06-GEOSYNTHETICS



GEOSYNTHETICS

Geosynthetics includes geotextile, geogrid, geomembrane, geonet, geocomposite, geocell, geomat, paving fabric, glass grid etc. Geofabrics made of natural fibres such as jute and coir is called natural geotextiles.

TYPES OF GEOSYNTHETICS

  1. Geotextiles: Any permeable synthetic fabric used with foundation, soil, rock and earth comes under this. The geotextile may be woven, non-woven or knitted fabric made from yarns or filaments of polypropylene, polyethylene or polyester or its compounds. Geotextiles are used in several areas like separation, filtration, drainage, reinforcement etc.
  2. Geogrids: They are netlike polymeric material used with foundation, soil, rock earth etc. Geogrids have relatively high strength, high modulus and low-creep sensitive polymers. The apertures varying from 10 to 100 mm size are either elongated ellipses, squares with rounded corners, rectangles etc. Geostrip is another form of geogrid in the form of strips made of high tenacity polyester yarn and contained in suitable polymer sheath. Geogrids and Geostrips are used as reinforcement in pavements and reinforced soil slopes.
  3. Geomembranes: It is an impermeable membrane used with foundation, soil, rock, earth etc., to control fluid migration. Geomembranes are made from PVC or polyethylene sheets which are duly protected from ultraviolet (UV) exposures by carbon black or any antioxidants and thermal stabilizers.
  4. Geonets: These are formed by continuous polymeric ribs at acute angle to one another. Geonets are used in combination with other geosynthetic materials to form a composite material.
  5. Geocomposites: They are manufactured by combination of two or more synthetic materials like geotextiles, geogrids, geonets, geomembranes etc. Drainage composites are popular materials under this category. Examples of geocomposites are prefabricated vertical drains (PVD)/band drains and fin drains.
  6. Geocells: It is a three dimensional structures with interconnected cells used in erosion control of slopes.
  7. Geosynthetic mats: There are two dimensional or three dimensional mats made of multi-filaments with apertures to allow growth of vegetation. Geosynthetic mats consist of UV stabilised, heat bonded non-degradable polyethylene/ polypropylene or similar fibres. These mats are sometimes reinforced with steel mesh to provide more strength against erosion in slopes or areas of heavy rain fall.
  8. Natural geotextiles: These are made from natural fibres like jute or coir. The jute or coir mats are further reinforced with polymeric netting stitched to either side to form a strong quilted mat.
  9. Paving fabric and glass grids: Paving fabrics are non-woven heat set material consisting of minimum 85% of polyolefin, polyester or polyamides by weight. Glass grids are composite glass fibre reinforced geogrids with continuous filament nonwoven geotextile, properly bonded. Glass grids are also made from bituminous material coated glass fibre geogrids with or without adhesive coating on one side.

QUALITY OF GEOSYNTHETICS

The following points shall be looked into for assuring the quality of the geosynthetic material for work:

  1. Every lot of material supplied shall be accompanied by manufacturer’s certificate.
  2. Further, the supplier shall provide a third party test report from an independent testing laboratory with valid accreditation.
  3. The manufacturer shall have ISO or CE (Conformité Européenne) certification for the manufacturing process and quality control of the geosynthetics.
  4. The testing of the material shall be in accordance with relevant IS, BS, ASTM or EN codes.
  5. The roll of geosynthetics shall contain the manufacturer’s name, roll number, grade, length, date  manufactured, product identification details etc.
  6. Each roll of geosynthetic shall be wrapped with tarpaulin or opaque plastic sheets for protection during transport and storage.
  7. During storage, geosynthetic rolls shall be kept above the ground adequately covered to protect from construction damage, precipitation, prolonged UV radiation, flames and sparks from welding etc.
  8. The outer most wrap of the geosynthetic rolls, if damaged, shall be discarded.
  9. Paving fabric used with bituminous overlays shall be completely dry prior to installation.

GEOTEXTILES FOR DRAINAGE, SEPARATION AND EROSION CONTROL

The following points shall be given importance in supplying and laying geotextiles for drainage, separation and erosion control.

  1. Geotextiles when used for drainage/filtration purposes shall be able to allow passage of water throughout its life.
  2. The minimum strength of geotextiles in terms of Minimum Average Roll Value (MARV) for different condition of installation shall be given as in table.(Ref: Table 700-1 of MoRTH Specifications).

