STRUCTURAL STRESS AND STRAIN ANALYSIS OF THE ORIFICE TUNNEL AFTER WET TEST XIAOLANGDI MULTIPURPOSE DAM PROJECT  

 Xinhua Hou   Lynn Wang

The Yellow River Water Hydroelectricity Development Company

Jiyuan City

Henan Province

China   454681

TEL: 86-0379-3905466   FAX: 86-0379-3905395

E-mail: Xeccid@Public2.Lyptt.Ha.Cn   

Abstract: This paper describes the structural stress and strain observations regarding the orifice tunnel wet test, Xiaolangdi multipurpose dam project. The impact was analyzed based on the data obtained. The test data provided a scientific basis for improving the orifice dissipation technology and also accumulated invaluable experience for future projects. 

Keywords: orifice tunnel, wet test stress strain observation

1    INTRODUCTION

The Orifice Tunnels in Xiaolangdi Multipurpose Dam Project were converted from diversion tunnels, the tunnel diameter being 14.5 m and the orifice ring diameter 10m. In the Mid Gate Chamber of the Orifice Tunnel, two eccentric articulated arc gates are installed. The maximum operating water head of the gate is 140 m, and the flow velocity near the out let of the gate is as high as 33-35 m/s. The theory of energy dissipation is that multiple stage orifice rings are constructed in the tunnel. When water flows through the orifice ring, sudden expansion makes the water turbulent, shear friction and collision occur inside the water, dissipation is realized by converting the kinetic energy to heat energy. In the 1980’s, the design institute invited concerned scientific research institutes both from home and abroad to carry out a large number of hydraulic and structural model tests and computation analysis. Orifice dissipation tests were also conducted in the sediment tunnel of Bikou Hydropower Project in Sichuan Province, providing scientific basis for Xiaolangdi Orifice Tunnel design and safe flood discharge evaluation.

The sediment tunnel in Bikou was originally designed as a pressure tunnel with a diameter of 4.4 m. The middle section before the turn was chosen as the test section, sheath lining was adopted, the diameter after lining was 3.8 m. Two stage orifice rings were installed in the tunnel, the inner diameter of the ring is 2.62m, and the distance between the rings is three times the tunnel diameter. The ratio of inner and outer diameter is 0.689, which is the same as that in Xiaolangdi Project. Also, the average flow velocity in the test section of the Orifice Tunnel and at the ring outlet is almost the same as that in Xiaolangdi Project design during the test period in the flood season. The silt content in the water during the flood season is also close to that in Xiaolangdi (See Reference in page 5). In spite of the model test in Bikou, taking the model scale effect into consideration, and to understand the actual dissipation effect and flood discharge safety in Xiaolangdi Orifice Tunnel, a prototype wet test was carried out in Orifice Tunnel No.1 when the reservoir water level reached 201 m. This paper makes the structural stress and strain analysis of the Orifice Tunnel after the wet test.

2    INSTRUMENTS LAYOUT

To learn the behavior of the orifice ring, tunnel lining, surrounding rock deformation and the external water pressure during the water discharge of the Orifice Tunnel, 90 different structural stress and strain instruments were installed in Orifice Tunnel No.1 according to the design. They were installed at the Orifice Ring No.3(0+217.00) , behind Orifice Ring No.3 (0+226), and behind the Mid Gate Chamber(0+398). Observation items include:

(1) Reinforcement bar stress: 26 reinforcement meters were installed in the inner and outer layer, both longitudinal and perpendicular. The meter is made by SINCO Company, USA, consisting of two symmetric VWPR spot weldable strain gauge, ranging –1500～1125 ;

(2) Concrete lining strain: 21 concrete stress meter and 4 zero stress meters were installed in the inner and outer layer, both longitudinal and perpendicular. The meter is made by SINCO Company, USA, VS vibrating wire type, ranging –1125～1125 ;

(3) Surrounding rock deformation: 16 multiple point displacement meters were installed behind Orifice Ring No.3 (0+226) and behind mid gate chamber (0+398) . The meter is made by Huggenberg Company, Switzerland, linear potentiometer type, ranging ±25 mm;

(4) External water pressure: 2 vibrating wire piezometer were installed behind Orifice Ring No.3(0+226), made by SINCO Company.

3    OBSERVATION METHOD AND APPROACH

The data from all the instruments in Orifice Tunnel No.1 were collected automatically through a 2380 MCU (Measurement Control Unit) made by Geomation Company, USA, each MCU is connected through cable, except that the data from the multiple point displacement meter was collected manually because of its low insulation resistance.

