculation ........................................................................................................... 34

3.3.3

Approximate calculation of the stresses .............................................................................. 39

3.3.4

Settlement tolerance of buildings ........................................................................................ 44

3.3.5

Causes of uneven settlements .............................................................................................. 47

3.3.6

Measuring settlements .......................................................................................................... 48

3.3.7

Defence against harmful settlements ................................................................................... 50


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3.4 Stability of shallow foundation .................................................................................................. 56

3.4.1

Slip safety ............................................................................................................................... 56

3.4.2 Uplift safety and failure ........................................................................................................ 59

3.5 Loads on shallow foundations ................................................................................................... 61

3.5.1

Dynamic effects ..................................................................................................................... 61

3.5.2

Effect of opening up underground voids ............................................................................ 63

3.5.3

Underwashing effect of ground water ................................................................................. 64

3.5.4 Freezing effect at cold stores ................................................................................................ 64

3.5.5

Foundation on shrinkage soil ............................................................................................... 64

3.5.6 Foundation on collapsible soil ............................................................................................. 66

3.5.7 Foundation on fill .................................................................................................................. 67

3.5.8

Foundation on organic soil ................................................................................................... 69

4

DEEP FOUNDATIONS ............................................................................................................................... 70

4.1

Pile foundations ........................................................................................................................... 70

4.1.1

Classification of piles ............................................................................................................ 70

4.1.2

Precast piles ............................................................................................................................ 71

4.1.3

Driving of precast piles can happen by ............................................................................... 72

4.1.4

Cast-in-place piles ................................................................................................................. 72

4.1.5

Design method of pile foundations ..................................................................................... 73

4.1.6 Calculation of expected maximum bearing capacity for the piles ................................... 73

4.2

Diaphragm wall foundation ....................................................................................................... 75

4.3 Cylinder and box caisson foundations ...................................................................................... 77

5

CONSTRUCTION OF FOUNDATIONS................................................................................................ 78

5.1 Retaining structures ..................................................................................................................... 78

5.1.1 Sloped excavation ................................................................................................................. 79

5.1.2

Props ....................................................................................................................................... 79

5.1.3

Sheet pile wall ........................................................................................................................ 83

5.1.4

Anchorage .............................................................................................................................. 84

5.1.5

Diaphragm walls ................................................................................................................... 88

5.1.6

Soil nailing ............................................................................................................................. 91

5.2 Dewatering the excavation ......................................................................................................... 92

5.2.1 Drainage in the open ............................................................................................................. 92

5.2.2 Water-pumping ..................................................................................................................... 94


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Preface

This Foundations note is aimed at helping the International and Hungarian university studentsconducting their studies in English at BUTE recalling the pieces of information mentioned atlectures and getting ready not only for exams but for real technical professional life.

The subject through this note presents a corner stone of geotechnics, showcasing the calculation ofsoil load bearing capacities, the preliminary design of shallow foundations, the problems that mayarise during construction and their solutions as well as basic technical correlations andtechnological processes.

Because of the wide scope of foundations of buildings and the diversity of the material to cover inthis subject, some questions are often discussed at a very basic, somewhat superficial level.Therefore, the note is to the point, containing only the essence every civil engineer is supposed to

be well aware of while practicing their profession.

We do hope however, that in spite of all these, the material covered in lectures and included in thenote will be able to make the university students be interested so that later on, as an engineerthroughout their professional career they constantly acquire new knowledge in connection withgeotechnics.

We hereby would like to say thank you to dm Kapcsos who largely contributed to the creation ofthis very note with his excellent command of English.

Budapest, May 30. 2014

Dr. Szendefy JnosDr. Farkas Jzsef

Jzsa Vendel


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1 INTRODUCTION

1.1 The role and function of the foundation

Every structure transmits its self weight and the imposed loads onto the subsoil therefore the

stability and structural strength is predominantly the function of how successfully thisconnection between the structure and the soil has been established. The structure, more

precisely, its foundation generates stresses(Figure 1.) and deformations in the soil. The soil is

compressed, the substructure settles. Uneven settlements create forces, stresses in the

superstructure that may result in cracks, yielding and passing Serviceability Limit State. In

extreme case,-due to overloading, even soil failure can take place in the soil layer below the

foundation. Numerous national and international cases could be mentioned from various

historic eras when the connection between the structure and the soil was not properly designed

(e.g.: Leaning tower of Pisa, Transcona silo, fermentation tanks of Nagykanizsa, etc.). The

foundationsare- usually subterranean- load bearing and load transmitting structural members

of buildings that transmit the loads of the whole structure to the soil.

Figure 1.: Stresses generated in the soil layer below the foundation

The function of the foundation: Transmitting the loads to the soil without damage sustained.

There can be shallow and deep foundations moreover we can speak of intermediate solutions

as well. The method of foundationdepends on:

- the subsoil;

- the groundwater;

- neighbouring buildings;

-

the structure type of the building;

- thermal effects;

- the circumstances of the construction

1.2 The properties of the foundations

When dimensioning foundations, the following limit states shall be taken into examination:

- Loss of general stability

- Soil failure under the foundation, punching, squeezing

-

Failure due to slip

- Mutual failure of the superstructure and the subsoil


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- Superstructure failure by foundation displacement

- Intolerably large settlements (Figure 2.),

- Intolerably large uplift, swelling by frost or other reasons

- Vibration of unacceptable proportions

- Simple, fast (mechanised), economical it shall be.

Figure 2.: Settlement of the substructure and its settlement difference

The foundation is a unique part, because:

- difficult to classify;

- is build under the surface level among difficult conditions

- difficult to repair

- a mistake in the foundation endangers the whole building

2 GROUND INVESTIGATION

Adequate foundation can only be constructed if the parameters of the soil and that of thegroundwater are known on the site. For a geotechnical designer these are as essential input data as the

function, capacity and site coverage of a building is for an architect. While for the structural engineers

the mechanical properties of concrete and steel are given, used as known factors in calculations, the

first step of geotechnical design is getting known the soils of the site and producing mechanical

parameters for them.

On the basis of the aforementioned, it is obvious that without proper soil exploration the

design of a foundation is impossible furthermore; an economical design can only be produced via

profound knowledge of soil properties.

The subsoil and groundwater parameters can be determined by on-site (in-situ) ground

investigation. The ground investigation can be split into two groups on the basis of direct and indirect

processes. In case of direct processes the soil stratification is explored directly, samples are taken from

each layer further examined in laboratories. In contrast, in case of indirect processes soil properties

and stratification is deduced from a know parameter.

The advantage of the direct process is that samples can be taken with which further

examinations in laboratories are possible however, with this method by all means the original

structure of the soil is changed to some extent moreover laboratory tests bring in additional disturbing

factors into the results. The essence of the indirect processes is to get the soils examined in their

original, in-situ state hence the examination can take place having their natural layout and originalstress states. Although, in that case pieces of information regarding soil properties can only be

Settlement

Uneven

settlement


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obtained indirectly by using an empirical correlation therefore sometimes the computed results are not

of appropriate accuracy. Nowadays, when indirect processes are more and more widespread and

available methods of soil exploration, I suppose, that the best option is to use the direct and indirect

processes together thus getting the most accurate picture of the examined site at hand.

