ABSTRACT
For a long time, pure plastic clays have been used extensively as the core material of most embankment dams all over the world. Nevertheless, the differences between the stiffness of the fine cohesive core materials and coarse granular shell materials, have caused many construction difficulties and design problems. Adding some granular soils to pure plastic clay will increase the stiffness of the clay material and decrease the differential behavior between these two zones of the earth dams.
In this paper the geotechnical and physical properties of the mixed clays, prepared for the core materials of the earth dam are explained and described. The behavior of this material in connection with the shell materials of the earth dams are studied and discussed. Finally, for a large storage embankment dam of Karkheh, with mixed clay core recently constructed in southwestern part of Iran, the overall performance of this material are assessed and justified.
Keywords: dams, embankments, clay, gravel, mixed-clay, field observations, finite element method
INTRODUCTION
The core of an embankment dam plays an important role in seepage control and stability of the dams. Although different impervious materials such as asphalt, bituminous materials, and concrete are sometimes used as core material, the differences between the nature of the earth dam main material (shell) and the such core materials usually causes a significant contrast in deformational behavior of these two zones. A better alternative to pure clay can be used, resulting in relatively more similar behavior between these two zones. Nevertheless, using highly pure plastic clay may enhance some other problems such as,
In order to tackle the above problems, the use of clays mixed with sand and/or gravel can be an appropriate alternative. The type and size of the granular materials to be added to the pure clay, the percentage of the mixture, the practical and theoretical considerations for this mixed materials are among the main issues need to be discussed and clarified. In this paper the results of a comprehensive study done to determine and describe the role and influences of these factors, and show the justification of this effective method to solve the above problems. The material used is referred to as mixed-clay in this paper. The idea was implemented in the construction of a large embankment dam recently built in southwestern part of Iran.
THE TYPE AND METHOD OF PREPARATION OF THE MIXED CLAY
The type of the clay material used in this study was the reconstructed cohesive soils originally exploited from different mudstone layers in the borrow areas in the southwest of Iran. These materials are usually used for clay core of earth dams built in this region of the country. These materials are highly plastic with LL = 40-80 and PL = 15-45. The average specific gravity of these materials is about 2.74 and their chemical (mineral) compositions are mainly illite. The coarse material added to clay, was the reconstructed sandy gravel (GW), which in a wider ranges mainly were used as the shell materials of the embankment dams in the region. The Cu and Cc parameters of the material are 60 and 2.4 respectively. The ranges of the particle size distribution of the cohesive and granular materials, which is available and used in the region together with the average selected material used in this study, are shown in figure (1).
Figure 1. The ranges of particle size distributions of the coarse and fine materials in the region
with the selected mixed-clay used in this study
For laboratory testing, considering the diameter of the specimens, the large sizes of the coarse material, which are not allowed to be in the specimen, are initially removed from the mass. Then the two types of the materials are homogenously mixed with each other based on the desired percentage of combination. For covering the possible ranges of the mixed materials, three combinations of clay/gravel, namely; 70/30, 60/40,and 50/50 were selected and used for laboratory testing. The mixed materials in each kind were compacted in the testing mould in five layers at its optimum moisture content. Therefore the wet tamping was the preparation method used for laboratory testing of the mixed clay specimens. For the field preparation of the mixed clay, alternative layers of clay and gravel of appropriate thicknesses to achieve the desired mixture were deposited in their optimum moisture content. These sandwich layers of clay and gravel were then cut and mixed by appropriate excavating machines. The so produced mixed clay materials, were then loaded and carried to the core zone of the dam by means of special truck, where they were compacted in layers of limited thickness to get the required density.
THE GEOTECHNICAL AND PHYSICAL CHARACTERISTICS OF THE MIXED CLAYS
Different laboratory and field tests were carried out to obtain the mechanical properties of the mixed clays used. In the laboratory, different static and dynamic tests were performed to measure the basic soil parameters and to study the influence of the added coarse material on the pure plastic clays. The ranges of the clay/gravel combination mentioned before were selected in a way that the coefficient of permeability of the mixed clay remains in an acceptable limit. Prior to selection the optimum mixture, some permeability tests were carried out on different mixed clays and the effect of adding gravel on the coefficient of permeability of the clay was studied. According to results of these tests (figure 2.0) the optimum mixture percentage in which neither the material becomes too permeable nor remains too soft, was found to be 60% clay and 40% gravel. This mixture was selected and used in the core of the largest earth dam of the country (Karkheh), as the new core material. In this paper the instrumentation records of this dam as the largest and first dam with mixed clay core in the country, were used to justify the implementation of this material in practice.
