Geotechnics for the Structural Engineer

(1)

GEOTECHNICS

FOR

THE STRUCTURAL

ENGINEER

DENIS H. CAMILLERI

dhcamill

@

maltanet

.net

BICC

(2)

The Development of Foundation limit State

Design

Before World War II codes of practice for

foundation engineering were used only in a

small number of countries.

In 1956

Brinch

Hansen used for the first time the

words

“limit design

“

limit design”

”

in a

geotechnical

context.

Brinch

linked the limit design concept closely to

the concept of partial safety factors, and he

introduced these two concepts in Danish

foundation of engineering practice.

(3)

Basis Behind Eurocode 7

The Limit state concept is today widely accepted as a

basis for codes of practice in structural engineering.

From the very beginning of the work on the

Eurocodes it was a foregone conclusion that the

Eurocodes should be written in the limit state design

format and that partial factors of safety should be

used.

Consequently it was decided that also those parts of

the Eurocodes which will be dealing with

geotechnical

aspects of design should be written in

the limit state format with the use of partial factors

of safety

(4)

Geotechnical Categories & Geotechnical Risk Higher Categories satisfied by

greater attention to the quality of the geotechnical investigations and the design

Table 1-

Geotechnical Categories related to geotechnical hazard and vulnerability levels

High risk of damage to

neighbouring

structures or services

Possible risk of damage

to neighbouring

structures or services

due, for example, to

excavations or piling

Negligible risk of damage

to or from neighbouring

structures or services and

negligible risk for life

Surroundings Surroundings Areas of high Areas of high earthquake hazard earthquake hazard Moderate earthquake Moderate earthquake

hazard where seismic

design code (EC8 Part

V) may be used

Areas with no or very low

earthquake hazard

Regional

Regional seismicityseismicity

Unusual or Unusual or exceptionally difficult exceptionally difficult ground conditions ground conditions requiring non

requiring non--routine routine investigations and

investigations and

tests.

Ground conditions and

properties can be

determined from routine

investigations and tests.

Known from comparable

experience to be

straightforward. Not

involving soft, loose or

compressible soil, loose

fill or sloping ground.

Ground conditions Ground conditions High High Moderate Moderate Low Low Geotechnical

Geotechnicalhazards hazards /vulnerability /risk /vulnerability /risk GC3 GC3 GC2 GC2 GC1 GC1 Geotechnical

Geotechnical CategoriesCategories Factors to be

Factors to be

considered

(5)

Table 1 (cont.)

-- Very large Very large

buildings

-- Large bridgesLarge bridges

-- Deep Deep excavations excavations -- Embankments Embankments on soft ground on soft ground Tunnels in soft or Tunnels in soft or highly permeable highly permeable ground ground Conventional: Conventional:

-- Spread and pile Spread and pile

foundations

-- Walls and other Walls and other

retaining structures

-- Bridge piers and Bridge piers and

abutments abutments Embankments and Embankments and earthworks earthworks

-- Simple 1 & 2 storey Simple 1 & 2 storey

structures and agricultural

buildings having maximum

design column load of 250kN

and maximum design wall load

of 100kN/m

-- Retaining walls and Retaining walls and

excavation supports where

ground level difference does not

exceed 2m exceed 2m Examples of Examples of structures structures More sophisticated More sophisticated analyses analyses

Routine calculations for

stability and stability and deformations based on deformations based on design procedures in design procedures in EC7 EC7

Prescriptive measures and

simplified design procedures

e.g. design bearing pressures

based on experience or

published presumed bearing

pressures. Stability of

deformation calculations may

not be necessary not be necessary Design Design procedures procedures Experienced Experienced geotechnical geotechnical specialist specialist Experienced qualified Experienced qualified person

person ––Civil EngineerCivil Engineer Person with appropriate

Person with appropriate

comparable experience comparable experience Expertise Expertise required required GC3 GC3 GC2 GC2 GC1 GC1 Geotechnical