The direct processes of soil exploration are the followings:

test pit drillings:

small diameterlarge diameter

The indirect processes of soil exploration can be classified as follows:

sounding processes:standard penetration test (SPT)dynamic probe test (DPL, DPM, DPH)

cone penetration test (CPT)vane shear test (VST)flat plate dilatometer test (DMT)pressuremeter test (PMT)

geophysical processesgeoelectricsradioisotope processgeoradarradio frequency processescross-hole, down-hole

refraction processes

2.1 Direct processes

At direct processes of soil exploration the soil stratification is directly seen and recorded,

samples are taken. By applying this method, the opportunity is there to record the colour and structure

of the soil layers (grainy, smooth, and laminated) alongside with the stratification as well as the

highest and the average groundwater level (GWL).

Figure 3.: Test pit excavation by digger Figure 4.: Retaining of the test pit


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The test pit, in which the tilting of the soil layers is visible by naked eye, their thickness can be

measured precisely and ideal for taking undisturbed sample, is obsolete as a method. Although

nowadays large amount of soil is excavated by machines being more effective than manual digging

previously, the stability of the test pit and the extent in case of a sloped pit raises concerns regarding

costs.

Additional disadvantage being its limited depth, as most excavators can only dig down to 3-5 m of

depth which value is further reduced by the presence of groundwater, below whose level a test pit

cannot be deepened. Do not construct test pits below the designed building. Because of recompaction

issues test pits are to be excavated outside of the contour of the designed building. In recent times, test

pits can only be found at foundation explorations at existing buildings.

Soil mechanical drillings can be done by drilling apparatus of small and large diameter. The

advantage of the small diameter drill is its portability and that it can be operated in areas inaccessible

for vehicles such as cellars and patios. The shipment of this tool is manageable by an average

passenger car which implies a great cost reduction.

Obviously, the classic manual drills have already been replaced by machines of various

manufacturers and brands. In Hungary, the small diameter drilling has been affiliated with the

apparatus manufactured by the firm Borro, therefore it is referred to as Borro-drilling in professional

circles.

Figure 5/a: Small diameter driller by one person Figure 5/b.: Borro driller (Mdosk Ltd.)

Although the portability of the small drilling tools has its back draws coming mainly from the

limited power (energy output) and downward force exerted by people. The method leads to the

drilling grinding to a halt in case of hard and dense soils consequently in many cases only a limited

depth can be reached. This kind of halt is also frequent in soils having considerably large grain size

diameters. Basically, this method is applicable for drillings between 3 and 8 meters of depth. During

the drilling, a spiral of 63 mm in diameter is utilised and the opportunity is there for taking samples of

moisture content, which enables the soil identification procedures in laboratories. Its scope includes

dwelling houses, industrial facilities of smaller scale, linear structures.


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The large diameter drills step over the limitations of small diameter drills, being usually truck

or lorry mounted drilling towers, having the self-weight of the truck as a reaction force and being

capable of exerting larger torque in drilling due to the high performance engines. Several alternative

type of this drilling method is known depending on the desired depth, the expected soil layers and the

method of sample taking. With the help of large diameter drills tens or hundreds of meters of depth

can be reached, the standard drill diameter ranging from 100 to 300 mm. When carrying out large

diameter drillings, undisturbed samples are available usually of 90-160 mm diameter alongside with

moisture content samples.

Advantages can sometimes turn to disadvantages as big machinery means big cost, bigger

crew and less number of accessible places. French and Italian firms are pioneers in manufacturing

medium self-propelled caterpillar drilling machines between large and small diameter. These

machines are capable of drilling large diameter boreholes and taking undisturbed samples from couple

of tens of meters of depth, thus are applicable in case of the majority of engineering structures. Such

smart machine is shown in the picture (Figure 7/b.)

Figure 7/a: Large diameter drilling machine

(Geovil Ltd.)Figure 7/b.:Joy drilling machines

(Geoszfra Ltd.)


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2.2 Indirect processes

The indirect processes perform measurements and present results on the basis of soil

properties or a type of resistance of the soil at hand.

By this, the chance is given to define the soil stratification, to approximate each soil type

(gravel, sand, silt, clay) to measure groundwater level and possible pore water pressure. Beside these,the shear strength, the horizontal earth pressure the Shear Modulus and the Modulus of Elasticity can

be concluded from measured parameters.

Because of the extent of this very subject, only the main features of these methods will be

presented, the analysis of the measured data is the scope of other subjects.

2.2.1 Sounding

Due to the various soils and soil states around the World the developed and applied sounding

methods can be very different from one another. This diversity is shown in the picture below. Only

the most widespread sounding methods will be discussed in detail.

The Standard Penetration Test,which may be a transition between the

drilling and the indirect processes is a

method used at drillings. During the drilling

at every meter the process is ceased and with

repeated hammer blows a cylinder is driven

into the soil. The number of hammer blows

assigned to a given depth is recorded while

simultaneously undisturbed samples can be

taken from the inside of the cylinder. The

procedure is presented in short in Figure 8.

Figure 8.: SPT sounding process


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Dynamic Probing is very similar to SPT sounding, where a special tip of 90 conical shape

and 4,37 cm in diameter located on a bar 3,2 cm in diameter is driven in by a 50 kg rammer dropped

from 50 cm of height. (DPH: Dynamic Probing Heavy) The most popular is the DPH but in the

function of the geometry of the tip of the probe and the kinetic energy of the hammer (weight and

dropping height) there exist Dynamic Probe Light (DPL) and Dynamic Probe Medium (DPM).

During the measurement the number of hammer shocks concerning 10 cm of penetration

depth is recorded and represented. Figure 9/b is depicting a sounding diagram. The resemblance of the

method SPT and DPH is showcased by the fact that the SPT30hammer shock number regarding 30

cm penetration depth at SPT is equal to the N20belonging to 20 cm of penetration depth at DPH

according to the literature.

Figure 9/a.: Dynamic probe

(www.tordrilling.co.uk)

Figure 9/b.: DPH sounding diagram

Recently, Cone Penetration Test (CPT) has assumed considerable proportions. The method

was named after the tip of the probe which is a cone of 10 cm2 and 60, driven into the soil at the

quasi-constant speed of 2 cm/s.

Figure 10.: CPT sounding process


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The counter weight necessary against the pressure is either maintained by the truck or by soil

anchors. Stress during the driving is measured at the probe tip and at the mantle of the cone that are

called tip resistance and mantle friction. The tips of some special tools of this kind are capable of

measuring pore water pressure and most recently emitting seismic waves. The measured data is

transmitted directly to computers.

The method is the

base of pile dimensioning

but more and more

correlations are concluded

regarding the shear

strength, the Modulus of

Elasticity or even the

yielding tendency of soils.

Figure 12. summarises the

most important data of

sounding. Figure 11.

shows a general set of data

obtainable after analysis.

Courtesy of Robertson

(Robertson 1986) even the

type of soils can be

identified from sounding

data with relatively highprecision.

Figure 11.: CPT results


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The Vane Shear Test is a sounding method used at soft cohesive soils and peats. With the help

of the correlations defined for this very method the undrained shear strength of soils can be

conveniently concluded in-situ. At the sounding a four-winged probe tip is rotated in the soil with

which around the mantle the soil is sheared. The force necessary for the rotation (torque) is measured

and recomputed into drained shear strength in the knowledge of the mantle surface area. The method

and the most important data is shown in Figure 12.

Figure 12.: VST process

Flat Dilatometer Test is all about obtaining parameters of the soil regarding horizontal stresses

and deformations. On the side of a blade like probe tip pushed into the undisturbed soil at the bottom

of a borehole, there is an inflatable membrane. The pressure required for the inflation and the

deformation is recorded. The short summary of the method is presented below in Figure 13.

