PIANC_Catalogue of prefabricated elements (2005)

8/14/2019 PIANC_Catalogue of prefabricated elements (2005)

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INTERNATIONAL NAVIGATION ASSOCIATION

CATALOGUE

OF

PREFABRICATED ELEMENTS

Report of Working Group 36

of the

MARITIME NAVIGATION COMMISSION

INTERNATIONAL NAVIGATIONASSOCIATION

ASSOCIATION INTERNATIONALE

DE NAVIGATION

2005

MarCom_ReportWG36 indd 1 9 15 2005 3:11:13 PMBblz-Marcom36 CR.indd 1Bblz-Marcom36+CR.indd 1 04-12-2006 16:23:1104-12-2006 16:23:11


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PIANC has Technical Commissions concerned with inland waterways and ports (InCom),

coastal and ocean waterways (including ports and harbours) (MarCom), environmental aspects

(EnviCom) and sport and pleasure navigation (RecCom).

This Report has been produced by an international Working Group convened by the Maritime

Navigation Commission (MarCom). Members of the Working Group represent several countries

and are acknowledged experts in their profession.

The objective of this report is to provide information and recommendations on good practice.

Conformity is not obligatory and engineering judgement should be used in its application,especially in special circumstances. This report should be seen as an expert guidance and state

of the art on this particular subject. PIANC disclaims all responsibility in case this report should

be presented as an official standard.

PIANC General Secretariat

Graaf de Ferraris-building 11thfloor

Boulevard du Roi Albert II 20, B.3

B-1000 Brussels

BELGIQUE

http://www.pianc-aipcn.org

VAT/TVA BE 408-287-945

ISBN 2-87223-152-8

All rights reserved



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PIANC/AIPCN MarCom Working Group 363

CONTENT

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

1.2 Terms of Reference . . . . . . . . . . . . . . . . . . . . . . . . .3

1.3 Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

1.4 Work of the PIANC Working Group 36 . . . . . . . . . 6

1.5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . .6

1.6 Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

2. ROLE OF PREFABRICATED ELEMENTS IN

MARITIME WORKS . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

2.2 Types of application considered in this catalogue .72.2.1 Breakwaters . . . . . . . . . . . . . . . . . . . . . . . . .7

2.2.2 Revetments, seawalls & coast protection . .8

2.2.3 Quays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

2.2.4 Bank protection . . . . . . . . . . . . . . . . . . . . . .9

3. CONSIDERATIONS FOR SELECTION . . . . . . . 10

3.1 Prefabricated elements for breakwaters . . . . . . . .10

3.1.1 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.2 Structural integrity . . . . . . . . . . . . . . . . . . .12

3.1.3 Hydraulic performance . . . . . . . . . . . . . . . . 13

3.1.4 Constraints . . . . . . . . . . . . . . . . . . . . . . . . .13

3.1.5 Maintenance . . . . . . . . . . . . . . . . . . . . . . . .133.1.6 Construction costs . . . . . . . . . . . . . . . . . . .14

3.1.7 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 Prefabricated elements for quays . . . . . . . . . . . . .15

3.2.1 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

3.2.2 Structural integrity . . . . . . . . . . . . . . . . . . . 16

3.2.3 Hydraulic performance . . . . . . . . . . . . . . .16

3.2.4 Maintenance . . . . . . . . . . . . . . . . . . . . . . . .16

3.2.5 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3 Prefabricated elements for revetments and seawalls 16

3.3.1 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16


3.3.3 Hydraulic performance . . . . . . . . . . . . . . . 18

3.3.4 Maintenance . . . . . . . . . . . . . . . . . . . . . . . .18

3.3.5 Materials . . . . . . . . . . . . . . . . . . . . . . . . . .183.4 Prefabricated elements for bank protection . . . . .18

3.4.1 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19


3.4.3 Hydraulic performance . . . . . . . . . . . . . . .20

3.4.4 Constraints . . . . . . . . . . . . . . . . . . . . . . . . .20

3.4.5 Maintenance . . . . . . . . . . . . . . . . . . . . . . . .20

3.4.6 Construction costs . . . . . . . . . . . . . . . . . . .21

3.4.7 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

APPENDIX (IN CD FORMAT). . . . . . . . . . . . . . . . . . 22

1. INTRODUCTION

1.1 Summary

The aim of the working group is to collect all the available

prefabricated elements up to date. This work is the basis for

the construction of a large Catalogue that can be updated

after distribution and reached by many professionals related

with ports and coastal engineering. Obviously, this cata-

logue will continue expanding in the future, so all engineers

are encouraged to cooperate and send new or different refer-

ences of prefabricated elements.

Types of applications considered in this catalogue

For the last four decades, the use of prefabricated elements

in the construction of port and coastal structures has becomea very common practice. Prefabricated elements provide

important advantages, such as improved hydraulic perform-

ance when compared with natural materials, ecological ben-

efits, cost reduction, construction efficiency, etc. As a con-

sequence, numerous new prefabricated units have been de-

signed for a wide variety of engineering applications, such

as breakwater protection, coastal erosion control, stability

of river banks, reflection damping on quays, attenuation of

waves, etc.

In this work, different applications of prefabricated ele-

ments in maritime and fluvial works are briefly described.

The structures are classified into four types:

a) Breakwaters

b) Revetments and seawalls & coast protection

c) Quays

d) Bank protection.

For each of these four types of structures some relevant char-

acteristics are described. This includes: types of prefabri-

cated elements; structural integrity; hydraulic performance;

constraints; maintenance; construction costs and materials.

The catalogue includes all the names of prefabricated ele-

ments known to the members of the WG at the moment.

Some of them have additional characteristics like: shape,

photograph, etc.; type of work; reference projects; bibliog-

raphy; invention and development and commercial refer-

ences.

1.2 Terms of Reference

In the last four decades the use of prefabricated elements in

the construction of port, coastal and waterway structures has

become a very common practice. Prefabricated elements



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PIANC/AIPCN MarCom Working Group 36 4

can represent important advantages not only from the struc-

tural point of view (hydraulic performance, stability under

extreme wave conditions) but also from many others (i.e.:ecological benefits, cost reduction, construction efficiency,

material availability).

As a consequence, a lot of new prefabricated units have

been designed for a wide variety of engineering applica-

tions (breakwater protection, coastal erosion control, stabil-

ity of river banks, reflection damping on quays, attenuation

of waves, etc.).

Coastal engineers and contractors are now facing the prob-

lem of identification and selection of the optimum product

for their specific work. Information on prefabricated ele-

ments is nowadays dispersed, not easily available and almostimpossible to be evaluated.

PIANC, as a non-profit international association, is in an

optimum position for producing a Catalogue of Prefab-

ricated Elements for Coastal and Port Engineering. This

document, which includes a list of products, is useful for

managers, port authorities, engineers, scientists and other

professionals.

The catalogue is focused on prefabricated units used for the

construction of the following types of structures:

a) Breakwaters

b) Revetments and seawalls

c) Quays

d) Waterways banks

The task of the Working Group has consisted of collecting

and processing technical and commercial information on all

types of prefabricated units, developed for the above men-

tioned purposes, that fulfil two requirements:

1) Commercial or technical references exist; and

2) the element has been used in an actual work.

The work of the group is published as a Catalogue that will

include a standardised form for each type or unit compris-

ing:

basic technical features (shape, dimensions, photo-

graphs, etc.)

list of references on technical performance

list of references of existing applications

commercial status (patent, information, commercial ad-

dress, etc.).

The Catalogue does not include detailed information (per-

formance indexes, response curves, etc.) about the technical

performances of the unit, but only gives references to the

most relevant published information. Therefore, the inclu-

sion of a certain type of element in this PIANC Catalogue

should not be deemed as confirmation of its technical qual-

ity or suitability for any particular application.

1.3 Members

This Catalogue was produced by the PIANC Marcom Work-

ing Group no. 36.

Members of the group have been the following:

Chairman:

Mr. Jos Mara Berenguer

BERENGUER INGENIEROS, S.L.

