Reducing risks in the investigation, design and construction of large concrete dams

Accepted Manuscript Reducing risks in the investigation, design and construction of large concrete dams E.T. Brown

PII:

S1674-7755(16)30221-9

DOI:

10.1016/j.jrmge.2016.11.002

Reference:

JRMGE 292

To appear in:

Journal of Rock Mechanics and Geotechnical Engineering

Received Date: 13 June 2016 Accepted Date: 10 November 2016

Please cite this article as: Brown ET, Reducing risks in the investigation, design and construction of large concrete dams, Journal of Rock Mechanics and Geotechnical Engineering (2016), doi: 10.1016/ j.jrmge.2016.11.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Reducing risks in the investigation, design and construction of large concrete dams E.T. Brown Golder Associates Pty. Ltd., Brisbane, Australia

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Received 13 June 2016; received in revised form 8 November 2016; accepted 10 November 2016

Abstract: An overview of the GeoSafe 2016 Symposium topic is provided using the example of large concrete dams for purposes of illustration. It is essential that the risks associated with large dams be evaluated rigorously and managed proactively at all stages of their lives so that the risk of failure remains As Low As Reasonably Practicable (ALARP). Rock engineering features of large concrete dams that require particular attention, assessment and monitoring during the investigation, design, construction, initial filling, in-service operation, and subsequent repair and upgrade stages of the lives of concrete dams are identified and illustrated by examples from recorded experiences. A number of major concrete dam failures, including that of the

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St. Francis dam, California, U.S.A., in 1928, have led to significant developments in rock mechanics and rock engineering knowledge and techniques, as well as in dam design and review processes. More recent advances include a range of analytical, numerical modelling, probabilistic, reliability, failure mode and risk assessment approaches.

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Keywords: concrete dam failure; dam engineering; risk assessment; rock engineering design; site investigation

The definition of large dams has varied over time. In the context of

Introduction

concrete dams, the International Commission on Large Dams (ICOLD)

This paper seeks to provide an overview of the GeoSafe 2016

defines a large dam as one which is “more than 15 metres in height

Symposium topics, Reducing Risks in Site Investigation, Modelling and

measured from the lowest point of the general foundations to the

Construction in Rock Engineering, using the example of the

“crest” of the dam”. This definition will be adopted here. The types of concrete dam to be considered here are generally gravity

While the rock engineering aspects of large concrete dams are

or arch dams, but may also include buttress dams and combined types.

generically similar to those of other types of projects involving

Concrete-faced rockfill dams will not be considered. Arch dams may be

engineering in rock, dams have a number of features which make them a

thin or thick and may be singly or doubly curved; some gravity dams

particularly important and rewarding topic of study. Because of their

may also be curved in plan. Generally, the concrete is placed in mass

sizes, their importance to society, and the potentially disastrous

concrete blocks, although more recently roller compacted concrete

consequences of their failure, it is essential that the risks associated with

(RCC) has been used to dam heights of about 200 m. Concrete gravity

large dams be assessed rigorously and managed proactively at all stages

dams rely on the weight of the concrete to withstand the forces imposed

of their lives so that the societal risk of dam failure remains As Low As

on the dam. Arch dams transmit these forces into the abutment

Reasonably Practicable (ALARP) (ANCOLD, 2003; Barker, 2011).

foundations by the structure’s arching action and, as a result, generally

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investigation, design, construction and operation of large concrete dams.

impose higher loads on the foundations. Modern concrete gravity and

the planning, site investigation, design (including, but not restricted to,

arch dams have galleries in the dam from which grout holes may be

modelling), construction, operation, and repair and upgrade stages of the

drilled to reduce the permeability of the foundation. A combination of

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It is necessary to recognise at the outset that the full consideration of

lives of concrete dam projects, involves a wider range of engineering

grouting and drainage reduces the uplift pressures within the dam

and scientific knowledge and skills than those usually represented in the

foundation and at the dam-foundation contacts (Fell et al., 2014).

disciplines of rock mechanics and rock engineering alone. Attention

Table 1 lists the key features of a number of large concrete dams. It

here will be focused, in the main, on rock mechanics and rock

includes the world’s highest concrete dams that either exist or are under

engineering considerations. A broad international perspective will be

construction, as well as a number of other historically important dams

taken having recourse to the extensive literature on the subject.

and dams referred to in this paper, listed in decreasing order of height.