(6)

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 redredare partial factors either given or implied in ENV version of Eare partial factors either given or implied in ENV version of EC7C7

Values 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

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 γ γ_A_A Accidental action Accidental action 0 0 0 0 0 0 0 0 0 0 γ γ_Q_Q Variable favourable Variable favourable action action 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.95 0.95 γ γ_G_G Permanent

Permanent fvourable fvourable action action 1.20 1.20 1.50 1.50 1.30 1.30 1.50 1.50 1.50 1.50 γ γ_Q_Q Variable

Variable unfvaourable unfvaourable action action 1.00 1.00 1.35 1.35 1.00 1.00 1.35 1.35 1.00 1.00 γ γ_G_G Permanent Permanent unfavourable action unfavourable action (HYD) (HYD) (EQU) (EQU) (GEO) (GEO) (STR) (STR) (UPL) (UPL) Partial load factors (

Partial load factors (γγ_F_F))

Case C3 Case C3 Case C2 Case C2 Case C Case C Case B Case B Case A Case A Factor Factor Parameter Parameter

(7)

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 EC77

Values 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

Case C3 Case C3 Case C2 Case C2 Case C Case C Case B Case B Case A Case A Factor Factor Parameter Parameter 1.40 1.40 1.00 1.00 1.40 1.40 1.00 1.00 1.40 1.40 γ γ_CPT_CPT CPT resistance CPT resistance 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 γ γ_g_g

Unit weight of ground

Unit weight of ground γγ

1.40 1.40 1.00 1.00 1.40 1.40 1.00 1.00 1.40 1.40 γ γ_plim_plim Pressuremeter

Pressuremeter limit limit

pressure

p

_lim_lim

1.40 1.40 1.00 1.00 1.40 1.40 1.00 1.00 1.20 1.20 γ γ_qu_qu Compressive strength Compressive strength

q

_u_u 1.40 1.40 1.00 1.00 1.40 1.40 1.00 1.00 1.20 1.20 γ γ_cu_cu Undrained

Undrained shear strength shear strength

c

_u_u

1.20 1.20 1.00 1.00 1.60 1.60 1.00 1.00 1.30 1.30 γ γ_c_c_’_’ Effective cohesion c Effective cohesion c’’ 1.20 1.20 1.00 1.00 1.25 1.25 1.00 1.00 1.10 1.10 γ γ_tan_tan_φ_φ’_’ Tan Tan φφ’’ (HYD) (HYD) (EQU) (EQU) (GEO) (GEO) (STR) (STR) (UPL) (UPL) Partial material factors

(8)

Table 2 (Cont.)

Values inValues inred red are partial factors either given or implied in ENV version of ECare partial factors either given or implied in ENV version of EC77

Values inValues ingreengreenare 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

1.00 1.00 1.40 1.40 1.60 1.60 1.00 1.00 1.40 1.40 γ γ_st_st

Pile Tensile resistance

Case C3 Case C3 Case C2 Case C2 Case C Case C Case B Case B Case A Case A Factor Factor Parameter Parameter 1.00 1.00 1.30 1.30 1.30 1.30 1.00 1.00 --** γ γ_t_t

Total pile resistance

1.00 1.00 1.20 1.20 1.50 1.50 1.00 1.00 1.30 1.30 γ γ_A_A Anchor pull

Anchor pull--out resistanceout resistance

1.00 1.00 1.30 1.30 1.30 1.30 1.00 1.00 --** γ γ_s_s

Pile shaft resistance

1.00 1.00 1.30 1.30 1.30 1.30 1.00 1.00 --** γ γ_b_b

Pile base resistance

1.00 1.00 1.40 1.40 1.00 1.00 1.00 1.00 --** γ γ_Re_Re Earth resistance Earth resistance 1.00 1.00 1.10 1.10 1.00 1.00 1.00 1.00 --** γ γ_rS_rS Sliding resistance Sliding resistance 1.00 1.00 1.40 1.40 1.00 1.00 1.00 1.00 --** γ γ_RV_RV Bearing resistance Bearing resistance (HYD) (HYD) (EQU) (EQU) (GEO) (GEO) (STR) (STR) (UPL) (UPL) Partial resistance factors

