Eurocode 7 – Good practice in geotechnical design
Transcript of Eurocode 7 – Good practice in geotechnical design
![Page 1: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/1.jpg)
Eurocode 7 –
Good practice in geotechnical design
Brian SimpsonArup
8th Lumb Lecture
![Page 2: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/2.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 3: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/3.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 4: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/4.jpg)
4
Limit state design
• “states beyond which the structure no longer satisfies the relevant design criteria”
• partial factor design ?
• probabilistic design ?
• concentration on what might go wrong
EN1990 3.3 (3)
States prior to structural collapse, which,
for simplicity, are considered in place of
the collapse itself, may be treated as
ultimate limit states.
![Page 5: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/5.jpg)
5
EN 1990 3.3 Ultimate limit states
Serious failures involving risk of injury or major cost.
Must be rendered very unlikely. An “unrealistic” possibility.
![Page 6: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/6.jpg)
6
EN 1990 3.3 Ultimate limit states
Serious failures involving risk of injury or major cost.
Must be rendered very unlikely. An “unrealistic” possibility.
![Page 7: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/7.jpg)
7
EN1997-1 2.4.7 Ultimate limit states – STR, GEO
![Page 8: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/8.jpg)
8
EN1990 3.4 Serviceability limit states
Inconveniences, disappointments and more manageable costs.
Should be rare, but it might be uneconomic to eliminate them
completely.
![Page 9: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/9.jpg)
9
EN1990 3.4 Serviceability limit states
Inconveniences, disappointments and more manageable costs.
Should be rare, but it might be uneconomic to eliminate them
completely.
![Page 10: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/10.jpg)
10
Limit state design
• An understanding of limit state design can be obtained by contrasting it with “working state design”.
• Working state design: Analyse the expected, working state, 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 at least unlikely).
![Page 11: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/11.jpg)
11
Soil failure without geometrical instability (large displacements)??
BP190a.28
![Page 12: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/12.jpg)
12
Fundamental limit state requirement
Ed ≤ Rd
E{ Fd ; Xd ; ad} = Ed ≤ Rd = R{ Fd ; Xd ; ad}
E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}
or E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = Rk/γR = RnφR (LRFD)
or γE Ek = Ed ≤ Rd = Rk/γR
so in total
γE E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}/γR
E = action effects d = design (= factored)
F = actions (loads) k = characteristic (= unfactored)
R = resistance (=capacity) rep = representative
X = material properties
a = dimensions/geometry
![Page 13: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/13.jpg)
13
Partial factors recommended in EN1997-1 Annex A (+UKNA)
Values of partial factors recommended in EN1997-1 Annex A
Design approach 1 Design approach 2 Design approach 3Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors DA2 - Comb 1 DA2 - Slopes DA3
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4 A1 M1 R2 A1 M=R2 A1 A2 M2 R3
Actions unfav 1,35 1,35 1,35 1,35
fav
unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3
Soil tan φ' 1,25 1,25 StructuralGeotech 1,25
Effective cohesion 1,25 1,25 actions actions 1,25
Undrained strength 1,4 1,4 1,4
Unconfined strength 1,4 1,4 1,4
Weight density
Spread Bearing 1,4
footings Sliding 1,1
Driven Base 1,3 1,1
piles Shaft (compression) 1,3 1,1
Total/combined
(compression)
1,3 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Bored Base 1,25 1,6 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,15 1,5 1,1
Shaft in tension 1,25 1,6 1,15 1,1
CFA Base 1,1 1,45 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,1 1,4 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Anchors Temporary 1,1 1,1 1,1
Permanent 1,1 1,1 1,1
Retaining Bearing capacity 1,4
walls Sliding resistance 1,1
Earth resistance 1,4
Slopes Earth resistance 1,1
indicates partial factor = 1.0 C:\BX\BX-C\EC7\[Factors.xls] 25-Nov-06 17:26
Permanent
Variable
(+ UKNA)
![Page 14: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/14.jpg)
14
Partial factors recommended in EN1997-1 Annex AValues of partial factors recommended in EN1997-1 Annex A
Design approach 1 Design approach 2 Design approach 3Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors DA2 - Comb 1 DA2 - Slopes DA3
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4 A1 M1 R2 A1 M=R2 A1 A2 M2 R3
Actions unfav 1,35 1,35 1,35 1,35
fav
unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3
Soil tan φ' 1,25 1,25 StructuralGeotech 1,25
Effective cohesion 1,25 1,25 actions actions 1,25
Undrained strength 1,4 1,4 1,4
Unconfined strength 1,4 1,4 1,4
Weight density
Spread Bearing 1,4
footings Sliding 1,1
Driven Base 1,3 1,1
piles Shaft (compression) 1,3 1,1
Total/combined
(compression)
1,3 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Bored Base 1,25 1,6 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,15 1,5 1,1
Shaft in tension 1,25 1,6 1,15 1,1
CFA Base 1,1 1,45 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,1 1,4 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Anchors Temporary 1,1 1,1 1,1
Permanent 1,1 1,1 1,1
Retaining Bearing capacity 1,4
walls Sliding resistance 1,1
Earth resistance 1,4
Slopes Earth resistance 1,1
indicates partial factor = 1.0 C:\BX\BX-C\EC7\[Factors.xls] 25-Nov-06 17:26
Permanent
Variable
![Page 15: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/15.jpg)
15
Partial factors for DA1 – UK National Annex
Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4
Actions unfav 1,35
fav
unfav 1,5 1,3 1,3
Soil tan φ' 1,25 1,25
Effective cohesion 1,25 1,25
Undrained strength 1,4 1,4
Unconfined strength 1,4 1,4
Weight density
Spread Bearing EC7
footings Sliding values
Driven Base 1,7/1.5 1,3
piles Shaft (compression) 1.5/1.3 1,3
Total/combined
(compression)
1.7/1.5 1,3
Shaft in tension 2.0/1.7 1.6
Bored Base 2.0/1.7 1,6
piles Shaft (compression) 1.6/1.4 1,3
Total/combined
(compression)
2.0/1.7 1.5
Shaft in tension 2.0/1.7 1.6
CFA Base As 1.45
piles Shaft (compression) for 1.3
Total/combined
(compression)
bored 1.4
Shaft in tension piles 1.6
Anchors Temporary 1,1 1,1
Permanent 1,1 1,1
Retaining Bearing capacity
walls Sliding resistance
Earth resistance
Slopes Earth resistance
indicates partial factor = 1.0
C:\BX\BX-C\EC7\[Factors.xls]
Permanent
Variable
UKNA
![Page 16: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/16.jpg)
17
2.4.7 Ultimate Limit States
1
7
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
![Page 17: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/17.jpg)
18
DA1 Combinations 1 and 2 correspond to STR and GEO?
