practice in geotechnical design

7/28/2019 Practice in Geotechnical Design

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Case studies for practicein geotechnical design

Lecture 4

Master courses


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New P10/2005

structure+basement+foundation+soil= structural system


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Loads from the structure

Designing infrastructure is entirely conditioned bythe complet structure analysis.

Loads delivered to the infrastructure areestablished in both fundamental and specialgroupings.

Any failure mechanism according to the specialgroupings of loads is restricted only within thestructure.


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Dimensions of the footing

Are established so that the contact pressures onthe footing have acceptable values, to preventdeveloping of any limit states, endangering thesafety or service of the construction.

Limit states within foundation soil can beregarded as the followings:

Ultimate Limit State (ULS)

Service Limit State (SLS)


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Limit States within Foundation SoilEC 7

SLS settlements withinacceptable values for thestructure

ULS sperated on 3 cases:

P 10

Case A loss of thestatic equilibrium:material or soil strength isirrelevant; Case B structure/structure

elements failure, includingfootings, piles, basementwalls: due to the materialfailure within thestructure; Case C soil failure;

Deformation Limit State (SLD)

SLD.U when the soildeformations are inacceptable

for the structure safety;SLD.EN when the soildeformations are affecting thestructure service

Bearing Capacity Limit State

(SLCP) ULS soil failure


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PressureSetllement dependency


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Acceptable pressures are considered to be:

a conventional pressure pconv;

a pressure to comply with the restrictions

regarding the SLD.U and SLD. EN; a pressure to comply with the restrictions

regarding the SLCP;

All these pressures are established based on bothconstruction and soil characteristic features


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Construction influencing factors:

a) The class of importance Special constructions, CS (class I and II, as within

STAS 10100/0-75);

Regular constructions, CO (class II, IV, and V);

b) The sensitivity to settlements Sensitive constructions to differential settlements

(CSEN);

Insensitive constructions to differential settlements;

c) The existence of deformation restrictionsduring service

Constructions with restrictions (CRE);

Constructions without restrictions.


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Soil influencing factors

Soil category: Adequate/good foundation soils (TB);

Inadequate/difficult foundation soils.


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Conditions to set the footing dimensions based on

the acceptable pressure concept:


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Limit statesrestrictions:A) STAREA LIMIT DE DEFORMAIE (S.L.D.)1) Starea limit a exploatrii normale (S.L.E.N.), prin verificarea terenului sub

efectul ncrcrilor totale de exploatare, n gruparea fundamental laaciuni corespunztoare (S.L.E.N.) prin restricia:

2) Starea limit ultim (S.L.U.) de rezisten i stabilitate, prin verificareaterenului de fundare sub efectul ncrcrilor totale corespunztoareS.L.U. prin restricia:

acompaniate de .A')Calculul dup presiuni convenionale sub ncrcrile din gruparea

fundamental de aciuni prin restricia:

B) STAREA LIMIT DE CAPACITATE PORTANT (S.L.C.P.) prin verificarea terenului sub efectul ncrcrile de gruparea specialde aciuni prin una din restriciile:

igii VnCP

tt

iigiiii VnnCnPn

ss plef pmp

max

convef pmp iigiiii VnnCnPn

iiiii EVnCP

crcef pmp


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Correction coefficients for pconvand ppl


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Cohesionless soils - pconv


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Cohesive soils - pconv


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Bearing capacity limit state (SLCP)


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*(o) N Nq Nc

o o 0,0 1,0 5,1

5 o 0,1 1,6 6,5

10 o 0,2 2,5 8,3

15 o 0,7 3,9 11,0

20 o 1,8 6,4 14,8

22 o30 2,7 8,2 17,5

25 o 4,1 10,7 20,7

27 o30 6,1 13,9 24,9

30 o 9,0 18,4 30,1

32 o30 13,6 24,6 37,0

35 o 20,4 33,7 46,1

37 o30 31,0 45,8 58,4

40 o 47,7 64,2 75,3

42 o30 75,0 91,9 99,3

45 o 120,5 134,9 133,9

qqqccccr iNqiNciNBp ** N1 N2 N3

0 0,00 1,00 3,14

2 0,03 1,12 3,32

4 0,06 1,25 3,51

6 0,10 1,39 3,71

8 0,14 1,55 3,93

10 0,18 1,73 4,17

12 0,23 1,94 4,42

14 0,29 2,17 4,69

16 0,36 2,43 5,00

18 0,43 2,72 5,31

20 0,51 3,06 5,66

22 0,61 3,44 6,04

24 0,72 3,84 6,45

26 0,84 4,37 6,90

28 0,98 4,93 7,40

301,15 5,59 7,95

32 1,34 6,35 8,55

34 1,55 7,21 9,21

36 1,81 8,25 9,98

38 2,11 9,44 10,80

40 2,46 10,84 11,73

42 2,87 12,50 12,77

44 3,37 14,48 13,06

45 3,66 15,64 14,64

)( 321 NcNqNBmp lpl

)3

2( 321 NcN

qqNBmp ielpl

Differences between ppl and pcr


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Coefficient of the working conditions


