The Effect of Impervious Clay Core Shape on the Stability of Embankment Dams

Geotech Geol Eng (211) 29:627 635 DOI 1.17/s176-11-9395-z TECHNICAL NOTE The Effect of Impervious Clay Core Shape on the Stability of Embankment Dams R. Nayebzadeh M. Mohammadi Received: 1 June 29 / Accepted: 4 February 211 / Published online: 4 March 211 Ó Springer Science+Business Media B.V. 211 Abstract One of the most important dangers that treat earth dams which can lead to interior failure over a prolonged period is the hydraulic fracturing factor. In the case of zoned dams, due to differences in stiffness of the core and its abutment zone, differential settlements occur between them. This factor is responsible for the arching phenomenon. Differential settlements between core and shell cause cracks within the core initially sub-surface, Those cracks may develop the first impounding causing internal erosion on the dam core. In this research, using a computer modeling of Ghavoshan rockfill dam (located the west part of Iran) as a case study computed by SIGMA/W program, the role of the dam core shape on those factors is demonstrated. It is found that an inclined core shape is preferred in a condition that is especially important settlements of construction during for dam body. The result of finite element analysis indicates desired conditions from the point of view of stress, deformation and resistance against hydraulic fracturing for the same width of R. Nayebzadeh Faculty of Engineering, Urmia University, P O Box 165, 57169-33111 Urmia, Iran e-mail: Rezvan_7@yahoo.com M. Mohammadi (&) Faculty of Engineering, Urmia University, P O Box 165, 57169-33111 Urmia, Iran e-mail: m.mohammadi@mail.urmia.ac.ir dam designs. Moreover, this can be higher priority for embankment dam designs. Keywords Embankment dam Clay core shape Arching Hydraulic fracturing SIGMA/W software 1 Introduction The main purpose of stability analysis of an embankment dam is to answer the following two fundamental questions: (1) How safe is the structure against a total or partial failure? (2) Will the deformations of the structure remain within tolerable limits for the operation and function of the structure? The integration of dam structure should be protected in its operation period or probable events that occur in the dam operation. The stability of an embankment dam should be secured by resting stress at acceptable levels and dam core integrity in all of anticipated events. One of the important subjects in embankment dams is arching phenomenon occurring in the interior body of the dam. Arching action is also one of the hydraulic fracturing factors for embankment dams (see Ng and Small 1999; Sherard 1991; Narita 2 for more detail). To minimize the

628 Geotech Geol Eng (211) 29:627 635 horizontal cracking potential due to the arching action such factors have important roles namely core size and shape, material selection and its placement. In this research, by considering much usage of clay material on the core part of the earth and rockfill dams is used for evaluation of the hydraulic fracturing problem and core arching phenomenon. The Ghavoshan storage dam located at the western part of Iran is selected as a case study that has been considered with changes at the geometric core shape from vertical to inclined one. 2 Background and Literature Survey Arching importance and its existence in rockfill dams was reported for the first time by Lofquist in the year 1951 (Ohne and Narita 1977). Using pressure measurements, he turned to considerable stress decrease of lateral and vertical pressures of rockfill dam having weak cores. Lofquist showed stress decrease is greatly relevant to core settlements towards the shell with the consequent load transfer from core towards the shell. By the year 196, a few considerations about load transfer subject has been taken into account, while by the year 1961, Nonvieller and Anognosti had developed stress theories in respect to the settlement of the core towards the shell (Ohne and Narita 1977). By the year 1976, Kulhawy and Gurtowski had considered load transfer and hydraulic fracturing phenomenon in zoned dams and found that load transfer within zoned dams will take place due to the hardness differences of the adjacent zones (see Ng and Small 1999; Sherard 1991 for more detail). However, it is believed that the arching phenomenon is still one of the key issues in embankment dams anguishing any civil and embankment dam engineers. 2.1 Zoned Dam Cores The seepage preventative in zoned embankment dams usually comprises a central or inclined compacted toward upstream impermeable earthen core or puddle clay core. Core size will depend on accessibility, locality, and the properties of material. It will also need to prevent high seepage gradient. Impermeable clay cores are constructed in embankment dam sections at three major location and shapes, namely: central vertical core, moderately sloping core, sloping (inclined) core (see Fig. 1a c). When core downstream slopes are at 1V:.5H or more towards upstream, it is called moderately sloping core. It is called sloping (inclined) core, if the downstream shell and core contain a self-stable slope about 1V:1.25H or less, This slope is usually used in rockfill dams that downstream rockfill shell is constructed in the form independent and the post time, the upstream filter and core are performed (French National Committee 1973; Reinus 1973). 2.2 Interior Erosion (Internal Failure) By uncontrolled releasing of the water stored in dam, it cab be distinct that a small amount of failure occurred. Any abnormal appearance at soil shear resistance which is against the original water operation suitably shows failure may be due to the differential settlements made by arching action. On the basis of studies given by Babb and Mermel (1968) after overtopping, a commonly important factor in embankment dam failures is piping (or hydraulic erosion) (Ohne and Fig. 1 Core shapes and their location in embankment dams sections