Figure 2. The results of permeability tests on the mixed-clay used.
Based on different static and dynamic tests carried out on the mixed clay specimens of above mixture (i.e.: 60% clay + 40% gravel), the geotechnical properties of the mixed clay were obtained. In order to compare the behavior of the pure plastic clay with that of the mixed clay, the results of the experiments are plotted for both pure and mixed samples in the same graphs. The stress-strain curves, the cyclic shear stress ratio (CSR) versus number of load cycles (N), the damping ratio (ζ), and shear modulus (G), versus shear strain are shown in figure (3). As
Figure 3. The results of various static and dynamic tests on pure and mixed-clays
can be seen from the figure, for both static and dynamic loading conditions, the mixed samples show quite stronger and stiffer behavior than the pure samples. It is also evident that the dynamic properties of the pure plastic clay increase considerably when is mixed with gravel, particularly in the small strain levels, which is the case in most earth project such as embankment dams.
THE GENERAL SPECIFICATIONS OF THE KARKHEH LARGE STORAGE DAM WITH THE MIXED CLAY CORE
The Karkheh is a large multi-purpose 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. The typical section of the dam is shown in figure (4). 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 4. The typical cross section of Karkheh Dam with mixed-clay core
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.
THE PREDICTION OF KARKHEH BEHAVIOR INVOLVING BOTH PURE AND MIXED CLAY CORE:
The stress-strain analysis of the Karkheh dam has been carried out by finite element method. The selected mesh for the analysis consists of 561 nodes and 526 elements for the total stress, and 1489 nodes and 892 elements for the effective stress analyses. The dam construction has been simulated by application of 14 layers and impounding has been derived into 7 stages. The conventional computer software of FEADAM84, and the newly developed in the country, CA2, were used for analyses. The results of the analyses by the two softwares were compared and showed a good degree of consistency ( figure5). The material parameters are taken from parabolic (Duncan & Chang) and hyperbolic approximation of the stress-strain curves from the triaxial tests carried out on shell and core (both pure and mixed clay) materials.
Figure 5. The analysis results for Karkheh Dam using FEADAM and CA2 software
For the foundation the linear elastic parameters have been applied. The total and effective stresses in the horizontal and vertical directions at different conditions and stages were predicted. The pore pressure build up at different stages of construction were also estimated. A typical vertical stress contours at the end of construction based on the total stress analysis for the dam with pure clay core is shown in figure (6). Also the pore pressure development in the core as well as the dam settlements at different construction stages for the pure clay core are shown in figures (7) and (8).
THE INSTRUMENTED BEHAVIOR OF THE KARKHEH EMBANKMENT DAM
Some 1000 instruments have been installed in 23 cross sections of the dam to monitor its behavior during and after construction. These devices consist of 26 inclinometers and
Figure 6. Contours of vertical stress (t/sq.m) at the end of construction from total stress analyses
Figure 7. The pore pressures (t/sq.m) at various stages of construction from effective stress analyses
Figure 8. Variation of dam settlement along the core axis at the end of each layer of construction, effective stress analyses
settlement recorders, located at 10 different sections for monitoring the vertical and horizontal displacement, 134 stand pipe piezometers, 68 rock piezometers, and 191 electrical piezometers for manually and electrically monitoring the pore pressure development within the foundation and core of the dam, and finally, 510 earth pressure cells for measuring the total earth pressure induced at different points and directions (5 various directions) of the dam body.
Continuous and regular readings are carried out immediately after mounting each instrument. At the time of writing this paper, the construction of the dam has been completed for 10 months. The impounding of the reservoir has been started for 1.5 years and the water level in the reservoir has reached to 180 m.a.s.l. correspondent to about 73 m. height of water which is about 35% of the total reservoir height for the normal water level of the dam.
Typical results of developed pore pressures within the core of the dam, based on the field data, are shown in figure (9). Also settlements of the dam body at different levels of embankment according to the instrumented records are plotted in figure (10). The pore pressure ratio and the arching ratio of the dam, based on the maximum developed pore pressure and vertical earth pressure of the dam are illustrated in figure (11).
