Table Of ContentCapital dredging
Proceedings of the conference organized
by the Institution of Civil Engineers and
held in Edinburgh on 15-16 May 1991
*! | Thomas Telford, London
Conference organized by the Institution of Civil Engineers and
co-sponsored by the British Section of CED A and the British
Section of PI ANC
Organizing Committee: R.S. Allen (Chairman), J. Barker, P.
Roland and M. Thorn
A CIP catalogue record for this book is available from the British Library
ISBN 978-0-7277-1654-5
First published 1991
© The Institution of Civil Engineers, 1991, unless otherwise stated.
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Published on behalf of the Institution of Civil Engineers by Thomas Telford Ltd,
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Contents
Study and design
1. Too much, or too little? R. HOLLAND 1
2. Scientific and engineering studies. T. N. BURT 15
Site investigation
3. Geotechnical investigations for capital dredging.
C. H. ADAM and P. T. POWER 33
4. Soil investigation. M. J. STONE 47
Contracts and claims
5. Government conditions of contract and claims.
M.R. LAWRENCE 63
6. A practical view of the dredging contract and
specification. W.R. MORRISON 75
7. Contracts and claims. J. UFF 89
8. The implications for nature conservation of capital
dredging projects. R. MITCHELL 97
Environmental considerations
9. The disposal of capital dredging—the role of the
licensing authority. J. A. CAMPBELL 111
Case studies
10. Dredging of Swash Channel, Poole.
R. N. APPLETON 125
12. Trench dredging for three long sea outfalls. J. KLOET
andJ.FAIRGRIEVE 139
13. Capital dredging of Harwich Harbour. R.S. ALLEN 153
Report of the conference 169
1. Too much, or too little?
R. HOLLAND, Director, Posford Duvivier
SYNOPSIS. When planning and preparing tender documents for
a capital dredging contract it is essential to understand the
client's real requirements, and following this to carry out all the
necessary investigations. These may include obtaining
Parliamentary powers, planning consents, environmental studies,
mathematical or physical modelling as well as boreholes,
hydrographic surveys and perhaps current metering. It is
important to appreciate how long some of this work may take.
Environmental issues are becoming more important, and it is
advisable for a developer to approach them in a positive way,
perhaps by carrying out an initial appraisal before being required
to do a full environmental assessment. The tender documents
should be as clear as possible and should include all available
information on ground conditions and any other data such as
currents, wave exceedance tables, etc. Borehole data should be
classified using accepted nomenclature such as that recommended
in the PIANC "Classification of soils and rocks to be dredged".
Method of measurement should take into account the way in
which the work is likely to be performed, so that the prices in
the bill reflect the likely cost of the work.
INTRODUCTION
1. This paper examines the work which must be done by a
consulting engineer before the tender documents for a capital
dredging contract can be put together. Although the requirement
for adequate soils information is generally appreciated, the need
for much of the other investigation work is sometimes overlooked
and the timescale often underestimated.
2. Figure 1 shows a typical activity chart for the preparation
of a major capital dredging project, and each of the main
activities will be discussed in the following paragraphs. Each
project will of course have its own peculiarities, and some
aspects may have been covered already before a consultant is
appointed. However, the important point to remember is that
someone has to do them!
Capital dredging. Thomas Telford, London, 1991
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PAPER 1: HOLLAND
THE CLIENT'S REQUIREMENTS
3. Understanding the client's requirements is the first
essential step in planning a dredging contract. Does he really
need a 20 km channel dredged to -18.0 m CD all the way up to
his proposed new berth, or could he more economically have a
-16.0 m channel and accept some ship delays in waiting for high
water?
4. Large capital dredging projects are usually the subject of a
feasibility study before any major expenditure is made on site
investigations or model studies. It is at this stage that the
client and the consultant should together develop the
requirements for the project, and this requires a knowledge of
the shipping industry as well as the dredging industry. Matters
which must be addressed include:
• what size and kind of ships does the client have to plan for
now and in the future? For example, a channel for bulk
carriers may be able to take advantage of tidal sailings,
whereas one for container vessels will almost certainly have to
accommodate them at any state of the tide
• has a proper investigation been carried out on the most
appropriate size of vessels to carry the products? - less
knowledgeable clients can sometimes be misled into planning for
the wrong size of vessel
• will a staged approach to the dredging over say 2-3 years be
acceptable, or is the maximum depth necessary immediately in
order to attract the trade at all?
• can the dredged material, if suitable, be used for reclamation
so as to reduce the total project cost by benefiting from extra
land value?
