Indian Journal of Fibre & Textile Research Vol. 25, September 2000, pp. 1 82- 1 94

Production method and characteristics of braided PVD

P K Banerjee" & J P Sampath Kumar Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

and

G Venkatappa Rao

Department of Civil Engineering, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

Received 6 August 1999; accepted 14 February 2000

The evolution of a new class of PVD having sheath of braided yams encasing a large number of core yarns is described. The flexibility and simplicity of the production process leading to the possibility of controlled variation in dimensional, mechanical and hydraulic properties of the product have been highlighted. A limited comparative evaluation of some typical variants of this drain made of jute sheath and coir yarns as core with three other commercial PVDs has been reported. The hygroscopic nature of jute and coir plays a significant role in altering the thickness, pore size, permeability and discharge capacity of the drain in the wet state under confining pressure.

Keywords: Braiding, Coir Jute, Prefabricated vertical drains, Soil consolidation

1 Introduction Pre-consolidation of soft soils is often done by the

installation of prefabricated vertical drains (PVDs) which provide the pore water an escape route. Wide ranges of PVDs, made of synthetic polymers differing in their quality particulars, are being marketed commercially. The modern PVD is made of a thin nonwoven filtering material enclosing a synthetic core having different profiles joined either at the edges or on one side by thermal bonding or by ultrasonic welding. The outer filter sheath allows pore water to seep through it while retaining the soil particles and the central core helps in easy draining of water. These drains, although made partly out of a nonwoven textile fabric, do not exhibit the shear behaviour typical of textile materials, which is very important for large buckling deformation of a tubular structure. A recent development in this field is a drain produced entirely from textile yarns and fabrics made of jute and coir, which are ecofriendly and available in abundance in South-East Asia. In the present paper, the structure as well as physical, mechanical and hydraulic behaviour of a new type of PVD made of yarns only are discussed in detail.

a To whom all the correspondence should be addressed. Phone: 659 1409; Fax: 09 1 -0 1 1 -6857757; E-mail:pkbt@hotmail.com! pkb@textile.iitd.ernet.in

2 Developments in PVDs The first vettical drain, developed by Kjellmann in

1 948 and made of corrugated cardboard covered by a paper sheath measuring 1 00 mm x 3 mm, completely replaced sand drains 1 •2 • Subsequent developments in materials and manufacturing processes led to the evolution of synthetic drains in 1 970' s made of materials such as polyethylene, polypropylene and polyester. The popularity of PVDs is due to the ease in storage and transport, rapid installation, l ighter install ing equipment required, high discharge capacity and non-clogging potentiaI2,3 .

Table 1 lists the various commercial drains available in the marketJ-6. Most of the drains are 100 mm wide and about 4 mm thick; the fi lter being made of either nonwoven polyester or polypropylene and the core is of either polypropylene or polyethylene.

A recent addition to the class of PVDs is a natural drain, called Fibredrain4-7, which is made up of a double layered jute hessian cloth, called Burlap, acting as filter, surrounding 4 coir yarns used for transporting water and held together by three longitudinal stitches7 . This drain developed at Singapore is 80-1 00 mm wide and 8- 1 0 mm thick. The characteristic properties of Fibredrain are given in Table 2.

3 Characteristics of PVDs The properties of a few commercially available

PVDs as tested5 are given in Table 3 . The main

BANERJEE et al.: PRODUCTION METHOD AND CHARACfERISTICS OF BRAIDED PVD 1 83

Table I-Dimensions and materials of some PVDs

Drain type Approx .width Approx. thickness Core Filter mm mm

Kjellmann 1 00 3 Cardboard Paper Geodrain 95 4 PE Cellulose or treated paper Castledrain boards 94 2.6 Polyolefin Nonwoven fabric . Mebradrain 95 3 PFlPP PP or PET

'

Alidrain 1 00 6-7 PFl plastic PET or cellulosic Colbonddrain 300 4 Nonwoven Nonwoven PET

1 00 6 PET PET Hitek 1 00 6 PE PP Tenax VDR 1 00 1 08/96 3.4 HDPE PP Flodrain 98 6.8 HDPE PP Amerdrain 98.5 4.4 PP PP

92 10 PP PP PVC 100 2 PVC None Hongdrain 100 6 PE PET Quickdrain 1 00 6 PE PP Bando 96 2.9 Paper PVC Desol 95 2 Polyolefin

