Assessment of the Foundation of Tunisia Ghezala Dam

Geotech Geol Eng DOI 10.1007/s10706-014-9825-9 ORIGINAL PAPER Assessment of the Foundation of Tunisia Ghezala Dam H. Karoui • M. Bouassida Received: 11 January 2014 / Accepted: 8 October 2014  Springer International Publishing Switzerland 2014 Abstract Ghezala Earth Dam was built in the city of Mateur in the North West area of Tunisia. The filling of the dam reservoir, with a storage capacity close to one hundred thousand cubic meters, started in 1985 and lasted for 2 years. The dam foundation is essentially composed of consistent clayey marl layer. At the lower level of the dam, reinforced concrete drainage, gallery was constructed to evacuate water, when needed, from the upstream side to downstream side. The gallery structure is composed of eleven 15 m length sections. During its service life (more than 26 years), an installed health monitoring system permitted, in particular, the measurement of consolidation settlement which has occurred in the soil foundation. This follow up system made it possible to observe the cracks which mainly developed progressively in the central portion of the drainage gallery due to differential settlements. In this context, a 2D numerical simulation of the behavior of dam foundation was performed by adopting the Mohr–Coulomb model for the dam’s material and the soil foundation. Numerical predictions of the consolidation settlement development were compared to those measured in situ and a good agreement was found between the two. H. Karoui
M. Bouassida (&) Université de Tunis El Manar, Ecole Nationale d’Ingénieurs de Tunis, LR14ES03, Ingénierie Géotechnique, BP 37 Le Belvédère, 1002 Tunis, Tunisia e-mail: mounir.bouassida@enit.rnu.tn This finding highlights the effectiveness of the adopted model for the studied case. Keywords Settlement
Gallery
Disorders
Elastoplastic
Consolidation
Piping 1 Introduction Detecting and analyzing the mechanisms of degradation that affect the structural health of dams is very important especially for safety reasons. The end result of the assessment is a list of recommended corrective actions to maintain dams in normal functioning conditions. Such an assessment cannot be performed without the installation of in situ measurement devices such as: piezometers, inclinometers, settlement gauges, etc. The collection of measurements over time, their treatment and their interpretation will enable to understand the functioning of the dam (Arfaoui 2007). In Tunisia 26 dams were built from 1928 to date. The assessment of these dams enabled the detection of degradations and allowed decision makers to adopt the best procedures for their maintenance and rehabilitation. Ghezala dam, object of this paper, is one of such dams; its drainage gallery was affected by severe cracks due to differential settlements. This paper presents a model that was used to predict the consolidation settlement of the dam foundation 123 Geotech Geol Eng that are believed to be main causes for the development of severe cracks and significant joints’ opening in the concrete drainage gallery observed at the downstream side. The paper layout is as follows: first, a presentation of Ghezala dam and the measured developed settlements are provided. Then, a numerical analysis, using plane strain conditions, is described. This analysis was carried out to simulate the observed behavior of the dam foundation. A comparison between the in situ measurements and the predicted results obtained by Plaxis finite element software (version 8) is presented. This comparison gives a helpful insight explaining the mechanism which had occurred during the operation of Ghezala dam over its 26 years of service life. Finally, the conclusions of the paper are drawn and presented. 2 Presentation of Ghezala Dam Ghezala dam was built from 1981 to 1984 as a homogeneous earth dam, 31 m in height and 560 m in length. It is founded on a compressible marl formation resting on calcareous stratum that is considered as an impervious and non-compressible layer. The elevation of High Water Level is 86.1 m (GNT Tunisian system). Ghezala dam provides a storage capacity of approximately one hundred thousand cubic meters devoted to the irrigation of 900 hectares of agriculture lands. A reinforced concrete drainage gallery, installed in 1983, crosses the dam structure. It is composed of eleven rectangular elements that are 15 m length, 2 m 9 2.5 m cross-section, and 0.6 m thick. The gallery is located 25 m