Mitigation Measures Evaluation for Concrete Faced Rockfill Dams. Juan E. Quiroz Mehdi Modares REC Conference - IIT

Transcription

1 1 Mitigation Measures Evaluation for Concrete Faced Rockfill Dams Juan E. Quiroz Mehdi Modares REC Conference - IIT

2 INTRODUCTION 2 Dams for water impoundment can be: Gravity Arch Masonry Roller Compacted Concrete (RCC) Embankment Earthfill Rockfill Concrete Faced Rockfill Dams or CFRD

3 CONCRETE FACED ROCKFILL DAMS (CFRD s) 3

4 CONCRETE FACED ROCKFILL DAMS (CFRD s) 4 Zones 1A and 1B protect the upstream concrete faces - usually cohesionless silt or fine sand. Zone 2 support the concrete faces - processed granular materials. Zones 3, 4, etc. are quarry rockfill zones. Zone 3A limits the void size. Zone 3B resists water pressure and controls face deflection. Zone 3C is composed by larger rocks.

5 CFRD s & FAILURES 5 The most attractive option for the designers Straightforward to construct Economical Adaptable to terrain geometry Materials in close proximity CFRD s with larger heights have increased tremendously Face slab damage was unprecedented until 2005 Failures usually occurred during the last few meters of filling

6 CFRD FAILURES CFRD ISSUE CAUSE Aguamilpa h=187m Barra Grande h=185m Campos Novos h=202m Itá h=125m Itapebi h=120m Mohale h=145m Tianshengqiao 1 h=178m Xingó h=150m Concrete facing cracking Concrete facing cracking Concrete facing cracking Slabs cracking Cracks parallel to the plinth Compression joint rupture Horizontal cracking Slabs cracking Rockfill deformability Joint failures Rockfill defformability Rockfill deformability Foundation geometry Rockfill deformability Construction sequence 6 Sharp geometry of the left abutment and material deformability (Ma and Cao 2007)

7 CAMPOS NOVOS CFRD FAILURE 7

8 MOHALE CFRD FAILURE 8

9 BARRA GRANDE CFRD FAILURE 9

10 TSQ1 CFRD FAILURE 10

11 CFRD s FAILURE CAUSES 11 Highly deformable rockfill cracking Concrete slabs follow rockfill deformation excessive stresses During impoundment, pressure increases additional stresses

12 BEHAVIOR 12

13 CURRENT PRACTICE 13

14 CURRENT PRACTICE 14 Designs are mostly based on common practice rather than rigorous analysis procedures. Face Slab thickness Head of Water (h) Face Slab Thickness (T) > 100m 0.3m h to 0.3m h 50m to 100m 0.3m < 50m 0.25m

15 CURRENT PRACTICE Vertical modulus of deformation, E v, is obtained from vertical settlements (Fitzpatrick, et al. 1985) 16 Transverse modulus of deformation, E t, is obtained from face slab deformation

16 CURRENT PRACTICE 17 Empirical approaches relate the ratio E t /E v with valley shape factor A/h 2 (Pinto and Marquez 1998).

17 CURRENT PRACTICE 18 Face slab deflection as a function of E v and valley shape factor A/h 2 (Pinto and Marquez 1998).

18 SUMMARY 19 Designs are mostly based on common practice. Due to the experienced structural failures of CFRD s, a more comprehensive methodology for analysis and design is needed. Numerous design changes and mitigation measures are required while construction is in progress to prevent failures. Structural analyses for estimating and comparing the slab stresses and the effectiveness of mitigation measures.

19 METHODOLOGY 20

20 METHODOLOGY 21 Analysis framework for CFRD s A comprehensive non-linear finite element analysis scheme for CFRD s that includes: Construction sequence Contact interactions Impounding of the reservoir Design changes evaluations Understand behavior

21 Material Calibration METHODOLOGY 22 Previous designs evaluation Precedent cases & experience Previous designs evaluation Precedent cases & experience No Foundation stiffness significantly larger than rockfill Yes No foundation required No Foundation stiffness significantly larger than rockfill Yes No foundation required Include foundation block Perform mesh sensitivity No Define base boundary conditions (e.g. Fixed, rollers, springs) Setup 3D FE model Incorporate costruction plan Foundation Interface required? Yes Define interface properties & rigid surface Implement stage sequence on FE model Evaluate material zones & properties Include foundation block Perform mesh sensitivity No Foundation Interface required? Yes Run analysis without slabs New CFRD design? NO (during construction) YES NO Validation: Settlements from model match measurements YES Define base boundary conditions (e.g. Fixed, rollers, springs) Define interface properties & rigid surface Evaluate results & behavior for: Arching effect Valley geometry - Shape factor Compare with previous dams Include concretefacing stages & Contact interfaces in model Setup 3D FE model Define slab thickness, joint locations & contact behavior Run FE model with slabs to EOC & FSL Consider mitigation measures and improvements to design: - Material properties stiffening - Zoning modifications - Contraction joints - Interfaces - Slabs thickness & detailing - Construction staging improvements Behavior Improvements Incorporate costruction plan Yes Slab stresses and behavior acceptable? No Final review and compare with previous CFRD's END

