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

Project Snapshot

An intake pump house draws raw water from a river and delivers it to a treatment plant or supply network, so it must sit safely at the edge of a moving, sediment-laden river and stay stable for decades. At Sultanganj in Bhagalpur district, Bihar, on the banks of the Ganga, such a structure required reliable filtration and bank protection at its water-facing works. Ocean Non Wovens supplied 6,300 square metres of Ocean Non-Woven Geotextile in the 2 mm polypropylene grade for the project. This case study explains what the geotextile does here, why it was chosen over a conventional granular filter, and how it should be installed to last.

Why Geosynthetics Are Used in Water Intake Infrastructure

A river intake is a demanding location. The structure and its approach channel are exposed to flowing water, changing river levels, sediment, and a saturated, often weak foundation. The banks around the intake and the channel have to be protected against erosion, and the water that moves in and out of the ground has to be managed so it does not wash out the soil or build up pressure behind walls and slopes.

For a long time this was handled with graded granular filters, layered beds of sand and gravel placed under stone pitching or behind walls to let water through while holding soil back. These work, but they need large volumes of well-graded aggregate and are hard to build to specification or place accurately underwater. A non-woven geotextile does the same filtration job in a single engineered layer, which is why it has become the standard filter at water intake and river-protection works. The International Geosynthetics Society, the Geosynthetic Institute and Indian guidance such as IRC:SP:59 all treat the geotextile filter as established practice.

Why a Non-Woven Geotextile Instead of a Granular Filter

The main reason for choosing the geotextile is that it replaces a thick, multi-layer granular filter with one thin, consistent sheet. A sand and gravel filter can be several hundred millimetres thick and must meet strict grading rules at every layer, which is difficult on a river bank and nearly impossible to verify once stone is placed on top. A needle-punched non-woven geotextile is manufactured to a controlled, repeatable opening size and permeability, so its filtration performance is known before it reaches the site.

There are practical gains as well. The geotextile is light, quick to lay, and easy to cut and overlap around an irregular structure, and it cuts the quantity of imported aggregate, which matters on a river site where good filter material may not be available locally. Because it is a manufactured product tested to standard, its behaviour is predictable in a way a site-mixed granular filter is not, and at an intake, where a filter failure means either erosion of the foundation or clogging of the intake, that predictability is worth a great deal.

Engineering Challenges at an Intake Pump House

Several conditions make this work difficult. The foundation soil at a river edge is usually soft, silty and saturated, so it offers weak support and erodes easily. The water table is high and moves with the river, driving seepage in and out of the bank and creating the risk of piping, where flowing water carries fine soil away and hollows out the ground. When the river level falls quickly after a flood, water trapped in the bank drains back and can drag soil with it, a rapid-drawdown condition that must be allowed for. The structure can settle unevenly on variable ground, the current and scour attack the bed and banks, and the water-facing works usually have to be built in the short dry-season window when river levels are low.

What the Ocean Non-Woven Geotextile Does Here

The 2 mm non-woven geotextile performs several related functions at the intake. Its main job is filtration. Placed beneath the stone pitching or riprap that protects the bank and channel, it lets water pass freely between the soil and the river while holding the bank soil in place. This is what prevents the piping and washout that would otherwise undermine the protection and the structure.

It also separates and drains. As a separator, it keeps the soft bank soil from mixing into the granular bedding and stone armour, so the protection keeps its strength and does not sink into the ground. As a drainage medium, it relieves the pore-water pressure that builds in a saturated bank during changing river levels, one of the main causes of slope failure at a river edge. Laid under stone, the fabric also acts as a cushion that protects the layers beneath. Together these functions give reliable bank protection and a stable foundation for the pump house works.

The Technical Basis

Geotextile filtration is governed by two requirements that work against each other, and good design balances them. The first is soil retention: the openings in the fabric, described by the apparent opening size and measured to ASTM D4751, must be small enough to hold back the soil particles. The second is permeability: the fabric must be more permeable than the soil, measured as permittivity to ASTM D4491, so that water passes through without pressure building up. A filter that retains soil but is too fine will clog, and a filter that drains freely but is too open will let soil pipe through. Matching the fabric to the specific bank soil is therefore the core of the design.

The same fabric provides separation by keeping two materials apart under load, and drainage by transmitting water within its thickness to a free outlet. A single tested sheet delivers all of this, removing the need for the thick graded granular filter that traditional design relied on. Its strength properties, grab tensile to ASTM D4632 and puncture resistance to ASTM D4833, confirm it can survive the stone placed on it and the loads it carries in service.

Site Practice and Common Mistakes

A geotextile filter only works if it is installed correctly, and this is where projects often go wrong. The subgrade has to be trimmed smooth and cleared of sharp debris, because a projecting stone can puncture the fabric when the armour is placed. Panels must be overlapped or seamed by the specified amount, and that overlap has to increase on soft ground and underwater where panels can separate, since a gap in the filter is a direct path for soil to escape. The fabric needs to be anchored, usually in a trench at the top of the slope, so it does not slide or lift before the stone is placed.

Placing the stone pitching is the most damaging step. Dropping heavy stone from a height can tear or puncture the fabric, so the drop height is limited and the work done carefully. The fabric should be laid, anchored and covered in a controlled sequence rather than left exposed, because non-woven polypropylene loses strength under prolonged ultraviolet exposure. Quality control means checking the delivered fabric for opening size, permittivity and mass per unit area against the specification, and checking overlaps and cover on site. The common failures are simple ones: punctured fabric, insufficient overlap, the wrong opening size for the soil, and fabric left exposed to the sun.

Long-Term Performance

Polypropylene non-woven geotextiles are durable in this service. They resist the chemical and biological conditions of river water and soil and are unaffected by the salts and mild acidity found in most ground. Under the cyclic wetting and drying and changing water pressure of a river bank, the fabric keeps its filtration and drainage function, and provided the opening size is correctly matched to the soil it continues to filter without clogging. It is stabilised against ultraviolet degradation for the exposure that occurs during construction, and once buried under stone it is protected for the long term. Because the filter is a permanent buried layer with no moving parts, maintenance is minimal, and it avoids the repeated erosion repair that a failed or absent filter would require.

Sustainability

Replacing a graded granular filter with a single geotextile layer reduces the volume of sand and gravel that must be quarried and hauled, lowering both cost and the carbon of material transport. On a river site with limited local aggregate, that saving is significant. The fabric is quick to install, which shortens construction, and its long service life reduces rebuilding. These environmental benefits come directly from using less bulk material and building faster.

Where Non-Woven Geotextiles Are Used

The same functions are used across many works. Non-woven geotextiles serve as filters behind retaining walls and in subsurface and edge drains, as separators beneath roads and embankments, in canal linings, in water and wastewater treatment plants, in landfill drainage and capping, and in river and coastal protection under stone armour. Anywhere water must be let through while soil is held back, the non-woven geotextile filter is the standard solution.

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 a water intake, a treatment plant, a canal or a riverbank, the company can supply the correct geotextile grade for the soil and hydraulic conditions, with technical support to match the opening size and permeability to the design. To discuss filter, separation or drainage geotextiles for your project, contact Ocean Non Wovens.