deep under the elevation of full reservoir (Fig. 1). By the end of dam construction, a settlement of 200 mm has been measured in the central part of the gallery. In four portions (60 m in length), several openings of separation joints (J3 to J8) and transverse cracks were observed as sketched in Fig. 1. Among the ten main cracks (C1 to C10) six of them have an opening greater than 10 mm (Fig. 1). In post first fill-up of the dam reservoir in 1985, repairing works of cracks have been executed in the gallery structure. Later on, a progressive settlement development has resulted from the triggered piping of estimated amount by 1 kg per week in 2003 of soil material deposited at the downstream side (Arfaoui 2007). The maximum settlement of 455 mm was measured in 2008 under the crest of the 123 dam (Fig. 2). Piping phenomenon is caused by water infiltration from the gallery within the dam foundation; it induces the transport of particles at the exit point of the downstream side. From August to October 1985, the first repairing activities were performed immediately before the first filling of the reservoir. A bentonite grout injection was used as a primary repairing action. The progressing development of settlement during the following years induced the opening of cracks. For this reason second repairing activities were undertaken from 1992 to 1993. This repair mainly consisted in the treatment of opened cracks by using water swelling product prepared from a monomer aqueous solution. The additional recorded movements were approximately 120 mm. The piping phenomenon induced transport of sediments observed at crack F3 triggered an emergency repairing action. This showed that the second repairing activities were unsuitable to stop the piping phenomenon; the decision makers favored the installation, at crack locations, of PVC drains surrounded by geotextile filter which allow the water drainage and prevent the fines transportation. This solution efficiently aided in stopping transport sediment. In 1993 retrofitting of the cracks was performed by using a technique that involves the injection of chemical expansive product. In April 1996, the rate of leakage increased from 3.9 to 5.1 l/s (Arfaoui 2007). In a second step, another injection technique was tested, but it also revealed to be unsuitable to stop the water leakage. In June 1996, the control office recommended the application of a suitable geotextile filter plate product, which needs to be renewed once the leakage of water is again observed in the gallery. In 2007, the repairing of cracks was performed by the usage of a new product that is a composite material using reinforced polymer by ceramic steel. This product improved the water-tightness of the drainage gallery, and the transport of fines was prevented with much reduced leakage rate as 5 l/min. Figure 2 groups the development of observed behavior concerning both the water storage and leakage from which resulted the piping phenomena. 3 Analysis of Settlement Measurements The settlement development of Ghezala dam foundation has been followed up by means of topographic recorders made up of twenty-five points installed Geotech Geol Eng Fig. 1 Cross section of Ghezala dam with localization of cracks in the gallery structure Fig. 2 Debit of leakage and loss of solid quantities of Ghezala dam underneath the sub base of the gallery at the time of dam construction in 1982. A plot of the cumulative settlement of the gallery, from 1982 to 2008, (Fig. 3) shows that the maximum settlement is 45 cm in the central portion of gallery located under the crest elevation of the dam. The settlement linearly decreases towards the upstream and downstream sides quasi proportionally with the reduction of the dam height. It is noticed that the settlement over the first 3 years of the gallery construction was approximately 26.5 cm (1985), and reached approximately 32.2 cm in 1987, which is equivalent to approximately 71 % of the current maximum settlement measured after 26 years of service. 123 Geotech Geol Eng Fig. 3 Evolution of recorded settlement underneath the gallery of Ghezala Dam as shown in Fig. 4 were analyzed: section (A–A) is at the upstream side with an equivalent water pressure of 25 m height; section (B–B) at the crest level of maximum dam elevation of 30 m and, section (C–C) at the downstream side (Al Husein 2001). The perfect elastoplastic Mohr–Coulomb behavior has been adopted for the compressible marl foundation layer and the fill clayey material used to construct the dam (Brinkgreve 2003). The geotechnical parameters used in this study are summarized in Table 1. Three, scenarios for the degree of saturation of the marl layer were considered. The idea behind the scenarios is to simulate three different periods according to water infiltration around the gallery and the consequent settlement development. 