Filtration That Lasts: Ocean Non-Woven Geotextile at the Sultanganj Intake Pump House

Project snapshot

At Sultanganj in Bhagalpur district, Bihar, an intake pump house on the bank of the Ganga needed a filter layer under the protection works on its water-facing side. Ocean Non Wovens supplied 6,300 square metres of Ocean Non-Woven Geotextile in the 2 mm polypropylene grade for the job.

An intake pump house draws raw water from the river and sends it to a treatment plant or supply network. It sits at the edge of a moving, sediment-laden river and has to stay stable for decades. This case study covers what the geotextile does at the intake, why it was used in place of a graded granular filter, and how it should be installed.

• Location: Sultanganj, Bhagalpur district, Bihar (Ganga bank)
• Structure: River intake pump house
• Product: Ocean Non-Woven Geotextile, 2 mm, polypropylene
• Quantity: 6,300 sq m
• Function: Filtration, separation and drainage under bank and channel protection

Why intakes use a geotextile filter

The banks around an intake and its approach channel face flowing water, changing river levels, sediment and a saturated foundation. They need protection against erosion, and the water moving in and out of the ground has to be managed so it neither washes soil out nor builds pressure behind walls and slopes.

The traditional answer is a graded granular filter: layers of sand and gravel under stone pitching or behind walls. It works, but it can be several hundred millimetres thick, every layer must meet grading rules, and it is hard to place accurately on a river bank or underwater and nearly impossible to check once stone is on top. A needle-punched non-woven geotextile does the same filtration job in one sheet with a controlled opening size and permeability, so its performance is known before it reaches site. Indian guidance such as IRC:SP:59 covers geotextile filters under stone protection, and the FHWA Geosynthetic Design and Construction Guidelines (FHWA-NHI-07-092) set out the filter criteria used to select one.

The sheet is also light, quick to lay, easy to cut and overlap around an irregular structure, and it reduces the imported aggregate a river site may not have locally.

Site conditions at a river intake

Foundation soil at a river edge is usually soft, silty and saturated, so it is weak and erodes easily. The water table moves with the river and drives seepage in and out of the bank, which can lead to piping: flowing water carries fine soil away and hollows out the ground. When the river falls quickly after a flood, water trapped in the bank drains back and can drag soil with it (rapid drawdown). Add uneven settlement on variable ground, scour from the current, and a short dry-season window for building the water-facing works.

What the geotextile does here

Filtration. Placed under the stone pitching or riprap that protects the bank and channel, the 2 mm geotextile lets water pass between soil and river while holding the bank soil in place. That is what stops piping and washout from undermining the protection and the structure.

Separation. It keeps the soft bank soil from mixing into the bedding and stone armour, so the armour keeps its thickness and does not sink into the ground.

Drainage. It relieves pore-water pressure in the saturated bank as river levels change, a common trigger for slope failure at a river edge.

Cushioning. Under stone, the fabric also protects the layers beneath from point loads.

Design basis: retention against permeability

Geotextile filter design balances two requirements that pull against each other. The fabric’s openings, described by the apparent opening size (AOS, ASTM D4751), must be small enough to retain the bank soil. Its permittivity (ASTM D4491) must be high enough that water passes more freely through the fabric than through the soil, so pressure does not build up. Too fine a fabric can clog; too open a fabric lets soil pipe through. Matching the fabric to the actual bank soil grading is the core of the design.

Strength properties such as grab tensile strength (ASTM D4632) and puncture resistance (ASTM D4833) show whether the fabric will survive the stone placed on it and the loads it carries in service.

Installation and common mistakes

A geotextile filter only works if it is installed correctly:

• Trim the subgrade smooth and remove sharp debris, or a projecting stone can puncture the fabric when armour is placed.
• Overlap or seam panels by the specified amount, and increase the overlap on soft ground and underwater, where panels can separate. A gap in the filter is a direct path for soil to escape.
• Anchor the fabric, usually in a trench at the top of the slope, so it does not slide or lift before the stone goes on.
• Limit the drop height of stone pitching. Dropping heavy stone from height is the step most likely to tear the fabric.
• Cover the fabric promptly. Non-woven polypropylene loses strength under prolonged UV exposure.
• On delivery, check opening size, permittivity and mass per unit area against the specification; on site, check overlaps and cover.

The usual failures are simple ones: punctured fabric, short overlaps, the wrong opening size for the soil, and fabric left in the sun.

Durability and maintenance

Polypropylene is resistant to the chemical and biological conditions of river water and most soils. Under cyclic wetting, drying and changing water pressure, the fabric keeps filtering and draining as long as its opening size suits the soil. It is UV-stabilised for the exposure that occurs during construction and is protected once buried under stone. As a buried layer with no moving parts, it needs little maintenance, and it avoids the repeated erosion repairs that a failed or missing filter causes.

Replacing a graded granular filter with one geotextile layer also cuts the sand and gravel that must be quarried and hauled, which lowers cost and transport emissions on a site with little local aggregate. The same filter function is used behind retaining walls, in subsurface and edge drains, under roads and embankments, in canal linings, in landfill drainage and capping, and under stone armour in river and coastal works.

About Ocean Non Wovens

Ocean Non Wovens manufactures and supplies non-woven and woven geotextiles, geobags, geocells, geogrids and HDPE liners, tested to Indian and international specifications, for water and environmental infrastructure, river and canal works, highways, railways, industrial developments and defence projects across India. For an intake, treatment plant, canal or riverbank, the team can help match a geotextile grade’s opening size and permeability to the soil and hydraulic conditions. More river and water projects are on the case studies page.