Geotech Geol Eng (211) 29:627 635 629 2.4 Hydraulic Fracturing Phenomenon Fig. 2 Differential settlement and cracking in core for embankment dam (Ohne and Narita 1977) Narita 1977; Babb and Mermel 1968). One of the factors is cracking due to negative effective stress arising from imported forces on dam body that can be estimated by Finite Element Method (FEM). 2.3 Load Transfer or Arching Phenomenon Since core is softer than shell, load transfer occurs from core to shell. As a result of this action, pore water pressure can become more than total stress within core (Sherard 1991; Ono and Yamada 1993). This action may lead to hydraulic fracturing and make of cracks due to excessive water pressure. There is also the possibility of piping in this case. Since more settlements are within core with respect to the shell causing differential settlements and core leaning to shell as a result of much transformation of loads. Consequently, it can create longitudinal cracks between them in beneath the surface. Figure 2 shows cracking on the zoned dams. If a zoned dam contains soft shell and hard core, its reverse occurs, in other words load will transfer from shell to core. This case of load transfer may cause over stresses on core and it can be led to the plastic yielding and brittle cracking on the core too. Load transfer is evaluated with calculation of vertical stresses (r v )in core toward overburden stresses (c t h)in each depth under crest. Less than 1 ratios shows that load of core will transfer on shell and transient zone, whereas more than 1 ratios indicates that load will transfer from shell and transient zone to core of dam. Arching coefficient (A coef. ) on dam core obtains from Eq. 1 as: A coef ¼ r v ð1þ c: h herein r v = Total vertical pressure (KPa), c = unit weight (KN/m 3 ), h = embankment height (m). One of the most important problems that dam designers confront with them is probability of the cracking in zoned dams. In recent years, hydraulic fracturing has been a matter of great concern in the design and construction of embankment dams (Ogita et al. 22). considerable attentions on this subject have been done with examination of previous samples. Extensive studies have been made on this subject, especially since failure of the Teton Dam (USA) occurred in the year 1976. Hydraulic fracturing can be considered equivalent to the well-known seepage failures such as quick sand and piping (Wang et al. 27). A typical pattern of cracking arises from arching, which engineers often encounter in the field, is shown illustrated in Fig. 3. With respect to Fig. 3, both stresses namely r 1 and r 3 decrease due to the arching action in the upper part of the core, which cause internal cracking. It can be seen that total stress circle becomes small (drop of r 1 may be larger in this case) and shifts left. The effective stress circle shifts left by the action of upstream water pressure (p w ) and touches failure envelope to occur cracking and seepage fracture. Two distinct patterns of hydraulic fracturing can be considered in embankment dams: One is the case where differential settlement after construction is contributive to cause cracking in the embankment and erosion takes place due to the flow of the reservoir water passing through open cracks. When embankment deformation is accompanied by differential settlement, tensile strains develop on the surface or in the interior of the embankment, and the minor principal stress (r 3 ) tends to decrease locally to open tension cracks. The criterion for the possibility of hydraulic fracturing in this case is given by the following condition: r 3 \ p t ð2þ where, p t is the tensile strength in terms of total stress. Corresponding stress state is indicated in Fig. 4a, where the initial stress circle (I) grows on the left side due to the decrease in r 3 and touches the failure envelop at the circle (II) to open tension cracks.