However, it must be recognised that the writer’s personal experience of

The highest dams listed are arch dams. It will be noted that a significant

the engineering of large concrete dams is restricted to his home country

number of the highest concrete dams are in China. In fact,

of Australia.

approximately 20% of the world’s large dams are Chinese. As indicated in the caption, the data in Table 1 were assembled by the author from a

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Large concrete dams

variety of sources; accordingly, their absolute accuracy cannot be guaranteed.

Corresponding author. Email: [email protected]

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ACCEPTED MANUSCRIPT Table 1. Key features of some large concrete dams (data assembled by the author from a variety of sources, including Wikipedia and the referenced papers). Height (m)

Type

Country

Construction completed

References

Bakhtiari

315

Double curvature arch

Iran

Unknown

Agharazi et al. (2012), Haftani et al. (2014)

Jinping I

305

Double curvature arch

China

2013

Song et al. (2011), Feng and Song (2015)

Xiaowan

292

Double curvature arch

China

2010

Wu et al. (2009), Wang et al. (2011), Lin et al. (2015)

Xiluodu

285.5

Double curvature arch

China

2013

Fan et al. (2015), Lin et al. (2016)

Grande Dixence

285

Gravity

Switzerland

1964

Herzog (1999)

Baihetan

277

Double curvature arch

China

2019-2020

Jiang et al. (2014)

Inguri

271.5

Double curvature arch

Georgia

1980

Savich et al. (1974), Mgalobelov and Lomov (1979)

Vaiont (disused)

261.6

Double curvature arch

Italy

1959

Müller (1968), Hendron and Patton (1987) Herzog (1999), Straubhaar et al. (1994) Wieland et al. (2008)

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Name

250

Double curvature arch

Switzerland

Deriner

249

Double curvature arch

Turkey

2012 (?)

Wudongde

240

Gravity

China

~2020

Hoover

221.5

Gravity arch

U.S.A.

1935

Malpasset (disused)

196.9

Double curvature arch

France

1954

Itaipu

196

Hollow gravity

Brazil/Paraguay

1984

de Barros et al. (1978), Cabrera (1988)

Three Gorges

181

Gravity

China

2006

Fan et al. (2011), Liu et al. (2003a, b)

St. Francis (disused)

62.5

Curved gravity

U.S.A.

1926

Rogers (1992, 2006), Nuss and Hansen (2014)

Xu et al. (2014)

Cadbury (2003), Giroux (2010), Rogers (2010) Londe (1987), Duffaut (2013)

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Mauvoisin

1957 (to 237 m); 1991 raised to 250 m

dams differ from many other rock engineering projects for which the

Key rock engineering risks associated with large concrete dams

flowchart of Fig. 1 is developed in that they are subjected to an

additional range of risks throughout their decades-long post-

3.1. General rock engineering risk framework

construction operating lives. This period includes the initial filling

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Risk factors will be identified and discussed within the governing framework for the identification, assessment and management of rock

engineering risk developed by Hudson and Feng (2015) (see Fig. 1). This flowchart treats the risks in two groups – those that may be reduced

before construction starts through site investigation and design measures

(referred to only as “modelling” in the flowchart), and those that may be

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reduced through construction procedures or adaptations. Large concrete

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period (usually measured in years), in-service operation and, almost invariably, repair, strengthening and upgrade for a range of reasons (ANCOLD, 1992; Xu and Benmokrane, 1996; Brown, 2015). In this paper, the risks associated with this post-construction period will be treated as an additional category to the two main risk categories shown in Fig. 1.

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Fig. 1. Governing flowchart of rock engineering risk factors enabling the development of risk-reduced design and risk-reduced construction (Hudson and Feng, 2015). 3.2. Key failure modes of concrete dams

•

Large concrete dams are designed to withstand a range of static,

Overtopping from extreme flood events or from reservoir slope failures, leading to structural failure, spillway failure or excessive

hydrological (or flood) and earthquake loadings (Engemoen et al., 2014;

scour.

Fell et al., 2014; Brown, 2015). Although the key failure modes of arch

•

and gravity dams may differ, accumulated experience (e.g. Gillan et al.,

Downstream sliding on pre-existing planes of weakness (see Fig. 2).

