Ehlers, Stefan

Moll, Stephen

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Conference Paper, Published VersionEhlers, Stefan; Moll, StephenStrengthening of Angat Rockfill Dam and Dykes due toIdentified Dam Safety DeficienciesDresdner Wasserbauliche MitteilungenZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:Technische Universität Dresden, Institut für Wasserbau und technischeHydromechanikVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/104625Vorgeschlagene Zitierweise/Suggested citation:Ehlers, Stefan; Moll, Stephen (2018): Strengthening of Angat Rockfill Dam and Dykes due toIdentified Dam Safety Deficiencies. In: Technische Universität Dresden, Institut fürWasserbau und technische Hydromechanik (Hg.): Wasserbauwerke im Bestand - Sanierung,Umbau, Ersatzneubau und Rückbau. Dresdner Wasserbauliche Mitteilungen 60. Dresden:Technische Universität Dresden, Institut für Wasserbau und technische Hydromechanik. S.277-286.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Technische Universität Dresden – Fakultät Bauingenieurwesen Institut für Wasserbau und Technische Hydromechanik 41. Dresdner Wasserbaukolloquium 2018 „Wasserbauwerke im Bestand – Sanierung, Umbau,    Ersatzneubau und Rückbau“    C1  Saal 3  Strengthening of Angat Rockfill Dam and Dykes due to Identified Dam Safety Deficiencies Stefan Ehlers Stephen Moll  The Angat mulitpurpose storage scheme is located on Luzon Island in the Philip-pines. Angat reservoir is impounded by a Main Dam, a Main Dyke and a Second-ary Dyke, which are rockfill dams with inclined earthfill core. The Main Dam has a height of about 130 m. Comprehensive safety evaluations and analyses revealed that international state-of-the-art dam safety requirements are not met. For strengthening of the embankment dams a stabilizing fill at the downstream slopes was designed and tendered. This paper informs about the current status of the construction works. In addition, the heightening of the core was initially fore-seen at the Main Dam. By performing additional geotechnical investigations and updated stability analysis it was possible to demonstrate that the core heightening is not required. Keywords: Dam safety assessment, dam strengthening, rehabilitation 1 Introduction The Angat storage scheme is located in a highly seismic area on Luzon Island in the Philippines, around 60 km upstream of Metro Manila. The multipurpose pro-ject was built in the 1960s and provides potable water to Metro Manila and wa-ter for irrigation to farms, serves as flood control storage, and generates hydroe-lectric power. The Angat reservoir is formed by the Main Dam, Main Dyke and Secondary Dyke (Figure 1). Main Dam, Main Dyke and Secondary Dyke are rockfill dams with inclined im-pervious core. The Main Dam has a height of about 130 m. The Main Dyke is located on the ridge of a hill and has a height of about 40 m in its center, where-as its downstream slope has about the same height as the Main Dam. The Sec-ondary Dyke is adjacent to the Main Dyke and has a height of about 20 m. The downstream slopes are steep with an inclination of 1(v):1.4(h) at the Main Dam and 1(v):1.3(h) at the dykes, respectively. Upstream and downstream of the core three filter zones were designed, each with a width of 3 m. 278 Strengthening of Angat Rockfill Dam and Dykes due to identified dam safety deficiencies    Figure 1: Angat reservoir formed by Main Dam, Main Dyke and Secondary Dyke; sta-tus before the strengthening works (Google Earth, 2015) 2 Safety Evaluations 2.1 Dam and dykes classification In 2015, Pöyry has carried out safety evaluations of the Angat dam and dykes. The safety of the Angat scheme is of major importance because a dam or dyke break, in the worst case, would lead to flooding of large areas downstream in-cluding Metro Manila with its about 13 million habitants. Loss of life and con-siderable damages at infrastructure would be expected. Angat dam and dykes are classified as large dams with extreme consequence rating. 2.2 Flood Safety The flood safety of the Angat multipurpose scheme was evaluated under consid-eration of the Probable Maximum Flood (PMF) and flood events with return pe-riods of 10,000 and 1,000 years. At the Angat scheme the PMF event is decisive for the evaluation of the flood safety. The PMF was determined on the basis of the Probable Maximum Pre-cipitation (PMP). A comprehensive meteorological PMP assessment provides the state-of-the-art framework for the determination of most extreme precipita-280 Strengthening of Angat Rockfill Dam and Dykes due to identified dam safety deficiencies                                           Angat Main DykeSteady State Seepage (El. 212.00) - Stability Analysis - DS Slope                  CD Parameters - Factor of Safety : 1.377Random fillClayFiltersRolled or Dumped RockfillDistance-190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70120130140150160170180190200210220230-190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70Elevation120130140150160170180190200210220230 Figure 3: Stability analysis result of the downstream slope of Angat Main Dyke 2.4 Earthquake stability and deformation analyses The Angat multipurpose scheme is located in a highly seismic area. Seismic sta-bility analyses were