STR and GEO both designed for the same partial factors
STR GEO
Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4
Actions unfav 1,35
fav
unfav 1,5 1,3 1,3
Soil tan φ' 1,25 1,25
Effective cohesion 1,25 1,25
Undrained strength 1,4 1,4
Unconfined strength 1,4 1,4
Weight density
Permanent
Variable
![Page 18: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/18.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 19: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/19.jpg)
20 ©20
Genting Highlands BP87.59 BP106.30 BP111.22 BP112.43 BP119.43 BP124-F3.9 BP130.33 BP145a.8
BP184.54
![Page 20: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/20.jpg)
Genting Highlands BP87.60 BP106.31 BP111.23 BP112.44 BP119.44 BP124-F3.10 BP130.34 BP145a.9
BP184.55
![Page 21: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/21.jpg)
FOS > 1 for characteristic soil strengths BP87.61
BP106.32 BP111.24 BP112.45
BP119.45 BP124-F3.11 BP130.35 BP145a.10
- but not big enough
![Page 22: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/22.jpg)
The slope and retaining wall are
all part of the same problem. BP87.62
BP106.33 BP111.25 BP112.46 BP119.46 BP124-F3.12
BP130.36 BP145a.11Structure and soil must be designed together - consistently.
![Page 23: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/23.jpg)
24
Partial factors for DA1 – UK and MS National Annex
Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4
Actions unfav 1,35
fav
unfav 1,5 1,3 1,3
Soil tan φ' 1,25 1,25
Effective cohesion 1,25 1,25
Undrained strength 1,4 1,4
Unconfined strength 1,4 1,4
Weight density
Spread Bearing EC7
footings Sliding values
Driven Base 1,7/1.5 1,3
piles Shaft (compression) 1.5/1.3 1,3
Total/combined
(compression)
1.7/1.5 1,3
Shaft in tension 2.0/1.7 1.6
Bored Base 2.0/1.7 1,6
piles Shaft (compression) 1.6/1.4 1,3
Total/combined
(compression)
2.0/1.7 1.5
Shaft in tension 2.0/1.7 1.6
CFA Base As 1.45
piles Shaft (compression) for 1.3
Total/combined
(compression)
bored 1.4
Shaft in tension piles 1.6
Anchors Temporary 1,1 1,1
Permanent 1,1 1,1
Retaining Bearing capacity
walls Sliding resistance
Earth resistance
Slopes Earth resistance
indicates partial factor = 1.0
C:\BX\BX-C\EC7\[Factors.xls]
Permanent
Variable
UKNA
• Should DA1-1 and DA1-2 give
the same result?
• Then what’s the point in doing
two calculations?
![Page 24: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/24.jpg)
EN1990 – choice of partial factor values BP145a.14
Design consistently at β standard deviations from the mean
![Page 25: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/25.jpg)
0.7 and 0.8 or 1.0 and 0.4 ? BP145a.15
Provided the uncertainties of loads and
resistances are reasonably similar …
… use this approach
But if one type of uncertainty is really dominant …
![Page 26: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/26.jpg)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]
Ratio of ββββ achieved to ββββ required
0.6
0.7
0.8
0.9
1
1.1
1.2
0 0.2 0.4 0.6 0.8 1
SA
FE
TY
RA
TIO
.
σE/(σR+σE)
ααααE=-0.7, ααααR=0.8
Less economic
Less safe
![Page 27: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/27.jpg)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]
Ratio of ββββ achieved to ββββ required
0.6
0.7
0.8
0.9
1
1.1
1.2
0 0.2 0.4 0.6 0.8 1
SA
FE
TY
RA
TIO
.
σE/(σR+σE)
ααααE=-0.7, ααααR=0.8Slope
stability
Typical
foundations
Tower
foundations
![Page 28: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/28.jpg)
0.7 and 0.8 or 1.0 and 0.4 ? BP145a.15
Provided the uncertainties of loads and
resistances are reasonably similar …
… use this approach
But if one type of uncertainty is really dominant …
![Page 29: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/29.jpg)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]
Ratio of ββββ achieved to ββββ required
0.6
0.7
0.8
0.9
1
1.1
1.2
0 0.2 0.4 0.6 0.8 1
SA
FE
TY
RA
TIO
.
σE/(σR+σE)
ααααE=-0.7, ααααR=0.8
ααααE=-0.4, ααααR=1.0 ααααE=-1.0, ααααR=0.4
Uneconomic
Unsafe
![Page 30: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/30.jpg)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]
Ratio of ββββ achieved to ββββ required
0.6
0.7
0.8
0.9
1
1.1
1.2
0 0.2 0.4 0.6 0.8 1
SA
FE
TY
RA
TIO
.
σE/(σR+σE)
ααααE=-0.7, ααααR=0.8
ααααE=-0.4, ααααR=1.0 ααααE=-1.0, ααααR=0.4
Uneconomic
Unsafe
![Page 31: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/31.jpg)
Combinations 1 and 2 in EC7 - DA1 BP106.34 BP111.53 BP112.47
BP119.47 BP124-F3.13 BP129.50 BP145a.22
• Just like load combinations, extended to include variables on the resistance side.
