A Comparison of Asphaltic Concrete Core Dams and Asphaltic Lining Dams with Clay Core Dams

 

S.M. Ali Zomorodian

Department of Water Engineering, University of Shiraz, Shiraz, Iran
E-mail: smazomorod@yahoo.com

Mohammad Charrakh

Department of Civil Engineering, Islamic Azad University, Estahban, Iran

and

M. Heidarpour

Irrigation Department, Isfahan University of Technology, Isfahan, Iran

ABSTRACT

Earthfill and rockfill dams with clay core have some problems such as voluminous material in cores. Therefore, they need a long time for construction, and during constructing this type of core moisture and compactibility must be controlled. Besides, clay is sensitive to climate. Other phenomena that may occur in clay core dams are piping and hydraulic fracture.

Due to these problems and for constructing more economical earth dams an alternative approach is developed by substituting clay core with asphaltic concrete core or asphaltic lining. They are mainly used in areas where natural impermeable materials of sufficient quality or quantity are not available.

This research considers presents the advantages of using asphalt in lining dams and in asphaltic concrete core dams. To show the ability of asphalt in dam industry, a comparison is dpne between an asphaltic concrete core dam and an asphaltic lining dam with a clay core dam is done. This comparsion includes seepage and stability analyses. These analyses have been performed by commercially available seepage analysis is done by Ansys software and the stability anlysis by Plaxis software.

Keywords: Asphaltic concrete core, Asphaltic lining, stability, seepage

INTRODUCTION

Bituminous core and bituminous lining are excelent waterproofing materials and well established in hydraulic engineering for many decades. Bituminous facings are used to waterproof the upstream faces of dams or embankments, or the bottoms of resevoirs which consist of materials of inadequate water-tightness (gravel or sandygravel soils, morainic or alluval soils and rockfill), as an alternative to waterproofing by means of natural materials (clay, silty clay, etc.) where natural impermeable materials of sufficient quality or quantity are not available[2].

Using natural materials, the watertightness is generally provided by in impervious core, while with bituminous materials (exept for a few recent applications) it is usally provided by a continuous watertight revetment on upstream face. The characteristics of these facings are manifold and are strictly related to the properties of the structure on which thay are applied, namely: Bituminouse materials generally satisfy these requirements rather well, even if some of them may seem to conflict.

Different types of structures encountered, variety of ambient conditions, different evaluations of requirements, variety of bituminous material available and different construction techniques have resulted in considerable differences in the features and in the design of the alternatives used for bituminous facings. It is also sometimes difficult to find out whether considerable differences in bituminous facing depended on an actual and rational interpretation of technical requirements or mainly on the sensitivity and the artistic inspiration of the designer [2].

Since clay core dams have been used for many years the efficiency of this type of dam is well documented. Therefore, in this research asphaltic concrete core dam and asphaltic facing dam are compared with clay core dam. To validate the result Maejaran dam which an asphaltic concrete core dam (located in the north of Iran in close to Ramsar city) is replaced to an asphaltic lining dam and a clay core dam. The dimensions of the dams are the same to each other. Cross section of each dams are present in Figures 1, 2 and 3.


Figure 1. The cross section of clay core dam


Figure 2. The cross section of asphaltic core dam


Figure 3. The cross section of asphaltic lining dam

SEEPAGE ANALYSIS

The quantity of water seeping through and under an earth dam, together with the distribution of the water pressure, can be estimated by using the theory of flow through porous substance. This theory is one of the most valuable analytical tools available to the engineer.

The computed amount of seepage is usefull in estimating the loss of water from reservoir. The estimated distribution of pressure in the pore water is used primarily in the analysis of stability against shear failure, and also occasionally to study the hydraulic gradient at the point of seepage discharge which gives a rough idea of the piping potential [3]

Principles and Calculations

Since the governing differential equation is the same, thermal conduction can be used to solve complex seepage problems which are difficult to solve graphically. The seepage medium is simulated by a thermodynamic conductor with the permeability cofficient substituted by conductivity cofficient [1].

Table 1 shows geotechnical parameters of the materials used in this analysis.

Table 1. Geotechnical parameters of the materials

The Results of Seepage Analysis

The first stage of seepage analysis is estimating the phreatic line. After determining the phreatic line the coordinates of points on the phreatic line are given to computer. The first result of seepage analysis are equipotential lines, which are shown in Figures 4, 5, and 6 [1].


Figure 4. Equipotential lines in clay core dam


Figure 5. Equipotential lines in asphaltic core dam

As shown in Fig. 5 the head of water after in contacting the asphaltic core is reduced to zero. An asphaltic core dam can pond during construction but a clay core dam can not. This is the main difference between a clay core dam and a asphaltic core dam. As shown in figure 6 because of the low permeability of asphalt the potential head of water after contacting with asphalt in surface is reduce. After the surface layer the role of horizental filter is repesented.