Costa Brava, 13

28034 Madrid

Espaa

phone: +34 91 736 40 87

fax : +34 91 734 43 76

e-mail: [email protected]

Co-Secretary:

Mr. Jos Ramn IribarrenSIPORT XXI, S.L.

Edificio Azasol, calle Chile, 8 of 104

28290 Las Matas (Madrid)

Espaa

phone: +34 91 630 70 73


Co-Secretary:

Mrs. Paula Zambrana Berho

BERENGUER INGENIEROS, S.L.

Costa Brava, 13

28034 Madrid

Espaaphone: +34 91 736 40 87

fax : +34 91 734 43 76


Members:

Mr.William N.H. Allsop

Howbery Park, Wallingford

Oxon

OX 10 8BA

phone: + 44 1491 82 22 30

fax: + 44 1491 82 55 39




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Mr. Hans.F. Burcharth

Sohngaardsholmsvej, 57

DK 900 AalborgDenmark

phone: + 45 96 35 84 82

fax: + 45 98 14 25 25

e-mail 1: [email protected]

e-mail 2: [email protected]

Mr. Arie Burggraaf

P.O. Box 32696

Braamfontein 2017

South Africa

phone: + 27 11 242 4029

fax: + 27 11 242 4029


Mr. Romeo Ciortan

IPTANA

36-38 Bd Dimicu Golescu

7100 Bucharest

Romania

phone: + 401 210 3542

fax : + 401 312 1416


Mr. Billy L. Edge

College Station,

Texas Tx 77843 - 3136

United States of Americaphone: + 19 79 845 4515 / 979 845 4516

fax: + 19 79 862 8162

Mr. Leopoldo Franco

Universit di Roma, 3

Via Vito Volterra, 62

00146 Roma

Italy

phone: 39 06 551 73 458


Mr. P. Galichon

Port Autonome du Havre

P.O. Box 1413F-76067 Le Havre CEDEX

France

phone: 33 35 21 7400


Mr. Minoru Hanzawa

2-7 Higashi-Nakanuki Tsuchiura

Ibaraki, 300 - 0006

Japan

phone: + 81 298 31 7411

fax + 81 298 31 7693


Mr. Frans Kapp

Entech Consultants Ltd.

P.O. Box 4137599 Stellenbosch

South Africa

phone: + 27 21 883 92 60

fax: + 27 21 883-32 12


Mr. Sverre Lorgen

SAM LORGEN AS

6002 Norway

phone: + 47 70 10 73 00

fax: + 47 70 10 73 01


Mr. Luc Maertens

Avenue des Communauts, 100

1200 Brussels

Belgique

phone:+ 32 2 4026 563

cellular: + 32 475 490 206

fax: + 32 2 4026 530


Ms. Kirsty J. McConnell

Howbery Park, Wallingford

OX 10 8BA OxonUnited Kingdom

phone: + 44 1491 82 22 30

fax: + 44 1491 82 55 39


Mr. Remouchamps

CAMET

Boulevard du Nord, 8

B-5000 Namur

Belgique

phone: + 32 81 77 29 70

fax: + 32 81 77 37 67


Mr. Krystian Pilarczyk

Vander Burghwegl, P.O. Box 5044

2600 GA Delft

The Netherlands

phone: + 31 15 25 18 427

fax: + 31 15 25 18 568/25 18 555




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1.4 Work of the PIANC Working Group 36

Most of the information required for completing the Cata-

logue was intended to be gathered from the research of thegroup members.

PIANC Marcom Working Group 36 has had the following

meetings:

LONDON (United Kingdom) 26, September, 2001

Meeting during the International Conference on Breakwa-

ters, Coastal Structures and Coastlines

BARCELONA (Spain) 5, April, 2002

Meeting at the Port of Barcelona

1.5 AcknowledgementsThe Chairman is grateful to the Barcelona Port Authority

for the attention to the Working Group 36 in the meeting at

the Port of Barcelona.

1.6 Foreword

Despite the work carried out by the Group, the present cata-

logue only includes a limited number of prefabricated ele-

ments that are commonly used in coastal and fluvial engi-

neering.

The WG realize that there are a considerable number of ele-

ments that have not been included in the final list of theReport. In most cases, this fact has been due to lack of in-

formation about the technical data of the unit or references

about actual applications.

That reason, together with the normal development of new

elements with time, should lead to a periodic updating of

the information contained in it. Therefore, the present report

must be considered as a first edition of a Catalogue on Pre-

fabricated Elements that must be the starting point for future

and more complete publications.

2. ROLE OF PREFABRICATED

ELEMENTS IN MARITIME WORKS

2.1 Background

Prefabricated elements have been used in maritime engi-

neering since ancient times. Phoenician and Greek engineers

used cut rocks with regular placement to build breakwaters

and seawalls, sometimes fastening neighbouring blocks

with metal joints and clamps. The weight of the blocks typi-

cally did not exceed one tonne in order to allow easy han-

dling with the lifting tackle available at the time. Later, the

Romans invented hydraulic cement and concrete technology

took its place in works at sea. Though concrete structures

were mostly cast in-situ within wooden forms or sunken

ship hulls, large mortar blocks could also be prefabricated in

the dry above an emerging sand mound to be washed awayor within watertight caissons before sinking on a prepared

foundation surface.

As the capacity of lifting cranes increased in the 19thCentu-

ry, heavier precast blocks could be placed for rubble mound

breakwaters or for blockwork seawalls and quaywalls. Their

shape was typically parallelepiped or cubic. At Leghorn

(Livorno) even the core of the curvilinear breakwater was

made with large regularly cut rock blocks in 1850. The size

of prefabricated blocks steadily increased up to 500t for the

solid cyclopean blocks used for vertical breakwaters in the

first part of the 19thCentury. Parallelepiped blocks of 150t

were used for the protection layer of the Port of Bilbao rub-ble mound outer breakwater. A tailor-made crane must be

constructed for placing such artificial concrete units.

The 20th Century showed the revival and development of

the technology of cellular reinforced concrete caissons (pre-

fabricated in yards and on fixed or floating platforms) and

the production of un-reinforced concrete blocks of various

shapes to be mainly used for breakwater armouring. In 1950

the first slender tetrapod block was developed. Economic

advantages in comparison with massive-type blocks pro-

moted its use in a large number of breakwaters all around

the world.

Another milestone occurred in the late 1970s when themassive Antifer cube and the slender Dolos were developed,

quickly followed by the hollow (multi-hole) block genera-

tion (Shed, Cob). Some catastrophic events occurred in the

1980s mainly due to the structural failure of slender ele-

ments, and this led the research again towards bulky units.

Finally in the 1990s other bulkier units like Accropode

(France), Core-loc (United States) were developed to op-

timise the hydraulic and structural properties for a stable,

durable, economic armour based on a single-layer design.

In some cases it was even the contractor, instead of the de-

signer, who proposed a new block shape to avoid payment

of royalties or to simplify the unit prefabrication, transportand placement.

The use of prefabricated elements for the construction of

quays developed on the basis of two new requirements;

deeper berths for larger ships and higher values of exploita-

tion loads. Roman engineers constructed a quay of 7 metres

depth using large geometric rocks in the port of Cesarea

Maritima (1thCentury, B.C.) Once the draught of commer-

cial ships exceeded 6-7 metres depth in the 19th century,

performance limits of the quays existing in ancient ports,

made from natural materials, were exceeded. At this limit,

the use of artificial prefabricated concrete blocks becomes



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necessary for the construction of berthing gravity struc-

tures.

In the 20thcentury, the use of prefabricated concrete cellular

caissons has become the most widely used solution for large

and deep quays all over the world. The possibility of using

specialised construction facilities that allow time and cost

reductions and floating plant for transportation and place-

ment, are important advantages of this technique. Several

types of caisson have been developed. Classification can be

made based on the horizontal section of the caisson (circu-

lar, parallelogram), the geometry of the cells (cylindrical,

parallelepipedic), the type of front face (ranurated, perfo-

rated, slotted, non-permeable, etc). Provided there are good

quality foundations, most of the quays in Europe are being

constructed with this technique.