To ensure the reliable operation of the MCU, insulation resistance tests were conducted to all the instruments to be connected to the MCU, and the MCU were debugged using the Geonet programmed software. Also to ensure accurate readings and normal operation of the MCU, and to collect as much data as possible, it was decided the collecting frequency would be once every 15 minutes before the test; during the test, once every 2 minutes for the 8 instruments in Orifice Ring No.3, once every 5 minutes for the rest; after the test, the frequency was adjusted to once every 30 minutes and lasted for 12 hours.

4    ORIFICE TUNNEL WET TEST PROCESS

Based on the impounding of the Xiaolangdi Reservoir, the tunnel started filling in the afternoon, April 26, 2000, lasting 12 hours. The official wet test started on April 26, 2000, the whole process is given in Table 1 (See page 5). During the test, the upstream water level in the intake was 210.18 ～210.28 m, and that in the downstream in the outlet was 135.90～134.30 m.

5    ANALYSIS ON DIFFERENT SECTION OBSERVATION RESULTS

(1) Orifice Rings No.3

When opening and closing the gate during the test, the flow status in the Orifice Tunnel changed. The average changes of stress and strain in each reinforcement bar and concrete strain meter are listed in Table 2 and Table 3 (See page 5 and 6).

Regarding the reinforcement bar itself, the maximum compression change amplitude was  5.71 MPa during the test, averaging 4.47 MPa; maximum tension amplitude was 6.45 MPa, averaging 5.23 MPa. The graph (see Figure 1) shows that the stress change of reinforcement bar belonged to low stress change category during the test. The reading of the stress meter reflected that the concrete was in compression mode, The average stress change amplitude of inner and outer layer longitudinal to the tunnel was 0.31 MPa while that perpendicular to the tunnel was 0.48 MPa. Considering that concrete and reinforcement bar deformed simultaneously, the structural stress change reflecting the interior force change by the reinforcement bar was 0.57 MPa. By comprehensive analysis it is concluded that no tensile stresses occurred in the concrete lining during the wet test, thus it is safe for further structural stress change.

(2) Observation Section 0+226 behind the Orifice Ring

During the test, the maximum amplitude of reinforcement bar stress was about 5.12 MPa, the measured value ranging –40～90 kN. Compared with the design tensile and compressible strength, the reinforcement stress change belonged to low stress change category during the wet test. Concrete strain meters indicated that the amplitude of concrete stress change was about 0.48 MPa. Considering that concrete and reinforcement bar deformed simultaneously, the structural stress change reflecting the interior force change by the reinforcement bar was 0.49 MPa maximum. This showed that the structural stress of concrete lining changed little during the test. Stable readings by the multiple point displacement meters indicated that no displacement occurred to the surrounding rock during the test, and the external water pressure remained unchanged.

(3) Observation Section 0+398 behind the Mid Gate Chamber

The maximum reinforcement bar stress change amplitude was 0.89 MPa behind the Mid-gate Chamber 0+398, average amplitude was less than 0.032 MPa, which was minor compared with the design tensile and compressible strength. Almost no stress change occurred to the concrete lining in this section during the test by the stable readings of the strain meters. Considering that concrete and reinforcement bar deformed simultaneously, the structural stress change reflecting the interior force change by the reinforcement bar was 0.047 MPa maximum.

6    CONCLUSIONS

By analyzing the observation results in different sections, when the reservoir water level was below 210 m, the stress change amplitude of concrete in orifice Ring and Orifice Tunnel was less than 0.6 MPa, the maximum reinforcement bar stress change amplitude was 5.71 MPa. These are little higher than those measured in Bikou Hydropower Project. The maximum amplitude of concrete strain meter was 20.6με(about 0.49 MPa ) and the maximum amplitude of reinforcement bar meter was 23.4 kg/cm² (about 2.34 MPa). Compared with the design criteria of this structure, it showed that the stress upon the structure was rather low, and there was no tensile stress inside the concrete. The measured results indicated there is enough safety reserve for the lining structure of the Orifice Tunnels.

In summary, the wet test of Xiaolangdi Orifice Tunnel has profound meaning. On the one hand, it is required for the safe operation of Xiaolangdi Project, while on the other hand, it will improve the technology of orifice dissipation, accumulating experience for future projects. 

Table 1    Filling process during the test

 Table 2    Static change value of reinforcement bar interior stress

Orifice Tunnel No.3        Unit：kN

Table 3    Static change value of concrete lining orifice tunnel No.3

Unit：

Fig. 1    Reinforcement stress change graph during the test

Reference

Xianru Wang. Research Report on the Orifice Dissipation Test in the Sediment Tunnel, Bikou Hydropower Project. Reconnaissance and Plan Design Institute, Yellow  River Water Conservancy Committee, November 1987.