Design of Tunnel Plugs for Hydropower Projects
Tough they are an important part of the dam construction process,
tunnel plugs are an oft-overlooked part of a project’s design.
By Chongjiang Du
Diversion tunnels and access adits to power tunnels at hydropower projects must be closed with concrete plugs
prior to initial reservoir impounding. Te primary responsibility of these plugs is to provide
a barrier against reservoir water throughout
the life of the project.
Although the volume of concrete used and
costs associated with the plug are insignifcant
in comparison to the total project costs, the
plug is an important, permanent element that
should receive proper consideration during a
project’s design. However, the schedule for
plug construction is usually tight because its
construction typically takes place when the
project is approaching completion.
Current design practices draw upon
decades of observation and practice, but there
is no consistent nor unifed design criteria or
engineering practice for tunnel plugs. Tis
article will draw upon the author’s experience
to explicate the subject, with the goal of providing guidance in the design of tunnel plugs
for hydroelectric facilities.
Positioning and structural features
Te positioning of tunnel plugs is primar-
ily governed by functional and operational
requirements, while economic considerations
are rarely a critical factor in the decision-mak-
ing process because of the limited concrete
volume and related works. Some secondary
factors — such as the completion schedule,
access requirements, water table level and local
geological conditions — could occasionally
afect the positioning of the plugs.
Generally speaking though, common
practices dictate that:
• For a diversion tunnel, the plug should be
arranged in the abutments beneath the
dam, near the dam axis;
• For permitting utilization of a diversion tun-
nel as a headrace, sediment sluicing or food
discharge tunnel, the plug should be located
immediately upstream of the intersection of
the diversion tunnel with the vertical/inclined
shaft and water conductor tunnel; and
• For an access adit to the power tunnel, the
plug should be arranged at the immediate
junction with the power tunnel.
In the latter two cases, additional concrete
should be placed for the necessary hydraulic
profle at the junction with the main tunnels.
Given the primary requirements to safely retain
the reservoir, a number of concrete outlines
may be chosen. In practice, two types of tunnel
plugs are frequently applied at hydro facilities:
• Solid concrete plug with or without a grout-
ing gallery (see Figure 1 on page 73); or
• Gated concrete plug (see Figure 2, page 73).
Te shape of the plug, its contact with
the surrounding rock and its stability are the
most important design features for providing
stability and favorable hydraulic conditions.
For better plug action and cutting the seepage path along the concrete/rock interface,
it is generally recommended to enlarge the
tunnel into a conical shape with the narrow
end facing downstream, then ftting it with a
wedge-shaped key. Another advantage of this
“bottle plug” key is that some possible minor
gap at the concrete/rock interface caused by
concrete shrinkage could be closed.
Te key should be provided at the concrete/
rock interface and within concrete to ensure
efective plug action by providing adequate bear-
ing of the plug concrete on the tunnel lining or
plug concrete/tunnel lining on the surrounding
rock. Te keyway should have a minimum depth
of 0.45 m. At a minimum the key should be
constructed at the bottom and side walls of the
plug, in case the construction schedule is tight.
For a tunnel excavated by tunnel boring
machine, the depth of the key becomes a ques-
tion of the required mechanical support. For
a tunnel excavated by drill-and-blast method,
the key should be 1 to 2 m deeper, depending
on the blasting techniques and design.
Te key in a diversion tunnel can be excavated before or after the diversion period,
depending on the construction schedule. If the
key is excavated before diversion, the keyway
should be flled up with lean concrete to ensure
the smooth fow of water during the diversion
period. Te lean concrete should be removed
before placing the plug concrete.
Te plug should be constructed as a monolithic structure with no transverse joint. To
facilitate grouting, a gallery may be arranged
in the plug. Adequate provisions should be
made for dewatering during construction. Te
concrete plug may be placed in lifts. To ensure
proper bonding between two successive lifts
of concrete, chipping or roughening the joints
should be performed, along with provisions
for suitable dowels. For gated concrete plugs,
reinforcement is generally required.
Typical design criteria for tunnel plugs
Tunnel plugs are signifcant engineering structures. Tey are characterized mechanically by
their own stress states and hydraulically by
water pressure and fows. Te design criteria
should be worked out considering both of
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has been evaluated and edited in accordance
with reviews conducted by two or more
professionals who have relevant expertise.
Tese peer reviewers judge manuscripts for
technical accuracy, usefulness, and overall
importance within the hydroelectric industry.