3.5.2 Initial Water Content
In the same study of confining pressure by Liao and Bhatia (2005), the water content
V increases from 100% to 400% as the maximum flow rate increases from 4 to 12 cm/min (Figure
3.32). The author concluded that the filter cake formed above the geotextile is thinner for the
slurry with higher water content and it is easier for the soil particles to pass through the geotextiles.
The conclusion is supported by final height of the soil column with a pressure of 35kPa after the
tests was 7.52, 4.45, 2.63, and 1.77cm for the specimens prepared at water contents of 100%, 200%,
300%, and 400%, respectively. v
^^ , fUA^
^-—^ rf ^
3.5.3 ^CoHtent of Air Bubbles J)
In the-past, th^pHrllcfe size and its distribution were assumed to be the critical properties
which affect the filtration rate. Kakwani (1983) observed that entrapped air bubbles could also
cause significant changes in filter cake structure and adverse effects on the filtration process.
The number and size of entrapped air bubbles are evaluated in the filter cakes. In the study of
Chi (et al., 1985), the particle size of the coal samples and other variables are held constant in
order to distinguish the effects of entrapped air bubbles. The size of coal sample fractions is
classified into -80 mesh, -42 mesh, -32 mesh, and -100 +200 mesh filter. These
"surface-volume mean particle sizes" (XP°rl), which is the diameter of a sphere having the same
surface-to-volume ratio as the particle, for four samples are 41.8 urn, 81.6 urn, 120.1 urn and
The experimental results give a visual image of the entrapped air bubbles inside the filter
cake shown in Figure 3.32. The air bubbles are easily recognizable under the microscope, and
it is black in circles with a white spot at the center of the two-dimensional cake sections (Chi et
and cake permeability decrease from a factor of 2 to nearly 4. When the volume percentage of
air bubbles increases from 0.5% to 3%, the filtration flux and cake permeability decrease by a
factor of 2 to 2.5. For dewatered cakes, the maximum percentage of entrapped air bubbles is
less than 2% with a narrow size distribution range from 150 to 240 um of air bubbles of
dewatered cake pore volume. The distinction of the volume percent of air bubbles in 'aerated',
'normal' and 'deaerated' dewatered cakes ranges from 0.4% to 2.0%, 0.4% to 2.0% and 0.6% to
0.9%, respectively, and are not as easily found as those in comparable filter cakes is 1.0% to
4.3%, 1.2% to 5.2% and 0.4% to 1.5% (Chi et al., 1985).
The size and volume percentage of air bubbles increase from the bottom to the top for
each filter cake. The increase is explained because air bubbles tend to rise in the filter cake
due to the buoyant force in the early stage of cake formation. Bigger air bubbles have larger
buoyant forces. Therefore, air bubbles migrate upward and are trapped in the upper layer of
the filter cake. The mean size distribution of air bubbles is different in dewatered cakes than in
filter cakes and dewatered cakes tend to have a more uniform size distribution. The rate of
filtration and tjredewatering are both adversely affected by the air bubbles in the filter cake
which is caused by the lack of connectedness with pores in cake matrix.
Chapter 4 GEOTEXTILE TUBE STATE OF THE ART
This section discusses designing geotextile tubes and testing that has been used to
evaluate the filtration behavior between geotextiles and soils.
4.2 Behavior of Geotextile Tubes
The parametric study done by Leshchinsky (et al., 1996) was used to gain a better a
understanding of the effect of different design parameters. In this study, the relationship among
ultimate tensile force, the height of the tube, and the pumping pressure will be discussed.
4.2.1 Ultimate Tension
Figure 4>U$nows the effects of the specified tensile force in the circumferential direction / / J
of the geotextile tube. When the cross section is a perfect circle, with diameter equal to
D YM1 = 2-9 m (9.5 ft), the required tensile force and pumping pressure increase toward
infinity. However, when T is as low as 14.6 kN/m (1,000 Ib/ft), the height is only 1.8 m.
Thus H is 63% of the maximum theoretical height, D. By increasing T to 87.9 kN/m
(6,023 Ib/ft), the tube height would increase to 2.6 m which is 89% of D. The cross sectional
area has little influence, while the height changes. This information gives clear design
implications if a certain volume of slurry is needed (Leshchinsky et al., 1996).
