ABSTRACT
In some large and particular water supply projects, the optimum utilization of the resources and materials often causes the project in some parts goes under service before construction is completed. This is usually very common in large dams, to start impounding while the construction of the dam is in progress. Although this will increase the benefit ratio of the project, and make the faster utilization of the project possible, which is of great importance, however, the adverse effect of such action on the safety of the project in some cases should not be neglected.
In large earth dams the pore pressure within the clay core will appear in some stages of filling. After that stage, it starts developing due to increasing the height of the dam. Usually large pore pressure will develop within the core, when more than half of the dam height is constructed. Due to consolidation process, it normally takes a long time the developed pore pressure dissipates. If impounding the dam parallel to the construction process at this stage is done, the excess build up of the pore pressure within the core may occur due to the head of the water in the reservoir and seeping the water in the dam. This in some condition may lead to excess pore pressure in the core, resulting in hydraulic fracturing of the dam.
In this paper, the influence of the impounding of the earth dams on the pore pressure development within the core, while the construction is under progress, is presented. A special and home developed computer software (CA2), has been used to model and analyze the earth dam to determine the pore pressure developing due to both impounding and constructing the dam at the same time. In order to calibrate the numerical model, the same geometric and geotechnical characteristics of the Karkheh large embankment dam, recently constructed in Iran, have been taken into account. Since this dam was subjected to the same condition (simultaneous construction and impounding), the analytical results, of the present study have been compared with instrumentation data of the dam, and some important conclusions have been made accordingly.
Keywords: Impounding, Instrumentation, Pore pressure, Construction, Embankment dam, Clay core, Seepage, Consolidation.
INTRODUCTION
Nowadays, water supply is one of the most important problems in many societies. The limitation of water resources in the world has caused to implement different methods to get maximum efficiency from the existing and new water supply projects. Large dams are usually designed and build to control the surface water for different purposes, mainly water supply. However, it takes a long time to construct and complete all parts of such dams. The time may be doubled if some financial and construction problems happen during constructing the project.
If utilization of such project in a short period is applicable, it may be of great help. What has to be taken into account in this extent are:
The first item can be easily evaluated based on the present level of filling, the induced reservoir volume, the hydrological system of the upstream region, and the rate of construction operation of the dam. considering these factors, the impounding can be controlled in a way that always a safe distance between the level of the filling and reservoir remains. But, the second item needs a special study to be done to investigate the influence of impounding on the dam behavior while the overburden is continuously increasing due to continuation of the filling of the dam.
In embankment dams the progress of filling, after some stages, will cause induction and development of considerable pore pressure within the core of the dam. If prior to completing the construction and relative dissipation of the pore pressure, the impounding starts, the increasing of water level in the reservoir may double the pore pressure in the core, due to seeping the water within the dam. As a results, it may lead to excess pore pressure in the core ending to hydraulic fracturing of the dam. In this respect a special study has been carried out to investigate the effect of simultaneous impounding and construction of an embankment dam, the detail of which is given in the following sections. Since the same process was applied to Karkheh Embankment Dam, the largest storage dam recently constructed in the south-western part of Iran, the specification of this dam has been used in this study. Also the instrumentation records of the Karkheh Dam have been used to verify the results of this study. In the next section the specifications of the selected embankment dam (Karkheh) are described.
THE GENERAL SPECIFICATIONS OF THE KARKHEH LARGE EMBANKMENT DAM
The Karkheh is a large multi purposes embankment dam (32 million cubic meters fill) of 3030 m. length and 127 m. height, which has been designed to store some 7800 million cubic meters of water at its maximum capacity. At the designing stage the upper portion of the core was going to be made of plastic clay, whereas, the lower portion consists of clay in mixture with sand and gravel (mixed clay). Later, it was decided to construct the whole core (two portions), from the mixed clay. The clay originates from mudstone layers in the borrow areas.