PE - Polyethylene, HDPE - High density polyethylene, PP - Polypropylene and PET - Polyester

Table 2-Properties of fibredrain7

Sheath (2 layers of jute burlap) Thread diameter, mm Mesh size per cm2

Weight, glm2

Core (4 coir strands) Diameter, mm

Drain Width, mm Thickness, mm Dry weight, glm2

Filter cover permeability, cmls Axial cover permeability, cmls

Tensile strength, kg

Free surface, mm2

Free volume, %

: 1 .5-2 : 3 double strands by 3

single strands 600-650

3-6

: 80- 1 00 : 8- 1 0 : 3000-3400 : >1 0-3

: > I 0-3 for confining pressure up to 330 kN/m2

: 500-700/80 mm wide drain at 8.7% strain

: 1 80 for 80 mm width : 50-60

characteristics governing the efficiency of PVDs are the filtration properties and the discharge capacity. Though a number of test methods are available for measuring these properties, none represents the behaviour of the drains in practice8. The filtration function includes retaining soil particles and allowing water to flow from the soil into the core9-1 1 . The openings in the filter sleeve should be fine enough to prevent soil particles from passing through theml2. The maximum opening size (095) based on apparent opening size (AOS) should be around 75 11m to retain

silt particles in the range of 1 5 to 30 11m (ref. 1 3) . The efficiency of PVDs could be reduced by blocking or blinding of the sleeve by the silt particles passing through the larger openings in the sleeve.

The filter, the core and the soil conditions are significant factors affecting the permeabi lity characteristics of pVDSI4. The water permeability of the sleeve is determined to a large extent by the pore size and the number of pores per unit area4. 1 5 . The structure of the sleeve, the manufacturing technique, the material and the composition also play a role in permeability. Many authors round the world. suggest that the permeability of the filters should be .,at least 1 0 times

h h b ' l ' f ' 1 1 1 1 3 1 6- 18 greater t an t e permea 1 tty 0 SOl . . . .

The pore water that reaches the core of the drain has to flow vertically. Rate of this flow, i .e. the discharge capacity of the drain, should be as high as possible. However, the discharge capacity is affected by the lateral stresses from the soi!3. IO, 19-2 1 . Subsequent settlements due to consolidation result in bending, buckling or kinking of the drains. Many experiments have been reported to determine the discharge capacity of drains in the buckled state. The results show that there is a considerable reduction in the discharge capacity due to bending and kinking. But it is important to observe how far this affects the performance of the drains in field conditions considering the factors l ike time taken by ti lt: Jrains to undergo deformation in the soil and the d·. , j r.:d discharge capacity

1 84 INDIAN J.FIllRE TEXT. RES., SEPTEMBER 2000

Table 3-Properties of some of the commercial synthetic drains5

Criteria Mebra Colbond Tenax F10drain Amer Required drain MD drain CX VDR I OO FD 6xl 00 drain 407 value 7407 1 000

Drain Width, mm 1 00 96.519 1 1 08/96 98- 1 0 1 98-99 Nominal

(±5%) Equivalent diam, mm 65.5 6 1 70.9 67.2 65.5 Nominal Dry mass, glm 76 66.4-67.8 85.6-86.4 86.7- 1 08.7 1 03.2- 1 07 Nominal (± IO%) Thickness at 2.94 4.63-4.86 3.33-3.62 5 .92-7.44 3.9-4.9 Nominal (±I O%) 20 kPa, mm Tensile strength (dry), kN 2. 1 1 -2.26 1 .74- 1 .89 2. 1 1 -2. 1 9 2.83-3.36 2.3-2.95 > 1 Strain at break (dry), % 49.4-66 24.6-28.7 22-52 37-57 9- 1 8 > 1 5 Discharge capacity (mIls) at 1 25 kPa, T=20°C

@ I OO mm head 79- 1 07 88.2- 1 2 1 57-60.6 140-237 1 63-223 > 1 0 @200 mm head 78-92 64.9-9 1 .3 48.5-50 1 00- 1 78 1 38- 1 89 > 1 0

Sleeve Permeability (mm/s) 0.29-0.6 1 1 . 1 5- 1 .64 0.24-0.36 0.22-0.42 0. 1 7-0.2 >50k(sllil)

