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Design and performance of the Yele asphalt-core rockfill dam ¨ eg, and Yingbo Zhang ¨ eg, Weibiao Wang, Kaare Ho

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Abstract: The planning, design, and performance of the Yele asphalt-core rockfill dam in Sichuan Province, China, are presented. The dam has a maximum height of 124.5 m, is located in a very seismic region with inclement climate, and is founded on a geologically complex foundation, partly resting on a deep and pervious alluvial overburden and partly on bedrock. With these site conditions only an embankment-type dam was considered feasible, and three different options were considered for the design of the impervious barrier: ( i) earth core, (ii) concrete facing, and ( iii) asphalt core. The third option was chosen. The design is based on extensive analyses and laboratory tests, and a special model test was performed to study the behavior of the connection between the narrow asphalt core and concrete plinth. An impoundment test was performed when the dam was 73 m high to test the imperviousness of the constructed core and the connections between (i) the core and plinth and ( ii) the plinth and foundation cut-off wall. An extensive field monitoring program was implemented. Design predictions are compared with field performance observations of the core and its interaction with the adjacent transition zones. Key words: embankment dam, asphalt core, laboratory tests, finite element analyses, dam deformations, field monitoring.

Re´sume´ : La planification, la conception et la performance du barrage de Yele dans la province de Sichuan en Chine sont pre´ sente´ es dans cet article. Le barrage en remblai rocheux de Yele, avec noyau bitumineux et une hauteur maximum de 124,5 m, est localise´ dans une re´ gion de tre` s haute sismicite´ et sous un climat incle´ ment. Le barrage est porte´ sur une fondation geólogiquement complexe, en partie sur une couche e´ e´ paisse et perme´ able d’alluvions et en partie sur du roc. Sous ces conditions, la seule alternative faisable e´ tait un barrage en remblai. Trois options ont e´ e´ te´ conside´ re´ es pour la conception de la barrie` re impermeáble : (i) un noyau en terre, ( ii) un reveˆtement de be´ ton et (iii) un noyau bitumineux. Cette dernie` re solution a e´ te´ retenue. Le dimensionnement est base´ sur de nombreuses analyses et essais en laboratoire, ainsi que sure une mode´ lisation expe´ rimentale particulie` re du comportement de l’interface entre le mince noyau central et la plinthe de be´ton. Un essai de mise en eau a e´ te´ fait quand le barrage a atteint une hauteur de 73 m afin de tester l’imperme´ l’imperme´ abilite´ du noyau bitumineux, et les interfaces noyau–plinthe et plinthe–mur de fondation. Un programme extensif d’observations a aussi e´ te´ adopte´ . Les pre´dictions faites lors de la conception sont compare´ compare´ es avec le comportement in situ du noyau et de ses interactions avec les zones de transition adjacentes. ´ s : barrage en remblai, noyau bitumineux, essais de laboratoire, e´ Mots-cle´ e´ le´ ments finis, barrage, observations in situ.

Introduction The first embankment dam with a compacted asphalt concrete core was built in Germany in 1961–1962, and The Internat ter nation ional al Jou Journa rnall on Hyd Hydrop ropowe owerr & Dam Damss (Sa (Saxeg xegaar aard d 2010) provides a listing of asphalt-core dams that have been built or are under construction in different countries. The International Commission on Large Dams (ICOLD) and others have summarized the experience with the design, construction ti on,, an and d pe perf rform orman ance ce of th this is ty type pe of da dam m (e (e.g .g., ., IC ICOL OLD D 1992; Hoëg 1993; Creegan and Monismith 1996; Scho¨ nian 1999; Hoëg et al. 2007; Wang 2008). Mostt asp Mos asphal halt-c t-core ore dam damss hav havee bee been n bui built lt in Eur Europe ope,, but China has also built and is currently building several dams of this type, among them the 170 m high Quxue Dam that Received 11 May 2009. Accepted 22 March 2010. Published on the NRC Research Press Web site at cgj.nrc.ca on 16 November 2010. W. Wang1 and Y. Zhang. Xi’an University of Technology, 5 Jinhua South Road, 710048 Xi’an, China. ¨ eg. Norwegian Geotechnical Institute (NGI), P.O. Box ëg. K. Ho 3930 Ullevaal Stadion, NO-0806, Oslo, Norway. 1

Corresponding Correspondi ng author (e-mail: wangweibia wangweibiao59@hotm [email protected]). ail.com).

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will be the highest so far. Spain, Saudi-Arabia, and Iran recently built their first such dams. Canada just completed an asphalt-core dam, the first of its kind in North America (Alicescu et al. 2008), and Hydro Que´ bec has decided to construct str uct several several mor moree emb embank ankmen mentt dam damss of thi thiss typ typee in the Provin Pro vince ce of Que Quebec bec (La Rom Romain ainee pro projec ject). t). Bra Brazil zil is cur cur-rently ren tly com comple pletin ting g its fir first st asp asphal halt-c t-core ore dam (Fo (Fozz de Cha Cha-peco), pec o), and several several dam damss of thi thiss typ typee are being considere considered d for a very large hydropower development in the Amazon region. Thiss pap Thi paper er pre presen sents ts the des design ign and per perfor forman mance ce of the Yele asphalt-core rockfill dam in China, describes the challenging site conditions and the studies performed to ensure the quality quality of the asphalt asphalt cor coree and its con connec nectio tion n wit with h the concrete concr ete plinth, and eval evaluates uates the perfo performanc rmancee of the core based bas ed on fie field ld mon monito itorin ring. g. Con Constr struct uction ion sta starte rted d in Apr April il 2001 and was completed in December 2005.

Yele Dam site conditions The Yele hydro project, on the very upper reach of the Nanya Nan ya Riv River er in the southwes southwestt of Sic Sichua huan n Pro Provin vince, ce, is one of six projects in a cascade development for electricity generation. The river is 49.5 km long with a hydraulic drop of

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1714 m and an exploitable hydropower potential estimated to be 700 MW. The topography of the Yele basin is well suited for building a reservoir with a catchment of 323 km 2. The annual mean river flow is 14.5 m 3 /s, which is composed mainly of rainfall, snowmelt, and groundwater. The river basin planning includes a Yele balancing reservoir for yearover-year storage and the following power stations: Yele (installed capacity of 240 MW), Liziping (120 MW), Yaoheba (123 MW), Nanguaqiao (120 MW), Ximagu (42 MW), and Daduhebian (60 MW). Therefore, the Yele reservoir will play a very important role in the Nanya River cascade development. At Yele the winter season is 6 to 7 months long and the rainy season is from May to October. Annually there are about 215 rainy days with a mean rainfall of 1830 mm and air relative humidity of 86%. The annual mean temperature is 7 C, ranging from –20 to +28 C. Figure 1 shows the geological conditions along the longitudinal section of the dam (Yu 2004; Hao and He 2008). The deep overburden from the bottom to the top may be classified into the following five groups: 8

8

(1) Q21I and Q22I — gravel with thin silty sand layers. (2) Q31II — overconsolidated and stiff cohesive soil containing a significant amount of stones, but with low permeability; thickness of 31–46 m. (3) Q32–1III — gravel with layers of loam; thickness of 46– 154 m. (4) Q32–2IV — gravel; thickness of 65–85 m. (5) Q32–3V — sandy silt layers with carbonized plant fragments; thickness of 90–107 m. On the left bank, under the 35–60 m overburden, there is fractured and jointed quartz diorite bedrock as shown in Fig. 1. The overburden is 55–160 m deep under the bottom of the valley and more than 220 m deep on the right bank.

