Keyword

1 General Introduction

Karuma Hydropower Station is located at the Kyoga Nile River in Kiryandongo District in Uganda. It consists of such buildings as the gate dam, water conveyance system, underground powerhouse and tailrace tunnel [5]. The maximum height of planning head hub dam gate is about 20 m and the normal storage level is 1030 m. The powerhouse dimension is 226.5 × 21 × 56.5 m (length × width × height) and the length of main tailrace tunnel is about 8.2 km. The preliminary planning installed capacity is 600 MW [2, 3].

Ventilation shaft 2# of Karuma Hydropower Station is located at the ventilation and safety tunnel (EVT), MAT, adit-1, adit-2, ventilation tunnel 2-1, ventilation 2-2 and ventilation 2-3. The excavation diameter of shaft 2# is successively 6.1 m, tunnel section 6.2 × 6.1 m [4].

Speculating according to the latest geology data, the ventilation shaft 2# is in the environment of Class II rock and the surrounding rock of ventilation tunnel 2-1 is in the environmental of Class III [1].

2 Calculation Illustration

2.1 Calculation Software

During calculation, the body of EVT, ventilation shaft and ventilation tunnel, and the bottom intersection are computed by three-dimension finite difference method.

2.2 Calculation Model

The bottom intersection of shaft adopts three-dimension calculation model, as is shown in Fig. 1. The ventilation bottom center is set as the model base point. The longitudinal and horizontal length of model are 70 m and 40 m, respectively. The grid element of calculation model is shown in Fig. 2.

Fig. 1
A three-dimensional model has a vertical cuboidal structure with three outlets. Another model alongside exhibits the internal connection of the outlets.

Calculation model

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Fig. 2
A three-dimensional model of a cylindrical shape with extended parts at the bottom, in the middle, and at the top.

The grid element of calculation model

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2.3 Material Property

Model geology data: the basic geology data and detailed rock parameter is listed in the Table 1.

Table 1 Physical mechanics parameter values of rock
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3 Calculation Results and Analysis

Intersection of EVT- ventilation tunnel 2-1 and ventilation shaft 2 is located at the stratum with class III rock. After excavation of the intersection of EVT- ventilation tunnel 2-1 and ventilation shaft 2, maximum displacement of 7.39 mm occurs at the intersection of ventilation shaft 2 and ventilation tunnel 2-2. Along the ventilation tunnel 2-1, it can be seen that after excavation, the maximum displacement occurs at the junction of ventilation shaft and ventilation tunnel 2-1, up to 4.53 mm (Fig. 3).

Fig. 3
A set of 5 contour plots labeled from a to e represents the zonal displacement magnitude across the following sections. a, along the ventilation tunnel 2 1. b, along the ventilation tunnel 2 2. c, Outer surface of E V T. d, Outer surface of ventilation tunnel 2 2. e, Outer surface of ventilation tunnel 2 3.figure 3

Displacement cloud picture of the bottom intersection of ventilation shaft

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After excavation, the range of plastic zone is large, and mainly concentrated in the left side and upper right side of EVT. The necessary supporting measures is applied through small advance pre-grouting duct, grouted anchor bar and so on. Meanwhile, the plastic zone of both side of shaft 2 is visible, as shown in Fig. 4.

Fig. 4
An illustration consists of a rectangle with a profile cut across the section. It represents different zone groups with different shades of colors.

Plastic zone cloud picture along the ventilation tunnel 2-1

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4 Conclusion

Rock stability of intersection of EVT- ventilation tunnel 2-1 and ventilation shaft 2# is analyzed through FLAC3D on the base of actual geology parameter and test value of crustal stress. The following conclusion is drawn:

  1. (1)

    After excavation, the maximum displacement 4.53 mm occurs at the upper-right junction of ventilation shaft and ventilation tunnel 2-1, from along the ventilation tunnel 2-1 perspective. The displacement value is related to crustal stress, physical mechanical parameters.

  2. (2)

    The plastic zone mainly concentrated in the left side and upper-right side of EVT.

  3. (3)

    Based on calculation results, without system support and local shotcrete with locking bolt at the intersection of EVT- Ventilation Tunnel 2-1 and Ventilation Shaft can satisfy the requirement of design.