@ 50 mm head, unloaded EOS

O!XI. llm 1 35 - 1 65 75- 140 1 70- 1 86 1 50-200 1 20- 1 30 Depending on soil 095. llm 1 55-200 80- 1 75 1 80-220 1 40- 1 60 type

"

Tensile(trans) & shear 1 .86-2.4 1 .73-2.08 4.3 1 -5 .35 3.05-6. 1 4 2.98-4.52 > 1 .5 strength of seam, kN Seam strain at break, % 3-8.2 3.5-4.4 8.5- 1 5 .4 1 0-40 4.5- 1 1 .5 > 1 0

at that particulac t�me. Very little work has been reported on this aspect. De Jager and Oostveen22 observed that the� .disch&rge capacity ·of the buckled drain should be .

'arounrl 75% of the normal drains.

They also observed that buckling mainly appears in the soft-top layers and hence the testing in the laboratory needs to be conducted at a low cell pressure of 100 kPa. Various methods to determine the discharge capacity of the drains are explained and 'the ideal test' is recommended in which the drains need to be tested for the discharge capacity by placing them in cylindrical clay sample8• Other important factors to be considered are duration of loading, method of applying the load and the hydraulic gradient. A hydraulic gradient between 0.2 and 0.5 for laminar flow in the laboratory testing is recommended 10 •

The tensile sv-ength of PVDs is a consideration during install�tion process. Reports suggest the need for a PVD tensile strength of 0.5-1 kN corresponding to a strain of 1 0% (refs 1 3 & 20).

4 Genesis of Brecodrain

The synthetic drains are prone to undergo considerable deformation during soil consolidation, such as

kinking and buckling which would result in drop in discharge capacity . The equal lengths on both sides of the buckled sheath cause compression of the inner layer and extension of the outer layer, resulting in unequal strains in the two layers of the sheath. Because of the sealing of the two layers along their edges as well as the inability of the individual layers to undergo shear deformation, this unbalanced strain cannot get distributed, resulting in stress concentration (Fig. 1 ) . This would result in small sharp kinks and folds on the inner layer of the sheath, leading to the possibility of localized failure of the drain due to opening up of the edges (Fig. 2), allowing the soil particles to pass through them. Moreover, bending of the rigid plastic core could also give rise to sharp folds, resulting in considerable obstruction in the planar flow. Such a ' problem is not expected to be encountered with a flexible textile drain which, on account of its ability to undergo both shear and bending deformations, would possibly not exhibit any significant change in the planar flow properties. Fibredrain, due to its robustness in addition to being a textile

BANERJEE et at.: PRODUCflON METHOD AND CHARACTERISTICS OF BRAIDED PYD 1 85

Fig. I -Photograph showing sress concentration along the sides

Fig. 2-Photograph showing opening up of edge

drain made of jute and coir, does not suffer from the above-said problems. It is, moreover, biodegradable and ecofriendly.

But the method of manufacturing Fibredrain involves a series of discontinuous steps like cutting the woven jute fabric, placing the coir yarns over two layers of such fabrics, folding the fabric layers back and then stitching longitudinally using a three-needle

industrial sewing machine23• This results in considerable manual handling and use of more number and varieties of machines that may add to the cost of the drain . Hence, a single-step manufacturing proce,ss is desirable which can produce a textile drain at a much faster rate. This motivated development of the new drain termed as Brecodrain at The Indian Institute of Technology, New Delhi, by employing braiding technology.

1 86 INDIAN J.FIBRE TEXT. RES., SEPTEMBER 2000

5 Brecodrain 5.1 Braiding

Braiding is the process of producing narrow width fabrics, like shoe laces, elastics, tapes and fancy braids, used as trimmings for clothes, furniture, etc24-27. It is defined as the process of cloth formation in which threads cross the fabric diagonally from side to side and at the same time pass over and under each other. In other words, threads interlace in such a manner that no adjacent threads make complete turns about each other28,29. The fabric so produced is termed as braid that can be obtained in either flat or tubular form, depending on the machine employed.

5.2 Principle of Braiding

The basic principle of maypole braiding or otherwise called 'Barmer dolly' braiding consists of two sets of bobbins, each moving in opposite direction. The yarns to be braided are wound onto bobbins that are moved bodily in a horizontal plane along a serpentine (or sinusoidal) path, causing the threads to interlace as they are withdrawn from one point above the bobbins.