* Circumference of tube, L = 9 [m]
* No outside water
* ' slnsy/ 'water — 1.2
* No safety factors on geosynthetic strength
= 87.5 kN/m j
= 875.0 kN/m = 0.83
— -0 98
W po = 52.4 kPa
po = 593.4 kPa
* Area= 6.51m2
Area -6.66 m2 = 2.6m
H = 2.9m
w =0 - 50
p0 = 4.8kPa
Area =5.56 m2
H = 1.8m
Figure 4.1: Effects of TU|t on Geometry of Tube (Leshchinsky et al., 1996).
Figure 4.2 illustrates the influence of the designed height H of the geotextile tube^, If
the desired tube height is 0.9 m, about 31 % of D, the required pumping pressure is nearly zero.
The required circumferential force is 2.6 kN/m (178 Ib/ft). However, if the desired tube height
is 2.7 m, about 94% of D, the required pumping pressure is about 122.8 kPa (17.8 psi). The
required circumferential force is approximately 190 kN/m (13,019 Ib/ft) (Leshchinsky et al.,
"After the pumping and as the slurry consolidates, experience indicates that the height
of the tube drops, while its maximum width increases very little" (Leshchinsky et al., 1996).
The dropping height of the tube can be very large, especially with fine-grained soil. An
H = Initial height of tub e
A H 0 = Change (drop) m height of tube
/T<$$ .. )' 0= Wtial sl«ny tatit Wii^tt ••/ X,
)* J '•'•
1.1 L2 U 14 IS & 1.7
( «sofl/ 'w/f
Figure 4.3: Drop in Height of Tube as Function of Density of Soil (Leshchinsky et al.,
4.2.3 Pumping Pressure
Figure 4.4 shows the effects of the pumping pressure of the geotexifile tube. If the
pumping pressure (po) is 34.5 kPa (5 psi), the tube is 2.5 m in height and 61.7 kN/m (4228 Ib/ft)
in circumferential force. However, if the pumping pressure increases to 103.5 kPa (15 psi), the
tube becorare J.I m in height, and the circumferential force increases sharply to 462.0 kN/m
mjfe Z7 it
(31,6551b/ft). From the observations of Leshchinsky (et al., 1996), pumpingxp'ressure beyond
35 kPa (5 psi) would not increase the tube height significantly, but the/gfrengthajbplied to the
geotextile increases exponentially. Typically, a pumping dredge line is about 300 kPa (44 psi)
or more. Strict field control is needed since the pumping pressure can build up a force which
can rupture the geotextile.
* Circumference of tube, L= 9 [in]
* No outside water
* ' slurry/ 'Wafer — 1 . 2
* No safety factors on geo synthetic strength
Tljlf = 462.0kN/m
w=°- 9 0 H = 2.5m
H = 2.7m
Area = 6.57rnz Area= 6.45m2
po = 6.9 kPa
= 18.1 kN/m
Area= 5.7r5 m2
Figure 4.4: Effect of po on Geometry of Tube (Leshchinsky et al., 1996).
Figure 4.4 and Figure 4.5 illustrate the relationship between the height of the tube and
the pumping pressure. It can be seen that p0 has the most significant influence on height at low
rC. Therefore, as the pr
relationship approaches the asymptote of H = D only when p0 increases/o infinity (Leshchinsky
et al., 1996).
Figure 4.6 shows the effectSyof pumping pressure on both T and Taxja|. It is shown that
as po increases, the difference offlie values between/the circumferential force and the axial force
become larger. The usage oranisotropic strength/can be economical when selecting
geotextiles with significantly different tensile forces T and Taxja|. Two geotextiles transfer
force through seams. Therefore, seam strength and seam efficiency are important factors
-^__ . ->
(Leshchinsky et al., 1996).
Figure 4.7 depicts the maximum theoretical height and minimum feasible height of a
tube with a circumference L. The maximum theoretical height, Hmax, is equal to the diameter
of a tube with a circular cross section. The minimum feasible height, H m j n , corresponds to a
case where the pumping pressure is about zero. This figure also indicates the range of feasible
heights for given circumferences.