In estuaries and rivers, soft soil conditions led to solutions

based on rigid or flexible wall structures made with wooden

piles and plates. Higher loads and depths required by larger

cargo and ships required the use of metallic piles, sheet piles

or concrete prefabricated piles. Since the 19thcentury when

Mitchell-type metal piles were introduced in the construc-

tion of maritime works, manufacturers all over the world

have developed a wide variety of prefabricated elements.

As well as in the case of prefabricated caissons, the amount

of different designs exceeds the scope of the present cata-

logue.

Prefabricated elements have been used also as an alternative

for the protection of river banks and channels. Vegetation

cover and rip-rap were traditionally used for this purpose.

Geotextile techniques and protective layers of prefabricated

elements have become more and more commonly used for

this purpose.

In general, the major advantages of prefabrication in mari-

time works can be summarised as follows:

standardised design and construction methods

less variation in quality and easier and more efficientquality control

facilitates or eliminates formwork, especially underwa-

ter

less dependence on weather conditions

reduction of construction time

reduction in cost.

On the contrary some disadvantages can be identified:

requirement of high standards of quality in material and

construction methods

availability of suitable construction equipment

narrow tolerances in put-in-place operations.

2.2 Types of application

considered in this catalogue

In the last four decades the use of prefabricated elements in

the construction of port and coastal structures has become

very common practice. Prefabricated elements provide im-

portant advantages, such as improved hydraulic perform-

ance when compared with natural materials, ecological

benefits, cost reduction, construction eff iciency, etc. As aconsequence, numerous new prefabricated units have been

designed for a wide variety of engineering applications,

such as breakwater protection, coastal erosion control, sta-

bility of river banks, reflection damping on quays, attenua-

tion of waves, etc.

In this chapter, the types of application of prefabricated ele-

ments in maritime and fluvial works are briefly described.

The structures are classified into four types:

a) Breakwaters

b) Revetments and seawalls & coast protection

c) Quays

d) Bank protection.

2.2.1 Breakwaters

Prefabricated elements have been commonly used for the

construction of the protective layer of rubble mound break-

waters. In some cases, artificial elements have also been

used for the core (Port of Gijn) or filter layers. On occa-

sion, superstructures and parapets have been constructed

with massive regularly placed prefabricated units.

Past PIANC Congresses, collected and resumed in PIANCs

Centennial Jubilee Memorial Book, have illustrated the

technical debates about the applicability of different tech-

niques. L.F.Vernon-Harcourt, H. Wortman, V. Benezit, J.

Lira, E.J. Castro, R. Iribarren, J. Larras, Hudson, A. Paape,

F. Abecasis, F. Vasco Costa, A. Torum, P.A. Hedar, and many

other excellent researchers and engineers established a solid

foundation for future development of coastal engineering.

Wave dissipating concrete blocks, such as Tetrapods and

Dolosse, are popular prefabricated elements used for some

time in rubble mound and composite breakwater construc-



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tion. The main role of such concrete blocks is to reduce

wave reflections and wave forces acting onto caissons. Their

roles have proved to be reliable in the course of their historyof about 50 years.

Recently, new types of breakwaters, such as vertical wave

screens and skirt breakwaters, etc., have been developed. A

wave screen is a porous vertical wall, usually constructed

using rectangular slats oriented in either a horizontal or

vertical direction and attached to vertical piles to support

structures. Wave screens can reduce wave transmission by

up to 80%. In addition, environmental considerations are

an important requirement for maritime structures. For ex-

ample, a new type of submerged breakwater, so called arti-

ficial reefs, composed of purpose-designed concrete frame

units has been invented and their effectiveness in providinga good environment for ecosystems has been proved in ac-

tual site applications.

Prefabricated large units have been traditionally used for the

construction of monolithic-type breakwaters. Many break-

waters in Japan, Italy and Spain are built based on the ad-

dition of large rectangular blocks or caissons. The floating

caisson technique, developed in the last half century, has

allowed the construction of breakwaters in deep water in a

very economical way, for example the South breakwater of

the Santa Cruz de Tenerife port (Spain) reaches the 60m wa-

ter depth contour. A wide number of configurations of pre-

fabricated units, aimed at improving hydraulic performance(Jarlan-type, slotted-type, curved slit-type, multi-cellular-

type, etc) have been developed recently, mainly in Japan.

2.2.2 Revetments, seawalls & coast protection

Coastal protection has long been a response in the fight of

man against wave action. Littoral erosion was recognized as

a loss of quality and surface of coastal lands. Former meth-

ods of coastal protection were always based on the hard-

ening of the natural erodible materials. Large amounts of

rip-rap and rocks were placed along eroded shores. In most

cases, long-term evolution of the coast produces the pro-

gressive degradation and failure of this type of protection.

The conceptual comprehension of littoral processes by en-

gineers in the 16th century promoted the adoption of new

types of remedial measures, such as groynes or detached

breakwaters.

As coastal protection becomes a vital strategy for land pro-

tection and reclamation, cheaper and safer approaches were

required. At present, two major types of protection measure

can be applied:

Soft measures: beach renourishment, algae plantation

Hard measures: seawalls, revetments, groynes, de-

tached breakwaters

Concrete blocks of relatively flat shape are major prefabri-

cated elements used as cover layer in the structures of revet-

ments, seawalls and other hard approaches to coast protec-

tion. They provide armour for slopes of natural soil and/or

rubble, protecting the structures from erosion and scouring

caused by wave attack. A wide variety of types of modular

blocks and cabled block types have also been developed and

patented in the last decades. Flexible materials have also

been used as a cover layer, e.g. bag blankets, stacked-bags,

fabric mattresses, and tubes, etc.

For the cover layer, stability against uplift forces and degra-

dation of the subsoil are major aspects to be carefully con-sidered in the design phases. Stone size and thickness of

under layer should be carefully selected.

In recent years, a wide variety of geotextiles has been de-

veloped and used as the filter layer of structures of revet-

ments, seawalls & coast protection. Geotextiles generally

allow the installation of sublayers or cover layers beyond

conventional filter rules. Geotextiles are easily damaged,

especially during installation, and are rather difficult to re-

pair. Therefore, special care must be taken when contacting

with the subsoil.

As for the design and construction of revetments using geo-

textiles, the documents, such as the PIANC reports of PTC I

WG4 and PTC II WG 21, can be used as guidelines.

2.2.3 Quays

The use of prefabricated elements for the construction of

quays derived from three major aims:

- to reach deeper depths for large vessels

- to improve cost-effective construction methods

- to reduce wave reflection for wave disturbance pur poses.

The most important developments for the first and second

aims were the development of the pile and sheet piling tech-

niques, and the prefabricated caisson technique.

Floating caissons, upright wave absorbing caissons and

modular blocks are popular prefabricated elements in quay

structures. Concrete sheet piles and concrete beams are also

found in quay structures. Other advantages of prefabricat-

ed elements in the construction of quays are derived from

improved technical performance (wave reflection), easier

construction methods, economics and lower environmental

impacts.



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Gravity quays are used in loading and unloading vessels.

These structures, when backfilled with soil, rely on the

structure weight to resist the resulting earth pressure. Themost common types of gravity quay are those constructed

by concrete blocks, those developed with floating caissons

and, for smaller vertical structures, steel sheet piling back-

filled with soil.

Caisson quays are prefabricated sand-filled concrete cais-

sons. They have different sizes and forms. Usually, they

depend on the available formwork of the construction com-

pany. The foundation must support the structure and resist

sand scour and usually consist of a mat or mound of rubble

stone. Depending on site conditions, caissons are generally

suitable for depths from about 5 to 8 m and they can reach

depth up to 26 m. Beyond this limit, pressures upon the

foundation may exceed the acceptable values, if it is formed

by rock.

Traditional design methods for caisson quays take into ac-

count the verification of safety factors for the main failure

modes:

- Overturning

- Sliding

- Settlement or collapse of foundation

- Global failure (caisson-foundation failure).

The failure modes are calculated for different action and

load combinations. Main variables affecting load combi-

nations are: caisson weight, hydrostatic and dynamic wave

forces, earth forces, mooring loads, storage overload, ma-

chinery movement, overload acting upon the caissons.