Figure 13.: FDT process


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The Pressuremeter Test is an indirect process widespread in French-speaking territories. The

method is basically similar to FDT however; here a cylindrical probe is inflated in whole. The

sounding also takes place in a borehole but in some cases self-drilling pressiometers can be found as

well. In the process the necessary pressure and the deformation is measured. The load bearing

capacity of the layered planes can be calculated from the measured data. The process is presented in

Figure 14.

Figure 14.: PST process


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2.2.2 Geophisical measurements

The geophysical methods measure soil stratification and the properties that of each layer from

the surface or from boreholes. During the measurements, resistance of some kind or signs appearing

due to that are measured, and by the analysis of the received data will only be the result plastic and

displayable.The analysis of the measurements is done on the basis complex physical correlations to which

the knowledge of geophysical properties of soils and bedrocks are indispensible. Civil engineers in

general only use the dataset of results computed by geophysicist.

Of the geophysical methods, only the mechanisms of the below listed ones and their ways to

extract data from them shall be summed up shortly:

Geophysical processes

geoelectricsgeoradarcross-hole, down-hole

refraction seismic survey

The geoelectrics, which is a direct circuit measurement method works on the principal of

Ohms Law. Various soil types have different measurable electrical resistances thus individual layers

can very well be told apart. Having measured the current and the electrical resistance between a pair

of anode and cathode, the distance between the poles can be enlarged and so the depth of

examination. Nowadays, the aforementioned obsolete method of pole distance enlargement is

replaced by the so called multielectrode measurement method. In the multielectrode method, as it

name indicates, tens or even hundreds of metal bars are driven into the soil some electrified by a

control unit while some serve as medium through which current is measured simultaneously. By thisprocedure data is gathered at exceptionally large number of sampling points. Depending on the

distribution, tens of meters of penetration can be achieved. It is practical to apply this method over a

large area as soil stratification mapping or 2,5-3D display determination of horizontal places of layer

borders is easy and fast.

Figure 15/a depicts the work principal while Figure 15/b. showcases an on-site measurement.

Figure 15/c. shows the visualization of the processed data.

Figure 15/a.: Method of geoelectrics 15/b.: Multielectrodes on site


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Figure 15/c.: Soilsection by mulitelectrode

Engineering georadar technology (GPR) is a measurement performed with different

frequencies. During the measurement a receiver measures the waves emitted by the transmitter and

reflected by the various soil layers. The method is somewhat limited possessing a ~3 m penetration

depth capability though this threshold can be examined with comparatively high definition of display.

The method is mainly used for mapping voids, loose zones and washed out spaces.The theory of the measurement is shown in Figure 16/a while Figure 16/b depicts ~1m thick

layer and a 3m deep public utility manhole.

Figure 16/a.: Method of GPR

Figure 16/b.: Fill and manhole in GPR results

The cross-hole and down-hole methods determine the shear wave propagation velocity. As EC8 took

effect in Hungary a bigger emphasis was laid on structure dimensioning against earthquakes. The

reaction spectra used at dimensioning were the function of soil properties with special regards to shear

wave propagation velocity. This can only be known on-site via cross-hole or down-hole

measurements. In case of both methods, waves are generated on one spot and measured on another.


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Figure 3.: Denotations of groundwater levels on the borehole log

According to Eurocode 7, the Design Groundwater Level (GWLd) is equal to the value

of the Characteristic Groundwater Level (GWLk), which is the estimated maximum GWL

raised by 0,5 m.

0,5,The water level expected during construction is called Construction Groundwater Level.

The permanent structures should be designed for the GWLd, but at the temporary structures

(e.g. sheetpile wall) can be used the construction GWL.

The chemical compositionof the soil shall be defined as well.(SO4, pH, Cl).

In general-if the suitability of a dewatering system cannot be justified and its operation

maintained. The design groundwater level value can be taken as the highest level ever

recorded which may very well be identical to the surface level. The groundwater types (Figure

19.)

- 1) Free surface;

- 2) Pressure groundwater;

- 3) Lower groundwater

floor;

-

4) Water dome;

- 5) Floating groundwater;

- 6) Pseudo groundwater.

Figure 4.: Groundwater types


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The groundwater level plays an important role in the following cases:

- Taking up the foundation level (above construction groundwater level)

- Excavation dewatering;

- Effect of the fluctuation of the water level on geophysical properties (strength,

compressibility)

- Danger of uplift;

- Backwater effect (swelling)(e.g.: Metro tunnel, underground garage);

- Interference in hydraulic ecology:

water withdrawal,

groundwater table sinking,

piping,

deforestation,

mining activity,

establishment of fishpond, reservoirs,

channelling.

2.3 The necessary extent of ground investigation

This extent is defined by the importance, the value, the sensitivity to settlement, and the

size of the building as well as subsoil properties. The exploration plan is to be constructed on

the bases of the aforementioned. The more complicated the building is, the worse the subsoil

conditions are, the more detailed the exploration should be.

The facility and the ground investigation can be:

-

point like (e.g.: monument);- linear (road, pipeline);

- areal (industrial site).

The explorations shall be designed under the foundation level down to such adepthwhere the

soil compression is negligible in terms of the stresses caused by the building and the

stratification of the soil.

To the design of the explorations the standard EN 1997-2:2007 ANNEX

B assigns the following directives:

1.

For high-rise structures and civil engineering projects, the largervalues of the following conditions should be applied

za6m

za3,0bf


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2. For raft foundation and structures with several

foundation elements whose effect in deeper

strata are superimposed on each other:

za1,5bB

where bB is the smaller side of the structure.

3. For piles the following three

conditions should be met:

za1,0bg

za5m

za3,0Df

where

Dfis the pile base diameter

bg is the smaller side of the rectangle

circumscribing the group of piles forming the

foundation at the level of the pile base

4. For small tunnels and caverns:

bAB


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6. For excavations where the piezometric surface and the groundwater tables are below

the excavation base, the larger value of the following conditions should be met:

za0,4h

za(t+2,0)m

wheret is the embedded length of the support,.

h is the excavation depth.

7. For excavations where the piezometric surface and the groundwater tables are above

the excavation base, the larger value of the following conditions should be met:

za(1,0H+2,0)m

za(t+2,0)m

where

t is the embedded length of the support,.

H is the height of the groundwater level above the excavation base.

General remark: Up to soil layer of good strength and adequate thickness shall be drilled.

Construction on bomb- site: The foundation level of the neighbouring buildings is to be

defined as well.

Floor- attachment: area and foundation level depth defined with open ditch exploration. The

product is the soil exploration report.

The recommended spacing of the investigations can be found at the Table 1.Table 1.

Type of buildingLayout of ground investigation

points

High rise buildings, industrial facilities Grid of 15-40 m

Buildings of big area Max grid of 60 m

Linear structures (road, railroad, canals, pipelines, causeways,

retaining walls)Grid of 20-200m

Special structures (e.g.: bridge, chimney, machine base) 2-6 explorations per substructure

Dams, barrages 25-75 m at the critical sections


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3 SHALLOW FOUNDATIONS

The foundations transmit the load of the building onto the soil. If the foundation is directly

part of the whole structure, for instance placed right under the wall (as an extention), then we

are speaking about shallow foundation. Should the load bearing soil layer be situated lower

below, pile foundation or diaphragma wall (deep foundation) is necessary to be installed asload transferring structural element.

Deepened shallow foundation: if the foundation is placed deeper than the necessary minimal

level of foundation by structural admissibility. For instance, a building having a cellar is

usually assigned with a foundation depth of 2,5-3,0 m.

