Equipe 2010 Foundation Design

8/8/2019 Equipe 2010 Foundation Design

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Geotechnical Foundation

Design

Peter Reading,

Technical Director

Equipe Training


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Geotechnical design


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There is an Intimate LifetimeRelationship

A BUILDING A SITEand

Manufactured to adefined layout using

selected very carefullyspecified materials

Location is selectedbut characteristics are

inheritednotspecified

THE GROUND

The Foundation for

success

Between


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Reasons for foundation failure

Application of a foundation load whichexceeds the ability of the soil to support

the load


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Shrinkage or swelling of thesoil


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Differential settlement


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Mining subsidence


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Frost action


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Excavation Failure

Singapore MRT April 30, 2004, 3:15 pm

Failure of excavationstruts


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Tunnel FailureSao Paulo tunnel collapse (Brazil)

Earth pressure design

miscalculation


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Reasons for failure

Excavation of basement

Without control of

groundwater


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With the right information manyproblems can be mitigated

It is essential to carry out a site investigation

The investigation should be made usingappropriate methods and the data obtained,interpreted and evaluated for the proposeddevelopment by someone who is competent andexperienced.

The investigation should be regarded as site-

specific Research and Development


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Geotechnical Design using Eurocode

The structural eurocodes are a European suite ofcodes for structural designdeveloped over more

than twenty five years. By 2010 they will have effectively replaced the current British

Standards as the primary basis for designing buildings

and civil engineering structures in the UK.

(Institute of Structural Engineers 2004)


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Design using Eurocode

Conventional design uses allowable stress, this is basedon applying a factor of safety to the ultimate stress .

Allowable Bearing pressure qna = Undrained shear strength x f

factor of safety

Eurocode uses limit state design,

There are basic requirements for limit state design for astructure.

It must sustain all likely actions and influences and mustremain fit for purpose and have adequate structuralresistance ,durability and serviceability for its design life.


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LIMIT STATE DESIGN

Working state design: Analyse the expected, workingstate, then apply margins of safety.

Limit state design: Analyse the unexpected states atwhich the structure has reached an unacceptable limit.

Make sure the limit states are unrealistic or unlikely.

Limit states can occur in the ground or the structure or acombination of both.


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Foundation Design

Historically the UK were at the forefront of foundation design.

Designs based on sound mathematical formula, usually theseare ultimate state formula . They calculate the loading which will

cause a catastrophic failure. Depending on the designers confidence in the derived values a

factor of safety can be applied to give working design values.

Typically factors of safety range from 1.2 to 3 or even higher.

Some designs are based on empirical relationships or limitingvalues, these are derived from observational methods which aredeemed to be adequately safe.


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The SPT test

This is a field test and isthe number of blowsfrom a 63.5kg hammerfalling 760mm used todriver a 30 degree coneor split spoon of 50.8mmdiameter.

Blows required to drive

the tool for 300mm arecounted, The number ofblows is referred to asthe penetration

resistance N value.


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Cone Penetration Tests (CPT)

Rigs are: 20tonne trucks (as shown) Lighter crawlers Demountable units

A cone is pushed into the ground ona rod string using hydraulic rams Cone resistance and side friction

continuously recorded electronically Piezocone also measures pore

water pressure

Provides a plot records againstdepth which can be interpreted

to give an indication of soil type


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.



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Empirically derived bearing pressure

The graphs here werederived by Terzaghi andPeck from SPT N values.The allowable bearingpressures are forfoundations placed on sandand presume a limitingsettlement of 25mm.


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Factors affecting foundation design

We intend to construct a foundation which

will not cause shear failure by

overstressing the ground. Will not suffer distress from excessivesettlement.

W

ill take account of the site constraints. Will be economically viable.


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Factors affecting foundation design

Changes in total stress will not causedeformation of an element of soil.

Deformation is the result of changes ineffective stress.


