Evaluation of seismic hazard, local site effect ... - Springer Link

BULLETIN

of the International Association of ENGINEERING GEOLOGY de I'Association Internationale de GI~OLOGIE DE L'INGt~NIEUR

Paris - No 52

-

0ct0bre 1995

EVALUATION OF SEISMIC HAZARD, LOCAL SITE EFFECT, LIQUEFACTION POTENTIAL, AND DYNAMIC P E R F O R M A N C E OF A WORLD EXAMPLE OF AN E M B A N K M E N T DAM CHARACTERIZED BY VERY COMPLEX AND UNIQUE FOUNDATIONS CONDITIONS: KARAMEH DAM IN THE JORDAN VALLEY t~VALUATION DU RISQUE SISMIQUE, DES EFFETS DE SITE, DU POTENTIEL DE LIQUI~FACTION ET DES PERFORMANCES DYNAMIQUES D'UN BARRAGE EN ENROCHEMENTS CONSTRUIT DANS DES CONDITIONS DE FONDATIONS TRl~S COMPLEXES: LE BARRAGE DE KARAMEH DANS LA VALLI~E DU JOURDAIN AZM S. AL-HOMOUD*

Abstract The geologic structures associated with the site of a 55.106 m ? Karameh e m b a n k m e n t dam (under construction in Jordan Valley) and the tectonic effects on dam foundation and reservoir margins were reviewed. The regional seismicity and history of events were analyzed and a design earthquake was established. At K a r a m c h dam site, the geotechnical conditions for the foundations formations (including liquefiable sands) a n d materials to be placed, the presence of the major Jordan Valley fault crossing the site. the h y d r o g e o l o g i c a l conditions with salty artesian aquifers and soluble soils make this site a unique, very complex one at the limit of feasibility. The under construction K a r a m e h dam is situated in the Dead Sea Rift (32.00~ 35.50aE), the b o u n d a r y between the A r a b i a n and African-Sinai plates. The primary seismic source contributing to the hazard at the K a r a m e h dam site is the active Jordan Valley fault which extends from the Dead Sea to the Sea of Galilee with an expected m a x i m u m earthquake magnitude of 7.8, and passes under the right abutment of the d a m body. A probabilistic method is used to evaluate the seismic hazard at the dam site. The Peak G r o u n d Acceleration (PGA) is selected as a measure of g r o u n d motion severity. Analysis is carried out for 50 %, 90 % and 95 % probability that is not being exceeded in a life time of 50, 100, and 200 years. A c c o r d i n g to guidelines of [ C O L D , P G A for a M a x i m u m Design Earthquake (MDE) was 0.50 g and for an Operating Basis Earthquake (OBE) it was 0.17 g. Local site effect analysis was carried out using the c o m p u t e r code S H A K E (Schnabel et al., 1972). The p e r f o r m a n c e of the dam under dynamic loading was analyzed. The factor of safety was evaluated for different cases of dam operation. N e w m a r k sliding block on a plane method was used to evaluate the displacement of the dam body c o n s i d e r i n g liquefaction potential. A P G A of 0.50 g associated with the MDE will trigger liquefaction of the sand layers existing in the dam foundation. Similarly, liquefaction may o c c u r in the dam foundation layers in case of 7.8 magnitude earthquake which is expected to result in a crest se'ttlement of 4.4 m. The event horizontal rupture d i s p l a c e m e n t for an earthquake of this magnitude will be about 12 m. To safely resist large earthquake events, it is r e c o m m e n d e d to have the following safety measures for the design of the dam: (1) free b o a r d should be increase at least to 7.0 m instead of the 5.0 m of the final design; (2) the foundation of the d a m should be stabilized against liquefaction hazard; and (3) the e m b a n k m e n t internal zoning should be designed to a c c o m o d a t c damage resulting from e a r t h q u a k e s of intensity greater than the O B E event. Critical elements of the e m b a k m e n t regarding safety are the core, d o w n s t r e a m fine filter, c h i m n e y drain and drainage blanket. The d o w n s t r e a m critical filter zone had a horizontal width of 5.0 m, enhanced to 7.0 m in the vicinity of the main fault. Stability analysis indicated that e m b a n k m e n t side slopes as fiat as I(V) : 8 (H) were required. Rdsumd U a r t i c l e expose le contexte g~ologique du barrage de Karameh, en construction darts la vallde du Jourdain (55 9 106 m3), ainsi que les consdquences des caracteristiques tectoniques sur les fondations et les flancs du rd.servoir. La s~ismicitd ainsi que I'histoire sismique de la r6gion ont ~te dtudi~es et un sdisme de projet ddfini. Le site du barrage, qui c o m p r e n d des sables susceptibles de liquefaction, qui est travcrs~ par la faille principale de la vall~e du Jourdain et qui sur le plan h y d r o g d o l o g i q u e contient des aquiFeres artdsiens salds et des sots solubles est tr~s complexe et ~. la limite de la faisabilitd. Le barrage en construction est situ0, sur le rift de la Mer Morte entre les plaques d ' A r a h i e et d'Afrique-Sin~//. La e s o u r c e sismique,, principale est la faille active du Jourdain, qui va de la Met Morte 5. la Mer de Galilee, avec un tremblement de terre potentiel de magnitude 7.8, et qui passe sous I'appui droit du barrage. Une mdthode probabilistc a dtd utilis~e pour dvaluer le risque sismique sur le site du barrage. Selon les directives du Comitd International des Grands Barrages, le pic d'acc~l~ration retenu pour le t r e m b l e m e n t de terre de projet le plus fort est de 0,50 g et pour un tremblement de terre n ' i n t e r r o m p a n t pas le f o n c t i o n n e m e n t du barrage de 0,17 g. U a n a l y s e des effets locaux de site a ete r~alisde a l ' a i d e du p r o g r a m m e S H A K E (Schnabel et autres, 1972). Le c o m p o r t e m e m du barrage sous charge d y n a m i q u e a ~t~ analyse. Le pic d ' a c c d l d r a t i o n de 0,50 g, correspondant au tremblement de terre de r~f~rence le plus fort provoquerait la liqudfaction des niveaux sableux de la fondation du barrage, et un tassement de la crete de 4,4 m. Le d @ l a c e m e n t horizontal serait dans ce meme cas de 12 m.

