Water Power and Dam Construction - Issue September 2009

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IWP& DC

32 SEPTEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

TUNNELLING

ERVICE LIFE

The 450mm bore Viking Johnson eccentric plug valve is designed to

give reliable service for many years in this almost inaccessible loca-

tion. Manufactured in cast iron using high pressure moulding tech-

niques the valve incorporates a nickel-welded seat for corrosion

resistance, specially profiled for low torque and extended valve life.

The valve is trunnion-supported and fully encapsulated in elas-

tomeric polymer with a valve body fully internally and externally lined

with epoxy. The eccentric plug design ensures that the ductile iron

plug – fitted on permanently lubricated austenitic stainless steel bear-

ings – rotates away from the seat as soon as movement begins, avoid-

ing scuffing and therefore extending the operational life of the valve.

“This complex project has required close co-ordination with a

number of specialist companies and personnel including Eric Wright

Civil Engineering, Shakespeare Engineering Supplies, Rotork Fluid

Systems, Consortium Underwater Engineers L td, Red 7 Marine

diving contractors and MWH project consultants working along-

side United Utilities capital maintenance, site operations and project

management teams,” says Graham Biggs, business development

manager for Viking Johnson. “The custom-built design and the engi-

neering quality of the new eccentric plug valve and its automation

and power system means that it will be very many years before such

an operation is required again to replace it.”

“Grizedale was a particularly challenging project,” adds James

Tresnan, United U tilities project manager. “Installing a 450mm

diameter valve 50m along a 1.7m high brick arch tunnel shou ld

never be classed as straightforward. But when the tunnel is sub-

merged 20m below water and the silt in it results in zero visibility

for the divers, then the degree of difficulty is amplified tenfold.

“This was the situation at Grizedale. Consequently it was key that

we worked with our contractors and suppliers from an early stage

to ensure that we simplified the construction activities whilst at the

same time achieving the required functionality.”

For further information visit www.vikingjohnson.com

RIZEDALE reservoir in Lancashire, UK, was built in 1866

by forming a 22m high, 120m long earth embankment

across the valley of Grizedale Brook to act as the wall of

the dam. A 1.3m wide, 1.7m high tunnel beneath the dam

takes water from the reservoir to a pump house from where it is

pumped up to the nearby Barnacre North Reservoir before it flows

under gravity to the treatment works at Franklaw.

A gate valve sited near the outlet has for many years controlled

water flow through the tunnel. However, recent inspections showed

that the valve was no longer adequate. In order to improve opera-

tional control of the water flow without reducing the water level in

the reservoir or potentially polluting the downstream watercourse

it was decided to install a secondary valve.

Viking Johnson was selected to supp ly this replac ement v alve

which, rather than being of a gate design, is of the ‘eccentric plug’

type. This will provide the reservoir’s operator, United Utilities, with

the ability to not only isolate but also control the flow when trans-

ferring water to Barnacre. The eccentric plug valve requires only a

one-quarter turn of the shaft between its fully open and fully closed

positions, therefore making it relatively easy to automate its opera-

tion for submerged duty.

In addition to the valve, Viking Johnson has provided a total solu-

tion including the bespoke actuation system, a specially-designed

hydraulic power pack and an emergency hand pump facility for the

hydraulic system which permits the valve to be operated in the event

of electrical supply failure. The system also includes almost 400m

of high specification stainless steel braided hydraulic hose lines run-

ning between the valve and the pump house.

IVING WORK

A team o f ten divers winched the valve and actuator, weighing

almost three quarters of a tonne, along the tunnel on a spe cially

designed trolley. Viking Johnson provided the diving c ompany

with a ‘dummy’ valve ahea d of the installation to permit the

divers to practice manoeuvri ng it in contro lled conditions with in

a training facility.

On site at Gr izedale the divers used a decompression chambe r

housed on a floating pontoon above the submerged tun nel

entrance, 20m beneath the surface, to permit them to work for up

to 70 minutes at a time.