Figure 6. Equipotential lines in asphaltic lining dam

Seepage Results

The main problem governing in desgining dam and one of the most important proposed of dam constucting is storing water and preventing water to seep.

In the clay core dam the total seepage registered at maximum reservoir level is 42 liters per second. In this case the total seepage registered at maximum reservoir level in the asphaltic core dam and asphaltic lining dam are 26 and 0.55 liters per second respectively. The hydraulic gradient in clay core dam is equal to 0.68 and in the asphaltic core dam and asphaltic lining dam equal to 0.29 and 0.19 respectively. The compersion of the results shows that the asphaltic lining dam has the best performance against piping phenomenen respect to asphaltic core dam.

STABILITY ANALYSIS

Finite element method is the best solution to solve slope stability analysis. In this method there is no need to estimat the shear slid lines. However in this method the shear slid line will estimat automaticly [1].

In additional to calculate the safety factor some other parameters such as deflection, plastic points, stress and etc. would have been calculated.

Stability analysis have been done in three states in earth and rockfill dams.

These states are discussed in the following sections.

During Construction

Slides of embankment dam during costruction have not occurred as frequently as slides during the operation of reservoir and have not resulted in failures of the catastrophic type. Nevertheless the pore pressures which develop in the embankment or foundation during construction may be higher than at any subsequent time and it is usually recommended to analyze the stability of the embankment for this condition. The construction condition is especially likely to be critical for dams on soft foundation [3]. The materials of dam in this case are simulated unconsolidated and undrained [4].

The minimum tolerable safety factor in this state is 1.25. The results of stability analys in this case are present in Table 2.

Table 2. Results of the stability analyses for the first condition

With respect to the minimum proposed safety factors, the calculated value for this condition is not acceptable for the dam with clay core.

Full Reservoir Stability Condition

Since a shear slide when the reservoir is full can be loaded to a disastrous failure, the stability analysis should be treated more conservatively then during construction and reservoir draw down condition for the full reservoir state only the downstream portion of the dam need to be analyzed since an upstream slope slide during full reservoir is theoretically conceivable only if the strength of the foundation were to be reduced very greatly by wetting. The materials of dam in this case are simulated drained. The minimum tolerable safety factor in this state is 1.5 [4]. The results of stability analysis in this case are presented in Table 3.

Table 3. Results of the stability analyses for the second condition

In this case as it is presented the results for clay core dam are not acceptable.

Rapid Draw-down

Rapid draw-down in every reservoir is possible because of war or floody situation or every other accident. Because of this the stability analyses in this case must be done.

The dams materials in this case are simulated consolidated and undrained. The minimom tolerable safety factor in this state is 1.2 [4]

The results of stability analysis in this case are presented in Table 4

Table 4. Results of the stability analyses for the third condition

CONCLUSIONS

Three dams with three different impermeable layers are compared. This comparison includes seepage control and stability analysis. The seepage analysis is done by ansys software. The comparison shows that the seepage flow in the clay core dam was the biggest. Since the permeability coefficient of clay is more than asphaltic concrete. Furthermore in the facing dam the seepage flow was the lowest because of the surface layer permeability that acts like water stop. The hydraulic gradient in facing dam is the lowest and it has a big safety factor against piping phenomena. The stability analyses of three dams are done by Plaxis software. The results of stability analysis show big safety factors for the asphaltic facing dam in comparison with clay core and asphaltic core dams.

Because of the advantages mentioned in the introduction of this paper and due to the results of analysis one can recommend to use asphalted facing dams instead of cloy core dams and asphaltic concrete core dams.

REFERENCES

  1. Charrakh, M. (2003) The Comparison of Asphaltic Concrete Core Dams with Asphaltic Lining Dams Regarding Seepage and Stability. M.Sc.thesis, Islamic Azad University, Iran
  2. ICOLD (1980) Bituminous Concrete Facings for Earth and Rock fill Dams” International commission on large Dams, Bulletin, no 32, Paris
  3. Sherard, J.L., R.J. Woodward, S.F. Gizienski, and W. A. Clevenger (1963) Earth and Earth-Rock Dams. John Wiley and Sons, Inc.
  4. Das, M. Braja (2001) Principles of Geotechnical Engineering. PWS-KENT Publishing Company, Boston, MA, USA.

ACKNOWLEDGEMENT

This paper is part of the research project supported by Islamic Azad University, Estahban, Iran. The authors wish to thank the university officials for their support.

 

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