• what is a realistic programme time for the project - will the
day after tomorrow be good enough? - what are the cost
implications of delays in the dredging contract or of carrying
it out too soon?
• how much information does the client have available already on
bed levels, ground conditions and currents and wave heights?
• has a financial viability study been carried out to determine if
savings in shipping costs are likely to outweigh dredging costs?
Is this study a first priority before much money is spent on
any further investigation work?
• does the client appreciate how much may have to be spent on
site investigations before tenders are invited? A figure of 1%
of the project value on boreholes alone would not be
unreasonable if complex strata exist.
• does the client have the necessary powers to carry out the
works, or is a Parliamentary Bill or Harbour Revision Order
necessary?
STUDY AND DESIGN
5. Having discussed these issues thoroughly with the client,
the consultant can proceed to carry out a preliminary feasibility
study using the minimum information which is available. It is
essential during this study to clarify the position with regard to
permission to carry out the work. In the UK, if a Parliamentary
Bill or Harbour Revision Order is necessary the time required
could be quite long - possibly several years, and the client would
have to decide whether to risk expenditure on any other
investigations whilst such powers are being obtained.
6. Assuming that the study shows the project to be viable, the
consultant can then proceed to the more detailed stage of the
investigation.
LICENCES AND LEGISLATIVE ISSUES
7. The requirements for licences and consents will vary from
country to country; those for the U.K. will be covered in Part 5
of the Maritime Structures Code - BS 6349 (ref. 1) and can be
summarised as follows:
• Disposal of dredged material at sea: A licence is required under
Part II of the 1985 Food and Environment Protection Act, and
the deposit of materials at sea is subject to the Oslo and
London Dumping Conventions. Many Port Authorities do of
course have licences for dumping material from maintenance
dredging, but a major capital project will require a new
licence.
• Licence to extract from the seabed: The Crown Estate
Commissioners (CEC), as owners of the seabed out to the 12
mile limit, usually have to be involved where there is
extraction of seabed material. The procedure is dependent on
whether or not the extraction is solely under navigational
powers. If material has to be obtained from a borrow area
separate from the navigational work, a licence from the CEC is
required. The issue of such a licence is however dependent on
the approval of the Department of the Environment following a
consultation process known as the Government View procedure.
The CEC charge a royalty on the quantity extracted. For
dredging work undertaken solely under navigational powers, no
licence, only a consent, is required from the CEC. Royalties
are only charged when there is beneficial use of dredged
material, not when it has to be dumped.
• Permission to dredge: If dredging work has to be undertaken
outside the limit of navigational powers, permission will be
required from the relevant port or harbour authority (assuming
it is not they who are carrying out the work) and also from
the Department of Transport, Marine Directorate. Some local
authorities have orders under Section 18 of the 1949 Coast
Protection Act which give them control over extraction in the
designated area, which is usually down to low water but in
some cases extends 3 miles off the coast. Local authority
consent is required for extraction from such an area.
Permission is also required from the land/seabed owner, usually
PAPER 1: HOLLAND
the Crown Estate.
• Permission to reclaim land: The situation varies in different
localities and also depends on the land/seabed owner involved.
The seabed owner is usually the Crown Estate who also own
about half the foreshore between high and low water marks.
Permission to reclaim on their land will usually be dependent
on reaching an agreement over the terms of a lease. Similar
permissions are obviously required from any landowner involved.
Other significant foreshore owners include the Duchies of
Cornwall and Lancaster. Reclamation of land will also require
planning permission from the local authority under the Town
and Country Planning Act 1971, and a licence from the
Ministry of Agriculture, Fisheries and Food under the Food and
Environment Protection Act 1985, Part II (or DAGS in
Scotland).
• Parliamentary Bills and Harbour Revision Orders: If part of the
project extends beyond the existing harbour limits or powers,
then a Bill or Revision Order will be required, and a decision
will have to be taken on which to apply for. Generally, for a
project which is likely to be non-controversial, a Harbour
Revision Order will be more appropriate, and should be
obtainable within 6-9 months for a works order. If the project
is likely to be controversial, then applying for a Parliamentary
Bill may be a more prudent course of action. The application
has to be deposited by 27th November in any year, and it may
be completed by July of the following year or after the summer
recess. The advantage of a Bill is that wider powers may be
obtained, and the developer is more in control of the
programme. A Harbour Revision Order, if opposed can take
several years to obtain, and the programme is in the hands of
the D.O.T. with little control by the developer.
• It should be emphasised that there will be local variations in
regulations which may alter some of the above requirements.