•

Abutment instability (see Fig. 2).

main, failures of concrete dams arise from one, or a combination, of the

•

Foundation seepage or erosion.

following:

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Concrete cracking and fracture due to overstressing.

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Seismically-induced stresses and displacements in the concrete

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Overturning under the action of water, silt or ice pressures, including uplift pressures.

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2011; Warren, 2011; Nuss and Hansen, 2014) indicates that, in the

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and/or the foundation.

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Fig. 2. Discontinuity-controlled sliding in the foundations and abutments of concrete dams (Wittke, 2014). Material deficiencies

Table 2 lists some generic causes of near-miss incidents and failures in both concrete and embankment dams. It will be noted that these

Operational errors

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causes arise from deficiencies in each of the planning, site investigation, design, construction (including material deficiencies) and operation stages of the life of a dam (Warren, 2011).

Table 2. Some generic causes of structural failure of dams (Warren, 2011). Generic cause Project planning Site investigation

• Design errors

• • • • • • •

Construction errors

• • •

Excessive construction loads Material inconsistency Premature deterioration Fabrication defects Structural alterations Operation beyond the scope of the design Change in structure use Inadequate surveillance, monitoring or maintenance

The risk factors leading to these various modes of failure can be

Examples • Lack of clear scope • Conflicting client expectations •

• • • • • • • •

identified and their effects are assessed during the design, construction and operational stages of the life of a dam. The sources of risk and

Inadequate scope or extent of ground investigations Misinterpretation of information

causes of failure may be obvious or may be more subtle. From a rock

Conceptual design errors Lack of redundancy Failure to identify all loads and load combinations Calculation errors Detailing deficiencies Specification deficiencies Failure to consider surveillance, monitoring and maintenance

persistence, shear strength and infilling of discontinuities in the

Inappropriate temporary works Improper sequencing Improper methods or timing of construction

and hydraulics of the catchment and the dam environs, including dam

engineering perspective, they relate mainly to the location, geometry, foundation and abutment rock masses (see Section 5), but they can include less obvious factors such as the shear and tensile strengths of concrete-rock interfaces (Lemos and Antunes, 2011; Krounis et al., 2015). In addition to the rock mechanics and rock engineering issues being concentrated on here, a range of issues related to the hydrology breach analyses (Begnudelli and Sanders, 2007; Veale and Davison,

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of techniques, including Failure Modes Assessment or Analysis (FMA),

cracking and deterioration with age of the concrete must also be taken

Failure Mode and Effects Analysis (FMEA) or Potential Failure Modes

into account (Nuss et al., 2008; Gillan et al., 2011; Lin et al., 2015;

Assessment (PFMA), often within a risk analysis and management

Quirion, 2015). The vitally important risk of rock scour represents a

framework (Ghanaat, 2004; Lund et al., 2014; Rogers et al., 2014). Fig.

combination of hydraulic and rock engineering issues (George and

3 shows a simple event tree illustrating a potential failure mode within

Annandale, 2006; Bollaert et al., 2012; George et al., 2014).

the context of a PFMA.

During all stages of the life of a dam project, the influence of these

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factors on the possible failure of the dam may be assessed using a range

Fig. 3. Event tree illustrating potential failure mode of a section of a gravity dam (Lund et al., 2014). 3.3. Frequency of occurrence and consequences of dam failures

of acceptable societal risk of dam failure to be ALARP. For major

In quantitative risk analyses for large engineering projects, it is

dams, this level may be set at a risk level of two orders of magnitude

standard practice to prepare a risk register listing hazards and risk

lower than, or one hundredth of, the individual risk tolerability limit, i.e.

factors (often in categories) and estimates of the frequency of

1:1,000,000 per annum for existing dams (e.g. DSC, 2006). 3.4. Examples of concrete dam failure

occurrence and the consequences of any resulting incidents (Brown and

As have others, the author has argued elsewhere that the study of

number of other engineering and societal activities, it is common

concrete dam failures, notably those of the Malpasset Dam (Londe,

practice to evaluate the frequency of occurrence in terms of the annual

1987; Duffaut, 2013) and the Vaiont reservoir slope (Müller, 1968;

probability of failure, F, and the associated annual number of fatalities,

Hendron and Patton, 1987) in December 1959 and October 1963,

N. Probability of failure calculations using a range of techniques have

respectively, provided great impetus for much needed advances being

become well-established in geotechnical and other branches of

made in rock mechanics knowledge and techniques (Brown, 2011).

engineering in recent years. Making estimates of the annual numbers of

More recently, Nuss and Hansen (2014) studied the failures of nine

expected fatalities has often relied on less formal procedures involving

concrete dams dating from 1928 to 2002, providing details of each dam,

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Booth, 2009). When carrying out risk analyses for large dams and for a

its failure, the result and an assessment of the causes. The nine cases

methods are now being introduced (Fiedler et al., 2014; Meneses et al.,

studied include examples of all of the major types of failure listed in

2015).