* Civil Engineering Department Jordan University o f Science and T e c h n o l o g y P.O. Box 3030 lrbid-Jordan Tel. : 962 2 295111 Ext. 2126 Fax : 962 2 295123

10 En vue de rdsister b. de forts tremblements de terre, les mesures suivantes ont dt6 recommanddes lors de la conception du barrage: (1) La revanche devrait fitre portde b. 7.0 m au lieu de 5.0 m : (2) Les niveaux de fondation doivent 6tre stabilis6s pour 6viler le risque de liqudfaction ; (3) La structure interne du barrage dolt ~,tre con~ue pour supporter les dommages rdsultant des tremblements de terre d'intensit6 sup6ricure b. l'intensit6 n'affectant pas le fonctionnement. Les dldments critiques concernant Ia sdcurit6 sont le noyau, le filtre fin en aval, les drains-cbemint~e, etla couverture drainante. La zone filtrante critique en aval a dt6 portde de 5 ~t 7 m dans le voisinage de la faille. L'analysc de stabilit6 a rnontrd que les pentes latdrales ne devaient pas ddpasser i pour 8.

Introduction

faults that trend parallel to the rift. Some individual faults have length over 40 km (Figure 2).

In Jorda, most of the constructed dams are of embankment type because of geological and economical reasons. Besides, embankment dams proved to be highly recommended for construction in earthquake prone regions. Furthermore these types of dams are not only economical but also very adaptable to dynamic effects because of their suitable ductibility.

Faults within the Jordan Valley Rift zone are considered active. The most affected areas are nearest the rift margins: and transition from the rift valley to the raised shoulders of the rift (regional fault-line scarp) occurred through the system of step-faults, flexural and grabens (Figure 3).

