**4. Connection techniques**

A reliable connection between the seepage control components within a rockfill dam and its overburden foundation is a prerequisite for a successful impervious system. Connection zones are weak places that require special design considerations. In this section, connection techniques used in different types of rockfill dams are briefly introduced.

#### **4.1. Connection to clay core**

For earth core rockfill dams, two basic design schemes can be used to connect the cutoff wall with the clay core. The simplest one is to insert the top of the cutoff wall directly into the clay core for a specified depth. The depth can be determined by the allowable hydraulic gradient along the interface between the cutoff wall and the surrounding soil. Inadequate inserting depth may result in seepage erosion along the contacting path. To avoid shear failure and cracks in the clay core adjacent to the inserting points, a zone of highly plastic clay is used to encapsulate the top of the cutoff wall, as shown in **Figure 8(a)**. Highly plastic clay is more deformable than the clay core, and it can absorb incompatible deformation between the cutoff wall and the core wall without sacrificing its impermeable performance, even under large shear strains.

high compressive stresses within the cutoff wall, but it removes the need for a complicated

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

http://dx.doi.org/10.5772/intechopen.78547

15

An asphalt core is a thin plate structure similar to a concrete cutoff wall. A connection between the two structures is often accomplished using a concrete base built on the top of the cutoff wall, as shown in **Figure 9(a)**. The location of the concerned section can be seen in **Figure 4**. An inverted trapezoidal cap is used to accommodate the enlarged foot of the asphalt core wall, with mastic asphalt used to ensure the cementation between the cap and the core wall. The top surface of the base is usually curved slightly downwards to ensure that the asphalt core does not spread. A gallery can also be incorporated into the concrete base for inspection, grouting and communication. Asphalt core has currently not been used in rockfill dams higher than 150 m in China, and the connection with concrete cutoff wall is usually simpler

Another distinct feature of the reviewed Yele ACRD is the use of two layers of concrete cutoff walls that are connected by a construction gallery in the right bank, as shown in **Figure 9(b)**. The upper cutoff wall was constructed on the ground, while the lower one in the gallery (6.0 × 6.5 m). Curtain grouting was also finished in this gallery. To connect the upper cutoff wall with the top of the gallery, joint curtain was constructed using a non-circulation descending stage grouting. Both cement and chemical slurries were pumped into the jointing soils under a maximum pressure of 4.5 MPa. Grouting operation did not cease until the perme-

If the toe plinth of a concrete face rockfill dam is built on overburden layers and a concrete wall is used to cut off the underground seepage, then a reliable connection between the face slabs

**Figure 9.** Connection techniques in the Yele ACRD. (a) Section A-A. (b) Section B-B.

water stop structure between the cutoff wall and the gallery floor.

**4.2. Connection to asphalt core**

than that in ECRDs as described above.

ability of the curtain reached *q* < 5 Lu.

**4.3. Connection to concrete face slabs**

The second connection method is to use a concrete gallery on the top of the cutoff wall, as shown in **Figure 8(b)** and **(c)**. Using a gallery near the base of the dam has several advantages. Curtain grouting can be performed within the gallery at the same time of dam filling, which may considerably shorten the construction time. Second, the gallery provides a possibility to enforce the foundation impervious system in the case that it does not function well. Without a gallery, repairing the underground seepage control component will be extremely difficult, if not impossible. The gallery can also be used to monitor the performance of the dam and it also provides a path to connect the left and right bank slopes. In the Changheba and Pubugou ECRDs, two concrete cutoff walls are used, one inserting into the clay core and the other connected with a gallery enclosed by highly plastic clay zones.

Careful designing, however, should be exercised when using a gallery. On the one hand, no structural joints are used in general for the riverbed monolith, and the gallery is vulnerable to cracks because of uneven settlement of the overburden layers. The gallery is usually extended into the rock banks, and the connection places of riverbed and rock segments often suffer large shear deformation and damage of water stops. On the other hand, connection of the concrete cutoff wall and the floor of the gallery also require special design considerations. In current practice in China, a rigid connection is mostly wide used, where the top of the concrete wall is reinforced and cast together with the floor of the gallery using an inverted trapezoidal transition cap, as shown in **Figure 8(b)** and **(c)**. A rigid connection may result in

**Figure 8.** Connection techniques for ECRD. (a) Xiaolangdi ECRD. (b) Changheba ECRD. (c) Luding ECRD.

high compressive stresses within the cutoff wall, but it removes the need for a complicated water stop structure between the cutoff wall and the gallery floor.