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Definitions

Ultimate bearing pressure (qt) Is the value of the bearing pressure at which the

ground will fail in shear

Maximum safe bearing pressure (qs ) Is the intensity of the applied pressure that thesoil will safely support without the risk of shearfailure

qs = qt / F

F depends on our confidence of the soil properties and canrange from 1.75 to 3. This approach may be fine under Eurocode. The factor can be

applied to the assessed part of the formula.


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Ultimate Bearing capacity

The three components of the Terzaghi (1943) bearingcapacity equation are:

gross qult = cNc+poNq+1/2 KBN K

where:cNc is due to cohesion and friction in the soilpoNq is due to surcharge and friction in the soil1/2 KBN K is due to self-weight and friction in thesoilpo = total overburden pressure at foundationlevel around the foundation (see fig 8.14)

K = bulk unit weight of soilB = width of foundationNc,Nq andN K are termed bearing capacity factorsand are related to the J value only.


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Adaptation of the basic Terzaghiequation

The Terzaghi solution can be applied to cases wherethe groundwater table is very deep, in this case totalstress is equal to the effective stress.

The solution can also apply to the undrained totalstress situation using cu and u .

For free draining soils such as sand where theexcess pore pressure under the footing is zero once

the load has been applied the Terzaghi equation canbe modified to

q = Nc +p'(Nq -1) + B K N K + p Where p = initial effective overburden pressure at the foundation level

p = initial total OBP and K = submerged unit weight


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Skemptons Bearing capacity equation

Skempton derived an equation from theTerzaghi method for clay soils

Qf = Su Ncu + q0

Where Ncu is Skemptons bearing capacityfactor and is equal to Nc.sc .dc

Where s is a shape factor = 1+ 0.2(B/L)

d is a depth factor =1+(0.053D/B) Nq = 1 N K = 0 and Nc = 5.14


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Skemptons Bearing capacity equationfor clay soils


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Bearing capacity in practice Cohesivesoils

It is necessary to consider the

ground condition within which the

foundation is to be placed .

If the soils are predominantly clay

pore pressure will increase as

the load is applied. Because permeability is lowdissipation will take a long time. There will be little gain

in strength therefore the condition is undrained. Theanalysis can be made in terms of total stress.

Overtime pore pressure will dissipate and there will bea gain in strength.


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Bearing capacity in practice Cohesivesoils

Where foundations are placed in a free draining soil such assand and gravel, pore pressure will dissipate as the load isapplied.

The soil will consolidate and gain in strength as constructionproceeds such that all

settlement is completed by the

time construction is complete.

In this situation a drained analysis can be made using effective

stress parameters.


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Bearing Capacity Factorsfor Foundations

The factors Nc, Nq andN K are dependant onthe angle of shearing resistance J.In cohesive soils J = 0 and c = SuThe total stress condition

therefore the N K term is = 0The Nq = 1In drained soils the calculation is in terms ofeffective stresses.

Nc,NqandN K > 0


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Use of the bearing capacity factors

Ultimate bearing pressure qult is dependant on the initialeffective overburden pressure at the foundation leveltherefore as foundation depth is increased then qult alsoincreases.

If groundwater is absent again p' is increased andhence qult is increased. Where the groundwater table isconsidered to lie at least 2B below the base of thefooting and will remain so then p' is equal to p, this willalso increase qult .

Caution must be used where J'is high because highvalues ofNq andN K will be obtained for small changesin J'


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Bearing capacity factors

Following on from the work of Terzaghi severalresearches modified the values of the factors Nc, Nq andN K and also took account of the footing depth andshape.

Terzaghi proposed a shape factor to be applied to theterms

Strip Circular Square

Sc 1.0 1.3 1.3

SK 1.0 0.6 0.8


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Variation of Shear strength with depth

In most cases the undrained shear strength increaseswith depth.

In soils made up of layers of different properties the

stress at each stratum needs to be checked to ensurethat the stratum is overstressed.

This is particularly important where softer soils underliefirmer soils. In these cases the load is taken to spread atan angle of 30o down to the weaker soil and bearing

capacity is checked in the usual way.


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Before Eurocode 7

Factor of Safety of 3 limits settlement to a controllable level.

For granular soils, ultimate bearing capacity is usually higher than for clays butthey derive the allowable bearing capacity on density (often from SPTs).


