Ground Improvement B: Geosynthetics

Question 1

What are the three main groups of geosynthetics?

Answer 1

Geotextiles

Geosynthetic barries (also called geomembranes)

Geogrids

Question 2

What are the common polymers used for geotextiles?

Answer 2

• Polypropylene (PP)
• Polyethylene (PE)
• Polyamide or Nylon (PA)
• Polyester (PET)

PP, PE, PA normally used for filters, separators and protection

PET stronger and stiffer, used in reinforcement geotextiles

Question 3

Give a method in which geotextiles are manufactured?

Answer 3

Weaving

Question 4

Give an example function of geotextiles?

Answer 4

Improving erosion control if the railway is next to the canal

Question 5

What do geogrids do?

Answer 5

• Ballast locks into the grid
• Acts as tensile strength system, stops water penetrating through
• Reduces settlement (e.g. might save one tamper per year)

Question 6

Give a method in which geogrids are manufactured?

Answer 6

Polyester

Normally covered with PVC to protect the fibres from mechanical/chemical damage

Question 7

What are the different types of geogrid?

Answer 7

Punched and extruded - high resistance to site damage, but large extensions at failure

Fortrac (PVC coated PET filaments) - low strain at failure, but can be damaged

Polyethylene coated PET - very strong, good damage resistance at high strengths

Question 8

What are the three types of material used for geosynthetic barriers (geomembranes)?

Answer 8

Polymeric

Bituminuous

Geosynthetic Clay Liners (GCLs)

Question 9

Give three materials that are used in geosynthetic barriers?

Answer 9

High density polyethylene (HDPE)

Linear low density polyethylene (LLDPE)

Ethylene copolymers (ECP)

Question 10

What can be done to geosynthetic barriers to improve their strength?

Answer 10

Inter laying bitumen, creating bituminous geosynthetic barriers

Mineral layers can be added to improve durability and increase surface roughness

Question 11

What are geosynthetic clay liners (GCLs) and what do they do?

Answer 11

GCLs are made by sandwiching a layer of bentonite rich clay between two sheets of geotextile

The bentonite clay absorbs and is hydrated by water; the thin, low permeability layer prevents the passage of fluids through the soil

Question 12

How can temperature affect polymers?

Answer 12

Higher temp = more strain in polymer

Too low temp = polymer can become brittle

Question 13

Summarise the trackbed application of geogrids and geotextiles

Answer 13

Question 14

Name the different rail infrastructure applications of geogrids and geotextiles

Answer 14

• Separation
• Filtration (drainage)
• Reinforcement (e.g. granular embankment)
• Protection
• Erosion control

Question 15

What is an advantage of using a geocell over a geogrid?

Answer 15

It provides a stiffer solution

However, it is more difficult to fill with material without the sides falling in

Question 16

Why do geosynthetics need to be tested?

Answer 16

Quality testing during production; need to confirm that geosynthetics with the correct properties are being incorporated into the works

Designers need standard tests to provide parameters for analysis

Question 17

Name the eight basic tests used on geosynthetics

Answer 17

• Tensile strength
• Apparent open size (AOS)
• Water flow normal to the plane
• Static puncture
• Tensile creep
• Dynamic puncture (drop cone)
• In-plane water flow
• Sand geosynthetic friction

Question 18

What do geomembrane joints need to be to be effective?

Name one geomembrane/barrier field test

Answer 18

Geomembrane joints must be as strong as the parent material, and resistant to the passage of the contained liquid

Spark testing

Question 19

What is XiTRACK a type of?

What does it do?

Answer 19

XiTRACK is a 3D polyurethane geocomposite

It is a liquid polymer that is applied to the ballast. The double polymer layer significantly reduces ground vibration

It also significantly reduces settlement and prevents track from moving laterally