Geosynthetics are polymeric (synthetic) materials used in civil, geotechnical, and environmental engineering applications. They offer engineered, predictable performance that natural materials like sand filters and gravel drains cannot achieve consistently. India's geosynthetics market is growing rapidly, driven by highway, railway embankment, and landfill construction under NHDP and Swachh Bharat Mission.
Classification of Geosynthetics
| Type | Description | Primary Functions |
|---|---|---|
| Geotextile (GTX) | Permeable fabric (woven/non-woven/knitted) | Separation, filtration, drainage, reinforcement |
| Geogrid (GGR) | Open grid structure with large apertures | Reinforcement (mechanical interlock with soil) |
| Geomembrane (GMB) | Thin impermeable sheet (HDPE, LLDPE, PVC) | Containment (lining, barrier) |
| Geonet (GN) | Open net structure with ribs | Drainage (in-plane flow) |
| Geocell (GC) | Honeycomb 3D cellular structure | Confinement, slope protection, load distribution |
| Geocomposite (GCL, GCD) | Combination of above (e.g., geotextile + geonet) | Multiple — separation + drainage |
| Geosynthetic Clay Liner (GCL) | Geotextile + bentonite core | Hydraulic barrier; low k = 10⁻¹¹ m/s |
Functions of Geosynthetics
1. Separation
Prevents intermixing of two dissimilar soils (e.g., subgrade and base course in roads). Without separation, fine subgrade soil pumps into coarser granular base under repeated traffic loading, contaminating the base and reducing structural performance.
Application: Non-woven geotextile placed directly on subgrade before gravel sub-base placement on soft ground.
2. Filtration
Allows water to pass while retaining soil particles. Equivalent to a graded granular filter (Terzaghi's criteria) but manufactured and predictable.
Design criteria: O₉₅ ≤ 2.5D₈₅ (filtration criterion) and k_geotextile ≥ k_soil (permeability criterion)
Application: Drainage layer behind retaining wall, around perforated drain pipes, in erosion control systems.
3. Drainage
Transmits water in the plane of the material (geotextile or geonet). Transmissivity (θ = in-plane flow rate) is the key design parameter.
Application: Vertical drain (prefabricated vertical drain, PVD) for soft ground consolidation; chimney drain in earth dams; subsoil drainage.
4. Reinforcement
Tensile elements embedded in soil to increase the composite strength, prevent lateral spreading, and improve bearing capacity or slope stability.
Application: Geogrid-reinforced embankment over soft soil; reinforced earth retaining wall (Tensar, Terramesh); geogrid-reinforced pavement base for reduction in base thickness.
5. Containment / Barrier
Impermeable geomembranes prevent fluid migration.
Application: Municipal solid waste landfill lining (base + side slopes); tailings dam; pond lining; canal lining.
6. Protection
Cushion/protection layers between geomembrane and sharp aggregate.
Geotextile Types
| Type | Manufacturing | Typical Tensile Strength | Best Functions |
|---|---|---|---|
| Woven PP/PET | Woven filaments or slit film | 30–200 kN/m | Reinforcement, separation |
| Non-woven PP (needle-punched) | Random fibre bonding by barbed needles | 10–50 kN/m | Filtration, separation, drainage |
| Non-woven PET (heat-bonded) | Heat-fused fibre matrix | 15–100 kN/m | Filtration, protection |
| Knitted | Interlooped yarns; stretchy | Variable | Erosion control, drainage |
Geogrid Design for Road Base Reinforcement
Geogrids interlock with aggregate particles, creating a mechanically stabilised layer (MSL). Benefits:
- Reduce base course thickness by 25–40% for equivalent structural performance
- Extend pavement life
- Allow construction on very soft subgrade (CBR < 3%)
Tensile Stiffness Ratio (TSR) or Traffic Benefit Ratio (TBR) are used to quantify improvement. NCHRP 4-38 method or AASHTO Appendix MM provide design procedures.
Geomembrane for Landfill Lining
Municipal Solid Waste (MSW) Landfill regulations in India (Municipal Solid Waste Rules 2016) require:
- Bottom liner: 1.5 mm HDPE geomembrane + 600 mm compacted clay (k ≤ 10⁻⁷ m/s)
- Leachate collection layer: Geonet composite or gravel drainage blanket above liner
- Top cap: HDPE geomembrane + drainage geocomposite + vegetated cover
HDPE (High Density Polyethylene) geomembrane: most chemical-resistant; 0.75–3 mm thickness; seamed by hot wedge welding; tested by air pressure lance.
Prefabricated Vertical Drains (PVD) for Soft Ground
PVDs (also called wick drains or band drains) are geosynthetic strip drains (100 mm × 6 mm cross-section) installed vertically in soft clay to accelerate consolidation:
- Drain spacing: 1.5–3.0 m in triangular or square pattern
- Radial drainage reduces time to 90% consolidation from years to months
- Mandrel-driven to depths of 20–30 m
- Used for embankment foundations, port reclamation (JNPT, Mundra Port), airport runways on soft ground
IS 15868 — Geosynthetics Standards in India
- IS 15868 Part 1: Geotextile test methods
- IS 15868 Part 3: Geomembrane specifications
- IRC 113:2018: Guidelines for use of geosynthetics in highways
- IS 14262: Geotextile tests (tensile, CBR puncture, peel, burst)
Key tests: Wide-width tensile (IS 14262), Apparent Opening Size (AOS/O₉₅), Permittivity, Transmissivity, CBR Puncture, UV resistance (geosynthetics degrade under UV if not covered promptly).
Frequently Asked Questions
Can a non-woven geotextile be used for reinforcement under a highway?
For separation and filtration, non-woven geotextiles are ideal. For structural reinforcement (requiring high stiffness under low strain), woven geotextiles or geogrids are more effective. Geogrids outperform geotextiles for reinforcement because their apertures allow mechanical interlock with aggregate particles, which non-woven textiles cannot achieve.
What is the design working life of HDPE geomembrane in a landfill?
HDPE geomembrane in a covered landfill liner system (protected from UV, contact temperatures 15–35°C, non-aggressive leachate) has a design life of 100+ years per EPA and GRI guidance. UV-exposed applications are much shorter (1–3 years if not covered). Chemical resistance varies by contaminant — always verify with chemical compatibility charts for the specific leachate/liquid.