geotechnics for the structural engineer

GEOTECHNICSGEOTECHNICSFOR FOR

THE STRUCTURAL THE STRUCTURAL ENGINEERENGINEER

DENIS H. CAMILLERIDENIS H. CAMILLERIdhcamilldhcamill@@maltanetmaltanet.net.netBICC BICC –– CPD 22/04/05CPD 22/04/05

The Development of Foundation limit State Design

Before World War II codes of practice for Before World War II codes of practice for foundation engineering were used only in a foundation engineering were used only in a small number of countries.small number of countries.

In 1956 In 1956 Brinch Brinch Hansen used for the first time the Hansen used for the first time the words words ““limit designlimit design”” in a in a geotechnical geotechnical context.context.

Brinch Brinch linked the limit design concept closely to linked the limit design concept closely to the concept of partial safety factors, and he the concept of partial safety factors, and he introduced these two concepts in Danish introduced these two concepts in Danish foundation of engineering practice.foundation of engineering practice.

Basis Behind Eurocode 7The Limit state concept is today widely accepted as a The Limit state concept is today widely accepted as a

basis for codes of practice in structural engineering. basis for codes of practice in structural engineering. From the very beginning of the work on the From the very beginning of the work on the Eurocodes it was a foregone conclusion that the Eurocodes it was a foregone conclusion that the Eurocodes should be written in the limit state design Eurocodes should be written in the limit state design format and that partial factors of safety should be format and that partial factors of safety should be used.used.

Consequently it was decided that also those parts of Consequently it was decided that also those parts of the Eurocodes which will be dealing with the Eurocodes which will be dealing with geotechnical geotechnical aspects of design should be written in aspects of design should be written in the limit state format with the use of partial factors the limit state format with the use of partial factors of safetyof safety

Geotechnical Categories & Geotechnical Risk Higher Categories satisfied by greater attention to the quality of the geotechnical investigations and the designTable 1-Geotechnical Categories related to geotechnical hazard and vulnerability levels

High risk of damage to High risk of damage to neighbouring neighbouring structures or servicesstructures or services

Possible risk of damage Possible risk of damage to neighbouring to neighbouring structures or services structures or services due, for example, to due, for example, to excavations or pilingexcavations or piling

Negligible risk of damage Negligible risk of damage to or from neighbouring to or from neighbouring structures or services and structures or services and negligible risk for lifenegligible risk for life

SurroundingsSurroundings

Areas of high Areas of high earthquake hazardearthquake hazard

Moderate earthquake Moderate earthquake hazard where seismic hazard where seismic design code (EC8 Part design code (EC8 Part V) may be usedV) may be used

Areas with no or very low Areas with no or very low earthquake hazardearthquake hazard

Regional Regional seismicityseismicity

Unusual or Unusual or exceptionally difficult exceptionally difficult ground conditions ground conditions requiring nonrequiring non--routine routine investigations and investigations and tests.tests.

Ground conditions and Ground conditions and properties can be properties can be determined from routine determined from routine investigations and tests.investigations and tests.

Known from comparable Known from comparable experience to be experience to be straightforward. Not straightforward. Not involving soft, loose or involving soft, loose or compressible soil, loose compressible soil, loose fill or sloping ground. fill or sloping ground.

Ground conditionsGround conditions

High High Moderate Moderate Low Low GeotechnicalGeotechnical hazards hazards /vulnerability /risk/vulnerability /risk

GC3GC3GC2GC2GC1GC1Geotechnical Geotechnical CategoriesCategoriesFactors to be Factors to be

consideredconsidered

Table 1 (cont.)