10 15 20 25 30 35 40 45 50
Circumference of Tube, L [m]
Figure 4.7: Extreme Values of Height of Tubes (Leshchinsky et al., 1996).
4.3 Design of Geotextile Throes I (—•>
With its environmental ^iend/y characteristics, geotextile tubes provide a solution to
/ / / *
problems in environmentally^se&smve areas. By sevym-g-^everal geotextiles together in the ,
shape of a tube, it could retain a large amount o materials^ith high water content. /They/are
highly permeable nattfre and their ability to be stacked are the two main advantages of geotextile,
when compared to settling ponds within a limited construction site.
There are numerous articles discussing the formulation for the tensile force of the
geotextile tube, but most are incomplete. Leshchinsky (et al., 1996) proposed an overview of a
complete formulation which indicated tensile force along the circumference and the geometry of
the cross-section of the geotextile tube. In his study, the design of geotextile tubes for strength,
durability, and permeability are provided. The design of geotextile tubes is based on the force
equilibrium of the geotextile. The governing assumptions for the formulation are as follow:
1. The tube is long and the cross-section is perpendicular to the surface. Thus the pressure loss
in the filling process, while draining is ignored, and the pumping pressure is used for the
2. The geotextile of the tube is thin, flexible, and weightless.
3. A hydrostatic state of stresses occurs in the geotextile tube.
4. No shear stresses generate between the slurry and the geotextile tube.
4.3.1 Strength Design
Leshchinsky (et al., 1996) presented a design method based on the force equilibrium along
the circumferential direction of the geotextile. The geotextile tube is assumed to be surrounded
by air and filled with one type of slurry. In Figure 4.8, the cross section is symmetrical with a
maximum height (H) at the centerline, maximum width (W), and a flat base that is in contact with
woven geotextiles, the safety factor of two is recommended (Leshchinsky et al., 1996). The
defective seam may render geotextile tubes ineffective, especially for clayey slurries. Seam
efficiency is defined:
E (%) = Tseam x 100 (Equation 4. 2)
= wide-width seam strength,
= wide-width geotextile strength.
18.104.22.168 Installation Damage
A$6I the installation of a geotextile tube, the high pressure of pumping fine-grained
material may lead to a rupture of the geotextile. Pumping pressures beyond 35 kPa (5 psi)
will increase the stress on the geotextile exponentially. A factor of safety is needed for
pumping pressure uncertainties, flaws in the geotextile material, and poor construction control
by the contractor. Excessive pumping pressure may cause local rupture adjacent to the seam
of the geotextile (Leshchinsky et al., 1996).
22.214.171.124 Chemical and Biological Degradation
Typically, geotextiles are inert to the slurry. The specified test in ASTM D 5322
(Standard Practice for Immersion Procedures for Evaluating the Chemical Resistance of
Geosynthetics to Liquids) can be used to verify the chemical damage cause by the slurry.
Furthermore, since the geotextile tubes might be exposed to the sun every day, the degradation
by ultraviolent radiation isvXso another consideration. The specified test is ASTM D 4355
(Standard Test Methocfior Deterioration of Geotextiles for Exposure to Ultraviolet Light and
Water). Additional carbon black may be added to extend the resistance to the deterioration
slowly caused bjlfU/' Biological degradation is another consideration, but it is not an issue
in most cases (Leshchinsky et al., 1996).
Creep is defined as the elongation of the geotextile under constant load./ In the
dewatering application, the geotextile tube would experience a maximum force from the initial
filling process and the force decreases as the slurry consolidates. However, creep is
continuously influenced by the force within the geotextile and it might rupture the geotextile
tube. The specified test is in ASTM D 5262 (Standard Test Method for Evaluating the
Unconfined Tension Creep and Creep Rupture Behavior of Geosynthetics). Reduction factor^)
.—^if applied to the use of the ultimate strength of the geotextile and this will prevent from creep
of the designed life of the geotextile tube. Reduction factor also depends on the type of
geotextile (Leshchinsky et al., 1996).