carried out in accordance to the recommendations of the International Commission on Large Dams (ICOLD, 2016). The Safety Evalua-tion Earthquake (SEE) turned out to be the crucial, which is the maximum level of ground motion for which the dam should be designed or analysed. There must not be any uncontrolled release of water when the dam is subjected to the seis-mic load imposed by the SEE. Three different deterministically determined earthquake time histories were considered in the analyses. The horizontal peak ground acceleration (PGAh) of the SEE at the dam sites on rock surface of up to 0.94 g (at the Main Dyke) has been determined (Pöyry, 2015). Nonlinear dynamic response analyses were carried out using the equivalent-linear method (Seed and Idriss, 1970). The permanent earthquake-induced dis-placements were estimated according to the calculation procedure proposed by Newmark (Newmark, 1965). Potential sliding masses were selected and their yield accelerations were calculated by performing static slope stability calcula-tions. The permanent displacements were calculated by integrating the time history of the relative sliding velocity produced by the acceleration pulses exceeding the yield level. Significant slip circle sliding displacements of up to almost 9 m have been calculated at the embankment dam slopes. Excess pore pressures produced in the core and the filter layers of dam and dykes during the earthquake reduce the effective stresses in these zones. How-Dresdner Wasserbauliche Mitteilungen, Heft 60 – 41. Dresdner Wasserbaukolloquium 2018 „Wasserbauwerke im Bestand – Sanierung, Umbau, Ersatzneubau und Rückbau“ 281    C1  Saal 3   ever it was possible to demonstrate that the post-earthquake slope stability is provided. 2.5 Evaluation of the earthquake safety The critical seismic failure modes of embankment dams are loss of freeboard (overtopping) and internal erosion: (i) Loss of freeboard due to crest settlement Sliding of the upstream and downstream slopes plus densification of the embankment material (Bureau, 1997) would lead to considerable crest set-tlement due to the SEE. The freeboard criteria were met for the Main Dyke and the Secondary Dyke but not for the Main Dam. (ii) Internal erosion due to offset of the filter zones The predicted slope movements would cause an offset of the filter zones. As the upstream and downstream filter zones might not remain functional after the SEE, seepage paths may develop along cracks and the critical slip surfaces leading to internal erosion. 2.6 Potential fault displacements The West Valley Fault (WVF), a potentially active fault, takes course in a dis-tance of only about 200 m to the left bank of the Main Dyke. During a strong earthquake deformations could occur at existing sub-vertical fault planes in the footprints of Angat dam and dykes. The maximum displacements were estimat-ed to 5.3 m by means of an empirical formula reflecting the strike-slip nature of the WVF (Wells et al., 1994). Angat dam and dykes might not be able to cope with such large deformations due to the hazard of a concentrated leak develop-ing suddenly along the rupture surface formed in the embankments. There is considerable uncertainty in estimating the magnitude of potential fault displacements. Therefore a morphotectonic study is carried out to better charac-terize the neotectonic environment of the project. The investigations started in 2016, and are supported by the Philippine Institute of Volcanology and Seismol-ogy (PHIVOLCS). 3 Rehabilitation Concept proposed in 2015 The downstream slopes of Main Dam, Main Dyke and Secondary Dyke did not fulfil the required level of safety. Therefore it was proposed to place a stabiliz-ing fill at the downstream slopes of Angat dam and dykes. The required average inclination of the stabilizing fill was determined to 1(v):1.6(h) (Figure 4). Dresdner Wasserbauliche Mitteilungen, Heft 60 – 41. Dresdner Wasserbaukolloquium 2018 „Wasserbauwerke im Bestand – Sanierung, Umbau, Ersatzneubau und Rückbau“ 283    C1  Saal 3   possible to demonstrate that the freeboard criteria is also met for the Main Dam in the SEE load case. The contractor on site was informed in September 2017 that the heightening of the Main Dam’s core is not required and shall not be car-ried out. Main deformations caused by a serious earthquake would occur in the very up-per part of Angat dam and dykes, where the water load and the hydraulic gradi-ent are small in general. The properties of the core and the three stage filter are favourable with regard to their ability to cope with deformations. It was there-fore concluded that the potential for internal erosion due to the SEE ground shaking is small. By spillway operation the reservoir level can be lowered rela-tively fast below the damaged dam zones. Table 1: Estimated deformations at Angat Main Dam and Main Dyke due to SEE  Sliding Movement Vertical  Settlement  Upstream Slope Downstream Slope Main Dam 4.7 m 2.0 m 4.6 m Main Dyke 2.1 m 0.8 m 2.0 m 5 Implementation of the strengthening works The strengthening works comprising mainly the stabilizing fill placement at the Main Dam, Main Dyke and Secondary Dyke were awarded as EPC contract. Preparatory works and quarry development commenced in October 