Owen Falls Movements

12
The effect of concrete expansion at Owen Falls power station, Uganda P. J. Mason, BSc , MPhil , PhD, CEng , FICE and J. D. Molyneux, BEng , CEng , MICE & Cracking of concrete at Owen Falls power station, Uganda, was diagnosed as being due to concrete expansion caused by alkali–aggregate reaction. Resulting struc- tural movements had caused local over- stressing and also deflections of installed plant and equipment. The degree of expansion varied according to the dierent cements used during construction. The processes of detecting and clarifying the implications of the various movements are explained, together with measures taken to mitigate immediate problems and provide adequate monitoring to areas of longer-term concern. Lessons are drawn for the guidance of others investigating similar phenomena. Keywords: dams, barrages & reservoirs; power stations (non-fossil fuel); rehabili- tation, reclamation & renovation Introduction Construction of the Owen Falls dam and power station complex in Uganda started on site in 1951. The works comprised the damming of the Victoria Nile a short distance downstream from its source at Lake Victoria. A right flank main dam and sluice structure is separated by high ground from a left flank power station with ten Kaplan turbines. The turbines were commis- sioned in stages, the first two in 1954 and the last in 1968, giving a total installed capacity of 150 MW. The power station has provided the overwhelming majority of the power in Uganda, plus additional power for export, right up to the present day. The location of the works is shown in Fig. 1. A downstream view of the power station is shown in Fig. 2 and an internal view along the machine hall in Fig. 3. A chronology of events and principal characteristics of the schemes are given in Tables 1 and 2 respec- tively. 2. In 1964 cracks were noticed in the con- crete around the generator housing on machine No. 4. Also in 1964, Lake Victoria, by then eectively impounded by the dam and power station, reached record levels with a rise of more than 2 m above the levels which had been carefully monitored since 1896. The Nile Waters Agreement required additional releases to be made through the sluices to reflect the level rise, leading also, therefore, to higher tailwater levels. The level rises increased both the loading and the uplift on the power station and other works. The fact that the cracks were first noticed in the same year that the rises occurred, led to the inevitable suspicion that some form of structural distress due to increased loading had taken place. As this seemed, at the time, to be limited to machine No. 4, no changes were made to arrangements for the subsequent installation of machines No. 9 and 10. 3. The power station and associated works deteriorated rapidly during the 1970s as Uganda passed through a period of political instability, firstly through the Amin regime (1971–79) and then through the Obote regime (1980–85). The works were not inspected in detail again by the original designers until 1983. By this time most turbines were operable only at reduced load. This was largely due to lack of spares and maintenance, although there were also some machine misalignments and clearance losses. The cracking in the power station had also increased significantly to include a major crack up to 25 mm wide through the machine hall floor and running the length of the power station (see Fig. 4). The cracking at machine No. 4 had increased significantly and was mirrored at all other machines. It should be noted that when the inspections took place in 1983, general social and security conditions in Uganda were still quite dire with no local hotels available and little infrastructure. Visiting engineers slept in the power station. 4. Refurbishment works started in 1988 and these included rewaterproofing the roof, making underwater repairs throughout the works, carrying out considerable amounts of stressed anchoring in the power station to stabilize the civil structure and generally refurbishing all the mechanical and electrical equipment. As part of this refurbishment the generators were uprated from 15 to 18 MW, increasing the total capacity of the station to 180 MW. Details of the stressed anchoring which was carried out are given elsewhere. 1 5. During 1990, continued monitoring of the cracks indicated that movement had not ceased and that the underlying cause might be other than simple structural overload. Indeed, the crack monitoring results showed a broadly linear trend of crack opening since 1973 which did not, for example, seem to vary with changes Peter J. Mason, Director, Binnie, Black & Veatch, Redhill (formerly Director, GIBB Ltd) J. Dominic Molyneux, Senior Project Engineer, GIBB Ltd, Reading 226 Proc. Instn Civ. Engrs Wat., Marit. & Energy, 1998, 130, Dec., 226–237 Paper 11726 Written discussion closes 15 April 1999

Transcript of Owen Falls Movements

Page 1: Owen Falls Movements

The effect of concrete expansion atOwen Falls power station, UgandaP. J. Mason, BSc, MPhil, PhD, CEng, FICE and J. D. Molyneux, BEng, CEng,MICE

& Cracking of concrete at Owen Falls power

station, Uganda, was diagnosed as being

due to concrete expansion caused by

alkali±aggregate reaction. Resulting struc-

tural movements had caused local over-

stressing and also de¯ections of installed

plant and equipment. The degree of

expansion varied according to the di�erent

cements used during construction. The

processes of detecting and clarifying the

implications of the various movements are

explained, together with measures taken

to mitigate immediate problems and

provide adequate monitoring to areas of

longer-term concern. Lessons are drawn

for the guidance of others investigating

similar phenomena.