• All designs must comply with both combinations in all respects, both geotechnical and structural
• Turkstra’s principle for load combinations -extended
Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4
Actions unfav 1,35
fav
unfav 1,5 1,3 1,3
Soil tan φ' 1,25 1,25
Effective cohesion 1,25 1,25
Undrained strength 1,4 1,4
Unconfined strength 1,4 1,4
Weight density
Permanent
Variable
![Page 32: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/32.jpg)
8th Lumb
Lecture
![Page 33: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/33.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 34: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/34.jpg)
35
Fundamental limit state requirement
Ed ≤ Rd
E{ Fd ; Xd ; ad} = Ed ≤ Rd = R{ Fd ; Xd ; ad}
E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}
or E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = Rk/γR = RnφR (LRFD)
or γE Ek = Ed ≤ Rd = Rk/γR
so in total
γE E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}/γR
E = action effects d = design (= factored)
F = actions (loads) k = characteristic (= unfactored)
R = resistance (=capacity) rep = representative
X = material properties
a = dimensions/geometry
![Page 35: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/35.jpg)
36
Fundamental limit state requirement
Ed ≤ Rd
E{ Fd ; Xd ; ad} = Ed ≤ Rd = R{ Fd ; Xd ; ad}
E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}
or E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = Rk/γR = RnφR (LRFD)
or γE Ek = Ed ≤ Rd = Rk/γR
so in total
γE E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}/γR
Concrete and steel: 2 standard deviations from the mean test result.
![Page 36: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/36.jpg)
37
Characteristic values in EC7
![Page 37: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/37.jpg)
38
Characteristic values in EC7 – definition (2.4.5.2)
![Page 38: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/38.jpg)
39
Characteristic values in EC7
2.4.3(4) also mentions:
![Page 39: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/39.jpg)
41
Characteristic values in EC7
![Page 40: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/40.jpg)
42
Characteristic values in EC7 – zone of ground
“Cautious” – worse than most probable.
Small building on estuarine beds near slope
![Page 41: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/41.jpg)
43
Characteristic values in EC7 – zone of ground
![Page 42: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/42.jpg)
44
Characteristic values in EC7 – zone of ground
2.4.5.2 Characteristic values of geotechnical parameters
Thoughtful interpretation – not simple averaging
7.6.2.2
![Page 43: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/43.jpg)
45
Characteristic values in EC7 – definition (2.4.5.2)
![Page 44: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/44.jpg)
46
0.5 SD below the mean?
A suggestion:
When:
• a limit state depends on the value of a parameter averaged over a large
amount of ground (ie a mean value), and
• the ground property varies in a homogeneous, random manner, and
• at least 10 test values are available
Then: A value 0.5SD below the mean of the test results provides a useful
indication of the characteristic value
(Contribution to Discussion Session 2.3, XIV ICSMFE, Hamburg. Balkema.,
Schneider H R (1997) Definition and determination of characteristic soil properties.
Discussion to ISSMFE Conference, Hamburg.)
![Page 45: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/45.jpg)
47
0.5 SD below the mean?
- a useful consideration, not a rule
C:\bx\EC7\[EC7.xls] 26-May-03 10:10
0
0.2
0.4
0.6
0.8
1
1.2
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
5% fractile of
test resuts
5% fractile of
mean values
Results of soil tests
MeanSD from mean
More remote when dependent on
specific small zone.
![Page 46: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/46.jpg)
48
A USA proposal – 25% fractile
C:\BX\BX-C\EC7\[EC7a.xls] 14-May-09 11:20
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
-3 -2 -1 0 1 2 3
TEST RESULTS (SD from mean)
PR
OB
AB
ILIT
Y D
EN
SIT
Y
5% fractile
of test
results
5% fractile of
mean values
75% exceedence
for tests
Results of soil tests
![Page 47: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/47.jpg)
49
Characteristic values in EC7
• NOT a fractile of the results of particular, specified laboratory tests on
specimens of material.
• A cautious estimate of the value affecting the occurrence of the limit state
• Take account of time effects, brittleness, soil fabric and structure, the effects
of construction processes and the extent of the body of ground involved in a
limit state
• The designer’s expertise and understanding of the ground are all encapsulated
in the characteristic value
• Consider both project-specific information and a wider body of geotechnical
knowledge and experience.
• Characteristic = moderately conservative = representative (BS8002) = what
good designers have always done.
![Page 48: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/48.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 49: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/49.jpg)
51
Square footing
Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
cu = 50 kPa
c’=0, ϕ’ = 25°
γ = 20 kN/m3
Gk = 700 kN
γ = 17 kN/m3 1m
Ultimate bearing capacity
Undrained: R/B = (π+2) cusu + q
Drained: R/B = c' Ncsc + q' Nqsq + 0.5 γ' B Nγsγ
Partial factors γCu = 1.4 γϕ=1.25
To satisfy ULS requirements:
Undrained: B = 1.73 m Working bearing pressure = 237 kPa
Drained: B = 2.02 m Working bearing pressure = 174 kPa
SLS:
B?
![Page 50: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/50.jpg)
52
6.6 Serviceability limit state design
![Page 51: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/51.jpg)
53
Square footing
Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
cu = 50 kPa
c’=0, ϕ’ = 25°
γ = 20 kN/m3
Gk = 700 kN
γ = 17 kN/m3 1m
Ultimate bearing capacity
Undrained: R/B = (π+2) cusu + q
Drained: R/B = c' Ncsc + q' Nqsq + 0.5 γ' B Nγsγ
Partial factors γCu = 1.4 γϕ=1.25
To satisfy ULS requirements:
Undrained: B = 1.73 m Working bearing pressure = 237 kPa
Drained: B = 2.02 m Working bearing pressure = 174 kPa
SLS:
SLS using cu/3 B = 2.44 m Working bearing pressure = 120 kPa
SLS using cu/2 B = 2.04 m Working bearing pressure = 171 kPa
B?
![Page 52: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/52.jpg)
54
Settlement prediction by Bolton et al
Vardanega, P.J. and Bolton, M.D. (2011) Strength mobilization in clays and silts. Canadian Geotechnical Journal
48(10):1485-1503.
McMahon, B.T., Haigh, S.K., Bolton, M.D. (2014) Bearing capacity and settlement of circular shallow foundations
using a nonlinear constitutive relationship. Canadian Geotechnical Journal 51 (9): 995-1003.
τmax
τ
cu/2 (M=2)
γγ (M=2)
τ/c
u
γ/γM=2
cu
![Page 53: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/53.jpg)
55
Square footing• Design for undrained ULS only –
likely to fail at SLS.
• Design for ULS drained –marginal at SLS
• cu/3 – small settlements
• cu/2 – non-linearLimiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
cu = 50 kPa
c’=0, ϕ’ = 25°
γ = 20 kN/m3
Gk = 700 kN
γ = 17 kN/m3 1m
B?