4 Evolution of Settlement of Gallery Structure 5.2 Numerical Modeling Figure 3 shows that the measured settlements from 1982 to 2008 went through three different stages. The first stage lasted approximately 1,000 days (between 1982 and 1985) during which the construction of the dam had induced, in the central portion of gallery, the highest rate of settlement of about 90 mm/year. The second stage was between 1985 and 1987 when full fill up of the dam reservoir had been maintained. In this stage, a moderate settlement development was recorded at a rate of 20–25 mm/year. In the third stage (between 1987 and 2008), the rate of settlement varied from 5 to 6 mm/year. Further, the decreased trend of settlement development leads to deduce that the primary consolidation is almost ended, and therefore the behavior of the foundation of Ghezala dam is stable. 5 Numerical Simulation of Gallery Behavior 5.1 Geotechnical Conditions of Ghezala Dam The measured settlements, during the service life of Ghezala dam, show that the short term undrained conditions prevail to explain the observed behavior of the gallery. The latter is modeled as a reinforced concrete frame having a 2 9 2.5 m2 rectangular cross section, 0.6 m thick and 170 m long (Fig. 5). In order to simulate the behavior of the gallery along a given cross section of the dam, that is subjected to various loading conditions (Renon 2002), a plane strain analysis is undertaken. Three different cross sections, 123 Numerical computations were conducted using the half model presented in Fig. 5 due to the geometrical and mechanical symmetries (Boidy 2002). Figure 5 shows the typical plane strain model including the rectangular cross section of the gallery overlaid by variable thickness of dam material or height of water. The width of the model was taken as 40 m (ten times greater than the gallery width), so that the prediction of soil behavior around the gallery will not be affected by the condition of zero horizontal displacement along the vertical border (Fig. 6). The thickness of the marl foundation layer was set equal to 11 m, while the thickness of the earth dam layer was variable depending on the studied section. The numerical mesh is composed by 15 nodes triangular elements (Hicher and Chang 2007). The gallery is modeled as a reinforced concrete plate element having a linear elastic behavior with normal rigidity EA and bending stiffness EI. The initial at rest state defined by: K0 = 1 - sin u0 is considered for all materials which were treated as normally consolidated soils. The plane strain hypothesis was used by considering the primary consolidation procedure in clayey marl layer and by assuming anisotropic hydraulic conductivity for all materials (vertical and horizontal permeability are: kv = 10-8m/s and kh = 10-7m/s, respectively). 5.3 Processed Loading The embankment is not primarily activated, and then it is activated by single layers in accordance with the Geotech Geol Eng Fig. 4 Gallery cross sections Table 1 Geotechnical parameters and materials constitutive of numerical model Material Sr (%) Clayey marl 100 Dike’s material cd (kN/m3) ch (kN/m3) C0 (kPa) u () E (MPa) 16 20 35 25 20 90 15.4 19.4 30 20 20 50 15.35 17.5 25 20 20 16 20 30 27 15 Fig. 6 Finite element mesh of 2D model generated by Plaxis software Fig. 5 Cross section of the dam staged construction of the dam. Such optimized numerical procedure reveals efficient to simulate the staged construction as it happens during the actual construction of embankments. Indeed, the construction of earth dam was modeled by ten layers built in 1,090 days in conformity with the executed project. The water level at the upstream side is modeled by uniformly distributed pressure on the upper side of embankment as shown in Fig. 7, which refer to the last stage of applied loading (Bouassida et al. 2011). Two numerical computations were carried out, by using the mechanical model above described, in order to predict the settlement of the gallery foundation. Each simulation included two loading phases, the initialization of stresses followed by, the dam weight loading. 