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Partial factors

Under the Eurocode factors of safety will be applied tovarious components.

The weighting of the factor is dependant on confidence

in the parameter and if the effect is favourable or not. There are three possible design approaches.

It is most likely the UK will adopt design approach 1.This has two combinations of factors both need to bechecked to ensure the design is adequate.

The final methods will not be confirmed until the Nationalannex has been published.

A work in progress....


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Some examples of partial factors

Partial factor

Design approach 1

Combination 1 Combination 2

K G 1.35 1.0

K G,fav 1.0 1.0

K 1.0 1.25

K cu 1.0 1.4

KRh 1.0 1.0

K G,fav /(KRh x K ) 1.0 0.8

KG /(KRh x K ) 1.35 0.8

1 /(KRh x Kcu ) 1.0 0.71


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Foundation design

Once we establish the allowable bearing pressure, thesize of the foundation can be obtained this is based onthe actual loads applied to the footing including factors ofsafety for the structure.

This may need to be an iterative process.

It may be found that a solution is not possible ie ourloads exceed the allowable load which may be applied. Ifthis is the case an alternative foundation solution may berequired.


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Settlement

Some soils reach failure at relatively small strains forexample stiff clays.

Others do not reach their failure stress until relatively

large strains have been developed. It is thereforeimportant to check that strain is not so large that it willcause structural distress.

In addition clay soils will dissipate pore pressures at a

slow rate whilst this will develop an increase in strengththe resulting consolidation may be undesirable.

Therefore once the foundation size and bearing pressurehas been obtained the footing needs to be checked toensure that settlement is not excessive.


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Stress strain relationship for soils ofvarious types


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The Behaviour of the Ground

The most significant

behaviour for most structures

Settlement

Total

Settlement

Differential

Settlement

Rate


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Settlement Depends on...

- Actual not maximum loads

- Distribution and duration of loading

- Area stressed and stress distribution- Strata thicknesses and compressibility's

- Strata levels relative to applied stress

- Strata type and stress history- Collapse of voids unrelated to applied

stress


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Stress Distributions BeneathFoundations

On SandOn Clay

Flexible foundationOn Clay


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Settlement calculation

Settlement prediction based

on Consolidation tests.

Settlement proportional to:

- mv co-efficient of volume decrease

- Stress applied

- Thickness of soils stressed


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Boussinesq equation

Distribution of stress with depth due to theapplication of a load at ground surface(Boussinesq)


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Distribution of vertical stress

Within Allowable Stress Levels


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Total settlement calculation

There are several basic formula used tocalculate total settlement

c = e H 1+ e

c = mv .H.p

c = Cc H log10 p'0

+p

1 + e0 p'0


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Calculation of settlement in amultilayered system

Using the distribution of stress with depthgraphs the effect of the stress applied by each

strata can be derived in proportion. Therefore by using the mv derived for each

layer the total settlement attributed to eachlayer can be calculated.

The total settlement should be less than thesettlement considered to be acceptable forthe structure.


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consolidation testing derivation of

parameters.

geotechnical laboratory testing awareness


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Consolidation testingVoids ratio load curves



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Calculation of Mv

The coefficient of volume compressibilityis derived from consolidation tests and is

defined as mv = V P

V

In the oedometer test a change in volumeis proportional to the change in heightthus

mv = H P


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Consolidation testingTaylor analysis forCv



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Oedometer consolidationCassagrande analysis forCv



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Consolidation testing

To relate test to voids ratio it is necessary to measurethe particle density.

Often assumed as 2.65 or 2.70

The choice of cv values should be determined from thebest fit analysis Taylor or Casagrande.

Note that permeability is given by

k= cv mv x 0.31 x 10-9 m/s

Thus as load is increased permeabilitydecreases

ie cv x mv must decrease.



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Organic soils

Low Particle Density so need to measure this to

calculate true voids ratios which can be very high

Need to measure the coefficient of secondaryconsolidation this is consolidation which continues

to occur after pore pressures have equilibriated. Thiscontinues for a very long time (longer than primaryconsolidation).