Settlement limits
![Page 54: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/54.jpg)
57
Square footing
Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)cu = 50 kPa
c’=0, ϕ’ = 25°
γ = 20 kN/m3
Gk = 700 kN
γ = 17 kN/m3 1m
B?
0
50
100
150
200
250
0
10
20
30
40
50
60
1.60 1.80 2.00 2.20 2.40 2.60
Bea
rin
g p
ressure
kP
a
Se
ttle
men
t
m
m
Footing width m
Undrained
Drained
Bolton
BoltonE/Cu=200
E/Cu=300
Bearing pressure
F=3F=2
Settlement limits
![Page 55: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/55.jpg)
58
Square footing • Design for undrained ULS only –likely to fail at SLS.
• Design for ULS drained –marginal at SLS
• cu/3 – small settlements
• cu/2 – non-linear
• Necessary to check both ULS and SLS: SLS may govern
Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
cu = 50 kPa
c’=0, ϕ’ = 25°
γ = 20 kN/m3
Gk = 700 kN
γ = 17 kN/m3 1m
B?
Settlement limits
![Page 56: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/56.jpg)
59
Grand Egyptian Museum
•Governed by SLS
![Page 57: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/57.jpg)
Coventry University Engineering and Computing Building
![Page 58: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/58.jpg)
Coventry University Engineering and Computing Building
Design for Collaborative Learning
Detailed Design
![Page 59: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/59.jpg)
62
•Sand and clays•Governed by long term bearing capacity (ULS)
•Careful consideration of relevant load combinations
![Page 60: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/60.jpg)
63
![Page 61: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/61.jpg)
Greengate Public Realm
- footbridge near Manchester
64
![Page 62: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/62.jpg)
65
Section 7 – Pile foundations
![Page 63: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/63.jpg)
66
SLS also covered by ULS factors
![Page 64: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/64.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 65: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/65.jpg)
Water has a way of seeping between any two theories!
![Page 66: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/66.jpg)
69
2.4.7 Ultimate Limit States
6
9
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
![Page 67: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/67.jpg)
70
2.4.7 Ultimate Limit States
7
0
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
a a
W W
M
F1 F2
b
![Page 68: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/68.jpg)
71
2.4.7 Ultimate Limit States
7
1
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
2.4.7.1 General
(1)P Where relevant, it shall be verified that the following limit states are not exceeded:
— loss of equilibrium of the structure or the ground, considered as a rigid body, in which the
strengths of structural materials and the ground are insignificant in providing resistance
(EQU);
— internal failure or excessive deformation of the structure or structural elements, including
e.g. footings, piles or basement walls, in which the strength of structural materials is
significant in providing resistance (STR);
— failure or excessive deformation of the ground, in which the strength of soil or rock is
significant in providing resistance (GEO);
— loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy)
or other vertical actions (UPL);
— hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
(HYD).
γγγγF = 1.0, 1.35, 1.5 etc
γγγγF = 1.0, 1.35, 1.5 etc
γγγγF,dst = 1.0/1.1, γγγγF,stb = 0.9
γγγγF,dst = 1.35, γγγγF,stb = 0.9
![Page 69: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/69.jpg)
72
“Design” water pressures in EC7
![Page 70: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/70.jpg)
73
2.4.2 – Actions The “single source principle”
(9)P Actions in which ground- and free-water forces predominate shall be
identified for special consideration with regard to deformations, fissuring,
variable permeability and erosion.
NOTE Unfavourable (or destabilising) and favourable (or stabilising)
permanent actions may in some situations be considered as coming from a
single source. If they are considered so, a single partial factor may be applied to
the sum of these actions or to the sum of their effects.
![Page 71: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/71.jpg)
3rd International Symposium on Geotechnical Safety and Risk,
Munich, June 2011
Geotechnical safety in relation to water pressures
B. Simpson
Arup Geotechnics, London, UK
N. Vogt
Technische Universität München,Zentrum Geotechnik, Munich, Germany
A. J. van Seters
Fugro GeoServices, The Netherlands
Simpson, B, Vogt, N & van Seters AJ (2011) Geotechnical safety in relation to water
pressures. Proc 3rd Int Symp on Geotechnical Safety and Risk, Munich, pp 501-517.
![Page 72: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/72.jpg)
Very “simple” problems
75
![Page 73: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/73.jpg)
Slightly more complex problems
76
![Page 74: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/74.jpg)
Explicitly accommodate the worst water pressures that could reasonably occur
1m rise in water level multiplies BM
by about 2.5 – outside the range
allowed by factors on the water
pressure or water force.
77
![Page 75: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/75.jpg)
Use of an offset in water level?
∆h
7
8
![Page 76: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/76.jpg)
HYD – Equation 2.9
79
EC7 {2.4.7.5(1)P} states: “When considering a limit state of failure due to heave
by seepage of water in the ground (HYD, see 10.3), it shall be verified, for every
relevant soil column, that the design value of the destabilising total pore water
pressure (udst;d ) at the bottom of the column, or the design value of the seepage
force (Sdst;d) in the column is less than or equal to the stabilising total vertical
stress (σstb;d) at the bottom of the column, or the submerged weight (G´stb;d) of
the same column:
udst;d ≤ σstb;d (2.9a) – total stress (at the bottom of the column)
Sdst;d ≤ G´stb;d (2.9b)” – effective weight (within the column)
σu
G’ S
z
![Page 77: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/77.jpg)
HYD – Equation 2.9
81
EC7 {2.4.7.5(1)P} states: “When considering a limit state of failure due to heave
by seepage of water in the ground (HYD, see 10.3), it shall be verified, for every
relevant soil column, that the design value of the destabilising total pore water
pressure (udst;d ) at the bottom of the column, or the design value of the seepage
force (Sdst;d) in the column is less than or equal to the stabilising total vertical
stress (σstb;d) at the bottom of the column, or the submerged weight (G´stb;d) of
the same column:
udst;d ≤ σstb;d (2.9a) – total stress (at the bottom of the column)
Sdst;d ≤ G´stb;d (2.9b)” – effective weight (within the column)
σu
G’ S
Annex A of EC7 provides values for partial factors to be used for HYD, γG;dst = 1.35
and γG;stb = 0.9. But the code does not state what quantities are to be factored.