Shallow foundation can be applied if:

- Near surface soil layer of adequate load bearing capacity and thickness there is;

- The layer close to surface has no big strength but deeper down the layers have no

better properties either hence the load of the building can be distributed over a

large area (slab foundation);- The load bearing capacity of the subsoil is small but the structure imposed on it is

not sensitive to building settlements and by applying near surface shallow

foundation the costly water table sinking and deep foundations can be avoided.

Deep foundation is to be designed only in case if the shallow foundation is not feasible

technically or could only be constructed with higher costs.

3.1 Types and structures of shallow foundations

3.1.1 Stripe foundations

Stripe foundations are built to continuously support the walls. In exceptional cases, their

bottom width can be identical to that of the wall but given that the load bearing capacity of the

soil is lower than that of the construction materials, they generally reach out sideways in

cantilever-like manner under the walls.

Stripe foundations can be made of: brickwork, rubble (ashlar), floating concrete, rammed

concrete and reinforced concrete.

The rammed concrete stripe foundation (in formwork or between soil walls) is presented in

Figure 21.

Figure 5.: Concrete stripe foundation


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Figure 6.: Types of stripe foundations classified according to their materials

3.1.2 Pad (point) foundations

Pads are placed under columns at framed structures. Their cross section is usually quadratic or

A/B = 1-3.5. Their choice of material and method of construction is similar to that of the

stripe foundations. Due to increased loads they are made of concrete or reinforced concrete.

Most frequently pad foundations contain a reinforcing steel mesh (Figure 23).

Figure 7.: Structure of pad foundations

At the prefabricated (precast) columns of industrial facilities the following versions are in use.

Figure 25. The chalice foundation seen at part a) is comparatively common. The steel or

reinforced concrete columns are placed into the chalice of the prefabricated pad then

concreted in.

Floating concreteConcreteConcrete

Stone (rubble) Brickwork Reinforced concrete


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Figure 8.: Types of pad foundations classified according to their materials

Figure 9.: Application of prefabricated columns

The connection of the steel column (pillar) and the concrete pad is shown at part b) the screw

connection is meant to transmit tensile stresses.

3.1.3 Raft slab foundations

Because of weaker soil conditions or mechanical reasons pillars are placed on a- usually

heavily reinforced- stripe like beam. (Figure 14.) It is made of reinforced concrete and

provides the building with longitudinal structural rigidity.

Figure 10.: Raft slab foundation

Special version of it being the ring foundation.(high rise buildings).

Brickwork Stone (rubble)

Concrete

Reinforced Concrete

Chalice foundation Joint of the steel column

Widening used incase of higher

pillar spacing


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3.1.4 Continuous footing (beam grid foundation)

System of intersecting raft slabs. (Figure 27.)

Figure 11.: Beam grid foundation

Constructed if the soil is weaker or enhanced rigidity is necessary in two directions. The

material being reinforced concrete.

3.1.5 Slab foundations

The slab foundations are transitive reinforced concrete structures under the whole building

(Figure 28.), that support walls and pillars alike.

Figure 12.: Slab foundations

Their construction only takes place if the loads of the building can only be transmitted over

the whole surface area otherwise the specific load would exceed the load bearing capacity of

the soil. Originating from their relatively small thickness, slabs are quite flexible in general. If

there are reinforcing bars inside the slab at the places of ribs but the thickness of the slab is the

same as the height of the ribs then the structure at hand is a concealed slab. Strength-wise the

convex vault in the bottom is more favourable though more complicated to make. At fully

cellared building if insulation against water pressure is needed as well, the slabs are

economical choices.

Flat Slab

Upper chord ribbed slab

Lower chord ribbed slab

Mushroom-like slab

structure


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3.1.6 Box foundations

Dwelling houses assembled of prefabricated house elements are only capable of sustaining

minor deformations without major damage suffered. Therefore in that case, the slab

foundations with the cellar walls and slabs built on them form one monolithic structure.

(Figure 29.)

Figure 13.: Box foundation

3.1.7 Shell foundations

A shell foundation is a special- material saving but labor- intensive, slab foundation. These are

mathematically very well described surfaces of single or double curvature that negotiate only

normal force but not bending moments.

3.2 Steps of shallow foundation design

Before the design of the foundation the plans of the building are to be examined from

structural and static point of view:

- How about rigidity and settlement sensitivity?

- The soil exploration and examination data possessed is of adequate kind?

The design is a question of technical and financial aspects.

3.2.1 Taking up the foundation level

Foundation level: The lower, supported surface of the shallow foundation.

Foundation depth: The vertical distance measured between the ground surface and the

foundation level.

The foundation system used is predominantly the function of the foundation level. While

designing, always the structurally necessary, minimal depth foundation shall be taken into

account first.

Requirements:

- the foundation level shall be below the frost line (freezing depth);

- should be on a load bearing soil just limitedly compressible;

- if possible, shall be above ground water level to avoid costs of dewatering and

insulation;

- should be located at the depth demanded by the structure (cellar, underground

garage, etc.);

- in case of variable-volume subsoils, the foundation level shall be above the drying

up level;

-

shall fit into the built environment.


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Frost line (freezing depth): the largest thickness of the soil layer cooled below 0 in the

winter. Under national circumstances the frost line:

- in granular soil: 0.8 m;

- Over 500 m above the Baltic Seal Level: 0.9 m;

- in cohesive soil: 1.0 m;

- in case of foundation on solid bedrock: 0.5 m.

Figure 14.: Taking up the foundation level at shallow foundations

3.2.2 Areal dimensioning on the bases of load bearing capacity

The horizontal extensions of the shallow foundations (length, width) are determined by the

design value of the load bearing capacity of the soil and the expected settlements. In practice,

firstly the dimensions are determined by load bearing requirements, and then in terms of the

aforementioned, the allowable settlements are checked.

a)

Load bearing capacity of shallow foundations

The load transmitted to the soil from a foundation of given position and size can be

determined:

- by loading test;

- by experiences acquired from the cases of neighbouring buildings;

- theoretically from formulae (codes);

-

by data from sounding (half empirical method).

At theoreticaldetermination, at first the mechanism of soil failure shall be made clear.

On flat terrain

In case of slab foundation

In case of pad foundationIn case of stripe foundation

On sloped terrain

Brick stripe foundationMonolith concrete stripe

foundation

On flat terrain in case of partially cellared building

In case of pad foundation In case of stripe foundation

Concrete fill-in


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b)

The mechanism of soil failure

If an ever growing load is imposed on a foundation, the soil below the foundation is getting

compressed more and more. Initially, the settlement is directly (linearly) proportional to the

force, the load.(Figure 31)

Figure 15.: The mechanism of the soil failure

Later on, the penetration increases dramatically (vertical and horizontal grain particle

displacement takes place as well), and a well defined slip surface- soil failure takes form as

the loaded area loses its support. The failure does not always happen the way it was

mentioned. There is:

- general shear failure;

- local shear failure;

-

impact (drilling-in).

c) Computation of the failure load

As a substructure we consider a stripe foundation with a vertical centric concentrated force on

it and a small foundation depth (t < 2b). This time, with regards to the soil mass above the

terrain being either backfill or loose surface layer; the shear strength of the soil above the

foundation level is modified in favour of safety.

Terzaghis failure stress formula is written in the following general form:

where: N

b, N

t s N

c bearing capacity coefficients, their value is the function of friction

angles computable and presentable in graphs and curves (Figure Figure 16.)