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Limitations Sample thickness is only 19mm.

Results may not reflect true soil structure

Drainage is in vertical direction.

Can only be carried out on fine grained soils.

Difficult not to smear drainage paths duringpreparation



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Differential settlement

Differential settlement will occur if

The foundations are not uniformly loaded

The strata is variable beneath thestructure

Shrinkage or swelling occurs associatedwith trees or vegetation

With careful design all these factors canbe eliminated or catered for in the design.


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Some examples of partial factors

Partial factor

Design approach 1

Combination 1 Combination 2

K G 1.35 1.0

K G,fav 1.0 1.0

K 1.0 1.25

K cu 1.0 1.4

KRh 1.0 1.0

K G,fav /(KRh x K ) 1.0 0.8

KG /(KRh x K ) 1.35 0.8

1 /(KRh x Kcu ) 1.0 0.71


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Example foundation design Spreadfoundation - Pad on clay

K s:k characteristic weight,density of soil = 20kN/m3

K c;k density of concrete

=24kN/m3

Cu:k characteristic value ofundrained shear strength= 60 kPa

Determine the width B of thefooting

ie Vd < Rd

0.5m

500kN

B

1.50m


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Example - pad foundationDesign approach 1 combination 1

1. Imposed vertical load = 500kN

2. Weight of foundation

Wt of concrete (1x0.5x0.5x24) +(2x2x0.5x24) = 54kN

3. Weight of backfill (2x2x1) (0.5x0.5x1) x

20 =

75kN

Total permanant load Vk =


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Example - pad foundation

design strength cudi = cuk /K cu = 60 /1.0

= 60 kPa Soil surcharge , design value adjacent to

footing

q di = q k x K G = (1.5 x 20) x 1.0

q di = 30kPa


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Example - pad foundation

Design bearing resistance

Rd1 = Rk / KRh

= Rk / 1.0 Rd1 = Rk After Terzaghi

Rd1 = 4 [ ( + 2) x 1.2 x60 +30]

Rd1 = 1601kN

Check ifV

d1 < Rd1 849kN


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Design load Vd2 = 1.0 x 629

Vd2 = 1.0 x 629

Vd2 = 629 kN Design strength

Cud2 = 60 / 1.4

= 43kPa

Soil surcharge

qd2 = qd2 x K G = (1.5 x 20 ) x 1.0

= 30 kPa


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design bearing resistance

Rd2 = R/ R K v

= R /1.0 Again after Terzaghi

Rd2= 4 [ ( + 2) x 1.2 x 43 +30]

= 1181kN Check if Vd2 < Rd2 629 < 1181

footing 2m x 2m is acceptable

(design app. 1 comb.1)


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What if......

Bearing Capacity is too low? or Settlement is too great?

or Ground is too variable?This may, and often does apply to

natural deposits on Greenfield

Sites but it is often the case on

Brownfield Sites where there aresubstantial deposits of Made

Ground or Fill and we need to look

for Solutions


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Fill and Phil are very similar

Character Profile

- Inconsistent

- Unreliable

- Generally weak

- Dodgy history

- Variably dense- Variably thick

- Sometimes smells

- Unpredictable

- Can take time to settle

- Can be aggressive

- Hangs around cities

- Doesnt recognise

normal laws- Operates under cover

- Can collapse suddenly

Phil


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We could try to improve Phil

.Use some method to increase the grounddensity and hence improve its loadcarrying capacity and reduce the effect of

settlement Compaction

Surcharging

Grouting

Stone columns (or similar)

Dynamic consolidation


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We could try to improve Phil

We can compact the ground will a heavyroller

Solutions Excavation and


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Solutions - Excavation andRecompaction

AdvantagesSimple technologyCan be used on sites of all

sizesCan be cost effectiveTreats all of the soil to the

required depthsCan closely control

treatment

DisadvantagesRequires storage areas on

siteCannot treat ground below

groundwater level without

lowering itRequires support of

excavation sides or batteredslopes

Exposes operatives topotential contamination


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Width > 2 x d

h = height of surcharge fill toapply stresses well inexcess of foundation, raft,floor or long term fillstresses.