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historical data and engineering judgement, although more formal

Section 3.2. The earliest of Nuss and Hansen’s (2014) case histories,

As illustrated in Fig. 4, F-N charts are plotted on log-log scales, with

that of the St. Francis dam, California, U.S.A., is not often referred to in

levels of risk indicated by the diagonal lines. Risk acceptability criteria

the modern rock mechanics and rock engineering literature, but has

or limits of tolerability for a range of engineering and societal activities

significant implications for dam engineering and civil engineering more

have been established by relevant authorities. Both individual risk and

broadly, particularly in the U.S.A. (Rogers, 1992, 2006; Petroski, 2003;

societal risk are often assessed (ANCOLD, 2003; Barker, 2011). A

Iglesia et al., 2008; Jackson, 2010). The following account is

number of examples, including dams, are shown in Fig. 4. The annual

paraphrased and abbreviated from that given by Nuss and Hansen

probabilities of failure of dams are generally less than 10-4 (White and

(2014).

Anderson, 2014) and, using the U.S.A. data shown on Fig. 4 may

The St. Francis dam was a curved concrete gravity dam, completed in

decrease to 10-6 for major dams for which the number of fatalities

May 1926. It had a structural height of 62.5 m, a crest length of 213 m,

associated with the dam failure may be in the order of 10,000. The

a crest width of 4.9 m, and a base width of 53.3 m (these dimensions

current Australian National Committee on Large Dams (ANCOLD)

have been converted to metres from those given in feet by Nuss and

individual risk tolerability limit is 1:10,000 per annum for existing dams

Hansen (2014)). As illustrated in Fig. 5a showing the dam on

(this corresponds to the lower limit of acceptable risk for open pit slope

completion, the face of the dam was stepped. Construction commenced

design shown in Fig. 4), and 1:100,000 per annum for new dams and

in 1924, but after a re-examination of the water supply requirements for

major upgrades (ANCOLD, 2003). It is standard practice to set the level

the growing city of Los Angeles, in July 1924 the dam height was

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ACCEPTED MANUSCRIPT increased by 3 m. In July 1925, after concrete had been placed for 11

drains only under its centre section. As shown in Fig. 5d, the centre section remained intact following the failure. There was no peer review

base width of the dam. The dam had no vertical contraction joints, no

of the design, and Rogers (1992) has suggested that the dam was

inspection galleries, no pressure grouting in the foundation and had

designed without any allowance being made for uplift.

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months, the height was increased by another 3 m without changing the

Fig. 4. Comparison of risk acceptability criteria with statistics (Steffen et al., 2006). The foundation consisted of two rock types. The canyon floor and the

assessment of the causes of the failure. As noted above, Rogers (1992) found that the dam was designed without appreciation of uplift theory

schistosity being essentially parallel to the canyon wall and dipping

which was known to dam engineers at the time (Igelsia et al., 2008). He

towards the canyon at about 35°. The upper half of the right side of the

also found that the dam base was not as wide as previously assumed,

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left abutment were of a relatively uniform schist with the planes of

foundation was in the Sespe Formation, a red conglomerate. The contact

that the designers were not aware of the left abutment paleo-mega-

between the two formations was provided by the San Francisquito Fault

landslide (Fig. 5b), or that the Sespe Formation on the right abutment

(Fig. 5d). The dam was placed on the fault with full knowledge of its

would slake when submerged. Failure was most plausibly due to the

existence.

reactivation of approximately 380,000 m3 of paleo-slide material. As the dam filled and uplift pressures developed under the left abutment, the slide probably began to move. The dam became increasingly unstable

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The St. Francis dam failed at 11:58 p.m. on 12 March 1928 during initial filling with no unusual weather conditions and no seismic

and the movements loaded the dam obliquely, permitting the inflow of

the reservoir depth was at 54 m, just 1 m below the spillway crest.

reservoir water along the heel of the dam, increasing the uplift

During this time, several upstream to downstream cracks appeared in

pressures. In short, many of the risks associated with the site and the

the dam, but they were dismissed as concrete cooling cracks and were

design were not recognised and reduced.