63 Geotech Geol Eng (211) 29:627 635 Fig. 3 Cracking arised in dam core due to the arching action (Ohne and Narita 1977) Fig. 4 The hydraulic fracturing phenomenon in embankment dams. a conditions for cracking: differential settlement after construction causes decrease in minor principal stress (r 3 ) which leads to open cracks and internal erosion in embankment. b conditions for seepage fracture: effective stresses in the core decrease as reservoir filling proceeds, where decrease in effective stress (r 3 ) beyond tensile strength (p ) causes open t cracks and erosion The other pattern is the case where pore water pressure in the core increases according as the reservoir filling proceeds and the effective stress (r ) decreases up to the effective tensile 3 strength (p ) to open hidden or latent cracks t and stress states are illustrated in Fig. 4b, where the initial stress circle (I) shifts to the left without diameter change and touches the envelope at the circle (II). The criterion in this case is given by: r \ 3 p t ð3þ Hydraulic fracturing has been considered with compare of principle stresses at construction final stage to hydrostatic pressures that occurs under reservoir loading. 3 A Brief Summary of Ghavoshan Dam (A Case Study) Ghavoshan rockfill dam has a vertical clay core including 125 m height that is upon Gave Roud River located the western part of Iran. It is 38 km away from Sanandaj city that it has been constructed to provide the drinking water of Kermanshah city and also needing water for irrigation. By consider of done tests and reports on stability analysis of dam foundation, it is assumed as rigid materials. 4 Geotechnical Parameters and Modeling of Dam Stress and strain behavior of dam materials has been considered by using finite element modeling and computer simulating by plane strain method (Krahn 28). So one case of stress transfer that is observed in dam section, it can be expected with this analysis, thus possible conditions of cracking potential can be estimated with the analysis. To simulate in analysis of dam stage construction, the dam height can be divided into ten layers (12.5 m) at critical section (Zienkiewicz and Naylor 1986). Tables 1 and 2 present some geotechnical parameters of the hyperbolic model usage for effective and total stress analyses.

Geotech Geol Eng (211) 29:627 635 631 Table 1 Geotechnical parameters for effective stress analysis Materials type c wet (kn/m 3 ) c sat (kn/m 3 ) K K ur n R f C (kn/m 2 ) / degree K(m/s) n% Clay core 19.42 19.91 95 175.8.77 27 1 9 1-8.36 Filter & transient zone 18.63 19.12 458 687.18.8 35.18 Shell 2.6 21.9 78 936.23.67 45.28 Table 2 Geotechnical parameters for total stress analysis Materials type c wet (kn/m 3 ) c sat (kn/m 3 ) K K ur n R f K b m C (kn/m 2 ) / degree Clay core 19.42 19.91 97 136.363.93 4.2 39.23 7 Filter and transient zone 18.63 19.12 458 687.3.8 2.2 35 Shell 2.6 21.9 78 936.23.67 34.2 45 Fig. 5 Critical section of Ghavoshan dam (Ghavoshan Powerhouse and Dam Design 24) Fig. 6 Vertical stress contours in the dam sections: a vertical core b inclined core Tables 1, 2 give some geotechnical parameters of dam core for stability analysis (Ghavoshan Powerhouse and Dam Design 24). Figure 5 shows major section of the Ghavoshan dam that in this research it is analyzed at downstream and upstream critical sections (Ghavoshan Powerhouse and Dam Design 24). 5 Analysis of the Results According to the analysis done (as Fig. 6a), it has illustrated a lack of stress compatibility in the case of the vertical core: load transfer from core to the shell below the surface creates the possibility of brittle cracking, representing a risk to the stability of the dam. Stress concentrations exist on both upstream and downstream sides of the vertical core, but in the case of the inclined core, they occur only on the upstream side, as it can be seen in Fig. 6b. Here, brittle cracking might occur, but on the downstream side, stress concentration is lower and the conditions are more favorable than the vertical core. According to the relevant stress analysis in the upstream part of an inclined core, stress whirlpool phenomenon (we call it) appears, in this special case.