h

d

Surcharge Fill

Soft, loose orvariableground

Competent Stratum

Solutions - Surcharge


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Advantages Treats all of the loaded area

uniformly Treats the full depth of problem

soils Proves the treated area Provides parameters for future

predictions Can rapidly treat unsaturated fills Does not require sophisticated

plant Simple concept

Surcharge - Simple Concepts


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Surcharging

Disadvantages Requires large quantities of fills requiring later disposal For saturated natural soils consolidation can be slow without

vertical drains Requires detailed monitoring of settlement patterns


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Load Settlement Data

Surcharging


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Speeding up the surcharging process


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Solutions - Vertical Drains


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Vertical Drains

Accelerates consolidationof layered soft cohesive

soilsAllows gain in strengthresulting fromconsolidation to be usedin stage loadingprogramme beneath

embankments / structuresQuick to installProvides control of

consolidation rates

AdvantagesDisadvantages

Requires working platform forrelatively high plant

Requires surface drainage layer

(usually)Requires surcharge to addresslong term settlements

Does not, of itself, strengthen orincrease density of treated soils

Requires instrumentation to

monitor pore water pressuresand settlement

Can provide linkages betweenpollutants and groundwater orgases


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Solutions - Vibrocompaction


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Vibro-compaction

Advantages Suitable for compacting loose

granular soils Can be adapted to treat made

ground by incorporatingcompacted stone columns

Known depth of treatment Accelerates consolidation rates

by providing drainage to linknatural preferential drainagepaths

Can treat foundation and floorslab areas

Reduces total and differentialsettlement

Helps unify ground conditions Avoids contact between site staff

and contaminated soils Can treat materials below water

table

Disadvantages Causes ground vibration which

can adversely affect localinstallations

Creates pathways for flows ofpotentially contaminated water

Can create pathways formigrations of ground bornegases

Requires import of granular soils Treats discrete vertical columns

rather than the whole area

Unsuitable for small sites Can be costly May require surcharge to deal

with settlements


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Vibro-compaction


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Variations on a theme

Vibro replacement and vibro

concreted columns

S C


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Solutions - Dynamic Compaction

S l ti D i C ti


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Solutions - Dynamic Compaction

D i C ti d t


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Dynamic Compaction - advantages

Applies high impact stresses much greater than foundation loadsCauses any voids to collapse and reduces risk of collapse settlementCan treat to 8m depthDoes not create preferential drainage paths

Reduces permeability of compacted soilsDoes not require disposal of soilsTreats the whole areaReduces post-construction settlementSimple technology

Suitable for uncompacted Made GroundCan treat floor and foundation areasCosts can be favourable in relation to piling

Avoids contact between contaminated soils and site personnel


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Disadvantages Causes substantial vibratory loads to adjacent properties and installation Requires import of fills to form working platform and fill voids Depth of effect can only be demonstrated by comparing pre and post treatment

investigation data

Requires large plant, considerable headroom and stable access and working area Adverse affects from disturbance of soft alluvial deposits Cannot be used in natural organic ground Unsuitable for small sites Can be costly


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Ground improvement by grouting


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Drilling and Grouting

Jet Grouting and Soil Mixing

These processes both disturb the soil and introduce grout intothe soil melange.

Jet grouting is carried out by jetting the soil with the grout inorder to break the soil down and produce a grout soil mix

Soil mixing uses a drag bit to break to soil up. Rigid blades on

the drill string mix in the Grout which is introduced through thedrill string


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Jet Grouting

This technique was invented in the UK, anddeveloped in Japan

The method uses a high pressure jet of waterto erode the soil.

the soil residue is then mixed with grout toform a soil grout mix often formed into

columns. The process can be used for a variety of

geotechnical solutions, it does not requirelarge plant and is therefore ideal for areas

where access is restricted.