Time

Plastic zones

Slip surface

Load

Elastic impact

Elasticzone

Plasticzone

Intermediatezone

s

e

tt

l

m

e

n

t

s

e

tt

l

m

e

n

t


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Lateral dimensioning of substructures

The design value of the vertical load coming from the self weight of the building is

compared to the design load bearing capacity of the soil. The load bearing capacity is

adequate if:

Ed< Rd

where

Ed The design value of loads

Rd The design value of resistance (soil failure resistance)

3.2.4 Vertical dimensioning of shallow foundations

Having determined the base width, the next step is the calculation of foundation height. This

can be comprehended as the mechanical dimensioning of the whole structure. To this, thestress distribution along the foundation level shall be known. The standard load of the shallow

foundations is the bending moment generated by the loads of the superstructure and the

support reaction forces of the soil. Shear forces may arise as well (at pillars there is the danger

of punching).

a) Distribution of base stresses

The base stress is the stress generated at the foundation level, in other words, the specific

value of resistance of the soil counteracting the loads of the facility.

The resultant of the base stresses shall be in equilibrium with the loads. i.e.:

- The resultant of the base stresses = external load;

- The moment acting on the foundation: M = 0.

Factors influencing the distribution:

- Features of the substructure (structural rigidity, shape, width), the structural

rigidity of the building, foundation depth;

- Soil properties (granular or cohesive);

- Measure of loading, distribution pattern, line and point of effect

At rigid substructure the location of the resultant is important.

At flexible substructure the distribution of the load is important (Figure Figure 17.).

Figure 17.: Base stress distribution at rigid and flexible foundations

Rigid Flexible


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Rigid foundations

Rigid foundations are such that their plane does not deform even under load. These concrete

slabs almost identical in width and height are rigid.

At cohesive soils, according to (Figure Figure 18. (b)) a saddle like base stress body is likely

(with small sideways deviation).

In dry, granular soils (e.g.: sand), grains under the substructure can be displaced sideways

(Figure l. c.), in which case the stress under the edges (in case of t = 0) can be reduced to zero.

The stress distribution will be parabolic. The effects of foundation depth increment are

depicted in Figure d.

Figure 18.: Base stress distribution under a rigid substructure

In practice, simplifications, approximations are made in general and the work is done by the

base stress distribution shown in Figure Figure 19. The base stress distribution does not

influence the load bearing capacity of the soil since the failure takes place inside the soil mass.

Figure 19.: Base Stress distribution under stripe foundations (simplified) in case of differenteccentricities (e)

Granular soil

Granular soil

Foundation depth

Cohesive soil


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Flexible foundations

Base stress distributions are shown in Figure Figure 20.In a sense the base stress distribution

is the mirror image of the load distribution.

Figure 20.: Base stress distribution in case of flexible substructures

b) Determination of the height of stripe and pad foundations on the bases of

construction rules

The k cantilever length is calculated applying the construction rule and bearing in mind the

geometry of the load bearing wall or pillar above the substructure (Figure Figure 21 .). Thequotients of necessary cantilever lengths concerning various soil types and substructure

heights is summarised in Table 3. Based on that, the necessary h height can be calculated

back.

Figure 21.

Table 1.

Soil Type (Load Bearing Capacity) k:h

Dense grainy soil (>=36)

Hard cohesive (Cu>=75kPa)

1:2

Grainy soils

Plastic cohesive (Cu>=40kPa)

1:1,5

Small capacity 1:1

3.2.5 Dimensioning of raft slab foundations

The crosswise dimensioning is identical to that of the stripe foundations.

Longitudinally however, raft slabs are more flexible and base stresses consequently are

modified in terms of that. In other words, in longitudinal direction raft slabs are to be

dimensioned on the bases of the principals regarding beams on flexible supports.


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3.3 Settlements

The settlements- as described before- are vertical displacements of buildings, foundations

related to an initial reference point in time and in space. The proper design of the foundation

includes the proof that the deformation sustained by the subsoil is of not that great of a

magnitude to react harmfully on the building.

Causes of settlements:

- static loads;

- dynamic loads and effects;

- effect of water present in the soil (fluctuation of GWL, groundwater flow, slump,

swelling, drying, pipe leakage);

- under-washing (the void creating effect of groundwater), mine, cellar, tunnel;

- landslide (near surface soil mass movement);

- chemical transformations (swelling, dissolving);

- Thermal effects (frost, cold stores, furnaces).

Only the magnitude of the expected settlements induced by static loads can be calculated with

relative precision. In the followings they will be dealt with.

3.3.1 Components and timescale of settlements

Under a quickly imposed static load the

settlement of the foundation is as follows: (Figure 38.)

Part a) presents the case of loads and stresses in

saturated soil. At loading, the pore water pressure rises

(u) and by tc amount of time later, it swings back topore water pressure free state.

In part b) the sk Primary compression can be seen.

This is the result of the shape changing of a loaded soil

mass without change in volume (particles pushed

aside). Significant in case of closed, large slab

foundations (e.g.: silos)

In sketch c) Consolidation (sc) is lasting from t = 0

until tc. The reason of it is the loss (push out) of the

water found in the voids, resulting in a volume loss.

The part d) phenomenon, the slow, creep-like growing

procedure is the Secondary compression (sm). This

type of compression is most common at overloaded,

soft, fat clays (high plasticity), and organic soils. (To

be omitted in Hungary.)

The sum of these three components in the function of

time yields the respective settlement as shown at part

e)

Figure 22.: Components and timescaleof settlements


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The largest part of settlements is caused by consolidation therefore the timescale of sc is

important. The lower the permeability and bigger the compressibility of the soil, the lower the

process of consolidation is. The compaction of sand takes place quick while the settlement of

buildings built on clay can take a longer period.(Figure Figure 23.).

Figure 23.: Consolidation curve

3.3.2 Settlement calculation

The first task of the settlement calculation is to determine the standard loads from the

viewpoint of settlements. The permanent load (usually the self weight/dead load) shall be

calculated precisely with detailed computations then the possible additional loads, their time

of action, their probability and their frequencies of reoccurring should be analysed. The

standard value of mobile loads regarding settlements depends on the permeability of the

subsoil, the time of action (e.g.: wind load) and the type of the building (dwelling house,

industrial facility, bridge, silo etc.) The substructure, the foundation, the pillar of a bridge etc.

is just a preload they do not exert effect on the superstructure later installed. Dynamicmultiplication factor is not used generally at cohesive soils. The loads are to be taken into

consideration without safety factors. Having fixed the load states, the stresses in the soli are to

be determined.

a)

Stress distribution in the soil mass under the foundation

Influenced by:

- the quality of the soil;-the magnitude of the load;-the size, shape and other properties of the substructure

Assumptions (simplifications):

- since stresses only reach a certain quotient of the failure stress, the soil is

considered elastic with Hookes law being valid: =Es- the soil is homogeneousand isotropic;

- the Esand is constant principal of superposition is valid.

Stress computing methods from theoretical bases are capable of computing with:

- concentrated (point) load;

- linear (distributed) load;

-

lane load;- closed areal load.

Time

Granular soil

Cohesive soilNormal consolidation

curves

s

e

tt

l

m

e

n

t


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b)

Case of concentrated force

Loads transmitted over a rather small area are similar to this. This case can be used atblock foundations calculating the excess stress under neighbouring foundations. Accordingto Boussinesq(Figure Figure 24.) at any point B of the elastic surface of a soil, the stressgenerated by a point- like vertical concentrated force F is:

3 2

, cos

In fact, the stress components have similar, more or less complicated formula as well

sx,sy,txy,

Figure 24.: Case of concentrated force

c) Case of linear load

The behaviour of a rail laid on the ground (crane track) is approaching the most this,again theoretical case. Applicable at approximate calculations of excess stresses appearingunder neighbouring stripe foundations.