Keywords: dams, barrages & reservoirs;

power stations (non-fossil fuel); rehabili-

tation, reclamation & renovation

IntroductionConstruction of the Owen Falls dam and power

station complex in Uganda started on site in

1951. The works comprised the damming of the

Victoria Nile a short distance downstream from

its source at Lake Victoria. A right ¯ank main

dam and sluice structure is separated by high

ground from a left ¯ank power station with ten

Kaplan turbines. The turbines were commis-

sioned in stages, the ®rst two in 1954 and the

last in 1968, giving a total installed capacity of

150 MW. The power station has provided the

overwhelming majority of the power in Uganda,

plus additional power for export, right up to the

present day. The location of the works is shown

in Fig. 1. A downstream view of the power

station is shown in Fig. 2 and an internal view

along the machine hall in Fig. 3. A chronology

of events and principal characteristics of the

schemes are given in Tables 1 and 2 respec-

tively.

2. In 1964 cracks were noticed in the con-

crete around the generator housing on machine

No. 4. Also in 1964, Lake Victoria, by then

e�ectively impounded by the dam and power

station, reached record levels with a rise of

more than 2 m above the levels which had been

carefully monitored since 1896. The Nile Waters

Agreement required additional releases to be

made through the sluices to re¯ect the level

rise, leading also, therefore, to higher tailwater

levels. The level rises increased both the

loading and the uplift on the power station and

other works. The fact that the cracks were ®rst

noticed in the same year that the rises occurred,

led to the inevitable suspicion that some form of

structural distress due to increased loading had

taken place. As this seemed, at the time, to be

limited to machine No. 4, no changes were made

to arrangements for the subsequent installation

of machines No. 9 and 10.

3. The power station and associated works

deteriorated rapidly during the 1970s as

Uganda passed through a period of political

instability, ®rstly through the Amin regime

(1971±79) and then through the Obote regime

(1980±85). The works were not inspected in

detail again by the original designers until

1983. By this time most turbines were operable

only at reduced load. This was largely due to

lack of spares and maintenance, although there

were also some machine misalignments and

clearance losses. The cracking in the power

station had also increased signi®cantly to

include a major crack up to 25 mm wide

through the machine hall ¯oor and running the

length of the power station (see Fig. 4). The

cracking at machine No. 4 had increased

signi®cantly and was mirrored at all other

machines. It should be noted that when the

inspections took place in 1983, general social

and security conditions in Uganda were still

quite dire with no local hotels available and

little infrastructure. Visiting engineers slept in

the power station.

4. Refurbishment works started in 1988 and

these included rewaterproo®ng the roof, making

underwater repairs throughout the works,

carrying out considerable amounts of stressed

anchoring in the power station to stabilize the

civil structure and generally refurbishing all

the mechanical and electrical equipment. As

part of this refurbishment the generators were

uprated from 15 to 18 MW, increasing the total

capacity of the station to 180 MW. Details of the

stressed anchoring which was carried out are

given elsewhere.1

5. During 1990, continued monitoring of the

cracks indicated that movement had not ceased

and that the underlying cause might be other

than simple structural overload. Indeed, the

crack monitoring results showed a broadly

linear trend of crack opening since 1973 which

did not, for example, seem to vary with changes

Peter J. Mason,

Director, Binnie,

Black & Veatch,

Redhill (formerly

Director, GIBB Ltd)

J. Dominic Molyneux,

Senior Project

Engineer, GIBB Ltd,

Reading

226

Proc. Instn Civ.

Engrs Wat., Marit.

& Energy, 1998,

130, Dec., 226±237

Paper 11726

Written discussion

closes 15 April 1999

Page 2: Owen Falls Movements

in lake level. The present lead author became

involved at this stage and the following paper

broadly outlines the review and work that was

subsequently carried out to clarify and diag-

nose the cause of distress and to put in place

appropriate mitigating and monitoring mea-

sures.

Analysis of movements6. During the initial inspections in 1983,

samples of spalled concrete had been obtained

and were examined for potential distress such

as that caused by alkali±aggregate reaction

(AAR). This included analysis by thin section.

At that time no such distress could be detected.

The review in 1990 therefore focused on taking

a broad overview of what visible signs of

movement had occurred in order to visualize

overall patterns and see if this could shed

further light on underlying mechanisms.

7. It was noted that the patterns of cracking

and movement were very similar at most

machines, although focused more heavily on

machines No. 5 to 10. The pattern was therefore

initially viewed from a two-dimensional per-

spective as superimposed on a cross-section

through the power station, arbitrarily taken on

the centreline of machine No. 8 (see Fig. 5).

8. Monitoring the absolute and vector direc-

tions of crack movements in various parts of

the power station indicated that the down-

stream wall of the station was rotating down-

stream about a hinge point immediately above

the draft tube (see Fig. 6). It should be noted

that the power station was initially cast with

just the upstream and downstream walls as

®rst-stage concrete and with the latter heavily

reinforced to resist tailwater levels. This per-

mitted the machines to be erected and concreted

at a subsequent, second, stage.

