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1. INTRODUCTION

River and coastal related disasters have increased in both frequency and

intensity in recent years. The effect of sea level rising as a result of global

warming has also become a major concern. As a result, the construction of

dikes, river or coastal protection structures, or the rehabilitation of existing

dikes or coastal protection structures has become an important part of disaster

mitigation strategies.

It is a challenge to geotechnical engineers to develop cost-effective

methods or techniques for river or coastal protective structures that can be

constructed speedily and conveniently, in particular suitable for disaster

mitigation. As dikes or other types of coastal protection structures are long, a

small improvement in the design could result in a significant amount of saving.

Three innovative dike construction methods are use of clay filled

geomats for dike construction, the use of prefabricated, semi-circular caissons

for offshore breakwater, and the use of suction caisson for dike construction.

Each method has its own merits and demerits.

2. DIKES OR BREAKWATERS

Dikes and breakwaters are coastal protection structures. The purpose of

offshore breakwater or dike is to prevent storm waves from reaching the beach

and to resist flood, to protect harbour, to avoid beach erosion etc. Now a day the

coastal related disasters are increasing. So these protection structures have to be

constructed in a cost effective manner. The three innovative methods of dike

construction are explained below.

3. GEOSYNTHETIC TUBES AND GEOMATS

As a counter measure against river and coastal related disasters in recent

years, there has been an increasing use of geotextile and geosynthetic materials

for river and coastal construction. Rubber dam, geosynthetic or geotextile tube

and geosynthetic mats (or geomats) are a few examples of geotextile materials.

3.1 GEOTEXTILES

Geotextiles are permeable fabrics which have the ability to separate,

filter, reinforce, protect or drain. It is made from polypropylene or polyester.

There are several methods for using geotextile or geosynthetic materials for the

construction of coastal structures such as breakwater and dikes.

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3.1.1 USE GEOTEXTILES ACTING AS FORMWORK FOR CEMENT MORTAR

UNITS CAST-IN-SITU

The mortar mix needs only be of sufficient compressive strength to

support the weight above and the moment from the side force of the waves.

Since the subsequent deterioration due to UV rays or other conditions is of little

concern. Thus the method tends to be cheaper than the conventional method.

3.1.2 USE OF WATER OR AIR INFLATED RUBBER BAGS FOR THE

CONSTRUCTION OF SMALL DAMS FOR IRRIGATION OR FLOOD CONTROL

PURPOSE

This method is normally used

only for small dikes up to 6m high.

This method has an advantage of

adjusting the height of the dike

easily by pumping and releasing air

or water in to rubber bags. For this

reason this method has been used

recently for the Ranspot storm

barrier in Netherlands. This rubber

dam in this application is 8.2m

above the base when it is full

inflated.

3.1.3 USE OF SAND OR DEHYDRATED SOIL FILLED GEOSYNTHETIC TUBES

One shortcoming with the use of geotextile tubes for the construction of

dikes is the difficulty to maintain the lateral stability of the dike. To overcome

this problem, sand or sandy soil is filled in the geotextile tube. For near shore or

off shore projects suction dredger is used to pump sand from the seabed or a

sandpit directly in to the geotextile tubes. Silty clay or soft clay has also been

used when sand is not readily available. In this case clayey fill would have to be

in slurry state in order to be pumped and flow in the tube. After pumping the

slurry in the geosynthetic tubes it act as a cheese cloth. Allowing seepage of

liquid out and retaining the solid particle. In this case the selections of the

geotextile used for the tubes or mats become important. The geotextile has to be

chosen to meet both the strength and filter design criteria. The apparent opening

size (AOS) of the geotextile needs to be selected to allow the pore pressure to

dissipate freely and yet retain the soil particles in the geosynthetic tubes or

geosynthetic mats.

Fig.1 water inflated rubber dam used for a river

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Fig.2 Sand filled geotextile tubes for shoreline protection

3.1.4 USE OF GEOMATS

It is difficult to maintain the lateral stability of the dike when geotubes

are stacked one on the top of another. One innovative idea developed recently is

the use of geotextile mattresses or geomats. In this case, the soil filled geotextile

bag form a mattress with its lateral dimensions much greater than the vertical

one. The use of geomats will overcome the lateral stability problem.