Maybe:γG;dst udst;k ≤ γG;stb σstb;k (2.9a)
and
γG;dst Sdst;k ≤ γG;stb G´stb;k (2.9b)
In this format, the factors are applied to different quantities in 2.9 a and b.
z
1.35/0.9 = 1.5
![Page 78: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/78.jpg)
HYD – Equation 2.9
82
Orr, TLL (2005) Model
Solutions for Eurocode 7
Workshop Examples.
Trinity College, Dublin.
H=?
1m
3m
7m
Uniform permeability
![Page 79: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/79.jpg)
Apply Apply Apply Apply γG;dst = 1.35 to: Apply Apply Apply Apply γγγγG;stbG;stbG;stbG;stb = 0.9 to: HHHH
Pore water pressure udst;k Total stress σstb;k 2.78
Seepage force Sdst;k Buoyant weight G´stb;k 6.84
Excess pore pressure udst;k - γwz Buoyant density 6.84
Excess head (udst;k - γwz) /γw Buoyant density 6.84
Excess pore pressure or excess head Total density 6.1
HYD – Equation 2.9
83
udst;d ≤ σstb;d (2.9a) – total stress (at the bottom of the column)
Sdst;d ≤ G´stb;d (2.9b)” – effective weight (within the column)
Orr, T.L.L. 2005.
Model Solutions for Eurocode 7
Workshop Examples.
Trinity College, Dublin.
γG;dst udst;k ≤ γG;stb σstb;k (2.9a)
γG;dst Sdst;k ≤ γG;stb G´stb;k (2.9b)
![Page 80: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/80.jpg)
HYD – Equation 2.9
84
udst;d ≤ σstb;d (2.9a) – total stress (at the bottom of the column)
Sdst;d ≤ G´stb;d (2.9b)” – effective weight (within the column)
Apply Apply Apply Apply γG;dst = 1.35 to: Apply Apply Apply Apply γγγγG;stbG;stbG;stbG;stb = 0.9 to: HHHH
Pore water pressure udst;k Total stress σstb;k 2.78
Seepage force Sdst;k Buoyant weight G´stb;k 6.84
Excess pore pressure udst;k - γwz Buoyant density 6.84
Excess head (udst;k - γwz) /γw Buoyant density 6.84
Excess pore pressure or excess head Total density 6.1
![Page 81: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/81.jpg)
Safety Against Hydraulic Heave (HYD in EC7)
Conclusions
Not good to factor total water pressures
- Factoring differential or excess water pressure may be OK. (ie excess over hydrostatic)
85
![Page 82: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/82.jpg)
Terzhagi’s rectangular block
tG'
S
b=t/2G' = buoyant weight
S = seepage force due to excess water pressure
Dimensions t x t/2
FT = G'/S
Das (1983) Fig 2.47
86
![Page 83: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/83.jpg)
Factors of safety for HYD
8
7
Das (1983) Fig 2.47
f
![Page 84: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/84.jpg)
Essential to assess correct water pressures (permeabilities)
-12
-10
-8
-6
-4
-2
0
2
4
6
1.00E-06 1.00E-05 1.00E-04 1.00E-03
Lev
el
(m
)
Permeability m/s
1.00E-06
∆∆∆∆h/t = 2FT ≈ 1.5
FT = 1.17
89
Permeability m/s
m…then FT seems to be irrelevant
![Page 85: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/85.jpg)
Why b=t/2? A narrower block would be more critical.
tG'
S
b=t/2G' = buoyant weight
S = seepage force due to excess water pressure
Dimensions t x t/2
FT = G'/S
Das (1983) Fig 2.47
Include friction on the side of the block?
90
![Page 86: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/86.jpg)
∆h = 6m
t = 3m
6m
b
t
∆∆∆∆h/t = 2
FT ≈ 1.5
Equipotentials for uniform permeability – FT = 1.5
Simpson, B & Katsigiannis, G (2015) Safety considerations for the HYD limit state.
Submitted for ECSMGE, Edinburgh.
![Page 87: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/87.jpg)
Effect of friction on the Terzaghi block
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
∆∆∆∆h/t = 2
FT ≈ 1.5 No friction
+ friction
b=t/2
![Page 88: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/88.jpg)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fact
or
of
safe
ty
Column width b (m)
Effect of friction on the Terzaghi block
∆∆∆∆h/t = 2No friction
b=t/2
![Page 89: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/89.jpg)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fact
or
of
safe
ty
Column width b (m)
Effect of friction on the Terzaghi block
∆∆∆∆h/t = 2No friction
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fact
or
of
safe
ty
Column width b (m)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
∆∆∆∆h/t = 2No friction
∆∆∆∆h/t = 3
b=t/2
![Page 90: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/90.jpg)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fact
or
of
safe
ty
Column width b (m)
Effect of friction on the Terzaghi block
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
∆∆∆∆h/t = 2No friction
∆∆∆∆h/t = 3∆∆∆∆h/t = 3.33± friction
b=t/2
![Page 91: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/91.jpg)
Effect of friction on the Terzaghi block
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8
Fa
cto
r o
f sa
fety
Column width b (m)
∆∆∆∆h/t = 2No friction
∆∆∆∆h/t = 3∆∆∆∆h/t = 3.33± friction
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6
Fa
cto
r o
f sa
fety
Column width b (m)
No friction
+ friction
∆∆∆∆h/t = 2
FT ≈ 1.5
![Page 92: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/92.jpg)
97
Conclusions of EG9
• Not good to factor total water pressures
- Factoring differential water pressure may be OK.
• Design for F=… is no use if the pore pressures (permeability distribution) are not properly understood.
• ULS design water pressure derived without factors (1% chance)
- No factors on effects of water pressure – eg seepage force S.
- But could be factors on structural effects of water pressures – eg BM
• Take directly assessed ULS design water pressures (1% chance) with factored strengths of materials. Consider all failure mechanisms. Simple!
• Special case: Terzaghi block – only consider one mechanism so add a factor of safety (1.5?).