9. It should also be noted that this down-

stream rotational movement was compatible

with two other observations. One was the main

longitudinal crack along the machine hall ¯oor.

This was up to 25 mm wide and, together with

other minor cracks, indicated a downstream

movement at that level of 32 mm. Secondly, the

overhead gantry crane rails were also known to

Mediterranean

Cairo

Egypt Red Sea

Sudan Khartoum

R. N

ile

KampalaOwenFalls

LakeVictoria

VictoriaNile

VictoriaNile

Tailrace

Headrace

Power station

Sluices

Main dam

Roadbridge

To Jinja

To Kampala

0 50 100 150

Scale: m

227

Fig. 1. Location plans

Fig. 2. Downstream

view of Owen Falls

power station

CONCRETE EXPANSION

AT OWEN POWER STATION

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Fig. 3. View down the machine hall

Table 1. Owen Falls dam and hydropower complexÐa brief chronology

Date Event

1935 River Nile examined for hydroelectric potential

1947 Further survey of hydropower potential by Ugandan government

1948 Uganda Electricity Board formed

1949 Owen Falls hydroelectric complex planned

1951 First concrete placed at Owen Falls

1954 Machine No. 1 commissioned in January and No. 2 in April

Inauguration ceremony by HM the Queen on 29 April

1955 Machine No. 3 commissioned in January and No. 4 in August

1957 Machine No. 5 commissioned in January and No. 6 in February

50-year power agreement reached with Kenya

1958 Machine No. 7 commissioned in May

1959 Machine No. 8 commissioned in July

1964 Lake Victoria reaches unprecedented levels in May

First awareness of concrete cracking around machine No. 4

1966 Machine No. 9 commissioned in May

1968 Machine No. 10 commissioned in July

1971 Idi Amin seizes power in Uganda in a military coup

1973 Crack gauging commenced in power station by local sta�

1978 Targets installed to monitor downstream wall movements

1979 Idi Amin ousted by rebel forces

1980 Milton Obote returned to power

1983 Reinspection of power complex by team of UK engineers

1985 Milton Obote overthrown

1988 Start of refurbishment works on site

Table 2. Owen Falls dam and hydropower complexÐprincipal characteristics

Description Dimensions

Lake Victoria:

Catchment area 267 000 km2

Lake area 67 000 km2

Lake mean depth 40 m

Turbine generators:

Total number 10

Design head range 17´5±22 m

Originally installed output per machine 15 MW

Original ¯ow per turbine 96 m3/s

(now uprated to 18 MW with corresponding ¯ow

increase)

Sluices:

Total number 6

Size per sluice 3 m 6 5´1 m high

Design discharge per sluice 212 m3/s

Principal dimensions:

Dam crest road level 1136´15 m asl

Upstream max. water level 1135´00 m asl

Upstream min. storage level 1131´90 m asl

Max. tailwater level 1114´35 m asl

Min. tailwater level 1112´80 m asl

Nominal min. dam foundation level 1108´00 m asl

Nominal min. power station foundation level 1100´00 m asl

Length of gravity dam 726 m

Length of machine hall (excluding loading bay) 167´6 m

Width of machine hall 16´5 m

asl = above sea level.

Fig. 4. View of the longitudinal crack in the

machine hall ¯oor

228

MASON AND MOLYNEUX

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be moving apart. Some years earlier, the over-

head crane had in fact jammed and crane

movement was reinstated by machining 9´5 mm

o� the bosses on one set of wheels to allow

them greater axle ¯oat.

10. It was also known that the power station

¯oor had risen in a number of locations. Again

these were concentrated between machines No.

6 and 10 but rises were as much as 76 mm (see

Fig. 7). Another indication of movement at

several locations was diagonal cracks in the

draft tube side walls (see Fig. 6).

11. After a reassessment of the evidence,

the 1990 review concluded that all these e�ects

could be broadly explained by an expansion of

the concrete around the machines. This would

exert a load on the downstream wall which was

one part of the structure relatively free to move.

It would also tend to cause cracking in the

original ®rst-stage concrete. Diagonal shear

cracking in the draft tube side walls would be

due to the force couple developed by the

expansive thrust downstream being resisted by

the draft tube foundations.

Initial modelling12. At this stage a simple two-dimensional

®nite element model was undertaken to assess

the e�ects of second-stage concrete expansion

around the turbines and their spiral casings

when viewed in the plan. The results are shown

in Fig. 8. It can be noted that for any single

machine, the concrete is unable to move later-

ally as it is restrained by neighbouring blocks,

nor can it move upstream. Movements are

therefore concentrated vertically and down-

stream. The downstream movement is ampli®ed

on the centrelines of the machines due to arch

action around the spiral casing. The model

therefore indicated the potential for the devel-

opment of voids downstream of the spirals and

also for vertical cracks between the ®rst- and

second-stage concrete in the stair wells between

the machines. Such vertical cracks were in fact

present on site. When the steel spiral casings

were subsequently drilled through downstream,

gaps between the steel and concrete of up to

15 mm were found.