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activity. Reservoir filling began on 1 March 1926 and by 10 May 1927

cement grouted. In February 1928, a number of leaks developed in the

Following the disaster, outcry led to the formation of the California

right abutment within the Sespe conglomerate and along the fault. On

Division of Safety of Dams and the requirement that there be more

10 March, 1928, large tension cracks were noted in the left abutment.

geological reviews of new designs which would be reviewed by an

Within 70 minutes of the initial failure, the entire reservoir had emptied

independent review board. The Civil Engineers Registration Bill

and an estimated 470 lives were lost, although it is still not certain that

requiring engineers to obtain licences to practice became law in August

this represents the total number (Petroski, 2003).

1929. All Federal and State of California dams higher than 15.2 m were

After the disaster, 13 separate panels investigated the cause of the

reviewed. Hoover dam almost did not get approved for construction

failure. More recent detailed investigations by J. David Rogers (Rogers,

because of the St. Francis Dam failure (Jackson, 2010; Nuss and

1992, 2006; Petroski, 2003) have now provided a more complete

Hansen, 2014).

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Fig. 5. (a) St. Francis dam on completion; (b) paleo-mega-landslide; (c) failure scenario; and (d) failure aftermath (Nuss and Hansen, 2014).

4.

engineering design approaches developed by Feng and Hudson (2004,

The overall planning, investigation, design, construction and post-construction processes

2011).

As noted in Section 3.1, the post-construction stages involving initial

filling, in-service operation and usual repair, strengthening and upgrade,

construction and post-construction processes followed for large concrete

do not always have the same significance in other types of rock

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In the author’s experience, the overall planning, investigation, design, dams, may be regarded as a combination of the processes illustrated and

engineering projects. Another significant difference is the nature of the

discussed by Bieniawski (1993) and Feng and Hudson (2004, 2011).

total design and documentation process which goes well beyond the analysis and modelling stage of Feng and Hudson’s rock mechanics modelling and rock engineering design flowchart.

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Fig. 6 reproduces the flowchart of rock mechanics modelling and rock

Fig. 6. Flowchart of rock mechanics modelling and rock engineering design approaches (Feng and Hudson, 2004, 2011).

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ACCEPTED MANUSCRIPT Because of the difficulty of including all of the elements of the

al. (2008), Shaffner et al. (2009), Friz et al. (2011), Shaffner (2011), Fell

overall process and the interactions of those elements in one flowchart,

et al. (2014), and Haftani et al. (2014).

the key steps in the overall process for large concrete dams will simply

Some specific geological, geotechnical and hydrogeological factors

be listed and, in some cases, outlined briefly here. Some of these stages

that may require consideration in any given case include:

will then be discussed in more detail in Sections 5-8 to follow. It should

•

be noted that the overall process does not proceed in a linear fashion as

the geological history of the site, including the presence of paleoslips (Müller, 1968; Macfarlane, 2009; Nuss and Hansen, 2014);

•

the following list might suggest. Several of the stages may proceed in parallel with a range of interactions between them and with other

the presence of geological risk factors such as karst and rocks that are susceptible to slaking (Rogers, 1992);

•

engineering disciplines, most notably hydraulic and structural engineering in the present case. It should also be noted that risk

in situ stresses, particularly in incised river valleys (Wu et al.,

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2009; Xu et al., 2014; Lin et al., 2015);

assessment and risk management processes are usually used and

•

the depth and degree of rock mass weathering;

updated almost continuously throughout the overall process, including

•

the locations and characteristics of faults, shear zones and other

during in-service operation (Stewart, 2000; Barker, 2011; Scott, 2011;

major discontinuities in the dam foundations and abutments (de

Sanz-Jiménez et al., 2012). The key steps in the overall process are:

Barros et al., 1978; Cabrera, 1988; Allen and Cluff, 2000;

Project purpose and objectives – identify the project’s purpose and

Sadowski, 2015);

performance objectives, establish project management approaches. •

the locations and shear strengths of sheeting joints (Hencher et al.,

2011; Jiang et al., 2014); 2011; Sadowski, 2015);

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•

Design concept – select dam type and system components,

•

Design approach – establish design methods and acceptance

formations such as columnar basalt (Wei et al., 2011; Jiang et al., 2014);

•

Design analysis and numerical modelling (see Section 6).