5.3 Machine Designed for Brecodrain Manufacture The principle of operation of the machine, devel

oped at IIT-Delhi, is similar to any other braiding machine intended to produce tubular braid, although it exhibits unique design features23• The heavy-duty spindles can carry flanged bobbins weighing around 2 kg, which are rotated along a pre-determined path to

.

achieve a biaxial braided tubular fabric. Such a fabric is highly deformable in axial as well as radial direction . Therefore, a provision was made in the braiding machine for a third set of axially-oriented yarns placed at regular intervals and maintained under appropriate tension to impart a very high rigidity in axial direction. This triaxially-braided tube forms the filtering sheath of the Brecodrain . This is produced continuously around a specially designed hollow mandrel fixed at the centre of the machine. A sheet of coir yarns (up to 20 in number) is also fed continuously to the braiding point into the hollow mandrel and thus forms the core of the drain . The mandrel gives the drain a pre-determined and uniform width along its length. The drain is finally flattened, compressed and then wound onto a fabric roll situated behind the machine. The fabric roll having a capacity of up to 1 10 m can be doffed without interrupting the production process.

Line Width

Plait Hei&ht

� Strand Width

Fig. 3-Structure of a typical Brecodrain sheath

5.4 Structure of Brecodrain Fig.3 shows the structure of Brecodrain sheath.

This drain is made up of a flattened sleeve, enclosing a core composed of a number of yarns. The core is thus covered on each of its two sides by a layer of braided fabric, henceforth termed as sheath layer. Each strand is composed of yarns of pre-determined count and number. The figure also shows the interlacement of axial yarns with the braiding yarns. The axial yarns are embedded in the structure. The core yarns lie parallel to the axial yarns inside the tube.

5.5 Characteristic Properties of Brecodrain It is clear from the construction process of Breco

drain that the count and number of yarns in the strand can be varied at will to achieve a drain sleeve of varying width, thickness and weight per linear meter.

. By changing the number and count of core yarns, the thickness, the strength and more importantly the discharge capacity of the drain can be varied. The axial yarns contribute to the tensile strength of the drain. The permeability and pore size distribution of the sheath can be controlled to a desired value by suitably choosing the number and count of braiding yams in tAe strand, braid angle, tension in the yarns and rate of take up. Some important properties of two typical samples of Brecodrain are given in Table 4.

6 Preliminary Experiments A unique feature of the technology involved in the

manufacture of Brecodrain is that it is possible to de-

BANERJEE et at. : PRODUCfION METHOD AND CHARACTERISTICS OF BRAIDED PVD 1 87

Table 4-Properties of Brecodrain samples

C

1 88 INDIAN I.FIBRE TEXT: RES .• SEPTEMBER 2000

In another study, wet specimens were tested for their thickness under a constant pressure of 2 kPa for a time interval of 10 min to few hours in order to determine the time required for the drain to swell completely.

7.2 Tensile Properties The strength and elongation-at-break of the drain

specimens were tested using Instron 4202 Universal Testing Machine. According to ASTM D 4595-86, the tensile properties of geotextiles need to be tested using 200 mm wide and 1 00 mm long strips. However, the drains being only 100 mm wide, a gauge length of 50 mm was chosen in order to maintain the same aspect ratio as prescribed by ASTM. The upper jaw was allowed to move at 5 mmlmin or 10% strain rate.

7.3 Pore Size Distribution 7.3.1 Dry Sieving

The drain samples were slit open along one of their edges and flattened without disturbing the structure of the sheath fabric. From this, a circular specimen of 200 mm diameter was cut and tested for pore size distribution using Ballotini as per ASTM D 475 1 -95. A special sieve frame had to be fabricated to accommodate these smaller diameter specimens.

7.3.2 Hydrodynamic Sieving The pore size of Brecodrain sheath in wet condi

tion was determined by using hydrodynamic sieving

apparatus30• A visual observation of the sheath reveals considerable difference in pore size value between its dry and wet states (Figs 4 and 5) because jute, being hygroscopic material, swells on wetting. Moreover, under field conditions the wet sheath would be compressed due to the pressure exerted by the soil, thus flattening the jute yams and further reducing the pore size. To simulate such a state of the sheath and estimate the resulting pore size distribution, specimens were immersed in a fabricated water trough (500 mm X 1 20 mm x 60 mm ), maintaining a constant normal pressure of 100 kPa over a period of 4-6 h.