2016 and placement of stabilizing fill at the Main Dam started in January 2017. The quarry is developed at the right river bank downstream of the Main Dam on the ridge of a mountain, adjacent to an old quarry used during dam construction. The quarried rock is fresh to slightly weathered and durable volcanic and sedi-mentary rock, mainly Basalt / Diorite and Sandstone / Breccia. Figure 5 shows an aerial photograph of the project site with the ongoing con-struction works. The downstream faces and abutments of Main Dam and dykes are heavily over-grown, so thorough vegetation clearing is the first task to be done before place-ment of any new fill. Before placement of stabilizing fill against the existing dam surface, loose rock blocks are scraped off from the existing dam face to en-sure a good bond and interlocking between old and new rockfill. Both the vege-tation clearing and the scraping off of loose material is done simultaneously and advancing with the heightening of the stabilizing fill. Dresdner Wasserbauliche Mitteilungen, Heft 60 – 41. Dresdner Wasserbaukolloquium 2018 „Wasserbauwerke im Bestand – Sanierung, Umbau, Ersatzneubau und Rückbau“ 285    C1  Saal 3   By the end of November 2017 the stabilizing fill placement at the Main Dam was completed up to EL 197.0 m asl and at the Main Dyke up to EL 175.0 m asl. Works at the Secondary Dyke will commence after the stabilizing fill placement at the Main Dyke is completed. This is to make use of the road on the downstream face of the Main Dyke as access road. New monitoring instruments, which will be installed, include surface displace-ment points at the downstream dam and dykes faces, standpipe piezometers, seepage weirs at the toe of the Main Dam and Main Dyke and new strong mo-tion recorder for earthquake monitoring. The works are scheduled to be completed by mid-2018.  Figure 6: Placement of stabilizing fill at the Main Dam 6 Conclusion Angat Main Dam, Main Dyke and Secondary Dyke are large dams classified as high hazard dams due to the large Angat reservoir volume and their location up-stream of Metro Manila with currently about 13 million habitants. A compre-hensive dam safety assessment was carried out to evaluate their compliance with international state-of-the-art dam safety requirements and their ability to safely withstand serious flood and earthquake events. Required strengthening works comprising the flattening of the downstream slopes by placing a stabilizing fill were designed and are currently under con-struction. After the completion of the construction works Angat dam and dykes will provide a sufficient level of safety with regard to flood and earthquake safe-ty in accordance to international guidelines. 286 Strengthening of Angat Rockfill Dam and Dykes due to identified dam safety deficiencies   A morphotectonic study is currently in progress to evaluate potential defor-mations along fault planes in the footprints of Angat dam and dykes due to strong earthquake events in more detail. The study is expected to be available in 2018 and based on the results the final dam safety evaluation will be concluded. 7 References Bureau, G. (1997): Evaluation Methods and Acceptability of Seismic Deformations in Embankment Dams. Proceedings of the 19th Congress on Large Dams, Florence, Italy, 1997. Geo-Slope International Ltd. (2008): Modelling with SEEP/W 2007, SLOPE/W 2007, SIGMA/W 2007, and QUAKE/W 2007, an engineering methodology. Calgary, Alberta, Canada, 2008. Geoogle Earth Pro (2015). Google Inc., 2015. International Commission on Large Dams (ICOLD, 2016): Selecting Parameters for Large Dams, Guidelines. Bulletin 148 (revision Bulletin 72), Paris, 2016. Newmark, N. M. (1965): Effects of Earthquakes on Dams and Embankments. Geotechnique, Vol. 15, No. 2, pp. 139-160. Pöyry (2015): Probabilistic and Deterministic Seismic Hazard Analysis, Final Report. Angat Dam Rehabilitation Study, Phase 1 Task 6.1, 2015. Seed, H. B. and Idriss, I. M. (1970): Soil Moduli and Damping Factors for Dynamic Response Analysis. Report No. EERC 70-10, Earthquake Engineering Research Center, University of California, Berkeley, USA. Wells, D. L.; Coppersmith, K. J. (1994): New empirical relationships among magnitude, rupture length, rupture width, rupture area and surface displacement. Bulletin of the Seismological Society of America, v. 84, no. 4, p. 974, 1994. World Meteorological Organisation (WMO, 1986): Manual for Estimation of Probable Maximum Precipitation. WMO No. 332, 1986. World Meteorological Organisation (WMO, 2009): Manual on Estimation of Probable Maximum Precipitation, WMO No. 1045, 2009.   Authors:  Dipl.-Ing. Stefan Ehlers  Pöyry Schweiz AG Herostrasse 12 8048 Zürich Schweiz  Mob.: +41 76 356 2088 Tel.: +41 44 355 5555 E-Mail: stefan.ehlers@poyry.com Dipl.-Ing. Stephen Moll  Pöyry Energy Ltd. 1126/2 New Petchburi Road Makkasan, Rajchthewi 10400 Bangkok, Thailand  Mob.: +41 76 356 2672 Mob.: +66 61 405 6826 E-Mail: stephen.moll@poyry.com   

Strengthening of Angat Rockfill Dam and Dykes due to Identified Dam Safety Deficiencies

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Strengthening of Angat Rockfill Dam and Dykes due to Identified Dam Safety Deficiencies

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