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Soil failure on limited depth - ppl


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PressureSetllement dependency


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Soil failure - pcr


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Tentativevalues of and c


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Example 1EC7

A

AB

D=1m

Gk

G =1000kNk

k

=350

k

=18kN/m3

c =0k

Case A

There is no buyoancy possibility; case A is irrelevant

Case B

The foundation width is based on the bearing capacity of the soil and

accordingly computed:

supGKqq

2 GsNB5,0sN'qB

For0

kd35

the bearing capacity factors are Nq=33,3 and N=45,2

The coefficients depending on the footing shape are s=0,7 and

sq= 1+sind=1,57

It results that:

35,11000B7,03,45185,057,13,33118B 2

and B=1,05m

Considering an uniform soil pressure distribution

onto the footing, the maximum bending moment for

the A-A cross section can be estimated as:

kNm2,1774/05,12/1350M'AA


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A

AB

D=1m

Gk

G =1000kNk

k=35

0

k

=18kN/m3

c =0k

Case CThe design value of the permanent load is

kNm100000,11000GGkkd

and

0kd

3,2935,1/tgarct

so that consequently

Nq = 16,9 and N=17,8

s=0,7 and sq= 1+sind=1,49

1000B7,08,17185,049,19,16118B 2

and B=1,29m

Consequently the maximum bending moment is:

kNm2184/29,12/35,11000M'AA

Foundation self weight is

kN402400,129,1 2

which certifyies neglecting it in the first place


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Example 1

new P10

A

AB

D=1m

Gk

G =1000kNkk=35

0

k=18kN/m3

c =0k


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Example 2EC 7

A A

A AB B

B

D=1m D=1m

Gk Gd

Gf

Qk Qd

G =400kNk Q =76,9kNkk=35

0

k=18kN/m3

c =0k

H=4m H=4m

e


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Case A

There is no buyoancy possibility; case A is irrelevant

Case B

The foundation width is based on the bearing capacity of the soil and accordingly computed for the

dimensions of the footing as B and B:

HQisN'B5,0isN'q'BBdqqq

HQeRd

Applying the partial coefficients from table 2.1.)loadleunfavourabfor(50,1

)loadfavourablefor(00,1

Q

inf,G

m30,2'B)2/'Be(2Band

m15,1R/HQe

kN4,11550,19,76QQ

kN40000,1400GGR

d

Qkd

inf,Gkd

For 0kd 35 the bearing capacity factors are Nq=33,3 and N=45,2The coefficients depending on the footing shape are:s=1-0,3B/B and sq= 1+(B/B)sind

508,0GQ7,01i

360,0GQ1i

3

ddq

3

dd

It results that B=0,39m

And 1,15>2,69/3 (e>B/3) it is recommanded to increase B with at least 10cm , so that:

B=0,39+2,30+2x0,10=2,89mThe verification for horizontal forces meets the restriction:

Sd > Hd (friction force onto the footing larger than the horizontal load)

dddQ280tgG

Considering an uniform soil pressure distribution onto the footing, the

maximum bending moment for the A-A cross section can be estimated as:

kNm46015,1400eRM'AA


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Case C

The design value of the permanent load is

m00,1ekN10030,19,76QQ

kNm40000,1400GG

Qkd

kkd

and B=B+2,00m

For 0kd 3,2935,1/tgarct

the values are Nq = 16,9 and N=17,8

s=1-0,3B/B and sq= 1+(B/B)sind

and the inclination coefficients:

562,0GQ7,01i

422,0GQ1i

3

ddq

3

dd

consequently B=0,65m and once again B=0,65+2,00+0,20=2,85m

Consequently the maximum bending moment is:

kNm4001400M'AA

Foundation self weight is

kN2012400,189,2 2

so it cannot be neglected.

Consequently, being favourable

00,1G

For the B case: e=0,89m, B=0,48m and B=2,26m

For the C case: e=0,78m, B=0,78m and B=2,32m and that validate the

supplementary computation with respect to the foundation self weight.


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Example 2application of the new P10


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Conclusive remarks

Different approach of soil-structureinteraction;

Apparently unique values of and c;

Value of settlement is assessed within aultimate limit state SLD.U;

Can a structural engineer perfom a

foundation design based a geotechnicalreport?

practice in geotechnical design

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