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Ancient Dam Built in Unfavorable Geology Analyzed
- 1. GEOTECHNICAL ANALYSIS OF AN ANCIENT DAM BUILT IN UNFAVOURABLE GEOLOGICAL CONDITIONS P. Croce, G. Modoni, A. Rasulo & M. Saroli University of Cassino and Southern Lazio Department of Civil and Mechanical Engineering
- 2. Alpe Cavalli dam Padova Al p Roma Cheggio Mt. e Ca va lli La ke Alpe Cavalli dam Alpe Cheggio Loranco River Cheggio Mt. Alpe Cavalli dam Alpe Cheggio Alpe Cavalli dam Alpe Cheggio Alpe Cavalli Lake Loranco River
- 3. ELECTRIC POWER PRODUCTION Alpe Cavalli reservoir Cingino Alpe Cavalli Camposecco ~ Campiccioli Antrona Antrona reserv. Rovesca power plant ~ Rovesca ALPE CAVALLI Qmax = 4.3 m3/s Campiccioli reservoir ∆h = 711.2 m P = 10.8 MW
- 4. Alpe Cavalli Reservoir Capacity: 8.35 10-6 m3 Altitude 1,500 m a.s.l. Loranco River Loranco River Calcareous Schists Morain
- 5. Geological Setting C’ view from downstream 4 1 3 C 5 2 1 2 1 5 4 3 1 Morain Deposit 2 3 Detritic Deposits 3 5 Alluvial Deposits Calcareous Schists Rocks 1 Amphibolitcs Rocks Dam
- 6. Geological Model view from upstream lake dam C’ Alluvial deposit Morain deposit Metamorphic rock Metamorphic rock Morain deposit Morain deposit Alluvial deposit Metamorphic rock Metamorphic rock
- 7. DEPTH (m) MORAIN DEPOSIT UNDISTURBED SAMPLING NOT FEASIBLE SILT SAND GRAVEL
- 8. DRY MASONRY DAM WITH IMPERVIOUS FACING Dam Height: 41.60 m - Crest Length: 165 m ro ck morain
- 11. SEEPAGE From the beginning of reservoir operation a relevant seepage was observed under the dam body and through the morain deposit. SHISTS MORAIN Left Shoulder Right Shoulder 10 years after dam completion extensive grouting was performed under the dam body
- 12. SEEPAGE THROUGH THE MORAIN Do w ns t re am Sp rin gs
- 13. PUMPING BACK TO THE RESERVOIR
- 14. WATER FLOW VERSUS RESERVOIR LEVEL average Z/Zaverage Q/Qaverage average year No Time-Lag
- 15. FLOW RATE VERSUS RESERVOIR LEVEL max impoundment river bed background flow
- 17. SEEPAGE BACK-ANALYSIS First Model: Homogeneous Permeability Second Model: Permeability Decreasing with Depth
- 18. SEEPAGE BACK-ANALYSIS Comparison with Experimental Data monitoring 1 layer 4 layers
- 20. DAM HORIZONTAL DISPLACEMENTS A’ (centre dam) cumulated displacement 1.6 mm/year C’: (close to left shoulder) no permanent displacement AIN ROCK MORAIN C’ A’
- 21. MORAIN HORIZONTAL DISPLACEMENTS back hor. movements forth .... 20 water level H - H min (m) 15 10 y = -0.0511x2 + 2.4767x R2 = 0.8929 5 0 0 2 4 6 8 10 12 14 W - W min (mm) morain movements versus reservoir level
- 22. FEM MODEL Seepage 0.000 50.000 350.000 400.000 100.000 150.000 200.000 250.000 300.000 350.000 250.000 200.000 0.000 Effective Stresses 50.000 100.000 150.000 200.000 250.000 300.000 450.000 500.000 150.000 50.000 Effective stresses 3 2 Extreme effective principal stress -3,31*10 kN/m 100.000 00.000 50.000 50.000 0.000 00.000 -50.000 Deformations 50.000 Shear strains Extreme shear strain 1,25 % 0.000 50.000 Displacements 400.000 450.000
- 23. CHOISE OF MORAIN YOUNG MODULUS Constant Modulus ? pressuremeter tests B.A. from previous cases SPOSTAMENTO - QUOTA INVASO Horizontal Movement (mm) H - H Level .... Water min (m) (m) 20 SPERIMENTALE Monitoring 15 E=190MPa E=170MPa E=150MPa E=130MPa E=110MPa E=90 MPa E=70 MPa 10 5 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 W - W min (mm)
- 24. Stiffness modulus (MPa) -50 50 0 E= Eo *z 0.5 250 350 Model Pressumeter tests 20 30 40 50 60 CHOISE OF MORAIN YOUNG MODULUS Modulus increasing with effective stress 70 80 90 w.l. I w.l. II w.l. III 100 water level w.l. IV Water level in the reservoir (m) . Depth below g.l. (m) . 10 150 25 20 15 10 5 0 0 5 10 15 Horizontal Displacements of Morain (mm) model measurement year 2001 measurement year 2002
- 26. POSSIBLE REMEDIAL WORKS Impervious Facing PARAMENTO IMPERMEABILE [m] 1520 [m] 1520 STRATO DRENANTE 1500 Foot Drainage 1500 1480 1480 1460 Rio Loranco 1460 Rio Loranco 1440 1420 1440 1420 1400 1400 1380 1380 1360 1360 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 20 440 [m] 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 [m] B) Drainage A) Waterproofing SPOSTAMENTO ORIZZONTALE 20 .... Rio Loranco H - H min (m) Grouting CONSOLIDAMENTO 1500 A B C [m] 1520 15 1480 1460 1440 1420 1400 no works FEM consolidato soil improv. FEM dreno drainage FEM 10 5 1380 FEM paramento impermeabile waterprooof 1360 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 [m] 0 C) Soil Improvement 0 2 4 6 8 10 12 14 16 W - W min (mm) DISPLACEMENTS 18 20 22
- 27. CONCLUSIONS a) In the Italian Alps there are several ancient masonry dams, similar to Alpe Cavalli, built almost a century ago and still in operation for electric power production. b) Continuous monitoring over almost a century has provided the essential data for back analysis. c) Careful back analysis was based on the peculiar geological model and was performed according to the principles of soil mechanics. d) The dam behaviour depends on the complex interaction among reservoir, subsoil and dam structure. e) Water level fluctuations induce cyclic movements of the morain deposit, which drags the right shoulder of the dam. f) Possible remedial works onto the morain deposit don’t seem to provide conclusive improvement. g) Continuous monitoring and back-analysis seem to be the most useful approach for safe operation of the reservoir.