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www.hydroworld.com June 2017 / HYDRO REVIEW 75
neglected and taken as a safety margin. Tus
the equation becomes:
Equation 3
Lreq = FShy • w Hw • A c • Pu
where:
• c is the cohesion intercept (kN/m²); and
• Pu is the perimeter of the plug at the
concrete-rock interface in unit length in
compression (m²).
• A is the cross sectional area of the plug (m²).
In practice, the factor of safety FSsf = 1. 5
to 3.0 has been used by various individuals
and agencies. Te author recommends the use
of values specifed by the U.S. Army Corps
of Engineers, 6 with FSsf = 2.0, 1. 7 and 1. 3
for usual, unusual and extreme load cases,
respectively, the same as that for the design
of gravity dams. Special attention should be
paid to the perimeter of the plug Pu. Only the
part of total perimeter P in compression can
be taken into account in the calculation. For a
tunnel with quadratic shape, the whole upper
face may not be in compression and should
be ruled out. If the tunnel has a circular/arch
upper face, the upper 120-degree area should
be subtracted.
Concrete or rock shear at plug
circumferential area
Te punching shear of the plug through con-
crete or rock mass at the plug circumferential
area is another frequently used criterion for
the determination of plug length:
Equation 4
Lreq = FSsh • w Hw • A [τ] • Pef
where:
• FSsh is factor of safety against shear failure;
• [ ] is design shear strength of concrete or
rock mass (kN/m²), whichever is less; and
• Pef is efective perimeter of the plug at
circumferential area in unit length (m²).
Te design shear strength of concrete can
be derived from ACI-350: = 0.170 •
and FSsh = 1. 4, in which fc’ is the concrete
compressive strength measured by crushing
cylindrical concrete specimens and =0.75 is
the strength reduction factor. Te project-specifc shear strength of rock mass obtained
on the basis of feld and laboratory tests is
recommended to be used. In the absence
of project-specifc data, shear strength may
be used. Te efective perimeter of the plug
depends on the construction quality, usually
Pef =(0.80 1.0)P.
Grouting
Grouting the surrounding rock mass and
concrete/rock interface is a primary step for
concrete plug construction. Te purpose of
grouting is to improve the quality of the rock
mass, reduce seepage and leakage, prevent
piping and downstream erosion, and ensure
better shear friction and shear resistance by
flling fssures and voids in the surrounding
rock and at the concrete/rock interface.
Curtain grouting
For the plugs of diversion tunnels located
beneath the dam, curtain grouting surrounding the plugs should be performed. Te ring
curtain grout fan should be extended to the
grout curtain at the dam’s foundation, forming
a seepage cutof to efectively increase the
length of seepage path around the plug. Tis
also decreases seepage passing through the
rock mass.