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3.1.5 USE OF GEOTEXTILE TUBES TO CONTROL BEACH EROSION

The beaches of the Northern coast of Yucatan in Mexico have been in a

permanent erosion process that has dramatically increased in the past 15 years.

Any restoring action did not affect the natural dynamic process that relates wave

climate-bathymetry-sediments. The solution was to generate a sand

accumulation process without interrupting alongshore sediment transport. Also

the solution had to be as flexible as possible, avoiding any rigid structures, so it

would easily absorb any physical media modification. Under these conditions

Fig.3 A picture showing the formation of a

dike using clay slurry filled geotextile

bags

Fig.4 A concrete mattress was cast in-situ with

geotextile as a mould to cover the dike

made of geomats.

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and for critical points geosynthetics were considered optimal for the beach

restoration project. Woven polypropylene geotextile tubes were designed to

work as low crested submerged structures. Their main function was to reduce

the incident wave energy on the beach, by controlling the wave breaking

process to the required level that maintains the dynamic balance on the

shoreline.

4. PREFABRICATED CAISSONS

The geotextile tube or mattress method may only be feasible

when dikes are to be constructed in relative shallow or quiet water. When water

is too deep or the wave is rough, gravity retaining structures using prefabricated

reinforced concrete segments or caissons may be a better option.

These concrete segments or caissons

have to be tall enough to match the

water depth and heavy enough to

provide stability against the waves.

When the concrete segments or

caissons are too heavy, the caissons

will cause settlement or bearing

capacity problems. This is particularly

the case when the foundation soil is

soft. The weak foundation soil can be

improved. However, it is difficult and

costly to treat soil offshore and over a

large area or distance. large area or distance.

Fig.5 Geotextile tube inducing wave

breaking for energy attenuation

Fig.6 View after sand accumulation

shoreward of geotextile tubes

Fig.7 Prefabricated caisson supported by

rubble mound and scour protection cover

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The concrete would provide a noncorrosive structure. The caisson would

be cast on the shore and floated into position. If the seabed is of soft soil it has

to be treated by using prefabricated vertical drains (PVDs) before the caisson

was installed. The semi-circular shaped caissons are more preferable.

Advantages of semi-circular shaped caissons are

1. The water pressure acting on the semi-circular shaped surface pass

through the centre of the circle. Thus the overturning moment

becomes much smaller and the vertical pressure distributions at the

base of the caisson become more uniform.

2. The lateral wave force acting on a semi-circular shaped breakwater is

smaller than that on a vertical breakwater of same height and thus the

cost for the construction of breakwater can be reduced.

In Yangtze Estuary coastal protection project, some dikes for navigation

purposes needed to be constructed. It was constructed at 40 km away from the

coast. The water depth ranged from 5.0 to 8.5 m. The design wave height was

3.32-5.90 m with a return period of 25 years. The total length of the dike was

about 17 km. The typical soil profile below the dike consisted of a 1.5-3.5 m

thick silt sand followed by 2-4m thick muddy clay and a layer of soft clay of

roughly 30 m thick underlying the muddy clay. The seabed soil was treated by

preloading using prefabricated vertical drains (PVDs) before the caissons were

installed. The caisson used was prefabricated reinforced concrete hollow

segment. It was semicircle shaped. The radius of the semicircle was 5.7m. The

advantage of using a semi-circular cross-section is that the direction of the

resultant wave force on the semi-circular shaped structure will always pass

through the centre of the circle, which will greatly improve the loading

condition of the structure. The hollow caisson would be filled with sand after

installation through a 600 mm diameter hole on top of the caisson. In order to

prevent the foundation soil from scouring, a geotextile sheet was used to cover

the seabed. A cushion which acted as the foundation bed was placed on top of

the geotextile. The cushion was 1-2m high. It was made of crushed stones of 1-

100 kg for the centre and 200-400 kg for the edge. After the caisson was placed,

berms were placed on two sides of the caisson. The berms were made of 400-

600 kg crushed stones.

5. SUCTION CAISSONS (Skirted foundation/buckets/suction anchors)

One of the disadvantages of the prefabricated concrete caisson method as

described above is the need to treat the soft seabed soil and the construction of

rubble mound which can be time consuming. Thus, the gravity caisson method

is not suitable for disaster mitigation purposes. Another method of building

breakwater or seawalls is the use of cylindrical steel or concrete suction

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Piles or caissons developed by the Tianjin Port Corporation, China. This

method is particularly suitable for the construction of breakwater on soft seabed

or in deep water. It takes less time to install than piles and being easier to

remove during decommissioning. So suction caissons are used extensively for

anchoring large offshore installations to the seafloor at great depths.