A leak-proof assembly of the fabric wrapped around the drum of the apparatus containing 50 g of sand of known particle size was placed in the water trough and rotated at 20 rpm for 75 min, forcing some of the sand particles to pass through the larger sheath openings. The sand thus passing through the sheath and collected in the trough was filtered, dried and weighed. This experiment was repeated with different pa;ticle sizes. An analysis of the percentage of sand particles of given size passing through the sheath yields the pore size distribution.

7.4 Permeability Test ASTM D-449 1 -85 specifies permittivity test using

constant head and fall ing head permeameters. The constant head test was carried out using a head of 50

f d · lO-l2 D . d mm 0 water un er varymg pressure' ' . eaJre water was used throughout the entire experiment.

Fig. 4-Photograph of dry sheath of Brecodrain

BANERJEE et al. : PRODUCTION METHOD AND CHARACTERISTICS OF BRAIDED PYD 1 89

7.5 Discharge Capacity Test The discharge capacity test was conducted using

the discharge capacity testing apparatus3 1 ,32 . A 100 mm long drain was compressed between two layers of clay under a given normal stress, allowed to consolidate and then the flow was measured.

8 Results and Discussion 8.1 Profile of Samples

The sample A was produced with eight yams per strand and had wide variation in width and a very high linear weight. Then�fore, the number was brought down to six per strand (Sample B 1 ) . This resulted in a decrease in thickness and linear weights,

Fig, 5-Photograph of wet sheath of Brecodrain

but the width was inconsistent. At this stage, the mandrel was introduced which h�lped in producing samples of uniform width (Samples B2-G) . To further reduce the weight, the number of yams per strand was further reduced to five (Sample C). In view of overlapping of yams in the strands of Samples B2 and C, resulting in increased thickness and weight of the drain, this number was further reduced to four. This value was maintained for further production of Samples D l - E.

Some samples, namely D 1 1 , E and F, were also produced with a larger mandrel. Sample E represents a drain of different braid structure. This diamond structure gave a sample with increased pore size at the intersecting points and, therefore, was discontinued. In another series, the count of the braiding yams was reduced (Samples F and G). Overlapping of yams could not be eliminated with a combination of reduced count and more number of yams in the strand (Sample F). So, the number was again changed to four (Sample G) and produced with smaller mandrel.

8.2 Dimensional Properties

Table 5 shows the dimensional properties of the samples produced.

. . 8.2.1 Effect of Mandrel !�' . ' ·Mandrel helps in producing a braid of uniform

width. The width of the braided tubes fluctuates con-siderably without a mandrel as can be seen frof!1 Samples A and B 1 ; the beneficial effect of introduc-tion of mandrel on width is clearly reflected by the Sample B2. As the diameter of the mandrel is in-creased, the width and the l inear weight of the sleeve

Table 5-Dimensional properties of Brecodrain samples produced

Drain Braiding yarn Core Mandrel Braid Width Thickness @20 kPa, mm Weight (ler linear meter,g type Count No, yams diam structure mm Drain Sheath Core Sleeve Total

Ib No. mm

A 30 8 8 No Regular 90- 1 06 1 5 ,23- 17,02 5 .65-6,23 . . 33 485-564 5 1 8-597 mandrel

B l 30 6 1 6 --do-- Regular 76-85 1 1 .94- 1 2.4 4,69-4.9 1 66 289 355:

B2 30 6 1 6 58.3 Regular 97-98 1 2.55- 1 2,73 4.25-4.78 66 362 428 C 30 5 1 6 58.3 Regular 97-98 1 0,26- 1 1 .0 3.87-4.33 66 299 365

D I 30 4 1 6 58.3 Regular 97-98 1 0.27- 1 0.82 3.33-3-.56 66 240 306 D2 30 4 1 2 58.3 Regular 97-98 1 0.06- 1 0.54 3 .33-3.56 49 240 289 D3 30 4 8 58.3 Regular 91-98 9.92- 1 0.22 3 .33-3.56 33 240 273 04 30 4 4 58.3 Regular 97-98 9.9 1 - 1 0.05 3.33-3.56 1 7 240 257 0 1 1 30 4 1 6 64. 1 Regular 1 02- 1 03 1 0.2- 1 0.7 3.3 1 -3 .63 66 250 3 1 6

E 30 4 1 6 64. 1 Diamond 1 02- 1 03 1 0.62- 1 0.93 3.44-3.83 66 282 348 F 20 6 1 6 64. 1 Regular 1 02- 1 03 1 0.2-1 0.68 3 . 1 6-3.44 66 245 3 1 1 G 8 4 1 6 58.3 Regular 97-98 8.01 -8 .71 2.36-2.72 66 147 2 1 3

1 90 INDIAN J.FIBRE TEXT. RES., SEPTEMBER 2000

also increase as shown by Samples 0 1 and 0 1 1 .

