Eurocode 7. Brian Simpson, Arup Geotechnics. Nordic Geotechnical Meeting Copenhagen, May 2012 BP198.1

Transcription

1 BP201.1 Nordic Geotechnical Meeting Copenhagen, May 2012 BP198.1 Eurocode 7 fundamental issues and some implications for users Brian Simpson, Arup Geotechnics 1

2 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 2

3 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 3

4 EN1990 Ultimate limit states BP87.48 BP BP111.8 BP BP124-T1.41 BP BP188.6 BP190a.20 BP198.7EN Ultimate limit states Serious failures involving risk of injury or major cost. Must be rendered very unlikely. An unrealistic possibility. 4

5 EN1990 Serviceability limit states BP87.49 BP BP111.9 BP BP124-T1.42 BP EN Serviceability limit states Inconveniences, disappointments and more manageable costs. Should be rare, but it might be uneconomic to eliminate them completely. 5

6 What sort of limit state is this? BP190a

7 2.4.7 Ultimate Limit States BP124-T 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). 7

8 2.4.7 Ultimate limit states EQU, UPL, HYD 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); a a internal failure or excessive deformation of the structure or structural elements, including W W e.g. footings, piles or basement walls, in which the strength of structural materials is M significant in providing resistance (STR); F1 F2 failure or excessive deformation of the ground, in which the strength of soil or rock is significant b 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). 8

9 Ultimate limit states STR, GEO

10 Fundamental limit state requirement E d R d E{ F ; d X ; a d d } = E d R d = R{ F ; d X ; a d d } E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d } Friction! E = action effects F = actions (loads) R = resistance (=capacity) X = material properties a = dimensions/geometry d = design (= factored) k = characteristic (= unfactored) rep = representative 10

11 11 EN Design values of actions

12 12 EN Design values of material or product properties

13 13 EN Design values of material or product properties

14 Fundamental limit state requirement E d R d E{ F ; d X ; a d d } = E d R d = R{ F ; d X ; a d d } E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d } or E{ F F rep ; X k / M ; a d } = E d R d = R k / R = R n R (LRFD) or E E k = E d R d = R k / R so in total E E{ F F rep ; X k / M ; a d } = E d E = action effects F = actions (loads) R = resistance (=capacity) X = material properties a = dimensions/geometry R d = R{ F F rep ; X k / M ; a d }/ R d = design (= factored) k = characteristic (= unfactored) rep = representative 14

15 EC Ultimate Limit States Verification of resistance for structural and ground limit states in persistent and transient situations General (1)P When considering a limit state of rupture or excessive deformation of a structural element or section of the ground (STR and GEO), it shall be verified that: E d R d (2.5) Design effects of actions (1) Partial factors on actions may be applied either to the actions themselves (F rep ) or to their effects (E): E d = E{ F F rep ; X k / M ; a d } or E d = E E{F rep ; X k / M ; a d }. (2.6a) (2.6b) 15

16 Design resistances Design resistances (1) Partial factors may be applied either to ground properties (X) or resistances (R) or to both, as follows: R d = R{ F F rep ; X k / M ; a d } or R d = R{ F F rep ; X k ; a d }/ R or R d = R{ F F rep ; X k / M ; a d }/ R (2.7a) (2.7b) (2.7c) Three different Design Approaches E E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d }/ R 16

17 Partial factors recommended in EN Annex A BP BP124-T2.14 BP124-A2.12 BP BP Values of partial factors recommended in EN Annex A Design approach 1 Design approach 2 Design approach 3 Combination Combination 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 Permanent unfav 1,35 1,35 1,35 1,35 fav Variable unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3 Soil tan ' 1,25 1,25 StructuraGeotech 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 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 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 17

18 BP BP124-T2.14 BP124-A2.12 BP BP Partial factors recommended in EN Annex A Values of partial factors recommended in EN Annex A Design approach 1 Design approach 2 Design approach 3 Combination Combination 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 Permanent unfav 1,35 1,35 1,35 1,35 fav Variable unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3 Soil tan ' 1,25 1,25 StructuraGeotech 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 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 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 18