22 Material Calibration Material Calibration METHODOLOGY 23 Implement stage sequence on FE model Previous designs evaluation Precedent cases & experience Evaluate material zones & properties No Include foundation block Perform mesh sensitivity Foundation stiffness significantly larger than rockfill No Yes No foundation required Foundation Interface required? Yes Run analysis without slabs Define base boundary conditions (e.g. Fixed, rollers, springs) Setup 3D FE model Define interface properties & rigid surface Incorporate costruction plan New CFRD design? YES Implement stage sequence on FE model Evaluate material zones & properties Run analysis without slabs NO (during construction) YES New CFRD design? NO (during construction) NO Validation: Settlements from model match measurements YES Evaluate results & behavior for: Arching effect Valley geometry Shape factor Compare with previous dams NO Validation: Settlements from model match measurements Include concretefacing stages & Contact interfaces in model Define slab thickness, joint locations & contact behavior Run FE model with slabs to EOC & FSL Slab stresses and Yes behavior acceptable? Final review and compare with previous CFRD's YES Evaluate results & behavior for: Arching effect Valley geometry - Shape factor Compare with previous dams Consider mitigation measures and improvements to design: - Material properties stiffening - Zoning modifications - Contraction joints - Interfaces - Slabs thickness & detailing - Construction staging improvements No Behavior Improvements END

23 Material Calibration METHODOLOGY 24 Previous designs evaluation Precedent cases & experience Include concrete facing stages & Contact interfaces in model No Foundation stiffness significantly larger than rockfill Yes No foundation required Define slab thickness, joint locations & contact behavior Run FE model with slabs to EOC & FSL Consider mitigation measures and improvements to design: - Material properties stiffening - Zoning modifications - Contraction joints - Interfaces - Slabs thickness & detailing - Construction staging improvements Behavior Improvements Include foundation block Perform mesh sensitivity No Define base boundary conditions (e.g. Fixed, rollers, springs) Setup 3D FE model Incorporate costruction plan Implement stage sequence on FE model Evaluate material zones & properties Run analysis without slabs New CFRD design? Foundation Interface required? YES Yes Define interface properties & rigid surface NO (during construction) Yes Slab stresses and behavior acceptable? No NO Validation: Settlements from model match measurements YES Evaluate results & behavior for: Arching effect Valley geometry - Shape factor Compare with previous dams Include concretefacing stages & Contact interfaces in model Final review and compare with previous CFRD's Define slab thickness, joint locations & contact behavior Run FE model with slabs to EOC & FSL Consider mitigation measures and improvements to design: - Material properties stiffening - Zoning modifications - Contraction joints - Interfaces - Slabs thickness & detailing - Construction staging improvements Behavior Improvements Yes Slab stresses and behavior acceptable? No END Final review and compare with previous CFRD's END

24 ADVANTAGES 25 The ability to investigate different scenarios, for new designs and during construction dams. Optimum scenarios are evaluated to make decisions at the design stage and to propose changes during construction.

25 NUMERICAL EXAMPLE 26

26 KÁRAHNJÚKAR CFRD LOCATION 27 Iceland

27 NUMERICAL EXAMPLE 28 The Kárahnjúkar CFRD (Iceland) is the tallest in Europe Height = 198m Length = 730m Installed capacity of 690MW Basalt bedrock foundation Full Supply Level (FSL) = 625m

28 DAM DURING CONSTRUCTION 29

29 ROCKFILL SEQUENCE (MAX SECTION) 30

30 FACING SEQUENCE 31

31 3D VIEW 32

32 3D VIEW 33

33 STAGING 34 Rendering FE Model

34 STAGING 35 Rendering FE Model

35 STAGING 36 Rendering FE Model

36 STAGING 37 Rendering FE Model

37 STAGING 38 Rendering FE Model

38 STAGING 39 Rendering FE Model

39 STAGING 40 Rendering FE Model

40 STAGING 41 Rendering FE Model

41 FINAL CONFIGURATION 42 Rendering FE Model

42 DURING CONSTRUCTION DETAIL 43

43 CONTACT FORMULATION 44 Behavior is defined on the two main directions Normal behavior Pressure-overclosure relationship for normal stress transfer Pressure as a function of clearance or opening Tangential behavior Based on classical Coulomb friction Shear stress at the interface is proportional to the normal stress

44 FILLER AT VERTICAL JOINTS 45 15mm bituminous filler material installed mostly on central joints

45 FILLER AT VERTICAL JOINTS 46 Behavior based on data from compression test

46 MATERIAL PROPERTIES 47

47 MATERIAL PROPERTIES 48

48 CALIBRATION 49 Measurements from instrumentation to determine material properties. Initial calibration performed to correlate measured settlements with analysis results. Settlement measured along three sections. Main section B maximum height. Slab stresses compared with incremental changes during impoundment.

49 INSTRUMENTATION 50 The instrumentation installed on the dam consisted of: Settlement Gauges: Hydraulic settlement gauges were installed for monitoring settlement of the embankment fill and face slabs. The settlement gauges measure vertical settlement of the fill below the installation elevation. Strain Meters: Strain meters were installed to monitor stresses and strains in the concrete face slabs of the CFRD.

50 CALIBRATION 51

51 RESULTS 52

52 ROCKFILL SETTLEMENTS 53

53 SETTLEMENT MEASURED vs COMPUTED 54

54 ARCHING STRESSES 55

55 INCREMENTAL SETTLEMENT DURING IMPOUNDMENT 56

56 Face Slab Deflection 57

57 DEFORMED SLAB MAGNIFIED x80 58

58 INCREMENTAL STRESSES 59 Section 1 Section 2 Section 3

59 MITIGATION MEASURES 60 Reduction of lift thickness in order to stiffen the crest. Addition of a horizontal contraction joint. Consideration of a wider fiber spacer between vertical slab joints. Addition of an asphalt layer material to partially reduce the friction between slabs and rockfill. Increase of central slab thicknesses by 10cm at the central portion of the slabs.

60 MITIGATION MEASURES 61 Section 2 total horizontal stresses with and without mitigation measures

61 THANK YOU 63