The shells of the dam consist of sand and gravel taken from alluvial deposits and from necessary excavations of conglomerates. The outer slopes of the dam are made of 1V:2.25H and 1V:2.5H upstream and downstream shells respectively. The filters act to protect the core from erosion to drain seepage water and to make a transition between the core and the shell. The core slopes 1V:0.25H both sides with the exception of the left abutment where it slopes 1V:0.3H between St. 1+025 and 1+145.
Figure 1. A Typical cross-section of the Karkheh Earth Dam
with the ground profile and position of the installed instruments
A plinth of lean concrete is designed to be placed between the core and foundation. The thickness is varying between 0.3 and 0.5 m. along the dam length. The function of the plinth is to exclude any erosion of fines from the core into the voids and joints of the foundation. A cut-off wall plastic concrete has been designed to control seepage in the foundation. The wall intersects the upper layers of conglomerate and penetrates into the one of the near horizontal mudstone layers. Exception may be at sections where the bottom of the cut-off wall is stepped to reach the next lower mudstone.
An inspection gallery is incorporated in the design between St.0+950 and 2+160 (the valley section of the dam). It is located some 7.5 m. downstream of the cut-off wall and has three accesses, at its ends and St.1+425. The gallery is dewatered by two pumping stations and via the middle access. It covers the diversion culvert below this structure. The inner clearance of the gallery is 3 m. in height and 2.2 m. in width. Such dimensions are sufficient to work inside with drill rigs in case of necessity. A typical cross-section of the Karkheh dam with the positions of installed instruments and mudstone layers in the dam foundation is shown in figure(1).
THE INSTRUMENTATION OF THE KARKHEH DAM
There are 23 instrumented sections in the Karkheh dam, 14 of which were anticipated at the designing stage. In an overall review of the instrumentation arrangements of the dam, after first trial impounding, another 9 sections were added to fill between the gaps and cover the dam important sections. Thus the total instrumented sections were increased to 23 starting from St 0+300, and ending to St. 2+850. The position of the instrumented sections along the dam axis is shown in fig No.(2).
Figure 2. The position of the instrumented sections along the dam axis .
The types of different instruments used in the Karkheh dam are as follows:
There are totally some 929 different instruments used in the Karkheh dam, the distributions of which are as follows:
The distribution of the above instruments in dam different cross-sections are given in Table 1.
Table 1. The Distribution of the Different Instruments Used in Karkheh Dam
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Section | Inclinometer (I) |
Earth pressure cell (PC) |
Stand-pipe piezometer (SP) |
Rock piezo- meter (RP) |
Electrical piezometer (EP) |
Total no. | Total no. | Total no. | Total no. | Total no. |
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1-1 | 1 | 20 | 8 | 2 | 6 |
2-2 | 1 | 20 | 10 | 7 | 8 |
3-3 | 1 | 30 | 11 | 6 | 10 |
4-4 | 3 | 75 | 10 | 4 | 21 |
5-5 | 4 | 75 | 10 | 5 | 27 |
6-6 | 4 | 40 | 11 | 5 | 27 |
7-7 | 4 | 65 | 8 | 2 | 25 |
8-8 | 3 | 30 | 10 | 4 | 19 |
9-9 | 3 | 65 | 11 | 5 | 19 |
10-10 | 2 | 30 | 11 | 4 | 15 |
11-11 | 0 | 20 | 6 | 0 | 4 |
12-12 | 0 | 10 | 4 | 0 | 2 |
13-13 | 0 | 20 | 8 | 2 | 6 |
14-14 | 0 | 10 | 6 | 2 | 2 |
15-15 | 0 | 0 | 2 | 2 | 0 |
16-16 | 0 | 0 | 2 | 4 | 0 |
17-17 | 0 | 0 | 0 | 4 | 0 |
18-18 | 0 | 0 | 0 | 2 | 0 |
19-19 | 0 | 0 | 0 | 2 | 0 |
20-20 | 0 | 0 | 0 | 2 | 0 |
21-21 | 0 | 0 | 0 | 2 | 0 |
22-22 | 0 | 0 | 2 | 2 | 0 |
23-23 | 0 | 0 | 2 | 0 | 0 |
TOTAL | 26 | 510 | 134 | 68 | 191 |
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THE CRITICAL SECTION SELECTED FOR STUDIES
As mentioned before, there are 23 different instrumented sections along the longitudinal axes of the Karkheh Dam. Considering the height of filling, depth of the river,and number of installed instruments, the most critical cross section of the dam is sec. 5-5 (st.1+230), at which the settlements and pore pressure development is maximum comparing with other sections. The section 5-5 with its instruments and the ground layers in the position of its foundation is shown in figure No.(1).