8.2.2 Effect of Number of Braiding Yarns in the Strand As the number of braiding yams is decreased from

6 to 5 and finally to 4 keeping the mandrel diameter constant, as in Samples B2, C and 0 1 respectively, the linear weight and the thickness of sleeve decrease (Figs 6 and 7). Similar results were obtained for the samples braided without mandrel (Samples A and B 1 ) . The reduction in thickness is caused by reduced riding of neighbouring yams whereas the reduction in weight is owing to less material being braided at any given instant.

8.2.3 Effect of Braid Structure There is an increase in thickness of the braid due to

change in braid structure from a regular construction (Sample 01 1 ) to the diamond structure (Sample E). In view of the limitation of the existing machine, the diamond structure resulted in strands of eight yams, leading to excessive riding amongst the yams as wel l as large pores at the intersection points. Hence, fur-

440 �----------------.

290 -t-----,....-----r-------i 4 5

No. of Braiding Yams 6

Fig. 6-Effect of number of braiding yams on linear weight of Brecotlrain

,_ l i . O E E '" � I II 6 · c: � .;! L ;- I U.2 .

9.8 +-------r-----.------r" 3 4 5 6

No. of Braiding Yams

ther experimentation with diamond structure was discontinued.

8.2.4 Effect of Count of Braiding Yarns An increase in mandrel diameter from 58.3 mm to

64. 1 mm resulted in an increase in l inear weight of sleeve by a factor of 1 .04 as revealed by the Samples 0 1 and O I L Extending this relationship to Sample B2, one

"would expect its l inear weight to become 377

g, if braided with larger mandrel . Comparing this value with that of Sample F, it is concluded that a 1 .5 times reduction in yam l inear density results in a near proportionate drop in fabric linear weight.

8.2.5 Effect of Number of Core Yarns The Samples 0 1 -04 reveal that a steady reduction

in number of coir yams resulted in a continuous drop not only in l inear weight of the drain but also in its thickness. The effect on thickness becomes insignificant below a certain number of coir yams, indicating riding amongst these yams in the event of reduced availability of lateral space.

8.2.6 Thickness Fig. 8 shows the thickness of the drain O l and of

the corresponding sheath in both dry and wet conditions measured under varying normal pressure. The sheath and drain are much thicker in wet state than in dry state for the same value of pressure. This is largely due to the hygroscopic nature of the materials. In the drain, around 80% change in thickness is caused by the swelling of sleeve while the remaining 20% is by the swelling of coir yams. As can be inferred from the figure, the dimensional behaviour of coir yams in dry and wet states under varying pressure is radically different from that of the jute sheath. Accordingly, the values of thickness of coir yam in dry and wet states were worked out and plotted sepa-

I& �------------------------� 1 6 1 4

e 12 g o 10 J.l 8 u � 6

--H- Wet Drain --+-Ory Drain --'-Wet Sheath -+-Dry Sheath I(

20 40 60 &0 100 120 140 160 180 200 Prusure (kPa)

Fig. 8-Thickness of Brecodrain and its sheath in wet and dry Fig. 7-Effect of number of braiding yams on sleeve thickness states

BANERJEE et ai. : PRODUCTION METHOD AND CHARACfERISTICS OF BRAIDED PVD 19 1

rately (Fig. 9). A perusal of the Figs 8 and 9 reveals that

- On wetting, the thickness of jute sheath as well as coir yam increases.

- On increasing pressure, the thickness of jute yams in the sheath as well as coir yams in the core decreases both in dry and wet states.

- On increasing pressure from 2 kPa to 200 kPa, reduction in the thickness of wet coir yam is almost three times as much as that of dry coir yam as a result of which the final thickness of wet coir yam at 200 kPa is lower than that of dry coir yam at the same pressure. This structural collapsing of the wet coir yams under pressure is expected to negatively affect the capillary action.

