Importance of using roller compacted concrete in technoeconomic investigation and design of small dams
Abstract
In recent years, and under constraints caused by persistent drought, Algeria has launched a new mobilization strategy for surface water resources from small and medium dams. However, by making a review of the studies and achievements of twenty small dams in the west of Algeria, some deficiencies appeared. In addition to reservoir siltation assessment, operation spillways have been the major constraint on the reliability of these types of dams. The objective of this paper is to use the roller compacted concrete (RCC) for small dams’ design for the benefit it offers and its ability to incorporate spillways. The development of this reflection was applied to the Khneg Azir earth dam situated in southwest of Algeria. Its uncontrolled lateral spillway has registered significant damage following the flood of October 2005, amounted, at that time, to more than 100 million Algerian dinars (1 million US Dollars). The present research encompasses a technical and economical comparative analysis concerning multiple criteria dam design types coupled with the conjugation of the spillways. Thus, on the basis of financial estimates calculated for all design types, the variant RCC remains competitive with that of the earth dam’s spillway isolated (Less than 40% of the cost). To assess the mechanical behavior of the foundations for both types of dams, (earth and RCC dams), numerical modeling has been undertaken, according to the comparative analysis of deformations in the foundations. Analysis of deformations showed that the average foundation deformations was between (0.052–0.85) m for earth dam and (0.023–0.373) m for RCC dam. These economical and technical considerations open up important prospects for the use of RCC in the design of small dams.
Keywords
Earth dam RCC dam Frontal spillway Lateral spillway DeformationsIntroduction
When the site conditions are favorable, the RCC dam finds its justification in multiple benefits: spillways easily combined with the dam, intensive mechanization of work reducing delivery times and its economic competitiveness (Zdiri et al. 2006). However, the great advantage of this type of dam is the integration of the spillways at the dam. This advantage gives a great opportunity at the relatively high flood sites. Thus, it can become competitive at the expense of other disadvantages: volume of the concrete, demands on the foundation deformation moduli, thermal stress, etc. (Goubet 1992).
One of the best solutions for dams’ design is RCC (Roller Compacting concrete) due to its economic benefits and construction time. In comparison with conventional concrete dam, RCC dam can decrease the costs in ranging from 25 to 50 percent. Constructing the spillway in dam’s body is costeffective comparing with the embankment dams that require using spillways in an abutment (Zarrin et al. 2016).
In Algeria, the means of conventional surface water resource mobilization has always been oriented towards large dams. Nevertheless, time’s realizations of these structures (studies, realization, financing, etc.) have led to a gap between needs and demands. Under the pressures and tensions caused by persistent drought, the government has committed significant investments for the mobilization of surface water resources from small dams. These structures, of a height between 10 and 25 m, are, exclusively, intended for the purpose of irrigation of small perimeters areas. They are designed in clay, homogeneous or with central core. The spillways are isolated from the dams.
The assessment of project flows by empirical models (no gauging of small watersheds) was the main cause of degradation observed on the spillways (Rouissat and Smail 2009a). The expertises carried out on twenty small dams in western Algeria showed that, in most cases, the recorded degradations are due to the inadequacy of spillway’s capacities (Rouissat and Smail 2009b).
The reflection undertaken in this paper concerns the analysis of using roller compacted concrete (RCC) in design of small dams.
Additionally, we propose a technoeconomic analysis between the spillways designs of small earth dams, and those incorporated in RCC dam’s body. The development of this reflection was applied to the case of Khneg Azir earth dam in southwest Algeria.
On the economical level, four variants were analyzed: earth dam with two types of spillways (lateral and frontal), fully submersible RCC dam and incorporated spillway on the dam. The design calculations carried out on the four variants have the objective of estimating the volumes of the works and consequently the costs of the various variants.
On the technical level, and particularly the aspect related to the deformation of the foundations, a numerical modeling has been developed for different deformation moduli to judge the adaptation of the RCC dam type according to the quality of the foundations.
Characteristics of Khneg Azir dam
The Khneg Azir small dam is located on Mellouk river, at the remote town of Kaf Lahmar about 50 km from the province of El Bayadh. The lambert site coordinates are (X = 329.30, Y = 363.30).

