How to Install a Geotextile Liner Under a Pond or Containment Liner

A geotextile liner, in pond and landfill work, is the needle-punched nonwoven fabric laid under (and sometimes over) a geomembrane to protect it from puncture. It is cheap next to the HDPE sheet it protects, and it is often installed in a hurry by the same crew that is waiting to start welding. Most puncture problems trace back to that hurry: stones left in the base, rolls dragged across the ground, joints that open up when the sheet goes on top. The tips below follow the order of work on site, from choosing the grade to placing cover, for an Indian contractor or site engineer.

Before the rolls arrive

### Pick the grade on puncture, not GSM alone

GSM is mass per square metre. It tells you roughly how much fibre is in the fabric, but the property that protects a geomembrane is resistance to a point load. Compare grades on CBR puncture strength (ASTM D6241) and thickness, and pick a heavier grade when:

• the base is murrum, gravelly or has angular stones that cannot all be removed;
• the water or waste above is deep, because every metre of water adds about 9.8 kPa pressing the sheet down on the fabric;
• gravel, riprap or concrete will be placed on top of the liner.

On Ocean’s projects, protection layers under pond geomembranes have typically been 250–400 GSM nonwovens, for example 400 GSM PET geotextile under a 1 mm HDPE liner at a water storage pond in Port Blair. Ocean’s geotextiles range covers PP and PET nonwovens from 90 to 1000 GSM. For how GSM relates to thickness, see how thick is a geotextile.

### Work out quantities with laps and run-outs

Order for the sloped area, not the plan area, and add the anchor trench, the overlaps and a margin for cutting.

Worked example (illustrative). A farm pond is 40 × 30 m at the top, 3 m deep, with side slopes of 1 vertical to 2 horizontal.

• The base is 28 × 18 m = 504 sq m.
• Each slope is √(6² + 3²) ≈ 6.71 m long. The two long slopes cover about 456 sq m and the two short ones about 322 sq m.
• A 1 m run-out at the crest plus a 0.5 × 0.5 m anchor trench adds about 2 m of fabric around the 140 m perimeter, or 280 sq m.
• Net area: about 1,562 sq m.
• With 4 m wide rolls lapped 300 mm, each roll covers 3.7 m of width, so add about 8%: roughly 1,690 sq m.
• Add about 5% for cutting around inlets and corners: about 1,775 sq m to order.

Run the same sum for the geomembrane, which has its own weld overlaps.

### Store the rolls properly

Keep rolls in their wrapping, off wet ground and out of the sun until they are needed. ASTM D4873 covers identification, storage and handling of geosynthetic rolls. Check every roll label against the delivery note and the approved grade before it goes into the pond.

Preparing the base

1. Excavate to shape and check levels. Make sure the crest is level all round, or the water line will expose fabric and liner on the high side.
2. Remove stones, roots and debris. Hand-pick the surface after excavation. Roots cut flush can regrow, so dig them out.
3. Fill soft spots and compact. Replace pockets of loose or organic soil with compacted granular fill.
4. Roll the surface smooth. Round off sharp changes of slope at the toe and crest.
5. Deal with water and gas. If groundwater may rise under the pond or the base holds organic matter, the design needs venting or drainage under the liner. A thin protection fabric alone will not carry that flow; our article on geomembranes for landfills and ponds explains why.

Laying the geotextile liner

1. Dig the anchor trench first. Roll the fabric over the crest into the trench so the top edge is held before the sheet goes on.
2. Unroll down the slope, not across it. Start at the crest and let the roll run to the toe. Laps then shed water and soil downhill and are less likely to be pulled open.
3. Never drag rolls across the base. Dragging picks up grit and can tear the fabric. Carry rolls on a spreader bar or lift them.
4. Lap every joint. A lap of 300 mm on firm, level ground is a common default; follow the drawings where they say more. On steep slopes and soft ground, heat-bond or sew the laps so they cannot slide apart.
5. Keep the fabric in full contact. It should follow the surface without bridging across hollows or being stretched tight over the toe.
6. Hold it down against wind. Use sandbags, not loose stones. Coastal and open sites in the pre-monsoon winds can lift a whole panel in minutes.
7. Inspect and patch. Walk the whole area before the geomembrane crew starts. Patch any tear with a piece of the same fabric that extends well beyond the damage on every side, and bond it in place.

Keep foot traffic to soft-soled shoes and keep vehicles off. A motorcycle crossing the fabric to save a walk round the pond can leave a stone pressed in exactly where the liner will sit.

Placing the geomembrane on top

• Deploy the sheet from the crest, the same way as the fabric, without dragging it over the geotextile.
• Lay and weld in the cooler hours where possible, so the sheet does not trap wrinkles as it cools.
• Make sure the geotextile laps are not pulled open as the sheet is moved.

The welding and seam testing that follow are a separate job; the lining article linked above covers them.

When to add a geotextile above the liner

A second geotextile layer on top of the geomembrane protects it from what is placed over it. It is worth adding:

• on slopes that will get a soil, gravel or concrete cover;
• under riprap, stone pitching or geocells at the waterline;
• at inlets, outlets and walkways where people or machines reach the liner;
• in fish and aquaculture ponds where the bottom is cleaned with tools.

A pond built with HDPE liner and PET geotextile at Nuh, Haryana and pond lining with 250 GSM geotextile in Bhopal are two examples of this kind of work. Ocean supplies the sheet itself as pond liners in a range of thicknesses.

Common installation mistakes

Frequently asked questions

Planning a pond or containment lining?

Send us the pond size, depth and base soil, and we will suggest a geotextile grade and liner thickness to match. Contact Ocean Non Wovens for a quote.