-- Very large Very large buildingsbuildings-- Large bridgesLarge bridges-- Deep Deep excavationsexcavations-- Embankments Embankments on soft groundon soft groundTunnels in soft or Tunnels in soft or highly permeable highly permeable groundground

Conventional:Conventional:-- Spread and pile Spread and pile foundationsfoundations-- Walls and other Walls and other retaining structuresretaining structures-- Bridge piers and Bridge piers and abutmentsabutmentsEmbankments and Embankments and earthworksearthworks

-- Simple 1 & 2 storey Simple 1 & 2 storey structures and agricultural structures and agricultural buildings having maximum buildings having maximum design column load of 250kN design column load of 250kN and maximum design wall load and maximum design wall load of 100kN/mof 100kN/m-- Retaining walls and Retaining walls and excavation supports where excavation supports where ground level difference does not ground level difference does not exceed 2mexceed 2m

Examples of Examples of structuresstructures

More sophisticated More sophisticated analyses analyses

Routine calculations for Routine calculations for stability and stability and deformations based on deformations based on design procedures in design procedures in EC7EC7

Prescriptive measures and Prescriptive measures and simplified design procedures simplified design procedures e.g. design bearing pressures e.g. design bearing pressures based on experience or based on experience or published presumed bearing published presumed bearing pressures. Stability of pressures. Stability of deformation calculations may deformation calculations may not be necessarynot be necessary

Design Design proceduresprocedures

Experienced Experienced geotechnicalgeotechnicalspecialistspecialist

Experienced qualified Experienced qualified person person –– Civil EngineerCivil Engineer

Person with appropriate Person with appropriate comparable experiencecomparable experience

Expertise Expertise requiredrequired

GC3GC3GC2GC2GC1GC1

Geotechnical Geotechnical CategoriesCategories

Ultimate Limite State (ULS) partial factors (persistant & transiet situations)

Table 2- Partial factors for ultimate limit states in persistent and transient situations

Values in Values in redred are partial factors either given or implied in ENV version of Eare partial factors either given or implied in ENV version of EC7C7Values in Values in green green are partial not in the ENV that may be in the EN versionare partial not in the ENV that may be in the EN version

1.001.001.001.001.001.001.001.001.001.00γγAAAccidental actionAccidental action

0000000000γγQQVariable favourable Variable favourable actionaction

1.001.001.001.001.001.001.001.000.950.95γγGGPermanent Permanent fvourable fvourable actionaction

1.201.201.501.501.301.301.501.501.501.50γγQQVariable Variable unfvaourable unfvaourable actionaction

1.001.001.351.351.001.001.351.351.001.00γγGGPermanent Permanent unfavourable actionunfavourable action

(HYD)(HYD)(EQU)(EQU)(GEO)(GEO)(STR)(STR)(UPL)(UPL)Partial load factors (Partial load factors (γγF F ))

Case C3Case C3Case C2Case C2Case CCase CCase BCase BCase ACase AFactorFactorParameterParameter

Table 2 (Cont.)

Values in Values in red red are partial factors either given or implied in ENV version of ECare partial factors either given or implied in ENV version of EC77Values in Values in green green are partial not in the ENV that may be in the EN versionare partial not in the ENV that may be in the EN version


1.401.401.001.001.401.401.001.001.401.40γγCPTCPTCPT resistanceCPT resistance

1.001.001.001.001.001.001.001.001.001.00γγggUnit weight of ground Unit weight of ground γγ

1.401.401.001.001.401.401.001.001.401.40γγplimplimPressuremeter Pressuremeter limit limit pressure pressure pplimlim

1.401.401.001.001.401.401.001.001.201.20γγququCompressive strength Compressive strength qquu

1.401.401.001.001.401.401.001.001.201.20γγcucuUndrained Undrained shear strength shear strength ccuu

1.201.201.001.001.601.601.001.001.301.30γγcc’’Effective cohesion cEffective cohesion c’’

1.201.201.001.001.251.251.001.001.101.10γγtantanφφ’’Tan Tan φφ’’(HYD)(HYD)(EQU)(EQU)(GEO)(GEO)(STR)(STR)(UPL)(UPL)Partial material factorsPartial material factors ((γγm m ))

Table 2 (Cont.)

