Saemankeum Comprehensive Tideland Reclamation Project

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Republic of Korea

Korean Research Institute

For Reclamation

Saemankeum

Comprehensive

Tideland Reclamation

Project

Feasibility

Study on

Hydraulic

Filling of Seadikes

November 1989

NEDECa

Netherlands Engineering Consultants

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Republic of Korea

Korean Research Institute

For Reclamation

Saemankeum

Comprehensive

Tideland Reclamation

Project

Feasibility

Study on

Hydraulic

Filling of Seadikes

November 1989

d\lEDECa

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i LIST OF CORTERTS LIST OF TABLES ii

LIST OF FIGURES iii

EXECUTIVE SUMMARY 1 1. INTRODUCTION 5 1.1 Background 5 1.2 Study objectives 5 1.3 Acknowledgments 6 2. REVIEW OF DATA 7 2.1 General 7

2.2 Characteristicsof fill sand and locations of borrow areas 8

2.3 Operationalconditions 11

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3. GENERAL DESIGN CONSIDERATIONS 14 3.1 Dam alignment and constructionstaging 14 3.2 Conceptual cross section 15 3.3 Aspects of hydraulic filling 15 3.4 Design requirementsfor hydraulic fill 17 3.5 Protection and filter elements 18 4. CONSTRUCTIONMETHODS WITH SEA-BORNEEQUIPMENT 21

4.1 General 21

4.2 Overview of dredging equipment 22

4.3 Selected options 24

4.4 Operationalconstraintsof main equipment 25 4.5 Fill stages and sandlosses 27 4.6 Equipment availabilityin Korea and abroad 30 5. EVALUATIONOF RECOMHENDEDSAND DIK! CONCEPT 31 5.1 Recommendedmethods for hydraulic fill 31

5.2 Equipment requirements 33

5.3 Quantities and costs of hydraulic fill 35

5.4 Constructionschedule 36

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LIST OF 'l'ABLES Table 2.1 Table 2.2 Table 2.3 Table 2.4 Table 2.5 Table 3.1 Table 3.2 Table 3.3 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5

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Table 5.1 Table 5.2 Table 5.3

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Characteristics of borrow sand area North Characteristics of borrow sand area Middle Characteristics of borrow sand area South

Monthly mean wind direction and velocity for Kunsan Station Tidal levels

Length and ground elevation of dam alignment Disposal slopes hydraulic fill

Liquefaction potential related to relative density Factors influencing the choice of execution methods and equipment type

Main characteristics of cutter dredges Operational limits of cutter dredges Specific workability of cutter dredges

Transport distance limits and production levels Construction stages of sea dike

Specifications of cutter dredge

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Hi LIST OF FIGURES

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Figure 3.1 Conceptual cross-section Figure 4.1a Open fill and bunded fill Figure 4.1b Demixing of fill sand

Figure 5.1 Construction stages sea dike construction

Figure 5.2 Direct hydraulic filling with underwater discharge pontoon Figure 5.3 Direct filling in an open fill with discharge pipe just above

high water

Figure 5.4 Schematic lay-out of transport line

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_{Feasibility Study on Bydraulic Filling of Seadikes}

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EXECUTlVE SUMMARY Introduction

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The Saemankeum Camprehensive Tideland Reclamation Project has as objective the reclamation of the extensive tidal flats in the estuaries of Dongjin river and Mankyung river for, among others, agricultural, urban and industrial use. The project includes the construction of a number of closure dams with a total length of some 34 km.

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Because of the large scale of the project and unfavourable site conditions the project requires skill and experience of a higher level than was

utilised in previous reclamation projects executed in Korea. Therefor the Agricultural Development Corporation (ADC) commissioned the Korean Research

Institute for Reclamation (KaIR) to study seadike construction and related aspects.

Study objective8

The scope of this study can be formulated as follows:

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The closure method and the conceptual design of the seadike (crest height. width. seaside slopes. etc.) are adopted from the KRIB. conceptual design.

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Review of project data

Suitable material for hydraulic filling are sand or sandy silt. Silt is less suitable and clayey silt must be regarded as unsuitable.

Potential sea-sand borrow areas are selected based upon the following criteria:

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It is concluded that:

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Basically, the sea-sand will be relatively easy to dredge and to transport through pipelines over considerable distances. The (very) fine texture of the sand (mean grain size: 100 microns) in combination with an uniform gradation will however impose restrictions on the process of filling. The major limiting factor for sea-borne operations is the wave climate at the project site, especially for locations exposed to north-western

directions. Workability of cutter-suction dredges is (preliminary) deter-mined based upon the available data of the wave climate, but it is

concluded that:

Furthermore it is essential to predict current strength and direction in the project area, including (potential) borrow areas.

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General design cOD.ideration.

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Given the natural boundary conditions of the site (mainly tidal boundary conditions) and the natural properties of the available sand it can be concluded that the under water slopes of the hydraulically filled sand body will be in the order of 1:15 and that the relative density of the under water part is expected to be between 35% and 45%.

The expected density indicates that the sand body might be subject to liquefaction during earthquakes with a ground surface acceleration of 0.10 g. It is therefor recommended:

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Couatructiou _thocla

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From the analysis of the specific project requirements, the conditions in the borrow areas and the characteristics of the available sand the follow-ing conclusions have been drawn:

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Fram the limited available wave data the workability of the chosen dredging equipment has been determined, the yearly overall workability is approxima-tely 70%, while in the winter season this varies between 50% and 60%.

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The expected losses of sand vary between 15% and 25%, depending on the different stages of fill operations. To minimise additional losses a number of recommendations have been made for the filling process:

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The hydraulic fill operations have been described in three stages:

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Quantitiea and coat of hydraulic fil!

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For the KRIR-proposed seadike alignment, the estimated quantities for the hydraulic fill are 42 million m3 netto The estimated loss amounts to some 8 million m3.

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5 1. IBTRODUC'l'IOR 1.1 Background

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The Saemankeum Comprehensive Tideland Reclamation Project aims at the reclamation of extensive tidal flats in the estuaries of Dongjin and Mankyung river for the purpose of tideland development and expansion of national land for agricultural, urban and industrial use, water resources development and desalinization and improvement of the agricultural environ-ment.

The project comprises the construction of a 34 km long closure dike across the delta of the Dongjin and Mankyung estuaries. Behind this dike a system of polders and a fresh water basin will be developeG.