4.4 Testing of Geotextile Tubes
4.4.1 Lab Testing
126.96.36.199 Falling-Head Dewatering Column Tests (FHDT)
Since a hanging bag test is hard to observe and evaluate filters cake because of the
possible disturbance during operations. Therefore, a falling-head dewatering column test
the chamber. Then the screws on the upper plate are tightened and the air pressure is applied
promptly to the required pressure. Graduated cylinders are used to collect and measure the
volume of filtrate with elapsed time. After the test is finished, the filtration device is
disassembled. The final height of the dewatered cake is measured and the total settlement is
calculated. Finally the analyses of the dewatered cake and the filtrate are conducted. The
filtration efficiency of this test is calculated by the total suspended solids of the discharge flow
to the initial total solids of the sample. The dewatering efficiency of this test is determined
by the initial percent solids and the final average percent solids.
Pressure Gauge Pressure Itilet
I "" 2a 1
Filtrate Outlet Support
Figure 4.12: Pressure Filtration Test Apparatus (Moo-Young et al., 2002).
188.8.131.52 Rapid Dewatering Test
The rapid dewatering test (RDT) is presented by the company TenCate. The primary
function of this test is to (TenCate, 2009):
• Evaluate the efficiency for different polymers
• Measure the volume of discharged fluid from the sludge.
through the fabric and sediment passing through in an elapsed time. It is also used to
evaluate the properties of the dewatered infill material and to examine the nature of the filter
cake. This test method is used to predict the behavior between the in situ specific fabric and
the specific infilled material. The intent of this test is to provide design information for a
designer or contractor before approving a particular fabric for a particular site (Koerner and
102 cm (40")
', •sanm 2.5«nin
Ho! Cut 163 5mf6»T
Figure 4.14: (a) Hanging bag test apparatus, (b) geotextile before fabrication and (c)
after fabrication (Koerner and Koerner, 2006).
Figure 4.14 shows the apparatus used for this test. A large bag was made by sewing a
fabric along its sides and bottom to form an enclosure that will support and contain a certain
amount of saturated infill material. The volume of the bags was approximately 175 L (40
gal). The bag opening has eight evenly spaced metal grommets around its upper perimeter
and is hung from the inside of the wooden frame. A pan is placed under the container to
collect sediment. The moisture content of the soil within the bag and particle sizes of the soil
passing through the bag can be tested back in the laboratory.
The Procedure) fM$ hanging bag testUvas provided by Koerner and Koerner (2006).
1. The hanging bag is first soaked in to the dredged area for pre-wetting.
2. The bag is attached to the wood frame with a distance less than 8 in. from the bottom of
the bag to the pan below.
3. After the bag has started to drain free water, a pan with a 3 in. deep and 24 in. diameter
was placed-below collecting fines.
4. 10 gaflloi^of site specific material and 40 gallpn^of sue water are prepared. Then the soil
^-^ V_^/ /
*~^4 material and the water are evenly mixed together. ^ //?
5. The premixed slurry is poured into the bag as the stopwatch began. Record the time
every 3 in. the slur
6. Hang the bag until water stops draining.
7. At the end, the pan is removed and the collection was transferred to a stainless steel drying
dish. The dish is brought back to the lab to determine the dry mass of sediment.
8. Measure the mass of the bag and obtain a representative sample from the inside of the
Evaluate the moisture content of the sample back in the laboratory.
9. Calculate the discharge flow rate with the increment of the elapsed time for a slurry level
drop of 3 in.
"No geotextile tube project should be designed without having conducted at least one
HBT under simulated site-specific conditions." (Koerner and Koerner, 2006). This test can
represent the behavior of the geotextile tube and gives the designer an opportunity to test
different flocculants and geotextiles. However, this test contains both hard working and time
consuming. With preparing samples, test setup, unknown dewatering periods, and data
analysis, it might take an entire day to do a single test.
184.108.40.206 Settling and Self-Weight Consolidation Column Test (SSCC)
The settling and self-weight consolidation column test was proposed by the U.S. Amy
Corps of Engineers (USAE, 1987). The purpose of this test is to determine settling velocity
(Vs) and self-weight consolidation coefficients (CF) of silty clay with different initial water
contents. In Figure 4.15, this test is conducted using an acrylic settling column with a height
of 45 cm and 10 cm in diameter (Shin and Oh, 2004).