Values inValues in red red are partial factors either given or implied in ENV version of ECare partial factors either given or implied in ENV version of EC77Values inValues in greengreen are partial not in the ENV that may be in the EN versionare partial not in the ENV that may be in the EN version** Partial factors that are not relevant for Case APartial factors that are not relevant for Case A

1.001.001.401.401.601.601.001.001.401.40γγststPile Tensile resistancePile Tensile resistance


1.001.001.301.301.301.301.001.00--**γγttTotal pile resistanceTotal pile resistance

1.001.001.201.201.501.501.001.001.301.30γγAAAnchor pullAnchor pull--out resistanceout resistance

1.001.001.301.301.301.301.001.00--**γγssPile shaft resistancePile shaft resistance

1.001.001.301.301.301.301.001.00--**γγbbPile base resistancePile base resistance

1.001.001.401.401.001.001.001.00--**γγReReEarth resistanceEarth resistance

1.001.001.101.101.001.001.001.00--**γγrSrSSliding resistanceSliding resistance

1.001.001.401.401.001.001.001.00--**γγRVRVBearing resistanceBearing resistance

(HYD)(HYD)(EQU)(EQU)(GEO)(GEO)(STR)(STR)(UPL)(UPL)Partial resistance factorsPartial resistance factors ((γγR R ))

Serviceability Limit State Calculations (SLS)Table 3 – Serviceability limits

Moderate to Moderate to severesevere


Severe to very Severe to very severesevere

15 to 2515 to 25

Increasing risk Increasing risk of structure of structure becoming becoming dangerousdangerous

Severe to Severe to dangerousdangerous

Severe to Severe to dangerousdangerous

Very severe to Very severe to dangerousdangerous

>25>25

ModerateModerateModerate to Moderate to severesevere


5 to 155 to 15

Serviceability of Serviceability of the building will the building will be affected, and be affected, and towards the towards the upper bound, upper bound, stability may stability may also be at riskalso be at risk

SlightSlightModerateModerateModerateModerate2 to 52 to 5

Accelerated Accelerated weathering to weathering to external featuresexternal features

Very slightVery slightSlight to Slight to moderatemoderate

Slight to Slight to moderatemoderate

1 to 21 to 2

Aesthetic onlyAesthetic onlyVery slightVery slightSlightSlightSlightSlight0.3 to 10.3 to 1

nonenoneInsignificantInsignificantVery slightVery slightVery slightVery slight0.1 to 0.30.1 to 0.3

NoneNoneInsignificantInsignificantInsignificantInsignificantInsignificantInsignificant> 0.1> 0.1

IndustrialIndustrialCommercial or Commercial or publicpublic

DwellingDwelling

Effect on Effect on structure and structure and building usebuilding use

Degree of damageDegree of damageCrack width mmCrack width mm

LIMIT STATE DESIGN –CHARACTERISTIC VALUE & DESIGN

STRENGTH

CHARACTERISTIC STRENGTH OF A CHARACTERISTIC STRENGTH OF A MATERIAL is the strength below which not MATERIAL is the strength below which not more than 5% (or 1 in 20) samples will fail.more than 5% (or 1 in 20) samples will fail.

CHARACTERISTIC STRENGTH = CHARACTERISTIC STRENGTH = MEAN VALUE MEAN VALUE –– 1.64 X Standard Deviation1.64 X Standard Deviation

DESIGN STRENGTH = DESIGN STRENGTH = CHARACTERISTIC STRENGTH CHARACTERISTIC STRENGTH ffuu

MATERIAL FACTOR OF SAFETY MATERIAL FACTOR OF SAFETY γγmm

EXAMPLE:EXAMPLE:Ten concrete cubes were prepared and tested by crushing in Ten concrete cubes were prepared and tested by crushing in

compression at 28 days. The following crushing strengths in N/mcompression at 28 days. The following crushing strengths in N/mmm22

were obtained:were obtained:44.5 47.3 42.1 39.6 47.3 46.7 43.8 49.7 45.2 42.744.5 47.3 42.1 39.6 47.3 46.7 43.8 49.7 45.2 42.7Mean strength Mean strength xxmm = = 448.9448.9 = = 44.9N44.9N/mm/mm22

1010Standard deviation = Standard deviation = √√[(x[(x--xxmm))22/(n/(n--1)] = 1)] = √√(80/0)(80/0)

= 2.98N/mm= 2.98N/mm22

Characteristic strength = 44.9 Characteristic strength = 44.9 –– (1.64 X 2.98)(1.64 X 2.98)= 40.0 N/mm= 40.0 N/mm22