•

Detailed design and documentation – design optimisation and

•

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construction materials, establish operating protocols, internal and external review, including by regulators (at this and other stages). •

Construction (see Section 7).

•

Initial filling – observation and monitoring, further feed-back to In-service operation – inspection, monitoring, maintenance, feed-

concrete-concrete and rock-concrete shear and tensile strengths (Lemos and Antunes, 2011; Krounis et al., 2015);

•

water pressure and in situ permeability testing (Barton and Quadros, 1997; Quiñones-Rozo, 2010; Farinha et al., 2011);

•

grouting tests and groutability assessment (Quadros and Correa

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Filho, 1995; Lopez-Molina et al., 2015; Zolfaghari et al., 2015);

Repair, strengthening and upgrade (see Section 8).

and

Site investigation and characterisation

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5.

rock mass strength and bearing capacity (Goodman and Ahlgren, 2000; Lawrence and Martin, 2007; Mehinrad et al., 2011);

•

back to design modification (see Section 8). •

rock mass deformation moduli (Read and Richards, 2007; Brown

and Marley, 2008; Quirion, 2015; Vibert and Ianos, 2015);

assessment, safety in design, construction planning, source

detailed design.

potential rock mass deformation and failure mechanisms (Deible

et al., 2010; Agharazi et al., 2012);

performance assessment, design documentation, constructability

•

the presence, properties and responses of particular rock mass

criteria, develop work packages, allocate work tasks, select consultants. •

the nature, geometries and shear strengths of joint sets (Friz et al.,

Geological, geotechnical and hydrogeological site investigation

including preliminary sizing. •

•

1967; Alonso et al., 2014; Sanei et al., 2015);

and characterisation (see Section 5). •

the locations and shear strengths of bedding planes (Maddox et al.,

Site selection – evaluate alternatives, select the site, identify key features and constraints of the site.

•

•

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•

•

monitoring uplift pressures under the dam (Spross et al., 2014).

6.

Analysis and design approaches

Adequate and effective geological, geotechnical and hydrogeological site investigation and site characterisation are essential to the success of

Almost all of the modelling approaches identified in Fig. 6 are, or

any rock engineering project. This is particularly the case with dams for

have been, used in large concrete dam analysis and modelling with the

which historical experience has shown that seemingly quite minor

possible exception of some systems and internet-based approaches. In

geological, geotechnical and hydrogeological details can have major

addition, a range of probabilistic, safety- and risk-based approaches are

impacts on the performance and stability of the dam (Terzaghi, 1929).

used. It is important to note that design analyses for dams are usually

In addition, the design and construction of large concrete dams requires

carried out for a number of particular load cases, often called normal or

consideration

and

usual, unusual and extreme (Brown, 2015). As well as static structural

hydrogeological factors that are not always of such significance in other

of

a

number

of

geological,

geotechnical

and water pressure loads for these cases, dynamic earthquake loading

rock engineering projects. Identifying and allowing for these factors in

may also be taken into account. Clearly, these various approaches and

the planning and design stages can be expected to reduce the levels of

load cases are intended to identify, assess and treat inherent risks. Fig. 7

risk associated with them. A number of these factors will be listed

shows a typical risk assessment process used for dams in Australia

below with references being given to examples of their occurrence and

(ANCOLD, 2003).

evaluation. Useful accounts of approaches to concrete dam site

Following the approach used in Section 5, the following is a list of

investigation and characterisation are given by FERC (2002), Powell et

some of the analysis and modelling approaches that have been used in large concrete dam design, commencing with traditional methods and

7

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then proceeding through numerical methods to probabilistic and riskbased approaches:

element methods (Kottenstette, 1997; Chen et al., 2003, 2004; Su

traditional 2D (two-dimensional) and 3D (three-dimensional)

et al., 2013);

static analyses of gravity dams (Herzog, 1999; FERC, 2002;

•

numerical hydromechanical analyses (Gimenes and Fernándes,

•

dynamic numerical analyses allowing for earthquake loading

Bretas et al., 2012; Fell et al., 2014); •

•

2006; Bretas et al., 2013; Hosseinzadeh et al., 2013);

traditional static analyses for arch dams, including abutment and thrust block stability analyses (USBR, 1977; Herzog, 1999;