2.2.4 Bank protection

Bank erosion is a natural geomorphological process, which

occurs in all channels. It is one of the mechanisms by which

a channel adjusts its size and shape to convey the discharge

and sediment supplied to it from the surrounding land. As

a natural process, bank erosion is generally beneficial, par-ticularly to the ecology of waterways. Erosion and deposi-

tion create a variety of habitats for flora and fauna, which

contribute to ecological diversity.

However, erosion adversely affects riparian landowners

whose land is lost, particularly where houses, factories or

other buildings on the bank are damaged or destroyed. The

loss of the bank also affects those who use it for grazing,

fishing or recreation.

The predominant method of bank protection on many water-

ways all over the world has been sheet piling. It is used on

navigable channels to protect against boatwash and in canals

to provide a watertight surface and prevent leakage.

Steel piling is, therefore, the most common method but has

a limited life before decay sets in. A frequent problem in

many waterways today is the failure of sheet piling installed

decades ago. Given its disadvantages of high cost, limited

life span and the fact that it does not provide a habitat for

flora and fauna, sheet piling can only be considered effec-

tive when assessed against a very narrow range of criteria.

There is therefore a need to consider alternative methods of

bank protection which are more environmentally sensitive

and, ideally, of lower cost.

Most canal banks have traditionally been protected by veg-

etation and stonework. Stone walls are used on most of theEnglish narrow canals and stone revetments on some of the

larger canals.

In tidal rivers, the main methods in current use are concrete

revetments with Reno mattresses or stone rip-rap to protect

the toe of the bank, blockstone and sheet piling.

The intent is to select decision-making so that cost-effec-

tive solutions to bank erosion problems can be developed

through integrating engineering, ecological and economic

considerations.

Concrete unit revetments combine the advantages of indi-

vidual concrete units or blocks that may be transported and

installed as modules with the coverage and protection of a

revetment. Revetments deflect wave energy, thus protecting

the bank from erosion.

The design of the revetment can be an open joint revetment:

simple precast blocks laid with no positive interconnection

between adjacent blocks. Stability of the revetment is then

dependent on the stability of the individual blocks.

Alternatively, the blocks can be interlocked. Interlocking

blocks have positive interconnection between neighbouring

blocks, helping to distribute loads and providing some re-

duction in unit weight. The resultant revetment has restrict-

ed flexibility. Various forms of blocks are available, locking

in plan and in elevation.

Blocks may also be held together by cables to form a large

flexible mat that may be laid by crane using a purpose-built

spreader frame. The blocks combine flexibility with restraint

under heavy loading. The mats are easy to lay underwater

and are less likely to be subject to progressive local failure.

Cables are made from steel or synthetic materials such as

polypropylene.

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3. CONSIDERATIONS

FOR SELECTION

3.1 Prefabricated elements for breakwaters

3.1.1 Types

Four broad categories of breakwater can be identified:

Rubble mound breakwaters

Vertical breakwaters

Mixed-type breakwaters

Curtain-wall breakwaters.

Rubble mound breakwaters

As mentioned in Chapter 2, rubble mound breakwaters are

the most common type of breakwater in the world. They

have been widely constructed in several forms and designs.

Vertical breakwaters have also been commonly constructed

in some countries, in particular in Japan, Spain and Italy.

Mixed type breakwaters, consisting of an upright section

covered with a wave-dissipating layer of blocks, have re-

cently been constructed, predominantly in Japan.

In the case of rubble mound-type breakwaters, the use ofprefabricated elements has been primarily in the formation

of the armour layer with a view to improve its resistance

against wave action or to overcome lack of appropriate nat-

ural rock units. Numerous different artificial armour units

have been developed since concrete cubes were first used

with this purpose.

Prefabricated armour units can be sub-divided into the fol-

lowing categories, according to the type of placement:

- Double or multiple-layer armour units randomly placed

- Single-layer armour units randomly placed

- Single-layer armour units orderly placed.

Four broad types of units exist, based on unit geometry:

- Massive or blocky units

- Bulky units

- Slender units

- Multi-hole cubes.

Superstructure.

Armour uni ts

Core

SINGLE-LAYER (order) ARMOUR

Toe block

Superstructure.

Armour units

Core

SINGLE-LAYER (random) ARMOUR

Superstructure.

Armour units

Core

MULTI-LAYER ARMOUR

Massive units, for example cubes, parallelepipedic and An-

tifer-type units, are usually placed as multiple-layer armour.

Resistance against wave action depends primarily on theself-weight of the unit and the interlocking degree with ad-

jacent units. If placed in a single layer, uplift forces caused

by water gradients must be compensated by self-weight and

friction forces.

Bulky units as e.g. Accropode, Haro, Betas, Seabee and oth-

ers, have been used as both multiple-layer and single-layer

armour. The stability of the armour layer is then based main-

ly on the high degree of interlock between adjacent units.

The recent trend of breakwater construction in deep water

and rough seas requires the use of large size blocks, and

another problem of the block strength has arisen lately.

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Slender units are vulnerable to cracking and breaking be-

cause their limited cross-sectional areas, as a solution of this

problem various types of high-strength concrete and rein-forcement have been considered (e.g. Dolos, Tetrapod).

MultiHole cubes, like Shed or Cob, are placed correctly in

patterns that exclude significant relative movements of the

blocks. Due to the slender structural members with rather

tiny cross sections, the limiting factors (excluding impacts)

for long-term durability are material deterioration, abrasion

on sandy coasts and fatigue due to wave loads.

Vertical breakwaters

Vertical breakwaters are usually constructed with sand-filled

caissons made of reinforced concrete, but blockwork types

made of stacked precast concrete blocks are also used.

The caisson itself is the prefabricated element more widely

used for the construction of these types of breakwater. A

large number of different designs have been developed. Var-

iations in the cross-section geometry (rectangular, semi-cir-

cular, trapezoidal, etc.), in the horizontal section (rectangu-

lar, cylindrical, triangular, etc.), in the geometry of the cells

(circular, square, hexagonal, etc.) or in the wall structure

(solid, perforated, slotted) leads to a broad classif ication.

CONVENTIONAL VERTICAL BREAKWATER

In-situ cast

concrete cap

CaissonScour protection

Bedding layer

Fill

In-situ castreinforced concrete

BLOCK WORK VERTICAL BREAKWATER

Block

Generally speaking, vertical breakwaters are less economi-

cal than rubble mound structures in the case of shallow wa-

ter but in deep water they become a cheaper solution.

Mixed type breakwaters

Two different types of breakwaters fall into this category:

- Vertically composite caisson breakwater

- Horizontally composite caisson breakwater.

VERTICAL COMPOSITE BREAKWATER

Rock armour

Rock fill

Caisson

Concrete armour units

HORIZONTAL COMPOSITE BREAKWATER

Caisson

For the first type, the caisson, almost equal to the one used

for a simple vertical breakwater, is placed on a relatively

high rubble mound foundation.

In the case of the horizontally composite type, the front of

the caisson is covered by armour units. This type is widely

used in Japan for shallow water zones. The armour reduces

wave impact forces on the caisson, wave reflections and

wave overtoppings.

Prefabricated units used for the cover layer are usually the

same as used for rubble mound breakwaters.

Curtain-wall breakwaters

Curtain-wall or wave screen breakwaters consist of an in-

clined or vertical curtain wall mounted on pile work. This

type of breakwater is applicable in mild wave climate on

sites with weak and soft subsoils. Almost all the principal

parts of a curtain breakwater (piles, curtain modules, con-

nectors) should be prefabricated.



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Useful information of all these type of breakwaters can be

found in the reports of PIANC Marcom WG 12 (1992) and

WG 28 (2003).

3.1.2 Structural integrity

When using prefabricated units in marine construction, the

following should be considered:

- Stability of the structure as a whole

- Integrity of the individual units.

Structural stability

Prefabricated units may require careful placement with nar-

row tolerances to ensure integrity. Preparation of underly-

ing material should ensure that the required tolerances are

met. It may be necessary to place prefabricated units on a

geotextile, particularly for those units where voids may be

large enough for underlying material to be lost. Guidance

is available on the use of geotextiles in the marine environ-

ment (PIANC, 1992).