A team of specialist divers recently

completed installation of an eccentric

plug valve to control water flow at

Grizedale reservoir in the UK

Divers control the flow

Caption to go here please xyxyxy

xyxyxy xyxy xyxy xyxy

Photos courtesy of United Utilities

The diving pontoon in position on the lake

The V iking Johnson valve and

trolley being lowered into the lake

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34 SEPTEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

TUNNELLING

EARTH PRESSURE BALANCE MACHINE

Earth Pressure Balance Machines (EPBMs) are shield machines spe-

cially designed for operation in soft ground conditions containing

water under pressure. Loose sedimentary deposits with large boul-

ders and a high water table will challenge ordinary TBMs, but the

EPBMs have an articulated shield that is sealed against the pressure

of water inflows up to 10 Bar. Robbins EPBMs control the stability

of the tunnel face and subsidence of the ground surface by moni-

toring and adjusting the pressure inside the cutterhead chamber to

achieve a balance with the pressure in front of the cutterhead, hence

the name ‘Earth Pressure Balance’.

The working area inside the EPBM is completely sealed against

the fluid pressure of the ground outside the machine.

A screw conveyor removes the fluidized muck behind the cutter-

head and in front of the ‘pressure bulkhead’. The screw conveyor’s

speed and discharge rate is controlled by the operator and is used to

control the pressure at the working face and match the muck dis-

charge rate to the advance rate of the EPBM.

The articulated joint between the forward shield and tail shield is

equipped with a high pressure seal that allows angular movement

between the shields and prevents water from seeping into the inte-

rior of the EPBM. The EPBM erects the segmented tunnel lining as

it advances. Specially designed high pressure seals in the tail shield

effectively seal the machine to the outside of the tunnel lining and

create a barrier against ground pressure.

When it becomes necessary to e n t e r the cutterhead chamber to

inspect the cutterhead or change cutting tools, the workers can safely

enter through a manlock while compressed air is used to maintain

earth pressure balance to support the working face.

The EPBM erects the segmented tunnel lining as it advances.

Specially designed high pressure seals in the tail shield effectively seal

the machine to the outside of the tunnel lining and create a barrier

against ground pressure. When it becomes necessary to enter the cut-

terhead chamber to inspect the cutterhead or change cutting tools, the

workers can safely enter through a manlock while compressed air is

used to maintain earth pressure balance to support the working face.

EPBMs can be steered through small turn radii when necessary by

employing the articulation joint and the copy cutter mounted in the

cutterhead. Robbins engineers will calculate the correct amount of

copy cutter extension and the articulation angle required for the par-

ticular radius requirements of a given tunnel.

www.robbinstbm.com

OR centuries, a lack of surface water has led to intensive use

of groundwater for irrigation in Baku, A zerbaijan. The

Samur-Apsheron Irrigation Project – developed by the

Azerbaijan Government’s Irrigation Department – will help

remedy current water problems by overhauling a main canal that

provides water supplies to Baku. New tunnels will be built using a

6.3m (20.7 ft) diamet er Robbins EPB (earth pressure balance)

machine – the first-ever TBM to be used in the country.

Contractor Azerkorpu awarded Robbins the contract for the com-

plete EPB boring system in June 2008, which includes the back-up,

cutting tools, segment molds, segment manufacturing plant, rolling

stock, ventilation system, spares, and operating personnel. Major

components of the machine including the main bearing, electrical,

and hydraulic systems were procured in the US, Europe, and Japan.

Assembly of the EPB was completed in April 2009 by the Robbins

EPB Division in Guangzhou, China.

Once launched, the TBM is e xpected to bore for eight to nin e

months in hard clay, silt, sand, and mixed face rock. The machine

features a mixed ground cutterhead with interchangeable disc cut-

ters and carbide bits. Four independent foam injection points in the

cutterhead evenly consolidate the face in varying ground conditions.

Multiple detection systems provide electronic infor mation about

clogged ports, preventing unnecessary cutter and cutterhead wear.

The machine will excavate one tunnel of approximately 3.5km.

As the TBM bores, it will line the tunnel with 300mm thick precast

concrete, universal type segments in a 5+1 arrangement. Once the

tunnels are complete in 2010, Azerkorpu plans to use the EPB on

several other projects in the country. The Samur-Apsheron Irrigation

Project consists of open canals and T BM-driven tunnels total ing

5.7km in length.

The network will convey water from the Samur River to use for

both i rrigation and the new Takhtaker pu hydro power station,

which will provide power to the Cusar District in Baku.

New tunnels will be excavated as part of

the Samur-Apsheron irrigation project in

Azerbaijan. A Robbins EPB – the first ever

TBM to be utilized in the country – will

play a major role in the project

A first for Azerbaijan

Key facts – Takhtakerpu hydro

station and storage reservoir

Dam parameters

Length – 1.18km

Height – 142.5m

Maximum width – 750m

Lake surface area – 8.7km

Water storage reservoir: Total displacement – 268.4Mm3,

Conservation zone - 218.9Mm

General description of works

Development of foundation ditch, in volume – 7,824,000m

Dam building.