13. The model also predicted that the mag-

ni®cation of upstream/downstream movement,

coupled with the lateral restraint against

expansion, would tend to produce ovality in the

water passages. This was also con®rmed by

clearance measurements around the tips of

the turbine blades, to the surrounding suction

cone.

Power station

Roof truss

Overhead crane

Draft tube deck

Generator

Lowerbracket

Columns

Tailrace

Spiralcasing

Turbinerunner

Suctioncone

Drafttube

Intake dam

229

Fig. 5. Typical cross-

section through the

power station

CONCRETE EXPANSION

AT OWEN POWER STATION

Page 5: Owen Falls Movements

Different concretes14. A remaining concern was why di�erent

areas of the station had developed movements

in di�erent ways. This applied not only to the

power station proper but also to other parts of

the associated works such as the main dam and

intake structures.

15. In order to clarify this issue, the historic

records of concrete pours were examined for

various parts of the work. It was noted that

midway through construction the cement type

changed from imported Rugby cement from the

UK, to Tororo cement from Uganda's ®rst

cement factory which was commissioned in

1953. Fig. 9 shows patterns of level rise with

areas where Tororo cement was used. The

relationship between level rise and cement type

is clearly apparent.

16. An analysis of the Tororo cement indi-

cated that it had been produced from the local

volcanic material carbonatite which is very rich

in both alkalis and potash. This was certainly a

contributory factor to the accelerated e�ects of

alkali-aggregate reaction in those areas where

the local cement was used. Another factor was

that the coarse aggregate, though originally

classi®ed by the Uganda Geological Survey

under the broad generic name amphibolite, was

in fact amphibol-schist. This contains small

particles of reactive materials such as strained

quartz in a non-reactive matrix.

17. Typically, with this form of reaction,

symptoms are not seen in the early days after

construction. Gradually, alkali pore water seeps

from the cement paste into the large aggregate.

This a�ects any reactive particles, producing

silica gel and leading to internal splits and

expansion. The resultant local cracks even-

tually join, giving the appearance of structural

cracking, rather than random crazing. This

process is known as alkali±silicate reaction

(ASR) or `slow±late' reaction. It was discovered

much later than the more common AAR,

generally associated with reactive sand.2 The

e�ects of ASR, or slow±late reaction, are

typically not seen until ®ve to ten years after

construction.

18. The ®rst- and second-stage concretes in

the power station are described on record

drawings as 5/1� mixes. These in turn were

originally speci®ed as ratios of 5 cwt (254 kg) of

cement to 12 cu. ft (0´34 m3) of sand to 20 cu. ft

(0´57 m3) of coarse aggregate of maximum size

1� in. (38 mm). This is di�cult to interpret

precisely in modern terms without knowing

original bulking factors and densities. An

analysis of concrete core samples from the

power station, however, revealed cement con-

tents averaging 300 kg/m3 of concrete, plus or

minus about 50 kg/m3. Water/cement ratios

were back-analysed as between 0´52 and 0´60.

More importantly, the analyses revealed excep-

tionally high, equivalent alkali (sodium oxide)

contents of 2´5% of the cement and 7´5 kg/m3 of

the concrete.

19. The reinforcement used in the power

station concrete was mild steel throughout. The

average reinforcement density was 46 kg/m3.

Interestingly, the second-stage concrete around

the machines was completely unreinforced.

Years later this too was considered to be a

possible contributory factor towards the crack-

ing, although with hindsight, nominal reinforce-

ment would have done little to restrain the

expansive forces that were subsequently gener-

ated.

Power station stability20. Once the general e�ects of expansion

had been diagnosed, it was felt important that

the overall stability of the basic structural shell

be re-examined. This was done by a simple

structural model with superimposed move-

ments, and incorporated a standard roof truss

(see Fig. 10). It was clear that movements had

0 10 20

Crack vector scale: mm

Note: Arrows indicate the direction and magnitude of crack opening

Pronounced shear cracks,hairline to 2 mm

230

Fig. 6. Cross-section

through the power

station showing

downstream rotation

based on the vector

measurement of

cracks, also shear

cracking in the draft

tube side walls

MASON AND MOLYNEUX

Page 6: Owen Falls Movements

generally started in the mid-1960s. This

allowed a rate of movement to be assumed of

approximately 1 mm per year horizontally at

machine hall ¯oor level. Overall stability and

structural adequacy were checked assuming a

further 30 years of expansion at the same rate.

21. Inherent stability of the overall struc-

ture was con®rmed but two local e�ects were

noted. One was that certain lower elements on

the roof trusses showed potential long-term

over-stress, depending on the potential yield of

structural connections on the upper wall. Sec-

ondly, it could be seen that the slender under-

water columns supporting the downstream deck

above the draft tubes were subject to rotation.