Lateral uncontrolled spillway

Flood frequency 1%: 292 m^{3}/s

Charge over the threshold: 2.5 m

Threshold: length 56 m

Trapezoidal channel: 150 m in length and 1.86% in slope

Fast channel: 32.5 m in length and 21% in slope

Basin of dissipation with drowned jump: 16.5 m in length and 5.5 m in deep (Hydro Projet Ouest 2000).
Methodology

Calculations and design of the spillways (earth dam) for two design types: lateral and frontal spillway (often adopted solution for small earth dams),

Projection, calculations and design of RCC dam’s spillway,

Analysis of the RCC dam’s stability,

Numerical modeling for both types of dams (earth and RCC dams) in order to launch a comparative study of behavior especially with regard to deformations with different types of foundations

Technical and economical comparative analysis between multiple criteria of types dam design coupled with the conjugation of the spillways.
Results and discussions
The objective of the hydraulic calculations is the determination of spillways geometries allowing estimating is financial costs.
Hydraulic resizing: lateral spillway type

Calculation of hydraulic parameters at the threshold (velocity, water level, and specific flow) by subdividing the threshold into 10 sections,

Determination of the flow parameters at the channel and fast channel by plotting the discharge capacity curves on these sections, and projection of a stilling basin by evaluation of its depth, hydraulic jump, jet lengths, and the combined depths of the hydraulic jump.
Results of hydraulic calculations—lateral spillway variant
Section  1  2  3  4 

Threshold  Channel  Fast channel  Basin of dissipation  
Bottom slope (%)  3.5  1.86  21  – 
Unit flow q (m^{3}/s.ml)  5.21  15  15  12.14 
Velocity V (m/s)  7  12  25  8.11 
Water level (m)  Input: 1.33 Output: 2.14  Input: 2.14 Output: 1.25  Input: 1.25 Output: 0.60  Input: 0.60 Output: 1.50 
Hydraulic resizing: frontal spillway type

Calculation of water level, velocity and Froude number at the downstream of the spillway by the USBR method.

Calculation of the hydraulic parameters at the level of the transition, of the convergent and the channel by plotting the discharge capacity curves on these sections and projection of a stilling basin by evaluation of its depth, hydraulic jump, jet lengths and the combined depths of the hydraulic jump.
Results of hydraulic calculations—frontal spillway variant
Section  1  2  3  4  5 

Threshold  Transition  Convergent  Channel  Basin of dissipation  
Bottom slope (%)  –  –  1.86  21  – 
Unit flow q (m^{3}/s.ml)  5.21  5.21  10  10  8.3 
Velocity V (m/s)  3.32  3.32  13  43  5.6 
Water level (m)  –  1.57  0.74  0.23  1.5 
Submersible RCC dam’s variant
To overcome the constraints related to the use of empirical models in the flood project evaluation for isolated spillways design of earth dams, a variant of a submersible RCC dam has been studied with a spillway incorporated in dam’s body. This variant is combined with ski jump for the energy dissipation.

Flow coefficient m = 0.5

Length of the dam crest: 370 m
Results of hydraulic calculations—RCC dam variant
Dumping zone  Downstream of the dam  

H_{1}(m)  V (m/s)  H _{2} (m)  Length (m) of jet (ski jump) 
RCC dam—length spill L = 370 m  
0.5  3.6  0.22  1.4 
RCC dam—length spill L = 185 m  
0.8  4.8  0.33  2.4 
RCC dam’s stability criterions
The stability against sliding and overturning of the RCC dam was tested for several types of foundations based on friction coefficients between the dam and its foundations.

Water load: 12.7 m

Dam’s crest: 6 m

Slope of dam upstream facing: 1H/1 V

Slope of dam downstream facing: 0.8H/1 V

Trapezoidal diagram of under pressure at foundations

Silting height: 6.35 m with a density of 17.7 KN/m^{3}

RCC density: 23.25 KN/m^{3}

Foundations cohesion: C = 60 KN/m^{2}.
Results of dam’s stability analysis—RCC variant
Foundations types  Hydrostatic strength (KN)  Sediment strength (KN)  Dam’s weight (KN)  Under pressure (KN)  K _{sliding}  K _{overturning} 

f = 0.22  791.1  316.5  2526.1  121.1  1.26  4.3 
f = 0.33  1.5  
f = 0.67  2.24 
Comparative financial analysis
Hydraulic calculations were used to determine the geometry of the various elements of the spillways. In fact, for the lateral and frontal spillways of the earth dam, the slopes were used to estimate the volumes of earthworks, and the specific flow rates allow evaluating the width’s elements. The heights of the walls were determined by evaluating the water levels in threshold, channel and fast channel.
Geometry parameters of spillways
Spillway’s types  Geometry parameters  