123 Geotech Geol Eng Fig. 7 Loading of the numerical model in plane strain condition Fig. 8 Evolution of settlement of the gallery: Section (A-A) 5.4 Interpretation of Results Measured and predicted settlements over time are compared for the three studied sections as shown in Figs. 8, 9 and 10. It is clear that the adopted numerical model retraces the overall observed behavior of the settlement development over time. The trend of the observed behavior was quasi identical for the three studied sections. In addition, the numerical results predicted what was observed in situ in that the rate of settlement development was very important in the period of 100–1,000 days after construction. Moreover, the adopted model shows that the magnitude of settlement depends on the thickness of dam material and water pressure when it is applied. Finally, the numerical model proves that the long term consolidation settlement will not affect the behavior of the 123 Fig. 9 Evolution of settlement of the gallery: Section (C-C) Fig. 10 Evolution of settlement of the gallery: Section (B-B) drainage gallery due to the insignificant calculated settlement between 1,000 and 10,000 days after construction: which is the same behavior in situ. Therefore, it can be concluded that the adopted mechanical model with its geotechnical parameters simulates thoroughly the observed behavior of Ghezala dam. 6 Solutions Against Piping The cracks opening and particle transportation can cause the leaching of clay material which induces a mechanism of ageing: • A pin-hole test should be performed for the dam clay material to assess their sensitivity against the risk of internal erosion. Geotech Geol Eng • • • • Plastic concrete diaphragm walls can be suggested to seal the body of the dam; The injection of low viscosity resine to the gallery extrados, by drilling the entire length of the gallery from its inside. This technique has been used for the treatment of leakage in the gallery (composed by 15 m long concrete wall) of Sayano Shushenskaya dam-Siberia, which is characterized by the opening of cracks and transporting of solids at a rate of 320 l/s. This technique has reduced the rate of sediment transport to 6 liter/second in post injection of resine. The gallery of the dam is not equipped with drainage devices at the contact dam-gallery. The rectangular section of the gallery does not ease the compaction of backfill material at the contact embankment-gallery. Thus, water infiltration may occur and contribute in favor of the instability of dam. 7 Conclusions The behavior of the foundation of Ghezala dam was simulated by using Plaxis code to explain the origin of disorders which had seriously affected the drainage gallery. Prevention solution has been formulated to stop the piping phenomena and to reduce the leakage flow of water from the gallery to the compressible marl layer. A 2D plane strain model made possible the simulation of observed settlements over 10,000 days underneath the gallery. The good agreement obtained between predicted and measured settlements leads to conclude that the adopted numerical modeling and mechanical characteristics of materials were suitable for the simulation of the behavior of Ghezala Dam. The numerical results as well as the measured values show that the development of consolidation settlement of the marl foundation layer of the gallery tends to be stabilized. References Al Husein M (2001) Etude du comportement différé des sols et ouvrages géotechniques. Thèse de doctorat, Université Joseph-Grenoble Arfaoui M (2007) Préparation des fissures du plot n4 de la galerie de prise d’eau du barrage Ghezala. Ministère de l’agriculture et des ressources en eaux. Tunis, 1–9 Boidy E (2002) Modélisation du comportement différé des cavités souterraines » Thèse de doctorat, Université Joseph-Grenoble I, pp 35–47 Bouassida M, Karoui H and Belaid M (2011) Pathology of foundation of Ghezala dam, a Tunisian case history. Proc. 15th African Regional Conference on Soil Mechanics and Geotechnical Engineering, 63–70 Brinkgreve R (2003) Manual of reference of Plaxis, Version 8 Hicher PY, Chang C (2007) A microstructural elastoplastic model for unsaturated granular materials. Int J Solids Struct 44(7–8):230–234 Mc Cann, M-W Jr et al (1997) National performance of dams program report. Hydropower Dams (4) Renon N (2002) Simulation numérique par éléments finis des grandes déformations des sols. Application à la Scarification. Thèse de doctorat, Ecole des Mines de Paris, pp 23–48 123