#### **4.2. Connection to asphalt core**

**4.1. Connection to clay core**

14 Dam Engineering

For earth core rockfill dams, two basic design schemes can be used to connect the cutoff wall with the clay core. The simplest one is to insert the top of the cutoff wall directly into the clay core for a specified depth. The depth can be determined by the allowable hydraulic gradient along the interface between the cutoff wall and the surrounding soil. Inadequate inserting depth may result in seepage erosion along the contacting path. To avoid shear failure and cracks in the clay core adjacent to the inserting points, a zone of highly plastic clay is used to encapsulate the top of the cutoff wall, as shown in **Figure 8(a)**. Highly plastic clay is more deformable than the clay core, and it can absorb incompatible deformation between the cutoff wall and the core wall without

The second connection method is to use a concrete gallery on the top of the cutoff wall, as shown in **Figure 8(b)** and **(c)**. Using a gallery near the base of the dam has several advantages. Curtain grouting can be performed within the gallery at the same time of dam filling, which may considerably shorten the construction time. Second, the gallery provides a possibility to enforce the foundation impervious system in the case that it does not function well. Without a gallery, repairing the underground seepage control component will be extremely difficult, if not impossible. The gallery can also be used to monitor the performance of the dam and it also provides a path to connect the left and right bank slopes. In the Changheba and Pubugou ECRDs, two concrete cutoff walls are used, one inserting into the clay core and the other con-

Careful designing, however, should be exercised when using a gallery. On the one hand, no structural joints are used in general for the riverbed monolith, and the gallery is vulnerable to cracks because of uneven settlement of the overburden layers. The gallery is usually extended into the rock banks, and the connection places of riverbed and rock segments often suffer large shear deformation and damage of water stops. On the other hand, connection of the concrete cutoff wall and the floor of the gallery also require special design considerations. In current practice in China, a rigid connection is mostly wide used, where the top of the concrete wall is reinforced and cast together with the floor of the gallery using an inverted trapezoidal transition cap, as shown in **Figure 8(b)** and **(c)**. A rigid connection may result in

**Figure 8.** Connection techniques for ECRD. (a) Xiaolangdi ECRD. (b) Changheba ECRD. (c) Luding ECRD.

sacrificing its impermeable performance, even under large shear strains.

nected with a gallery enclosed by highly plastic clay zones.

An asphalt core is a thin plate structure similar to a concrete cutoff wall. A connection between the two structures is often accomplished using a concrete base built on the top of the cutoff wall, as shown in **Figure 9(a)**. The location of the concerned section can be seen in **Figure 4**. An inverted trapezoidal cap is used to accommodate the enlarged foot of the asphalt core wall, with mastic asphalt used to ensure the cementation between the cap and the core wall. The top surface of the base is usually curved slightly downwards to ensure that the asphalt core does not spread. A gallery can also be incorporated into the concrete base for inspection, grouting and communication. Asphalt core has currently not been used in rockfill dams higher than 150 m in China, and the connection with concrete cutoff wall is usually simpler than that in ECRDs as described above.

Another distinct feature of the reviewed Yele ACRD is the use of two layers of concrete cutoff walls that are connected by a construction gallery in the right bank, as shown in **Figure 9(b)**. The upper cutoff wall was constructed on the ground, while the lower one in the gallery (6.0 × 6.5 m). Curtain grouting was also finished in this gallery. To connect the upper cutoff wall with the top of the gallery, joint curtain was constructed using a non-circulation descending stage grouting. Both cement and chemical slurries were pumped into the jointing soils under a maximum pressure of 4.5 MPa. Grouting operation did not cease until the permeability of the curtain reached *q* < 5 Lu.