Tunnel construction.

Emergency escape canal construction.

Construction of the powerhouse.

The project began in 2006 and is expected to be complete in 2010.

The 6.3m Robbins EPB will be Azerbaijan’s first ever TBM

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36 SEPTEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

SEISMIC ANALYSIS

• High intensity of ground shaking and long duration.

• Large number of rockfalls blocking access roads to dams, causing

damage at dam sites and locations of appurtenant struc tures,

destroying masts of transmission lines, aqueducts, etc.

• Landslide dams blocking rivers at several locations.

• Overtopping of run-of-river power plants.

• Failure of hydromechanical equipment (gates, penstock), etc.

It was expected from the very beginning that mass movements (rock-

falls, landslides) would be an important feature in this mountainous

terrain with steep valleys.

Sichuan is the province with the largest hydro power potential in

China with some 6678 dams and reservoirs. Some of the extraordi-

nary projects completed recently are the 240m high Ertan arch dam,

the 15m high Zipingpu CFRD, and the 132m high Shapai arch dam,

the world’s highest RCC arch dam. Several very large projects are

either under c onstruction or u nder de sign such as the 278m high

Xiluodu arch dam (12.6GW), the 186m high Pubugou earth core

rockfill dam (3.6GW) an d the 305m high Jinp ing 1 arch dam

(3.6GW). However, only Zipingpu and Shapai dams were located

relatively close to the ruptured fault segments and have experienced

strong ground shaking as discussed in the subsequent sections. The

N the afternoon of 12 May 2008 a magnitude 8 earthquake

occurred in China’s Sichuan province with the epicentre at a dis-

tance of 17km from the Zipingpu CFRD. The earthquake rup-

tured a 240km long segment of the Longmenshan fault system

separating the Tibetan Plateau from the Chengdu Basin and is

referred to as the Wenchuan earthquake. The main seismological

features and damage to buildings and infrastructure of this earth-

quake are described by Wang et al. (2008) and others. A summary

of dams and reservo irs affected by this destructive eart hquake is

given in Table 1. A more detailed description of the effects on large

dams is given by Chen (2008) and Xu (2008).

The main features of interest to dam engineers are as follows:

• Local magnitude: 8

• Maximum recorded peak ground acceleration: 958cm/s

(hori-

zontal component) and 948cm/s

(vertical component);

• Duration of strong ground shaking 90 sec to 120 sec; and

• Maximum fault movement at surface: 4.7m (horizontal) and 4.8m

(vertical).

The ma in phenomena of this earthquake relevant for dams are

as follows:

Lessons learnt from the

Wenchuan earthquake

Several dams and power plants damaged by the earthquake which occurred in China’s

Sichuan province on 12 May 2008 were visited during a joint mission of the Chinese

Committee on Large Dams and ICOLD’s Committee on Seismic Aspects of Dam Design

from 29 March-4 April 2009. Here, Martin Wieland and Chen Houqun discuss the

unique features of this earthquake and the damage caused, while addressing the question

of the earthquake being triggered by Zipingpu reservoir

Figure 1 – Damage to the gate

structure on top of the intake

towers at Zipingpu dam

SEISMIC ANALYSIS

WWW.WATERPOWERMAGAZINE.COM SEPTEMBER 2009 37

other dams, as well as Three Gorges, were several 100km away from

the epicentre and no damage was reported at these projects.

The Wenchuan earthquake affected more dams t h a n any other

previous earthquakes. It is also the most important seismic event to

have occurred since the creation of the International Commission

on Large Dams’ (ICOLD) earthquake committee some 40 years ago.

Therefore, plans for a joint mission were discussed with Chinese col-

leagues at the ICOLD Annual Meeting in Sofia, B u l gar i a in early

June 2008. From presentations at the event however, it became clear

that due to the large number of rockfalls and the interruption of

roads in the worst hit area, a visit to major dams was not possible

for the foreseeable future.

Finally at the 80th Anniversary Meeting of ICOLD in Paris in

ovember 2008, a date could be fixed for a smal l joint ICOLD -

CHINCOLD mission to take place in March an d April 2009,

approximately 11 months after the earthquake.