19 19 Source: Andrew Bond, chair of SC7

20 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 EG8 Harmonisation Andrew Bond, UK Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 20

21 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 21

22 Characteristic values in EC Characteristic values of geotechnical parameters (1)P The selection of characteristic values for geotechnical parameters shall be based on derived values resulting from laboratory and field tests, complemented by well-established experience. (2)P The characteristic value of a geotechnical parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state. (3)P The greater variance of c' compared to that of tan ' shall be considered when their characteristic values are determined. (4)P The selection of characteristic values for geotechnical parameters shall take account of the following: -- geological and other background information, such as data from previous projects; -- the variability of the measured property values and other relevant information, e.g. from existing knowledge; -- the extent of the field and laboratory investigation; -- the type and number of samples; -- the extent of the zone of ground governing the behaviour of the geotechnical structure at the limit state being considered; -- the ability of the geotechnical structure to transfer loads from weak to strong zones in the ground. (5) Characteristic values can be lower values, which are less than the most probable values, or upper values, which are greater. (6)P For each calculation, the most unfavourable combination of lower and upper values of independent parameters shall be used. (7) The zone of ground governing the behaviour of a geotechnical structure at a limit state is usually much larger than a test sample or the zone of ground affected in an in situ test. Consequently the value of the governing parameter is often the mean of a range of values covering a large surface or volume of the ground. The characteristic value should be a cautious estimate of this mean value. (8) If the behaviour of the geotechnical structure at the limit state considered is governed by the lowest or highest value of the ground property, the characteristic value should be a cautious estimate of the lowest or highest value occurring in the zone governing the behaviour. (9) When selecting the zone of ground governing the behaviour of a geotechnical structure at a limit state, it should be considered that this limit state may depend on the behaviour of the supported structure. For instance, when considering a bearing resistance ultimate limit state for a building resting on several footings, the governing parameter should be the mean strength over each individual zone of ground under a footing, if the building is unable to resist a local failure. If, however, the building is stiff and strong enough, the governing parameter should be the mean of these mean values over the entire zone or part of the zone of ground under the building. (10) If statistical methods are employed in the selection of characteristic values for ground properties, such methods should differentiate between local and regional sampling and should allow the use of a priori knowledge of comparable ground properties. (11) If statistical methods are used, the characteristic value should be derived such that the calculated probability of a worse value governing the occurrence of the limit state under consideration is not greater than 5%. NOTE In this respect, a cautious estimate of the mean value is a selection of the mean value of the limited set of geotechnical parameter values, with a confidence level of 95%; where local failure is concerned, a cautious estimate of the low value is a 5% fractile. (12)P When using standard tables of characteristic values related to soil investigation parameters, the characteristic value shall be selected as a very cautious value. 22

23 23 Characteristic values in EC7 definition ( )

24 Characteristic values in EC (4) also mentions: 24

25 25 EN Design values of material or product properties

26 Characteristic values in EC7 zone of ground Cautious worse than most probable. Small building on estuarine beds near slope 26

27 Characteristic values in EC7 zone of ground CPT results in variable deposit 27

28 Characteristic values in EC7 zone of ground Characteristic values of geotechnical parameters Thoughtful interpretation not simple averaging 28

29 29 Characteristic values in EC7 definition ( )

30 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. 30

31 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.) 31

32 0.5 SD below the mean? - a useful consideration, not a rule % fractile of mean values More remote when dependent on specific small zone % fractile of test resuts SD from mean C:\bx\EC7\[EC7.xls] 26-May-03 10:10 Results of soil tests Mean 32

33 A USA proposal 25% fractile 0.5 PROBABILITY DENSITY % fractile of test results 5% fractile of mean values 75% exceedence for tests Results of soil tests TEST RESULTS (SD from mean) C:\BX\BX-C\EC7\[EC7a.xls] 14-May-09 11:20 33