Lateral  Threshold  Channel  Fast channel  Basin of dissipation 
Length: 56 m Initial width: 1 m Final width: 19.5 m  Length: 150 m Width: 19.5 m  Length: 32 m Width: 19.5 m  Hydraulic jump length: 16.5 m Jet length: 32.3 m  
Frontal  Transition  Convergent  Channel  Basin of dissipation 
Length: 56 m Width: 56 m  Length: 67 m Width: 56–29 m  Length: 59.5 m Width: 29 m  Hydraulic jump length: 16.3 m Jet length: 34.3 m 
Quantity of construction works
Variant  Earthworks  Embankments  Concrete  Drainage 

Earth dam Lateral spillway  V = 31,676 m^{3}  V = 88,000 m^{3}  V = 4265 m^{3}  V = 4 052 m^{3} 
Earth dam Frontal spillway  V = 40,853 m^{3}  V = 88,000 m^{3}  V = 6151 m^{3}  V = 5 807 m^{3} 
RCC dam Length spill L = 370 m  V = 2 218 m^{3}  (RCC) V = 16,615 m^{3}  
RCC dam Length spill L = 185 m  V = 2 313 m^{3}  (RCC) V = 13 170 m^{3} 
Analysis of dams’ deformations
The homogeneous earth dams have the great advantage in case of quantitative and qualitative availability of waterproof materials, and for their economic competitiveness. The constraints related to this type of dams lies in their sensitivity to design floods with great prejudice for the safety of the spillway and of the dam against flood. This security is particularly compromised if the evaluation of project flood is empirical.
The use of RCC for gravity profiles has many advantages; in particular the incorporation of the spillway in dam’s body. However, for construction, gravity dams need good foundations and topography to perform better throughout in their lifetime (Rampure and Mangulkar 2016).
The concrete dams require the foundation and abutments to have sufficient bearing capacity, and deformation rigidity. Thus, the determination of foundation geology is one of the key issues for these dams (Renkun 2016).
To analyze the influence of this last parameter on the technical and economical choice of the dam’s variant, a numerical modeling hinging has been undertaken using the ANSYS computer code to analyze the mechanical behavior of earth and RCC dam’s variants.
The ANSYS calculation code used for modeling is based on the finite element method. The criterion of plasticity used usually in ANSYS code is Von Mises stress criterion for ductile materials.
For the analysis of the dams’ deformations, ANSYS code proposes the Drucker–Prager stress criterion which constitutes a generalization of Von Mises criterion taking into account the first invariant of the stress tensor and the second invariant of the deviatoric stress tensor.
Characteristics of dam’s materials
Parameters  Earth dam  RCC dam 

Deformation modulus (MPa)  28  250 
Poison coefficient  0.3  0.20 
Density (kg/m^{3})  1800  2000 
Conclusion
All hydraulic calculations engaged on the different variants were used to define the geometry elements of spillways.
Checking the structural stability of RCC gravity profile against sliding and overturning, led to the stability criteria ensured according to the profile designed for different types of support foundations.
The calculations and developed hydraulic checks were used to evaluate of volumes of work completed by their financial estimate, and to establish an economic comparison of different variants analyzed.
Thus, on the basis of financial estimates developed for all variants, the RCC dam remains competitive with that of the earth dam with isolated spillway.
Numerical modeling has led, according to the deformation analysis in the foundation, to a comparison of the proposed variants. Deformations calculated by modeling resulted in average values between (0.052–0.85) m and (0.023–0.373) m, respectively, for the earth and RCC dams’ variants.
Finally, according to the results obtained, the use of RCC for the design of small dams in Algeria is an interesting solution, which will solve the constraints to the design of this type of structures with isolated spillways.
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