Thirteen foreign dam and earthquake experts from A ustria,

Canada, Japan, Switzerland, the UK, and the US participated in this

mission, called the International Seminar on Earthquake and Dam

Safety. The local arrangements were made by Jia Jinsheng (ICOLD

President) and Chen Houqun on behalf of CHINCOLD. The pro-

gram consisted of a one day seminar in Beijing where the partici-

pants were briefed on the Wenchuan earthquake followed by a three

day visit of dam and run-of-river power plants in Sichuan, with a

final one day seminar in Chengdu where dam owners and designers

presented specific dam projects affected by the earthquake. Finally,

a discussion was held in which (i) the personal experience gained

from the mission, (ii) the lessons learnt, (iii) the topics for possible

cooperation of ICOLD with CHINCOLD, and (iv) a paper on reser-

voirs and the Wenchuan earthquake were discussed.

At the beginning, some participants may have wondered if they

could see anything at all so many months after the earthquake.

However, it became clear to everybody after the first day’s visit to

Zipingpu CFRD – the dam which was most easily accessible – that

these concerns were not justified. During the field visit, seeing the

huge number of mass movements and the seri ous dam age to the

transportation infrastructure, it became very obvious that a visit of

the important dams and hydro power projects would not have been

possible at an earlier da te. As a matter of fact, traffic conditions

along the main highway in the epicentral region were still very dif-

ficult leading to hours of waiting at some critical spots and it must

be assumed that it was not possible until recently to move heavy con-

struction equipment into some of the valleys where hydro power

projects are located.

Therefore, the da mage observe d in t hese remote sites was st ill

‘fresh’, as can be seen from some of the photos.

This joint mission was a great success although only a fraction of

dams da maged by the earthquake could be visited or discussed

during the two seminars. This visit was really a unique opportunity

for all participants interested in dams and earthquake safety.

In t he subseque nt sections the main lessons learnt fr om the

Wenchuan earthquake by the authors are presented.

PECIAL FEATURES OF EARTHQUAKE HAZARD IN

THE CASE OF STORAGE DAMS

The exposure of dams to strong earthquakes may be a few seconds

– at most – during their lifespan. Because strong earthquakes occur

very rarely, few dam engineers or dam owners have any experience

with them.

Strong earthquakes can affect a large area and many dams may

be subjected to strong ground shaking. This has been the case for

the Wenchuan earthquake. Different types of dams have been dam-

aged; therefore the lessons learnt from this earthquake will be rele-

vant for the whole dam industry.

It is well known that many existing dams were designed against

earthquakes using seismic design criteria and methods of dynamic

nalysis which are considered obsolete, or even incorrect, today. This

may also be true in China and it is expected that the seismic design

criteria and methods of analysis and design will be reviewed and

modified in the near future, as this has been common practice almost

universally after a strong earthquake. In general it is easier to con-

vince a dam owner to implement modern flood design criteria for

his project, because the flood hazard is visible whereas the earth-

quake hazard is invisible.

The Wenchuan earthquake has confirmed that earthquakes are

multiple hazards, which may have the following features in the case

of storage dams:

• Ground shaking causing vibrations in dams, appurtenant struc-

tures and equipment, and their foundations (Figure 1).

• Fault movements in the dam foundation or discontinuities in dam

foundation near major faults which can be activated causing struc-

tural distortions.

• Fault displacement in the reservoir bottom causing water waves

in the reservoir or loss of freeboard.

• Mass movements (rockfalls, landslides).

• Ground movements and settlements due to liquefaction, densifi-

cation of soil, causing distortions in dams.

Other effects such as water waves and reservoir oscillations are gen-

erally of lesser importance for the earthquake safety of a dam.

Usually the main hazard, which is addressed in codes and regula-

tions, is the earthquake ground shaking. It causes stresses, deforma-

tions, cracking, sliding, overturning, etc. The other hazards may not

be addressed because it is rather difficult to get any quantitative data

of these hazards. Most analysts have difficulties with such hazards.

ASS MOVEMENTS AND LANDSLIDE DAMS

Due to steep slopes in the area, there were thousands of rockfalls and

landslides in the Longmenshan fault zone. Over 2500 people were

killed in two landslides in Wangjiayan, Beichuan county, Sichuan, each

with an estimated volume of 10Mm

(Wang et al., 2008).