Diver examination had revealed that many of

these columns were severely distressed with

cracking, exposed reinforcement and consider-

able degradation of the concrete.

22. Measures to repair these columns were

discussed at some length and underwater trials

were carried out using epoxy repair techniques.

Dewatering by limpet co�erdams was also

considered. Eventually, however, the best way

forward emerged as a result of a focused, value

engineering study.

23. The value engineering study initially

considered various ways in which the down-

stream columns could be repaired but then,

alternatively, what other measures might be

available to give deck support. It was quickly

realized that drilling through the deck and

installing alternative circular steel columns

with structural connections to the tops of the

existing columns was a much more secure and

cost-e�ective way to proceed. Not only were the

repairs rapid but underwater diving time was

minimized. This was particularly important

given that the power station was in continual

use, with the client loath to cut back on

generation at any time unless absolutely essen-

tial. The repairs were successfully completed to

time and budget and at approximately half the

estimated cost of repairing the original columns

under water (see Fig. 11).

24. A visual inspection of the roof trusses

with regard to line, condition of paintwork,

rivets, etc. revealed no obvious signs of dis-

tress. With the evidence of expansion from

measurements of the crane rail gauge, and the

numerical modelling, this left the status of

the trusses uncertain. Therefore, before any

possible strengthening works were initiated,

extensometers were installed along the main,

lower truss elements to monitor movements and

provide evidence to justify any further expen-

diture.

25. Invar steel, bar extensometers, 5 m long,

were attached by clamping to the central lower

elements of the trusses 15 m above the machine

hall ¯oor. The instruments are read by trained

local monitoring sta� using a portable dial

gauge with the overhead gantry crane as a

mobile platform. Temperature is recorded along

with the dial gauge readings so that measure-

ments can be adjusted for the thermal expan-

Dow

nstr

eam

Ups

trea

m

Key section

E D F

C

B A

E

D

Fd(Fu shown dotted)

B

A

C

Machine

centreline

50 mm verticalmovement scale

Set

1 Set

2 Set

3 Set

4 Set

5 Set

6 Set

7 Set

8 Set

9 Set

10

Note: 1. Fu - Immediately upstream of main floor crack 2. Fd - Immediately downstream of main floor crack

231

Fig. 7. Isometric view

of machine hall ¯oor

rises

CONCRETE EXPANSION

AT OWEN POWER STATION

Page 7: Owen Falls Movements

sion of the trusses. In fact, since the average

daytime temperature in Uganda varies little

during the year and the temperature in the

power house tends to be regulated even more,

corrections tend to be minor.

26. To date, results have been inconclusive

but suggest that, although movements may

occur, they could be subject to relief elsewhere.

Indeed, visual inspection did reveal some

cracking in the concrete columns supporting the

trusses. Another factor which may have helped

to relieve the build-up of stress in the trusses is

that the power station roo®ng was replaced

after the 1983 inspections. The original precast

concrete slab covering was replaced with a thin

synthetic membrane which has substantially

reduced the dead load carried by the trusses.

Turbine movement and three-dimensional modelling27. A remaining aspect, critical to the long-

term reliable operation of the station, was

understanding how historic and future expan-

sion might a�ect the turbines and generators. It

was noted, for example, that the rise in machine

hall ¯oor level had caused the turbine runners

to lift by as much as 76 mm compared to their

original elevation. A number of alternative

means to allow this situation to be redressed

were considered, including breaking out sup-

porting sole plates and generally lowering

them. There was, however, concern that this

might damage the structural integrity of the

concrete support.

28. It should be noted that in the same way

that the concrete was arching downstream in

plan between machines (see Fig. 8), it was also

arching vertically above each machine (see Fig.

12). This had led to horizontal cracking around

the turbine pits. This in turn meant that for

many years the generators had, in fact, been

sited on structural arches, or domes, founded

between the sets, rather than with the principal

load passing vertically down through the

turbine stay vanes. The fact that operation had

continued for most of the station's life in this

situation suggested that it was in fact accept-

able. Moreover, the situation is undoubtedly

reproduced in many of the other hydropower

stations in the world, similarly a�ected by

AAR. Attempts to grout such cracks would

have little e�ect, given that movements would

inevitably continue.

29. It was decided to produce a three-

dimensional model of a turbine block (see Fig.

13) and model the expansion using various

parameters for concrete and for restraint of

associated neighbouring concrete and steel

elements. This was done by varying the para-

meters and correlating them against measured

site movements on various concrete levels

around the turbines, notably those for the

stator, lower bracket and stay ring sole plates

(see Fig. 13). Detailed reference levels were

available for all three of these from the original

construction records. It was established that the

best correlations occurred with the Poisson's

ratio for the concrete, increased from 0´17 to

0´49, in e�ect allowing incompressible ¯uid,

rather than elastic, movement of the concrete.