Dam type selection and design For the difficult geological foundation conditions with an irregular and compressible overburden and with the high regional seismicity, only an embankment-type dam was considered feasible. Three options were examined for the impervious barrier in a rockfill dam: ( i) earth core (ECRD), (ii) upstream concrete facing (CFRD), and ( iii) asphalt core (ACRD). To decide among these options, emphasis was placed on costs, sensitivity to severe weather conditions during construction, earthquake resistance, and compatibility with the geological conditions that may cause significant differential settlements across the valley. Hoëg et al. (2007) provides a general discussion of the relative merits of the three options. In the rainy and cold Yele area at high altitude (crest elevation (el.) 2654.5 metres above sea level (m.asl.)), the water content of the earth core material in the local borrow was 10% more than that required for optimum compaction. It would be difficult and time-consuming to reduce the water content, and core placement would have to be stopped during the frequent rainy periods. For the CFRD option, the concrete slabs would be placed after the upstream slope was completed, and it was 2 3

considered difficult to protect the upstream slope from damage by sudden heavy rains during the dam construction period. Furthermore, the impounding could not commence until the dam with a concrete face was completed. This was a disadvantage at the Yele site, where the reservoir would take a long time to fill as the rate of annual river flow is low. The core for an ACRD may be constructed during periods of rain and cold weather. During heavy rains, the asphalt mix is stored in hot silos. When the heavy rain stops, the asphalt-core construction can be restarted immediately after cleaning and heating the asphalt surface, without the long delay associated with the earth core. An infrared heater is mounted in the front of the core paver. The CFRD requires a longer concrete plinth than the ACRD, and if large differential settlements occur, leakage may develop in the joints between the slabs and in perimeter joints due to rupture of water stops. At the Yele site, such settlements could be caused by the nonuniform geological foundation conditions and severe earthquakes, which may cause large in-plane stresses in the concrete face. On the other hand, the ACRD with a central asphalt concrete core, if properly designed, is considered sufficiently flexible and ductile to be able to accommodate differential settlements without cracking. For the site and environmental conditions at Yele, the ECRD was estimated to cost approximately 10% more than the ACRD, while the CFRD was estimated to cost around 10% less than the ACRD. Among the three options, all aspects considered, the ACRD was selected as the most suitable (Hao and He 2003). At the start of the preliminary design of the Yele Dam in 1990, there were only a few asphalt-core dams of similar height: the High Island West and East Dams in Hong Kong (95 and 105 m, respectively), the Finstertal dam in Austria (150 m, but with a core height of only 96 m due to a rock ridge under the core), and the Storvatn Dam in Norway (90 m). In 1990 the Storglomvatn Dam (125 m high) in Norway was in the final design stage (construction was completed in 1997) long before the start of construction of the Yele Dam (Hoëg et al. 2007). However, the Yele Dam was to be designed and built for a site with much more complex foundation conditions than any of the previous dams and is located in a region with much higher seismicity. Figure 2 shows the maximum area cross section of the Yele Dam. The asphalt core in that cross section is 120 m high and the total dam height 124.5 m. Figure 3 shows a plan view of the dam with the locations of cross-sections A–G shown, and Fig. 4 shows a longitudinal section giving the locations of the same cross sections. As designed, the Yele asphalt-core rockfill dam has a crest length of 411 m with a 300 m long seepage cut-off wall extension over the right bank. Due to the very high seismicity of the region (Sichuan Province) with an assumed peak horizontal ground acceleration of 0.45 g2 at the Yele site, the dam is designed with gentle slopes of 1V:2H upstream (where V represents vertical and H represent horizontal) and 1V:2.2H downstream, and a wide crest (14 m). In addition, as an earthquake-resistant measure, geo-grids 3 were placed hori-

See Appendix A. See Appendix B. Published by NRC Research Press

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Fig. 1. Geological cross section of the Yele Dam foundation and abutments. 1, gravel with silty sand layers; 2, stiff, overconsolidated cohesive soils with stones; 3, gravel with layers of loam; 4, gravel; 5, sandy soil with loam and carbonized plant fragments; 6, quartz diorite bedrock; 7, crevice–lineament. W.L., water level.


Fig. 2. Cross section of the Yele Dam (this is section D as shown in Figs. 3 and 4). All dimensions in metres. 1, asphalt core; 2, transition zone; 3, rockfill (I); 4, rockfill (II); 5, natural gravel or rockfill (III); 6, toe berm (22 m in thickness and 215 m in length); 7, observation gallery for field instrumentation; 8, concrete cut-off wall. (There is no grout curtain under section D, see Fig. 5.)

zontally to reinforce the top 30 m of the dam (from el. 2624.5 m.asl. to the dam crest at el. 2654.5 m.asl.). Within the upper 20 m, the vertical height difference between the geo-grids is 1 m and within the lower 10 m is 2 m. Furthermore, to strengthen the lower part of the upstream dam slope against large deformations and potential sliding during an earthquake, 40 m long geo-grids were placed horizontally from the upstream dam face between el. 2594 m.asl. and el. 2603 m.asl. The vertical height difference between these geo-grids is 1.5 m.

Design and construction of impervious barriers in the Yele dam foundation Figure 5 shows the complex system of impervious barriers installed in the foundation to reduce and control the under-

seepage. The foundation barriers may be divided into three main sections from the left to the right bank: left bank barrier section, river bed barrier section, and right bank barrier section. After excavating the top of the overburden at the left bank, a 20–60 m deep concrete cut-off wall was constructed through the overburden, down to the sloping diorite bedrock. A grout curtain was injected into the quartz diorite through the concrete cut-off wall. A 150 m long and 80 m deep grout curtain was injected into the quartz diorite from the construction gallery (No. 7 shown on the left side of Fig. 5). For the river bed overburden, a 30–60 m deep concrete cut-off wall was brought 5 m down into the relatively impervious soil layer Q 31II shown in Fig. 1. For the right bank, the overburden is so deep that the water barriers had to be built in four stages. The upper first barrier is the 15 m high concrete wall extension built in the open excavation; Published by NRC Research Press

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Fig. 3. Plan view of Yele Dam and location of monitoring instruments. 1, displacement bolts; 2, observation gallery; 3, piezometers; 4, access galleries; 5, drainage gallery; 6, grouting gallery; 7, observation huts. Locations of cross-sections A–G are also shown in Fig. 4.


Fig. 4. Longitudinal section of the asphalt core for Yele Dam showing locations of cross-sections A–G (see also Fig. 3).

the second barrier is the concrete cut-off wall with a depth of 70 m down to the top of the second level construction gallery; the third is the 60–84 m deep concrete cut-off wall installed from the second level construction gallery, and the fourth is the grout curtain with a maximum depth of 120 m installed through the concrete cut-off wall. For more details about the very complex system of cut-off walls and grout curtains, an extremely demanding task, refer to Chen (2003) and Hao and He (2008).