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In Korea, there is a firm base of engineering and construction skill of closure works build up during the recent construction of similar tideland reclamation works. With the present project, in view of its large scale and more unfavourable environmental conditions, the necessary skill and

experience enters into the second and/or third generation of closure works, requiring both a higher level of tecbnology in design and construction as weIl as specific additional technology with regard to sea dike construc-tion.

In recognizing this, the Agricultural Development Corporation (ADC)

commissioned the Korean Research Institute for Reclamation (KRIR) to study the closure works and the sea dike construction of the Saemankeum project.

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Frequently sand bodies are constructed in seas, estuaries, lakes and rivers by hydraulic fill to serve as breakwaters, harbour extensions, artificial islands, closure dams, etc. The low price of the construction material is often an attractive reason for using this type of structure. Less attrac-tive factors are however:

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the gentie slopes caused by the construction method

the low relative density of the soil which introduces the risk of flow slides (liquefaction).

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During closure works in the past only a limited use was made of seaborne equipment for dike construction and it was feIt that additional know-how and experience in this field should be gained.

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1.2 Study objectives

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The objectives of the present study are to ·carry out the study for cross section of the sand closure sea dike by dredging and by sea-borne equip-ment, which is to be placed back side of the quarry stone fill" and to

·carry out the study for the sea borne equipment for the seadike construc-tion" •

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Basically, the study focuses on the specific design and construction aspects of the seadike (partly) constructed with sand borrowed from the adjoining sea bottom. This seasand is placed at the back side of a quarry

stone fill which is build out by dump trucks and/or seaborne equipment.

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The basic concept of the sea side slopes, protective and filter e1ements, as designed by the KRIR, is not inc1usive the study objectives and wi11 therefore not be reviewed in the present study.

During the c10sure of the tida1 channe1s and shoa1s, extensive bottom

protection is required to avoid unacceptab1e erosion of the remaining gaps. This is dealt with in detail during Nedeco's appraisa1 mission (Technical Appraisal Report, May 1989).

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The recommendations for the sandfil1 are based upon the Dutch know1edge and experience on hydrau1ic fi11ing gained in Dutch and international projects with the use of modern, high output, equipment.

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1.3 Acknov1edgments

We wou1d 1ike to express sincere thanks to the KRIR for the excellent organization of the assignment in Korea and the preparation of the data compi1ations before and during the assignment.

Especia11y we wou1d 1ike to thank Mr. Yoon, managing director, and Mr. Cho, director, for their stimu1ating and open communication during the briefings and discussions.

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2. REVIEW OF DATA 2.1 General

For the purpose of this study, the KRIR prepared a compilation of basic and design data concerning seadike design and construction. The information and data comprises the following sections:

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A-I Proposed plan of sand-fill of the seadike A-2 Construction of sand-fill sea-dike

8-1 Tides and tidal currents 8-2 Wind Data

8-3 Transportation of the construction materials

8-4 Alignment of sea-dike and location of boring holes 8-5 Standard penetration test data

8-6 Laboratory soil mechanics test data 8-7 Kumgang project soil mechanics data

8-8 Laboratory soil mechanics data (gradation curves) Wave climate data.

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Furthermore, basic and design data for the whole of the project is avail-able from the following reports:

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1. Saemankeum. Comprehensive Tideland Reclamation Project (Feasibility Study), December 1988, Agricultural Development Corporation. Summary in English.

2. Outline of the Studies Sea Dike Construction Plan Saemankeum

Project, February, 1989, Korea Research Institute for Reclamation. Summary in English.

3. Saemankeum Comprehensive Tideland Reclamation Project. Technical Appraisal Report, May 1989, Netherlands Engineering Consultants.

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The present study is basically concerned with the construction phase of the seadikes. The dimensions of the dike, in terms of crest height and width, seaside slopes and slope protections, are adopted from the KRIR conceptual design.

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To determ!ne the feasibility of the use of seasand for the seadikes two groups of essential data can be distinguished.

The first group comprises information on the characteristics and engineer-ing properties of the borrow sand, the locations of the borrow areas and the quality of the subsoil at the proposed seadike alignment.

The second group holds the information on the operational conditions for sea-born equipment at the borrow areas and construction site. The review of the available data will be restricted to both groups and will be dealt with separately in the next sections.

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2.2 Characteristics of fill sand and location of borrow areas.

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In 1987 and 1988 a large number of borings were carried out, mainly in the proposed seadike alignments. A limited number of borings were carried out at potential seasand borrow areas. In figure 1 of annex B-4 "Alignment of seadike and location of borehole's· four distinct borrow areas are indi-cated. At the moment, however, no firm decisions have been made for the locations of the borrow areas nor guidelines are formulated to select the locations. The only restrietion at present is that borrow area's should be located seaward of the seadike a1ignment.

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In the present study the general characterization of the borrow sand will therefore not be restricted to the indicated areas alone, but will be based upon the results of all the bor!ngs in the area. Borings which are showing the most suitable sand for the hydraulic filling will be indicated.

In general the subsoil of the Dongjin and Mankyung river delta consists of varying layers of silty sand, silt and clayey silt. For the largest portion of the seadike alignment a weathered (rock) zone is encountered at a level of approximately 40 m below Mean Sea Level (MSL).

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Suitable material for hydraulic filling are sand or sandy silt. (Unified soil classification SM). Silt layers are less suitable and clayey silt layers must be regarded as unsuitable due to the high clay content. Potential borrow areas are sele~ted using the following criteria:

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Clay content Silt content

< 5 microns) less than: 3 %

< 74 microns) less than: 25 %

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These criteria are based upon the minimum requirements for the hydraulic fill sand (see section 3).

The project area is broadly divided into three area's:

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area NORTH (between Sinsi and Bieung Chu Island) area MIDDLE (between Duri and Sinsi Island)

area SOUTH (between Daehangri and Duri Island)

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For each area the available information is reviewed considering the above quality criteria and bearing in mind that borrow areas should be close to the seadike for cost-effective reasons. Seasand with a higher silt and clay content (above 25% and 3% respectively) is regarded as less suitable, but could be used for the filling when special techniques, such as cycloning are employed. In the present study, however, the proposed filling methods will be based upon the use of suitable sand as defined above.

Area ~ORTH.

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In totallO borehole locations show potential suitable borrow sand. These are listed in annex 1. In general (silty) sand layers are found up to a depth of 5.0 to 10 m below the seabottom. Deeper layers are less suitable due to an increasing silt and clay content.

Waterdepths are ranging between 8 m (minimal) and 11 to 12 m (maximal) to MSL.

