Optimization of Prestressed Anchor Cable Layout for Left Pier of Jinchuan Hydropower Station

With the increase of the flood discharge capacity of the flood discharge structure, the size of the flood discharge orifice is widened, which leads to the increasing thrust of the radial gate. A large tensile stress zone will be formed at the connection between the anchor block and the pier, which may lead to concrete cracking. The optimization design of the prestressed anchor cable of the pier has always been a concern of the engineering community. In this paper, the left pier of Jinchuan Hydropower Station is taken as the research object, and the finite element simulation model is established. The layout position of prestressed anchor cable is optimized, and different layout schemes of prestressed anchor cable on the left pier are designed and calculated. The results show that when the amount of main anchor cable is the same, it should be arranged outside the pier as far as possible; under the condition that the number of main anchor cables on the inner side of the pier is constant, when the direction of the main anchor cable force is consistent with the direction of the radial gate thrust, the main anchor cable prestress plays the greatest role; the arrangement of balance cable has no obvious effect on the tension of concrete at pier neck. It can provide reference for structural design.


Introduction
Prestressed anchor cable technology after decades of development has become relatively mature, anchor cable technology can enhance the strength and stability of the soil itself, and can reduce the volume and weight of the structure, improve economic efficiency and safety, especially in the slope reinforcement [1-2] achieved good results.With the development of water conservancy and hydropower technology, the increase of the sluice gate hole and the increase of the water head, the requirements for the safe operation of the pier are getting higher and higher, thus the construction technology of prestressed pier anchor cable [3].The function optimization of the prestressed anchor cable of the pier has become the research direction of many scholars.Zhang W [4] improved the efficiency of the anchor cable by optimizing the tensioning process and construction method; Lu K [5] optimized the prestressed anchor cable according to the dip angle of anchor cable, bond diameter and prestress, and improved the safety factor of slope.Mao J L [6] and Ren G Q [7] studied the prestressed anchor cable of gate pier of Laos Nantun 1 Hydropower Station and Tongzilin Hydropower Station respectively, and met the engineering requirements by optimizing the construction technology according to the actual situation.Based on the engineering background of Jinchuan Hydropower Station, this paper designs and calculates different layout schemes of prestressed anchor cables on the left pier of spillway weir sluice section, which can provide reference for structural design.

Engineering Overview
The Jinchuan Hydropower Station is located in the upper reaches of the Dadu River in Jinchuan County, Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province.The dam site is about 13 km from the downstream Jinchuan County.The Jinchuan Hydropower Station is developed by dam type, the dam height is more than 100 m.
Spillway weir gate pier prestressed anchor arrangement is divided into primary and secondary anchor, fan-shaped arrangement.The main anchor cables of the left pier are arranged in 3 rows, 7 bundles outside, 5 bundles inside, and 3 bundles of balanced anchor cables, as shown in Figure 1.Main anchor rope adopts OVM15-27 anchorage system, single beam locking tonnage 4600kN, permanent tonnage 3900kN.Pulling anchor ratio is 3900×12/21800 = 2.15 ( without considering the balance anchor cable ).The thrust of a single arc door hinge is shown in table 1

Finite Element Model and Calculation Conditions
The left pier is simulated and analyzed by ABAQUS finite element software.The left pier is divided into 88,332 hexahedral 8-node elements, as shown in Figure 2.
Figure 2. Finite element model.Three typical working conditions are selected, among which condition 1 is the normal operation condition, without considering the effect of prestressed anchor cable ( normal water storage level, normal closing of arc gate to retain water ); condition 2 is the normal operating conditions, considering the role of prestressed anchor cable ( prestressed permanent tonnage, normal water level, the arc door is normally closed water ); condition 3 is the moment of lifting the door ( normal water level, the arc door has just opened the flood discharge ).
The maximum tensile stress of the pier neck concrete along the direction of the main anchor cable under various working conditions is shown in Table 2.It can be seen from Table 2 that under working condition 1, the maximum tensile stress of the pier neck along the direction of the main anchor cable is 5.3 MPa.Under working condition 2, the tensile stress of this part is 0.7 MPa, indicating that after the prestressed anchor cable is used, the stress of the pier neck is greatly reduced, and the prestress effect is good.Under working condition 3, the tensile stress of this part is 1.4 MPa, which is a dangerous working condition.Therefore, this working condition is selected to optimize the different arrangement of anchor cables.

Study on the Influence of Different Arrangement Positions of Main Anchor Cable
According to the different arrangement positions of the main anchor cable, 14 schemes are designed, as shown in Figure 3 and Table 3. Taking the working condition 3 as the research object, the finite element simulation analysis is carried out.The maximum tensile stress of the concrete in the neck of the pier along the direction of the main anchor cable under each scheme is sorted out as shown in Table 4, and the stress change curve of the part under scheme 4 and scheme 5-14 is drawn as shown in Figure 4.
From Table 4 and Figure 4, it can be seen that the maximum principal tensile stress of the lower neck of Scheme 1 is 0.717 MPa, and the maximum principal tensile stress of the lower neck of Scheme 2 is 0.987 MPa.Compared with the original scheme, the stress of the pier neck decreases significantly, but due to the increase of the amount of main anchor cable, the two schemes are not economical.The maximum tensile stress of this part under scheme 3 is 1.336 MPa, and the maximum tensile stress of this part under scheme 4 is 1.243 MPa.Comparing the two schemes, when the amount of main anchor cable is the same, it should be arranged as far as possible on the outside.Compared with other layout schemes, the tensile stress at the neck of the pier under scheme 4 is the smallest, indicating that the inner main anchor cable should be arranged downward as far as possible when the number of main anchor cables is constant, which is mainly determined by the thrust direction of the radial gate.When the direction of the main anchor cable force is consistent with the direction of the radial gate thrust, the main anchor cable prestress plays the greatest role.

Study on the Influence of Different Layout Schemes of Balance Cable on the Stress of Pier Neck
In order to study the influence of different arrangement schemes of balance cable on the stress of pier neck, four schemes are designed respectively, as shown in Fig. 5.The maximum tensile stress of pier neck under each scheme is calculated, as shown in Table 5.It can be seen from Table 5 that when the arrangement of the balance cable is different, the maximum tensile stress of the concrete at the pier neck along the direction of the main anchor cable does not change much, indicating that the arrangement of the balance cable has no obvious effect on the concrete tension at this part.

Conclusion
In this paper, the left pier of Jinchuan hydropower station is taken as the research object, and the finite element simulation model is established.The layout position of prestressed anchor cable is optimized.
The following conclusions can be obtained: (1) After the use of prestressed anchor cable, pier neck stress decreased significantly, prestressed effect is good; (2) When the amount of main anchor cable is the same, it should be arranged outside the pier as far as possible; (3) Under the condition that the number of the main anchor cable in the inner side of the pier is constant, when the direction of the main anchor cable force is consistent with the direction of the gate thrust, the main anchor cable prestress plays the greatest role; (4) The arrangement of balance cable has no obvious effect on the tension of pier neck concrete.

Acknowledgments
Many thanks to my teachers and students for their help with this article and for encouraging me, yes I was able to write this article.

Figure 5 .
Figure 5. Diagram of different arrangement of balance cable.Table5.The maximum tensile stress table unit of the left pier neck concrete along the main anchor cable direction under each calculation scheme.unit : MPa

Table 2 .
Maximum tensile stress of neck concrete along the direction of main cable.unit : MPa

Table 4 .
Maximum tensile stress of left pier neck concrete along the direction of main cable.

Table 5 .
The maximum tensile stress table unit of the left pier neck concrete along the main anchor cable direction under each calculation scheme.