Design of Tunnel Plugs for Hydropower Projects

Design of Tunnel Plugs for Hydropower Projects

Though 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. The 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 insignificant 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 unified design criteria or engineering practice for tunnel plugs. This 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

The positioning of tunnel plugs is primarily governed by functional and operational requirements, while economic considerations are rarely a critical factor in the decision-making 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 affect 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 tunnel as a headrace, sediment sluicing or flood 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.
Design of Tunnel Plugs for Hydropower Projects

In the latter two cases, additional concrete should be placed for the necessary hydraulic profile 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 grouting gallery (see Figure 1 on page 73); or
  • Gated concrete plug (see Figure 2, page 73).

The 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 fitting 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.

The key should be provided at the concrete/rock interface and within concrete to ensure effective plug action by providing adequate bearing of the plug concrete on the tunnel lining or plug concrete/tunnel lining on the surrounding rock. The 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 question 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.

The 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 filled up with lean concrete to ensure the smooth flow of water during the diversion period. The lean concrete should be removed before placing the plug concrete.

The 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. The 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 significant engineering structures. They are characterized mechanically by their own stress states and hydraulically by water pressure and flows. The design criteria should be worked out considering both of these requirements.

Diverse design criteria are used by varying individuals and agencies, but in general plugs should be designed using the following principals and requirements. The plug:

  • Is a permanent element and should be designed using the same criteria as the corresponding water-retaining structures, such as the dam or power intake;
  • Should be reliable and stable during the project life;
  • Should be resistant to hydrofracturing of surrounding rock;
  • Should be able to prevent excessive seepage passing through the plug and adjacent rock mass;
  • Should be strong enough to withstand the actual pressure; and
  • Should be safe against sliding within the structure.

The concrete plug can function as a water barrier when it tightly bonds to its surrounding rock, forming an integral mass. Behavior of the plug is closely related to its surrounding rock mass. Therefore, it is necessary to thoroughly treat the surrounding rock mass and concrete/rock interface so as to get good rock conditions and to eliminate as many small water-bearing fractures as possible

Moreover, caution is urged over the design interface between the temporary work and permanent structure because diversion tunnels and adits are designed and constructed as temporary works, whereas the concrete plug is a permanent structure. This, it is often necessary to treat the surrounding rock.

Hydrofracturing

Hydrofracturing is an event that produces fractures in a sound rock while hydrojacking or uplift by the opening of existing cracks of joints due to high-pressure water. This phenomenon is well-documented. Eliminating the risk of hydrofracturing is a fundamental task for the engineer involved in the design of tunnels and tunnel plugs. A number of criteria have been developed to guide the selection of the minimum cover to prevent hydrofracturing, of which one generally accepted is the Norwegian cover criterion.1

The minimum allowable depth of cover shall be as follows:

Equation 1

Design of Tunnel Plugs for Hydropower Projects

 

where:

  • Hr is required rock cover (m);
  • FShy is factor of safety for hydrofracturing;
  • Hw is static water head (m);
  • w is unit weight of water (kN/m³);
  • r is unit weight of rock (kN/m³); and
  • is slope angle of the mountain.

This equation will provide a factor of safety of 1.3 against uplift or hydraulic jacking. Checks using this equation should consider both lateral and vertical cover and make sure there are no major interconnected discontinuities or significant deformable zones near the plug.

In practice, the hydrofracturing check for the plug can be omitted in many cases. These include when the plug is located in the abutment beneath the dam (hydrofracturing will not occur) and when the plug is located in an access adit at the junction with the power tunnel (the hydrofracturing check was performed during design of the tunnel). When in doubt, the hydrofracturing check should be performed.

Determination of plug length

Determining the proper plug length is an essential part of the design work. It is unanimously accepted that the static load from water pressure can be considered as only load for the plug design. The plug should be designed for the maximum reservoir level, or the full supply level plus water hammer, depending on which is critical.

A safety factor should be designated to cover any imperfections and uncertainties in the acquisition of parameters, in construction and other rare situations. Water pressure acting on the plug, depending on the project, varies from low head (10 m) to very high head (above 1,000 m). Other actions — such as rock stresses, groundwater pressure and grouting pressure — are insignificant. Earthquake loading — even in the event of a major earthquake — is not critical for plugs deep underground of 30 to 50 m or more because the acceleration is significantly lessened and the required factor of safety is low for the transient conditions. However, seismic loads could become important if the plugs are located near the tunnel inlet or outlet in a strong seismic region. In addition, water hammer caused by major earthquakes should be checked.