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General characteristics are:

Table 2.1 Characteristics borrow sand area North

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Mean graindiameter D50 (micron) Clay content

Silt content SPT

100 (90mu minimum and 125 mu maximum)

< 1% 15 to 20 %

30 to 45 (dense)

---Conclusions area North:

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There seems to be sufficient suitable sand to chose optimal borrow area locations in terms of distance to the seadike alignment. It is however recommended to carry out additional investigations after the final alignment have been chosen.

Bottom levels in the area do not impose restrictions on the choice of dredging equipment.

The most suitable sand is found at the upper layers of the subsoil stratum.

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Area MIDDLE

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Only 5 borehole locations show potential suitable borrow sand. The selected boreholes are listed in annex 1. Compared to area NORTH the silt content is somewhat higher at the selected locations, which probably explains the lower SPT values. As for area NORTH, deeper layers below 5.0 to 10m below seabottom are less suitable or unsuitable. Waterdepths are ranging between

5 m (minimal) and 9 m (maximal) to MSL.

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General characteristics are:Table 2.2 Characteristics borrow sand area Middle

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Mean graindiameter D50 (micron) Clay content

Silt content SPT

95 (90mu minimum and 115mu maximum)

<1%

20 to 25 %

20 to 30 (medium dense)

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---Conclusions area Middle:

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As the alignment of the seadike has not been chosen yet, no firm conclusion can be drawn if there will be sufficient suitable sand for the hydraulic fill in this particular area. The borings, showing suitable sand, are located near and in the most western proposed alignment which will limit the use of these areas. In view of this. the most eastern alignment of the seadike is probably to be prefer-red.

Additional investigations in an early stage are strongly recommended to support the final choice of the alignment considering the

locations of borrow areas.

As for area North, the most suitable sand is probably found at the upper layers of the subsoil stratum.

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Area SOUTH

At the area South, the available data only show a limited area where

suitable borrow sand is present. The selected boreholes are listed in annex 1. The general characteristics are approximately as for area Middle. Water-depths are around 4 m to HSL.

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Conclusions area South:

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There area serious doubts if sufficient suitable sand is available for hydraulic filling of the particular seadike sections.

As for area Middle, additional investigations in an early stage are strongly recommended to support the final choice of the alignment considering potential borrow area locations.

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Basically, the seasand will be relatively easy to dredge and to transport through pipelines over considerable distances. The (very) fine texture of the sand in combination with a uniform gradation will however impose

restrietions on the process of filling. A large part of the study focusses therefore on the disposal stage of the dredging process.

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2.3 Operational conditions a) general climatic conditions

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The climate of Korea is characterized by the Asian Monsoon. The climate in winter is dominated by the continental climate from the southeastern region of China and Mongolia. The climate in summer is dominated by the moist air moving northwestwards form the Pacific towards low pressure areas in

Central Asia.

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winter season spring season summer season autumn season December-February March-May June-August September-November

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Monthly mean wind- directions and velocities for the Kunsan meteorological station are listed below.

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Table 2.4 Monthly Mean Wind Direction and Wind Velocity for Kunsan Station (Unit of wind velocity: mIs)

---Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Velocity

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4.0 4.6 4.8 4.5 4.2 3.9 3.6 3.8 3.8 3.8 4.0 4.0 Direction: WNW WNW WNW WNW WNW W WNW NE WNW NW WNW WNW

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The general climatic conditions will not limit dredging and/or sea-borne operations, except during typhoons. Typhoons are irregularly occurring from a South-western direction. No information is available on the frequency of ocurrance of typhoons. Adequate sheltering of the sea-borne equipment is necessary during these events.

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b) bathymetry

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Natural bottomlevels for the proposed (major) sea dike sections are varying between MSL and at lowest 15 m below MSL.

At the potentia1 borrow areas at the seaside of the a1ignment the natura1 bottomleve1 is generally lower than 5 m below MSL. On1y limited areas are above this level.

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The general bathymetry does not limit seaborne operations or mobilization of equipment. On1y at minor seadike sections bottom levels restricts the use of seaborne equipment during all stages of the tide. In general there is good access to the project area, both from the seaside and in1and. c) tides

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The tidal ranges only are slight1y varying throughout the project area. The fo1lowing tida1 levels are adopted in the present study:

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Table 2.5 Tidal levels

---High Water Ordinary Spring Tide (HWOST) ₊ 2.90 m High Water Ordinary Mean Tide (HWOMT) ₊ 2.09 m High Water Ordinary Neap Tide (HWONT) ₊ 1.26 m

Mean Sea Level (MSL) - 0.04 m

Low Water Ordinary Neap Tide (LWONT) - 1.34 m

Low Water Ordinary Mean Tide (LWOMT) - 2.16 m

Low Water Ordinary Spring Tide (LWOST) - 2.98 m Tidal ranges: Neap 2.59 m

Mean 4.24 m

Spring 5.89 m

The tide is semi-diurnal with a shorter period of flood tidal rise than the period of ebb tidal fall.

The relatively large tidal ranges do no influence sea-borne operations as such, except restricted navigation during low-water stages at some area's. They are however an unfavourable condition for the hydraulic filling

process of the seadikes, especially in combination with the (very) fine texture of the borrow sand.

d) tidal currents

In the natural state, the tidal currents during flood tide are occurring from a south-western direction. The maximum current velocity during mean water spring ranges between 1.00 and 1.70 mis at the deeper channels. At

the more shallow parts the velocities are generally below 1.00 mis. The ebb tidal velocities are somewhat lower and ranges between 0.60 and

1.45 mis at the deeper channels.

During the project execution, as a result of the gradual closure of the tidal shoals and channels, the flow patterns and current velocities will change. With the decreasing cross sectional area, the tidal flows will be more concentrated towards the remaining channel and shoal sections.

Changing flow patterns could create new channels by the eroding forces of the current. This could effect operational conditions at the borrow areas. A reliable prediction of current strength and direction at the project area

including potential borrow areas is necessary to evaluate operational

conditions during all stages of the closure. Employing a mathematical model for this is recommended.

e) waves

At two locations wave heights are measured with a wave gauge equipped with a pressure sensor. For both stations processed data is available for the period December 1988 to June 1989.

Location A, located north-east of Sinsi island, is exposed to ocean waves running from the north-western and northern directions and represents therefore the most unfavourable operational conditions.

Location B, located at the south shore of Sinsi Island is sheltered for these wave directions.