Figure 9. The pore pressures developed within the mixed-clay core
at different times as measured by piezometers mounted in the dam
COMPARISON BETWEEN THE DAM BEHAVIOR WITH MIXED AND PURE PLASTIC CLAY CORE
Since the Karkheh large embankment dam was constructed by a complete mixed clay core, all instrumented data collected from the site are associated merely with the behavior of an earth dam involving mixed core of clay (60%) and Gravel (40%). In comparison, the analytical data cover the behavior of the dam having both mixed and pure clay core. In order to verify the analytical results, some important responses of the dam for the same condition was compared with the field data and the method was calibrated initially. Then the analyses for the pure clay core were developed and the results compared with the field data to study the influences of
Figure 10. Settlement of the mixed-clay core at different construction stages measured by the settlement recorders mounted in the dam
Figure 11. The pore pressure and the arching ratios of the dam with mixed-clay core estimated based on the field data at different times
The effects of the mixed clay core on the general dam behavior. Such comparative curves are shown in figures (12) to (14).
As can be seen from the figures, the maximum settlement of the core at the end of construction is about 165 cm. for the dam with pure plastic clay, while it has reached to about 115 cm. for the mixed clay core. The maximum pore pressure predicted inside the pure clay core is about 1500 KPa. whereas, this parameter has been measured about 1000 KPa. at the same position and time for the mixed clay core by means of piezometers mounted in the core. The maximum horizontal displacement of the upstream shell at the end of construction for the dam with pure clay core has been estimated about 66 cm. This value for the dam with mixed
Figure 12. Variation of the core settlement with dam elevation for both pure and mixed-clay materials at the end of construction
Figure 13. The pore pressure development within the core at the end of construction for both pure and mixed-clay materials
clay core has been reduced to about 34 cm. Similar trend can be observed for the earth pressures and the pore pressures developed within the core of the dam. Referring to the arching ratio and the pore pressure ratio as the direct indicators of the pressure changes, will clarify the influence of the mixed clay on the development of these parameters. The arching ratio of the dam at the end of construction for pure clay core is about 0.46. This ratio for the same stage of the constructed dam with mixed clay core has increased to about 0.59 showing stiffer behavior of the core. The pore pressure ratio at the end of construction for the dam with pure clay core is estimated about Ru=0.70. This factor has reduced to 0.42 for the dam with mixed clay core.
Figure 14. Horizontal displacements of the upstream shell of the dam with pure and mixed-clay core at the end of construction
SUMMARY AND CONCLUSIONS
To reduce the differential behavior between core and shells in embankment dams, a new core material has been introduced. This material, which is called mixed clay, can be simply provided by adding 40% of the gravel material to pure plastic clay in a homogenous form. The so-called mixed clay is able to do all duties expected for the core of the earth dams in a more suitable way than the pure clay cores. In the Karkheh large embankment dam recently built in southwest of Iran, using mixed clay, resulted in 30% reduction of core settlement, 33% reduction in pore pressure build up, and 22% reduction in the arching phenomenon in the dam compared with those predicted for the dam if it would have been constructed by the pure plastic clay core.
REFERENCES
Mirghasemi, A.A., Tarkeshdooz, N., & Dadgostania, S. (2000) “Pore pressure monitoring within foundation and impervious zone of Karkheh Dam during construction,” Proc. of 20th Int. Conf. on Large Dams, ICOLD, Volume 3, PP.259-274, September, Beijing, China
Insttute of Water Conservancy, China (1994) “Static and dynamic property of sandy gravel and mudstone material for Karkheh project,” Technical report, Earthquake Dept., Institute of water conservancy & hydraulic power research. August 1994, Beijing, China.
IIEES of Iran (1995) “Results of cyclic triaxial and resonant column tests on core material and mudstone of Karkheh Dam,” Technical report, October, Tehran, Iran.
HBRC of Iran (1996) “Performing Dynamic tests on the soil materials of Karkheh Dam,” Final report, The Housing & Building research center of Iran, October, Tehran, Iran.
Mahab Ghods Consulting Eng. (2000) “Evaluation of instrumentation records of the dam body and foundation of Karkheh (after impounding),” June, Tehran, Iran.
Mahab Ghods Consulting Eng. (2001) “The report of instrumentation performance of Karkheh Dam during years of 1998 to 2001,” Technical report, October, Tehran-Iran.
Mir Mohammad Hosseini, S.M., and N. Tarkeshdooz (2000), “A Comparison between predicted and instrumented deformations of Karkheh Embankment Dam,” Proc. of 20th Int. Conf. on Large Dams, ICOLD, Volume 3, PP.275-288, September, China.
Niroomand, H. (2000) ”The evaluation of Karkheh behavior during construction according to instrumentation records,” MSc. thesis submitted in the Technical Faculty of Tehran University, January.
![]() | |
© 2002 ejge |