Design strength = Design strength = 40.040.0 = = 40.040.0γγmm 1.51.5

= = 26.7N/mm26.7N/mm22

OutputOutputCalculationsCalculationsRefRef

Date: 05/02Date: 05/02Drawing Ref: Done by: DHCDrawing Ref: Done by: DHC

The Characteristic Value of the angle of shearing resistance The Characteristic Value of the angle of shearing resistance ∅∅’’K is K is required for a 10m depth of ground consisting of sand for which required for a 10m depth of ground consisting of sand for which the the following following ∅∅’’K K values were determined from 10 values were determined from 10 traxial traxial tests: 33tests: 33°°, 35, 35°°, , 33.533.5°°, 32.5, 32.5°°, 37.5, 37.5°°, 34.5, 34.5°°,36.0,36.0°°, 31.5, 31.5°°, 37, 37°°, 33.5, 33.5°°To find the 95% confidence level, for soil properties, as only aTo find the 95% confidence level, for soil properties, as only a small small portion of the total volume involved in a design situation is teportion of the total volume involved in a design situation is tested, it is sted, it is not possible to rely on Normal Distribution.not possible to rely on Normal Distribution.For a small sample size the Student t value for a 95% confidenceFor a small sample size the Student t value for a 95% confidence level level may be used to determine that Xmay be used to determine that XKK value, given byvalue, given byXXKK = = XXmm [ l[ l--tV tV ] = ] = XXmm -- ttσσ

√√n n √√n n Some typical values of V for different soil properties given by Some typical values of V for different soil properties given by

Part of StructurePart of Structure

CHARACTERISTIC VALUE DETERMINATIONCHARACTERISTIC VALUE DETERMINATION

Job ref:Job ref:ProjectProject

FOUNDATION CPD COURSEFOUNDATION CPD COURSEBICCBICCBUILDING INDUSTRYBUILDING INDUSTRYCONSULTATIVECONSULTATIVECOUNCILCOUNCIL

Soil Property Range of typical

V values Recommended V Value if limited Test results available

tanφ’ 0.05 – 0.15 0.10 c’ 0.30 – 0.50 0.40 cu 0.20 – 0.40 0.30 mv 0.20 – 0.70 0.40 γ (unit weight) 0.01 – 0.10 0

Average angle of shearing resistance Average angle of shearing resistance ∅∅’’AVAV = 34.4= 34.4°°With a Standard Deviation With a Standard Deviation σσ = 1.97= 1.97°°Coeff Coeff of variation V = 0.057of variation V = 0.057

Student t for a 95% confidence levelStudent t for a 95% confidence levelwith 10 test results = 2.with 10 test results = 2.2626∅∅’’KK = 34.4 = 34.4 -- 1.97 X 2.26 / 1.97 X 2.26 / √√10 = 33.010 = 33.0°°

The Design Value XThe Design Value XDD = = XXkk//γγmm

Applying the Applying the γγmm = 1.25 for Case C in Table 2= 1.25 for Case C in Table 2∅∅’’cc = arc tan (tan = arc tan (tan ∅∅’’KK ) / 1.25 = 27.8) / 1.25 = 27.8°°

OutputOutputCalculationsCalculationsRefRef

Drawing Ref: Done by: DHCDrawing Ref: Done by: DHC

Part of StructurePart of Structure

CHARACTERISTIC & DESIGN VALUE CHARACTERISTIC & DESIGN VALUE DETERMINATIONDETERMINATION

Job ref:Job ref:ProjectProject

FOUNDATION CPD COURSEFOUNDATION CPD COURSEBICCBICCBUILDING INDUSTRYBUILDING INDUSTRYCONSULTATIVECONSULTATIVECOUNCILCOUNCIL

The t values are given in Table 4

Basic Cohesive Soil Founding Pressures

Shallow Foundation occurs when founding depth (D) is Shallow Foundation occurs when founding depth (D) is less than width (B)less than width (B)

D/B < 1 or when d<3m (may not be applicable for rafts)D/B < 1 or when d<3m (may not be applicable for rafts)For For undrained undrained conditions, the base resistance conditions, the base resistance qqBB per unit per unit

areaareaShallow foundation Shallow foundation qqBB = 5c= 5cuu + + γγssDDDeep foundation Deep foundation qqBB = 9c= 9cuu + + γγssDDFor the general soil type use the EC7 For the general soil type use the EC7 BrinchBrinch--Hansen Hansen

equation.equation.