Karameh dam site is one such dam. It is located on Wadi Mallaha which is adjacent to the Jordan River near the town of Karameh, on the eastern side of the Jordan Valley Rift (Figure 1). The dam is currently under construction. The project will involve 107 m 3 of zoned earthfill with central clayey core, a spillway and drawoff structures including a twin pipeline in a 5.2 m diameter tunnel 440 m long, a pumping station, and a twin 1.2 m diameter pipeline fot eh King Abdullah Canal. The dam is 41.0 m high, 800.0 m wide at base and the storage capacity is 55.0 million cubic meter. At Karameh dam site, the geotechnical conditions for the foundations formations and materials to be placed, and the presence of the major Jordan fault crossing the site, make this site a unique, very complex one. Jordan is an eathquake prone region. Consequently, Karamch dam was designed taking into consideration dynamic loading and associated hazards that include embankment acceleration zoning, foundation liquefaction risk, and rockhead rupture. Magnitude of design earthquake was estimated according to guidelines of ICOLD (1989), that are based on probabilistic seismic hazard analysis. This paper deals with an evaluation of seismic hazard, local site effect and dynamic performance of Karameh embankment dam. Faults Systems

in J o r d a n

In general, two networks of fault system exist in Jordan, one orthogonal and other diagonal. Each network has two systems of fault trends of the same tectonic age, as shown on Figure 2. The most prominent structural feature in Jordan is the Dead Sea-Jordan Rift System. This transform fault system, more than 1 000 km long, links the Zagros-Taurus zone of plate convergence to the north with the Red sea to the south where crustal spreading occurs. The Dead Sea Rift (Jordan valley Rift) was formed during the Cenozoic break up of the once continuous AraboAfrican continent. The plate boundary movement in Jordan was strike-slip with a total of around 105 km left lateral movement (Sunna, 1994). Major faults occur along the margins of the Jordan Valley Rifts which are complexly deformed within both the zone and individual

Outside the rift zone faults are considered inactive (passive) where an absence of geomorphic features typically associated with active faults. Jordan Valley is a linear depression, approximately 1015 km wide on the south. The width of the valley is controlled by the presence of parallel faults dipping toward Jordan River to form a graben. The length of the valley is 360 km approximately. Four main fault systems are recognized, as follows: N-S Faults This system includes a number of distinctive regional and areal faults that occur as wide (to 8 km) fracture zones. The N-S faults generally occur parallel to the Jordan Rift valley fault (Wadi Araba-Jordan River) and were formed concurrently with the Rift faults. Although these faults extend to great depths, the N-S faults do not traverse the younger sedimentary rocks formation. The N-S faults are usually concealed by sediments and their location detection requires subsurface exploration. N W - S E Faults The faults of this system are dominant over large parts of Jordan and extend far beyond the borders of neighbouring countries such as Saudi Arabia and Syria. Examples are the Sirhan fault zone, extending southward into Saudi Arabia and Northwards to Lake Tebrias for a distance of more than 300 km. The NW-SE fault system occurs parallel to Dead Sea fault and persists in the deep crustal rocks to the mantle of earth. These faults are mainly tensional in character and are believed to have formed concurrently with the Red Sea fault. Faults of the NW-SE system generally traverse the younger sedimentary rocks and in many localities can be identified in surface outcrops. The distance between individual faults of this system are not constant, and the features tend to bend as they approach the Jordan Rift Zone. E-W Faults The faults of this system have variable extensions, and some of the faults continue across the Saudi Arabia borders. The distance between the main faults of system is 35-40 km, such as the Zarqa, Amman, Siwaqa, and Zarqa-Ma'in faults. These are basement faults but also

11 may extend up to the surface for a limited distance. Many of the E-W faults following this trend and are believed to have dextral movement along their planes.

At Karameh dam site, the geotechnical conditions for the foundations formations and materials to be placed, and the presence of the major Jordan Fault crossing the site, make this site a unique, very complex one.

NE-SW Faults Regional G e o l o g y

The faults and lineaments following this trend cross/traverse the other three systems (N-S, NW-SE and E-W), and represent the youngest tectonically activity system. These faults are mainly compressional features and frequently associated with folding.

