Foundation and Abutment Reinforcing for Concrete Dams
Foundation and Abutment Reinforcing for Concrete Dams
Foundation and Abutment Reinforcing for Concrete Dams
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<strong>Foundation</strong> <strong>and</strong> <strong>Abutment</strong><br />
<strong>Rein<strong>for</strong>cing</strong> <strong>for</strong> <strong>Concrete</strong> <strong>Dams</strong><br />
National Dam Safety Program - Technical Seminar No. 20<br />
“Overtopping of <strong>Dams</strong>”<br />
National Emergency Training Center, February 21, 2013<br />
Tom Hepler, Technical Specialist
Rock Rein<strong>for</strong>cement<br />
• A free-falling jet impinging on a rock<br />
abutment below a concrete dam will<br />
transfer <strong>for</strong>ces to the surface <strong>and</strong> into the<br />
cracks, joints, <strong>and</strong> fissures of the rock<br />
• Rock rein<strong>for</strong>cement alone may be suitable<br />
protection in lieu of a mass concrete slab<br />
• Rock bolts <strong>and</strong> post-tensioned anchors may<br />
be installed to hold rock blocks in place
Design Philosophy<br />
Current design philosophy is to:<br />
• Determine the energy of the overtopping<br />
jet at the point of impingement on the rock<br />
surface based on the geometry <strong>and</strong><br />
characteristics of the jet<br />
• Determine whether the rock can withst<strong>and</strong><br />
the <strong>for</strong>ce of the impinging jet without<br />
fracturing or plucking
Characteristics of Jet<br />
Hydraulic properties included in design:<br />
• Initial depth, velocity, discharge, aeration,<br />
turbulence, angle, <strong>and</strong> shape of jet<br />
• Jet break up, aeration, <strong>and</strong> spread during fall<br />
• Jet velocity, depth, <strong>and</strong> angle at impingement<br />
with rock surface<br />
A jet overtopping a concrete dam is usually<br />
of low initial turbulence <strong>and</strong> velocity
Rock Scour Mechanisms<br />
• Brittle fracture – explosive break-up of rock<br />
• Fatigue (sub-critical) failure – slow break-up<br />
• Dynamic impulsion – plucking of individual<br />
rock blocks from matrix due to pressure<br />
imbalances between top <strong>and</strong> bottom of block<br />
due to impinging jet; natural frequency of rock<br />
fissures (greater than 100 Hz) will not lead to<br />
amplification of dynamic pressures due to<br />
resonance with turbulent water frequency
Pressure of Impinging Jet<br />
• Pressure induced by impinging jet <strong>for</strong><br />
dynamic impulsion is proportional to the<br />
unit weight of water, the velocity head of<br />
the impinging jet, <strong>and</strong> the total dynamic<br />
pressure coefficient<br />
• Rock rein<strong>for</strong>cement would be designed to<br />
restrain vulnerable rock blocks (generally<br />
applicable to competent rock with widely<br />
spaced joints)
Rock Joint Orientation
Case Study – <strong>Abutment</strong><br />
Rein<strong>for</strong>cement <strong>for</strong> <strong>Concrete</strong><br />
Dam Overtopping
Gibson Dam<br />
• Located on the North Fork of the Sun<br />
River, about 30 miles NW of Augusta MT<br />
• <strong>Concrete</strong> arch dam with a structural height<br />
of 199 feet<br />
• Overtopped on June 8-9, 1964 during a<br />
rain-on-snow flood<br />
• Dam was overtopped <strong>for</strong> about 20 hours;<br />
maximum depth of 3.2 feet<br />
• Minor erosion damage on abutments
Overtopping of Gibson Dam (1964)
Gibson Rock Scour Potential<br />
Based on a detailed evaluation following this<br />
event:<br />
• Erodibility index of the dolomite abutment<br />
rock was estimated to be between 5,100<br />
<strong>and</strong> 12,000<br />
• Streampower of impinging jet estimated to<br />
be 43 KW/m 2 on the upper abutments <strong>and</strong><br />
258 KW/m 2 on the lower abutments
Stream Power vs Erosion Index<br />
Damage<br />
Gibson Dam,<br />
1964 overtopping<br />
No Damage
Gibson Dam Modifications<br />
• Event raised concerns about larger floods that<br />
could erode the abutments <strong>and</strong> undermine the<br />
dam, or allow foundation blocks to daylight<br />
• Modifications were completed in 1982,<br />
designed <strong>for</strong> up to 12 feet of overtopping<br />
• Included excavation of unstable rock on both<br />
abutments, installation of groutable rock bolts<br />
to stabilize jointed rock, <strong>and</strong> placement of<br />
concrete caps on both abutments
Gibson Dam Modifications<br />
• Cap on right abutment was designed with a<br />
maximum thickness of 2.5 feet, but with no<br />
waterstops between joints <strong>and</strong> no drainage<br />
• Design aspects of the cap are undocumented<br />
• Left abutment protection less extensive; two<br />
prominent joints filled with concrete <strong>and</strong> rock<br />
bolting over remaining abutment area<br />
• Eight splitter piers constructed on top of dam<br />
to divide flow <strong>and</strong> provide aeration <strong>for</strong> jet
Gibson Dam<br />
Overtopping Protection
Gibson Dam Splitter Piers
Completed Gibson Dam (1982)
Questions?