In the case of storage dams the following features are possible and

many have been observed during the Wenchuan earthquake:

Table 1: Classification of damage to dams and reservoirs (Wang et al., 2008)

Province or Total number Number of damaged Classification of damage

municipality of reservoirs reservoirs Dangerous situations of dam-break Highly dangerous situations Dangerous situations

Sichuan 6678 1996 69 310 1617

Chongqing 2824 352 2 350

Shaanxi 1036 126 17 109

Yunnan 5422 51 2 49

Gansu 297 81 81

Guizhou 2105 12 12

Hubei 5804 25 25

Hunan 11435 23 23

Total 35601 2666 69 331 2266

38 SEPTEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

• Mass movements (rockfalls with large rocks) causing damage to

spillway piers (cracks), retaining walls (overturning) (Figure 2),

powerhouses (cracking and puncturing and distortions) (Figure 3),

electro-mechanical equipment (Figure 4), hydromechanical equip-

ment such as gates (Figure 5) and surface sections of penstocks,

damaging aqueduct, overturning of masts of transmission li nes

(Figure 6), etc.

• Mass movements into the reservoir causing impulse waves in the

reservoir.

• Mass movements blocking rive rs and formin g landslid e lakes

whose failure may lead to overtopping of run-of-river power plants

or the inundation of powerhouses with equipment.

• Mass movements blocking access roads to dam sites and appur-

tenant structures (Figure 7).

During t h e Wenchuan earthquake some 33 major landslide lakes

were created. Tangjiashan landslide dam with a height of 124m and

a volume of about 2Mm

, which stored 320Mm

of water, caused

a major threat and great efforts were needed to release the water in

a controlled way without causing a secondary disaster in the down-

stream valley and the city of Meijiang. The maximum discharge was

6500m

/sec which corresponded roughly to a flood with a return

period of 200 years.

Every time a strong earthquake occurs the design guidelines have

to be reviewed as new phenomena appear, which may have previ-

ously been overlooked. For example, during the Wenchuan earth-

quake the problem of mass movements (mainly rockfalls in very

steep mountains) and landslide lakes have shown to be very impor-

tant new features of strong earthquakes. In addition, an unprece-

dented large number of dams and run-of-river power plants have

been affected by this e arthquake. The Wenchuan earthquake has

confirmed and demonstrated that dams, spillways and appurtenant

structures must be able to withstand the multiple effects of strong

earthquakes.

EISMIC PERFORMANCE OF LARGE DAMS SUB

JECTED TO STRONG GROUND SHAKING

In the subsequent sections, two large dams with heights exceeding

100m which were subjected to very strong ground shaking are briefly

discussed. They are the Shapai RCC arch dam and the Zipingpu con-

crete face rockfill dam (CFRD).

As RCC and CFRD dams are very competitive dam types today,

and since none of them have been subjected to strong ground shak-

ing in the past, the lessons learnt from the behaviour of these two

dams during the 2008 Wenchuan earthquake are considered very

important for the dam community.

HAPAI

RCC

ARCH DAM

During the Wenchuan earthquake, the 132m hig h S hapai dam,

the world’s highest RCC a rch dam, was subjected to s trong

ground shaking.

The dam is a three-centered arch structure with vertical upstream

face. The crest length and thickness are 250.25m and 9.5m respec-

tively. The maximum thickness at the base is 28m. The dam is locat-

ed 30 km away from the epicenter and during the e arthquake the

reservoir was at normal water level of 1866m. There are two verti-

cal contraction joints and two induced joints with a spacing of about

50m. The dam is founded on granite and granodiorite and was com-

pleted in 2003. The seismic design of the dam was based on a hor-

izontal peak ground acceleration of 0.138g.

During the authors’ visit in early April 2009 no signs of damage

could be observed, however, the reinforced concrete building on the

crest and the superstructur es of the intake towers suffered some

repairable inelastic deformations. The powerhouse located several

kilometers downstream was severely damaged by high velocity rock-

falls (Figure 3) and the movement joint of the penstock failed, caus-

ing flooding of the powerhouse.