(9)

Serviceability Limit State Calculations (SLS)

Table 3 – Serviceability limits

Moderate to Moderate to severe severe Moderate to Moderate to severe severe Severe to very Severe to very severe severe 15 to 25 15 to 25 Increasing risk Increasing risk of structure of structure becoming becoming dangerous dangerous Severe to Severe to dangerous dangerous Severe to Severe to dangerous dangerous Very severe to Very severe to dangerous dangerous >25 >25 Moderate Moderate Moderate to Moderate to severe severe Moderate to Moderate to severe severe 5 to 15 5 to 15 Serviceability of Serviceability of

the building will

be affected, and be affected, and towards the towards the upper bound, upper bound, stability may stability may also be at risk also be at risk Slight Slight Moderate Moderate Moderate Moderate 2 to 5 2 to 5 Accelerated Accelerated weathering to weathering to external features external features Very slight Very slight Slight to Slight to moderate moderate Slight to Slight to moderate moderate 1 to 2 1 to 2 Aesthetic only Aesthetic only Very slight Very slight Slight Slight Slight Slight 0.3 to 1 0.3 to 1 none none Insignificant Insignificant Very slight Very slight Very slight Very slight 0.1 to 0.3 0.1 to 0.3 None None Insignificant Insignificant Insignificant Insignificant Insignificant Insignificant > 0.1 > 0.1 Industrial Industrial Commercial or Commercial or public public Dwelling Dwelling Effect on Effect on structure and structure and building use building use Degree of damage Degree of damage Crack width mm Crack width mm

(10)

LIMIT STATE DESIGN –

CHARACTERISTIC VALUE & DESIGN

STRENGTH

CHARACTERISTIC STRENGTH OF A

MATERIAL is the strength below which not

more than 5% (or 1 in 20) samples will fail.

CHARACTERISTIC STRENGTH =

MEAN VALUE

MEAN VALUE –

–

1.64 X Standard Deviation

DESIGN STRENGTH =

CHARACTERISTIC STRENGTH

f

_u_u

MATERIAL FACTOR OF SAFETY

MATERIAL FACTOR OF SAFETY γ

γ

_m_m

(11)

EXAMPLE:

Ten concrete cubes were prepared and tested by crushing in

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

m

22

were obtained:

44.5 47.3 42.1 39.6 47.3 46.7 43.8 49.7 45.2 42.7

Mean strength

Mean strength x

x

_m_m

= 448.9

=

448.9 = 44.9N

=

44.9N/mm

/mm

22

10

10 Standard deviation =

Standard deviation =

√

[(x-

[(x

-x

x

_m_m

)

22

_/(n-

_/(n

_{-1)] =}

_{1)] =}

√

_√

_(80/0)

= 2.98N/mm

22

Characteristic strength = 44.9

Characteristic strength = 44.9 –

–

(1.64 X 2.98)

= 40.0 N/mm

22

Design strength =

Design strength = 40.0

40.0 = 40.0

=

40.0 γ

γ

_m_m

1.5

1.5 =

=

26.7N/mm

22

(12)

Output Output Calculations Calculations Ref Ref Date: 05/02 Date: 05/02

Drawing Ref: Done by: DHC Drawing 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_Kvalues were determined from 10 values were determined from 10 traxial traxial tests: 33tests: 33°°, 35, 35°°, , 33.5