The rift valley, originated in Miocene time and within it was deposited a thick sequence of lacustrine (lake) deposits and associated evaporates 9 Most of the evaporates were deposited towards the centre of the rift

'

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ARAB

' ~ - ~ t ~ , - B. ~27m

and hr, e coarse

J - C ~ YX./v._.;-~

f

A:lernating bcncs

2-10rn Ihiek of shff [aminaled clayey SILT !O silly CLAY SAttO

~J-M~~/>,J'\?

3 000 00~

=-

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1 t.~

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Ix..

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3 000 50~

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b~ I

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17 halite. Moreover, geological sections show the existence of continuous layers of loose liquefiable sand.

37

Seismicity of dam site and design earthquake

36

Jordan lies in the area of the Jordan-Dead Sea Rift which extends from the Arabian Shield in the south to the highly movable Zagros Mountain range in the north. Geological and magnetic feature points to a relative plate movement of approximately 107 km along the rift boundary.

35

341

Information about the past distribution of earthquakes is used as a basis for seismic hazard estimates. Yuceman (1985), Qa'dan (1987), Abou-Karaki (1987), A1-Tarazi (1992) and A1-Homoud e t al. (1994) collected valuable data bout historical and 20th century instrumentally recorded earthquakes for Jordan and vicinity. Table i shows the historical and instrumentally recorded earthquakes in and around the dam site. This table indicates that the site of the dam has been effected by major earthquakes of a magnitudes greater than 6.0 in the past.

33

32

31

29301 . ~ i

The historical earthquakes that occurred in the site of the dam (32.00~ and 35.50~ along Jordan Valley Fault (J.V.F.) indicate that an eathquake of magnitude greater than 6.5 is expected to occur in the dam site with a return period of (600-800) years. These historical earthquakes occurred in the years 33, 748, 1546 and 1834 (see Table 1).

..........i

28

2732

i 33

~ ~ R e d Sea 34 35

36

37

38

39

Fig. 7 : Faults Considered to Assess the Seismic Hazard of Karameh Dam in Jordan According to Line Source Model.

The Karameh dam site is situated in the Dead Sea Ruft, the boundary between the Arabian and African-Sinai Plates. The primary seismic source contributing to the hazard at the Karameh dam site is the active Jordan Valley Fault, which extends from the Dead Sea to the Sea of Galilee, and passes under the site (Figures 3, 4 and 6). Of particular recent relevance to the dam site, because of its accurately located epicentre at 31.90~ 35.52~ is the magnitude 4.7 earthquake of 25th January 1985. This earthquake had a focal depth of 27 km and caused

Table 2 : Fault Classification Criteria (Cluff et al., 1982) Slip Rate (ram/year)

Class

Magnitude (Ms)

Recurrence Interval (Year)

_> 10.0

k 1.0

k I00

> 1026

>_ 7.5

500

k

5,0

k i.0

k 100

2 1026

k 7.5

~ 1000

1B

k 10.0

< 1,0

k 100

a 1026

k 7.0

2

1.0-10,0

k 1.0

50-200

k 1025

k 7.0

100-1000

1.0-10.0

< 1.0

50-200

a 1025

k 7.0

< 100

1.0-10.0 0.5-5.0

k 5.0 0.1-3.0

100 10-I00

> 1025

> 7.0

~ 1025

~ 6.5

~ 500-5000

4

0.1-I.0

0.01-1.0

1-50

~

1024

~ 5.6

1000-10000

4A

0.1-1.0

~ 1.0

~ l0

~ 1025

~ 6.5

1000-10000

-

-

1

tA

2A

2B 3

5

Slip by Rupture [ Seismic Event (m) Length (km) Moment (Dyne-cm)