![Page 93: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/93.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors• Retaining structures – numerical analysis
• The future
![Page 94: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/94.jpg)
99
Ground anchors –
EC7 Section 8
and the new UKNA
![Page 95: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/95.jpg)
EC7 Section 8 – Anchorages (existing)
![Page 96: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/96.jpg)
EC7 “Evolution Groups”EG0 Management and oversight
EG1 Anchors
EG2 Maintenance and simplification
EG3 Model solutions
EG4 Numerical models
EG5 Reinforced soil
EG6 Seismic design
EG7 Pile design
EG8 Harmonization
EG9 Water pressures
EG10 Calculation models
EG11 Characterization
EG12 Tunnelling
EG13 Rock mechanics
EG14 Ground improvement
![Page 97: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/97.jpg)
Three European documents
• EN 1537
Execution of special geotechnical work – Ground anchors
• EN ISO 22477-5 Geotechnical investigation and testing — Testing of geotechnical structures — Part 5:
Testing of anchorages
• Eurocode 7 – EN 1997-1 – Section 8 – Anchors + UKNA
• BS 8081 – Ground anchorages (being revised as NCCI)
And the existing British code
![Page 98: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/98.jpg)
103
EN 1997-1:2004/A1:2013 Eurocode 7 (2004) with amendment (2013)
Section 8 - Anchors
8.1 General
8.2 Limit states
8.3 Design situations and actions
8.4 Design and construction considerations
8.5 Limit state design of anchors
8.6 Tests on anchors
8.7 Lock-off load for pre-stressed anchors
8.8 Supervision, monitoring and maintenance
+UKNA
Motherhood and apple pie?
![Page 99: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/99.jpg)
104
Time
Anchor
force
The life story of a ground anchor
![Page 100: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/100.jpg)
105
Fserv – the maximum anchor force, including effect of lock off load, and sufficient to prevent
a serviceability limit state in the supported structure
Time
Anchor
force
Pre-load and testing
EN 22477-5
BS 8081
Working life
Proof
load, PpCheck
behaviour
Lock-off
load, P0
FServ
FServ
FULS
;k
;d
γγγγServ
;k
SLS
ULS: EULS;d = max (FULS;d , Fserv;d)
Sufficient to prevent supported structure exceeding ULS
Sufficient to prevent supported structure exceeding SLS
Characteristic (k): a cautious estimate of what is likely to happen
FULS;d
FULS – the force required to prevent any ultimate limit state in the supported structure
;dFServ
![Page 101: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/101.jpg)
106
![Page 102: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/102.jpg)
107
• Investigation or suitability tests must be used to check EULS;d
• Investigation tests not used much on small contracts. Suitability tests on working anchors.
• Investigation or suitability tests may optionally check behaviour at Fserv;k (NA)
• All grouted anchors must have acceptance tests
• Acceptance tests may check EULS;d and/or Fserv;k (NA)
Take the worst
Small factor γa;ULS
![Page 103: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/103.jpg)
108
Take the worst
Small factor γa;ULS
![Page 104: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/104.jpg)
110
![Page 105: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/105.jpg)
111
CEN value: ξULS = 1.0
UK value: ξULS = 1.35 Fserv;k/EULS;d
< 1.0, if EULS;d > 1.35Fserv;k
›› Fserv;k??
CEN value: γa;ULS = 1.1 = UK value
So RULS;m = 1.1xRULS;d ≥ 1.1EULS;d
So RULS;m = 1.1x (RULS;d ≥ EULS;d)x1.35 Fserv;k/EULS;d ≈ 1.5FServ;k
![Page 106: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/106.jpg)
112
Fserv – the maximum anchor force, including effect of lock off load, and sufficient to prevent
a serviceability limit state in the supported structure
Time
Anchor
force
Pre-load and testing
EN 22477-5
BS 8081
Working life
Proof
load, PpCheck
behaviour
Lock-off
load, P0
FServ
FServ ;k
γγγγServ
;k
SLS
ULS: EULS;d = max (FULS;d , Fserv;d)
Sufficient to prevent supported structure exceeding SLS
Characteristic (k): a cautious estimate of what is likely to happen
FULS;d
FULS – the force required to prevent any ultimate limit state in the supported structure
;dFServ
;dFULS
RULS;m
1.5
![Page 107: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/107.jpg)
113
RULS;m = 1.1x (RULS;d ≥ EULS;d)x1.35 Fserv;k/EULS;d = 1.5FServ;k
RSLS;m = Fserv;k
Advice on design of anchors to achieve these
performance requirements will be provided in
BS 8081 (2015).
![Page 108: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/108.jpg)
114
Summary
• Anchor validation based only on testing – no reliance on calculations.
• No requirement for big overall FOS.
• But contractor will need to be confident that every anchor will pass the acceptance test. Low creep at fairly high loads.
• So he might introduce extra margins to be sure of this.
• EC7 gives the test criteria, but doesn’t advise how to achieve them. BS8081 will do this.
![Page 109: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/109.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values of
soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 110: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/110.jpg)
How retaining walls fail – ULS (Eurocode 7) BP140.11a
Which governs – ULS or SLS? Always SLS?
![Page 111: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/111.jpg)
3:41 pm
![Page 112: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/112.jpg)
3:41 pm
![Page 113: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/113.jpg)
119
9.8 Serviceability limit state design
![Page 114: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/114.jpg)
120
9.8.2 Displacements BP168-4.33
![Page 115: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/115.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values of
soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis• The future
![Page 116: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/116.jpg)
122
9.8.2 Displacements BP168-4.33
Numerical analysis often used for SLS.
Nothing new in EC7.
![Page 117: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/117.jpg)
123
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
• EC7 Evolution Group 4, chaired by Dr Andrew Lees (Cyprus)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Simpson, B and Junaideen, SM (2013) Use of numerical analysis with Eurocode 7.
18th South East Asia Geotechnical Conference, Singapore.