(Lemos and Gomes, 2007; Scott and Mills-Bria, 2008; Chopra,

Dickson and Loar, 2011);

2012); •

allowance for uplift pressures in static analyses (Ebeling et al., 2000; Iglesia et al., 2008; McKay and Lopez, 2013);

•

physical model tests (Fumagalli, 1968; Liu et al., 2003b; Lin et al.,

2013); •

2015); •

static continuum and equivalent continuum numerical (FEM,

risk- and safety-based methods, including failure modes, fault tree and uncertainty analyses (Ghanaat, 2004; Barker, 2011; Scott,

FDM) methods (Herzog, 1999; Liu et al., 2003a; Wittke, 2014); •

probability, reliability and fragility methods (Ellingwood and Tekie, 2001; Peyras et al., 2012; Westberg Wilde and Johansson,

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•

discontinuum analyses including block theory, DDA and block

2011; Lund et al., 2014); and •

discontinuum numerical (DEM) methods (Lemos, 2011, 2014; Farinha et al., 2012);

dam breach (or break) and consequence or impact analyses (Begnudelli and Sanders, 2007; QDEWS, 2012; Veale and

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Davison, 2013).

Fig. 7. Typical risk assessment process for a dam (ANCOLD, 2003). 7.

A significant feature of the stress and deformation analyses carried out for large concrete dams is the need to check for cracking of the dam-

Construction As shown in Hudson and Feng’s (2015) flowchart for the

foundation interface at the heel of the dam (McKay and Lopez, 2013).

identification and assessment of rock engineering risk factors (Fig. 1), in

Among other things, this can modify the distribution of uplift pressures

the general case, it may be expected that during construction, further

under the dam (Fell et al., 2014).

risk factors will be identified and the associated risks reduced. This has

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ACCEPTED MANUSCRIPT certainly been the experience with large concrete dams. For example,

interfaces. An important part of the foundation preparation process is

geological features in the dam foundation may only be detected and

grouting, usually in the form of consolidation grouting closer to the

their properties fully evaluated following river diversion during

foundation surface and deeper curtain grouting (Fell et al., 2014). In

construction (Souza Lima and Abrahão, 1982). Fig. 8 shows one of a

addition, drainage holes are generally installed. The second major stage

number of potential sliding surfaces in the foundation of a RCC dam

which may bring its own risks is the placement of the concrete.

identified, confirmed or adjusted during foundation excavation (Ginther

The following are some examples of specific construction issues and

et al., 2014). On the other hand, as illustrated in Table 2, inadequate

their treatments:

construction practices and materials deficiencies can contribute to dam

•

foundation excavation, preparation and clean-up (Savich et al.,

•

treatment of fissures and faults, for example, by replacement or

incidents and failures.

RI PT

1974; Fell et al., 2014; Fan et al., 2015);

Following construction planning and site establishment, the first stage of the construction process proper is the excavation and preparation of

reinforcement with concrete (Carlile, 1966; Song et al., 2013;

the foundation (Fell et al., 2014). It is in this process that some

Feng and Song, 2015); •

previously unidentified risk factors may be identified and treated (the author has had recent experience of such a case in a dam upgrade

identification or confirmation of potential sliding surfaces (Ginther et al., 2014);

M AN U

SC

project). Special treatments are sometimes required at the rock-concrete

•

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Fig. 8. Potential sliding surface identified in the foundation of the Wyaralong Dam, Queensland, Australia, during construction (Ginther et al., 2014). treatment of rock-concrete interfaces to prevent sliding, for

• meet changes in safety standards;

example, by the use of shear keys or concrete plugs (Souza Lima

• overcome deficiencies in design and construction;

and Abrahão, 1982; Cabrera, 1988; Hatton et al., 1991; Yu et al.,

• recover loss of strength due to concrete deterioration; and

2005);

• raise the heights of dams.

control of seepage during construction (Malyshev et al., 1979);

consolidation and curtain grouting (Weaver and Bruce, 2007; Fan

•

concrete

EP

• •

Xu and Benmokrane (1996) had found that most of the reasons for

et al., 2011; Lin et al., 2016);

8.

including

temperature

strengthening existing concrete dams did not arise from rock control

and

engineering risk factors, although remedying deficiencies in the original

construction joint formation (Cadbury, 2003; Giroux, 2010; Wang

design (e.g. not allowing adequately for uplift) and strengthening dam

AC C

•

placement,

et al., 2011); and

foundations and abutments did. Concrete deterioration arising from

monitoring during construction and initial filling (Song et al.,

alkali-aggregate reaction and freeze-thaw cycles has been discussed by

2011; Hu et al., 2012; Chen et al., 2016).