Stability of the prefabricated units is normally achieved by

selecting a unit size or weight that is sufficient to resist the

hydraulic loading the structure will experience. For large

concrete armour units used in breakwater and revetment

construction, stability may depend on some, or all, of the

following depending on the shape of the unit:

- weight or mass as is the case for rock;

- interlock due to complex geometry: this can bring

economies as less weight and hence material may be re-

quired;

- energy dissipation this is often the case with hollow

blocks; and also with interlocking units where voids

between randomly placed units assist in dissipating

energy.

Assessment of stability of concrete armour units is primarily

based on methods originally developed for rock armour. R.

Iribarren and Nogales (1965) extended the formula original-ly developed by Castro (1935) to parallelopipedic blocks.

The prediction method of Hudson as given in the Shore Pro-

tection Manual (CERC, 1973, 1977, 1984) was originally

developed for rock armour. Extensive physical model test-

ing over the years has derived values of the KDcoefficient

for rock and a range of concrete armour unit types. These

are typically quoted by unit manufacturers, in design guid-

ance available in the literature e.g. CIRIA/CUR (1984),

CUR (1995), SPM (2003) or in national design standards.

Further work on the assessment of armour unit stability

was undertaken by several researchers replacing Hudsons

formula (see references). Due to the wide variety of units

available, and their varying response to wave conditions,

structure geometry and other variables, in many cases it is

necessary to undertake project-specific physical modelling

studies of armour stability.

Integrity of individual units

Prefabricated armour units are generally made of conven-

tional unreinforced (mass) concrete, except some multi-hole

cubes where fibre reinforcement is used.

As the size of individual units grows with the aim of resist-

ing higher storm waves, some large rubble mound break-

waters have experienced damage due to the breakage of theunits. In most cases breakage took place before the hydrau-

lic stability of intact units in the armour layer expired. It can

be deduced that there is an imbalance between the strength

(structural integrity) of the units and the hydraulic stability

(resistance to displacements) of the armour layer.

The integrity of individual prefabricated units will depend

on concrete (or other material) quality, which should be ad-

equate for use in the marine environment. Besides stresses

caused by mechanical and hydraulic loads, another problem

related to the structural integrity of concrete armour units is

the thermal stress developed during the process of curing.

Slender and big-size units are more sensitive to crackingphenomena, due to the temperature gradients created by the

hydration process.

Fatigue of concrete structures should also be considered

when repeated stress variations are significant. The waves

will cause pulsating and impact forces on the armour units

and thus significant stress variations.

As discussed above, the units selected should be of adequate

size to ensure stability under hydraulic loading. Movement

of units under storm conditions may lead to abrasion or deg-

radation, ultimately resulting in their failure. However, it is

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advisable to limit the size of the slender-type units in order

not to exceed acceptable stress levels.

In very dynamic environments, consideration should be giv-

en to the potential for abrasion by mobile sediment, which

may over time lead to a reduction in performance.

3.1.3 Hydraulic performance

There are three major factors that should be considered

when evaluating the hydraulic performance of prefabricated

units for rubble mound breakwaters:

- the ability to attenuate wave run-up and overtopping

- the ability to absorb the energy of waves as they break on

the slope, thus diminishing wave reflections

- the ability to control wave transmission.

The wave run-up level is one of the most important factors

affecting the design of coastal structures because it deter-

mines the design crest level of the structure in cases where

no or minor overtopping is acceptable.

The use of prefabricated armour blocks in breakwaters nor-

mally tends to increase the surface roughness and the poros-

ity when they are randomly placed. Both factors result in

the reduction of wave run-up and wave reflections. If the

armour is formed by units placed in a certain pattern or in an

orderly way, both porosity and roughness may decrease. Asa consequence, run-up and reflections increase.

Global porosity of the breakwater cross-section has an im-

portant influence on several hydraulic phenomena like ar-

mour stability, transmission, reflection or run-up. Single-

layer armour solutions normally result in a lower global po-

rosity that must be compensated by increasing the porosity

or thickness of the inner layers, if high porosity is required

in the design.

Vertical and upright breakwaters have several hydraulic

disadvantages over rubble mound breakwaters. They have

very high reflection and run-up coefficients, unless the crest

is sufficiently low to allow significant wave transmission.Wave reflections induce agitation on the neighbouring water

areas and, frequently this becomes an important problem for

fishery activities, navigation and preservation of the eco-

logical conditions of the sea bed.

3.1.4 Constraints

Risk of failure

Historical trends in the construction of rubble mound break-

waters using prefabricated elements show a tendency to

reduce the total amount of concrete by reducing the unit

weight of the individual unit and / or limiting the number

of layers of the armour. This tendency moves the structure

from a flexible to a rigid behaviour. As a consequence, fail-

ure modes may vary from gradual displacements to suddenand global collapse. This failure mode must be carefully as-

sessed in the design process of a breakwater protected with

a single layer armour.

Aesthetics

In some locations, prefabricated elements may be less pref-

erable on aesthetic grounds than natural materials. In an at-

tempt to overcome this, some types of units have been devel-

oped that either have a surface dressing of natural materials

or are finished to give the appearance of natural materials.

In other circumstances, local opinion may favour geometric

forms of construction using repeating shapes, that are easy

to form using, say, hollow cube armour placed in an orderlyway.

Environmental impact

The coastal and fluvial zone is usually a fragile and limited

environment that can be affected in a serious and irrevers-

ible way. Fabrication of prefabricated elements in dedicated

locations away from areas to be protected can avoid or at-

tenuate impacts on sensitive environment areas by factors

such as construction traffic, water quality, noise, air pollu-

tion, amongst others.

3.1.5 Maintenance

Maritime facilities and structures generally remain in serv-

ice for long periods of time, during which their functions

must be maintained. It is thus essential not only to give due

consideration when initially designing the structures, but

also to carry out appropriate maintenance after the facilities

have been put into service.

In order to maintain the functions of maritime structures at

a satisfactory service level and to prevent deterioration of

the safety of such structures, maintenance including inspec-

tions, evaluations, repairs, etc. should be carried out, in line

with the specific characteristics of the maritime structures.

Deterioration of the strength of concrete should be consid-ered for concrete structures and the corrosion rate should

be considered for steel structures. For other materials, e.g.

geotextiles, the deterioration or damage of fabric material

caused by aging and/or chemical effects by acid or other

substances should be taken into account.

Repair of maritime structures can sometimes incur higher

costs than the initial construction. For example, it is usu-

ally very difficult or sometimes almost impossible to repair

the underlayer of revetments. When selecting prefabricated

elements, ease of repair and cost of maintenance should be

taken into account.



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When selecting and designing a structure, it is necessary to

give due consideration to the requirements for future main-

tenance and to select the types of structures and materials sothat future maintenance will be easily executed. This aspect

should be reflected in the detailed design.

With respect to prefabricated concrete armour elements, in

most cases huge problems are found for repairing broken

units if land access along the breakwater is not possible.

Substitution of deteriorated units is not always an easy task

when they are strongly interlocked.

3.1.6 Construction costs

Construction costs can be influenced by the following vari-

ables:

Material availability

Prefabricated elements may be used where appropriate natu-

ral materials (e.g. narrow grade rock armour or wider grade

rip-rap) are not readily available. For example, for break-

water or revetment construction, prefabricated armour units

may be used where rock of adequate size, quantity or quality

is not readily available. Or perhaps, pre-cast concrete ele-

ments might be used for a wave screen where timber is not

available or might be rapidly damaged by borers.

Construction access

Placement of prefabricated elements might be preferable to

in-situ construction where access is restricted to short dura-

tions by tide conditions or wave attack or where construc-

tion requires to take place under water.

Tolerances

In order to obtain the required performance and structural

integrity from prefabricated units, it will often be necessary

to place them to narrow tolerances, specified grids. This

should be considered in conjunction with access, labour and

plant availability to ensure these requirements can readily

be achieved.

Labour

The degree of skill required for installation of prefabricated

elements should be carefully reviewed. Particular systems

may require careful installation to manufacturers specifica-

tion. This may be important where unskilled labour is to be

used.