 Design of Tunnel Plugs for Hydropower Projects

Plugs should be so designed that their lengths satisfy the criteria described earlier. There are two primary categories of principals to determine the length of the tunnel plugs, namely:

  • Punching shear failure mode, in which the plug should have sufficient structural capacity to carry the static load from the water pressure; and
  • Hydraulic failure mode, in which the plug should satisfy the requirements in terms of hydraulic gradient within concrete, at the concrete/rock interface, and through the rock mass.

In the past, many empirical formulas were used to determine the plug length:

  • Should not be less than the excavated diameter of the tunnel.2 If the tunnel is not circular, the equivalent diameter Deq should be used;
  • For low head projects, should be equal to 2 to 2.5 times the excavated diameter of the tunnel used;3
  • Should be equal to 2% to 5% times the static water head for high head projects;4 and
  • The required plug length Lreq = mHwDeq, where the coefficient m=0.0125 to 0.02.

Nevertheless, the following practice is becoming more prevalent in today’s designs. First, candidate lengths are calculated using the methods described below. Then, the candidate with the largest length is adopted as the final plug length. It should be noted that the finite element method is not deemed necessary to be used in the determination of plug length because of its complexity.

Hydraulic gradient

Concrete plugs are designed to provide a barrier to axial water flow. This function requires that the plug has a low hydraulic permeability. The interface between the plug and rock should be sufficiently sealed. Unfavorably oriented geological structures are avoided at the location for the plug or sealed by grouting. The maximum linear hydraulic gradient along the plug axis — that is, the ratio of water head to plug length — can be considered as a measure of leakage. Higher gradients may lead to unacceptably high leakage, piping or downstream erosion. The values can be used to determine the required plug length.

For an individual tunnel plug, the required plug length is then determined using the following formula:

Equation 2

 Design of Tunnel Plugs for Hydropower Projects

 

Regrettably, not much attention has been paid to the hydraulic gradient requirements in the design of tunnel plugs. Thus it is strongly suggested to take into account this criterion in the design.

Shear friction along concrete/rock interfaces

The shear friction against sliding along the concrete/rock interface is a commonly accepted criterion for determining the plug length. The required length is derived on the basis of the primary principal Vn ≥ FSsf • Vu, in which Vn is the shear-friction resistance of the concrete-rock interface, FSsf is a factor of safety against sliding, and Vu is the water load acting on the plug.

The shear friction resistance includes two primary contributions: frictional resistance and cohesion. The frictional resistance depends on the effective weight of the concrete plug. Because the weight of the concrete plug is comparably small, it is usually neglected and taken as a safety margin. Thus the equation becomes:

Equation 3

 Design of Tunnel Plugs for Hydropower Projects

 

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. The author recommends the use of values specified 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

The punching shear of the plug through concrete or rock mass at the plug circumferential area is another frequently used criterion for the determination of plug length:

Equation 4

 Design of Tunnel Plugs for Hydropower Projects

 

where:

  • FSsh is factor of safety against shear failure;
  • [] is design shear strength of concrete or rock mass (kN/m²), whichever is less; and
  • Peff is effective perimeter of the plug at circumferential area in unit length (m²).

The 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. The project-specific shear strength of rock mass obtained on the basis of field and laboratory tests is recommended to be used. In the absence of project-specific data, shear strength may be used. The effective perimeter of the plug depends on the construction quality, usually Peff =(0.80 ~ 1.0)P.

Grouting

Grouting the surrounding rock mass and concrete/rock interface is a primary step for concrete plug construction. The 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 filling fissures 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. The ring curtain grout fan should be extended to the grout curtain at the dam’s foundation, forming a seepage cutoff to effectively increase the length of seepage path around the plug. This also decreases seepage passing through the rock mass.

Consolidation grouting

In poor rock mass with a high permeability, consolidation grouting should be performed to improve the bearing rock and to reduce seepage. The necessity, extent and details of the consolidation grouting should be individually determined according to the local geological conditions. Consolidation grouting should be conducted either prior to the placement of the plug concrete or after the concrete gains the required strength. The latter is preferred, if the construction schedule permits.

Contract grouting and backfilling grouting

A backfilling grouting system containing feed and return pipes, vent headers and outlets should be installed prior to placing plug concrete. Water stops should be installed at the upstream and downstream end of the plug. Through the pre-installed grouting system, the cavities in the overt area can be backfilled with a grout pressure of 3 to 5 bars at three to seven days after placing the plug concrete. It should be noted that backfilling grouting is not performed at many projects, while the cavities and voids in the overt area are filled during the contact grouting.