Fig. 10 shows that around 90% of swelling of the sheath is completed in less than 90 min, whereas complete swelling takes about 4 h . The swelling potential of the jute sheath is high, resulting in finer pores in the wet condition.

8.3 Tensile Properties Fig. 1 1 shows load-elongation behaviour of a typi

cal drain (Sample D l ) and the corresponding sleeve. Of the two peaks recorded in the load-elongation curve of the drain, the first peak represents the load taken up by the axial yams. The first break was observed at about 3.6 kN at an elongation of 9-10%. After the failure of al l axial yams around the first peak, the load is transferred to the core yams. Beyond the second peak, the core yams start rupturing, resulting in rapid drop in load carrying capacity as well as elongation of the braided sheath which gets deformed into a thick rope like structure due to the straightening of braiding yams. It is observed from the above that the initial segment of load-elongation curve of a Brecodrain satisfies the tensile strength requirements of a PVD.

The tensile behaviour of the sleeve alone is similar to that of the entire drain, except for the absence of the second peak due to coir yams. This clearly indicates that the axial yams are the load carrying elements in case of Brecodrain and change in the quality of these yams would result in change in the loadelongation behaviour of the drain .

8.4 Pore Size Distribution Fig. 1 2 shows the pore size distribution of two

types of sheath ( D 1 and G) in dry as well as in wet state. The dry sieving plot of sheath D 1 appears very

6.0r. ;......----...;..---�------I 5.5

I so. 1 45 .g 4.0 . � 3.5

-+- Dry state ...... W!t state

3.0t-----.-----.------:'::"-�-�200 o 50 100 150 Pressure (kPa)

Fig. 9-Thickness of coir yarn in dry and wet states

�.o_---------------__. 4.'

i: 4.6 & i 4.4 ] 4.2 .!! � 4:.0

Pressure, 2 k Po

3.& 3.6 +-_.,...--,-----.----r----.----I

o so 100 ISO 200 250 Time (miD)

Fig. I O-Effect of time on swelling of sheath

o o 0 0 I . ' I 3 - - - ---. - -"'!-- - - - - - - - - - � - - - - - - - - - --�- -- ----- - - �- - - - - - . - - -I ."""... : : ! . ! 2 • • • • • . ,d . . . . - : ·::·.·:·+:·:.�-.� - - .J . . . . --. . . . -\. - . - . . - - - - · ; , u./ uLj =� tu · :l : ::o+· ··. : : .

• 0 I 0 • 0 0 o 0 10 20 30 40

EIoDptlo. (%) 50

Fig. l l-Load-elongation behaviour of Brecodrain D I and its sheath

· 100 -r------___,_-.-------,:-___.-------, ?<

. . * . . Type G (Hy�m

1 92 INDIAN J.FIBRE TEXT: RES., SEPTEMBER 2000

different from that of sheath G. This is because the sheath D l produced out of 30 lb jute yam ,has an over-jammed structure, exhibiting riding of yams at their intersection points. The resulting pores at these points become substantially larger, causing skewness in the pore size distribution. However, the sheath G, having a balanced structure, exhibits a more uniform distribution of pore sizes. However, these sheaths exhibit similar pore size distribution when subjected to hydrodynamic sieving test. It is very clear that the pores become smaller when the sheath swells in wet state. The hydrodynamic sieving test conducted on sheath D 1 that was kept in water under pressure showed further reduction in average pore size. The material under such conditions can swell only in the lateral direction, thereby exhibiting further reduction in pore size. It is also evident from the plots of hydrodynamic sieving with and without pressure that the smaller pores are affected to larger extent than the bigger pores due to the swelling and flattening of threads. The hydrodynamic 095 values of both the samples are 60-75 % lower than their corresponding 095 values in dry state. S imilar results were reported attributing this difference to the nature of test methods only33. However, the results obtained in the present study point to the swelling potential of the material in water as being an important component in influencing the pore size of a geotextile in wet state, although the actual share of this component in the reduction in value of 095 has not yet been established.