I

< 1.0 < 1.0

5

100

1000

k I0000

I 2 100000

18 slight damage to the west of the Jordan River. It was also felt in southern L e b a n o n and western Jordan. The average rate of slip of about 1 c m / y e a r (that is : meter per 1000 year, 100 m e t e r per 10000 years) is inferred (Ambraseys, 1982; Ben-Manahim, 1981; M c k e n z i e et al., 1970; and M c k e n z i e , 1972). This is a representative value of the last 10000 years and the present day. The rate of uplift is 4 mrrdyear. Clearly, this is a world class strike-slip fault. The probable length of rupture for a given e v e n t is 130 km. Faults can be classified into six types according to their activities, starting from 1 to 6 as given in Table 2 where 1 represents the most active type (Cluff et al., 1982). A fault with slip rate >_ 10 mm/year, slip per event _> 1 m, rupture length > 1 0 0 k m , m a g n i t u d e M s > 7 . 5 earthquakes and recurrence interval > 1000 years is classified as class 1A. The Dead Sea Fault is classified as class 1A fault, and therefore, is considered as an active fault.

Probabilistic Seismic H a z a r d Analysis. Generally, the e v a l u a t i o n of seismic hazard at a given site, follows four s t e p s : Shah and Dong, (1984) identification of seismic sources, estimation of recurrence relationship of earthquakes, description of attenuation of ground motion with distance, and site severity assessment. For the purpose of this study, each of these steps will be described below :

Identification of Seismic Sources Identification of potential seismic sources is essentially boxed on local geology, tectonic history and seismicity. A close relation between earthquakes and active faulting on the surface is present. In fact, the association of earthquakes with w e l l - d e f i n e d faults forms very important basis for identifying line sources. It is noted that the identification of seismic source zones is not an easy task and should be based on an interdisciplinary approach that i n v o l v e s various geological and civil enrineering disciplines. As result of an e x t e n s i v e survey and discussions with experts in this field as well as taking into consideration the available literature, nine seismic sources have been identified to help analyze the seismic hazard of dams

-t

32

33

34

35

36

Name

b-value

[3-value

39

in Jordan. T h e s e sources are shown in Figure 7, and are as follows : 1) Dead Sea Fault (No. 1) 2) Wadi Araba Fault (No. 2)

L~ (yearly)

m0

ml

] Focal depth t (km)

1

Dead Sea-Jordan River

0.89

2.06

0.310+ 0.07

4.0

7.5

15

2

Wadi Araba

0.77

1.81

0.150+0.04

4.0

6.7

15

3

Wadi Sirhan

0.79

1.88

0.054+0.01

4.0

7.5

10

4

Wadi Farah

0.92

2.13

0.072_++.+.0.02 _

4.0

6.7

10

5

Northern Fault

0.69

1.60

0.260+ 0.06

4.0

7.6

6

Red Sea Faults

0.55

1.25

0.39 + 0.05

4.0

6.6

24

7

Al-Galiel Fault

0.75

1.78

0.059+ 0.02

4.0

6.8

10

8

SE-Mediterranean

0.58

1.33

0.110+0.02

4.0

7.26

15

0.57

1.31

0.790+0.12

4.0

7.5

30

9 ]Cyprus Fault

38

Fig. 8 : Seismotectonic Map of the Dead Sea-Jordan Transform Fault System Showing Locations of the Historical Earthquakes (Prior to 1900).

Table 3 : Seismic Hazard Parameters Used in This Study Zone

37

[

15

19 37

(Figure 9) in this study. These data are used to calculate the seismicity parameters for the identified faults in the studied area.

36

Estimation of Seismic Hazard Parameters

x

Seismic hazard parameters are estimated by using the method developed by Kijko and Sellevoll (1989) with an assumed threshold magnitude mo = 4.0. The method gives the b parameter of the Gutenberg-Richter formula (Gutenber and Richter, 1965), the annual rate of earthquakes ),.4, and the upper bound magnitude mi. The method combines data from the largest historical earthquakes with complete instrumental data of variable threshold magnitude. Table 3 summarizes the results of the analysis so that the required parameters can be obtained. These parameters are used in the analysis as part of the input information for the computer program FRISK (McGuire, 1978).

35 ~ ~ ~-

34

0

+X

"

X

X+

A~ k

~4q

~-

,~..~,i-+

'~ . . . . . . . . . . . . . . . .

