simpson (2014) design for problems dominated by water pressures - latest eurocode developments

8/19/2019 Simpson (2014) Design for Problems Dominated by Water Pressures - Latest Eurocode Developments

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Design for problems dominated

by water pressures – latestEurocode developments

Brian Simpson

Arup Fellow

EC7 workshop, 18 March 2014



2

Piezometer lost and not replaced

Water has a way of seeping between any two theories!

Some examples of failures

EC7 requirements

Recent thinking on safetyHYD – Equation 2.9

EC7 – Evolution Group 9



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Piezometer lost and not replaced

Water has a way of seeping between any two theories!


EC7 requirements





4

Levees and river banks



Dubai Dry Dock BP184.14



(b)Silty fine sand

Sandy gravel

Very silty layer Toe drain (sandy gravel)

(a)Riprap Sandy gravelSand and gravel

Sand and gravel

SandSheetpile



(b)Silty fine sand

Sandy gravel

Very silty layer Toe drain (sandy gravel)

(a)Riprap Sandy gravelSand and gravel

Sand and gravel

SandSheetpile

Permeability distribution



Dubai Dry Dock BP184.16

Factors of safety ?!?



H1 H2

W=Vg

U1

U2

F = (U1+W)/U2

or

F = W/(U2-U1)

H1=4m, H2=5m

g=2gw

F = 1.2

F = 2.0

H1=19m, H2=20m

g=2gw

F = 1.05

F = 2.0

= (H1gw+W)/H2gw

= W/(H2-H1)gw



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Piezometer lost and not replacedWater has a way of seeping between any two theories!


EC7 requirements





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 issignificant 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 issignificant 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



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18



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(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.



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EC7 requirements





Geotechnical safety in relation to water pressures

B. Simpson

Arup Geotechnics, London, UK

N. VogtTechnische Universität München, Zentrum Geotechnik, Munich, Germany

A. J. van Seters

Fugro GeoServices, The Netherlands

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22



24



25



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27

1m rise in water level multiplies BM

by about 2.5 –

outside the rangeallowed by factors on the water

pressure or water force.



28

1. gG;stb & gG,dst 2. Buoyant weight

3. Factor water

density

4. Water unfactored

Ustb

Udst



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• Should generally be avoided

• Use unfactored water pressures and forces?

• Don’t factor density but factor pressures (AvS)?• Don’t factor pressures but factor forces (NV)?

• At some point, equilibrium is not preserved. Butthe question is – where?

• The problems are less for DA1, but not removed.



Dh

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2 0 4

0 42



Number of piles required (normalised). (a) unfactored,

(b) pile resistance factored, (c) gG = 1.35 on water pressure,(d) water table adjusted, (e) UPL,

0

1

2

0 0.2 0.4 0.6 0.8 1

n . R

k / W w

h/D

(a)

(b)

(c)

(d)

(e)

(f)

0

0.2

0.4

0.1 0.2 0.3 0.4

n . R

k / W w

h/D

(a)

(b)

(c)

(d)(e)

(f)

h/D

(a)

(b)

(c) (d)(e)

(f)

h/D0.2 0.3 0.4

0

0.1

0.2

0.4

0

1

2

n · R k

/ W w

n · R k

/ W w

0 0.2 0.4 0.6 0.8 1

32



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The possibility of a reduced factor on favourable weight, perhaps

between 0.8 and 0.9 should be considered.

2 0 4

0 42



Number of piles required (normalised). (a) unfactored,

(b) pile resistance factored, (c) gG = 1.35 on water pressure,(d) water table adjusted, (e) UPL, (f) gG;fav = 0.8 on weight.

0

1

0 0.2 0.4 0.6 0.8 1

n . R

k / W w

h/D

(a)

(b)

(c)

(d)

(e)

(f)

0

0.2

0.4

0.1 0.2 0.3 0.4

n . R

k / W w

h/D

(a)

(b)

(c)

(d)(e)

(f)

h/D

(a)

(b)

(c) (d)(e)

(f)

h/D0.2 0.3 0.4

0

0.1

0.2

0.4

0

1

2

n · R k

/ W w

n · R k

/ W w

0 0.2 0.4 0.6 0.8 1

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EC7 2.4.7.3.2(2)

In some design situations, the application of partial factors to actions coming

from or through the soil (such as earth or water pressures) could lead to designvalues which are unreasonable or even physically impossible. In these

situations, the factors may be applied directly to the effects of actions derived

from representative values of the actions.

Apply only to structural

bending moments etc, or

more generally?



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• Need robustness

• Accommodate the worst water pressures

• Apply the “single source” principle

•

Don’t factor water density (?) • Use of an offset in water level (?)

• Reduced factor on favourable weight (?)

• The “star” approach (?)