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The measuring period is too short to abstract statistical relevant (long term) average wave conditions. The data can only be used to get a first indication of the operational wave conditions. It is understood that no statistical data is available from other sources.

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Wave conditions, in terms of operational conditions, are generally descri-bed by the joint probability of wave direction, wave height and wave

period, in which the waveheight is the significant waveheight Hs (the average waveheight of the one/third highest waves) and the wave period is the zero-up crossing wave period Tz as determined from the wave spectra. Besides the significant waveheight, the waveperiod (or in fact the wave-length) is a major influencing factor for the operational conditions of sea-borne equipment (section 4.4).

With regard to the waveperiods (or wavelength) three conditions can be distinguished:

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waves with periods less than 6 seconds (small period waves) waves with periods in between 6 and 8 seconds and (intermediate periods)

waves with a period larger than 8 seconds (long period waves, including swell conditions)

A summary of the available wave data is presented in annex 2.

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For the months July through November no data are available yet. Based upon the seasonal climate and considering the variation in monthly mean wind directions and windvelocities the following comparisons are assumed: Wave conditions of : November equals December,

October equals March, September equals May, August equals June, July equals June,

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In the present study, the workability of dredging equipment will be based upon the at present available data of the waveclimate. The data is however insufficient for a reliable determination of workability. Data collection in this field should be continued and it is recommended to install additio-nal devices, for instance waveriders, at a number of off-shore locations.

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3. GENERAL DESIGH COHSIDERATIOHS

3.1 Dam Alignmrnt and general construction staging

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During the feasibility study for the Saemankeum project several possible dam alignments were studied. The length and ground elevation of the proposed sea dike sections, as proposed by the KRIR, are shown in the following table.

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Table 3.1 Length and ground elevation of dam alignment

---Name of Ground elevation 1988

sea dike Location Length Lowest Highest Average

---No. 1 Daehangni-Namgaryukdo 4,392 -15.00 0.00

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4.84 2 Namgaryukdo-Boukgaryukdo 529

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2.80 0.00

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0.10 3 Boukgaryukdo-Agdo 4,876 -11.55

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2.00

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6.18

4 Agdo-Douksan Island 310

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0.50 0.00

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1.19

5 Douksan Island-Duri Island 920

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4.20 0.00

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1.90 6 Duri Island-Sinsi !stand 6,552 -15.00

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5.30 -11.50 7 Sins! Island-Yam! Island 2,500

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6.30

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2.90

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5.10 8 Yam! Island-Bieung Island 11,500

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9.70

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3.50

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7.03 9 Bieung Island-Noraesoum 1,620

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5.50

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3.50

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4.10 10 Noraesoum-Oushik Island 740 _{+ 1.00 + 4.00 + 2.38} 11 Oushik Island-Naecho Isl. 1,170 _{+ 0.60 + 1.50}

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_{+ 1.10}

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Each sea dike section will have its own specific circumstances and demands, but the same general principles of hydraulic filling will be applicable to most of the sections. In the present study attention is focused on seadike nO.,8 which is the largest in length and exposed to the dominant wind and wave directions.

In the KRIR-plans it is envisaged to construct seadike no.8 in four stages. These stages refer to the construction conditions:

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Stage 4:

dam construction without placing bottom protection

bottom protection should be placed only directly under the quarry stone fill

in order to prevent foundation scouring of the closure gap, bottom protection should be placed prior to the quarry stone dumping operations. final closure Stage 1: Stage 2:

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Stage 3:

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The staging of the sea dike construction only indirectly influences the hydraulic filling. Actual building out of the seadike will be done by

constructing a quarry stone embankment at the sea side of the dike. Working methods may include dumping by barges and/or trucks or with pontoon-mounted crane's with barge transport. The quarry stone is extracted from nearby quarries.

The following dimensions are envisaged for the quarry run embankment:

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crest level crest width side slopes + 4.00 m MSL 8.00 m 1 : 1 to 1 1.2

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This working method creates, in principle, favourable working conditions for hydraulic filling of the dike body during all of the closure stages. During moderate sea conditions, the fill area is effectively sheltered from wave and current attack and the embankment permits a good access to the disposal site(s).

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It is however recommended to analyze the required dimensions of the quarry stone embankment under sea conditions still likely to occur during the construction phase. During more severe sea conditions, wave overtopping and spray may seriously hinder the construction operations. The stability of the quarry stone embankment alone should be assured, because an eventual failure (collapsing or breaching) of the embankment would have a detrimen-tal effect on the hydraulic fill behind.

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3.2 Conceptual Cross-section

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The conceptual cross-section of the seadike as designed by the KRIR is thebasis for the present study on hydraulic filling. The main features of this design are as follows (see figure 3.1):

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Crest height ₊ 11.0 m HSL (indicative) width 4.0 m slopes 1:3 Road berm level ₊ 4.0 m HSL width 32 m

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3.3 Aspects of hydraulic filling

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During the last 15 years more than ten large sand dams have been con-structed in the estuaries in the South-Western part of the Netherlands within the framework of the "Delta works". During recent years an extensive research program was executed by the "Rijkswaterstaat' and the Delft

University of Technology in collaboration with Delft Hydraulics and Delft Geotechnics.

The following conclusions were drawn for the construction of sand bodies in seas or rivers by hydraulic fill, when the sand is released above the

water:

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The slope resulting from the sedimentation above the water line is nearly equal to the equilibrium slope, i.e. the slope at which a sand-water mixture flows, if the average sedimentation is zero. This slope is mainly determined by grainsize and specific mixture flow rate, i.e. the flow per unit width.

Coarse sand and low specific flow rates, i.e. a weIl spread flow over the full width of the dumping site, result in relatively steep slopes.

The most gentle slopes often occur in the tidal zone; particularly when the waterdepth is small, the tidal difference is large and when

the mixture flow is concentrated in channels during low water. The (equilibrium) slope below the water line is much steeper. The grainsize is of primary importance for the underwater slope. The slope is also influenced by water depth and by other operation features, but these effects could not be quantified yet.

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The sand which has settled below the water level generally has a low relative density and may often be susceptible to liquefaction. The construction methad was not observed to influence the density. Above the waterlevel, the density of the sand is relatively high and the sand is seldom susceptible to liquefaction.