The soil is prepared with various water contents derived from the mixing ratio of the
soil and salt water mixture. The predetermined amount of dredged silty clay is first measured
and introduced into a settling and self-weight consolidation column. A ruler is attached
vertically to the column to measure the depth from the water surface to the interface of the
water and slurry.^and to recorcTthe self-weight consolidation settling time. The initial
Sfnperature of the" salt' water is measured and compared to the slurry at the end of the test. t
——— " ' f
is difficult to estimate the preliminary settling process, so the SSCC test was performed to
estimate this process. Jfter the preliminary settling ends, the valves are opened, while the
supernatant water is extracted from the column and continue to perform the self-weight
consolidation test?) The depth from the water surface to the interface of the self-weight
consolidation settlement is recorded at different times.
u. SLOPED ZONE SETTLING VELOCITY
UJ COMPRESS/ON SETTLING
Figure 4.16: Typical zone settling process (Shin and Oh, 2004).
From the plot in Figure 4.16, Vs can be determined by the slope of the initial portion of
the line. Regression analysis for both CF and Vs can be estimated fronvthis-test:
Furthermore, the field behavior for both CF and Vs can be predicted as discussed in section
4.5 Case Study
Koerner and Koerner (2005) contrasted three case histories in which geotextile tubes were
used in three different field performances, and the results of hanging bag tests and the pressure
filtration tests were also provided. Soil properties and fabric properties are tabulated in Table
4. land Table 4.2.
Site No.l was located at a beach resort along the Atlantic Ocean and geotextile tubes were
constructed in September 1999. The purpose of this project is to protect the shoreline transition
between the boardwalk and the beach by using geotextile tubes. The geotextile tubes at this site
are made of woven polyester geotextile (Fabric A-l). The geotextile tubes are filled with sand
which is classified as poorly graded (SP) with high permeability. Because so much water is
emerging and expelling from the tube, sacrificial sheets of plastic, scour aprons, and anchors
were utilized to keep the water from undermining the geotextile tube in this design. The
geotextile tubes performed very well in this application resisting Hurricanes Floyd and Isabelle
within 4 years after they were built.
Figure 4.18: (a) Hanging bag test and (b) Geotextile tube at site No.2 (Koerner and Koerner,
Site No. 2 was located at a marina inland from the Atlantic coastline where geotextile
tubes were placed in December 1999. ( i n this application, the silted up marina is solved by using
geotextile tubes for dewatering silts and clays in that areaA The geotextile tubes are made of
woven polypropylene geotextile (Fabric A-2) and the filling material is classified as silt with low
plasticity (ML). The authors were not satisfied with the result. In addition to the low
y due to thefine-grainedmaterials, the oily slurry makes the dewatering process even
worse since it accumulates rapidly and forms the shiny filter cake inside the fabric/ A flocculant
(polyacrylamide) was added to enhance the permeability. Finally, Koerner and Koerner (2005)
concluded that the results were relatively successful even though the increasing time and expense
for adding flocculant.
Figure 4.19: (a) Hanging bag test and (b) Geotextile tube at site No.3 (Koerner and Koerner,
Site No. 3 was located at an industrial site near the Great Lakes. Geotextile tubes were
placed in the summer of 2000. This project uses geotextile tubes for dewatering lagooned ash
which it is classified as well graded sandy silt (SW). Geotextile tubes are made of woven
polypropylene geotextile (Fabric B-l). Koerner and Koerner (2005) concluded that the result is
good to fair in this application even though the final moisture content did not go below 30%.
The soil at each site was tested by both hanging bag test (HBT) and pressure filtration test
(PFT) in the laboratory. The HBT has been discussed in section 220.127.116.11 and PFT is in section
18.104.22.168. In PFT, a mixture of 125 ml of 4% polymer flocculant and a surfactant mixture of 125
ml of 4% NaPOs were also used separately in the test to compare with water and soil alone. The
results are shown in Table 4.4 and 4.5
Table 4.3: Average HBT permeabilities for the site soils
(ksoii) (Koerner and Koerner, 2005).