Asphalt-core design investigations After the asphalt-core option was selected in the early design stage, a special test program was prepared to study the following aspects in more detail (Sun and Wang 1994):

(1) Suitability of aggregates of local quartz diorite and local natural sands. (2) Optimum asphalt mix design with the available aggregates, filler materials, and bitumen grade. (3) Triaxial compression stress–strain–strength behaviour of alternative mix designs. (4) Testing of tensile, bending, and creep behavior of the asphalt mix. (5) Resistance of asphalt concrete to cyclic loading simulating earthquake shaking. The quartz diorite quarry is located 3 km downstream of the dam site nearby an access road, while a dolomite quarry is located in the reservoir area 16 km upstream of the dam site. A special access road would have to be built to use the Published by NRC Research Press

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Fig. 5. Water barriers in the Yele Dam foundation. All dimensions in metres. 1, crest; 2, asphalt core; 3, ground surface; 4, excavation line; 5, concrete cut-off wall; 6, grout curtain; 7, construction gallery used for construction of grout curtain and concrete cut-off wall; 8, concrete cut-off wall extension built in the open excavation.


dolomite quarry. Dolomite is alkaline and has very good adhesion to bitumen (grade 5, Chinese boiling test, standard DL/T 5362-2006 (People’s Republic of China National Development and Reform Commission 2006)). It is considered to be very suitable as an aggregate in asphalt concrete. Quartz diorite is slightly acidic and therefore has weaker adhesion to bitumen (grade 4). However, as shown by Wang et al. (2010), for hydraulic asphalt concrete with air porosity <3%, somewhat poorer aggregate–bitumen adhesion has no significant effects on the stress–strain–strength behaviour and weathering resistance. Therefore, aggregates from the quartz diorite quarry were considered satisfactory, and this was a much more economical solution. Hydraulic asphalt to be used in a dam core should be virtually impervious, flexible, and workable. The aggregate composition complies with Fuller’s gradation curve improved with a fine grain component smaller than 0.075 mm (filler material) (Hoëg 1993). To ensure very low permeability (about 10 –11 m/s), the air porosity in the dam core should be less than 3% (Hoëg 1993; Wang and Hoëg 2009). Triaxial compression tests should be carried out under different confining stresses to assure that the asphalt concrete exhibits flexible and ductile (not strain-softening) behaviour required to adjust to dam deformations caused by static and dynamic loads and differential foundation settlements. For the Yele Dam asphalt core, the bitumen content (type AH-70 in Chinese standard DL/T 5411-2009 (People’s Republic of China National Energy Administration 2009)) is 6.3% by total weight and the filler content is 12%. About 30% of the fine aggregates (2.36–0.075 mm) consist of natural sands (rounded particles) to improve the workability of the asphalt concrete. Many of the details of these experimental studies and test results are reported by Wang (2008), who investigated the permeability of asphalt concrete as a function of imposed shear strains. Two-and three-dimensional finite element analyses to study stresses, strains, and deformations in different embankment zones were also performed. 4 4

An additional test program was undertaken to focus on the effects of shear displacements causing possible leakage at the joint between the asphalt core and the concrete plinth (Wang and Sun 1997, 1999). Special attention was given to the mix proportions of the sandy asphalt mastic placed on the core–plinth interface. Figure 6 shows the design of the critical asphalt core– plinth connection and the top of the concrete cut-off wall in the foundation. A model (1:10 scale) was built to test the core–plinth interface when it was subjected to shear displacements in the downstream direction and high water pressure (Fig. 7). The asphalt core in the model was 240 mm wide at the bottom, 120 mm wide at the top, and 330 mm in height. The asphalt mastic layer between the core and the concrete slab was 20 mm thick. The mastic mix consisted of bitumen type AH-70, limestone filler, and river sand in the proportions 1:2:1, respectively. The core–plinth connection model tests were run at a temperature of 7 C. As shown in Fig. 7, the water pressure at the interface between the asphalt concrete core and the concrete plinth was kept at 0.3 MPa (i.e., 30 m of head) during most of the test. The vertical stress at the base of the asphalt core in the Yele Dam design was computed by finite element analyses to be 1.65 MPa. The vertical stress applied in the model was increased gradually up to 1.65 MPa within 30 min. Then the concrete slab (plinth) was pushed horizontally. The shear stress required to make the slab move relative to the base of the core was 0.6 MPa, and the slab displacement rate was then kept at 0.1 mm/min. The resulting shear stress on the mastic layer was increased from 0.6 to 1.35 MPa during the shearing process. After 200 min of testing, the vertical displacement of the core and horizontal displacement at the interface were 17.4 and 20.5 mm, respectively. No leakage was detected even when the water pressure was increased from 0.3 to 1.0 MPa at the end of the test. The slab was then pushed at 1 mm/min to reach a shear displacement of 22 mm, i.e., 9% of the core thickness. This was the maximum shear displacement the model allowed. No leakage 8

See Appendix C. Published by NRC Research Press

1370 Fig. 6. Structural connection between the asphalt core and concrete plinth (all dimensions in metres; left side of figure is upstream, right is downstream). 1, asphalt core; 2, transition zone; 3, 1–2 cm thick sandy asphalt mastic; 4, geo-membrane covering foundation to upstream dam toe; 5, silt; 6, filter and drainage layer; 7, concrete cut-off wall; 8, reinforced concrete plinth; 9, foundation overburden.



dams had been completed in China, most of them were small and the cores had been constructed manually or with simple and improvised equipment. There was a lack of expertise and available modern equipment to build large asphalt-core dams. However, at the time, the asphalt core for the Maopingxi Dam (part of the Three Gorges Project) with a height of 105 m was under construction using a modern asphalt-core paver purchased from the Norwegian contractor Kolo Veidekke a.s. The Yele asphalt-core construction presented a special challenge because of the cold and rainy weather and a very tight construction schedule. A Chinese asphalt paver was built and construction procedures were developed for placing the asphalt core during the night and at air temperatures down to –5 C. The design of the asphalt-core paver was made very similar to the Norwegian one used for placing the Maopingxi Dam asphalt core (Hoëg 1993). Several job trials were undertaken before asphalt-core construction started. The asphalt-core paver places simultaneously the asphalt core and the adjacent supporting transition zones. The total width that could be placed by the new paver was 3.8 m; thus, the transition zones on either side of the core were each 1.3 m wide at the bottom and 1.6 m wide at the top. The core and adjacent transition zones were built up and compacted in 26 cm thick layers (compacted thickness). During dam construction, the core top elevation was at all times above that of the embankment rockfill and did not slow down the rapid construction progress. The Yele asphalt-core construction was started in November 2003 and was completed in November 2005. Systematic quality control of the asphalt core was carried out throughout the construction period to ensure that the air porosity of the asphalt concrete in place was less than the specified 3%, which gives a virtually impervious core (Hoëg 1993). Zones, compaction specifications, and quality control for the Yele Dam are shown in Table 1. Figure 8 shows the progress of dam construction and impounding until October 2007, and the operation until November 2008 is shown in the first figure in the section titled ‘‘Field performance observations of the asphalt core’’. The Yele Dam embankment construction started in April 2001 and in December 2004 the dam reached el. 2603 m.asl., which is 51.5 m below the crest of the dam (el. 2654.5 m.asl.). In January 2005 a special impounding test was started. At that time the water level was at el. 2552 m.asl. behind the upstream cofferdam, as shown in Fig. 8. After one month of impounding, the water level was raised 35.5 m (to el. 2587 m.asl.). Then the water was lowered to its original level at el. 2552 m.asl. Observations were made of the pore-water pressures on the downstream side of the core, of the deformations of the core, and of the strains in the concrete plinth and cut-off wall during the raising and lowering of the reservoir while the embankment height was kept constant at el. 2603 m.asl. The pore-water pressures on the downstream side of the core wall were measured to be close to zero during the impounding test, and the deformations of the asphalt core and the strains in the concrete plinth and cut-off wall were very small (see later discussion of performance observations). On 9 March 2005 the reservoir water level was raised again and reached el. 2634 m.asl. on 8

was detected during the model testing desspite the large imposed shear strains. The vertical stress, shear stress, vertical displacement, and shear displacement versus time for the model test are shown in Fig. 7. When the model was removed from the testing apparatus, some of the mastic was discovered to have extruded, and the mastic layer thickness had reduced to 12 mm from the initial 20 mm. However, no cracks or fissures were detected at the core–plinth interface (Chu et al. 2004). The test results showed that the behavior at the interface was satisfactory even for shear distortions much larger than anticipated in the field.