Hydraulic / structural performance

Many prefabricated units have been specifically developed

and optimised for hydraulic / structural performance and

may therefore present good technical solutions which use

less materials. Particular examples of this are randomly

placed concrete units for slope protection. They rely on theircomplex geometry and interlock as well as mass to provide

stability and may therefore be more economical than a rock

solution where interlock is less and mass is the main factor

in providing stability.

Plant

It may be necessary to use specialist plant for placement of

prefabricated unit. Consideration should be given to wheth-

er this plant will be locally available.

Logistics

Elements may be delivered to site prefabricated or alterna-

tively it may be necessary to fabricate the units close to (or

on) site, in a project-specific casting yard. Sufficient (level

and firm) land must be available for forming the units, re-

moving moulds, curing and storage in sufficient quantities

to allow construction to proceed without delay.

Fabrication cost and fees

Consideration should be given to the cost of manufacturing

or hiring moulds for prefabricated units if they are to be cast

on site. It may also be necessary to obtain consent for use

of a particular unit and in some cases a licence fee must be

paid.

3.1.7 Materials

Materials to be used in structures and foundation works are

selected after giving due consideration to the external forces

acting on them, deterioration with time, lifetime of struc-

tures, shape of structures, workability, cost, impact on the

environment, and other matters.

Concrete

Concrete is the most popular material in the field of pre-

fabricated elements. Conventional unreinforced concrete is

used for massive and bulky units and steel bar reinforcedconcrete is used for high interlocking blocks and vertical

wall blocks. Pre-stressed concrete is also used for concrete

sheet piles and beams. Recently, recyclable resources, such

as slag and/or coal ash, are considered as concrete materials

as replacing cement, sand or aggregate.

Unreinforced concrete is a brittle material with a low tensile

strength (1.53.0 Mpa) and a compressive strength, which is

one order of magnitude larger. As the reason for breakage of

units is due to tensile stresses it is therefore important that

tensile performance requirements are reflected in the speci-

fications for concrete to be used in armour unit fabrication.



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3.2 Prefabricated elements for quays

A great number of structural parts in a quay can be prefab-

ricated. Historically, piles were the first precast element tobe used with the purpose of enabling foundations in soft

ground conditions. Subsequently concrete caissons and

sheet piles were introduced.

3.2.1 Types

From the structural point of view, three broad categories of

quays can be established:

Gravity quays

Curtain-wall quays

Open-Piled quays.

Gravity quays

Gravity quays are the most primitive but may be the most

economical type if sea bed soils are strong enough to resist

high foundation loads. Three main types can de identified:

- Caisson quays

- Blockwork quays

- Cribwork quays.

GRAVITY QUAY. Caisson-type

Backfill

Superstructure

Fill

Bedding

Caisson

GRAVITY QUAY. Blockwork-type

BackfillBedding

Fill

Dissipating block

Superstructure

Design and construction of caisson quays is very similar to

vertical breakwaters (see chapter 3.1). A large number of

different designs of caissons have been developed. As theberth line has to be straight rectangular caissons are pre-

dominant. Main variations consist in the geometry of the

cells (circular, square, hexagonal, etc.).

Concrete blocks of different forms have been developed for

the construction of blockwork-type quays, with the aim of

minimising wave reflections. In the case of cribwork-type

structures, designs have been dictated, primarily, by the use

of readily available construction facilities.

Cribwork structures consist of the formation of a box by

interlocking prefabricated straight elements of steel or con-

crete and then in-filling to act as a gravity quay.

Cellular and floating caissons, wave-attenuating blocks and

crib-pieces are usually prefabricated elements in gravity

quays.

Curtain wall quays

Steel sheet piles are widely used for the construction of cur-

tain wall quays. A wide number of steel sheet piles have

been developed (Larssen, Hoesch, flat-web section, box sec-

tion, Z-sections, I-sections, etc.) The use of steel sheet piles

as a prefabricated element in quay construction is describedin detail in PIANC Bulletin n 59.

Steel sheet piles

Fill

Anchor

CURTAIN WALL QUAY

Superstructure

Scour protection

Open piled quays

Open-piled quays are commonly used in ports around the

world and are commonly used in soft soil areas. Piles are,

in some cases, prefabricated (steel tubes, pre-stressed con-

crete) and put in place by drilling or driving. Useful infor-

mation on the use of prefabricated piles is contained in PI-

ANC Bulletin n 54.



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Superstructure

Pile

Slope protection Fill

OPEN-PILED QUAY


Analysis of the structural stability of quays strongly dependson the specific type.

Failure modes can be classified in two main groups:

- Overall stability modes

- Local failure.

Seaward overturning and sliding, together with global struc-

ture-soil slip and settlement are included in the first group.

Most of them apply for all type of quays.

Local modes of failures are more in relation to the strengthof the prefabricated elements used in the formation of the

structure. Breakage of elements (blocks, piles, sheet piles,

etc.) depends mainly on the loads acting on and the strength

of the material used.

Conventional unreinforced concrete, if well fabricated, usu-

ally shows an acceptable level of resistance against long-

term loads such as corrosion or fatigue. Other material such

as reinforced concrete, pre-stressed concrete, timber or steel

are more sensible to deterioration (corrosion), in particular

in the intertidal and splash zones.

3.2.3 Hydraulic performanceGravity quays and curtain wall quays reflect some propor-

tion of the wave incident energy. If significant, this process

can generate high levels of wave disturbance that can affect

the operation and safety of berthed ships.

The energy of incident waves can be partly dissipated by

turbulence in holes and slots opened in the front face of

the quay. Changes in the wave phase can also contribute to

reducing wave disturbance. These two mechanisms are the

basis of the behaviour of attenuating solutions as e.g. at-

tenuating blocks, perforated and slotted walls, non-straight

walls, etc.

Hydraulic performance of the armoured slope under open

piled quays against the action of waves is very similar to

those described for breakwaters (see Section 3.1.3).

3.2.4 Maintenance

In order to limit deformations or settlements of structures

used for berthing, particularly in areas with a high degree of

exposure to hydraulic conditions or aggressive agents, regu-

lar inspections are required.

The principal aims of the survey are to determine:

- the structural integrity of elements of the structure

- the appearance of deterioration processes

- indication of movements, deformations and settlement

- indication of scouring processes at the toe of the quay.

3.2.5 Materials

Reinforced concrete

One of the main considerations in the design and production

of reinforced concrete is to achieve the appropriate cover

to reinforcement bars. The provision of a sufficient cover

thickness is the most positive way of reducing the risk of

corrosion damage. A nominal cover thickness of 50 mm isconsidered to be a minimum and is only suitable for very

mild and controlled conditions. For severe exposure condi-

tions it may be recommended to at least double the cover.

Different National standards and the publication EN 206-

1 (European Committee for Standardization, 2000) can be

used as guidelines.

Reinforced concrete quality is also influenced by the cement

type, the mix quality as determined by the water-cement ra-

tio and the placing tolerance that can be achieved.

3.3 Prefabricated elements for

revetments and seawalls

3.3.1 Types

Slope revetments may be divided into several categories e.g.:

Natural material (sand, clay and grass)

Protection by loose units (gravel, rip-rap)

Protection by concrete or asphalt slabs

Protection by interlocking units (concrete blocks and

mats).



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Concrete blocks of relatively flat shape are prefabricated el-

ements that are commonly used as a cover layer in the struc-

tures of revetments, seawalls and coast protection. Theyprovide armour for slopes of natural soil and/or rubble, pro-

tecting the structures from erosion and scouring caused by

wave attack. A wide variety of types of modular blocks and

cabled block types have also been developed and patented

in the last decades. Flexible materials have been also used

as a cover layer, e.g. bag blankets, stacked-bags, fabric mat-

tresses, and tubes, etc.

For the cover layer, stability against uplift forces and degra-

dation of the subsoil are major aspects to be carefully con-

sidered in the design phases. Stone size and thickness of

under layer should be carefully selected.