Reductions in moduli were made elsewhere

where concrete was in tension and the struc-

tural restraint therefore provided by internal

steel reinforcement. The only machine restraint

found to be signi®cant was that provided by the

heavy, steel, lower bracket.

30. The model was essentially linear and

therefore somewhat of an approximation to the

inevitable non-linearity of the stress condition

actually occurring. It did, however, give a very

good indication of stress values, which were

broadly permissible, and also of patterns of

movement which replicated the downstream

rotation (see Fig. 14), machine hall ¯oor rises

and ovality of spiral casing and turbine pas-

sages, all predicted and measured earlier.

31. In order to restore machine operation

and to ensure future reliability, it was decided

Late

ral r

estr

aint

with

dow

nstr

eam

slid

ing

Fixed

Expanded profile

2D finite element expansion study

StairwellDownstream wall

Mass concrete

Differential expansiondownstream leadingto crack in stairwellbetween downstreamwall and mass concrete

Boundary of idealizedfinite element modelshown hatched

Upstream

Key plan on spiral

232

Fig. 8. Two-

dimensional ®nite

element model of

concrete expansion in

plan, on the centreline

of a turbine spiral

casing

MASON AND MOLYNEUX

Page 8: Owen Falls Movements

to insert spacers in the main turbine/generator

shafts coupled with packing over the main sole

plates. The spacers were designed to accommo-

date future movement, over the next 25±30

years, with the compensatory packing under the

sole plates being progressively removed as

expansion continued. This is discussed in more

detail elsewhere3 and was developed jointly by

the civil and mechanical consultants.

32. Another result from this phase of the

work was establishing from surveys the actual

rises for the stator, lower bracket and stay ring

sole plates. It became clear from the values

obtained that the rises could be extrapolated

down to zero at approximately the base level of

the second-stage concrete around the machines.

This was a particularly useful ®nding as it

highlighted that the primary expansive force

was probably from the second-stage concrete

and was being exerted on the comparatively

less a�ected ®rst-stage concrete. The e�ect is

highlighted in Fig. 9 where the lower gallery,

founded on ®rst-stage concrete, has remained

close to design level whereas the upper gallery,

founded on second-stage concrete, has shown a

marked stepped rise from machines No. 6 to 10.

Stressed anchor monitoring33. In view of the likelihood of continued

concrete expansion, it was decided to use the

three-dimensional ®nite element model to

assess possible stress build-up in the stressed

anchors. Appropriate model node point separa-

tions were checked against the expansions

needed to calibrate the model against turbine

embedded part movements. The stress

increases that were predicted using this

approach were checked by carrying out lift-o�

tests on 24 representative anchors, out of a total

of approximately 170 installed.

34. The loads measured in the lift-o� tests

were generally lower than those predicted by

the ®nite element model. This result has been

found elsewhere in similar cases, although the

reasons are unclear. It may be related to strain

relief movements in the concrete immediately

under the end anchor plates, given the nature of

the AAR-a�ected concrete.

35. On six selected anchor heads, the cables

were completely de-stressed, permanent load

cells incorporated and the anchors re-stressed.

These will give a constant future record of

stress movements which can be used to assess

the likely condition of the anchors throughout

the station.

Future monitoring36. As described previously, monitoring at

Owen Falls began in 1973 with crack gauges.

Since then, as the potential problems were

recognized, a rather ad hoc monitoring regime

evolved. During 1994, with the newly acquired

understanding of movement causes, an e�ort

was made to rationalize all the existing instru-

mentation. A monitoring scheme was designed

to provide the information necessary to monitor

the concrete expansion and con®rm the safety

and serviceability of the dam and hydroelectric

power station. The scheme was implemented in

1994 as part of a contract to improve drainage

at the dam and also to investigate concrete

condition throughout the works.

37. A new network of thermally balanced,

double-skinned survey pillars and geodetic

survey targets was constructed. The geodetic

survey network is used to locate the power

station and draft tube deck to within an

accuracy of 2 mm. Such accuracy is deemed

su�cient to monitor drift movements caused by

the expanding concrete or any indication of

structural distress.

38. Survey points for a precise levelling

traverse through the power station were also

installed. These will enable monitoring of the

concrete and the machines and help evaluate

40

20

0

–20

20

0

–20

'Ris

e' in

mill

imet

ers

Set 10 Set 8

Set 9Set 7

Set 6

Set 5

Set 4

Set 3

Set 2

Set 1

Set 10

Set 8Set 9

Set 7 Set 6

Set 5

Set 4Set 3 Set 2

Set 1

Upper gallery

Lower gallery

2nd stage concretewith Tororo cement

2nd stage concretewith imported cement

Upper gallery1119·53

1117·09

1107·49

Stage 2

Stage 3

Stage 1

1115·26

1111·61

1109·17

Lowergallery

233

Fig. 9. Level rises in

relation to cement

type (elevations in m)

CONCRETE EXPANSION

AT OWEN POWER STATION

Page 9: Owen Falls Movements

the need to change the sole plate packers

installed to accommodate this movement. As

the refurbishment is completed and the need for

expert expatriate sta� diminishes, it is intended

that all future monitoring work will be carried

out by local personnel. A comprehensive

manual was written describing the monitoring

requirements and outlining the history and

purpose of each of the 16 types of monitoring

required.