34 peak, critical state or residual? Residual

35 Angle of shearing resistance c?

36 peak, critical state or residual? The value that will prevent the exceedance of the limit state. Caution about progressive loss of density or strength. So caution about >38º. Residual Suggestion: The ULS design value of the shear strength should never be greater than a cautious (ie characteristic ) estimate of the critical state strength of the material. Interfaces between structure and ground (wall friction, sliding etc) use critical state value as characteristic value, and apply normal factors. Useful guidance in BS8002 ( withdrawn )

37 peak, critical state or residual? Residual

38 peak, critical state or residual? The value that will prevent the exceedance of the limit state. Caution about progressive loss of density or strength. So caution about >38º. Suggestion: The ULS design value of the shear strength should never be greater than a cautious (ie characteristic ) estimate of the critical state strength of the material. Interfaces between structure and ground (wall friction, sliding etc) use critical state value as characteristic value, and apply normal factors. Useful guidance in BS8002 ( withdrawn )

39 peak, critical state or residual? Critical state Useful guidance in BS8002 ( withdrawn )

40 The best GI is as many different tests as possible. Paul Mayne So we have to be able to consider results from different tests together. Some more definite than others. May conflict. 40

41 Data from more than one source Bored pile

42 Conflicting data The code drafters could not have known about this uncertainty

43 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters EG11 - Lovisa Moritz, Sweden Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 43

44 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 44

45 Dubai Dry Dock BP184.14

46 Dubai Dry Dock BP Factors of safety?!?

47

48 Construction basin in Western Australia

49

50

51

52

53

54

55

56

57

58 Water has a way of seeping between any two theories!

59 Piping failure

60 60

61 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). 61

62 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); a a internal failure or excessive deformation of the structure or structural elements, including W W e.g. footings, piles or basement walls, in which the strength of structural materials is M significant in providing resistance (STR); F1 F2 failure or excessive deformation of the ground, in which the strength of soil or rock is significant b 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). 62

63 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); F = 1.0, 1.35, 1.5 etc failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance (GEO); F = 1.0, 1.35, 1.5 etc loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions (UPL); F,dst = 1.0/1.1, F,stb = 0.9 hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients (HYD). F,dst = 1.35, F,stb =

64 64

65 (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. 65

66 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 66

67 67

68 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 Introduction Case histories of failures caused by water pressure Existing geotechnical codes and guidance documents Requirements of EC7 Some fundamental considerations Examples Recommendations 68

69 69

70 70

71 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 71

72 72

73 1m rise in water level multiplies BM by about 2.5 outside the range allowed by factors on the water pressure or water force. 73

74 U stb G;stb & G,dst 2. Buoyant weight 3. Factor water density 4. Water unfactored U dst 74

75 G;stb & G,dst 2. Buoyant weight 3. Factor water density 4. Water unfactored F variable F permanent 75

76 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. But the question is where? The problems are less for DA1, but not removed. 76

77 h 77

78 n R k /W w n.rk/ww 2 1 (a) (b) (c) (d) (e) (f) n R k /W w n.rk/ww (c) (e) (d) (f) (b) (a) h/d h/d Number of piles required (normalised). (a) unfactored, (b) pile resistance factored, (c) G = 1.35 on water pressure, (d) water table adjusted, (e) UPL, h/d h/d 78

79 The possibility of a reduced factor on favourable weight, perhaps between 0.8 and 0.9 should be considered. 79

80 n R k /W w n.rk/ww 2 1 (a) (b) (c) (d) (e) (f) n R k /W w n.rk/ww (c) (e) (d) (f) (b) (a) h/d h/d Number of piles required (normalised). (a) unfactored, (b) pile resistance factored, (c) G = 1.35 on water pressure, (d) water table adjusted, (e) UPL, (f) G;fav = 0.8 on weight h/d h/d 80

81 EC (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 design values 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? 81