SEISMIC ANALYSIS

Left to right: Figure 5 – Complete failure of the radial gate in the spillway opening of a r un-of-river power plant (hydraulic pistons are still there but gate was

washed away); Figure 6 – Failure of the aqueduct near Shapai power plant; Figure 7 – Controlled release of water from Tangjiashan landslide lake

Left to right: Figure 2 – View of a run-of-river plant with traces of mass movements in the background; Figure 3 – The effect of rockfalls on the power plant at

Shapai dam; Figure 4 – Equipment damaged by rocks in Shapai powerhouse

IPINGPU CONCRETE FACE ROCKFILL DAM

The 156m high Zipingpu CFRD is one of the largest CFRDs in

China and one of the key projects for water supply and irrigation

for the Chengdu basin. The dam was completed in 2006 and repre-

sented the la t est CFR D technology in China (Figure 8). The dam

was designed for an intensity of VIII (Chinese scale), with a design

peak ground acceleration of 0.26g. The epicentral distance of the

dam was 17km. During the earthquake the reservoir level was low

and its volume was 300Mm

. Under normal operation conditions

the reservoir volume is 1100Mm

. The crest of the dam and the con-

crete face were damaged. Of greatest interest was the damage to the

concrete face and waterproofing system which consisted of damage

of the vertical joints and the offset of the horizontal joint between

he second and third stage slab construction (Figures 9 and 10).

After the earthquake th e ma ximum sett lement at the dam crest

was 735mm and the horizontal deflection in downstream direc-

tion was 180mm. The cross-canyon deformat ion of bo th abut-

ments was 102mm. Because of the low water level in the reservoir

at the t ime of the earthquake, it is difficult to estimate what the

earthquake behaviour of the dam, the concrete face, and the water-

proofing system would have been if the reservoir was full.

ISCUSSION OF THE EARTHQUAKE HAZARD

The ICOLD Committee on Seismic Aspects of Dam Design has recent-

ly revised Bulletin 62 entitled ‘Inspection of Dams following

Earthquake’. This bulletin can be used for the systematic inspection

of dams that have experienced strong ground shaking (ICOLD, 2008).

For existing and new dams it is recommended a safety plan be pre-

pared in which all hazards affecting a dam project are listed sys-

tematically and the action to be taken for each hazard is discussed.

As discussed earlier, the Wenchuan earthquake has shown the vul-

nerability of dams and especially appurtenant structures and spill-

ways to rockfall, it is therefore necessary to pay more attention to

this hazard in mountainous regions.

Among dam engineers, the conce pt of the maximum credible

earthquake (MCE ) or the safety evaluation earthquake is well

known, however, th e integral safety concept for dam s, which

includes the following elements: (i) structural safety, (ii) safety mon-

itoring, (iii) operational safety and maintenance, and (iv) emergency

planning, is less known to other people involved in dam projects.

In assessing the safety of slopes little reference is made to the sta-

bility of sl opes at the dam site and the reservoir under the MCE

ground motions. Usually the slope failure hazard is classified as mod-

erate, small, and very small. Such a qualitative description is inade-

quate as it is necessary to discuss the different failure scenarios under

very strong ground shaking and the consequences of such failures.

The discussion of the consequences is often omitted.

The rockfall hazard is generally underestimated. Earthquake-trig-

gered rockfalls and slides of steep slopes are usually shallow; there-

fore, protection of such slopes is technically and economically feasible.

For the seismic hazard assessment of dam sites and major infra-

WWW.WATERPOWERMAGAZINE.COM SEPTEMBER 2009 39

SEISMIC ANALYSIS

structure projects seismologists have developed sophisticated models.

At present, the emphasis is on probabilistic seismic hazard analysis.

The objective is to specify the hazard for different types of design

earthquakes. The result is basically a uniform hazard spectrum for

the acceleration at the ground/rock surface and a de-aggregation of

the hazard in terms of magnitude and epicentral distance. The engi-

neer would need the acceleration time histories of different design

earthquakes. The last step, i.e. the conversion of the uniform hazard

spectrum in to different sets of acceleration time histories, is still

under development.

As discussed in the previous section the seismic hazard is not only

ground shaking. It is much more complex. Therefore, focussing on

the ground shaking may not lead to an improved earthquake safety

s other important hazards may be neglected. The situation is some-

how comparable with the development of computer programs for

the dynamic analysis of dams, which, although they were very

advanced, the elementary information on material parameters and

their strength properties was inadequate and there were, and still

are, very large uncertainties involved in estimating the safety evalu-

ation earthquake ground motion. It w as realised that further

progress in the methods of dynamic analysis would not lead to an

improvement of the seismic safety of dams. Therefore, in the seis-

mic hazard assessment of dam sites, a thorough study of all hazards

is needed. Ground shaking may not necessarily be the dominant one.