Dam construction and simultaneous reservoir impounding The vertical core wall, located 3.7 m upstream of the dam centerline, was designed to be 1.20 m wide at the bottom and decreasing gradually to 0.60 m at the top (el. 2653 m.asl.). The base of the core is flared out against the plinth to a width of 2.40 m at the core–plinth interface. Similarly, the core is flared out against the plinth at the abutments to twice the core width at that elevation. Although before the year 2000 more than 10 asphalt-core


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Fig. 7. Model test to study behaviour of core–plinth interface when subjected to interface shear displacement and high water pressure. (a) Model of the asphalt core–plinth connection. ( b) Measured stresses and displacements versus time for the 1:10 scale model of the asphalt core–plinth connection. Disp., displacement.


26 October 2005. The embankment reached the crest elevation 2654.5 m.asl. in December 2005. During operation and power generation the first year, the water level dropped to el. 2600 m.asl., but rose again to el. 2642.5 m.asl. by 12 December 2006 (see Fig. 8). On 23 April 2007 the water level had dropped to el. 2609 m.asl., but on 23 October 2007 it rose again to el. 2648 m.asl., which is 2 m below full supply water level at el. 2650 m.asl. During construction, impounding and operation of the dam was monitored by means of a comprehensive instrumentation system as described below.

Dam monitoring and performance observations The monitoring system consists of measuring dam body

deformations, seepage through the core, foundation and abutments, water pressures in the abutments and foundation, stresses and strains in the asphalt core, in the concrete plinth and in the cut-off wall, temperatures inside the core, and accelerations during any earthquake shaking (Chen 2003; NRBHDC 2007; Chen et al. 2009).

Measured dam surface displacements during and after construction Figure 3 shows the arrangement of displacement observation bolts on the dam surface. The 99 bolts are installed along seven longitudinal lines, one on the upstream slope, two on the dam crest, one on the top of core, and three on the downstream slope. The horizontal distance between bolts is 50 m, corresponding to cross-sections B, C, D, E, and F shown on Figs. 3 and 4. The geodetic surveys for displacePublished by NRC Research Press

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Table 1. Zoning, compaction specifications and quality control for Yele Dam (standards SL274-2001 (The Ministry of Water Resources of the People’s Republic of China 2002) and DL/T 5411-2009 (People’s Republic of China National Energy Administration 2009)).


Layer thickness (m) 0.20–0.26 0.20–0.26

Compaction by vibratory roller 8 passes, 1.5 t 4 passes, 2 t

0.20–0.26

4 passes, 2 t

1.0–1.2

8 passes, 20 t

Quarried rock (0–800 mm)

1.0–1.2

8 passes, 20 t

Sho uld er (III)

Natu ral g ravel o r q uarried rock (0– 80 0 mm)

1.0 –1 .2

8 p as ses, 2 0 t

Toe berm

Rock debris (0–1200 mm)

1.0–1.2

8 passes, 20 t

Zone (see Fig. 2) Asphalt core Transition zone A 1.3–1.6 m Transition zone B 2.0–4.0 m Shoulder (I)

Material Bitumen AH-70, aggregate (0–20 mm) Gravel (0–80 mm, grain size < 5 mm passing 20%– 40%; grain size < 0.075 mm passing  10% Gravel (0–150 mm, grain size < 5 mm passing 10%–20%; grain size < 0.075 mm passing  3% Quarried rock (0–800 mm)

Shoulder (II)

Fig. 8. Progress of dam construction and reservoir impounding. (Note: the reservoir was held back by the upstream cofferdam before the impounding test started at el. 2552 m.asl.)

ment monitoring used six fixed benchmarks in the vicinity of the dam. Only a few of the dam surface deformation measurements will be discussed, as in this paper the focus is on the behaviour of the asphalt core and its interaction with the transition zones. As an example, the movements of the bolts on the upstream berm at el. 2620 m. asl., a distance of 79 m from the vertical dam axis, are shown in Table 2. The settlements have been measured since 25 July 2005 when the dam rockfill was at el.2627.8 m.asl. After placement of additional rockfill (26.7 m) to the top of the embankment, and after reservoir raising and lowering during the construction and operation (see Fig. 8), the settlements of the bolts along this berm on 10 September 2005 and 9 May 2007 are as shown in Table 2. Over the almost 2 year observation period, the measurements in Table 1 seem very consistent with the largest settlement of 82 mm at the maximum section D. On the left bank, the reduction in effective stresses due to impounding caused the berm to heave 9 mm. On the right bank with the deep overburden, the reduction of the effective stresses due to impounding caused the berm to heave 25 mm. Figure 9 shows the settlements of the bolts on the downstream berms at el. 2594.5 m.asl. and el. 2624.5 m.asl. at the end of September 2007 (20 months after end of construc-

Quality control Air porosity < 3.0% Dry density ‡ 20.6 kN/m3, porosity  20% Porosity  22% Dry density ‡ 21.9 kN/m3, porosity  24% Dry density ‡ 22.5 kN/m3, porosity  22% Dry d en sity ‡ 21.1 kN/m3, porosity  22% (upstream), 24% (downstream) Dry density ‡ 19.6 kN/m3

tion). The maximum settlement (combination of construction and post-construction settlements) is 60 mm between crosssections D and E. It should be noted that the bolt displacements at the lower berm have been measured since 12 November 2005 (when installed) and at the higher berm since 15 February 2006. Post-construction displacement observations at the top of the core (3.7 m upstream of the dam centerline), upstream crown points, and downstream crown points were started on 21 June 2006. Unfortunately, this is 0.5 years after the end of dam construction, so the post-construction displacements during the first months are not included. The post-construction displacements recorded on 27 September 2007 are shown in Fig. 10. The maximum post-construction settlement is measured to be 45 mm at the upstream crown point in cross-section E. However, the settlement may actually have been almost twice that if one were to include the post-construction settlements during the first 0.5 years after the end of construction. During the post-construction observation period until October 2007, the dam had experienced two cycles of water level rising and lowering of around 40 m (Fig. 8). As shown in Fig. 10a, the core crest settlement is 40 mm at section D. The settlement is slightly more at the upstream crown point and slightly more near the right bank than near the left bank. The downstream crown point shows about 10 mm less settlement than the core. The post-construction horizontal displacements of the crown points and top of core are shown in Fig. 10b. The downstream crown points show more displacement than the upstream crown point and the top of the core. The maximum is about 27 mm in cross-section D. The downstream crown point shows less post-construction settlement, but more downstream displacement than the upstream crown point, which agrees with the observations from many other central core rockfill dams (Hoëg et al. 2007). This is due to the effects of impounding on the behaviour of the upstream fill. Up until January 2009, the dam had experienced three cycles of water level rising and lowering, and the dam had been subjected to 10 earthquakes of various magnitudes (Chen et al. 2009; Zhao et al. 2009). The maximum postconstruction settlement occurred at the upstream point of the crest at section D (maximum cross section of the dam), and the value was about 14 cm in June 2008. For comparison, Published by NRC Research Press

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Table 2. Settlement (settlm.) of bolts on the upstream berm at el.2620 m.asl. Distance (in m) from dam crest on left bank, el.2654.5 m.asl.