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Based on the research and experience during several projects the following disposal slopes during the hydraulic filling can be expected:

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GrainsizeTable 3.2 DisposalAbove waterslopes Below water smooth sea Below water rough sea

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---fine sand 1:50 to 1:100 1:10 to 1:15 1:15 to 1:30 60 mu

-

200 mu medium sand 1:25 to 1:50 1:5 to 1:10 1:10 to 1:15 200 mu

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600 mu coarse sand 1:10 to 1:25 1:3 to 1:5 1:5 to 1:10 600 mu

-

2000 mu gravel 1:5 to 1:10 1:2 1:3 to 1:6 > 2000 mu

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As can be observed in the above table the (average) grainsize of the fill mate rial is of primary importance. The actual slopes, however, also depends on operational factors. The most important factors are:

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careful maintenance of the guide bunds, na break in production,

avoidance of channel formation at the dumping site in order to obtain low specific sand transport rates at the water line.

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A less attractive factor of hydraulic filling is the low density of the underwater part of the sand body. Prom bath laboratory and field measure-ments it is found that the relative density (Dr) ranges between 30% to 45%.

The relative density is defined as follows:

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Dr - (emax - e)/(emax - emin)

*

100 %, in which emax - the maximum porosity

emin - the minimum porosity

e - the actual porosity of the fill

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Some typical values for Dr (%) are:

0 - 15 % very loose

15 - 35 % loose 35 - 65 % medium 65 - 85 % dense 85 - 100 % very dense

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The compaction of the underwater part of the sand body is loose to medium, which is an unfavourable condition for the stability (liquefaction) of the

sand body.

In the next section this will be dealt with in more detail.

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In general. the relative density of the sand body above the water line is (much) higher. which is due to the densification effect of the equipment used during the hydraulic filling. According to available data the sand above the water table has a 4 to 5 % lower porosity than the underwater sand.

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Recently. the construction of the Kumgang estuary dam in Korea was com-pleted. The dam was (partly) hydraulically filled with sand borrowed from the adjoint river bot tom. Oensity tests showed a relative density of above 70 % for the dry part of the fill.

The properties of the fill sand are comparable with those of the seasand found at the Saemankeum project site. with. however, a somewhat higher mean grain diameter (150 micron versus 100 micron for the Saemankeum project).

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3.4 Design reguirements for hydraulic fill

General design requirements for hydraulically filled sand bodies reflects the aspects mentioned in the previous section. viz. gentle disposal slopes and a low relative density for the underwater part. The actual design of the seadike should be drawn up considering the specific features of hydraulic filling. For instance. demanding high densities for the under-water part of the seadike will lead to high execution costs because a

(expensive) densification program have to be included.

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A minimum relative density requirement for the underwater part of the hydraulic fill follows from the liquefaction potentialof the sand body. Under earthquake loading the loose to medium dense sand may compact.

increasing the pore water pressure and causing a loss in shear strength. As yet no generally accepted unified criterion has been developed for li-quefaction potential, although attempts have been made to relate it to relative density (table 3.3)

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Table 3.3 Liquefaction potential related to relative density Or (after Seed and Idriss)

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Maximum ground surface acceleration Liquefaction very likely Liquefaction depends on soil type an earthquake magnitude Liquefaction very unlikely

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0.10 g 0.15 g 0.20 g 0.25 g Or < 33% Or < 48% Or < 60% Or < 70% 33% < Or < 54% 48% < Or < 73% 60% < Dr < 85% 70% < Dr < 92% Or > 54% Or > 73% Or > 85% Or > 92%

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For the Saemankeum project a maximum ground surface acceleration of 0.10 g is adopted for the design. A first approximation of the minimum relative density required for the underwater part of the sandfill would be Or > 35

%. Adopting this value indicates that the under-water-part would still be suspectable to liquefaction and that special attention have to be given to the stability of this part.

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Basic design requirements are:

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applying gentie side slopes for the underwater part of the sand body. A slope inclination of approximately 1 : 15 is preferred both for stability and execution reasons, or

applying rock or gravel fill embankments at both sides of the seadike. These rock or gravel embankments should be designed con-sidering a partial loss of shear strength of the underwater part of the sand body due to possible seismic activity, resulting in

increased horizontal forces.

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Because of the liquefaction potential, hydraulic filling of the seadike requires appropiate in-situ quality control procedures during the construc-tion. Density checks of the underwater sand body most conveniently can be done by Cone Penetration Testing (CPT). Procedures for densification of the sand body should on fore-hand be evaluated and prescribed and should be incorperated in the quality control and assurance (project) plan.

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As to the fill material itself the following basic quality standards for suitability are generally adopted:

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clay fraction Silt fraction organic fraction less than 3 % less than 15 % less than 2 % 3.5 Protection and filter elements

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The functional requirements of protection and filter elements are to ascertain stability of the sand body. The outer layer must be stabie the prevailing open boundary conditions, viz. waves, tides, current, and precipation. The inner layer should protect the base material to washed out (filter requirement).

under wind be

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In view of the fine texture of the fill sand the use of synthetic filters (geotextiles) is indicated.

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Because the (fine) sand body is virtually non resistance against the prevailing sea- and climatic conditions, protective elements should be placed as soon as possible after the hydraulic filling of the dike. This is especially true in the tidal zone, where the sand body suffers from wave attack in combination with rising and falling waterlevels. These combined actions tends to flatten out the side slopes and a considerable loss of sand may occur.

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During the construction phase, the dry part of the sand body is exposed to wind and precipation. Wind attack may cause some sand loss and could cause hindrance to construction operations, for instance as a result of reduced visibility. A temporary protection of the sand body with e.g. straw or soil is often applied in such cases.

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Associated with the use of the sea sand for the dike is the resulting

relative high permeability of the sand body. Especially for those dike

sections where a considerable waterlevel difference will be created (polder

sections) the risk of seepage and piping is introduced. During the

hydrau-lic filling of the upper parts of the dike also a considerable head

difference results. This imposes strict requirements on the sand-tightness

of the filter and protection elements. The sand-tightness criterion should

be combined with a sufficient water permeability of the elements to avoid

uplifting water pressure.

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20 §

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u Q) GIl I GIl GIl o ~ u ... ~

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Cl. Q) u § (..)

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4. CONSTRUCTIOR METIIODSWITa SEA-BORHE EQUIPMEHT

4.1 General

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The choice of execution method and type of equipment is determined by several factors which can broadly be divided in three groups: project

requirements/constraints, conditions of the borrow area and characteristics

of the borrow area. In table 4.1 a general overview of factors which

influences the choice of execution method and equipment is presented.