33.5°°, 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 a

To find the 95% confidence level, for soil properties, as only a small small portion of the total volume involved in a design situation is te

portion 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% confidence

For a small sample size the Student t value for a 95% confidence level level may be used to determine that X

may be used to determine that X_K_K value, given byvalue, given by X

X_K_K = = XX_m_m [ l[ l--tV tV ] = ] = XX_m_m -- ttσσ

√

√n n √√n n

Some typical values of V for different soil properties given by

Part of Structure Part of Structure

CHARACTERISTIC VALUE DETERMINATION CHARACTERISTIC VALUE DETERMINATION

Job ref: Job ref: Project Project FOUNDATION CPD COURSE FOUNDATION CPD COURSE BICC BICC BUILDING INDUSTRY BUILDING INDUSTRY CONSULTATIVE CONSULTATIVE COUNCIL COUNCIL

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

(13)

Average angle of shearing resistance

∅

’

_AV_AV

= 34.4°

= 34.4

°

With a Standard Deviation

σ

= 1.97°

= 1.97

°

Coeff

Coeff of variation V = 0.057

of variation V = 0.057

Student t for a 95% confidence level

with 10 test results = 2.

with 10 test results = 2.26

26 ∅

∅

’

_K_K

= 34.4 -

= 34.4

-

1.97 X 2.26 /

√

10 = 33.0°

10 = 33.0

°

The Design Value X

_D_D

= X

=

X

_k_k

/

γ

_m_m

Applying the

γ

_m_m

= 1.25 for Case C in Table 2

∅

’

_c_c

= arc tan (tan

∅

’

_K_K

) / 1.25 = 27.8°

) / 1.25 = 27.8

°

Output Output Calculations Calculations Ref Ref

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

Part of Structure Part of Structure

CHARACTERISTIC & DESIGN VALUE CHARACTERISTIC & DESIGN VALUE DETERMINATION DETERMINATION Job ref: Job ref: Project Project FOUNDATION CPD COURSE FOUNDATION CPD COURSE BICC BICC BUILDING INDUSTRY BUILDING INDUSTRY CONSULTATIVE CONSULTATIVE COUNCIL COUNCIL

(14)

(15)

Basic Cohesive Soil Founding Pressures

Shallow Foundation occurs when founding depth (D) is

less than width (B)

D/B < 1 or when d<3m (may not be applicable for rafts)

For

undrained

conditions, the base resistance

q

_B

per unit

area

Shallow foundation

q

_B

= 5c

_u

+

γ

_s

D

Deep foundation

q

_B

= 9c

_u

+

γ

_s

D

For the general soil type use the EC7

Brinch

-

Hansen

equation.

(16)

(17)

(18)

MALTESE CLAYS CHARACTERISTICS

Referring to Mr. A.

Cassar

A&CE, from various

insitu

tests carried out using SPT and laboratory

tests on recovered samples, Maltese clays may be

described as stiff to very stiff in its natural state,

having an average C value of 100KN/m

2

2 _{, with a}

_{, with a}

lower limit of 50 and an upper limit of 200.

Also the plastic limit (PL) of clay is given at 23%,

with the liquid limit (LL) at 70% (

Bonello

1988).

The plasticity index (PI) is thus given by

PI = LL

(19)

MALTESE CLAYS CHARACTERISTICS

-continued

From the

Casagrande

plasticity chart this is

classified as an inorganic clay of high plasticity.

From BS 8004 table 1, stiff clays have a presumed

alloweable

bearing value of 150 to 300KN/m

2

2 _,

_,

whilst very stiff clays have values varying from

300 to 600 KN/m

2

2 _.

_.

For a PL at 23% and a high clay content, the

shrinkage and swelling potential of Maltese clays

is classified at high, usually showing cracks on

drying.