8.5 Permeability Characteristics Fig. 1 3 shows the permeability characteristics of

different sheaths under varying pressure. The sheaths of Fibredrain and Brecodrain made of natural fibre have higher permeability than the nonwoven synthetic sheath. This is due to the structure of the sheath of the drains. The woven sheath of Fibredrain and the braided sheaths of Brecodrain have larger pores and hence exhibit high permeability values in the beginning. But as the pressure is increased, the swollen yams tend to either partially or completely cover the pores, thereby reducing the permeability of the sheaths. This is substantiated by visual examination of Figs 4 and 5. The drop in permeability of the drains made of natural fibres is very rapid with small increment in pressure up to 40-80 kPa. Later, the drop in permeability is more gradual. Although Fibredrain has lower permeability than Brecodrains in

0.7 �-----------------., 1 0.6 !. o.s � � 0.4 II e 0.3 l! "a 0.2

_Type DI " . "Type G -6--Colbonddnin -M--Fibredrain

.. � U o.I��� • •• • � . . . . . . . . � . . . . . � 40 80 120 16()

Pr ... ure (kPa)

Fig. 1 3-Coefficients of permeability of diffrent sheaths

70 60

.-. SO ' :l: . ! 40 .. 2." J 30 .� ':::' 20

10 0

0

.......-'Colboncidroin __ TtDaxdrain � :Coi, Fib«drain -*46 'Colr 8m;Qdrain

50 100 ISO " 200 250 300 350 Pressure (kPa)

Fig. 14-Discharge capicity of various drains

the beginning, it however shows higher permeability at high pressures, i.e. the percentage drop in the value with the increase in pressure is less. Synthetic nonwoven sheath tested here behaves entirely different. It has a more stable and low value of permeability throughout the entire range of pressure.

Under pressure and in wet conditions, the smaller pore size of the Brecodrain sheath undergoes larger percentage change as compared to the larger opening size of the Fibredrain sheath. This is corroborated by the pore size distribution of the Brecodrain sheath tested under wet condition both with and without pressure. The nonwoven sheath of the Colbonddrain, although having much smaller pore size than those of Brecodrain and Fibredian and hence a much lower permeability, suffers very marginal deformation in wet condition and under pressure.

8.6 Discharge Capacity Fig. 14 shows the discharge capacity of Brecodrain

(Dl ), Fibredrain and two synthetic drains under varying lateral pressure. It is quite clear that the discharge capacity of two natural drains under investi-

BANERJEE et af.: PRODUCTION METHOD AND CHARACfERISTICS OF BRAIDED PVD 1 93

gation is much lower than those of the synthetic drains under similar pressures. But Brecodrain (01 ) showed more discharge capacity than Fibredrain under similar conditions. The observations after the experiments showed that the particles of clay were restrained from entering into the core of Brecodrain (01) even at a pressure of 350 kPa applied for more than 24 h. More detailed investigation on discharge capacity of various types of Brecodrain is under progress.

9 Conclusions The essential features of the production process of

Brecodrain are embodied in (i) the method of manufacture of drains by the principle of braiding, and (ii) the flexibility in designing of the products.

All commercially available PVOs, including the Fibredrain, require various kinds of machines and techniques for the production of drain. But the braiding technique described in this paper can be employed to produce drains on a single machine from suitable raw materials in one single operation. Although the present drain under investigation has been made from natural fibers, it must be clearly underlined that drains can also be made from synthetic materials, employing similar technique. The focus is thus more on the advantages of the technique of production which in addition to being a single-stage process also permits changing the width, weight per linear meter, strength, permeability and discharge capacity of the drains by changing the number and count of sheath and core yams as well as machine settings. Also, being a purely textile product, such drains would not be negatively affected by the buckling process expected during consolidation of soil .

The jute-coir Brecodrain exhibits greater sensitivity to moisture and pressure as compared to some commercial synthetic drains. Thus, the thickness of the drain increases significantly on wetting while the pore size values in the sheath decrease considerably on wetting under pressure. The permeability of the sheath decreases and the discharge capacity is negatively affected due to the collapse of the coir yams under high pressure in wet condition. Evidently, the hygroscopic nature of the raw materials causes all these changes, the extent of which, depending on the nature of application, may or may not be critical and need to be balanced against the ecofriendliness as well as other advantages of these natural materials vis-a-vis synthetic materials. In the extreme case,

when the susceptibility of these materials to moisture and pressure outweighs their natural advantages, the braided drains may still be constructed from polypropylene orland polyester materials to exploit the advantages of the technique outlined here.

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