The features of different types of dams to effectively resist seismic

ground motion are listed in ICOLD (2001). To observe these gen-

eral desi gn recommendations is highly recommended as they are

often more effective than any sophisticated dynamic analyses of a

dam system where some of these guidelines have been ignored.

ARGE RESERVOIRS AND SEISMICITY IN

ICHUAN

Shortly after the 12 May 2008 earthquake, reports appeared which

suggested that it was triggered by the impounding and operation of

the Zipingpu reservoir. The main arguments for this suggestion are,

firstly, the observation that large reservoirs can trigger earthquakes,

secondly, that the epicentre of the Wenchuan earthquake is located

17km from the Zipingpu dam and, thirdly, a small part of one branch

of the reservoir intersects the fault, which ruptured during the earth-

quake. However, if a tectonic fault is close to failure then it is almost

impossible to determine the exact factors triggering a major earth-

quake. This is also the case for the Wenchuan earthquake. Additional

research may shed more light into what triggered the earthquake.

The following text was prepared at the seminar in Chengdu during

the last day of the ICOLD-CHINCOLD mission.

The m aximum water level of the Zipingpu res ervoir (elevation

875.4m) has not exceeded the natural water level (elevation 877m)

where the Min River crosses the Beichuan-Yinxiu fault. Therefore,

the original hydrogeological conditions of the Beichuan-Yinxiu fault

have not been affected by the impounding of the Zipingpu reservoir.

In August 2004, 13 months before reservoir impounding, an earth-

quake monitoring network with seven fixed stations was set up in the

Left to right: Figure 8 – Zipingpu CFRD: view of downstream fac e with office bui lding an d powerhouse wit h debris from parap et wall at crest and rocks

dislocated during t he earthquake ; Figure 9 – Damage along the ve rtical joints at Zipingpu; Fi gure 10 – Repair of the dams’ longitudinal joint

40 SEPTEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

reservoir region. In October 2006, the reservoir water level reached

elevation 875.4m, 1.6m below normal storage level. The corre-

sponding reservoir volume was 900Mm

. In July 2006 and May

2007, the water level was lowered to elevation 820m. Before the

Wenchuan earthquake (30 April 2008), the reservoir water level was

at elevation 828.95m, the corresponding reservoir volume was

300Mm

. From the statistics of the annual seismic activity of the reser-

voir obtained from the Department of Reservoir Earthquake Research

of the Sichuan Seismological Bureau, the frequency and intensity of

seismic activity after impounding of the Zipingpu reservoir was almost

unchanged before and after reservoir impounding. The recorded seis-

micity has also no relation with the reservoir water level variation.

This phen omenon is known as reservo ir-triggered seismicity

(

RTS). The main mechanism for RTS is the release of tectonic stress-

es due to changes in stress and stre n g th properties in fault planes

caused by the reservoir. This requires that the fault that can produce

an earthquake is already near to failure; so that the added weight or

pore pressure build up d u e to reservoir impounding can trigger a

seismic event. Seismic events can also be triggered by the collapse

of underground cavities in mining areas or liquid injection.

Reservoir-triggered seismicity has been considered in the dam engi-

neering community since 1935, when the first documented case of

RTS was cr eated at La ke Mead, the res ervoir at t he 220m high

Hoover arch dam in the US. Since then several strong earthquakes

in India, China, Greece and Zambia have occurred which are sus-

pected of being reservoir triggered. The maximum magnitude of an

RTS event that has been identified to date is 6.3. The exact number

of reservoir triggered cases is not known but ICOLD believes that

the number of accepted cases is today somewhere between 40 and

100. The relationship between reservoirs and seismicity continues

to be debated in the scientific community.

The increasing height of dams and size of reservoirs may con-

tribute to the possibility of earthquakes being triggered. ICOLD rec-

ommends that RTS needs to be considered for large dams over 100m

in height. ICOLD has also published a bulletin presenting the state

of knowledge on reservoirs and seismicity.

According to ICOLD recommendations, high hazard dams are

designed to safely withstand ground motions caused by the maxi-

mum credible earthquake (MCE). Therefore RTS seismicity is not a

direct safety problem for a well-designed dam as the maximum reser-

voir-trigge red earthquake cannot be stronger than the MCE.

However, RTS may still be a problem for other structures, buildings

and appurtenant works because they may have a much lower earth-

quake resistance than the dam.

