Geotube
Geotube
Geotube
Jack Fowler, Ph.D., PE1, Thomas C. Stephens2, Mario Santiago3, and Pieter de Bruin4
ABSTRACT
The Amwaj Islands Project, Venice style resort involves the development of a new island off the northeast shore of
Muharraq Island in Bahrain. The key element in the successful design, construction and completion of the Amwaj
Island development project was the use of sand filled Geotubes to form the island perimeter for containment of 12
million cubic meter of dredged sand that formed the basic platform for the development project. The island
perimeter was constructed by hydraulically filling Geotubes with local sands from proposed navigation channels
and marinas serving residents and businesses on the main island of Bahrain. The development is strategically
located on the northeastern end of Muharraq-Bahrain between the Persian Gulf and Bahrain Sea. Amwaj Island
development will provide a variety of amenities such as living in beach front properties, hotels, restaurants,
recreation parks, theaters, marinas, and golf courses. The island will also provide a neighborhood mall and
entertainment center and will not only cater to the young but will also accommodate the elderly as a retirement
destination.
The Ossis Property Developers (Ossis) is responsible for land reclamation of 2.79 million square meters of open
sea in a shallow reef area to form land at the north east of Muharraq Island, Bahrain. Engineers from Ossis, TC
Nicolon Corporation in the US and The Netherlands were responsible for the planning and design of the sand
filled Geotube containment structures for the island perimeter and shoreline and foreshore protection systems.
Submerged Geotubes are also to be filled with sand along the islands northwest, northern, and northeast perimeter
and to be protected with rip rap stone to serve as offshore segmented breakwater structures. Approximately 10
kilometers of 2.0 to 2.6 meter high Geotubes are being used to successfully construct this project. Infrastructure
developments included access roads, bridges, marinas, communication systems, electrical power network, water
supply, and sewage collection and treatment systems.
The project cost for development of the entire land area for phase one was $70 million for land cost, $80 million
for land development, and $8.2 million for contingencies for a total cost of $158 million ($57 per square meter).
The net land area useable for development after accounting for lagoons, wastage and service requirements was
only 50 percent.
Keywords: Dredged Material, Island Perimeter, Containment Island, Disposal Area, Geotubes, Geotube
Breakwater Structures
INTRODUCTION
One of the first Geotube applications in Bahrain was for the construction of a dredged material containment island
designated as Amwaj (Arabic for waves). The site (Figure 1) is strategically located 1.6 kilometers off shore at the
northeastern end of Muharraq-Bahrain. This location along the islands is boasted to be the purest and clearest of
waters along the Bahrain coastline. The development consists of dredging cohesion-less ocean sediments
reclaimed from proposed navigation channels and marinas. The reclamation project will provide new land for
associated civil engineering infrastructures for the construction of five and four star hotels and restaurants, golf
course, homes, condominiums and many amenities. The island perimeter consists of various
Geotec Associates, 5000 Lowery Rd, Vicksburg, MS 39180, 601-636-5475, Geotechnical Consultant
TC Mirafi Corporation, Commerce, GA
3
Ossis Property Developers, Bahrain
4
TC Mirafi BV, Almelo, The Netherlands
2
The geotechnical site investigation consisted of boreholes, in situ testing and laboratory tests and an appraisal of
ground and site conditions and recommendations for foundation design and construction. The borings were
located near proposed bridge abutments where piling would subsequently be used for support. All field work and
laboratory work was supervised by a geotechnical engineer from Al Hoty Analytical Services, Manama, Bahrain.
Field Work
Field work consisted of drilling five boreholes with a combination cable tool percussion (shell auger), boring and
rotary core drilling methods. Boreholes were drilled from a flat deck barge and water was added to assist in boring
and as a flushing medium for the rotary coring. The boreholes were distributed uniformly over the foot print of the
island. Depths of boreholes varied from 17.5 to 20.0 m. Standard Penetration tests were conducted to determine
the relative density of the essentially granular soils and in-situ strengths of the cohesive solids and rock. The
resulting N values varied from 22 to 83 blows per 450 mm. Two of the borings indicated some loose grainular
materials and there were some carbonate rich sediments that are normally described as clay but were describe
here as mud.
Laboratory Tests
Laboratory tests included particle size distribution of twenty one representative samples of the borings. The
particle size distribution test indicated that the materials were principally granular material varying from a fine
medium sand to a medium to coarse gravel. Unconfined compression test were carried out on nineteen specimens
of rock cores together with bulk density and moisture content measurements. Point load test were performed on
twelve irregular shaped rock samples and stress vs. strain tests on four other samples. Chemical test for sulphates,
pH and chloride concentration were conducted on water samples.
Foundation Conditions
The main island of Bahrain consists of a partially eroded dome of sedimentary rock deposits that are of the Eocene
age and are flanked by formation of the Pleistocene and recent age. Geological records indicate that this site is
underlined by the Ras Aqr formation. This under-layment consist of cap rock limestone/sandstone, mud flood
deposits (unconsolidated soils) and carbonate rich, fine grained, rock formation.
Medium dense sand was encountered on the sea bed at borehole 1 and 4 to depths of 3.20 m and 4.6 m
respectively. The sand from the sea bed was slightly silty to fine to coarse grained, and contained gravel size shell
debris and fragments of calcarenite (carbonate sandstone). In borehole no. 2 the materials became slightly clayey
below 1.0 m and below 1.50 m to 2.50 m they became more loose and granular.
Boreholes nos. 2, 3 and 5 penetrated through layers of moderately weak to moderately strong calcarenite
(carbonate sandstone). This layer was thin to medium bedded and vuggy, i.e. it contained numerous scattered
small cavities. These layer thicknesses varied from 0.30 m (borehole no. 2) to 0.50 m (borehole no. 3).