Date 25 Jul 2005 10 Sep 2005 9 May 2007

Water level (m.asl.) 2584.5 2619.3 2610.1

Embankment level (m.asl.) 2627.8 2637.8 2654.5

100*

120 (B)

170 (C)

220 (D)

270 (E)

320 (F)

343*

Settlm. (mm) 0 –4 –9

Settlm. (mm) 0 1 16

Settlm. (mm) 0 23 52



Settlm. (mm) 0 6 33

Settlm. (mm) 0 –14 –25

Note: B, C, D, E, and F refer to section locations shown on Figs. 3 and 4. *Points located over the left and right banks just inside the embankment.


Fig. 9. Settlement of bolts on the downstream slope. (Observation periods: el. 2594.5 m.asl., 12 November 2005 – 30 September 2007; el. 2624.5 m.asl., 15 February 2006 – 29 September 2007.)

the settlement of the upstream point of the crest for the Storglomvatn asphalt-core rockfill dam, also 125 m high, was 18 cm after the first 2 years of operation (Hoëg et al. 2007). Table 3 shows the settlements of the upstream points of the crest before and after the Wenchuan earthquake, 12 May 2008. When the Wenchuan earthquake struck, the Yele reservoir level was near the minimum operating level el. 2600 m.asl. The maximum additional crest settlement during the earthquake was about 15 mm at section D. The Yele Dam site is located 258 km from the epicenter of the Wenchuan earthquake (magnitude 8.0) and the intensity (Chinese scale) at the dam site was less than VI (Chen et al. 2009; Zhao et al. 2009). According to the monitored accelerations from nine strong-motion seismographs installed on and in the Yele Dam, the calculated maximum settlement, horizontal displacement (downstream direction), and longitudinal displacement (along dam axis) of the dam crest induced by the Wenchuan earthquake were 19, 25, and 17 mm, respectively. The several other earthquakes that have occurred since the end of construction have had insignificant effects on the dam. In summary, the dam surface displacements show uniform and consistent deformation patterns even after having experienced the Wenchuan earthquake, and the observed settlements are smaller than expected. This must mean that the rockfill and gravel are of high quality, the embankment was

Fig. 10. Post-construction settlement and horizontal displacement of crown and top of core. ( a) Post-construction settlement at the top of the core and the upstream and downstream points on the crown. (b) Post-construction horizontal displacement in the downstream direction at the top of the core and the upstream and downstream points on the crown. (Measurements started 0.5 years after the end of dam construction.)

very well compacted, and that the alluvial overburden in the foundation is less compressible than anticipated.

Measured deformations inside the dam Five observation huts on the dam crest and five on the Published by NRC Research Press

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Can. Geotech. J. Vol. 47, 2010 Table 3. Settlements of the upstream points of the crest before and after the Wenchuan earthquake, 12 May 2008. Distance (in m) from dam crest on left bank el.2654.5 m.asl.

Date 25 Apr 2008 19 May 2008

Water level (m.asl.) 2602.0 2603.7

67*

120 (B)

170 (C)

220 (D)

270 (E)

320 (F)

365*

Settlm. (mm) 31.5 32.6

Settlm. (mm) 88.3 94.1

Settlm. (mm) 116.1 123.5

Settlm. (mm) 128.8 143.4

Settlm. (mm) 128.2 136.0

Settlm. (mm) 102.9 114.6

Settlm. (mm) 56.6 58.2

*Points located over the left and right banks outside the embankment.


berms on the downstream slope were constructed (see Fig. 3). They are used for collecting all the measurements of displacements inside the dam. The vertical displacements inside the dam are obtained by water level settlement gages and the horizontal displacements by extensometer measurements relative to the movements of the bolts on the observation huts. The reinforced concrete observation gallery inside the downstream dam body is located 8 m downstream from the dam axis. The base slab is at el. 2560 m.asl., 30 m above the dam plinth, which ensures that it is well above the line of saturation (see Figs. 2 and 3). The gallery is used to collect measurements of pore pressures in the dam foundation. The instrumentation leads that collect strain measurements in the plinth and cut-off wall are routed through a prefabricated vertical concrete pipe, which is located in the downstream shell near the right abutment. The instrumentation leads that collect measurements taken in the core and transition zone are routed vertically through the transition zone. During dam construction, the instrumentation leads were temporarily protected by vertical steel pipes. Three vertical pipes with a total of 29 electromagnetic rings were installed inside the downstream transition zone behind the core, to measure local vertical and horizontal displacements. The rings were arranged with an individual height difference of 10 m. Special gages were installed at the upstream and downstream interfaces between the core and transition zones to measure differential settlements between the core and transition zones at different elevations. The gages were modified joint meters used in concrete structures with one end anchored in the core while the other was fixed in the transition zone. The gages were only installed over the lower part of the core in each cross section (over the lower 25 m in cross-section D and over the lower 15 m in sections B and F). Vertical strain meters were also installed on the upstream and downstream faces of the core over these same lower parts to measure strains in the asphalt concrete. Total pressure cells were installed at the bottom of the asphalt core on top of the plinth near cross-sections A and D. Furthermore, shear displacements were measured by gages installed at the interface between the core and concrete plinth to determine the shear distortions at the interface. The structural connection between the asphalt core and reinforced concrete plinth is shown in Fig. 6. Strain meters were mounted on the reinforcing steel in the plinth to measure steel stresses, and there were four lines of optic fibre sensors to monitor potential cracking in the concrete plinth. Observed settlements in the downstream transition zone in August 2007 (i.e., 19 months after the end of construction)

Fig. 11. Settlement inside the transition zone behind the core at three cross sections. Observation date 3 August 2007.

are shown in Fig. 11. The maximum settlements, including the foundation settlements, at sections B, D, and F were 560, 1060, and 550 mm, respectively. The vertical strain in the transition zone varied significantly over the height, but the average compressive strains were 0.9%, 1.3%, and 0.8% in the three sections, respectively. The maximum horizontal displacements in the downstream transition zone in the downstream direction (normal to the dam axis) at sections B, D, and F were 30, 207, and 33 mm, respectively. In the longitudinal direction (along the dam axis), the maximum displacements towards the right bank in these three sections were 94, –240, and –144 mm, respectively. The measurements of strains in the plinth (as of January 2009) indicate low values and no cracking has occurred. Strain meters are also installed in the cut-off wall at four transverse sections, and the observed results indicate very satisfactory performance (Wu et al. 2009; Zhao et al. 2009; Zheng and Wang 2009).

Field performance observations of the asphalt core The measured settlement differences between the lower part of the asphalt core and the adjacent transition zones, and the maximum measured compressive strains in the upstream and downstream faces of the core, are shown in Table 4. The observation date is 18 October 2007, when the reservoir level was at el. 2647.5 m.asl. Since then and up to November 2008, the settlement differences between the core and transition zone and the compressive strains in the core have shown almost no change (Wang and Zheng 2009). Published by NRC Research Press

Wang et al.

1375 Table 4. Measured settlement differences between the asphalt core and the upstream and downstream transition zones and measured compressive strains in the core.