Adequate monitoring of RTS prior, during an d after impound-

ment of a reservoir provides the most conclusive evide nce as to

whether or not water impoundment causes triggered earthquakes.

To help distinguish between background seismicity and RTS, mon-

itoring should start well in advance of impounding of the reservoir.

From the observational engineering viewpoint, the RTS cases are

characterized by the following main features, which can be used as

a checklist to determine if RTS is occurring:

• a) The seismic events monitored during and after impounding are

more frequent than the background seismicity before impounding.

• b) With the increase and larger oscillations of storage levels, the

frequency and magnitude of RTS phenomena increases.

• c) In most cases, the triggered events tend to be scaled down, after

peaking, towards ambient background activity.

• d) In a number of triggered cases, the observed intensity of shak-

ing sharply decreased with distance from the epicenters, which usu-

ally cluster around the reservoir. In most cases the activity starts

soon after the beginning of impounding and grows with reservoir

levels, restarting as a rule after quick changes in reservoir levels.

• e) It has been proposed to use as a diagnostic tool the relationship

between frequency and magnitude of seismic events, with more

smaller events indicating triggered seismicity, although this indi-

cator is considered controversial.

• f) It is considered that more triggered events are linked to normal

and strike slip faulting than to thrust faulting.

• g) Relatively shallow earthquakes are the most likely outcome of

RTS. But, this statement has to be understood as a general trend,

as significant exceptions are possible, as in the case of Aswan reser-

voir, where RTS phenomena were documented at a depth of 25km.

In the case of Zipingpu, on this basis the possibility of the Wenchuan

Earthquake being RTS is very unlikely.

ONCLUSIONS

Based on the recent observations made in China, the following con-

clusions can be drawn:

. Following a strong earthquake, the seismic hazard at dam sites

have to be increased and the seismic design guidelines have to be

reviewed and updated.

2. In mountainous regions large rockfalls have to be expected, which

hinder access to dams after an earthquake for many weeks and

months, and rockfall s can cause substantial damage to surface

powerhouses, gates, piers, and appurtenant structures.

3. The earthquake hazard has many features which are site-specif-

ic, therefore a standard approach in which emphasis is only put

on ground shaking is problematic.

4. The concrete face of CFRD dams are vulnerable to strong ground

shaking mainly due to large in-plane forces.

5. Seismic instrumentation is still lacking in most dams, even in large

modern dams.

6. Every time a strong earthquake occurs, new features show up

which have been overlooked in the past by dam engineers.

7. Methods for the assessment of slopes in steep valleys subjected to

very strong ground shaking need further development.

Martin Wieland, Chairman ICOLD Committee on Seismic

Aspects of Dam Design, Poyry Energy Ltd

martin.wieland@poyry.com

Chen Houqun, Vice-Chairman, ICOLD Committee on

Seismic Aspects of Dam Design, Academician, China

Academy of Engineering, China Institute of Water

Resources and Hydropower Research, Beijing, China.

chenhq@iwhr.com

The photos showing the damage and repair works of the

concrete face of Zipingpu CFRD were obtained from Xu

Zeping of IWHR, Beijing

SEISMIC ANALYSIS

IWP& DC

References

[1] Chen H. (2008), Consideration of dam safety after Wenchuan

earthquake in China, Proc. 14th World conference on Earthquake

Engineering, Special session S13 Seismic Aspects of Large Embankment

and Concrete Dams, Beijing, China

[2] ICOLD (2001), Design features of dams to effectively resist seismic

ground motion, Guidelines, Bulletin 120, Committee on Seismic Aspects of

Dam Design, International Commission on Large Dams (ICOLD), Paris, France

[3] ICOLD (2008), Inspection of dams following earthquake, guidelines,

revised Bulletin 62, Committee on Seismic Aspects of Dam Design,

International Commission on Large Dams (ICOLD), Paris, France

[4] ICOLD (2009), Reservoirs and seismicity: State of knowledge, Bulletin

137, Committee on Seismic Aspects of Dam Design, International

Commission on Large Dams (ICOLD), Paris, France

[5] Wang Z. et al. (2008), General introduction to engineering damage

during Wenchuan earthquake, Journal of Earthquake Engineering and

Engineering Vibration, Vol. 28 supplement compiled by China Earthquake

Administration, October

[6] Xu Z. (2008), Performance of Zipingpu CFRD during the strong

earthquake, Proc. 10th International Symposium on Landslides and

Engineered Slopes, Xian, China, June 30 to July 4