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Jet Grouting

Jet Grouting


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Has been used to carry out works such as ;-

Embankment stabilisation

Underpinning of foundation

To provide foundation for new structures

In curtains to provide a barrier to fluid flows

Remedial works to dams cut off curtains and cofferdams

Solidification of contaminated soil or as a cut off.

Jet Grouting


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Can be carried out using either single jet; double jetor triple jet systems

Single jet Uses a 5 to 10 mm diameter nozzle on a hollow stem

grout tube the grout jet is used to erode the soil to createa slurry mix of grout and soil

Double jet Similar to the single jet method but the grout jet is

shrouded in a jet of air this improves cutting of the soil

Triple jet This uses an air shrouded water jet to erode the soil

grout is injected by a lower jet

Jet Grouting


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Compaction Grouting

1. Compaction Grouting

This is carried out by injecting a stiff grout or paste into theground in order to cause displacement of the soil mass.

The method is used to increase

the compaction of loose soils and

improve their ability to carry loads.


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Compaction Grouting

1. Compaction Grouting

This is carried out by injecting a stiff grout or paste into theground in order to cause displacement of the soil mass.

The method is used to increase

the compaction of loose soils and

improve their ability to carry loads.


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Compensation Grouting

Compensation Grouting is aspecialist form of hydrofracture.

It involves injecting grout intothe ground during tunnelling to

reduce or eliminate the effects of surface settlement which alwaysoccurs when tunnelling is beingcarried out.

The process was developedduring the 90s and was used in

London tunnels for the jubilee lineextension were constructed, beneathmany important structures.

In order to ensure the buildingswere not damages compensationgrouting was carried out

A new underground station was builtclose to the tower of Big Ben

(The Clock Tower that is as Big Ben is the name of the bell !!)

Amazing Fact eh?


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Compensation grouting uses permeation, hydrofracture;intrusion and compaction grouting methods to minimise the effectsof tunnelling.

The difference between this and other forms of grouting is thatthe grouting takes place as the tunnel is advanced to preventsettlement.

Usually an initial injection is carried out to stiffen the ground .



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As the tunnel or excavation proceeds very accurate surfacemeasurements are made. Grout injection starts when apredetermined settlement is recorded.

Grouting and monitoring are carried out simultaneously to ensure settlements are not exceeded.

It may be necessary to carry on grouting for some time after the tunnel

face has passed a particular point.

The process is complex and is normally controlled by computerised systems.


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Solutions - Piling


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Solutions - Piling

AdvantagesEasiest way to form a deep foundationConvenient way to bypass soft variable soils

and use strength/density of underlyingdepositsWide variety of proprietary driven and bored

techniques available

Wide range of diameter and hence loadcapacitiesCan use in groups to take heavy loadsConsiderable experience in their use

Can enhance lateral stabilit of foundations


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Pile Support is achieved via:


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Pile Support is achieved via:

a shear strength mobilised on the surface

of the shaft of the pile, the contribution ofeach stratum depending on its strength,

density and frictional characteristics.

bearing capacity at the base of the pile,

called end bearing.


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Pile design

Shaft resistance and base

resistance are mobilised at

different amounts of

settlement.

The full shaft resistance is

mobilised at a pile head

settlement of about 1-2% of

the pile diameter. To mobilise the full base

resistance the pile must be

pushed down about 10-

20% of the diameter.

Pile classification


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their method of installation i.e. driven, bored, jacked

their material type i.e.. timber, steel or concrete, pre-cast

or cast in situ, full length or segmental

the plant used to install them such as driven, tripod,auger, under-reamed, continuous flight auger

their size i.e.. small diameter bored, large diameter bored,

under-reamed, mini-piling

their effects during installation i.e.. displacement orreplacement

the way they provide load capacity i.e.. end bearing, friction

piles, uplift piles, raking piles.

Pile classification


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Various uses of pile groups

The zone of the soil stressed around a single

pile is much smaller than around and beneath a


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p

pile group

the installation method has lesseffect on group behaviour thanon single pile behaviour

compressible layers existingbeneath the base of a pile group

will produce settlement of thegroup while they may not effectthe result of a single pile loadtest

extrapolation from theperformance of a single pile load

test to the behaviour of a groupmust be treated with caution.