 Design of Tunnel Plugs for Hydropower Projects

Regardless, the concrete plug should be contact-grouted thoroughly to ensure full contact at the concrete/rock interface. For this purpose, holes of 50 mm to 76 mm in diameter should be drilled 30 cm to 50 cm deep into rock in the crown portion. The pipes of contract grouting should be inserted into the holes and introduced into the gallery or tunnel downstream. The contact grouting should be performed prior to the initial reservoir impounding as the temperature of the plug concrete drops to a stable temperature, but at least 1.5 months after concrete placement when a post-cooling system is not installed. To reduce the lapsed time for dropping the concrete’s temperature, the installation of a post-cooling system is strongly suggested.

For curtain grouting and consolidation grouting, fan holes are required to be drilled into the rock mass through concrete. The grout holes should be staggered. In addition, holes should be drilled at the downstream end of the plug to ensure that any seepage that bypasses the plug is released to the tunnel in a controlled manner.

Temperature control

Because the plug concrete belongs to mass concrete, temperature control during concrete placement is necessary to determine the time when the contact grouting commences. Due to the outlines of the plug and its boundary conditions, no section-through crack may occur in transverse and longitudinal directions as demonstrated by construction practices in normal conditions. Like other mass concrete structures, the temperature control measures for the plug concrete usually include:

  • Using low-heat cement and/or shrinkage-compensating concrete;
  • Reducing the cement content;
  • Using fly ash or other pozzolan;
  • Lowering the placement temperature by pre-cooling;
  • Placing concrete in low temperature seasons; and
  • Post-cooling using cooling pipes embedded into the concrete.

It should be noted that the post-cooling system, as illustrated in Figure 3, is not used in most plugs, although it is preferable — especially if the pre-cooling of concrete is not carried out. Precautions should be taken due to the fact that concrete strength at an early stage will be decreased by using fly ash or other pozzolan, which may influence the construction schedule. Thermometers can be installed into the plug concrete to monitor temperature variations.

Shrinkage-compensating concrete for tunnel plugs

The application of shrinkage-compensating concrete has become increasingly more common in the construction of concrete plugs during recent years. This type of concrete is made with an expansive cement or expansive component system, which creates an autogenous volume expansion of concrete to compensate for possible shrinkage while cooling.

In this regard, magnesium oxide-based concrete is to be highlighted.7 MgO concrete is made by adding lightly burnt MgO powder to the concrete mix, which exhibits some autogenous volume expansion that can effectively compensate for volume shrinkage while temperatures drop, and for the autogenous volume shrinkage of Portland cement itself.

With this property, the risk of gap formation at the overt of the tunnel plug is decreased or even avoided, so that the bonding of concrete to the surrounding rock and the seepage resistance is improved. As a result, the temperature control can be simplified, and post-cooling may not be necessary. Therefore, it is highly advisable to construct tunnel plugs using the shrinkage-compensating concrete.

Concluding remarks

The criteria for the design and construction of tunnel plugs are summarized. In determination of the plug length, the method and formulas provided in this article should be used. Grouting the surrounding rock and concrete/rock interface as well as temperature control for the plug concrete construction are emphasized.

Notes

1Benson, R.P., “Design of Unlined and Lined Pressure Tunnels,” Tunnelling and Underground Space Technology, Volume 4, No. 2, 1989, pages 155-170.

2“Design and Construction of Tunnel. Plugs — Code of Practice. (First Revision),” Bureau of Indian Standards, 2004.

3Gan, W.X., “Design of plugs for hydraulic tunnels” (in Chinese), Yangtze River, Volume 32, No. 5, 2001, pages 34-36.

4Bergh-Christensen, J., E. Broch, and A. Ravlo, “Norwegian high pressure concrete plugs,” in Norwegian Hydropower Tunnelling II, Norwegian Tunnelling Society Publication No. 22, 2013.

5Benson, R.P., “Design of Unlined and Lined Pressure Tunnels,” Tunnelling and Underground Space Technology, Volume 4, No.2,1 989, pages 155-170.

6“Gravity Dam Design,” Engineer Manual EM 1110-2-2200, U.S. Army Corps of Engineers, Washington D.C., 1995.

7Du, C., “A Review of magnesium oxide in concrete,” Concrete International (ACI), Volume 27, No. 12, 2005, pages 45-50.


Chongjiang Du is a concrete and dams expert with Lahmeyer International GmbH.

● Peer Reviewed This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.

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