MALTESE CLAYS CHARACTERISTICS

Referring to Mr. A.Referring to Mr. A. CassarCassar A&CE, from variousA&CE, from variousinsituinsitu tests carried out using SPT and laboratory tests carried out using SPT and laboratory tests on recovered samples, Maltese clays may be tests on recovered samples, Maltese clays may be described as stiff to very stiff in its natural state, described as stiff to very stiff in its natural state, having an average C value of 100KN/mhaving an average C value of 100KN/m22, with a , with a lower limit of 50 and an upper limit of 200. lower limit of 50 and an upper limit of 200.

Also the plastic limit (PL) of clay is given at 23%, Also the plastic limit (PL) of clay is given at 23%, with the liquid limit (LL) at 70% (with the liquid limit (LL) at 70% (BonelloBonello 1988). 1988).

The plasticity index (PI) is thus given byThe plasticity index (PI) is thus given byPI = LL PI = LL –– PL = 47% PL = 47%

MALTESE CLAYS CHARACTERISTICS -continued

From theFrom the CasagrandeCasagrande plasticity chart this is plasticity chart this is classified as an inorganic clay of high plasticity.classified as an inorganic clay of high plasticity.

From BS 8004 table 1, stiff clays have a presumedFrom BS 8004 table 1, stiff clays have a presumedalloweablealloweable bearing value of 150 to 300KN/mbearing value of 150 to 300KN/m22, , whilst very stiff clays have values varying from whilst very stiff clays have values varying from 300 to 600 KN/m300 to 600 KN/m22..

For a PL at 23% and a high clay content, the For a PL at 23% and a high clay content, the shrinkage and swelling potential of Maltese clays shrinkage and swelling potential of Maltese clays is classified at high, usually showing cracks on is classified at high, usually showing cracks on drying.drying.

Blue Clay FormationMineralogic Composition

41413324.524.531.531.53232898934534529.029.0LonLon--don don clayclay

575700303013.013.03131787813713736.036.0Blue Blue ClayClay

Smectite Smectite %%

Chlorite Chlorite %%

Kaolinite Kaolinite %%

Illite Illite %%

Placticity Placticity limitlimit

%%

Liquid Liquid limitlimit

%%

Undrained Undrained shear shear str str

kPakPa

Water Water Content Content

(%)(%)

Clay Clay typetype

Maximum burial depth: Blue clay: c 400m London Clay: c500m

Source: Saviour Scerri - geologist

Blue Clay – Geotechnical characteristics

3423421101101.461.461.911.91CVCV0.150.1548482626747433335.505.50

41541520201.491.491.951.95CHCH0.070.0742422727696930301.001.00

3053051101101.431.431.901.90CVCV0.120.1249492727767633335.505.50

28528520201.451.451.911.91CVCV0.110.1145452727727232321.001.00

3343341761761.451.451.921.92CVCV0.130.1346462828747434348.808.80

2662661041041.441.441.921.92CVCV0.160.1649492525747433335.205.20

2512511701701.441.441.911.91CVCV0.160.1645452626717133338.508.50

24324380801.401.401.901.90CVCV0.150.1548482929777736364.004.00

CCuu ––KN/mKN/m22

Lateral Lateral PressPress

Dry Dry WeightWeight

Bulk Bulk WeightWeight

Soil Soil ClassClass

LILIPIPIPLPLLLLLMoisture Moisture ContentContent

Sample Sample Depth Depth

mm

Source: Saviour Scerri -geologist

Blue Clay FormationBlue Clay has a high clay contentBlue Clay has a high clay content

•• Shrinkage due to desiccation is high and may Shrinkage due to desiccation is high and may reach 3m in depthreach 3m in depth

•• Deep cracks are producedDeep cracks are produced•• Clay loses all its cohesionClay loses all its cohesion•• Subsequent saturation produces clay slipsSubsequent saturation produces clay slips