As can be seen the rock piezometers (RP) for monitoring the behavior of cut-off wall are installed on both sides of the wall between mudstone layers(-1) and (-2) and also above mudstone (-1). To measure the pore pressure within the core 10, 6, 4, 5, and 2 vibrating wire piezometers are mounted at levels 107, 135, 165, 185, and 205 (masl) respectively. The total earth pressure at this section are measured in five directions by means of 8 and 7 cluster load cells, installed at levels 107 and 185(masl) respectively. Four inclinometers (2 in downstream shell, 1 in the core and 1 in the upstream shell) are mounted for monitoring the vertical and horizontal displacements of the dam. Finally 5 stand pipe piezometers for doing double check of the pore pressures have been installed at different positions of the section.
EVALUATION OF MEASURED PORE PRESSURES
IN THE SELECTED SECTION
The pore pressures developed within the core in section 5-5 at the lowest level (107), at different filling stage are plotted in figures 3 to 5 according to the records of the piezometers installed in the downstream , central, and upstream zones of the core respectively.
As can bee seen from the figures, pore pressures in all three zones of the core increase as the height of the core increases. This increment in the central zone of the core is quite large and considerable comparing with that in the other two sides of the clay core. The other point has to be mentioned here is the evident changes in piezometric level of the upstream piezometers proportional with the reservoir changes after starting the impounding (ie; the filling level of 202). Whereas the pore pressure in the central zone of the core and to some extent in the downstream zone of the core, do not show this kind of changes and keep increasing with the same rate before impounding as the level of filling increases. This means that the impounding has not yet affected the half down of the core and the pore pressure changes in this part, specially center of the core, is only due to increasing the height of the core as the construction of the dam is in progress. This fact is more evident in figure
6, in which the changes of the pore pressure within the selected section have been plotted at three different stages of construction. In this figure the variation of the pore pressure at the core lowest level (107 masl) are plotted once before impounding (filling level of 187) and twice after impounding (filling levels of 207 and 217). As can be seen, the upstream side of the core has been affected by the impounding while the downstream side has not been affected remarkably.
Figure 3. Pore pressure variations in section 5-5 at Elev.107
for different filling levels in the downstream.
Figure 4. Pore pressure variations in section 5-5 at Elev.107
for different filling levels in the central zone.
Figure 5. Pore pressure variations in section 5-5 at Elev.107
for different filling levels in the upstream.
Figure 6. Pore pressure variations in section 5-5 at Elev.107
for different filling levels within the core.
NUMERICAL ANALYSIS OF THE SELECTED SECTION
To investigate the pore pressure changes within the core of the selected section due to parallel construction and impounding, the computer package of CA2 (Continuum Analysis 2Dimensional) has been used for numerical analysis. The CA2 is a home developed finite difference package capable of modeling the elasto-plastic behavior of the geotechnical structures. It can also be used to do the effective stress analysis and model the unsteady state seepage within the porous media.