632 Geotech Geol Eng (211) 29:627 635 Fig. 7 Evaluation of horizontal cracks in the dam core comparing with vertical stress and hydrostatic pressure: a vertical core b inclined core Elevation (m) 14 12 1 8 6 4 2 Hydraulic Fracturing P.W.P Total Vertical Stress 5 1 15 2 25 Pressure (kpa) Elevation (m) 14 12 1 8 6 4 2 Hydraulic Fracturing P.W.P Total Vetical Stress 5 1 15 2 25 Pressure (kpa) Fig. 8 Evaluation of vertical cracks in dam s core comparing with horizontal stress and hydrostatic pressure: a vertical core b inclined core Elevation (m) 14 12 1 8 6 4 2 Hydraulic Fracturing P.W.P Total Horizontal Stress 5 1 15 2 Pressure (kpa) Elevation (m) 14 12 1 8 6 4 2 Hydraulic Fracturing P.W.P Total Horizontal Stress 5 1 15 2 Pressure (kpa) This phenomenon causes the concentration of stress and plastic yielding in an inclined core (see Fig. 6b). The probability of hydraulic fracturing becomes critical when the reservoir reaches its top level quickly and the core has not sufficient time for consolidation. Nobari and Duncan indicated that rapid reservoir filling does not cause considerable changes of core stresses (Nobari and Duncan 1972). Then the present research compares the core major stresses at the end of construction in the upstream surface along with hydrostatic pressures due to full reservoir loading that proceeded to detectable hydraulic fracturing (see Figs 7, 8). These Figures illustrate the relationship between reservoir water pressure (pore water pressure) and total vertical stress and consequently hydraulic fracturing. Horizontal cracks represent an important problem, because they are not observable and dam impairment may occur before they become detectable. Evaluation of vertical hydraulic fracturing in both of dam cores does not suggest cracking of the inclined core. but in the vertical core and comparison between horizontal stresses and hydrostatic pressures of the reservoir water at dam greater elevation indicates that the possibility of vertical cracking and long-term risk on the stability of the dam exist. In a case of hydraulic fracturing or cracking arising from arching, because of these cracks probability of destruction of core integrity exists. Although total collapse may not be occurred, however the operation of the dam could be at risk, because the possibility of obvious crack removes don t exist with passing of time. In a case of occurring hydraulic fracturing or deformation and or unsymmetrical displacement that dam endure duration of this movements it will be caused various cracks in dam. The importance of deformations is for this reason that cracking potential after construction of dam dependent to them. By reason of unsymmetrical settlement related to the different zones, longitudinal cracks often occur in zoned dams. Longitudinal cracks can develop parallel of dam axis at the excessive length. with considering the analysis made in the Fig. 9, concentration of settlement contours in both of sides of vertical core indicated differential settlements between core and shell that can be led to longitudinal cracks in the body of dam, but the settlement concentrations have seen in

Geotech Geol Eng (211) 29:627 635 633 Fig. 9 Vertical displacement contours in dams having: a vertical core b inclined core Fig. 1 Horizontal displacement contours in dams having: a vertical core b inclined core Fig. 11 Upstream core horizontal displacements in contact with shell: a vertical core, b inclined core Y-Coordinate (m) X-Displacement 14 12 1 -.6 -.5 -.4 -.3 -.2 -.1.1.2.3 X-Displacement (m) 8 6 4 2 Y-Coordiante (m) X-Displacement 14 12 1 -.6 -.5 -.4 -.3 -.2 -.1.1.2.3 X-Displacement (m) 8 6 4 2 downstream side of the inclined core that made differential cracks in back of reservoir. Horizontal displacement contours have been presented at Fig. 1 in two dams with vertical and inclined core sections. Presented in Fig. 11, horizontal displacements within upstream of dam cores in contact with the shell and also presented in Fig. 12 vertical displacements at central part of dams height. 6 Concluding On the basis of presented analyses and results in previous sections, the main conclusions are: 1. Concentration of settlement contours in both side of vertical core represents differential settlements between core and shell that can be led to longitudinal cracks in dam body but they are not seen in downstream of inclined core. However, differential settlements create cracks on joint surface in core that can be opened at the first reservoir filling and also it can be led to the piping phenomenon in core. 2. If reservoir reaches oneself to the top level quickly, there is hydraulic fracturing probability in upstream side of the vertical core dam that too possibility of destruction on obvious cracks risks don t exists with lapse of time, consequently vertical core shows unsuitable conditions for this action.