Figure 25.: Case of linear load


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d)

Case of lane load

This type is common under stripe foundations of walls. Naturally, the biggest vertical stress is

at the axis of symmetry of the lane.

sin cos Mitchellsdeduction regarding stresses under the stripe foundations of walls may be seen in

Figure Figure 26.

Figure 26.: Case of lane load

At the offset from plane load over dx width-area:

magnitude of force is acting whose only feature in the figure is the final result. Again, of

course the greatest vertical stress is at the axis of symmetry of the lane. (Figure Figure 27. ).

The angles appearing in the formula are meant to be unit less.

Figure 27. : Case of lane load


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e)

Case of closed areal load

This case is the most frequent in engineering practice (block, slab foundation). One of the

simplest cases: Stress determination zof uniformly loaded (p), slab of r radius measured in

the centre. Deduction here is done by starting out of Boussinesqs correlations of point-like

loading (Figure Figure 28.).

1

Figure 28.: Determination of stresses under a disc (Frhlich).

Steinbrenner was the first to derive interrelations on right- angled rectangular

substructures. Direct utilisation of the results of his complicated derivation would beproblematic therefore a graph is used to determine the vertical stresses fast (Figure Figure

29.).

Figure 29.: Calculation of vertical stresses with the help of the graph

First of all, L/B and z/B ratios are calculated, where z is the depth under the foundation level

of the examined points (at which points stresses are looked for). The value of z/B is taken onthe vertical axis then progress is to be made horizontally towards the corresponding L/B


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curve. The point of intersection is projected to the horizontal axis where a z / p ratio can be

read from which in the knowledge of p base stress, z is computable. With the help of the

graph, the stresses under the corner points are obtainable as well.

At settlement calculation the vertical stresses (average stresses) generated in the line of the so

called characteristic point is used. Hence graphs were constructed in order to acquire the

stress generated under the characteristic point of the right angled rectangular foundations as

well (Kany) (Table Table 2.), which can be seen here in a tabular form. (B = the smaller base

width).

Table 2.: Stress determination under the characteristic point

0 0,2 0,4 0,6 0,8 1

0 1,000 1,000 1,000 1,000 1,000 1,000

0,05 0,990 0,990 0,989 0,988 0,985 0,981

0,1 0,945 0,944 0,941 0,932 0,918 0,898

0,2 0,826 0,824 0,804 0,770 0,731 0,6940,3 0,739 0,730 0,689 0,637 0,593 0,557

0,4 0,677 0,660 0,601 0,544 0,502 0,470

0,5 0,630 0,603 0,532 0,477 0,438 0,409

0,6 0,590 0,553 0,477 0,425 0,389 0,362

0,8 0,524 0,469 0,392 0,348 0,316 0,289

1 0,467 0,399 0,329 0,290 0,260 0,234

1,5 0,360 0,278 0,226 0,193 0,166 0,144

2 0,288 0,206 0,163 0,134 0,111 0,094

3 0,203 0,128 0,095 0,072 0,057 0,047

4 0,155 0,088 0,060 0,044 0,034 0,028

5 0,125 0,065 0,041 0,029 0,023 0,018

6 0,113 0,056 0,035 0,024 0,020 0,015

7 0,100 0,047 0,029 0,020 0,016 0,013

8 0,088 0,039 0,023 0,016 0,013 0,010

9 0,075 0,030 0,017 0,012 0,009 0,008

10 0,063 0,021 0,011 0,008 0,006 0,005

12 0,056 0,018 0,009 0,006 0,005 0,004

14 0,050 0,015 0,007 0,005 0,004 0,003

16 0,044 0,012 0,006 0,004 0,003 0,002

18 0,038 0,009 0,004 0,003 0,002 0,001

20 0,032 0,006 0,003 0,002 0,001 0,001

B/L

z/B


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3.3.3 Approximate calculation of the stresses

The aforementioned methods of calculation were somewhat complicated inspiring the

practical engineers to come up with simpler, more convenient methods of stress calculation

especially in connection with stripe foundations.

a)

Closed region bounded by straight lines

The sidelines z= 0 enclose an angle with the vertical and there is no limit in depth. Between

the sidelines at any z depth uniformly z stress is generated (Figure Figure 30.).

Figure 30.: Stress body (diagram) bounded by straight lines

On the bases of vertical equilibrium statement:

2 tg

2 tg By convention = , tg= 0.5 is assumed, but in the national practice = 30 , or = 45 is

more widespread.

b) Jkys Limiting Depth Theory

Figure 31.: Jky approximate method

Stress is dissipated linearly

Stress reduction


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2 1 2 up to the limiting depth (below which no stress from the surface load is generated)

Therefore at stripe foundation:

m0= 2 B, (L )

at square pillars:

m0= B, (L = B)

Between the verticals of the edges of the substructure (based on the principal of similar

triangles):

that is:

The distance from the vertical statement of equilibrium:

12 2

Having substituted zback:

c) Determination of settlements

Basic correlations

Vertical specific deformation of the medium according to the elasticity of materials:

1

It it is not the E Modulus of Elasticity but 1 1 1 2

the compression modulus directly obtained from compression curves is what the calculation is

done with, then the effect of sx, systresses has already been taken into account as well thus

the formula at hand is:

1


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d) Calculation in practice

Out of the three components of settlements caused by static loads, as mentioned before the

Secondary compression is neglected under normal circumstances (sm 0). The traditionalsettlement calculation does not disentangle Primary compressionfrom initial consolidation

as these two are calculated together exclusively via laboratory compression tests. At normally

consolidated materials calculated traditionally, 60-95% is consolidation (sc), while the rest

being Primary compression (sk).

Steps of the traditional settlement calculation:

- Taking up the foundation depth, taking up the area, calculation of average base

stress, sketching soil stratification;

- Fixing the standard load combination from the viewpoint of settlements;

-

Determining the vertical self weight stresses with regards to GWL.- Determining the off-load of the layers under the foundation caused by the

excavation (cellar or footing pit);

- Calculating the distribution of the vertical normal stress in the function of depth in

the axis of the substructure (under the foundation level);

- Defining the compressibility of each layer (perhaps as the sum of sub results of

layers laminated);

- Obtaining the total settlement as the sum of individual ones

The compressibility of each layer can be calculated either by compression curves or making a

good use of the compression modulus.

1. Calculation with compression curves

The essence of this method of calculation is depicted in Figure Figure 32.

Figure 32.: Settlement calculation


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In terms of soil layers under the foundation level (1, 2, i. n.) the self weight stress diagram can

be drawn (hii) as well as the diagram of stress dissipation (z) keeping in mind the off-loading (z 0= p t0) . To the horizontal axis of the compression curve of the examined i-th soil layer, the self weight stress acting in the midline of the layer is projected ( g 1, g2, gn) ,

taking into consideration the fact that the layer was not unloaded prior to the construction, by

that. As a continuation ofgithe average stress (z1,z2,zn) generated by the building at the

midline of the layer (in case of lamination, the sub layer) is surveyed.

Readings are made on the vertical axis showing the increment of load induced specific

deformation (1, 2, n), according which the compression of the layer:

si= hii.

The settlement of the foundation:

.2. Calculation with compression modulus

Summarised in Figure 37.