Nuss et al. (2008) and Wieland (2010). Christensen (1974) reported an unusual case in which leaching of calcite from the concrete and from the

Operation, repair, strengthening and upgrade

consolidation grouting and grout curtains, caused the blocking of

The time taken for the initial filling of large concrete dams may be

foundation drains with resulting increases in uplift pressures. More

measured in years and then their operational lives will generally stretch

recently, Brown (2015) has discussed the issue of meeting changes in

for several decades. It is essential that during this time, the dam be kept

safety standards. A wide range of measures may be used to upgrade and improve the

under surveillance, monitored and repaired as necessary. Recent

performance of existing concrete dams. They include:

examples of monitoring during the life of a dam are given by Wittke et al. (2012), Rogers et al. (2014), and Spross et al. (2014). At some stage

•

raising the heights of dam walls;

in the dam’s life, it is likely that it will require strengthening or

•

raising the heights of parapet walls on the dams or spillways, including spillway training walls;

upgrading. Xu and Benmokrane (1996) carried out a major review of •

the then state-of-the-art of the strengthening of existing concrete dams using post-tensioned anchors. They listed the main reasons for strengthening existing dams as being to:

9

increasing spillway widths and/or depths;

ACCEPTED MANUSCRIPT •

anchoring gravity dams or the bases of arch dams by posttensioned anchors to increase resistance to downstream sliding and/or overturning;

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thickening and concrete buttressing of dam walls;

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different ways. In: Technology Roadmap for Rock Mechanics. Proceedings of the E.T. (Ted) Brown is a graduate of the Universities of Melbourne (BE 1960; MEngSc 1964), Queensland (PhD 1969) and London (DSc (Eng) 1985). He began his engineering career in the then State Electricity Commission of Victoria’s brown coal mining operations in 1960. After several years at what became James Cook University in Townsville, he joined the staff of the Royal School of Mines, Imperial College of Science and Technology, London, in 1975. He was appointed Professor of Rock Mechanics in 1979, and served as Dean of the Royal School of Mines (1983-1986) and Head, Department of Mineral Resources Engineering (1985-1987). In October 1987, Professor Brown returned to Australia as the University of Queensland’s first full-time Dean of Engineering. He became Deputy Vice-Chancellor of the University in October 1990 and Senior Deputy ViceChancellor in January 1996. Since retiring early from that position in early 2001, he has worked as a Senior Consultant to Golder Associates Pty Ltd. and had served on a number of Boards. Professor Brown has wide international experience as a researcher, teacher, consultant and writer on rock mechanics and its applications in the mining, civil engineering and energy resources industries. He served as Chairman of the British Geotechnical Society in 1982 and 1983, and as President of the International Society for Rock Mechanics from 1983 to 1987. He was elected an International Fellow of The Royal Academy of Engineering, UK, in 1989, and a Fellow of the Australian Academy of Technological Sciences and Engineering in 1990. In 2001, he was appointed a Companion in the Order of Australia for “services to the engineering profession as a world expert in rock mechanics and to scholarship through promotion of the highest academic and professional standards.” He was awarded the Consolidated Gold Fields Gold Medal of the then Institute of Mining & Metallurgy in 1984; a Centenary Medal by the Australian Government in 2003 “for service to Australian society in mining and civil engineering”; the John Jaeger Memorial Award of the Australian Geomechanics Society in 2004; the President’s Award of the Australasian Institute of Mining & Metallurgy for 2006; the International Society for Rock Mechanics’ highest honour, the Müller Award, in 2007; the SME Rock Mechanics Award in 2010; and the Douglas Hay Medal of the Institute of Metals, Minerals & Mining in 2013.

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Conflict of interest

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The authors wish to confirm that there are no known conflicts of interests

associated with this publication and there has been no significant financial support

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for this work that could have influenced its outcome.

Dr. E.T. Brown

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