Fabric A-1 Fabric A-2 Fabric B-l Fabric B-2
(cm/sec) (cm/sec) (cm/sec) (cm/sec)
Site 1 0.3 0.2 0.1 0.4
Site 2 0.005 0.002 0.003 0.004
Site3 0.07 0.05 0.04 0.08
Table 4.4: Average PFT permeabilities for the site soils
(Koerner and Koerner, 2005).
Fabric Fabric Fabric Fabric
A-l A-2 B-l B-2
(cm/sec) (cm/sec) (cm/sec) (cm/sec)
water/soil alone 2.2 2.21 2.1 2.2
w/flocculant 2 2 2.1 2
w/surfactant 2.1 2.2 2 2.1
water/soil alone 0.002 0.004 0.004 0.006
w/flocculant 0.006 0.007 0.005 0.008
w/surfactant 0.001 0.003 0.002 0.005
water/soil alone 0.09 0.1 0.1 0.2
w/flocculant 0.15 0.14 0.13 0.2
w/surfactant 0.08 0.08 0.07 0.15 7
In conclusion, Site No. 1 had the best results since the geotextile tube contained granular
beach sand because of its immediate settlement and fast drainage. However, the result in Site
/ r No. 2 was not satisfied in the field test and lab tests. )lt can be indicated jthat the material is the
v <- ^
finest with the lowest permeability and the highest plasticity index. Furthermorefit generated
the thickest filter cake inside the geotextile tube. More research is needed to test similar
materials by improving the dewatering behavior of the geotextile tube.
Chapter 5 FURTHER RESEARCH
Geotextile tuoes have been used for dewatering dredged materials commercially for over
forty years. They have been designed by software and tested in laboratory experiments, but
more research-needed due to predictions of the geotextile tube shape being highly empirical.
I r—v ~^>
|iV | Several aspects of research may be considered as follows. /
In the aspect of consolidation, theories that are related to consolidated geotextile
based on TerzagjH^Theory, such as constant rate of loading, constant rate of strain, and radial
consolidation (on/nor/zontal plane. No research has been done to study these aspects of
consolidation. Most researchers have done studies based on empirical experiments, such as the
volume reduction method and the settling and self-weight consolidation method. Even though a
problem had been proposed by Shin and Oh (2004), the volume reduction method has been
widely used commercially due to the simplified way it estimates the consolidation of a geotextile
tube. The settling and self-weight consolidation method has a better solution to predict the
shape of the tube, since this method is used for dredged materials. However, this method
requires more complicated knowledge and calculations to understand and more tests are required
with different materials to develop a general solution.
In the aspect of filtration behavior, a lot of criteria and testing can be used for determining
the filtration behavior of geotextiles. Researchers had found om that POA is a better way to
understand the filtration behavior for woven geotextiles whereas AOS is preferred for nonwoven.
Hanging bag tests are the most recommended test to have a pretty good idea of the filtration
behavior of a geotextile tube, but the disadvantage of this test is that it is a case by case study in
which the result only fits to the specific geotextile and the fine-grained soil used in the test.
More studies on the interaction between the geotextile and fine-grained are needed to set up a
reliability-based design for geotextile tubes.
In regards to cake formation, the critical dewatering behavior of geotextile tubes is due to
the formation of the filter cake which is built inside. Many theories related to cake filtration
have been done, such as conventional cake filtration theory and multiphase filtration theory. No
research has been done on cake filtration directly related to the behavior of geotextile tubes.
/ (_ Frkeraids are additives which hold more particles together and increase the porosity of
the filter cake. HenceTit improves the permeability of the filter cake. Sabah and Erkan (2006)
had done a lot of studies on different filter aids. The results were affected by different filter_aids,
the amount of dosage used, and the pH condition.^ There has no study been done directly related
0' to the behavior of the geotextile tube with using a filter aid. Standard criteria may be developed
*in the future. —- r— /
In the aspect of construction, pumping pressure is the most important issue that-had been"
reported. It is known that with the increasing pumping pressure, the tension increases
exponentially and may rupture the geotextile tube. More studies are needed to set up a standard
process for determining the pumping pressure that should be used with different kinds of
s^Td filled materials.