A grid of 101x55 has been used as the finite element mesh for the selected section. The number of selected layers was 13 from the foundation level. The thickness of each layer was 10 meters, which has been divided into 4 series of mesh in 2.5 meters thickness. Since the aim of the current study is to model the impounding of an embankment dam while it is under construction, the analysis has to be done in a way that a new and certain level of water in the reservoir has to be taken into account for each filling level. According to the figure no. (7), which shows the construction stages of the Karkheh Dam, it has taken about 100 days to build a layer of 10meters thickness. Therefore, the average field rate of filling can be considered equal to 10 centimeters per each day. In order to apply the consolidation process in the numerical analyses, instead of giving 10 days time for 10 meters layer to be built gradually, it has been postulated that this layer has been built at once under the full overburden in 50 days. Hence in the layer analyses, after constructing each layer, it has been mechanically analyzed to get the pore pressures due to overburden of filling, and then for a period of 50 days, it has been analyzed for both consolidation and mechanical effects simultaneously.
The results of the numerical analyses have been plotted in figures 8 to 11. The development of the pore pressure within the cross-section of the dam (5-5) at the lowest level (107), have been calculated and shown in these figures in different conditions. While in figure 8 and 9 the development of pore pressures merely due to construction are illustrated, in figure 10 and 11, the pore pressure developments due to both construction and impoundment have been plotted and compared separately. It is quite evident that impounding the dam while it is under construction will cause the symmetrical variation of the pore pressures within the core due to overburden pressure of the fillings changes to unsymmetrical shape. It can be seen that the simultaneous impoundment of the dam will cause the pore pressure increases in the upstream zone of the core, while in the central and downstream zones the pore pressures remains unchanged. The more the height of filling increases, the more increase in the pore pressure happens in upstream zone of the core in this condition.
Figure 7. The Dam filling progress with time (1375 until 1380).
Note. The date format is dd/mm/yy and in Iranian calendar 1375 is Roman 1996 and 1380 is Roman 2000
Also note how the century digits (in this case 1300) are also omitted.
COMPARISON BETWEEN ANALYTICAL RESULTS
AND FIELD DATA
The results of the measured pore pressures within the core in the selected section at two different construction stages together with the analytical results are plotted in figures 12 and 13. In figure 12 the pore pressure variation are presented when the filling elevation of the dam is about 187 (masl). At this stage the dam has not been impounded, and the plotted field and analytical data are those developed only due to overburden of the filling. Whereas, in figure 13 the data belong to the stage at which the filling elevation has reached to 217(masl) and the dam has been impounded. For this reason the analytical data are calculated and plotted in this figure for both with and without impounding in this condition.
As can be seen from the figures, the overall trend of the field data and analytical results are the same, although some differences between the exact values are evident. This may be attributed to the limitations of the used package in changing the elasticity modulus and the coefficient of permeability of the soil layers due to increasing the height of dam filling. The other point has to be mentioned is the greater analytical pore pressure developed in upstream side of the core (including the impounding of the dam), compared with those measured by the Piezometers. This may be due to filter effect in connection with unsteady state of the seepage in the first impounding, which takes a long time to be changed to steady state seepage, and it will affect the pore pressure in this zone remarkably.
Figure 8. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.157.
Figure 9. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.187.
Figure 10. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.207.
Figure 11. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.227.
SUMMARY AND CONCLUSIONS
The construction stages of large embankment dams usually takes a long time to be completed. However, impounding the dam while the construction is in progress may be an effective way to shorten this period towards getting some advantages of the project before ending the dam construction. The effect of impounding on pore pressure build up in the core besides the considerable amount of the pore pressure usually develop during construction due to overburden of the filling may be of great concern for this decision making. In this paper the pore pressure development in the most critical cross-section of an embankment dam due to simultaneous impounding and construction of the dam has been studied and investigated.
A home-developed computer package (CA2) has been used to analyze the dam, as well as the field data collected from the largest embankment dam (Karkheh) recently built in southwestern region of Iran. According to the obtained results, the simultaneous impounding and construction does not have any effect on the pore pressure developed at the central zone of the core due to overburden, and it will only increase the pore pressure in the upstream side of the core, where the pore pressure is always small due to influence of the transition filter zone. The amount of increment is not such a value to cause any problem for the core and is always less than the piezometric level in the dam reservoir.
Figure 12. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.187.
Figure 13. Pore pressure variations in sec.5-5 at Elev.107
when filling reached to Elev.217.
REFERENCES
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