•

A “middle 2/3rds” rule (?) • Use engineering expertise

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Values of partial factors recommended in EN1997-1 Annex A

Design approach 1 Design approach 2 Design approach 3

Combination 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 f' 1,25 1,25 Structur Geotech 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 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 1,15 1,5 1,1


CFA Base 1,1 1,45 1,1

piles Shaft (compression) 1,0 1,3 1,1

Total/combined 1,1 1,4 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

Combination 1

Use worst likely waterpressures.

Apply factors, either towater pressures (ugh!)or to structural actioneffects such asbending moments andprop forces (DA1-1*)

Combination 2Use the worst crediblewater pressures,unfactored.



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EC7 requirements





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http://localhost/var/www/apps/conversion/tmp/scratch_7/Hydraulischer_Grundbruch_Aulbach_RWTH_Aachen-BX2.mp4



6m

3m

6m

b

t



P bili fil



C:\BX\SAFE\Eq2_9-SAFE\SafeRe28-Oct-13 16:50

-12

-10

-8

-6

-4

-2

0

2

4

6

8

1.00E-06 1.00E-05 1.00E-04 1.00E-03

L e v e l ( m )

Permeability m/s

Permeability profiles

Runs 1, 2, 5

Runs 3, 8

Run 4

Run 9

Run 10

Run 12

R 13 5:1 aniso

*

*

**

*

* Runs either failed to converge, with soil “boiling”, or converged with substantial areas of zero

effective stress. Run 13 had constant horizontal permeability of 1E-5m/s (as Run 5), but with

vertical permeability reduced by a factor of 5.

4

3,8

9

10

12

1 ,

2 ,

5 ,

1 3

*

These

runs

unstable

despite

“F=1.5”



Safety Against Hydraulic Heave (HYD in EC7)

Conclusions

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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 properlyunderstood.



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EC7 requirements



EC7 – Evolution Group 9 – Water pressures



• Prof Tim Lansivaara - Finland

• Julien Habert - France

• Bernhard Odenwald - Germany

• Prof Antonio Gens - Spain

• Henrik Möller - Sweden

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B. Simpson

Arup Geotechnics, London, UK

N. Vogt

Technische Universität München,

Zentrum Geotechnik, Munich, Germany

A. J. van Seters

Fugro GeoServices, The Netherlands

EC7 Evolution Group 9 Water pressuresDeveloping ideas for a revision to be published 2020-ish

ater press res



water pressures

characteristic and design values (1)

agreement: no partial factors on

- unit weight of water or

- water pressures

agreement on characteristic water pressure:

The characteristic piezometric water levels and accordingly the

characteristic values of water pressures shall correspond to a

recurrence period at least equal to the durationof the design situation of the structure. (e.g. 100 years)This has to be done by considering hydrological, hydrogeological and

environemental information together with statistical analysis, if suitable data is available.

agreement on accidential situations (technical, not natural)

water pressures



water pressures


agreement on design value of water pressure:

The ultimate limit state design value of ground water pressures shall represent a

specified rare probability in the duration of the design situation (e.g. 10000 years)

It may be derived either

- by direct assessment.

- by adding a margin to the characteristic piezometric water level

NOTE: The value of the specified probability may be set by the national annex.

NOTE The recommended value of the probability is 1 %.

water pressures



water pressures


first agreement on handling hydraulic failure and uplift:

No factors on effects of water pressure should be used

for HYD- and UPL- (and EQU-) Verifications

– maybe we need changed partial factors on stabilising forces to count for robustness

- phrasing is still missing for final agreement.

- It is not a factor of 1.00

- no factor shall be used, but directly effects due to design water table

water pressures



water pressures


first agreement on effects of actions on constructions:

Ultimate limit state design values

for STR- (and GEO-?) limit statesmay also be based on application

of partial factors to structural effects

due to characteristic water pressures.

In this case it should be checked if an extreme raised water table could directly

affect the construction.

- phrasing is still missing for final agreement.



• Need robustness



•

Don’t factor water density (?) • Use of an offset in water level (?)

• Reduced factor on favourable weight (?)

• The “star” approach (?)

•

A “middle 2/3rds” rule (?) • Use engineering expertise

52



• Need robustness



•

on’t factor water density • Use of an offset in water level

• Reduced factor on favourable weight

• The “star” approach

•

A “middle 2/3rds” rule • Use engineering expertise

53



• Need robustness



•

on’t factor water density • Use of an offset in water level



•


54



• Need robustness



• on’t factor water density

• Use of an offset in water level



•


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• Need robustness







•


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• Need robustness







•


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• Need robustness







•

A “middle 2/3rds” rule • Use engineering expertise – THINK

simpson (2014) design for problems dominated by water pressures - latest eurocode developments

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