Foundation and Abutment Reinforcing for Concrete Dams

Foundation and Abutment
Reinforcing for Concrete Dams
National Dam Safety Program - Technical Seminar No. 20
“Overtopping of Dams”
National Emergency Training Center, February 21, 2013
Tom Hepler, Technical Specialist

Rock Reinforcement
• A free-falling jet impinging on a rock
abutment below a concrete dam will
transfer forces to the surface and into the
cracks, joints, and fissures of the rock
• Rock reinforcement alone may be suitable
protection in lieu of a mass concrete slab
• Rock bolts and post-tensioned anchors may
be installed to hold rock blocks in place

Design Philosophy
Current design philosophy is to:
• Determine the energy of the overtopping
jet at the point of impingement on the rock
surface based on the geometry and
characteristics of the jet
• Determine whether the rock can withstand
the force of the impinging jet without
fracturing or plucking

Characteristics of Jet
Hydraulic properties included in design:
• Initial depth, velocity, discharge, aeration,
turbulence, angle, and shape of jet
• Jet break up, aeration, and spread during fall
• Jet velocity, depth, and angle at impingement
with rock surface
A jet overtopping a concrete dam is usually
of low initial turbulence and velocity

Rock Scour Mechanisms
• Brittle fracture – explosive break-up of rock
• Fatigue (sub-critical) failure – slow break-up
• Dynamic impulsion – plucking of individual
rock blocks from matrix due to pressure
imbalances between top and bottom of block
due to impinging jet; natural frequency of rock
fissures (greater than 100 Hz) will not lead to
amplification of dynamic pressures due to
resonance with turbulent water frequency

Pressure of Impinging Jet
• Pressure induced by impinging jet for
dynamic impulsion is proportional to the
unit weight of water, the velocity head of
the impinging jet, and the total dynamic
pressure coefficient
• Rock reinforcement would be designed to
restrain vulnerable rock blocks (generally
applicable to competent rock with widely
spaced joints)

Rock Joint Orientation

Case Study – Abutment
Reinforcement for Concrete
Dam Overtopping

Gibson Dam
• Located on the North Fork of the Sun
River, about 30 miles NW of Augusta MT
• Concrete arch dam with a structural height
of 199 feet
• Overtopped on June 8-9, 1964 during a
rain-on-snow flood
• Dam was overtopped for about 20 hours;
maximum depth of 3.2 feet
• Minor erosion damage on abutments

Overtopping of Gibson Dam (1964)

Gibson Rock Scour Potential
Based on a detailed evaluation following this
event:
• Erodibility index of the dolomite abutment
rock was estimated to be between 5,100
and 12,000
• Streampower of impinging jet estimated to
be 43 KW/m 2 on the upper abutments and
258 KW/m 2 on the lower abutments

Stream Power vs Erosion Index
Damage
Gibson Dam,
1964 overtopping
No Damage

Gibson Dam Modifications
• Event raised concerns about larger floods that
could erode the abutments and undermine the
dam, or allow foundation blocks to daylight
• Modifications were completed in 1982,
designed for up to 12 feet of overtopping
• Included excavation of unstable rock on both
abutments, installation of groutable rock bolts
to stabilize jointed rock, and placement of
concrete caps on both abutments

Gibson Dam Modifications
• Cap on right abutment was designed with a
maximum thickness of 2.5 feet, but with no
waterstops between joints and no drainage
• Design aspects of the cap are undocumented
• Left abutment protection less extensive; two
prominent joints filled with concrete and rock
bolting over remaining abutment area
• Eight splitter piers constructed on top of dam
to divide flow and provide aeration for jet

Gibson Dam
Overtopping Protection

Gibson Dam Splitter Piers

Completed Gibson Dam (1982)

Questions?