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Table 4.1 Factors influencing the choice of execution method andequipment type

Factor: Influence on:

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a. Project requirements/constraints

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- type of fill and quantity

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- environmental restrictions

in borrow area

choice of transportation method (for example direct placement by pipelines vs. dumping by barges) choice of type and number of plants in production system

transportation method. (for example pipeline transport with or without boosters vs. barge or trailer) method of dredging

- desired completion time - transportation length

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b. Conditions borrow area

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- total transportation length from borrow area to fill site - wind, waves and currents - water depth and dimensions

borrow area

transportation method workability

type of dredger

---c. Characteristics borrow material

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- porosity, cohesiveness- thickness of layers to be

dredged

- grain size distribution

excavation method

type, mobility of dredger

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capacity of production system disposal methods at fill site

quality of fil!

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In the next sections the influencing factors will be reviewed starting off with a brief overview of common dredging equipment.

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4.1 Overview of dredging equipment

In general dredging processes can be divided in three consecutive stages:

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a. !he excavation stage, during which the material is dislodged from the bottom (primary extraction), raised to the waterlevel and deposited in the means of transportation (secondary extraction). b. !he transportation stage, during which the dredged material is

carried from the dredging site to the disposal site.

c. !he disposal stage, during which the transported material is actually placed in the disposal or reclamation site.

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All of the three processes are interrelated, but depending on the actual

project demands, process a} or c} maY be regarded as the primary process reflecting the main purpose of the dredging work. For the Saemankeum project the disposal stage is the primary process.

A general review of main dredging plant, including both mechanical and hydraulic plant, used in common dredging practice is given below.

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Mechanical dredging plant, such as dipper dredges, backhoes, grab dredges and bucket chain dredges have the follawing general characteristics:

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low capacity, either due to low installed horsepawer or due to the discontinuous extraction process

extraction process easy for fine sand and silt, but low production due to spoiling and loss of the finer fractions

low sensitivity towards cobbles and boulders

always barge transport required from dredging site to disposal site

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The Saemankeum project requires large volumes of fill mate rial to be

dredged. This effectively rules out the utilization of the above described mechanical dredging plant.

Hydraulic dredging plant includes: plain-suction dredges,

trailing-suction hopper dredges. cutter-suction dredges

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In plain-suction dredging the seabed material is dislodged by the combined

activities of gravity forces and water jets. Those dredges have the following general characteristics:

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not suitable for cohesive soils, densely packed sand and sand layers

with a lower than hydrostatic pore water pressure.

the productivity increases with the bank height of the sand layer to be dredged. For smaller bank heights the deep-suction dredge is replaced by a dustpan dredge, which is characterized by a wide suction mouth and increased water jet power.

in free flawing sand plain-suction dredges produce at lower unit rates than cutter-suction dredges, because of the lower costs associated with the extraction stage of the dredging process. Hawever, for projects involving relatively large transport distan-ces, total unit rates do not differ significantly because the partial costs associated with the extraction stage of the dredging process are relatively unimportant compared to the partial costs associated with the transportation.

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The trailer-auction hopper dredge is at present the only dredge, which can operate in longer period waves above 1 m significant wave height. Trailers are self-propelled ocean-going vessels. The dislodged seabed material is pumped into the vessel's own holde When the hold is filled with spoil, the

trailer sails to the disposal or reclamation site, where the load is either

dumped via bottom valves or pumped ashore. As result of the large

invest-ments associated with the dredging and transportation functions of the

trailer, the reclamation stage is a relatively expensive part of the entire

process. For larger quantities it is more economical to dump the material

in a rehandling pit in front of the reclamation site and to utilize a

separate hydraulic dredge for the actual reclamation stage.

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The cutter-suction dredge is known as the workhorse of the dredging

industry. It is by far the most common and versatile of all dredges. The

cutter-suction dredge or ·cutter· combines the advantage of hydraulic

dredges (continuous dredging process and thus high transport capacity) with

the advantage of some mechanical dredges (ability to dredge harder

mate-rial).

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4.3 Selected options

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Considering the above characterizations of major dredging plant and the prevailing site and project conditions, the following options for the extraction, transportation and disposal stages are selected for the hydraulic filling of the seadikes:

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Option Excavation Transportation Disposal

1 Cutter dredge pipeline discharge pipe

2 Cutter dredge pipeline underwater discharge pontoon

dumping 3 Cutter dredge barges

For the excavation stage the cutter dredge is the obvious choice. The most suitable borrow sand is found in the upper layers of the subsoil up to a depth of 5.0 to 10.0 m below the seabottom and is at some locations densely packed. Plane-suction dredging is therefore less attractive. The envisaged near-by location of the borrow area's rules out a cost-effective use of trailers.

Suitable cutter dredges can broadly be divided in two typical categories with characteristics as listed in table 4.2. The categorical division is based upon the composition of the dutch based dredging fleet. In view of the project dimensions and demands 'small' cutter dredges are left out of consideration.

Table 4.2 Main characteristics of cutter dredges

Characteristic Medium Large

Power on cutter 700 - 1000 HP 2,000 - 3,000 HP Power on pump 3,000 - 6,000 HP 8,000 - 10,000 HP

length over all 75 m 105 m

Width 12 m 18 m

Minimum draught 2.5 m 4.5 m

Maximum draught 3.5 m 5.5 m

Max. working depth 20 m 25 m

The choice between medium or large sized cutters will be discussed in section 5.2 of the report.

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For the transportation stage two options are available: pipeline transport or barge transport. The choice between barge or pipeline transport is in first instance a matter of economics. For relative short transportation distances pipeline transport will be cheaper. For longer distances barge transport becomes competitive because production levels of pipeline transport drops with increasing pipeline length and additional booster stations are required. For cutters, as indicated in table 4.2 and for fine sand the break-even point lies in the order at a transportation distance of 7 to 10 kilometer. With the use of modern, high output, cutters the use of pipeline transport is indicated. Option 3 will be less attractive.

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For the disposal stage two options are available: direct discharging from above the high water level or discharging beneath the waterlevel with an underwater discharge pontoon. Both methods are suitable and will be described in section 4.5 of the report.

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4.4 Operational constraints of main eguipment

Operational constraints of seaborne equipment includes draft, workability limits regarding current and waves and nominal production levels regarding transport distances. For the Saemankeum project the major operational

constraint will be the limited workability of cutter dredgers under exposed wave conditions and the transport distance from bo~row area to the disposal area. Potential borrow areas show sufficient waterdepth to allow medium and large cutters to operate.

