Introduction
With the development of economy, hydropower or water conservancy projects in use do not meet the needs any more. It is necessary to develop new projects, either by new dam building or old dam heightening. However, engineers paid more attention to dam heightening for the reason that the investment is less, and the quantities can be adequately controlled. Above all, the storage volume of reservoir can be substantially increased after the dam is heightened [
1]. There are a lot of projects on dam heightening abroad, such as Kurobe Dam in Japan, Ksob Dam in Canada, Vado Dam in Spain, and dan Mauvoi Dam in Switzerland. However, dam heightening projects in China are few, and most of them are gravity dams or earth dams. The special problems in arch dam heightening depend on the dams’ structural characteristics and heightening methods. These problems and their solutions are worth studying intensively.
There are two methods for arch dam heightening:
1) New concrete is poured on the downstream face of arch dam from toe to crest, and then, the original crest will be heightened to reach the required height. The original downstream of the dam face becomes the joint between new and old concrete dam body.
2) New concrete is poured on the upstream face of dam from heel to crest. The original upstream face becomes the joint between new and old concrete dam body.
The first is used more widely since it can ensure the overall stability of the dam and the normal operation of the reservoir do not need to be interrupted during dam heightening construction [
2].
Key problems and solutions in arch dam heightening
Key problems of arch dam heightening
Compared with gravity dams, arch dams are much thinner, and they are constrained strongly by rock foundation. Therefore, stress controlling on dam surface is the most important problem during the process of arch dam heightening. When the arch dam is been heightening, because of the constraints between new and original dam sections, as well as the temperature difference between new concrete and old one, three problems that are harmful for the dam’s safety may appear. These problems include: stress state getting worse at dam heel, cracking on new added concrete dam surface, and weak bonding between new added concrete and old dam.
Stress state getting worse at dam heel
It has been proven that the maximum tensile stress occurs at dam heel during the operation of arch dam. If the tensile stress increases, the safety of the dam will be affected directly. The first factor that causes the tensile stress increase at the dam heel is the water pressure, which is increased by reservoir level rising after dam heightening. The second is temperature changing. It is caused by hydration heat of new concrete. The fresh concrete temperature may increase in the early stage of heightening and fall to a quasi-stationary state during operation period. The new dam sections at the downstream face of the dam have a contraction when the concrete cools down, which may make the tensile stress on the upstream face of the dam increase.
Cracking on fresh concrete surface
The new section of the dam is constrained strongly by the original section and rock foundation, as shown in Fig. 1. When the dam is contracting, cracks may appear on the downstream face.
Weak bonding between fresh and old concretes
As it is known that the elastic modulus and the rigidity of new concrete are different from old one, therefore, the deformation of new and old sections may not keep in step when the boundary conditions changes. The dislocation of new and old section of dam can be easily caused by high normal tensile stress and tangential shearing stress.
The high self-weight of new concrete section is liable to lead to the tangential shear stress out of limits.
The drop of reservoir level during dam operation would cause the rebound deformation of dam, and high normal tensile stress may occur on joint surface.
The boundary temperature’s periodic change, which may cause temperature deformation along with the dam slope and axis, is the main reason of high normal tensile stress. For instance, the boundary temperature declining in winter and the normal tensile stress may occur on the edge of joint surface, as shown in Fig. 2. Vice versa, in summer, the normal tensile stress may appear in the middle of joint surface for the expand distortion of new dam section affected by boundary temperature rising, which is shown in Fig. 3.
Solutions to arch dam heightening
Structural measures
Structural measures mainly refer to setting contraction joints in new concrete section in order to limit high stress. In addition, anchorage can be used on joint face to resist normal tensile stress and tangential sheer stress.
Construction measures
Restricting reservoir level and strengthening the combination of fresh and old concrete are two principal construction measures during dam heightening. Limiting the reservoir level is to minimize upstream hydrostatic pressure, which leads to the decrease of tensile stress at dam heel. The joint surface should be treated by chiseling and laying mortar to strengthen the combination in order to improve tensile strength.
Temperature controlling measures
As the stresses of arch dam is affected by the temperature difference between inside dam and outside, it becomes important to take some reasonable measures to control the temperature difference. Reducing the thickness of concrete layer and increasing the interruption period are necessary. The thickness of concrete layer is determined by the structure, formwork erection, and temperature control measures. For the dam concrete, a layer of 2-3 m seems appropriate. Considering the requirements of thermal discharge, crack controlling, and construction operation, the interruption can be 7-9 d for 2-m concrete layer and 9-11 d for 3-m one. The temperature rising caused by hydration heat should be reduced in the process of construction. In addition, surface protection and maintenance should also be paid more attention. It is well known that surface protection is an effective way to limit the cracks of concrete dam.
Analysis for Geba arch heightening with finite element method
Project descriptions
The elevation of the existing dam crest is 1156.0 m, which will be raised by 1.2 m after heightening (see Fig. 4). Then, a concrete parapet wall (1.2 m high) with the crest elevation (1158.4 m) will be built on the new dam crest. The nonspillway sections of the dam are to be thickened by 2 m, with C15 concrete on the downstream side. The original downstream surface will be chiseled to ensure the combination of fresh and the old concrete. The contraction joints are set, with a spacing distance of 15.6 m, which will be grouted after constructing. On the abutments, the dam is embedded in the mountain rock at around 1-2 m.
Three structural measures are taken in the heightening project of Geba arch dam. First, five contraction joints are set in the new dam section, as shown in Fig. 5. Second, a concrete cushion is placed under the new dam section, which is 12 m wide and 3.1 m high. Last, the anchorage with a diameter of 20 mm is set across the joint interface, the length of which is 2 m, and the spacing is 1.5 m.