Figure 33.: Settlement calculation

As seen before:

1

This calculation is to be carried out with every layer (or/and sub layer) then summed up with

regards to compression moduli (Es):

Stripe foundation


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.

If the dissipation of stress is considered linear based on Jkys theory (Figure l. right hand

side), then the approximate value of the settlement is obtained easily:

. 2 e)

Limiting depth

It is exclusively the Theory of Jky that delineates the depth, below which (excess) stress from

the surface load is not generated under the foundation level (m0). Practically speaking, that

means that only with that depth shall settlement calculations be carried out. In practical cases

anyway it is advisable to draw a line of depth to which extent the load creates soil

deformation. The codes of most countries regard the depth m0the limiting depth, where:

thus the load generated stress is equal to the n quotient of geostatic pressure (self weight

stress). In the national practice, n = 5 calculating (0,2 hii). In Germany and in the USAn=10 value is in use. If the width of the base is B > 10 m (slab foundation), then experiences

show that:

m0= B B / 2

Taking up a limiting depth is justifiable (cohesive granular soils).

f)

Taking collapse into consideration

It has already been learnt in the subject soil mechanics that soils having macro pores and loose

structure (loess, loose sand, fill) sustain settlements exceptionally fast slump (collapse) under

load, or wetting. With the compression curve constructed by the Oedometer slump test, the

excess settlement under the foundation level may be calculated (Figure Figure 34.).

sr= rh, where his the thickness of the slumping layer

This incremental settlement value shall be added to the settlement value calculated from staticloads.

Figure 34.: Settlement from collapse

Soaking


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3.3.4 Settlement tolerance of buildings

Having calculated the settlements, received data is to be examined if it is of allowable

magnitude for the building at hand. The settlement in absolute sense (elevation-wise) may

cause trouble at, for instance, the sewage system and other public work connections, or

adjacent buildings. Uneven settlements may result in warpage, curl, tilting or deflection of any

kind and the excess load coming from all of them (moment, shearing).

The settlement tolerance of the building depends on:

- the structure;

- the dimensions;

- the function.

Structurally: The statically indeterminate structures- multi supported beams, frame girders,

arc girders etc.- are more sensitive to settlements. Buildings made of prefabricated housing

block are also sensible because of the corrosion protection of the steel joints. In terms of

dimensions those buildings having a centre of gravity high (water towers, smokestacks) are

the most endangered and sensitive.

By function, those facilities are sensitive whose un-cracked state is a precondition of safe

operation (tanks, pools, nuclear power plant).

Under the term of EN 1997-1 Threshold limits for deformations and displacements of load

bearing structures, annex H:

Components of substructure displacements to consider: settlement, settlement difference (or

relative settlement), rotation, tilting, relative deflection, relative rotation, horizontal

displacement and amplitude of vibration. For the concepts of the various substructure

displacements and deformations, see Figure H1. ( Figure 35.).

It is highly unlikely that the maximum allowable relative rotation of open frame structures,

filled in frame structures and load bearing or continuous brick walls is the same, nevertheless

these values, by all likelihood, may be found in the regime between 1/2000 and 1/300 in order

to avoid reaching Serviceability Limit State. The 1/500 maximal relative rotation is a value

most building can tolerate. The relative rotation value that is probable to reach the Ultimate

Limit State is approximately of 1/150 magnitude.

These proportions refer the trough-like displacements as shown in Figure Figure 35. In case of

opposite, crest-like displacements (i.e: edges settle more than the plain between them) it is advisable to

allow only the half of the values mentioned before.

In case of general load bearing structures on individual foundations, total settlements smaller

or equal to 50 mm are tolerable (at granular soils, at cohesive soils the limit is 71 mm).

Settlements exceeding that are only allowable if the relative rotations stay within the tolerable

threshold and if the total settlement causes no problem at public utility connections in load

bearing parts or does not involve leaning etc. The settlement differences are usually of themagnitude of one third or one half of the calculated total settlements (due to loads and the


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variance in soil properties).

These settlement restricting principals refer to ordinary, general structures. They are not to be

applied at buildings being extraordinary or having a significantly uneven loading.

Figure 35.: EN 1997 appendix H1. Substructure displacement concepts

In the table of national annex NA1. (see table Table 3.) all the limit values shall be perceivedas the as the ratio of settlement differences between the critical points of a building and the

distance between these point, according to the following:

- relative rotation is to be calculated from the settlement difference between two

arbitrary,

- tilting is computable from the settlement difference of two terminal points of a

rigid building,

- the relative deflection is calculated by connecting an internal point with a terminal

one. The excess settlements of points with regards to the line set out by the above;

over the distance is the relative deflection.- the relative inflexion is comprehended similarly to relative deflection if the internal

point remains above the line connecting the terminal points (edges).

According to Hungarian practice, the R curvature radius created by uneven settlements of

ordinary buildings is related to the L length and H height: R / (L H) as follows:

- no cracking expected even without partial defense if R / (L H) > 0,25,

- partial defense (e.g.: lower crown)prevents cracking if R / (L H) > 0,06,

- crack though appear, cause no mortal peril if R / (L H) > 0,04,

- cracks cause no mortal peril in case of partial defence R / (L H) > 0,01.

The R radius is to be determined as a circle set out by three structurally significant points.

Settlement

Settlement difference

Rotation

Relative deflection

Deflection ratio

Angular displacement

Relative rotation

(angular distortion)

Leaning


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Table 3. EN 1997-1 NA21.

Settlement tolerances of buildings

Structural and foundational characteristic of the building Nature of deformation

Deformation limit value

If the consolidation is

Fast Slow

Buildings withload bearingframework

Statically indeterminate reinforcedconcrete or steel frameworks

Relative rotation 0,002

Statically indeterminate reinforced

concrete or steel frameworks with brickfilled-in terminal pillar rows

Relative rotation 0,0007 0,001

Statically determinate frame structuresRelative rotation 0,005

Buildings

without loadbearing

framework

Prefabricated large housing block or

unreinforced brick wall without framestructure

Relative deflection 0,0007 0,001

Relative inflexion 0,00035 0,0005

Reinforced concrete prefabricated largehousing block or brick wall with steel coreor reinforcement

Relative deflection 0,001 0,0013

Relative inflexion 0,0005 0,0006

Single-storey industrial buildings or similar structuresRelative deflection 0,001

Relative inflexion 0,0005High centroid rigid buildings or buildings with rigid foundation Tilting 0,01 L/H

Crane tracks(rail)

Longitudinally Relative rotation 0,004

Transversely Relative rotation 0,004

Informative values of allowable settlements of building:

- load bearing brick walls: 8-10 cm;

- brick wall with rc. crown: 10-15 cm;

- rc. and steel framed buildings: 10 cm;

- buildings on slab foundation: 20-30 cm;

-

high centroid buildings (smokestack, silo): 20-30 cm;- pillar framed, 1-2 storey industrial buildings:

- 6 m- of pillar spacing: 6-8 cm;

- 12 m-of pillar spacing: 9-12 cm.

Generally, settlement differences cause trouble. At buildings standing on cohesive soil (clay)

approximately one and a half time bigger settlement differences are allowable than on

granular soil (construction materials tolerate better slow consolidation).


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3.3.5 Causes of uneven settlements

As mentioned before, the majority of building damage is due to uneven settlements. The

classification of causes is shown in Figure Figure 36.

Figure 36.: Uneven settlement causes

The cracks, towards the larger

settlement (more precisely towards the

part situated lower after the motion)

elevate. Naturally, harmful settlements

may not be only caused by static loads

but other effects such as water, dynamic

loads, shrinkage etc. First cracks appear

at the weakest spots of the walls of the

houses (doorways, windows, between

corner points). Doors, windows get stuck,

glasses cracked. Figure Figure 37.