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Background to Eurocode

HISTORICAL CONTEXT


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HISTORICAL CONTEXT

1957: EEC Treaty of Rome 1975: Commission of the EC - Eurocodes 1981: European national geotechnical societies

1987: First draft EC7 1989: Comit Europen de Normalisation

(CEN/TC250/SC7)

1994/5: ENV1997 (European Vornorm) 2004 : EN1997-1 + National Annex (2006)

2006 : EN1997-2 + National Annex (2007)

LEVELLING THE FIELD - MOBILITY


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STATUS OF THE STANDARDS

The BS EN ISOs have supremacy over existingnational standards, eg BS 5930 & 1377

To comply with the principle of supersession, nationalstandards must not conflict with BS EN ISOs

For example BS 5930 & 1377 will be progressively superseded

and revised as Eurocode related standards arepublished

Progress to date: 5930 Section 6, ie soil and rock description, has been

revised to accommodate changes and is with BSI forpublication (early 2008?)

1377 Part 9 already published in revised form (July 2007), iewith DP and SPT covered purely by onward referral to the BS

EN ISOs


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THE EUROCODE SYSTEM

BS EN 1990 Eurocode 0 Basis of design BS EN 1991 Eurocode 1 Actions on structures BSEN 1992 Eurocode 2 Concrete structures BS EN 1993 Eurocode 3 Steel structures BS EN 1994 Eurocode 4 Composite structures BS EN 1995 Eurocode 5 Timber structures BS EN 1996 Eurocode 6 Masonry structures BS EN 1997 Eurocode 7 Geotechnical design BS EN 1998 Eurocode 8 Design of structures for

earthquake resistance

BS EN 1999 Eurocode 9 Aluminium alloystructures


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EN 1990 2002


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EN 1990 :2002

Describes the Principles and requirements for safety,serviceability and durability of structures.

It is based on the limit state concept used in conjunctionwith a partial factor method.

For the design of new structures, EN 1990 is intended tobe used, for direct application, together with EurocodesEN 1991 to 1999

EUROCODE 1997: GEOTECHNICAL


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EUROCODE 1997: GEOTECHNICALDESIGN (EC7)

EC7 is in two parts

BS EN 1997-1: General Rules

BS EN 1997-2: Ground Investigation and Testing

EC7 Part 2 calls up several other series of BS EN ISO standards

14688: Identification and classification of soil

14689: Identification and classification of rock

22475: Sampling by drilling and excavation andgroundwater measurements

22476: Field Testing

These are normative references, ie they are mandatory

Eurocode 1997 or EC7


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Eurocode 1997 or EC7

Part 1 published. National Annex in preparation

Part 2 published. National Annex to be prepared thisyear

As a special concession 1997 has been allowed tocoexist with National Standards until 2010

Other Standards no concession - implementationrequired within 6 months of publication

National Foreword to set out rules and no more

EC 1997 P t 1


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EC 1997 Part 1

Comprises 12 sections and Annexes A to J

Section 1 General

Section 2 Basis of Geotechnical design

Section 3 Geotechnical data

Section 4 Supervision of construction,monitoring and maintenance

Section 5 Fill, Dewatering, groundimprovement and reinforcement

Section 6 Spread Foundations

Section 7 Pile foundations

EC 1997 P t 1 ( t)


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EC 1997 Part 1 (cont)

Section 8 Anchorages

Section 9 Retaining structures

Section 10 Hydraulic Failure

Section 11 Overall Stability

Section 12 Embankments

Annex A Recommended partial factors of safety

Annexes B to J supplementary information and guidanceincluding internationally applied calculation methods

EC 7 Part 1


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EC 7 Part 1

Section 2 deals with Geotechnical design

2.1 Design requirements

2.2 Design situations2.3 Durability2.4 Geotechnical design by calculation2.5 Design by prescriptive measures2.6 Load tests and tests on models

2.7 Observational method2.8 Geotechnical Design Report

PARTS OF EUROCODE 7


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PARTS OF EUROCODE 7

EN 1997-1

General Rules

EN 1997-2

Geotechnical Investigation and Testing

TC 341

Test Standards

TC 288

Execution of geotechnical works



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Conventional design uses allowable stress, this is basedon applying a factor of safety to the ultimate stress .