39. Monitoring generally is intended to

prevent events developing to undesirable levels

by giving warning in time for preventative

action to be taken. In order to serve this

purpose, monitoring data must be analysed and

interpreted promptly after instruments have

been read. It is no comfort to analyse data after

a disaster and be able to show that warning

signs were given; in fact, it may merely

demonstrate negligence.

40. At Owen Falls, two levels of alert have

been speci®ed for each type of monitoring. At

the (®rst) warning level, monitoring results

should be double-checked, frequency of read-

ings increased and a general increased level of

vigilance instigated. Depending on the circum-

stances, for instance if a number of independent

instruments demonstrate similar unusual

trends, it may be appropriate to notify the

power station management. At the (second)

alarm level, immediate action is required. All

instruments which could provide supporting

information should be rechecked and advice

should be taken on any action required to

safeguard the works.

41. The alert levels are assigned to pick up

short-term problems. Long-term trends of data

must be assessed annually and alert levels

adjusted accordingly. The actual values

assigned alter from instrument to instrument

depending on the usual scatter of results. Some

scatter is unavoidable due to seasonal ¯uctua-

tions, vibrations, accuracy of instruments,

human factors, etc. but they can be minimized

with careful procedures. The scatter of results

for any instrumentation scheme dictates the

minimum warning levels that can be set. Wider

bounds may be set if structural considerations

govern.

42. As a default case at Owen Falls,

warning levels were set at values more than

1´64 standard deviations above or below the

trend line in question. Assuming a normal

distribution, such values can be expected 10%

of the time. Alarm levels were set at 1´96

standard deviations above or below the trend

line in question, representing 5% of the time.

Supplementary criteria were set for extens-

ometer and crane gauge readings. Piezometer

readings were related to margins above tail-

water level. Drainage ¯ows were considered to

require judgmental assessment on an individual

basis.

43. For new instruments there is no scatter

of past readings to use to determine the

minimum alert levels. In order to evaluate the

expected scatter it was proposed that readings

were initially taken frequently. Several read-

ings in an hour would be necessary to establish

scatter due to vibration, hourly readings would

establish variation due to movements of the

sun, daily readings might be required to

measure variations due to reservoir level, and

monthly readings would be necessary to evalu-

ate seasonable changes. A knowledge of these

in¯uences is important in subsequently asses-

sing individual readings.

44. The long-term frequency of monitoring

must be a balance. Too frequent, and the

collected data will be overwhelming. Too sparce

and important movements could be missed. At

Owen Falls there are over 260 crack gauges.

These instruments provide the longest history

of movements in the complex but on the other

hand do not necessarily provide the best

measure of deformation in the short term

because of periodic stress build-up and sudden

relief. These instruments are therefore moni-

tored infrequently so that the history of move-

Deflected frame(exaggerated)

Original line ofpower station frame

Expandingconcrete

0 50 100

Deflection scale

234

Fig. 10. Structural

frame model of the

power station with a

32 mm horizontal

displacement

downstream at

machine hall ¯oor

level

MASON AND MOLYNEUX

Page 10: Owen Falls Movements

ment is continued, but they are not used for

day-to-day monitoring. Measurements of the

overhead crane gauge combine all the crack

movements for any individual set and provide a

much easier way of assessing overall expansive

movements.

Conclusions and lessons learnt45. The development and diagnosis of AAR,

or ASR, at Owen Falls power station, followed

on remarkably parallel lines to the similar

diagnosis at Mactaquac power station in

Canada.4 In both cases, movements and crack-

ing were noticed. In both cases, initial conclu-

sions featured structural movements and

loadings or foundation movement. In both

cases, initial assessments of alkali-aggregate

reaction proved negative. In both cases, long-

term ASR or slow±late reaction proved to be the

case. The Mactaquac power station started

operation in 1968, problems were noticed in

1979, and the ASR eventually diagnosed in

1986.

46. In such cases, the ®rst signs of distress

are often maloperation of equipment and/or

some form of cracking. These cracks are then

monitored. The long-term trend of crack

development may be broadly linear; however,

it will often appear as a stepped pattern due

to normal measuring errors and due to the

build-up and release of pressure as silica gel

is formed. Furthermore, movement of one

crack may temporarily halt while the move-

ment is accommodated instead by an adjacent

crack.