In recent years, a wide variety of geotextiles has been devel-

oped and used as the f ilter layer of structures of revetments,

seawalls & coast protection. Geotextiles generally allow the

installation of sublayers or cover layers beyond conventional

filter rules. Geotextiles are easily damaged, especially dur-

ing installation, and are rather difficult to repair. Therefore,

special care must be taken when contacting with the sub-

soil.

As for the design and construction of revetments using geo-

textiles, the documents, such as the PIANC reports of PTC I

WG 4 and PTC II WG 21, can be used as guidelines.


Marine structures such as revetments, seawalls and coast

protection are often constructed from a core of granular f ill

material, protected by a series of filter and armour layers.

In-situ material e.g. banks or coastal dunes, may be repro-

filed before protective layers are placed. Alternatively, earth

retaining structures may be constructed, such as for quay

walls.

In order to ensure that the structure remains stable the fol-

lowing issues should be considered in design.

It is essential to ensure that the core or in-situ material isadequately compacted and that there are no voids, which

may lead to deformation or settlement of the structure dur-

ing its life.

When designing filter, underlayers and armour layers, the

engineer should ensure that filter criteria are met to prevent

loss of fines from underlying material and adequate perme-

ability to prevent build up of hydraulic pressures with the

structure.

Where the structure has a sloping face this should not ex-

ceed the natural angle of friction of the fill material.

The stability of a revetment protection against the attack of

waves depends on such factors including friction, cohesion,

weight of the units, interlocking and mechanical strength.The stability of the revetment strongly depends on the sort/

composition of the sublayers and the subsoil conditions. As

a consequence, they must therefore be regarded as a whole

system.

As a rule of thumb, the permeability of the different layers

of the revetment must increase from underneath to top. As

granular filters are mostly more expensive and difficult to

realize within the required limits, a geotextile may be sub-

stituted instead of a graded stone layer.

Under wave attack, instability of artificially paved revet-

ments occurs at the peak of the maximum down rush, whereuplift forces are higher, just before the arrival of the next

wave front. If the protection layer is pervious uplift pres-

sures are strongly reduced. In this case, instability will oc-

cur due to the combined effect of uplift and impact forces

caused by wave breaking over the revetment.

For the dimension of a revetment the following failure

modes must be taken into account:

Sliding of the upper (prefabricated units) layer

Extraction of the units by uplift forces. Self-weight and

interlocking forces should be greater than uplift pres-

sures caused by water gradients

Global equilibrium (geotechnical instability). The revet-

ment, as a whole, including sublayers and subsoils must

be in equilibr ium.

Water gradients due to incoming waves caused by wind

action or passing vessels may induce uplift forces acting

on the units.

Numerous proprietary concrete blockwork systems are

available for use as bank protection and revetment armour.

Design guidance for stability is often very specific to the

particular block type. Generic methods are available for de-termining the block size required for stability under wave

attack, based on physical model tests undertaken by Klein

Breteler & Bezuijen (1991) (also see PIANC (1992).

Klein Breteler & Bezuijens method can be used to predict

block thickness for a wide range of support conditions, but

requires careful categorisation of underlayer materials. The

range of uncertainty in tabulated values of the stability coef-

ficient Sbis relatively wide. In exposed locations, this can

result in blocks of significant thickness. Guidance should

therefore be sought from potential product suppliers who

may have product-specific design guidance that takes into



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consideration the contribution to stability of other factors

such as interlocking, inter-block friction etc.

Where proprietary concrete blocks are to be used for bank

protection, they should be designed for stability under the

expected flow velocities. Guidance is given by 0CIRIA

(1987) on limiting flow velocities for various block thick-

nesses.


Wave run-up and overtopping depends on several factors:

wave height (+) and period (+), angle of approach (-), sur-

face roughness (-) of the upper layer, permeability of the

layers (-), slope and profile shape. In general, milder slopes

lead to lower run-up elevations.

Energy reflected from incoming waves generally increases

with the Iribarren number ( ). The wave reflection coef-

ficient also increases with steeper slopes and diminishes as

the surface roughness and permeability increases.

Prefabricated units with arms, legs, holes or protruding

forms contributes to attenuate the energy of the incident

waves, thus reducing reflection, run-up and overtopping.

3.3.4 Maintenance

Multiple hydraulic interactions between inner fill, filter lay-

ers, protective layer, bed soil, joints and other variables thatconverge in a revetment, mean that regular and frequent sur-

veys should be carried out to ensure integrity of the struc-

ture.

Surveys should check for the following:

Deformation of the revetment layer. This could warn

about the failure of the subsoil and inner layers. Core

material may be settling or flowing out through the filter

layers.

Loose of revetment units. Due to the role of interlocking

on the stability of the outer layer of the protection, the

displacement of an individual unit could lead to rapidfailure. Substitution with prefabricated or cast-in-place

units may be required.

Settlement of the crest level of the bank. This may indi-

cate loose core materials, scouring of the toe or geotech-

nical instability of the bank.

3.3.5 Materials

The following materials are commonly used in the construc-

tion of seawalls and revetments: sand, gravel, quarry rock,

industrial waste-products (slags, minestone, etc.), timber,

concrete, asphalt, geotextile, etc. Useful information about

the standards and specifications for these materials can be

found in several publications (SPM, 1984, TAW/CUR, 1984,CIRIA, 1986, PIANC 1987a).

Prefabricated elements used in seawall and revetments are

usually made of conventional unreinforced concrete.

3.4 Prefabricated elements

for bank protection

The strategies for controlling bank erosion can be classified

into six types:

1. Allowed natural adjustment; permitting erosion to con-

tinue and monitoring that the acceptable expectationsare being met.

2. Management; based on addressing the causes of the

problem.

3. Relocation; based on moving the affected activities to a

less vulnerable location.

4. Bioengineering; based on utilising the engineering role

of vegetation to stabilise the bank.

5. Biotechnical engineering; based on combining the engi-

neering role of vegetation with the structural benefits ofinert materials.

6. Structural engineering including not only bank rein-

forcement measures but also others oriented to control

the flow.

The strategy chosen should take account of the consequenc-

es of bank failure. Where these are rated as severe, the risk

associated with the failure of any strategy is high. A low-risk

strategy is therefore appropriate. For example, where flood

defence is in question or navigation threatened, structural

engineering is likely to be the only appropriate strategy.

Where the consequences of bank erosion are less signifi-

cant, a riskier solution may be more appropriate because ofits lower cost and, compared with structural engineering, its

greater benefit to ecological habitat and landscape.

Allowed natural adjustment should be the first option con-

sidered in any situation. It is particularly appropriate where

any other approach requires a level of investment, which

cannot be justified in economic or environmental benefits

or where the intervention would cause bank instability

downstream or upstream.

Where natural adjustment is not acceptable, the second op-

tion should always be positive management of the bank.



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A structural engineering strategy, sometimes termed hard

engineering, includes the use of steel, concrete or timber

piling, often to create vertical banks. Other materials in-clude rubber tyres and stones. It is particularly appropriate

wherever there is a risk of:

flooding of surrounding land

damage to structures

damage to property, towpaths, roads, railways

damage to canal lining with consequent loss of water in

the channel through leakage

rapid scour of the channel bed material.

Structural solutions are suitable where:

- flow velocities are extremely high

- porewater pressures encourage movement of the lower

bank

- strong tidal currents occur

- boatwash is high and cannot be reduced by management

of the volume of traff ic and type of craft

- drawdown is frequent and rapid with large fluctuationsin f low depth.

3.4.1 Types

Structural solutions for bank protection fall broadly into the

following categories:

stone revetments, concrete bags and gabions

timber and sheet piling

gravity walls and in situ concrete revetments

concrete unit revetments.

The concrete unit revetment is one of the categories of the so

called structural solution.

Within this category, a wide number of prefabricated ele-

ments have been developed under different trade names (see

below).

Concrete unit revetments combine the advantages of indi-

vidual concrete units or blocks that may be transported and

installed as modules with the coverage and protection of a

revetment. The revetment deflects wave energy, thus pro-

tecting the bank from erosion.

Bank protection using concrete units can be achieved by

three different approaches, as for coastal revetments dis-

cussed in Section 3.3.

Open joint revetment

Simple precast blocks are laid with no positive form of in-

terconnection between adjacent blocks. Stability of the re-

vetment is dependent on stability of individual blocks.