The two construction measures using in the heightening work are controlling the upstream reservoir level to the dead water level and treating the joint surface by chiseling and laying mortar to improve the jointing strength.
Finally, two measures to control the temperature are attributed to the heightening project: one is to control the thickness of concrete layer as 3 m, and the other is to limit the interruption period as 9 d. Additionally, in order to reduce the temperature gradient between inside and outside the dam, the polystyrene foam boards are adopted as the thermal insulation to cover on the dam’s surface.
Simulation analysis by finite element method
Calculating parameters
By the finite element method, the simulation analysis for Geba arch dam heightening is carried out, and the parameters are listed in Table 1 [
3,
4].
Finite element method model of Geba arch dam
According to the actual configuration and foundation conditions of Geba arch dam, the model for the three-dimensional finite element analysis is designed [
5] (see Figs. 6 and 7). The upstream side, arch abutments, and bedrock are 1.5 times of the height of the dam, as well as the downstream side is 2 times. The total number of elements is 43439, and the nodes are 48948. Simulation calculations are carried out by a finite element method software, and the change of concrete hydration heat and boundary temperature over time are also taken into account. Several kinds of loads, such as the deadweight of the dam, hydrostatic pressure, temperature load, and creep deformation are considered as a result of a high-computational precision.
Key problems
In this paper, with the purpose of verifying the existence of the problems mentioned before and the feasibility and the effectiveness of the solutions, the simulation analysis for Geba arch dam heightening is carried out by the three-dimensional finite element method.
Analysis of stresses at dam heel
Three operation conditions with their load combinations that are simulated are as follows:
1) Operation condition before the dam heightening. The load combination is
Dead weight of structure+ hydrostatic pressure of normal water level+ temperature drop+ uplift+ silt and wave pressure.
2) Operation condition after the dam heightening with normal reservoir water level. The load combination is
Dead weight of structure+ hydrostatic pressure of normal water level+ temperature drop+ uplift+ silt and wave pressure.
3) Operation condition after the dam heightening with dead reservoir water level. The load combination is
Dead weight of structure+ hydrostatic pressure of dead water level+ temperature drop+ uplift+ silt and wave pressure.
Figures 8 to 10 are the contours of first principal stress on upstream face before and after dam heightening (tensile stress is positive) [
6]. It can be seen that the principal tensile stress at dam heel increases after dam heightening. The value of the maximum principal tensile stress is 1.45 MPa if the dam is raised to a normal reservoir water level, but the value of it is only 1.34 MPa when the dam is raised to a dead water level. Therefore, it can be concluded that the tensile stress state at dam heel becomes worse after dam heightening, and the reservoir level controlling can play an important role in reducing the negative effects of stress deterioration during the operation of Geba dam after dam heightening [
7].
Analysis of cracking on fresh concrete surface
The tensile strength of the fresh concrete is low when the concreting has just been completed. At the same moment, a high tensile stress may appear on the surface of new section of the dam, which results from the difference temperature between inside and outside the dam. What is worse is that it can lead to concrete cracks [
8]. It should be noticed that the low boundary temperature at the time 300 d later from the beginning of the dam heightening is the main cause of cracks on the new section of the dam.
It is necessary for the project to adopt the polystyrene foam board as insulating cover on the dam face after the dam heightening and before winter. In Figs. 11 and 12, the maximum principal tensile stress on the new dam surface is 0.67 MPa once the cast finished, and its value is 0.25 MPa at the time 300 d later from the beginning of dam heightening. The tensile stress does not exceed the C15 concrete’ tensile strength. The cracks may not occur anymore. From the results, it has been proven obviously that the reasonable methods on temperature controlling during project construction may help prevent the surface of the new dam from cracking.
Analysis of bonding between fresh and old concrete
The high tangential shearing stress and normal tensile stress may directly lead to the opening of joint interface between the old and new concrete [
9]. Two reasons of this appearance are presented as follows:
The vertical shearing stress on the joint surface is due to the dead weight of the new dam section. The method of setting concrete cushion in the restrained areas is adopted in the project. According to that in Fig. 13, the maximum tangential shearing stress is only 0.12 MPa that appears on the top of the cushion and does not exceed the shearing strength of C15 concrete. Therefore, a part of the new dam’s dead weight can be supported by the concrete cushion, and some negative effects by geological condition can also be reduced drastically by the cushion.
The opening of joint face between the old and new concrete is result from high normal tensile stress. The solution is the anchor rod with a diameter of 20 mm.
In Figs. 14 and 15, the maximum normal tensile stress appearing at the edge of joint face is 0.2 MPa at the moment of heightening finished, and it is only 0.16 MPa at 300 d later from the beginning of dam heightening. According to this result, the maximum normal tensile stress is 0.2 MPa in the course of the dam heightening, and it is shown adequately that the bearing capacity of the anchor rod can meet the tensile requirement of the joint face completely.
Conclusions
Three problems affecting the dam’s safety during the arch dam heightening include stress degeneration at the dam heel, cracking on the new concrete surface, and weak bonding between the fresh and old concrete. The problems can be solved in three ways: structural measures, construction measures, and temperature control measures. Simulation analysis of Geba arch dam heightening with 3D finite element method has proven the effectiveness of the reasonable engineering measures to solve the problems in arch dam heightening. Two conclusions are summarized as follows:
1) Three key problems do exist in the process of arch dam heightening and the effects are obvious.
2) For the dam heightening, it is significant to adopt reasonable measures for reducing the negative effects caused by those key problems. It is proven by analytical results that the solutions in Geba project are play an important role in limiting adverse effects, which can also be used as reference by some other similar projects.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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