Section B

D 1 1 / 1 2 / 4 0 n o 8 1 . 8 6 . 6 5 1 . 2 1 2 y b m o c . s . s y l e r n p h o e c r s a u e s l a e r n o c r s r n . e p w r w o F w m o r f d e d a o l n w o D . J . h c e t o e G . n a C

F

Elevation (m.asl.) 2594 2604.4 2609.5 2534.4 2548 2558 2585 2595 2600

Settlement difference between the core and transition zone (mm)*

Compressive strain in the core (%)

Upstream 59 7 29 — 20 48 50 39 39

Upstream face — 2.9 — — 2.5 — — — 2.6

Downstream 38 59 20 — 20 55 40 30 34

Downstream face — 1.2 — 3.1 3.1 2.7 — — 2.9

*Positive value means that the core has settled more than the transition zone.

The data show that there are large variations in the measured differential settlements, but the asphalt core has settled more than the transition zone in all measurements. The maximum recorded settlement difference in November 2008 was about 60 mm. Figure 12 shows the differential settlement curves with time at el. 2558 m. asl. in section D. The curves show that the settlement differences took place during the dam construction period, and there is virtually no measured increase in differential settlements after the end of construction. Figure 13 shows vertical core strain versus time. The curves show that around 90% of the asphalt strains took place during the dam construction and impounding period, and there is minimal measured increase in the core strains during the subsequent 3 year operation period. The compressive vertical strains on the upstream and downstream sides of the asphalt core in the lower 25 m of the core wall in cross-section D were measured to be 2.5%–3%, while the average compressive strain in the downstream transition zone over the corresponding height was 1.7%. As shown in the figure, the 12 May 2008 Wenchuan earthquake had insignificant effects on the strains in the core. The temperature of the core is observed with 14 temperature sensors at sections B, D, and F. During the first month after asphalt concrete placement, the temperature dropped from around 160 C to less than 20 C and gradually reduced with time. The temperature at the bottom of section D was 13.7 C in December 2004, 12.4 C in December 2005, 9.5 C in December 2007, and 9.4 C in November 2008. In July 2009, the temperatures at different points in the core were in the range 7.2–12.5 C (Wang and Zheng 2009). 8

8

8

8

8

8

8

Evaluation of asphalt core performance Interaction between asphalt core and adjacent transition zones The vertical compression stresses measured by the total pressure cells at the bottom of the core are 0.65 MPa near cross-section A and 2.1 MPa near section D (see Fig. 4). These values have stayed almost constant from the end of dam construction (December 2005) to the latest observation in November 2008, showing only very small variations caused by fluctuations in the reservoir level (Wang and

Zheng 2009). The measured stresses are 90% (section A) and 70% (section D) of the stresses computed by multiplying the local height of the core with the unit weight of the material above. The stress computed by the finite element analyses for cross-section D was 1.65 MPa, which is considerably less than the measured stress of 2.1 MPa. This means that the arching effect between the core and the stiffer transition zones is smaller than that modeled by the finite element analyses. This is probably due to the inadequate modeling of the viscoelastic–plastic behavior (with temperature and time) of the asphalt core during construction. The measured arching effect corresponds to a small average vertical, upward shear stress of around 4 kPa on both sides of the core. The constitutive modeling of asphalt concrete behavior for use in numerical analyses must be improved to give more reliable analyses in better agreement with field observations. Interesting comparisons may be made between the field measurements from the Yele Dam and the Maopingxi Dam. The Maopingxi asphalt-core dam is 105 m high with a crest length of 1840 m. The dam was built from 1997 to 2003 and was extensively instrumented (Xu et al. 2009). The measured results, 5 years after end of construction, indicate that the maximum settlement difference between the core and the transition zones was 48 mm, and that occurred 14 m above the core base. The vertical strains on the upstream and downstream side of the asphalt core are all compressive with a maximum value of 4%. The compression stress measured at the core bottom against the concrete plinth is 1.5 MPa for the maximum dam section, which is 60% of the stress computed by simply multiplying the local height of the core with the unit weight of the material above. Shear displacements at the core–plinth interface normal to the plinth in the downstream direction were measured to be less than 2 mm (Zou et al. 2008). When considering the differences in dam geometry, zoning, and material properties between the Yele Dam and Maopingxi Dam, one may conclude that the measured behaviour of the two asphalt cores seem very consistent with each other. It is a difficult task to take measurements of differential settlements between the hot core and the adjacent transition zones and to measure strains in the core itself, but it has also Published by NRC Research Press

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Fig. 12. Settlement differences between asphalt core and transition zones versus time at el. 2558 m.asl. at section D. Latest observation date 1 November 2008.


Fig. 13. Strains at upstream (up) and downstream (down) faces of the core versus time at different heights above the plinth at section D. Latest observation date 1 November 2008.

been attempted for some earlier dams. Measurements at Storvatn Dam, Norway (Adikari et al. 1988), showed smaller differential settlements (10–30 mm) than in the Yele and Maopingxi dams, and in the case of Dhu¨nn Dam, Germany, there was virtually no measured differential settlement (Strobl and Schmid 1993). For earth-core dams there is concern about the arching effect in the core between the adjacent filter zones and a possibly significant reduction in effective stresses in the core, which then may crack due to hydraulic fracturing. This is of much less concern in a core of ductile asphalt concrete, which also has some tensile strength, and the International Commission on Large Dams (ICOLD 1992) states that hydraulic fracturing cannot occur in an asphalt concrete core. In the Yele Dam core, the total stresses towards the bottom of the core are higher than the corresponding water pressures, so the effective stresses are positive. However, in other situations there may be a concern about hydraulic fracturing. Therefore, the authors are currently carrying out laboratory experiments

at Xi’an University of Technology to study whether conditions may arise that potentially could lead to the phenomenon of hydraulic fracturing in an asphalt concrete core.

Back-analysis of strains in the core based on measured dam and foundation settlements For a dam resting on a compressible foundation where large differential settlements may occur, whether transverse cracks or fissures may develop through the asphalt-core wall needs to be considered. Differential settlements were definitely a concern for the Yele Dam at the design stage. However, the field measurements show that the foundation settlements are relatively small, and the settlement profile across the valley is rather gradual and almost symmetrical about the centerline of the valley. A simplified back-analysis of the shear strains in the core was performed using the observed foundation settlements 2 years after construction. The finite element analysis of strains in the asphalt core is based on the following simplifications and assumptions: Published by NRC Research Press

Wang et al.