![Page 118: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/118.jpg)
124
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 119: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/119.jpg)
125
Partial factors recommended in EN1997-1 Annex A (+UKNA)
Values of partial factors recommended in EN1997-1 Annex A
Design approach 1 Design approach 2 Design approach 3Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors DA2 - Comb 1 DA2 - Slopes DA3
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4 A1 M1 R2 A1 M=R2 A1 A2 M2 R3
Actions unfav 1,35 1,35 1,35 1,35
fav
unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3
Soil tan φ' 1,25 1,25 StructuralGeotech 1,25
Effective cohesion 1,25 1,25 actions actions 1,25
Undrained strength 1,4 1,4 1,4
Unconfined strength 1,4 1,4 1,4
Weight density
Spread Bearing 1,4
footings Sliding 1,1
Driven Base 1,3 1,1
piles Shaft (compression) 1,3 1,1
Total/combined
(compression)
1,3 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Bored Base 1,25 1,6 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,15 1,5 1,1
Shaft in tension 1,25 1,6 1,15 1,1
CFA Base 1,1 1,45 1,1
piles Shaft (compression) 1,0 1,3 1,1
Total/combined
(compression)
1,1 1,4 1,1
Shaft in tension 1,25 1,6 1,15 1,1
Anchors Temporary 1,1 1,1 1,1
Permanent 1,1 1,1 1,1
Retaining Bearing capacity 1,4
walls Sliding resistance 1,1
Earth resistance 1,4
Slopes Earth resistance 1,1
indicates partial factor = 1.0 C:\BX\BX-C\EC7\[Factors.xls] 25-Nov-06 17:26
Permanent
Variable
(+ UKNA)
• Easy to factor primary input –
material strengths and actions
• Difficult to factor geotechnical
resistances and action effects
DA2
unsuitable for
numerical
analysis
![Page 120: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/120.jpg)
126
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 121: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/121.jpg)
127
Fundamental limit state requirement
Ed ≤ Rd
E{ Fd ; Xd ; ad} = Ed ≤ Rd = R{ Fd ; Xd ; ad}
E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}
or E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = Rk/γR = RnφR (LRFD)
or γE Ek = Ed ≤ Rd = Rk/γR
so in total
γE E{γF Frep; Xk/γM; ad} = Ed ≤ Rd = R{γF Frep; Xk/γM; ad}/γR
• (a) Reduce strength, increase the loads, and check equilibrium
OR
• (b) Find the remaining FOS?
OR
• (b) “c-φ reduction”
![Page 122: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/122.jpg)
129
Pre-factored strength, or c-φφφφ reduction?Max wall displacement 48mm
Max wall displacement 48mmMax wall displacement 48mm
γφ = 1.25
0
10m
500 1000 1500
BENDING MOMENT
kNm/m
γϕ = 1.25γϕ = 1.45
MR;d
xbcap5-Dec11ab.sfd
Large displacement
γφ = 1.45
![Page 123: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/123.jpg)
130
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 124: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/124.jpg)
131
Wrong failure mechanism?
• There is no “right” failure mechanism
• Because failure isn’t the “right” answer!
• EC7 is interested in proving success, not failure.
• Finding FOS useful for design refinement, but not for final verification.
• Plastic models of structural elements useful in ULS analysis.
Max wall displacement 48mmMax wall displacement 48mmMax wall displacement 48mm
γφ = 1.25
xbcap5-Dec11ab.sfd
Large displacement
γφ = 1.45
![Page 125: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/125.jpg)
132
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 126: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/126.jpg)
133
Factoring advanced models – φ′, c′, cu not explicit parameters
• eg Cam Clay, BRICK, Lade etc
• Change to Mohr-Coulomb for the factored calculation?
• If γc′=γφ′ this is the code factor on drained strength, however derived.
• Consider: is the model’s drained strength more or less reliable than those used in conventional practice?
- eg the model might take good account of combinations of principal stresses, direction (anisotropy), stress level etc.
- Possibly modify factors in the light of this.
![Page 127: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/127.jpg)
134
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 128: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/128.jpg)
135
Undrained strength in effective stress models
Reliable computation of undrained strength from effective stress parameters is very difficult.
EC7 generally requires a higher factor on undrained strength (eg 1.4 on cu) than on effective stress parameters (eg 1.25 on c′, tanφ′).
![Page 129: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/129.jpg)
136
cu/1.4 doubles bending moment when sensitive
![Page 130: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/130.jpg)
137
Undrained strength in effective stress models
• Reliable computation of undrained strength from effective stress parameters is very difficult.
• EC7 generally requires a higher factor on undrained strength (eg1.4 on cu) than on effective stress parameters (eg 1.25 on c′, tanφ′).
• The drafters assumed that effective stress parameters would be used only for drained states.
• The higher factor (eg 1.4) was considered appropriate for characteristic values of cu based on measurement, which is generally more reliable than values computed from effective stress parameters.
• So it is unreasonable to adopt a lower value for the latter.
![Page 131: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/131.jpg)
138
Time-dependent analysis
• Beyond EC7!
• Geotechnical category 3
![Page 132: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/132.jpg)
139
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 133: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/133.jpg)
140
Ko
• In reality, K0 is not a simple function of soil strength (φ').
• So it is not sensible, and not a Eurocode requirement, to factor K0 or vary it as a function of φ'. In situ stresses are taken as a separate parameter – an action.
![Page 134: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/134.jpg)
141
Soil stiffness
• CIRIA Report C580 recommends that stiffness should be reduced (halved) for ULS analysis. No other publication has a similar requirement.
• The reason for this was that larger strains may be mobilised in ULS analyses – it was not an additional safety margin.
• This reasoning may apply to Strategy 1, but not so clearly to Strategy 2 since, in many cases, most of the displacement has already taken place when the strength is reduced. If the soil is close to failure, stiffness will not be important.
• So reduction of stiffness for ULS analysis is not recommended.
ττττ
γγγγ
![Page 135: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/135.jpg)
142
• Use of numerical methods for ULS
− Can numerical methods be used for all design approaches?
− How should strength factors be applied?
− Does FEM give the wrong failure mechanism?
− Use of advanced soil models for ULS
− Undrained behaviour and consolidation
− K0 and soil stiffness
− Staged construction
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
![Page 136: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/136.jpg)
143
ULS for staged construction – single propped example
Factor material strengths
Initial state
Excavate to 5m –
wall cantilevering
Could be critical for wall
bending moment
Install prop at
4m depth
Excavate to 10m
Could be critical for wall length, bending moment
and prop force
Compute using
characteristic parameters
Compute using
factored parameters
Excavate to 5m –wall cantilevering
Install prop at 4m depth
Excavate to 10m
No further factors on strut forces or BMs
Compute using factored strength
Apply factors on strut forces or BMs
Compute using unfactored parameters
No further factors on strut forces or BMs
Initial state?