Sole plateloads

Generator

Turbinepit crack

Probable(arching)load path

Suction cone

Draft tube

235

Fig. 11. Detail of a

typical bracket

connection between

old and new draft

tube deck support

columns

Fig. 12. Longitudinal

section through the

power station showing

structural arching

support to the

generators and

stators, and

horizontal cracking

around the turbine

pits

CONCRETE EXPANSION

AT OWEN POWER STATION

Page 11: Owen Falls Movements

47. Apart from indicating whether the

movement is stable, accelerating or decelerat-

ing, the absolute monitoring of such cracks

does not always provide much useful informa-

tion. Inevitably, they are a secondary e�ect due

to expansion of concrete elsewhere. Of far more

immediate use and guidance is the overall

measurement of broad dimensions such as

height above foundation. In the case of Owen

Falls the ampli®ed width, as measured at the

crane rail, is probably the best indicator of

overall movement. The main gantry crane is

used as a stable, though temperature-depen-

dent, measuring bar and plates at either end are

®xed and used to measure o�sets to the rails. It

was also noted that one end of the power

station, at the loading bay beyond machine

No. 1, was completely uncracked and could be

taken as an original datum. Measurements of

crane rail separation on the machine centrelines

taken both proceeding down the station and

back and then averaged, gave a remarkably

good and consistent pattern of overall long-

term movements. These could be viewed

against global strains measured elsewhere, for

example overall dam height increases above

foundation.

48. It can be seen that the diagnosis of

alkali±aggregate reaction, eventually con®rmed

by concrete analysis, was accompanied by a

considerable amount of structural detective

work and intuitive analysis in which the skills

of the engineer were used to guide mathemat-

ical modelling rather than vice versa. This

must always be recommended as the way to

proceed.

49. Above all, the analyses were carried out

fully to understand the nature of the structure

both in its present state and how it would

continue to develop over the next 25±30 years,

which represents the economic life of the

installed turbines. They con®rmed that, with

regular monitoring, maintenance and adjust-

ment, the power station could continue to

operate e�ectively.

Acknowledgements50. The works described in this paper

were carried out between 1990 and 1997 when

the present lead author was a technical

director and subsequently director of GIBB

Ltd with responsibilities for the hands-on

direction of the contract at Owen Falls on

behalf of GIBB. The second author led

investigation work at site during 1994 includ-

ing coordination of the monitoring system and

rechecking stressed anchor loads. Particular

mention should be made of Mark Henning of

GIBB who so ably carried out the ®nite

element analyses; also of Peter Murray, the

GIBB inspector on site. Peter's career started

in Scotland with the manufacture of some of

the ®rst Owen Falls turbines. In the 1960s he

was based on site with the turbine Contractor

for the installation of later sets. Before

retiring in 1996 he spent eight years at Owen

Falls supervising their refurbishment. The

experience and knowledge of such men is

invaluable.

51. Mention should also be made of Dr Bill

French of Queen Mary and West®eld College

who so ably carried out the analysis of concrete

specimens taken from the site.

52. The work was carried out in conjunction

with Kennedy and Donkin Ltd of the UK who

were the lead consultants and particularly

responsible for the electro-mechanical aspects.

Richard Meileniewski of Kennedy and Donkin

Spiral

Suctioncone

Drafttube

Stator sole platesLower bracket sole plates

Stay ring

Fig. 13. Cut-away isometric view of the mesh for

the three-dimensional ®nite element model

Downstream

Suctioncone

Draft tube

Deflectedprofile

Originalprofile

Fig. 14. Section through the three-dimensional

®nite element model demonstrating downstream

rotation and ¯oor rise with second-stage

concrete expansion

236

MASON AND MOLYNEUX

Page 12: Owen Falls Movements

carried out the detailed appraisal of turbine

movements from the electro-mechanical per-

spective. Electro-mechanical work in the power

station was funded principally by the UK

Department for International Development

(DFID), formerly the Overseas Development

Administration (ODA). Other works were

funded by the World Bank and the Common-

wealth Development Corporation. Lastly, the

authors would like to express their appreciation

to the Uganda Electricity Board, and in parti-

cular to Mr Alex Mugoya, for their cooperation

throughout the period of the authors' involve-

ment, and for their agreement to the publication

of this paper.

References1. ARCANGELLIRCANGELLI E. and STELLATELLA C. The use of pre-

stressed anchors at the Owen Falls refurbishment.

Water Power & Dam Construction, 1993, 45, 23±

30.

2. ICOLD. Alkali-aggregate Reaction in Concrete

DamsÐReview and Recommendation. International

Committee on Large Dams, Bulletin 79, Paris,

1991.

3. MUGOYAUGOYA A. Keeping hydro units aligned. Hydro

Review Worldwide, 1994, 2, No. 4, winter.

4. HAYWOODAYWOOD D. G., THOMPSONHOMPSON G. A., RIGBEYIGBEY S. J.

and STEELETEELE R. R. Engineering and construction

options for the management of slow/late alkali-

aggregate reactive concrete. International

Committee on Large Dams, Proceedings of the

16th Congress, San Francisco, 1968, Q62, R33,

575±588.

237

CONCRETE EXPANSION

AT OWEN POWER STATION