Uneven loading

Different foundationmethod

Equilibrium state of anexisting building

disrupted

Uneven soil

stratification

Stress superposition

Opportunity for soil

sideways movement

enhanced

Deep

excavation

Piles

Hard clay

Soft claySand

Gravel


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Figure 37.: Cracks from uneven settlements

Hairline cracks are spoken about if the crack width is smaller than 0,1 mm. With rendering

(plastering) cracks of 5-15 mm may be repaired. Although above 25 mm crack width

restoration or reconstruction is necessary.

3.3.6 Measuring settlements

The settlements of significant buildings shall be measured from the start of the construction.

Measurement: generally by levelling (with levelling rods and surveyors levels of 0,1 mm

accuracy).

May be used for measuring settlement differences:

- footing (Figure l. Figure 38.);

-

window sill of the facade;- slabs;

- dependent corridors;

- line of windows;

- sidewalk.

Figure 38.:Result of measuring the footing of the building

For long term measurements, benchmarks can be built into the walls and pillars. (Figure

Figure 39.)

Figure 39.: Surveying benchmark in the wall

Line of settlements

Wall plain

GypsumLevelling rod position


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In the Figure 56. typical settlement curves can be seen. In the settlements of building (a)

standing on granular soil (sand), overwhelmingly take place during construction. In case of

cohesive subsoil(clay), settlements are significant having finished construction as well (b). In

case (c), the curve converges to a diagonal tangent, the slow deformation leads to failure.

Figure 40.: Typical settlement curves

A common task of engineering practice is to define if the cracked building is moving

(momentarily) during site visit. In this case, a gypsum patch shall be rendered over the crack

(Figure l. Figure 41./a), and if cracked after a few days then the settlement is still taking place.It is advisable to render the gypsum over the crack and push a thin glass on it. If the crack

widens both the gypsum and the glass will be broken (Figure l. Figure 41./b.). By this way,

only the fact of the motion may be concluded, its magnitude and direction may not.

With the method seen in figure (c) even the vector of the displacement can be determined. In

the vicinity of the crack three gypsum patches are placed in which noticeable crosses are

carved. Distances x and z are measured with 0.1 mm accuracy. By regular examinations it can

be defined if the crack width increases moreover, the displacement vector can be constructed

with which the nature and cause of the motion is revealed.

Final (constant) load

Sand subsoil

Clay subsoil

Load

Settlement

t time


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Figure 41.: Settlement measurement with gypsum patch

3.3.7 Defence against harmful settlements

If the calculated (expected) settlements or settlement differences are not allowable for the

building then defence shall be built. The types of it:

a)

Applying smaller base stress

A foundation of far larger area than what the soil bearing capacity would require is

constructed. Under a wider surface, smaller stresses are generated. In many cases, results fall

short of expectations as settlements are not mitigated by the magnitude the designer thought.

Partly because the wider substructure meant bigger self weight but mainly because stressescausing compression propagate deeper as well.

b)

Deepening the foundation level

This option is taken into consideration if there is a good bearing capacity soil layer not too

deep and the preliminary settlement calculations yield larger values than allowable at the

foundation level taken up in the near surface compressible layer (Figure Figure 42.).

Figure 42.: Taking up a lower foundation level

This method is economical if the underground premises may be utilised.

It has dual effect:- foundation level is put on a load bearing layer;

Gravel

Soft Clay


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- weight of excavated soil mass increased

Figure 43.: Role of the excess soil excavation

The fact that the excavated excess soil mass significantly reduces settlements can be made a

good use of in designing practice. In Figure58/a. a section of a two-storey-cellared building

can be seen. In part b. of the figure the depicted consolidation curve shows that the settlement

of the building only started when during construction a certain load level was reached.

In this way, an almost perfectly settlement free foundation can be constructed. Namely, if the

mass of the soil excavated from the place of the cellar is greater than that of the building to be

placed in then the resultant of the effective stresses on the subsoil is negative, hence the

building motionless.

c) Soil replacement

The highly compressible original soil below foundation level is excavated in part or in wholeand a replacement soil is put in its place (more favourable properties, sandy gravel or sand

usually) with proper compaction (Figure Figure 44.), and with geogrid, geoweb, geotextile or

composite separating layer if possible.

Figure 44.: Settlement mitigation with soil replacement

Soil replacement may only be done above GWL otherwise the compaction is impossible to be

performed.

d) Restraining sideways movement

In case of loose granular and soft cohesive soils with near surface foundation level, the

sideways movement of soil particles (dodging) from under the foundation may cause

Total load

CompressibleSoft Clay

Wellcompacted

granular

soil


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considerable settlements.

Prevention:

- surrounding the whole foundation with steel sheet piles;

- impose surface load on surfaces about to bulge due to sideways movement.

Sheet piling is especially effective if the piles can be driven into deeper, solid soil layers and

thus a cantilever like behaviour is maintained.

e)

Soil stabilisation

The artificial physical property enhancement of sub-foundation soil layers prone to settle. This

method is when certain substances are put (injected) into the voids of the soil but it is also soil

stabilisation when properties are changed for the better by compaction.

1. Mechanical stabilisation

Deep compaction is used for enhancing stability and reducing compressibility of loose

granular soils and fills. At vibroflotation also widely used in Hungary, a vibration generating

cylinder of 38 cm diameter is proceeding downwards in the soil due to its self weight. Its

motion is also helped by water grouting. In the vicinity of the hole, now created, the soil is

compacted. After that, alongside with the gradual pullout of the vibrator, gravel, grit, mine

tailings and sand is poured into the hole and is compacted by the upward proceeding vibrator

(Figure l. Figure 45.).

Figure 45.: Vibroflotation1

Geodraines, more and more frequently used around the World facilitate the faster

consolidation of cohesive soils. The wick drain like a large sewing machine- grouts or

drives a rigid bar into the soil and simultaneously a polyethylene stripe (geodrain) covered in a

filtration paper (textile). After the removal of the rigid bar, the geodrains remain in (Figure l.

Figure 46.).

1http://www.youtube.com/watch?v=0SR8BMbOpAg

Vibrator

Extension

pipe

Compacted zone

Slump cone

Replacement mat.


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Figure 46.: Geodrain placement2

(The geodrain stripes wrapped up on a bobbin similar to that of the sewing machine are then

cut on the surface.) In most cases, the ribbed plastic drain stripes collect the water of the

cohesive subsoil. With this method, the quick pore water pressure alleviation or permanent

elimination of cohesive soils and building settlement acceleration is obtainable. It is widely

used at motorway embankments where with preloading the desired settlement can be even

more speeded up (Figure Figure 47.).

Figure 47.: Mutual application of drain and preload

2. Soil stabilisation with injection

The injection materials are grouted through drilled or driven pipes into the soil under pressure.

The injecting material can be:- laitance;

- soluble glass (Sodium silicate based),

- Acrilymid,

- Lignosulphite - Lignosulphate,

- Fenoplast;

- Aminoplast;

- other substance

Cement grouting:

The perforated grouting pipes are placed 50-100 cm distance from one another transmitting

2http://www.youtube.com/watch?v=WP-4_5gMb14

Without drain

With drain, without preloading

Preloading

Drain+Preload

Time

Construction phase

Settlement

s

e

tt

lm

e

n

t

7/17/2019 F

Foundation Engineering

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