Operational limits wave and current conditions

In table 4.3 an indicative review is presented of the operational limits of cutter dredgers. These limits are valid for dredging sand with an optimal heading towards the wave direction. As can be noticed besides the wave height also the wavelength is a major influencing factor.

Indicated conditional working limits depend on the cutters' hull dimensions and mass. For detailed analysis of working limits, sophisticated calcula-tion models of (hull) motions under different waveheight and wavelength combinations became recently available in the Netherlands.

Table 4.3 Operational limits of Cutter dredges in sand Significant Wave Height (m) Wave Period (s) Hs < 0.50 m Tz < 6 s 6 - 8 s > 8 s 0.50 - 1.00 m Tz < 6 s 6 - 8 s > 8 s 1.00 - 1.50 m Tz < 6 s 6 - 8 s > 8 s all waves Tz > 12 s Uncond. Cond. No Unconditional working Conditional working No working possible

Medium size Large size

Uncond. Uncond. Cond. Uncond. Uncond. Uncond. Cond. No No Uncond. Cond. No No No No Cond. No No No No

The workability of the cutter dredges is determined considering the above limits and the actual waveclimate. Workability is the least in the winter months November to February, 50 % to 60 % for medium size cutters and 55 %

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The overall workability on an year-round basis for both wave gauge stations is given in the table below.

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Table 4.4 Workability of cutter dredgers. Yearly average Equipment Type Station A

mn

~x Station B

mn

~x

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Medium Size Large Size 65 % 70 % 70 % 85 % 60 % 75 % 75 % 90 %

It is reminded that the above figures on workability are based upon a limited period of measured wave data and are therefore only rough indica-tions. Furthermore it is assumed for above figures that the cutters are operating on spuds. Overall workability can be improved by working on anchors in stead of spuds.

The present current velocity levels do not form a significant constraint for operational conditions of the main dredging plant. They could, however, effect dredging related activities such as construction of submerged

pipeline sections. The location of borrow areas and the pipeline routes should therefore be selected with due consideration to the flow patterns and velocities during all stages of the closure works.

Transport distances and nominal production

The transport distance from borrow area to disposal area is a major

influencing factor for the choice of equipment and transportation methode Without the use of additional pumping stations in the transport line

(boosters) the maximum transport distance of cutter dredges is limited by the instalied pump power. Beyond a certain limit, depending among others on the grain-size of the pumped material, effective production drops and other means of transport may become more cost-effective. Some typical production

levels of cutter dredges in relation to the transport distance are given in table 4.5.

Table 4.5 Typical transport distance limits and production level

---Equipment type

Production in cubic metres per effective pump-hour Distance < 2500 m 3000 m 4000 m 5000 m 7000 m

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Medium size Large size 2700 5000 2500 4500 2400 4000 2000 3500 1500 3000

--

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_---The above figures are based on:

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Most suitable pump impeller diameter for each distance Standard dredging depth

No production limit by the cutter power and swing/step capacity fine sand (approx. 100 microns)

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For distances over 7 to 10 km barge transport becomes competitive. If the average distance for the total quantity is over, say, 4 km the use of (a) boosterstation(s) could increases the cost-effectiveness of pipeline transport.

Because it is envisaged that near-by borrow areas will be used barge transport will be less cost-effective than pipeline transportation.

Only for special cases barge transport could be considered: due to over-flowing of fine material during barge loading, the percentage fines in the fill sand decreases, which is a more favourable condition to meet density requirements.

4.5 Fill stages and sandlosses

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Important aspects when a hydraulic fill is carried out are the slopes af ter filling, the demixing of the material and the accessibility. These aspects are directly related to the mean grain diameter of the fill sand and to the amount of fines in the material.

In the Netherlands, the amount of fines is generally defined as the

percentage of material with a smaller diameter than 64 microns. The amount of fines in the fill sand is off course dependent on the soil characteris-tics of the borrow area, but also depends on how the borrow area was

dredged, i.e. direct pumping or transportation in barges, with the result-ing overflow of the fines.

Acceptable amounts of fines in the fill ranges between 15 to 20%. A too large amount of fines or accumulated pockets of fines are not wanted

because of the poor drainage properties, resulting in extreme low densities and a virtual unaccessible reclamation area.

Construction methods and staging should therefore aim at a reduction of the amount of fines in the fill to an acceptable level and on avoiding ac-cumulation of pockets of fines.

Resulting disposal slopes is dealt with in section 3.3 of the report. In the present study a underwater slope of 1 : 15 is adopted.

With respect to the fill area a principle distinction can be made between an open fill and a bunded fill (see figure 4.1 a).

Discharging in an open fill allows the fines to be washed out of the fill area, resulting in an upgrading of the fill characteristics. This may however result in a relatively high loss of sand.

A bunded fill reduces the loss of sand, but introduces the risk accumula-ting pockets of fines. In view of the envisaged properties of the borrow sand and the relative high construction costs of underwater (gravel or crushed stone) bunds, an open fill is recommended. Reducing the amounts of fines through barge transport or other techniques for the major part of the filling is at present not indicated.

During the filling process a gradual demixing of the material takes place. Nearby the dis charge pipe the coarser material will settie while finer fractions are deposited further down the disposal slope (see figure 4.1 b). To avoid that lower parts of the seadike are filled only with the finer fractions of the borrow sand, the maximum height of the disposal slopes should be limited. In the present study, this maximum height is taken as approximately 10 m.

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Considering the above aspects. the following hydraulic fill stages are advied:

a) below -6.0 m MSL Direct filling with an underwater discharge pontoon in an open fill b) between -6.0 and +3.5 m MSL: Direct filling with discharge pipe

just above high water in an open fill c) above +3.5 m MSL Direct filling with discharge pipe at

required level

The sandlosses occurring during the hydraulic filling are mainly due to the washing out of fines and due to (unexpected) gentle disposal slopes.

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In common practice the minimum loss of sand is approximated to equal the percentage of fines in the borrow sand when direct discharging in an open fill is concerned. Significant losses as a result of current action is not to be expected since the hydraulic fill area is effectively protected by the quarry stone embankment. Af ter the final closure of the seadike however some current flow may occur behind the quarry stone embankment as aresult of the relative high permeability of the embankment and the water head difference. Based upon the characteristics of the borrow sand and based upon general experience the following loss of sand, on an overall basis, can be expected during the distinguished hydraulic fill stages:

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a) b) c)

Direct filling with under water discharge pontoon Direct filling in an open fill

Direct filling upper part

15 % 25 % 25 %

To minimize additional losses as a result of (unexpected) gentie disposal slopes and to maintain quality standards for the fill, the following basic rules for direct hydraulic filling apply:

Avoid the formation of gullies and channels on the disposal slope. Relative steep slopes can be achieved by spreading the sand-water mixture over the full width of the disposal area.

