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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?