ENVIRONMENTAL ISSUES
8. Environmental issues are now of much greater concern, and
can have a disastrous effect on a proposed capital dredging
project if not handled properly. In America the environmental
legislation is very strict, and some ports have had great difficulty
in deepening their navigation channels due to the limitations on
disposal of contaminated material either on land or at sea. In
South Carolina U.S. Army Corps of Engineers recently ran into
difficulties after commencing a dredging project at the Port of
Charleston due to a perceived risk of the local federally
protected sea turtles being sucked into the hopper dredge.
9. Unfortunately, in the U.K. there are no clear guidelines on
whether an environmental assessment is definitely required for a
dredging (or dredging and reclamation) project. Each case has to
be considered on its merits. The 1988 Statutory Instrument No.
1336 - The Harbour Works (Assessment of Environmental Effects)
Regulations implement for England and Wales the EEC Council
STUDY AND DESIGN
Directive 85/337/EEC of 27 June 1985 in respect of the procedure
for making harbour revision orders and harbour empowerment
orders. This gives the Secretary of State the power to determine
if an environmental assessment is needed. This Statutory
Instrument applies to dredging work undertaken under
navigational powers. The exact condition set down in the
relevant Navigation Act may require environmental studies in
their own right to ensure compliance.
10. It is beneficial for a developer to take a proactive role in
considering environmental issues, rather than waiting anxiously
for objections to be raised or hoping that no one will notice
what is going on. It is preferable to arrange for an
environmental impact appraisal to be prepared for submission with
the application for permission to dredge. This will help to
eliminate opposition (and this means saving time) and will
contain:
• A statement of what is proposed
• A statement of which parts of the environment are likely to be
affected during the construction contract and afterwards during
the operational phase
• Which effects are likely to be significant
• What measures could be taken to mitigate the impact
and
• Suggestions for a post-development audit
11. Typical concerns which may have to be investigated are:
• What benthic species/communities are present, and are there
any species of national and/or international importance in the
area to be dredged, the area surrounding the dredged zone or
in the area receiving the dredge spoil?
• Would any of the species/communities identified be affected by
smothering as sediment settles out of suspension?
• What is the physical/chemical nature of the area to be dredged
and the area receiving the dredge spoil. Are there any
contaminants or high organic contents in the sediment?
• Will there be an adverse impact on the ecosystem(s) (ground
water quality included) where sediment is to be dumped?
12. Using the dredged material for reclamation may be
required as an integral part of the overall capital dredging
project, or as a way of reducing costs because of long distances
to the approved dumping ground. If this is done, it will be
necessary to decide if the material is likely to be classed as
contaminated. There are no guide values on concentrations which
apply specifically to dredged spoil for reclamation purposes.
Instead, those guide values for re-developing on contaminated
land are being used.
13. Such values are set by the DoE via their ICRCL
(Interdepartmental Committee on the Redevelopment of
Contaminated Land) and based on the concept of "trigger
PAPER 1: HOLLAND
concentrations" which in turn depend upon the intended use of
the site (ref. 2). Concentrations above the trigger values mean
that some action or consideration will be necessary.
14. Dredge spoil may be tested for a variety of contaminants
and their levels are compared with the "threshold" and/or
"action" concentrations, as suggested by ICRCL. Because the
risks posed by contamination are difficult to quantify, an indirect
method based on "threshold" and "action" trigger concentrations
has been devised. Their purpose is to assist in selecting the
most appropriate use for the site (and effectively the reclaimed
site) and in deciding whether remedial action is required. Figure
2 illustrates the approach, and the guide values are given in the
ICRCL publication.
Importance
of hazard
Significance of risk depends
on intended use and form of
development
Use professional judgement to
decide whether action is needed
/ Action /
/ required /
/ Risk no greater than ///
/ is normally accepted ///
z Treat as uncontaminated /
/ no action required /
Y///////////A.
Threshold Action Concentration
value value of contaminant
in soil
FIGURE 2 - INTERPRETATION OF 'TRIGGER CONCENTRATIONS"
FOR CONTAMINATED MATERIALS
15. Apart from the ICRCL guide values, some prefer to use
the so-called "Kelly-table" - named after R. T. Kelly who
submitted the paper "Site Investigation and Materials Problems" in
the Chemical Industry Conference in October 1979 (ref. 3).
Typical examples are given in Table 1 below.
16. It will be noted that there is room for judgement on
acceptable levels, and this again emphasises the need to
investigate the matter early on in the conceptual stage.
