Design of Earth Dams. Earth Dam Components

Transcription

1 Design of Eart Dams Eart Dam Components 1

2 Design Requirements Types of Eart Dams

3 Oter Types of Dams Concrete Roller Compacted concrete Debris flows Coice of Type Depends on Availability of materials Trucking increases cost Homogeneous Eart Dam Lots of low K material Till Zoned Limited core material Rock available Upstream construction Use mine tailings as a construction material 3

4 Foundation Requirements 1. Strong foundation Minimal differential settlement No bearing capacity sear failure Sand/Gravel or rock. Low ydraulic conductivity Silt and/or Clay, non-fractured rock Since 1 and are often incompatible we need to do foundation treatment Foundation Treatment Soil Cut-offs walls Seet pile walls Slurry trenc Impervious upstream blanket Removal and replacement Densification Zone grouting Rock Removal and replacement of upper fractured rock Grouting Impervious upstream blanket 4

5 Hydraulic Efficiency Install upstream and down stream peizometers E H = Head loss across barrier Total ead loss = Δ Efficiencies greater tan 90% ave been attained Foundation Treatment (Mitcell) 5

6 Foundation Treatment (Mitcell) Foundation Preparation Want smoot transition to minimize negative skin friction and soil arcing as tat can led to ydraulic fracturing as well as core cracking 6

7 7 Foundation Preparation Foundation Distortion F L L B B Δ = + + = ε H m v v γ ε = Sear Strain (τ) = ε v -ε 1%

8 Hydraulic Fracturing Occurs wen te porewater pressure is equal to te minimum embankment effective stress σ = Kσ = KγH E ε > μ = v Hγ w were K~0.5 for compacted materials E is te tensile modulus obtained from a triaxial extension test ε = orizontal strain Linear and Plastic Deformations 8

9 Soil Arcing Terzagi 1943 Soil Arcing Terzagi Trap Door Note significant reduction is vertical stress 9

10 Soil Arcing Terzagi Trap Door Soil Arcing 10

11 Approximation of Vertical Stress in DAMS σ v Lγ 1 e K tanφ K = ( tanφ ) Z L Were L is te widt of te valley K=constant ~1 Z is dept from dam crest Approximation of Vertical Stress in DAMS If H>L ten z/l >1 and K=1 σ v = Lγ tanφ 11

12 Hydraulic Fracturing if μ Hγ w for full reservoir σ Kσ v were K ~ tanφ σ Lγ Hydraulic Fracturing ten Lγ σ ' = 0 wen = γ wh γ note γ w tereσ ' 0 wen L = H Note: ydraulic fracturing will occur wen L is approximately equal to H A few dams exists were average L < average H 1

13 Design of Cutoffs Must prevent ydraulic fracturing σ >porewater pressure Ten L cutoff > H cutoff σ v = γh dam 1 H cutoff L cutoff Foundation Preparation 1. To prevent ydraulic fracturing must: Have no vertical sear boundaries tat can create differential settlement Smoot profile along base of dam Cord Lengt greater tan te eigt of dam Cutoffs wit cord lengts greater tan cutoff eigt. Hydraulic conductivity tat will allow water to pond beind te dam 3. Strong enoug to minimize distortional strains to less tan 1 percent 13

14 Design of Zoned Dams Rock fill will be muc stiffer tan core. As core settles arcing will occur in te core. Design of Zoned Dams To prevent core arcing and ydraulic fracturing we need: 1. wide enoug core. need sear layer between core and sell to prevent internal core cracking 14

15 How wide of core do we need? were: W is te widt of core Z is dept below crest γ core is unit weigt of core Zγ core σ v = GZ 1+ D tw G is te sear modulus of filter material t is widt of filter material D c is confined modulus of core = 1/m v c Can Geotextiles Prevent Sear Note t is very small for geotextiles tis makes GZ very large DctW Resulting is significant arcing in core creating core cracking and ydraulic fracturing. To prevent ydraulic fracturing need GZ < 0.35 D tw c 15

16 Metods to Prevent Core Cracking Wider cores Greater filter widt Greater core compaction Benced cores to increase vertical stress Transverse Core Cracking Develops wen D/S sell material distorts to resist U/S water pressures transmitted by core Limit to less tan 1% by aving 00Hγ w G < cot β Were G is modulus of sell material H is dam eigt β is down stream slope angle β 16

17 Tranverse Cracking Often dams designed wit u/s arc. Tis keep d/s in compression Design Limits δ v /Hcotβ Impact No cracking Tin reinforced concrete may crack Longitudinal cracking in dry cores Transverse cracks in cores and oblique core to sell cracks Longitudinal cracks in wet cores & jointed concrete facing Danger of transverse cracks and piping failures 17