Allowable Bearing pressure qna = Undrained shear strength x f

factor of safety

Eurocode uses limit state design, There are basic requirements for limit state design for a

structure.

It must sustain all likely actions and influences and must

remain fit for purpose and have adequate structuralresistance ,durability and serviceability for its design life.

Basis for design


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Basis for design

Design Working life is defined as:-

the period for which a structure or part of it is to be usedfor its intended purpose with anticipated maintenance butwithout major repair being necessary. [EN1990

1.5.2.8]

Design working life for most structures is 50 years,

Temporary structure are generally 10 to 15 years andsignificant structures 120years

LIMIT STATE DESIGN


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LIMIT STATE DESIGN

Working state design: Analyse the expected, workingstate, then apply margins of safety.

Limit state design: Analyse the unexpected states at

which the structure has reached an unacceptable limit.

Make sure the limit states are unrealistic or unlikely.

Limit states can occur in the ground or the structure or acombination of both.

Limit States


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Limit States

Ultimate limit states are concerned with the safety ofpeople and the structure [EN 19903.3pt1]

There are three limit states concerned with structures

Limit state EQU dealing with static equilibrium

Limit state STR dealing with excessive deformationloss of stability

Limit state FAT dealing with fatigue mechanismsand time related effects

Limit States geotechnical


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Limit States - geotechnical

There are three geotechnical Limit states referred to inthe code,

Limit state GEO relates to or excessive movement

of the ground Limit state HYD relates to hydraulic heave erosion

and piping in the ground caused byhydraulic gradients

Limit state UPL relates to loss of equilibrium due touplift by water pressure or othervertical actions.

Ultimate limit state


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Ultimate limit state

This is derived from a combination of thesix limit states

EQU STR FAT

GEO HYD URL

Design to Eurocode


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Basic requirements - limit state

The structure must be able to withstanddisproportionate damage which might be caused byadverse events such as Explosion

Impact

Human error

In addition the design must be sufficient to avoidor limit the potential for damage by avoiding or

eliminating hazards. Avoiding sudden collapse (employing redundant

members)

Designing for accidental removal of members

Basic Design Requirements of


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BS EN 1997

For each and every geotechnical situation thestandard requires that no relevant limit state conditionis exceeded.

The following factors need to be taken into account:-

Site conditions considering groundmovements and stability

Surrounding influences ( adjacent

structures, vegetation, contamination etc) Ground conditions

Groundwater

Environmental influences (hydrology,

BS EN1997th t


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assumes that.

Data is collected, recorded and interpreted by appropriatelyqualified personnel;

Structures are designed by appropriately qualified andexperienced personnel;

Continuity and communication exists between thepersonnel involved in data-collection, design andconstruction;

Supervision and quality control are provided; Execution is carried out according to the relevant standards

and specifications by personnel having the appropriate skill

and experience; Construction materials and products are used as specified; The structure will be adequately maintained; The structure will be used for the designed purpose.

OBTAINING PARAMETERS


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OBTAINING PARAMETERS

Field or laboratory test result

Derivation Derived values of geotechnical

parameter

Characterisation Characteristic value

Factorisation

Desi n Value

CHARACTERISTIC VALUES


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The selection of characteristic values for geotechnicalparameters shall take account of the following:

geological and other backgroundinformation.

the variability of the measured propertyvalues.

the extent of the field and laboratory

investigation. the type and number of samples. the extent of the zone of ground governing

the behaviour of the geotechnical structureat the limit state being considered.



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Bear in mind that

soil and rock structure may play a different rolein the test and in the structure;

many geotechnical parameters are not trueconstants but depend on stress level and modeof deformation;

time effects;

the softening effect of water on soil or rockstrength;

the brittleness or ductility of the soil and rock

Equipe 2010 Foundation Design

Documents

Transcript of Equipe 2010 Foundation Design