82 Depending on other factors, it might be necessary to enhance the loads derived from water pressure (or load effects such as an uplifting force), even when they are very certain. Problem in cases where it is directly equivalent to factoring water pressures 82

83 A middle 2/3rds rule could be considered. Effective resultant force must lie within the middle 2/3rds of the base. 83

84 84

85 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 85

86 Values of partial factors recommended in EN Annex A Combination 1 Apply factors, either to Design approach 1 Design approach 2 Design approach 3 Combination Combination 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 Permanent unfav 1,35 1,35 1,35 1,35 Soil fav Variable unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3 water pressures (ugh!) tan ' 1,25 1,25 StructuraGeotech 1,25 Effective cohesion 1,25 1,25 actions actions 1,25 Undrained strength 1,4 1,4 or to structural action 1,4 Unconfined strength 1,4 1,4 1,4 Weight density effects such as Spread Bearing 1,4 footings Sliding 1,1 bending moments and Driven Base 1,3 1,1 piles Shaft (compression) 1,3 1,1 Total/combined 1,3 prop 1,1 forces (DA1-1*) 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 Shaft in tension 1,25 1,6 Combination 1,15 2 1,1 CFA Base 1,1 1,45 1,1 piles Shaft (compression) 1,0 1,3 1,1 Use the worst credible Total/combined 1,1 1,4 1,1 Shaft in tension 1,25 1,6 1,15 1,1 Anchors Temporary 1,1 1,1 water 1,1 pressures, Permanent 1,1 1,1 1,1 Retaining Bearing capacity unfactored. 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 86

87 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 87

88 z u G S EC7 { (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 (u dst;d ) at the bottom of the column, or the design value of the seepage force (S dst;d ) in the column is less than or equal to the stabilising total vertical stress ( stb;d ) at the bottom of the column, or the submerged weight (G stb;d ) of the same column: u dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) 88

89 z u G S Annex A of EC7 provides values for partial factors to be used for HYD, G;dst = 1.35 and G;stb = 0.9. But the code does not state what quantities are to be factored. Maybe: EC7 { (1)P} states: When considering a limit state of failure due to heave by G;dst seepage u dst;k of G;stb water stb;k in the (2.9a) ground (HYD, see 10.3), it shall be verified, for every and relevant soil column, that the design value of the destabilising total pore water pressure G;dst S dst;k (u dst;d G;stb ) at G stb;k the bottom (2.9b) of the column, or the design value of the seepage force (S dst;d ) in the column is less than or equal to the stabilising total vertical In stress this format, ( stb;d ) at the the factors bottom are of applied the column, to different or the quantities submerged in weight 2.9 a and b. (G stb;d ) of the same column: u dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) 89

90 H=? 7m 1m 3m Uniform permeability 90

91 u dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) Apply G;dst = 1.35 to: Apply G;stb = 0.9 to: H Pore water pressure u dst;k Total stress stb;k 2.78 Seepage force S dst;k Buoyant weight G stb;k 6.84 Excess pore pressure u dst;k - w z Buoyant density 6.84 Excess head (u dst;k - w z) / w Buoyant Orr, density T.L.L G;dst u dst;k G;stb stb;k (2.9a) 6.84 Model Solutions for Eurocode 7 Excess pore pressure or excess head Total Workshop density Examples. 6.1 G;dst S dst;k G;stb G stb;k (2.9b) Trinity College, Dublin. 91

92 u dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) Apply G;dst = 1.35 to: Apply G;stb = 0.9 to: H Pore water pressure u dst;k Total stress stb;k 2.78 Seepage force S dst;k Buoyant weight G stb;k 6.84 Excess pore pressure u dst;k - w z Buoyant density 6.84 Excess head (u dst;k - w z) / w Buoyant density 6.84 Excess pore pressure or excess head Total density

93 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures EG9 Norbert Vogt, Germany The EQU limit state Numerical analysis for ULS design 93