Coarser material will settle nearby the discharge pipe. For wider disposal areas the use of more discharge pipes, simultaneously or alternating in use, is recommended to avoid that a large part of the body is filled with only the finer fraction of the borrow sand. The outer sections, at the side slopes of the seadike, are prefer-ably filled during flood tide (rising water level) and high water levels to assure that disposal slopes are at a minimum.

It is noted that highly experienced contractor's site staff is required at the disposal area to meet the standards.

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29 '.~: _..-' -~. . (OSSot tines ~':':"':"~.•~.-:...;.,;.:z··:-·,.._....;..-_~:..~.:. .._.... _._._ <s>,'. - ,- . ;. ... .:___:

Figure 4.1a Open fill and bunded fill

... ...

-~~

.' '. '" IN~ - ,-:.- :~' ...q

_-

.~-'.-':

~:

_::::--....

...

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4.6 Eguipment availability in Korea and abroad

The Saemankeum projects typically demands the use of medium to large size dredging equipment in view of the exposed working conditions and the total quantity to be dredged. A first impression of the suitability of a certain cutter is given by the total installed power. A minimum requirement of 6000 HP installed power is adopted for the review of equipment availability.

The review is restricted to Korea, Japan, The Netherlands and Belgium.

Country Number of units

over 6000 HP Number of Contractors

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Korea Japan Belgium The Netherlands 3 24 11 30 1 6 3 9

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The Korean based dredging fleet consists mainly out of cutter suction dredges with a large variety of sizes, grab dredges, barges and auxiliary equipment.

The dredging fleet is private owned and contractors has national and inter-national experience in dredging works. At present, only a limited number of cutters would be suitable for the project.

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Compared to the Korean and Japanese dredging fleet, the West-European dredging fleet is at some points more sophisticated resulting in a higher average production compared to installed power. Operational costs are however somewhat higher. In the present report the costs and production level of the dredging equipment will be based upon the appliable standards of the Dutch based dredging fleet.

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5. EVALUATION OF HYDRAULIC FILL CONCEPT 5.1 Recommended methods for hydraulic fill

For the construction of the seadike, 5 consecutive stages are envisaged as depicted in figure 5.1 and described in table 5.1.

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Table 5.1 STAGE 1-STAGE 2

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STAGE 3STAGE 4 -STAGE

5-I

Construction stages sea dike construction

Placing of bottom protection underneath quarry run embankment, if applicable.

Construction of quarry run embankment by truck and/or with seaborne equipment.

Placing of filter layers behind the embankment.

(seadike sections where bottom level is below - 6.0 m MSL) Direct hydraulic fill with underwater discharge pontoon from bottom up to a level of - 6.0 m MSL.

Direct hydraulic filling in an open fill with discharge pipes just above high water.

Direct hydraulic filling of upper part with discharge pipes on a level of + 7.0 m MSL.

Shaping of inner slopes and placing slope protection Construction of seaside slope protection

Construction of seadike crest and shaping to design specifications.

Construction of protective layers.

The following methods are recommended for the hydraulic filling during stage 2, 3 and 4.

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STAGE 2 Direct hydraulic filling with underwater discharge pantoon

Direct filling with an underwater discharge pontoon is only foreseen for the deeper parts of the seadike alignment. These lower parts are filled up to a level of - 6.0 m MSL. permitting sufficient waterdepths during all of the stages of the tide. The underwater discharge pontoon is connected to the delivery line with a flexible floating pipe line section. The pontoon is positioned with the use of 4 anchored cables connected to winches mounted on the pontoon (see figure 5.2).

The discharge pipe is depth-adjustable, lowered or hoisted to the required discharging depth with a cable and is equipped with a diffusor to assure optimal spreading of the fill material.

Principle filling sequence is in a transversal direction. Depending on the maximum cable length in view of the winch capacity the anchors have to be

replaced every 60 to 80 meter.

The pontoon is equipped with a position finding system to aid the filling process.

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STAGE 3 Direct hydraulic filling in an open fill vith discharges pipes just above high water.

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Starting from the land or an already constructed dike section a dam head is built up behind the quarry stone embankment with a level of just above high water (+ 3.50 m MSL). From here the sand body is filled horizontally in longitudal direction. Three discharge pipe lines are envisaged, connected through valves with the delivery line. For convenience the discharge pipes are denoted as 1, 2 and 3 (see figure 5.3). The elevation of the discharge

pipes is + 3.50 m MSL for the two outer pipes and +3.00 m MSL for the

middle one to assure that the sand-water mixture will be concentrated in a forward direction.

To minimize sandlosses a strict working sequence should be applied con-sidering the basic 'rules' for hydraulic filling (section 4.6). The recommended working sequence is as follows:

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During rising water (from 3 hours before until high water) delivery line no 3 is used, assuring minimal (underwater) disposal slopes. The filling is aimed on the creation of underwaterbund as depicted in the figure. This discharge pipe should always be 20 to 40 meter ahead of the other pipes.

During falling"water (from high water until approx. 4 hours after)

dis-charge pipe no 1 is used to fill the area behind the quarry stone embank-ment. The filling will be guided by bulldozers to avoid the formation of gullies. In the meantime, when waterlevel permits, guidebunds are formed at

the waterside by means of a hydraulic crane and wheelloader.

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During the remaining 5 hours of the tide (falling to low water and r~s~ng tide) discharge pipe no 2 is used. Because of the somewhat lower fill level than at the sides the filling is directed in a forward direction and

sideward loss of material is minimized by the bund, built out ahead by discharge pipe no 3. The filling will be guided by bulldozers.

During the filling, work continues at the formation of a guidebund at the waterside of the fill area. The inner slope (at the disposal site) of the guide bund is protected with propypoleen sheet to avoid erosion by the discharge during filling.

Actual shaping of the water side profile according to design specification is done by a hydraulic crane.

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STAGE 4 Direct hydraulic filling of upper part vith discharge pipes on a

level of + 7.0 m HSL.

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During stage 4 two discharge pipes will be used. Before filling at both sides of the area guidebunds are formed by hydraulic cranes. The bunds are protected with propypoleen sheet to avoid erosion. The elevation of the