Source: Saviour Scerri - geologist

Preparing A Clay Founding LayerIn order to eliminate seasonal ground movement In order to eliminate seasonal ground movement (heave or shrinkage) a min. foundation depth of 0.9m (heave or shrinkage) a min. foundation depth of 0.9m is suggestedis suggestedWhen constructing foundations in very dry weather, When constructing foundations in very dry weather, care must be taken to ensure superstructure loads are care must be taken to ensure superstructure loads are applied as soon as possibleapplied as soon as possibleFoundations are to be placed at a sufficient distance Foundations are to be placed at a sufficient distance from trees. To reduce above damage due to from trees. To reduce above damage due to subsidence or heave, foundations should be placed at a subsidence or heave, foundations should be placed at a distance away of 0.5H, being the mature tree height.distance away of 0.5H, being the mature tree height.For trees such as the poplar, oak, elm, willow and For trees such as the poplar, oak, elm, willow and eucalyptus the distance should be doubled to Heucalyptus the distance should be doubled to H

Constructing a Raft FoundationRaft foundations should be placed on fully Raft foundations should be placed on fully compacted draining infill separated by a polythene compacted draining infill separated by a polythene sheet not exceeding 1.0m in depth. The raft and sheet not exceeding 1.0m in depth. The raft and fully compacted fill tend to act compositely in fully compacted fill tend to act compositely in resisting the heave forces. Heave movement is resisting the heave forces. Heave movement is reduced by removing the most desiccated clay reduced by removing the most desiccated clay layer.layer.For protection against the possibility of future tree For protection against the possibility of future tree planting producing damaging ground movement the planting producing damaging ground movement the bored pile foundation is more suitable. The upper bored pile foundation is more suitable. The upper part of the pile shaft in the clay desiccation zone part of the pile shaft in the clay desiccation zone should be sleeved to reduce uplift selling forcesshould be sleeved to reduce uplift selling forcesHeaving pressures in clays may be up to 200KN/mHeaving pressures in clays may be up to 200KN/m22

Indirect Design Methods

This is the traditional method used in most countries. In This is the traditional method used in most countries. In this method calculations are carried out at characteristic this method calculations are carried out at characteristic stress levels (CP 2004 stress levels (CP 2004 –– table 1 enclosed) with table 1 enclosed) with unfactored unfactored load and ground parameters.load and ground parameters.

Although EC7 does not provide provision for this method, Although EC7 does not provide provision for this method, it is expected to be included in the revised version.it is expected to be included in the revised version.

Foundations on rock applicable to this method, although Foundations on rock applicable to this method, although Annex G of EC7 gives presumed bearing resistances Annex G of EC7 gives presumed bearing resistances dependant on the rocks compressive strength and dependant on the rocks compressive strength and discontinuity spacing.discontinuity spacing.

Foundation Settlement EC7 – Appendix F

Adjusted elasticity method s= Adjusted elasticity method s= pBfpBf//EEmm (cohesive & (cohesive & nonnon--cohesive)cohesive)

p is elastic bearing pressure linearly distributedp is elastic bearing pressure linearly distributedf is the settlement coefficient?f is the settlement coefficient?EEmm is the soil modulus of elasticityis the soil modulus of elasticityAppendix H outlines structural deformation & Appendix H outlines structural deformation &

foundation movement foundation movement

ALLOWABLE SETTLEMENTS & ROTATIONS

For normal structures with isolated foundations total For normal structures with isolated foundations total settlements up to 50mm acceptable. A max relative settlements up to 50mm acceptable. A max relative rotation of 1/500 acceptable for most structures, given rotation of 1/500 acceptable for most structures, given in EC7. in EC7.

Other sourcesOther sources’’ max raft total settlement of clay up to max raft total settlement of clay up to 125mm with differential settlements of 45mm 125mm with differential settlements of 45mm acceptable. For sand, total given at 50mm and acceptable. For sand, total given at 50mm and differential at 30mm. differential at 30mm.

Isolated foundations max. deflection on clay given at Isolated foundations max. deflection on clay given at 75mm (sand 50mm). 75mm (sand 50mm).

Brick buildings total settlement quoted at 75Brick buildings total settlement quoted at 75--100mm. 100mm. Angular distortion of 1/300 also quoted.Angular distortion of 1/300 also quoted.

geotechnics for the structural engineer

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