94 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 94

95 95 Paper on the EQU problem

96 EN1990 Single source and EQU a a W W M F 1 F 2 b Note 3 : The characteristic values of all permanent actions from one source are multiplied by G,sup if the total resulting action effect is unfavourable and G,inf if the total resulting action effect is favourable. For example, all actions originating from the self weight of the structure may be considered as coming from one source ; this also applies if different materials are involved. 96

97 a a W W M F 1 F 2 b 97

98 What if EQU not balanced? How to involve material strength? W a a W Three views: 1. Material demand only calculated when design W s balanced. For a=5b M/Wa F/W M F 1 F 2 b 2. Nonsense! Must apply G = 1.35, , (tension) 3. Treat EQU as another load case. Check M, F 1, F 2 for G = 0.9, 1.1 OR G = 1.15, G a,k G b,k P k.stb a/2 b 98

99 Overturning on infinitely strong rock. b W Dry sand (loose) F Infinitely strong rock For finite strength, check Wx1.0; Fx1.35 (or material factor on tan of sand). Check ground and structure.? For infinite strength, check overturning for Wx0.9; Fx1.1? Less severe design loading because rock infinitely strong? Less severe loading for overturning than for sliding? Even the structure is not infinitely strong. So not a real problem? But no problem if EQU treated as another load case. 99

100 Soil strength also involved Q (1.5) 1 2 Homogeneous dry sand W (0.9 < 1.0) h Inclination of slip surface < d Inclination of slip surface > d d=0 Nobody wants EQU to rule. Could make a special rule to prevent EQU check.? Better to require the check in principle and arrange factors so that it will never be critical. 100

101 EN1997 UKNA Partial material factors reduced for EQU 101

102 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state TC250 group including chair of SC7 Numerical analysis for ULS design 102

103 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design 103

104 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design What to factor and when to factor? 104

105 BP124-T2.14 BP124-A2.12 BP BP BP Partial factors recommended in EN Annex A (+UKNA) Values of partial factors recommended in EN Annex A (+ UKNA) Design approach 1 Design approach 2 Design approach 3 Combination Combination 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 Permanent unfav 1,35 1,35 1,35 1,35 fav Variable unfav 1,5 1,3 1,3 1,5 1,5 1,5 1,3 Soil tan ' 1,25 1,25 StructuraGeotech 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 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 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 Easy to factor 1,1 primary input 1,1 1,1 Retaining Bearing capacity 1,4 walls Sliding resistance material strengths and actions 1,1 Earth resistance 1,4 Slopes Earth resistance 1,1 Difficult to factor geotechnical resistances and action effects indicates partial factor = 1.0 C:\BX\BX-C\EC7\[Factors.xls] 25-Nov-06 17:26 105

106 Fundamental limit state requirement BP193.8 E d R d E{ F ; d X ; a d d } = E d R d = R{ F ; d X ; a d d } E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d } or E{ F F rep ; X k / M ; a d } = E d R d = R k / R = R n R (LRFD) or E E k = E d R d = R k / R so in total E E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d }/ R Reduce strength, increase the loads, and check equilibrium OR Find the remaining FOS? OR c- reduction 106

107 Pre-factored strength, or c- reduction? Max wall displacement 48mm Large displacement Max wall displacement 48mm xbcap5-dec11ab.sfd = 1.25 = = 1.25 = m M R;d BENDING MOMENT knm/m 107

108 Wrong failure mechanism? Max wall displacement 48mm Large displacement Max wall displacement 48mm xbcap5-dec11ab.sfd = 1.25 = 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.