634 Geotech Geol Eng (211) 29:627 635 Fig. 12 Vertical displacements in core middle height: a vertical core, b inclined core Y-Settlement (m) Differential Settlement 2 -.5 25 3 35 4-1 -1.5-2 -2.5-3 -3.5-4 -4.5 Differential Settlement -5 X-Coordinate (m) Y-Settlement (m) Diffrential Settlement -.5 2 25 3 35 4-1 -1.5-2 -2.5-3 -3.5-4 -4.5 Diffrential Settlement -5 X-Coordinate (m) 3. Using stress analyses, vertical core dam shows excess stress decrease because of the occurrence of arching in both sides namely downstream and upstream core. at near of foundation within shell, It then prepares shear stress concentration and plastic yield conditions. It consequently makes risk on the stability of the dam. In a case of inclined core, it may be seen only shear stress concentration and plastic yield conditions at the upstream side, which we call it stress whirlpool phenomenon. 4. Upper half of vertical core dam indicates that the shell lean towards to the core which can be led to brittle cracks on the dam crest. An interesting point is that within inclined core dam in its upper section, horizontal displacements of shell creates only in smallness zone and it shows some suitable conditions from the viewpoint of no cracking on the crest. 5. By considering the horizontal hydraulic fracturing in two cores, it doesn t exist in both of them. The vertical core stresses show excess difference towards water pressure, but differences between stresses and water pressures in an inclined core may increase in lower depth locally. 6. After discovering on the stress transfer problems from compressible thin core to its adjacent zones on dam section, from the viewpoint of settlements and total stability, it is concluded that an inclined core will show better behaviors than a vertical core. This is because of making stresses by rockfill zones of upstream and full reservoir and also seepage does exist naturally for consolidation of the core. Then arching potential will decrease in cases of that core is more compressible than rock fill. Acknowledgments The present study is funded by the University of Urmia. The support is gratefully acknowledged. Authors would also like to acknowledge Dr. Antony W. Wakefield of the University of Stanford of UK, for his valuable comments and discussions. Appendix Index n% = porosity percentage K = hydraulic conductivity c wet = Unit weight of wet soil c sat = Unit weight of saturated soil / = Friction angle of soil C = cohesive strength of soil Nonlinear Elastic (Hyperbolic) Model R f = ratio between the asymptote to the hyperbolic curve and the maximum shear strength K = modulus number describing the soil stiffness K ur = unloading-reloading modulus n = a value describing the rate of change of the soil stiffness as a function of the confining stress.

Geotech Geol Eng (211) 29:627 635 635 K b = bulk modulus m = a value describing the rate of change of the bulk mudulus as a function of the confining stress. References Babb AO, Mermel TW (1968) Catalog of dam disasters, failures and accidents. US Bureau of Reclamation, USA French National Committee (1973) International watertight cores. Proceedings 11th ICOLD congress, vol III, Q42, R28, Madrid Ghavoshan Powerhouse and Dam Design (24) Second process: technical reports, attachment 4. Mahab Ghodss Consultant Engineers, Iran Krahn J (28) Stress-deformation modeling with SIGMA/W, 3rd edn. An engineering methodology. GEO-SLOPE International Ltd, Calgary, Alberta Narita K (2) Design and construction of Embankment dams. Department of Civil Engineering, Aichi Institute of Technology, Aichi Ng AKL, Small JC (1999) A case study of hydraulic fracturing using finite element methods. Can Geotech J NRC Canada 36(5):861 875 Nobari ES, Duncan JM (1972) Movements in dams due to reservoir filling. Proceedings ASCE special conference on: performance of earth and earth supported structures. Purdue University, Lafayette, pp 797 815 Ogita S, Okumura T, Narita K, One Y (22) Hydraulic fracturing in earth and rock-fill dams. Bulletin of Aichi Institute of Technology, Part B, no. 37, pp 13 Ohne Y, Narita K (1977) Discussion on cracking and hydraulic fracturing in fill-type dams. Special session 8, 9th ICSMFE Ono K, Yamada M (1993) Analysis of arching action in granular mass. Geotechnique 43(1):15 12 Reinus E (1973) Some stability properties of having an inclined core. Proceedings of 11th ICOLD congress, vol II, Q42, R-2, Madrid Sherard JL (1991) Cracking and hydraulic fracturing in Embankment dams. US Army Corps of Engineers, pp 31 327 Wang J-J, Zhu J-G, Mroueh H, Chiu CF (27) Hydraulic fracturing of rock-fill dam. Multi-Science Publishing Co Ltd, Internat J Multiphy, vol 1,No. 2, pp. 199 219 (21) Zienkiewicz OC, Naylor DJ (1986) Static analysis of embankment dams, ICOLD, Bulletin No. 53