A bedrock was encountered directly beneath these materials and it contained calcisiltite (carbonate siltstone) at
all boring locations. The calcisiltite was very weak to moderately weak, sand and often clayey and thinly to
medium bedded. Some inclusions of clay calcarenite, calcilutite (carbonate mud stone) and siliceous sand stone
were found. Borehole no. 2 passed through a stratum of weak sandy, very silty, calcilutite layer at 17.00 m to
17.50 m below the sea level bed. All other boreholes penetrated through the calcisiltite stratum at depths of 19.4
to 20.00 m.
Foundation Design
Detailed foundation design on the reclaim areas for residential, hotels, restaurants, condominiums, roadways,
bridges and leisure uses are still in the preliminary design phases. The five boreholes were located at the bridge
sites spanning between a number of artificial islands. No structural details are available at this time. Where
foundation materials are loose or soft, spread footings and pile support will be required.
ENVIRONMENTAL ISSUES
Environmental issues were address through an environmental impact study during the planning and design of
Amwaj Island prepared in a Master Plan and Environmental Impact Assessment by Buro Happold in February
2001 (Happold, 2001). Their report provides the hydraulic boundary conditions for the project. The existing land
on the Bahrain Island consists of only 2 percent arable (fit for tillage), 2 percent permanent cropland, 6 percent
pasture land, no forest land and 90 percent other uses. Only 10 square kilometers of land is irrigated. Current
environmental issues desertification resulting from the degradation of limited arable land, periods of droughts and
dust storms are of concern. Coastal degradation, damage to coral and sea vegetation caused by oil spills and other
discharges from large tanker, oil refineries, and distribution stations have been monitored and controlled. Bahrain
has no natural fresh water resources therefore water needs are limited to ground water and sea water. Periodic
dust storms and droughts are natural hazards when residing in arid areas. Close proximity to Middle Eastern
petroleum resources and its strategic location to the Persian Gulf by which much of the Western worlds petroleum
must travel to reach open oceans also expose Bahrain to limited environmental risk.
Prior to construction approval of the Amwaj Island Development these issues and potential turbidity issues during
dredging and filling the Geotubes were addressed. A turbidity curtain was used during dredging and filling of the
Geotubes to control turbidity. Damage to the sea bottom resulting from dredging and placement of dredged
material was also mitigated for the development.
TURBIDITY MITIGATING MEASURES
The contractor agreed to observe a number of limitations and take appropriate measures to ensure minimum
disturbance and siltation to the surrounding aquatic environment. Since environmental issues were of prime
importance, the owner opted to use Geotubes instead of a quarry rock retention dike. The Geotubes themselves
were considered to aid in controlling the turbidity during dredging and reclamation. The contractor agreed to
provide turbidity curtains for siltation control during dredging in order to control silt emissions to the limits of the
contract specifications.
The turbidity curtains are essentially an in-water silt fence that is designed to prevent the flow of the disturbed sea
bottom during dredging and filling the Geotubes. They are not designed to hold back the current flow but are
designed to control the turbidity and silt migration caused during dredging and reclamation.
Wave breaking offshore is critical to the wave intensity at Amwaj Island. If the waves break off shore this means
that the wave will not propagate onto the shelf. The wave height on the reef platform is restricted by the water
depth. This wave breaking process is depth limited and provides considerable protection to the reef platform,
shoreline and the proposed development. The depth limited wave height versus water depth for MHWS for an
extreme storm condition, where there is a 1/50 per year storm surge on the reef platform is shown tabulated below:
Condition:
Prevailing annual
Extreme storm (1/50)
2.4
3.9
1.9
3.4
1.5
2.7
breakwaters, G is 0.25 times Lr or about 75 m. The distance offshore is X = Lr = 300 m. The distance to the beach
shoreline is about 340 m from the hard boundary of the island. The principles and definitions of the reef design are
shown in Figure 5. There are eleven 300 m long segmented Geotube breakwater reefs planned as off shore
protection.
Figure 6. Offshore Breakwater Structure for the Shallow and Dewater Areas
The two layers of sand filled Geotubes will form the perimeter of the proposed hydraulically filled containment
island and roadways for the Amwaj Islands project (Figures 7 and 8). During filling of the Geotubes, excess
dredge water was filtered by the close weave pattern of the Geotube geotextile fabric. As the dredged material
was pumped into the Geotube, it formed its own filter medium that filtered fines less than 0.063 mm. This filter
cake was formed on the inside of the Geotubes very quickly allowing only clear water to exit. Minimum
disturbance was caused by the dredging operation but this disturbance was mitigated by the application of the
turbidity curtains.
TEST METHOD
Apparent Opening
ASTM D 4751
Permeability
ASTM D 4491
Puncture
ASTM D 4833
Wide Width Tensile
ASTM D 4595
in both principal
directions
Wide Width Tensile
ASTM D 4595
Elongation in any
Principal Direction
Burst Strength
ASTM D 3786
Trapezoidal Tear ASTM D 4533
Ultraviolet Degradation ASTM D 4355
(percent of ultimate strength retained
Seam Strength
ASTM D 4884
Weight per Square
Meter
UNIT
MINIMUM AVERAGE
TEST VALUE
VALUE
U.S. Sieve
sec-1
kN (lbs)
kN/m (lbs/in)
#40
0.20
2.67 (600)
175 x 175 (1000 x 1000)
15(maximum)
kPa (psi)
kN (lbs)
80%
10,320 (1500)
2.67 x 2.67 (600 x 600)
kN/m (lbs/in)
gr/m (oz/sy)
10
11
12
13
14