109 Factoring advanced models, c, c u not explicit parameters eg Cam Clay, BRICK etc Change to Mohr-Coulomb for the factored calculation? If c = this is the 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. 109

110 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 c u ) than on effective stress parameters (eg 1.25 on c, tan ). 110

111 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 c u ) than on effective stress parameters (eg 1.25 on c, tan ). 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 c u 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. 111

112 Time-dependent analysis Beyond EC7! 112

113 ULS for staged construction single propped excavation Compute using factored strength Compute using unfactored characteristic parameters parameters Compute using factored parameters Factor material strengths Initial state? Excavate to 5m wall cantilevering Initial state Excavate to 5m wall cantilevering Could be critical for wall bending moment Install prop at 4m depth Excavate to 10m No further factors on strut forces or BMs Install prop at 4m depth Excavate to 10m Apply factors on strut forces or BMs Could be critical for wall length, bending moment and prop force No further factors on strut forces or BMs 113

114 Staged construction FE with partial facotrs Compute using factored strength Compute using unfactored characteristic parameters parameters Compute using factored parameters Factor material strengths Better continuity and consistency? Initial state? Suits some software. Excavate to 5m wall cantilevering Only one definition of soil properties. Install prop at 4m depth But: conservatism at one stage might be unconservative at Excavate to 10m another. No further factors on strut forces or BMs Initial state Suits some software. Excavate to 5m Combines wall cantilevering SLS analysis Can be used with more Install advanced prop at 4m depth soil models Excavate to 10m Apply factors on strut forces or BMs Could be critical for wall bending moment Could factor soil strengths or take another approach Could be critical for wall length, bending moment and prop force No further factors on strut forces or BMs 114

115 Simpson, B and Hocombe, T (2010) Implications of modern design codes for earth retaining structures. Proc ER2010, ASCE Earth Retention Conference 3, Seattle, Aug Florence Rail Station 25m deep, 50m wide, 550m long Mezzanine level prop High groundwater level 115

116 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. 116

117 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. 117

118 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. 118

119 BP BP BP BP124-T2.13 BP124-A2.11 Partial factors for DA1 UK National Annex BP Design approach 1 Combination Combination Combination 2 - piles & anchors A1 M1 R1 A2 M2 R1 A2 M1 or M2 R4 Actions Permanent unfav 1,35 fav Variable unfav 1,5 1,3 1,3 Soil tan ' 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 1.7/1.5 1,3 Shaft in tension 2.0/ Bored Base 2.0/1.7 1,6 piles Shaft (compression) 1.6/1.4 1,3 Total/combined 2.0/ Shaft in tension 2.0/ CFA Base As 1.45 piles Shaft (compression) for 1.3 Total/combined 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] 119

120 ULS for staged construction single propped excavation Compute using factored strength Compute using unfactored characteristic parameters parameters Compute using factored parameters Factor material strengths Initial state? Excavate to 5m wall cantilevering Initial state Excavate to 5m wall cantilevering Could be critical for wall bending moment Install prop at 4m depth Excavate to 10m No further factors on strut forces or BMs Install prop at 4m depth Excavate to 10m Apply factors on strut forces or BMs Could be critical for wall length, bending moment and prop force No further factors on strut forces or BMs 120

121 Florence Station comparison of bending moments 45 Prop and excavation levels Stage Level (m) ,000-4,000-2, ,000 4,000 6,000 8,000 Bending Moment (knm/m) C1 C1 C1 C2, factors all stages C2, factors all stages C2, factors all stages C2,factoring stages separately C2, factors all stages C2, factoring stages separately 121

122 BP198.2 BP201.2 Eurocode 7 fundamental issues and some implications for users Safety format of EC7 Characteristic and design values of soil parameters Design in situations dominated by water pressures The EQU limit state Numerical analysis for ULS design EG4 Andrew Lees - Cyprus 122

123 Concluding remarks Code-drafter s challenge: To standardise and regulate where it is sensible to do so, while encouraging maximum exploitation of engineering expertise. Characteristic values and water pressures: full use of human knowledge, sometimes guided but never governed by statistics. Partial factors: society s safety margin (and ours!). FEM the tool of the future code must not restrict. EC7 still evolving! 123

124 BP201.1 Nordic Geotechnical Meeting Copenhagen, May 2012 BP198.1 Eurocode 7 fundamental issues and some implications for users Brian Simpson, Arup Geotechnics Thanks for your attention 124