Suction caissons are a better alternative to driven piles in deep water

because of technical challenges and costs associated with the installation

equipment for driven piles. Heavy lift vessels can be avoided, simplifying and

shortening the installation procedure. Another advantage is that there is more

control over the installation process. Therefore, the location of the anchors on

the seabed is fixed and known with accuracy. Suction caissons also provide a

greater resistance to vertical and lateral loads than driven piles and drag anchors

because of the larger diameters typically used. Initial penetration of the suction

caisson into the seabed occurs due to the self-weight. Subsequent penetration is

by the “suction” created by pumping water out from the inside of the caisson. A

submersible pump attached to the top of the sealed caisson applies suction

pressure. By evacuating water from the inside, a pressure differential is created.

The limiting value of this pressure differential, such that cavitation does not

occur, is the sum of the atmospheric pressure and hydrostatic pressure outside

the caisson. In very deep waters, large penetration or suction pressures can be

created, which is only limited by the capacity of the pump. Once the required

depths are reached, the pumps can be disconnected and retrieved.

Fig.8 Layout of the suction caissons: (a)

viewing from the bottom of the suction

Caissons; (b) viewing from the top of the

concrete suction caissons with the top

Cylinders used as part of the breakwater

Fig.9 Installation of concrete suction

caissons.

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This method has been used recently in the Tianjin Port, China for an offshore

breakwater project.

The soil profile at the site consisted of up to 25 m soft soils where the

water content of the soil was generally higher than the liquid limit of the soil.

Four 12m in diameter prefabricated reinforced concrete cylinders were

connected together using 4 concrete walls to form one unit as shown in Fig 8a.

The top opening of the four cylinders was sealed by a precast concrete plate as

shown in Fig 8a. The unit formed by the four cylinders was towed to the

required position and sunk onto the seabed by ballasting. Suction was then

applied simultaneously to the inner chambers of the four cylinders which were

sealed at the bottom by the seabed clay. Under the suction and the hydrostatic

pressure, the four cylinders were dragged down to penetrate into the seabed soil.

The suction was removed after the penetration depth of the cylinder was

sufficient. The amount of penetration required was calculated in accordance

with the bearing capacity that had to be provided by the shaft friction. After the

4 cylinder units were installed, another row of prefabricated concrete cylinders

of the same diameter was installed on top of the bottom units as shown in Fig

8b. The operations for the installation of the lower 4 cylinder unit and the upper

cylinders are presented in Figs. 9 and 10 respectively. The breakwater after

installation is shown in Fig. 11.

Although a new development, the use of suction caissons for offshore

dike or breakwater construction has shown a number of advantages over the

existing methods. In addition to avoiding the treatment of the seabed soil, it also

seems to be more economical as compared with the semi-circular concrete

caisson method. The settlement of the caisson is also much smaller. However,

this method may only be applicable to sites with a relatively thick layer of soft

soil. Furthermore, it required special machines to cast, transport, and install the

suction caissons. More study on the design and analysis aspects is also required

before this new approach can be developed into an established technology.

Fig 11. Breakwater after installation. Fig. 10. Installation of upper cylinders to

form breakwater.

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6. CONCLUSION

The main features of each method are

1) The geotextile mattress or geomat method is suitable for the construction of

breakwater or dikes in shallow water. In this method, the dike is constructed by

stacking several layers of flat geotextile mattresses that are formed by pumping

either sandy or clayey slurry into pre-sewed geotextile bags. The main

advantage of this method is that it can provide much better lateral stability as

compared to the geotextile tube method.

2) The prefabricated concrete caisson method is suitable for the construction of

seawall or sea dike in relatively rough water. However, when the seabed is

relatively weak, the soil has to be improved.

3) The suction caisson method is new, but promising. It is particularly suitable

for dikes constructed in relatively deep water. With the use of suction skirt, the

treatment of soft seabed soil is no longer required. However, more field

experience needs to be established. More theoretical studies are also required to

guide the design and construction.

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7. REFERENCES

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