(1) Two-dimensional (2-D) plane strain analyses have been undertaken using the software SIGMA/W in GeoStudio 2004. (2) The settlements along the plinth, as shown in Fig. 14, have been increased in proportion to the dam height (as the dam was constructed in 13 layers). (3) The unit weight of the asphalt core (25 kN/m 3) has been reduced to an equivalent unit weight of 18.4 kN/m 3. This is done to simulate the effect of skin friction (arching) between the core and transition zone on either side of the core. The magnitude of the reduction was determined by using the total stress measured at the base of the core in section D. Having observed the settlements at sections B, D, and F in the downstream transition zone and the settlement differences between the asphalt core and transition zone (Fig. 12 and Table 4), one may estimate the settlements of the asphalt core at these three sections. The comparisons between the calculated (by the 2-D finite element analyses) and observed settlements in the core at sections B, D, and F are shown in Fig. 15. In the analysis, the equivalent Young’s modulus for the core was taken equal to 45 MPa and Poisson’s ratio equal to 0.4 based on the laboratory test results presented by Wang (2008) for the asphalt concrete used in the core of the Yele Dam. The observed and calculated settlements at these three sections match reasonably well below el. 2625 m.asl., but not towards the top of the core. As geo-grids were placed horizontally over the top 30 m of the dam, they confine the horizontal displacements of the top part of the dam body, which then undergoes smaller settlements. This is not modeled in the back-analysis and is one reason why the calculated settlements over the top 30 m are much bigger than the observed settlements. The back-calculated shear stresses and shear strains in the core are shown in Fig. 16. The results from the back-analysis confirm that the most critical location for the asphalt core is 160 m from the left bank, where the bottom of the cut-off wall leaves the rock base and goes over to the more compressible overburden. The computed stresses and strains at this location are shown in Table 5. The shear stresses are almost symmetrical around the deepest section of the dam due to the rather symmetrical foundation settlement pattern that was measured and used in the analysis (Fig. 14). The settlements are only slightly larger on the right bank than on the left bank, where the depth to bedrock is much smaller. Stress–strain–strength tests were performed on 100 mm diameter samples drilled out of the asphalt core during construction. The results from strain-controlled compression triaxial tests, keeping the lateral confining stress constant during each test, are shown in Fig. 17. The tests were run at 7 C. The stress–strain curves show a very ductile asphalt concrete behavior with insignificant strain-softening even for tests with very low confining stress (Wang 2008). This is characteristic of the behavior of hydraulic asphalt concrete with a bitumen content between 6.5% and 7.5% (by total weight), 12%–15% filler content, an aggregate grain-size curve that satisfies the Fuller distribution of particle sizes, and maximum aggregate size between 16 and 20 mm. The behavior is that of a ductile, viscoelastic–plastic material 8

1377 Fig. 14. Measured settlements along the plinth at end of construction and impounding. (These settlements are used as input in the finite element back-analysis.)

Fig. 15. Comparisons between the measured and calculated settlements in the core at sections B, D, and F.

with self-healing (self-sealing) properties should any fissures or cracks occur due to excessive shear distortions. Eberlaste Dam, Austria, was one of the first asphalt-core embankment dams ever built (1962–1964). It rests on a deep and compressible alluvial foundation, and large differential settlements have taken place under the dam, causing significant shear distortions in the asphalt core (Hoëg 1995). However, even in that case, no leakage due to cracking in the core has occurred. As the designers of the Eberlaste Dam had anticipated large differential settlements, they specified the use of an especially soft grade of bitumen in the asphalt concrete to be able to accommodate large shear distortions without cracking (Rieno¨ssl 1973). This is one of the advantages of an asphalt-core embankment dam: the geomechanical properties of the asphalt concrete may to a certain extent be tailored to the specific design conditions, making it well suited for use in a dam water barrier. Based on the finite element analysis results presented in Table 5 and the test results for the Yele asphalt-core specimens, one may conclude that the computed stress and strain states inside the core are safely within a stress–strain range where there is no danger of cracking due to high shear stresses or significant shear dilation that could increase the Published by NRC Research Press

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Fig. 16. Computed maximum shear stresses and strains in the core. ( a) Maximum shear stress contours (kPa) in the asphalt core from the back-analysis. ( b) Maximum shear strain contours in the asphalt core from the back-analysis.


Table 5. Stress and strain state in the most critical location of the asphalt core at section C.

State Stress (MPa) Strain (%)

Vertical 1.73 1.9

Long itud inal 1.08 –0.15

Hor.–v ert. sh ear –0.19 1.18

Max . N/A 2.04

Max. shear 0.38 2.34

Major principal stress 1.78 N/A

Minor principal stress 1.08 N/A

Ratio of major to minor principal stress 1.65 N/A

Note: Hor.–vert., horizontal–vertical; N/A, not applicable.

Fig. 17. Triaxial compression test results from 100 mm diameter samples drilled out of the Yele Dam core during construction. Results presented for different levels of confining stress.

permeability of the core. The measured field behaviour of the core–plinth interface in the Yele Dam is also very reassuring.

Asphalt core as impervious barrier Thirteen pizeometers were installed in the downstream transition zone adjacent to the core at sections B, C, D, and E, and 15 piezometers were installed upstream and downstream near the joint between the asphalt core and concrete plinth at these same sections. Fifteen piezometers were installed in the foundation under sections B, C, and D, and 20 piezometers were installed from the observation gallery down to 2 m below the dam base. About 200 m downstream of the dam toe, a measuring weir was installed to measure the seepage rate coming from the river bed section, left side of the dam, and left abutment. Most, if not all, of the seepage through the right side of the dam and through the right abutment is assumed to be collected and measured by the 12 weirs installed on the right Published by NRC Research Press

Wang et al.


bank in the drainage gallery, construction gallery, access galleries, and a drainage ditch (see Fig. 3). During the impounding test, very little seepage was recorded and the pore pressures measured on the downstream side of the asphalt core above the plinth were zero or negligible up to a reservoir level of about el. 2630 m.asl. When the reservoir level exceeded el. 2633 m.asl., there was a significant increase in seepage through the right bank. When the reservoir was at el. 2648 m.asl. (i.e., only 2 m below full supply level), the pressure at plinth level on the upstream side of the core was 118 m while the pressure head in the river bed at the downstream side of the core was still only 7 m under section D. However, the pore-water pressure in the foundation under section F on the right bank was observed to be high, increasing with the reservoir level. The significant seepage beneath the cut-off wall at this section was of concern, and in early 2006 a deep drainage well was installed in the foundation through the observation gallery. In December 2007, when the reservoir was at el. 2650 m.asl. (i.e., full supply level), the maximum total seepage was 358 L/s, which is still smaller than the maximum seepage value of 500 L/s anticipated during design (Wang et al. 2009). From May to September 2008, additional grouting was carried out, and new drainage wells were drilled in the drainage gallery in the right bank. When the reservoir was at full supply level again in November 2008, the total seepage was reduced to 277 L/s. Based on the measured pore pressures in the downstream dam body, at the plinth level downstream, and on the results from the impounding test, one may conclude that insignificant seepage is coming through the asphalt core and the core–plinth interface.

Concluding remarks For the complex foundation and inclement climatic conditions at the Yele Dam site, which is in a highly seismic region, a rockfill dam with a central core of asphalt concrete was selected rather than a dam with an earth core or concrete facing. The design of the dam has been presented in this paper with an emphasis on the design and construction of the asphalt concrete core. The properties of asphalt concrete may to a certain extent be tailored to specific design and site requirements, and field experience and research show that asphalt concrete is a ‘‘forgiving’’ material very well suited for use in the impervious core of an embankment dam. An extensive field monitoring program was implemented for Yele Dam, and the recorded results have been compared with those of other high rockfill dams with an asphalt core. Special attention has been given to the interaction between the core and adjacent transition zones. Based on the field measurements, back-analyses, tests on the properties of the asphalt concrete and the joint between the core and plinth, one may conclude that the asphalt core of the Yele Dam performs very well. There are no indications of any leakage through the core or at the joint between the asphalt core and concrete plinth above the foundation cut-off wall. However, as anticipated at this geologically very difficult site, there is some leakage under the dam in spite of the ex-

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tensive use of deep cut-off walls and curtain grouting. In late 2008 the leakage amounted to about 280 L/s. Continuous surveillance is taking place to study and control the development of this underseepage.