-+.v

x

33 x

+

32

iman

•

o

Attenuation of Peak Ground Acceleration (PGA)

31

30

x

x x2

,. 29 x

28

LEGEND

....9...........:- r2.0 2 kin), it is generally acknowledged that the close in peak accelerations are nearly independent of magnitude for magnitudes greater than about 6.5 (Seed and Idriss, 1982). The close in records, on deep alluvium, recorded from the 1979 Imperial Valley earthquake (M = 6.8) are appropriate according to tlendron (1992) for estimating motions from the Jordan Valley Fault which is also located in a rift filled with deep alluvium. The measured peak acceleration showed that the mean peak motion measured at 1 km from the fault was about 0.50 g and that the range of measured values at distance of 1 km was from 0.38 to 0.70 g. A mean value of 0.50 g was taken by ttendron (1992) for analysis of the Karameh dam. For the Imperial Valley earthquake the ratio of peak horizontal acceleration, peak horizontal particle velocity and peak horizontal displacement occur in the ratio of approximately 1 g, 125 cm/sec, and 90 cm. Thus, for a 0.5 g acceleration, the peak horizontal particle velocity and the peak horizontal displacement expected are 62 cm/sec and 45 cm, respectively. These values were used by Hendron (1992) at the base of the dam for calculation of the deformations of the dam under seismic motion. Assuming larger strains and considering the damping associated with these motions, the motion of the dam was considered to be those of the ground without amplification effects. At Lower San Fernando Dam, for example, it has been shown that the foundation accelerations as well as the crest accelerations of the dam were initially 0.55 g at the beginning of the shaking (Hendron, 1992). Below the upstream berm at elevation - 3 1 5 , the cyclic stress ratio applied to any sand layer by the M.D.E. in

the L.L formation was evaluated to be : %vg / ~v = 0.65. Under the downstream berm at elevation --315, the cyclic stress ratio in the L.L formation was evaluated to be %vg / ~ = 0.325. Considering the relationship developed by Seed and Idriss (1982) for evaluating the stress ratio Cavg/ N, causing liquefaction for each corrected Standard Penetration value N~, it was concluded that NI values would have to be about 30 under the upstream berm and about 25 under the downstream berm in order to prevent the "triggering" of cyclic-mobility, if the sands were clean. If the sands had 15 % fines, then the values of NI necessary to prevent "triggering" under the upstream and downstream - 3 1 5 berms would be about 20 and 25, respectively. Hendron (1992) concluded that "triggering" will occur in sand layers under both the upstream and downstream portions of the dam since an evaluation of the foundation sand in the L.L. formation under the maximum section of the dam, N1 values on the order of 12-18 or Dutch Cone values qc on the order of (6 MPa) to (9 MPa) conducted at depths corresponding to vertical "effective" stress levels of (0.1 MPa). Since it is concluded that the sand layers in the L.L. formation are likely to trigger, then the stability of the dam must be checked using the "undrained" residual shear strength of the sand layers after "triggering" according to Seed and Idriss (I982). Considering a representative value of Nl = 15 (qc (7.5 MPa)) then the "undrained" residual shear strength Su was estimated to be about (24 to 29 kPa). If after consolidation of the shells and the core of the dam it can be assumed that the strength parameters can be approximated by c' = 0, 0' = 30~ then for the 8:1 average downstream slope it was shown that the required undrained shear strength necessary for static equilibrium after earthquake shaking along a horizontal sand seam in the foundation is Su (16 kPa). The static Factor of Safety (ES.) after shaking was evaluated to be 1.5 to 1.8. Consistent with the above is a downstream wedge of soil with an active force PA of (51 MN/m). If the residual "undrained" shear strength after triggering is (26 kPa) then, the value of the Newmark dynamic resistance for horizontal sliding in N W - - ( 3 6 MN/m) and since the weight of the wedge, W, is (1 250 MN/m), then the N value is 0.03. Therefore the yield acceleration is Ng = 0.03 g.