Minimum geotextile strength property requirements

Installation

condition

Type

                                Strength property requirement (MARV)

Grab strength

in Newton (N)

as per ASTM

D 4632/

IS:13162

Part 5

Tear strength

in Newton (N)

as per ASTM

D 4533/IS:

14293

Puncture

strength in

Newton (N)

as per

IS:13162

Part 4

Burst

strength in

Newton (N) as

per ASTM

D 3786/

IS:1966

Elongation at failure

<50%

>50%

<50%

>50%

<50%

>50%

<50%

> 50%

Harsh

Type I

1400

900

500

350

500

350

3600

1700

Moderate

Type II

1100

700

400

250

400

250

2700

1300

Less severe

Type III

800

500

300

180

300

180

2100

950

NOTE:

  1. All numeric values in the above table represent MARV in weaker principal direction. The MARV is derived statistically as the average value minus two standard deviations.

  2. When the geotextiles are joined together by field sewing, the seam strength shall be at least 60% of the material’s tensile strength. All field seams shall be sewn with thread as strong as the material in the fabric.

  1. When used for the separation of layers, the intermixing of the two layers shall be prevented throughout the life period.
  2. When used below stone pitching or stone filter mattresses over slopes, geotextiles shall allow the water to flow out but shall prevent loss of soil fines.
  3. The puncture strength in Newton (N) as per ASTM D 6241 shall be as given in table. Puncture strength requirement for geotextiles Strength property requirement (MARV)

Puncture strength requirement for geotextiles

Installation Condition

Strength property requirement (MARV)

Puncture strength in Newton (N)

Elongation at failure

< 50%

> 50%

Harsh

2800

2000

Moderate

2250

1400

Less severe

1700

1000

The material shall satisfy the UV stability requirements specified in table (Ref: Table 700-2 of MoRTH Specifications).

Requirement of UV stability

Sl.No

Properties of fabric

Requirements (Retained strength)

1

Grab strength

Not less than 70% after 500 hrs

of exposure

2

Tear strength

3

Puncture strength

4

Burst strength

The physical requirement of geotextile shall conform to the requirements specified in table (Ref: Table 700-3 of MoRTH Specifications).

Geotextile Requirements for subsurface drainage

In-situ soil

passing 0.075

mm sieve (%)

Permittivity, per sec, as

per ASTM D 4491/

IS:14324

Maximum apparent

opening size, mm ASTM D

4751/IS:14294

<15

0.5

0.43

15 to 50

0.2

0.25

>50

0.1

0.22

The geotextiles for different subgrade soil condition shall meet the requirement given in tables (Ref: Tables 700-4 & 5 of MoRTH Specifications).

Geotextile requirements for separation (Subgrade soaked CBR >3)

Sl. No. Geotextile property Requirement
1 Permittivity as per ASTM D 4491  0.02 Sec-1 (per sec)
2 Maximum apparent opening size 0.60 mm

Geotextiles selected for erosion control shall have the property specified in table (Ref: Table 700-6 of MoRTH Specifications).

Geotextile requirements for separation (Subgrade soaked CBR ≤3)

Sl. No. Geotextile property Requirement
1 Permittivity as per ASTM D 4491  0.05 Sec-1 (per sec)
2 Maximum apparent opening size 0.43
  1. Geotextiles placed shall be covered within 14 days.
  2. Care shall be taken to avoid damages to the geotextiles. Damages if any shall be rectified by providing patches. 
  3. Overlaps of adjacent rolls shall be minimum 450 mm. If placed under water, the overlap shall be minimum 1.0 m.
  4. Wrinkles or folds in the fabric shall be removed before covering.
  5. During slope protection works, geotextiles shall not be damaged by stone or stone filled mattresses placed over it.

GEOGRIDS IN SUBBASE REINFORCEMENT

The following points shall be carefully considered in using geogrids for subbase of pavements and soil reinforcing.

  1. The requirement under table (Ref: Table 700-7 of MoRTH Specifications) shall be met for geogrids used to reinforced subbase layers of flexible pavements.

Property

Code

Unit

Requirement

Stiffness at 0.5%

strain

ISO-10319

kN/m

≥350; both in machine and

cross- machine direction.

Tensile strength @

2% strain

ASTM D 6637

kN/m

≥15% of Tult; both in machine

and cross-machine direction.

Tensile strength @

5% strain

ASTM D 6637

kN/m

≥ 20% of Tult; both in machine

and cross-machine direction.

Junction efficiency

for extruded

geogrids

GRI-GG2-87 or

ASTM-WK 14256

 

-

90% of rib ultimate tensile

strength.

Ultraviolet stability

ASTM D 4355

-

70% after 500 hours exposure.

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