foundation engineering
DESCRIPTION
foudationTRANSCRIPT
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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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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
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