Strategy 1 Strategy 2
![Page 137: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/137.jpg)
151
Florence Rail Station- 25m deep, 50m wide,
550m long
- Mezzanine level prop
- High groundwater level
Simpson, B and Hocombe, T
(2010) Implications of modern
design codes for earth retaining
structures. Proc ER2010,
ASCE Earth Retention
Conference 3, Seattle, Aug
2010.
![Page 138: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/138.jpg)
152
Eurocode case study: High speed rail station, Florence, Italy
• 454m long, 52m wide and 27 to 32m deep
• 1.2 to 1.6m thick diaphragm walls
• Three levels of temporary strutting.
![Page 139: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/139.jpg)
153
Eurocode case study: High speed rail station, Florence, Italy
• SLS analyzed as if London Clay using the BRICK model.
• Time dependent swelling and consolidation.
• Eurocode 7, DA1, Combinations 1 and 2 analysed using FE and Oasys FREW.
![Page 140: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/140.jpg)
154
Eurocode case study: High speed rail station, Florence, Italy
• Eurocode 7 readily used with FE for this large project.
• Geotechnical and structural design readily coordinated.
![Page 141: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/141.jpg)
155
Partial factors for DA1 - UKNADesign approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors
A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4
Actions unfav 1,35
fav
unfav 1,5 1,3 1,3
Soil tan φ' 1,25 1,25
Effective cohesion 1,25 1,25
Undrained strength 1,4 1,4
Unconfined strength 1,4 1,4
Weight density
Spread Bearing EC7
footings Sliding values
Driven Base 1,7/1.5 1,3
piles Shaft (compression) 1.5/1.3 1,3
Total/combined
(compression)
1.7/1.5 1,3
Shaft in tension 2.0/1.7 1.6
Bored Base 2.0/1.7 1,6
piles Shaft (compression) 1.6/1.4 1,3
Total/combined
(compression)
2.0/1.7 1.5
Shaft in tension 2.0/1.7 1.6
CFA Base As 1.45
piles Shaft (compression) for 1.3
Total/combined
(compression)
bored 1.4
Shaft in tension piles 1.6
Anchors Temporary 1,1 1,1
Permanent 1,1 1,1
Retaining Bearing capacity
walls Sliding resistance
Earth resistance
Slopes Earth resistance
indicates partial factor = 1.0
C:\BX\BX-C\EC7\[Factors.xls]
Permanent
Variable
1,7/1.5
1.5/1.3
1.7/1.5
2.0/1.7
2.0/1.7
1.6/1.4
2.0/1.7
2.0/1.7
As
for
bored
piles
1,1
1,1
1.87/1.65
1.65/1.43
1.87/1.65
2.20/1.87
2.20/1.87
1.76/1.54
2.20/1.87
2.20/1.87
UKNA MSNA
![Page 142: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/142.jpg)
156
ULS for staged construction – single propped example
Factor material strengths
Initial state
Excavate to 5m –
wall cantilevering
Could be critical for wall
bending moment
Install prop at
4m depth
Excavate to 10m
Could be critical for wall length, bending moment
and prop force
Compute using
characteristic parameters
Compute using
factored parameters
Excavate to 5m –wall cantilevering
Install prop at 4m depth
Excavate to 10m
No further factors on strut forces or BMs
Compute using factored strength
Apply factors on strut forces or BMs
Compute using unfactored parameters
No further factors on strut forces or BMs
Initial state?
Strategy 1 Strategy 2
![Page 143: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/143.jpg)
157
Florence Station – comparison of bending moments
5
10
15
20
25
30
35
40
45
-6,000 -4,000 -2,000 0 2,000 4,000 6,000 8,000
Le
ve
l (m
)
Bending Moment (kNm/m)
C1 C1
C2, factors all stages C2, factors all stages
Prop and excavation levels
Stage 1 2 3
5
10
15
20
25
30
35
40
45
-6,000 -4,000 -2,000 0 2,000 4,000 6,000 8,000
Le
ve
l (m
)
Bending Moment (kNm/m)
C1 C1
C2, factors all stages C2, factors all stages
C2,factoring stages separately C2, factoring stages separately
Prop and excavation levels
Stage 1 2 3
(Strategy 1)
(Strategy 2)
C2, Strategy 2
C2, Strategy 1
![Page 144: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/144.jpg)
158
Summary – numerical analysis
• FEM analysis of SLS is conventional – nothing new.
• FEM can also be used for ULS
• Design Approach 1 is well suited to this.
• Difficult to distinguish favourable and unfavourable actions from the ground – the “star” approach for these.
• The code requirement is best checked by applying fixed factors to strength – method (a).
• “c- φ reduction” might be useful for design refinement – method (b).
• Plastic modelling of the structure would be beneficial.
• When advanced soil models are used, it may be best to switch to Mohr-Coulomb for the ULS check.
![Page 145: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/145.jpg)
159
Summary – numerical analysis
• Great care is needed in modelling undrained situations using effective stress parameters – requires a good advanced model.
• The full value of γCu should be applied for undrained materials.
• Factoring of K0 and stiffness is not recommended.
• “Strategy 2” – applying factors to stages individually – is recommended.
- Analyse DA1-1 first, then check critical stages for DA1-2.
- Computing effort might be reduced if stages for which DA1-2 is critical can be established for a given range of situations.
• EC7 Evolution Group
![Page 146: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/146.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
![Page 147: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/147.jpg)
161
The future
• Evolution groups => extensive revisions of most sections
• About to start re-drafting for 2020(?)
• Reorganised into three parts: General, Testing, Specific elements
• Harmonisation – simplifying the Design Approaches
• Consequence classes – variations to partial factors (1.25 → 1.2?)
• Additional sections
- Reinforced ground
- Ground improvement
- Rock mechanics
• Numerical analysis – section or sub-section
![Page 148: Eurocode 7 – Good practice in geotechnical design](https://reader038.fdocuments.us/reader038/viewer/2022102518/584b871e1a28ab85738d298b/html5/thumbnails/148.jpg)
Eurocode 7 – Good practice in geotechnical design
8th Lumb Lecture
• Limit state design
• Holistic design – structures and ground
• Practical approach to characteristic values
of soil parameters
• ULS and SLS design requirements
• Water pressures
• Ground anchors
• Retaining structures – numerical analysis
• The future
Thanks for
listening