30 If it is assumed that triggering takes place during the maximum acceleration of 0.5 g with particle velocity of (0.60 m/s) early in shaking, then it is still reasonable that there could be a significant time of shaking where the maximum acceleration might be about 0.30 g and the particle velocity would be about (0.37 m/s). Adopting Newmark (1965) "Sliding block" model, the dynamic inelastic residual movement due to the remainder if ground shaking after triggering was estimated as (2.30m). Similar calculations for the 1:10 slope upstream indicate a yield acceleration of 0.037 g and a dynamic displacement of (1.50 m). Thus the total spreading due to the upstream and downstream slopes could be on the order of 4 m. Thus the crest settlement due to embankment spreading could be on the order of the same magnitude.

Conclusions The proposed Karameh dam is situated in an area of high seismic potential and will be founded on, and constructed of unique soils. 1. Foundation of the proposed Karameh dam involves the following complex conditions : permeability, and seepage; seismicity and li.quefaction; artesian and saline groundwater; downstream sinkholes; foundation settlements, and dam deformation and tectonic activities. Therefore, this project carries high level of risk. The design of embankment considered the dynamic loading and associated hazards that include embankment acceleration zoning, foundation liquefaction risk, and rockhead rupture hazards. 2. Site of proposed Karameh dam is traversed by the major Jordan Valley fault of the Jordan Valley Rift Zone. This strike-slip fault, with numerous secondary faults and fractures forms a zonal fault, rather than a single fault which passes under the right part of the dam. Major earthquakes have occurred at the dam site and being within the active Jordan Valley Rift zone necessitated a special design of dynamic loading. 3. Based on the seismotectonic study in the vicinity of Karameh Dam conducted by Tapponnier (1992) and the analysis of satellite and aerial images, the following conclusions on seismic hazard and rupture at the dam site can be drawn : a. The Jordan Valley Fault, which will cross the future dam right abutment, is the main seismogenic fault at the site. It is paralleled by a subordinate strand that bears clear evidence of metric vertical motion. b. The rate of horizontal motion along the main Jordan Valley fault strand is about 1 cm/year (that i s : 10 meters per 1000 years). Clearly this is a world class strike-slip fault. c. The rate of vertical motion (uplift of the western compartment relative to the eastern one) reaches 4 m m / y e a r at the right abutment of the dam, probably by combined incremental folding of the

d.

e. f. g.

Ghor-el-Katar anticlines (2-3 mm/year) and a discrete thrust c o m p o n e n t of slip (2-3 mm/year) on the Jordan Valley Fault. The surface trace of the fault bears evidence for large event d i s p l a c e m e n t (8-15 m), at least several centuries old. The length of rupture for a given events is probably 130 kin. This implies that earthquakes (M > 7 . 8 occur along the Jordan Valley Fault. The recurrence times of such earthquakes might be 6-800 years if the event displacement is 68 meters, consistent w i t h the historical seismicity if the I 540 B.C., 756 B.C., 31 B.C., 746 A.D. and 1 546 A.D. are all considered to have occurred on the Jordan Valley Fault (Ben Menahem, 1991). The magnitude of such events should be >_ 7.6. If the 31 B.C., and 1 546 A.D. earthquakes are considered to h a v e occurred on the other fautls (Ambraseys a n d Kariz, 1992), for instance Mount Carmel and J e r i c o Faults, which is considered more likely, then the average recurrence time of the other e v e n t s (which are larger) is 1 150 + 350 years. The magnitude of such events would be above 8. This would be characterized by displacements of about 12m. The last one being the bet-Shean 746 A.D. event, the occurrence of such an earthquake in the next century should be quite probable.