(20)

Blue Clay Formation

Mineralogic Composition

41

3

24.5

31.5

32

89

345

29.0

29.0 Lon

Lon-

-don

don

clay

57

0

30

13.0

31

78

137

36.0

36.0 Blue

Blue

Clay

Smectite

%

Chlorite

%

Kaolinite

%

Illite

%

Placticity

limit

%

Liquid

limit

%

Undrained

shear

str

kPa

Water

Content

(%)

Clay

type

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

Source: Saviour Scerri - geologist

(21)

Blue Clay – Geotechnical characteristics

342

110

1.46

1.91

1.91 CV

CV

0.15

48

26

74

33

5.50

415

20

1.49

1.95

1.95 CH

CH

0.07

42

27

69

30

1.00

305

110

1.43

1.90

1.90 CV

CV

0.12

49

27

76

33

5.50

285

20

1.45

1.91

1.91 CV

CV

0.11

45

27

72

32

1.00

334

176

1.45

1.92

1.92 CV

CV

0.13

46

28

74

34

8.80

266

104

1.44

1.92

1.92 CV

CV

0.16

49

25

74

33

5.20

251

170

1.44

1.91

1.91 CV

CV

0.16

45

26

71

33

8.50

243

80

1.40

1.90

1.90 CV

CV

0.15

48

29

77

36

4.00

4.00 C

C

_u_u

–

KN/m

22

Lateral

Press

Dry

Weight

Bulk

Weight

Soil

Class

LI

PI

PL

LL

Moisture

Content

Sample

Depth

m

(22)

Blue Clay Formation

_{Blue 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 depth

•

• Deep cracks are produced

Deep cracks are produced

•

• Clay loses all its cohesion

Clay loses all its cohesion

•

• Subsequent saturation produces clay slips

Subsequent saturation produces clay slips

(23)

Preparing A Clay Founding Layer

In order to eliminate seasonal ground movement

_{In order to eliminate seasonal ground movement}

(heave or shrinkage) a min. foundation depth of 0.9m

is suggested

_{When constructing foundations in very dry weather,}

care must be taken to ensure superstructure loads are

applied as soon as possible

_{Foundations are to be placed at a sufficient distance}

from trees. To reduce above damage due to

subsidence or heave, foundations should be placed at a

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 H

(24)

(25)

(26)

(27)

Constructing a Raft Foundation

Raft foundations should be placed on fully

_{Raft foundations should be placed on fully}

compacted draining infill separated by a polythene

sheet not exceeding 1.0m in depth. The raft and

fully compacted fill tend to act compositely in

resisting the heave forces. Heave movement is

reduced by removing the most desiccated clay

layer.

_{For protection against the possibility of future tree}

planting producing damaging ground movement the

bored pile foundation is more suitable. The upper

part of the pile shaft in the clay desiccation zone

should be sleeved to reduce uplift selling forces

(28)

(29)

(30)

Indirect Design Methods

This is the traditional method used in most countries. In

this method calculations are carried out at characteristic

stress levels (CP 2004

–

table 1 enclosed) with

unfactored

load and ground parameters.

Although EC7 does not provide provision for this method,

it is expected to be included in the revised version.

Foundations on rock applicable to this method, although

Annex G of EC7 gives presumed bearing resistances

dependant on the rocks compressive strength and

discontinuity spacing.

(31)

Foundation Settlement

EC7 – Appendix F

Adjusted elasticity method s=

pBf

/

E

_m

(cohesive &

non

-

cohesive)

p is elastic bearing pressure linearly distributed

f is the settlement coefficient?

E

_m

is the soil modulus of elasticity

Appendix H outlines structural deformation &

foundation movement

(32)

ALLOWABLE SETTLEMENTS &

ROTATIONS

For normal structures with isolated foundations total

settlements up to 50mm acceptable. A max relative

rotation of 1/500 acceptable for most structures, given

in EC7.

Other sources

’

max raft total settlement of clay up to

125mm with differential settlements of 45mm

acceptable. For sand, total given at 50mm and

differential at 30mm.

Isolated foundations max. deflection on clay given at

75mm (sand 50mm).

Brick buildings total settlement quoted at 75

-

100mm.

Angular distortion of 1/300 also quoted.