Acknowledgements The first and the third authors would like to thank Professors Sun Zhentian and Wu Liyan and the late Professors Ding Purong and Yang Quanmin at Xi’an University of Technology, Xi’an, People’s Republic of China, for their cooperation during several research programs on the Yele asphalt core since 1991. The authors thank the Nanya River Basin Hydro-Electric Development Cooperation, the dam owner, for permission to present the performance observations of the Yele Dam.

References Adikari, G.S.N., Valstad, T., Kjaernsli, B., and Hoëg, K. 1988. Behaviour of Storvatn Dam, Norway. A case of prediction versus performance. In Proceedings of the 5th Australia-New Zealand Conference on Geomechanics, Sydney, Australia, 22–26 August 1988. Institution of Engineers, Barton, Australia. pp. 86–92. Alicescu, V., Tournier, J.P.S., and Vannobel, P. 2008. Design and construction of Nemiscau-1 Dam, the first asphalt core rockfill dam in North-America. (Proceedings of the Canadian Dam Association 2008 Annual Conference. Winnipeg, Man., 27 September – 2 October 2008.) Canadian Dam Association Bulletin, 21(1): 6–12. Chen, X.G. 2003. Monitoring design for the asphalt core rockfill dam of Yele hydropower station. Sichuan Water Power, December 2003. [In Chinese.] Chen, J.C., Wang, X.B., He, Y.L., and Xiong, K. 2009. Analysis of the data from earthquake monitoring of Yele Dam. In Proceedings of the 1st International Symposium on Rockfill Dams, Chengdu, China, 18–20 October 2009. China Water Power Press, Beijing. pp. 851–858. Chu, W., Yu, L.S., and Wu, L.Y. 2004. Joint structure model test of asphalt concrete core of embankment dams. Journal of Northwest Hydroelectric Power, 20(1): 23–26. [In Chinese.] Creegan, P.J., and Monismith, C.L. 1996. Asphalt concrete barriers for embankment dams. American Society of Civil Engineers (ASCE) Press, New York. Hao, Y.L., and He, S.B. 2003. Layout of Yele hydropower station. Sichuan Water Power, December 2003. [In Chinese.] Hao, Y.L., and He, S.B. 2008. Design of Yele asphalt concrete core rockfill dam. Dam construction in China – state of the art. Chinese National Committee on Large Dams, Beijing. pp. 226–233. Hoëg, K. 1993. Asphaltic concrete cores for embankment dams. Stikka Press, Oslo, Norway. Hoëg, K. 1995. Transverse cracking in embankment dams. A literature and finite element study. Norwegian Geotechnical Institute, Oslo, Norway. NGI Report 532060. Hoëg, K., Valstad, T., Kjaernsli, B., and Ruud, A.M. 2007. Asphalt core embankment dams: recent case studies and research. International Journal on Hydropower and Dams, 13(5): 112–119. ICOLD. 1992. Bituminous cores for fill dams. Bulletin 84. International Commission on Large Dams (ICOLD), Paris. NRBHDC. 2007. The monitoring results of Yele project. The Nanya River Basin Hydro-electric Development Cooperation (NRBHDC), Chengdou, China. November 2007 report. [In Chinese.] People’s Republic of China National Development and Reform Commission. 2006. Test code for hydraulic bitumen concrete. Published by NRC Research Press

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Appendix A The Yele Dam is located in the northern part of the seismically active fault zone of the Anning River region, and the reservoir is about 2 km west of the Anning River downstream active faults. The basic seismic intensity of the dam site is VIII (Chinese scale) and the design intensity is IX with a peak horizontal ground acceleration of 0.45 g. The Yele Dam seismic analyses were carried out by taking the peak horizontal rock ground acceleration of 0.45 g in the river direction and 0.3 g in vertical acceleration. The coupling coefficient of earthquake horizontal and vertical accelerations of 0.5 was used. The earthquake was assumed to last 40 s in a time-domain analysis. The predicted maximum earthquake-induced settlement, horizontal displacement (downstream direction), and longitudinal displacement (along the dam axis) of the dam crest were 62, 188, and 52 mm, respectively. Based on the results of the analyses, the dam design as presented in this paper is considered to be very earthquake resistant.

Appendix B In the original design, the earthquake resistance was increased by using reinforced concrete beams in the top part of the dam. As this was found to be impractical from a construction point of view, it was decided to use geo-grid reinforcement instead. That was the first time geo-grids were to be used to increase earthquake resistance in an embankment dam in China. The type and physical parameters of the geogrids used are: maximum tensile strength ‡ 250 MPa; maximum longitudinal tensile load ‡ 150 kN/m; maximum transverse tensile load ‡ 80 kN/m; maximum tensile strain ‡ 8%; tensile load ‡ 60 kN/m at a tensile strain of 3%. The tensile strength at joints was specified to be ‡ 50 kN/m. When using geo-grids in such designs, the material of the geo-grids should be made of polypropylene or high-density polythene and an oxygen-resistance agent should be added to prevent the geo-grid from ageing.


Wang et al.

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Appendix C Table C1. Dam fill material parameters used in Duncan–Chang model (Duncan and Chang 1970) in finite element analysis.


Material (see Figs. 1 and 2) Q21I Q22I Q31II Q32–1III Q32–2III Q32–3IV Asphalt core Transition zone (dry) Transition zone (wet) Upstream rockfill (dry) Upstream rockfill (wet) Downstream rockfill (II) Downstream rockfill (I) Toe berm Downstream rockfill (III)

’

Rf 0.65 0.65 0.68 0.70 0.59 0.65 0.76 0.67 0.67 0.72 0.72 0.65 0.72 0.65 0.75

K 1950 1800 900 1100 1300 900 850 1200 1080 1000 900 1200 1000 800 800

n 0.76 0.72 0.74 0.78 0.76 0.73 0.33 0.52 0.52 0.5 0.5 0.45 0.5 0.45 0.4

G 0.38 0.35 0.38 0.38 0.39 0.38 0.38 0.32 0.32 0.33 0.33 0.31 0.33 0.31 0.28

F –0.023 –0.023 –0.026 –0.04 –0.035 0.02 0.05 0.06 0.06 0.06 0.06 0.03 0.06 0.05 0.05

D 4.5 3.8 4.3 5.6 5.9 5.7 15 5 5 6 6 3 6 3 3

Kur 3800 3600 2200 2200 2600 2200 1200 2400 2100 1800 1600 2000 1800 1800 1600

K0

DK

40 40 37 38 39 37 27 43 41 48 46 50 48 48 36

0 0 0 0 0 0 0 5 5 5 5 5 5 5 3

C (kPa) 70 70 80 80 60 60 200 0 0 0 0 0 0 0 0

Density (g/cm3) 2.42 2.42 2.45 2.24 2.35 2.20 2.43 2.2 2.2 2.2 2.2 2.25 2.2 2.3 2.2

Note: Rf , ratio between the asymptote to the hyperbolic curve and the maximum shear strength; K , modulus number describing the material stiffness; n, a value describing the rate of change of the material stiffness as a function of the confining stress; G, F , and D, test parameters related to material volume change; K ur, modulus number used during unloading and reloading; K0, material friction angle in degrees; DK, increase of material friction angle in degrees; C , cohesion intercept. ’

Reference Duncan, J.M., and Chang, C.-Y. 1970. Nonlinear analysis of stress and strains in soils. Journal of the Soil Mechanics and Foundations Division, ASCE, 96(5): 1629–1653.


Asphalt Core Rock Fill Dam

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