Backfill

Close-jointed block

OPEN JOINT BANK PROTECTION

Interlocking blocks

Interlocking blocks have positive interconnection between

neighbouring blocks. The resultant revetment has restrictedflexibility. Geometry and physical size of blocks are fac-

tors that must be considered if there is a curvature required.

Blocks are laid by hand.

Fundation toe Free-draining material

INTERLOCKING BLOCKS BANK PROTECTION

Concrete block

Cable-tied

Blocks are held together by cables to form a large flexible

mat that may be laid by crane using purpose-built spreader-

frame. The blocks combine flexibility with restraint under

heavy loading. The mats are easy to lay underwater and are

less likely to be subject to progressive local failure. Cables



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are made from steel or synthetic materials such as polypro-

pylene.

CABLE-TIED PROTECTION

Filter

Geotextile

Block

Connecting cable


Two major types of loads may cause instability of a bank

protection:

- wave attack caused by passing vessels or wind-gener-

ated waves

- shear forces generated by currents caused by river flow,

tidal variations and passing vessels.

The resistance behaviour of a bank protection under the at-

tack of waves is similar to those described in Section 3.3.2.As the stability of the protection strongly depends on the

sort/composition of the sublayers, the subsoil conditions

and the bed stability, it must, therefore, be regarded as a

whole system.

Water flowing over a bed of sediment at the toe of the bank

protection exerts forces on the grains that tend to move or

entrain them. If the resultant effect of disturbing forces

(drag and lift forces) becomes greater than stabilising forces

(gravity and cohesion) particles start to move and scouring

is initiated.

Shear stress forces induced by current flow also act on the

cover layer units. Connected or interlocking units can gener-

ally be lighter than loose or free units to achieve the same

degree of resistance. Stability of free placed blocks can be

improved by washing the joints by a granular grout. Regular

maintenance is essential if this is vital to the stability of the

structure.

Exposed edges, such as bed protection at scour holes, edges

of a toe protection and transitions between adjacent revet-

ment systems should be carefully assessed.


The roughness of the protection layer is one of the main

factors affecting current flow. Turbulence generated in thewater layer close to the surface of the bank induces loss of

energy and velocity.

With respect to reflection and run-up of waves, trends are

similar to those outlined for coastal revetments (see Sec-

tion 3.3.3).

Prefabricated units with arms, legs, holes or protruding

forms contribute to attenuation of the energy of the flow or

waves.

3.4.4 Constraints

In the case of revetments constructed with concrete units,

attention must be paid to ensure that there is adequate drain-age from the bank through the structure to prevent the build-

up of porewater pressures, which can lead to the failure of

the complete bank along with the structure.

For revetments with slopes steeper than 1 in 3 the geotechni-

cal instability can be a decisive factor and should be exam-

ined properly.

Concrete unit revetments often protect the bank without re-

ducing the energy of the flowing water, and can result in the

transference of erosion problem to another bank section fur-

ther downstream. Special attention must therefore be paid in

the protection of either ends of the structure.

From the aesthetic point of view, structural solutions based

on the multiple repetition of individual forms are poorly

evaluated. Vegetation raising in joints or holes can mitigate

against the visual impact of the structure.

3.4.5 Maintenance

Maintenance should focus on maintaining the overall integ-

rity of the revetment. Three major modes of start of failure

must be observed in regular inspections:

- Deformation of the surface upper layer. This could be

evidence of the failure of the subsoil and inner layers.Core material may be settling or flowing out through the

filter layers.

- Loss of revetment units. Due to the role of interlocking

in the stability of the outer layer of the protection, the

displacement of an individual unit could lead to rapid

failure. Substitution with prefabricated or cast-in-place

units is required.

- Settlement of the crest level of the bank. Loss of core

material, scouring of the toe or geotechnical instability

of the bank could be occurring.



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3.4.6 Construction costs

Generally speaking, revetments made of prefabricated ele-

ments are more costly than those made of natural materials,unless no quarries are in the vicinity of the site.

The cost of concrete unit revetments depends on several fac-

tors:

- Source of materials

- Suitable run length

- Machinery available for unit placing

- Manual labour required for underlayer preparation

- Dimensions and fabrication costs of the prefabricated

units.

3.4.7 Materials

Material usually used in the construction of the prefabri-

cated units for revetments is generally mass concrete. As no

relevant tensile stresses are expected from the flow action no

special strength performances are required for the concrete.

Gabions are used for bank and slope protection with stones

as core material. Stone filled bags and nets are also used as

prefabricated elements for seawalls, coast and bank protec-

tion. In those types of elements, a smaller size of stones canbe utilized compared with those to be used individually.

REFERENCES

BIBLIOGRAPHY

Allsop N.W.H. (1995) Vertical walls and breakwaters: op-

timisation to improve vessel safety and wave disturbance by

reducing wave reflections Chapter 10 in Wave Forces on

Inclined and Vertical Wall Structures, pp 232-258, ed. Koba-

yashi N. & Demirbilek Z., ISBN 0-7844-0080-6, ASCE,

New York.

British Standards Institution (1991) Maritime Structures -

Part 7: Guide to the design and construction of breakwaters

BS 6349: Part 7.

British Standards Institution (2000) Maritime Structures

- Part 1: Code of practice for general criteria BS 6349:

Part 1.

Besley P.B., Stewart T, & Allsop N.W.H. (1998) Overtop-

ping of vertical structures: new methods to account for shal-

low water conditions Proceedings of Int. Conf. on Coast-

lines, Structures & Breakwaters 98, Institution of Civil En-

gineers, pp46-57, publn. Thomas Telford, London.

Besley P. (1999) Wave overtopping of seawalls: design and

assessment manual, prepared by HR Wallingford limited

for the Environment Agency, R&D Technical Report W178,

Bristol, UK.

Burcharth, H.F., K. dAgremond, Van der Meer, J.W.

(2000).

Burcharth, H.F.(1984). Fatigue in breakwater concrete ar-

mour units. Proc. 19th International Conference on Coastal

Engineering, Houston, Texas.

CIRIA (1987) Design of reinforced grass waterways

H.W.M. Hewlett et al Construction Industry Research andInformation Association, Report 61, London, UK.

CIRIA/CUR (1994) Manual on the use of rock in coastal

and shoreline engineering CIRIA Special Publication 83 /

CUR Report 154, CIRIA, London.

Coastal Engineering Manual Part VI, (2002). U.S. Army

Corps of Engineers, Washington D.C.

CUR (1995) Manual on the use of rock in hydraulic en-

gineering Report 169, Balkema, Rotterdam. ISBN 90 410

6050.

Escarameia M. (1998) River & channel revetments a de-

sign manual publn. Thomas Telford, London, UK. ISBN 0

7277 2691 9.

Gardener J.D. & Townend I.H. (1988) Slotted vertical

screen breakwaters Proc. Conf. Design of Breakwaters,

Eastborne, ICE, 1988.

Gardner J, Townend I.W. & Fleming C.A. (1986) Design

of a Slotted Vertical Screen Breakwater Chapter 138, Pro-

ceedings ICCE, Publn. ASCE, New York.

Goda Y. (1974) A new method of wave pressure calcula-

tion for the design of composite breakwaters. Proc. 14thInt. Coastal Eng. Conf. ASCE, New York.

Goda Y. (1985) Random seas and design of maritime

structures University of Tokyo Press, Tokyo.

Goda Y. (2000) Random seas and maritime structures, 2nd

edition ISBN 981-02-3256-X, World Scientific Publishing,

Singapore.

Hudson R.Y. (1974) Concrete armour units for protection

against wave attack Miscellaneous Paper H-74-2, Water-

ways Experiment Station, Vicksburg.



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Klein Breteler M. & Pilarczyk K.(1998) Alternative Re-

vetments, Chapter in Pilarczyk K. W. (1998) Dikes and

revetments design, maintenance and safety assessment,Balkema, The Netherlands.

Kriebel, D. L. (1992) Vertical Wave Barriers: Wave Trans-

Documents

PIANC_Catalogue of prefabricated elements (2005)