4. The available data r e g a r d i n g seismicity of Jordan from both historical and the instrumentally recorded earthquake files were r e v i e w e d . These have been used to estimate the s e i s m i c hazard parameters for each of the nine distinct seismic sources with potential earthquake activity. Results of seismic hazard analysis were presented in the form of Peak Ground Acceleration (PGA) values at the bedrock. The P G A values were computed for 50, 100 and 200 years e c o n o m i c life of the dams and 50 %, 90 % and 95 % probability of non-exceedence. For 100 years life time a n d 50 % probability of nonexceedence, the PGA v a l u e at Karameh dam site amounts to a p p r o x i m a t e l y 0.170 g. For 90 % and 95 % probability of n o n - e x c e e d e n c e , the PGA values reach 0.355 g and 0.466 g , respectively. Earthquake risks at the dam site w e r e considered according to guidelines of ICOLD (1989) : A Maximum Design Earthquake (MDE); Peak Ground Acceleration was 0.50 g at the surface outcrop; and an Operating Basis Earthquake (OBE); Peak Ground Acceleration was 0.17 g. 5. The dam has been d e s i g n e d to withstand a horizontal rupture movement of 4 m in the event of an earthquake. This is appropriate from a probabilistic analysis of likely earthquake i n d u c e d movements at the site. However, based on a deterministic or observational approach, the d e s i g n should be for a 10 to 12 m movement. 6. To safely resist large e a r t h q u a k e events, it is recommended to have the f o l l o w i n g additional safety measures for the design of the dam:

31 a. T h e f r e e b o a r d s h o u l d b e i n c r e a s e d at l e a s t to 7 . 0 m , i n s t e a d o f t h e 5 . 0 m in t h e f i n a l d e s i g n o f t h e d a m in o r d e r to a c c o m o d a t e t h e f o l l o w i n g : possible spreading of the dam after earthquake t r i g g e r i n g w h i c h will i n v o l v e a w e d g e s l u m p down movement of the crest of about 4.0 m, p o s s i b l e w a v e e f f e c t o f 1.5 to 2 . 0 m , a n d t h e down-throw that may result from the fault movement. b. T h e f o u n d a t i o n o f t h e d a m s h o u l d b e s t a b i l i z e d against liquefaction hazard by densification of t h e s a n d l a y e r s , o r b y i n s t a l l i n g r e l i e f w e l l s to dissipate the excess pore water pressure during e a r t h q u a k e s , if p o s s i b l e as t h e e x c a v a t i o n f o r t h e p r o j e c t p r o c e e d , all s a n d l a y e r s l o c a t e d at a c l o s e to f o r m a t i o n l e v e l u n d e r t h e e m b a n k m e n t and w h i c h a r e s u s c e p t i b l e to l i q u e f a c t i o n s h o u l d be removed during excavation. c. T h e e m b a n k m e n t i n t e r n a l z o n i n g s h o u l d be d e s i g n e d to a c c o m o d a t e d a m a g e r e s u l t i n g f r o m e a r thquakes of intensity greater than the OBE event. Critical elements of the embankment regarding s a f e t y a r e t h e c o r e , d o w n s t r e a m f i n e filter, c h i m ney drain and drainage blanket. The downstream critical filter zone had a horizontal width of 5 m, e n h a c e d to 7 m in t h e v i n i c i t y o f t h e m a i n f a u l t . Stability analysis indicated that embankment side s l o p e as f l a t as 1 (V): 8 ( H ) w e r e r e q u i r e d . 7. It is l i k e l y t h a t a P G A o f 0 . 5 0 g a s s o c i a t e d w i t h the M D E will t r i g g e r l i q u e f a c t i o n o f t h e s a n d l a y e r s e x i s t i n g in t h e d a m f o u n d a t i o n . S l o p e s t a b i l i t y a n a l y s i s i n d i c a t e d e e p f a i l u r e p l a n e s in t h e f o u n d a t i o n zone, which indicates that the foundation need relief w o r k s in t h e d o w n s t r e a m to l o w e r t h e p o r e p r e s s u r e .

Acknowledgment The author wish to extend his sincere thanks to the following Jordanian Governmental Authorities f o r supplementing the information used in this study: Ministry of Water Resources attd Irrigation, Jordan Valley Authority. and Natural Resources Authority. The author extend his sincere thanks to the Deanship of Scientific Research at Jordan University of Science and Technology for financing this study under grant No. 35/92. The contents of this paper reflects the view o f the author and do not necessarly reflect the official views or policies o f Governmental Authorities in Jordan.

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