Building Roads on Difficult Ground: Ocean Non-Woven Geotextile 150 GSM on a Tanzania Infrastructure Project

Project snapshot

On a road and infrastructure project in Tanzania, the main obstacle was the ground beneath the alignment. Ocean Non Wovens supplied 15,350 square metres of Ocean Non-Woven Geotextile in the 150 GSM polypropylene grade, used mainly for separation and filtration between the weak subgrade and the granular layers above. This case study covers what the geotextile does, why it was chosen, and how it protects the road over its service life.

Why geosynthetics are used on roads in emerging markets

Road builders across much of East Africa face the same difficulty Indian engineers know well. The natural ground is often soft, fine and moisture-sensitive, and good crushed aggregate has to be quarried and hauled long distances at high cost. A geotextile placed at formation level improves the ground in place and keeps the imported aggregate doing its job, so a thinner pavement can be built with less material. The International Geosynthetics Society and the US Federal Highway Administration (FHWA) treat separation and filtration geotextiles as established road-building practice.

What a 150 GSM non-woven geotextile does

A non-woven geotextile is a permeable fabric made from polypropylene fibres needle-punched into a felt-like sheet. It separates, filters and drains; it does not reinforce like a geogrid or hold back water like a membrane. The 150 GSM grade (a mass of 150 grams per square metre) is a light to medium weight suited to separating a subgrade from a granular layer on roads and similar works.

On this project the fabric did three linked jobs. Its primary role was separation: placed between the soft subgrade and the sub-base, it kept the two materials from mixing under construction and traffic loads. When soft soil pumps up into the aggregate and the aggregate punches down into the soil, the granular layer loses its strength and the road fails from within. Its second role was filtration, letting water pass while holding soil particles back, which stops the slow loss of fines that weakens a formation. Its third role was drainage, giving pore water a path to escape instead of building pressure in a saturated subgrade.

Why a geotextile instead of conventional methods

Without a geotextile, the usual answers to a weak, wet subgrade are to dig it out and replace it, or to build an extra-thick granular layer to allow for the contamination that will follow. Excavation and replacement move large volumes of earth and need a lot of imported fill. An over-thick aggregate layer still deteriorates once subgrade soil starts migrating into it, because neither method deals with the boundary between soft soil and good aggregate.

A 150 GSM non-woven geotextile treats that boundary directly and at low cost. It is light, quick to lay and made to a known, repeatable specification. Because it keeps the granular layer clean, the pavement section can be thinner, which cuts the quantity of aggregate to be quarried and transported. Where good stone is scarce and haulage is expensive, that saving adds up.

Ground and site conditions

Several conditions made this project a good candidate for a separation geotextile. Much of the region is underlain by fine, moisture-sensitive soils, and many parts of Tanzania have expansive black cotton soil, which swells when wet, shrinks when dry and gives very poor support to a pavement. A subgrade like this deforms under load and pumps when water is present. Heavy seasonal rainfall saturates the formation and drives the pore-water pressures behind pumping and rutting. The road also had to carry heavy commercial and construction traffic, which concentrates loads on any weakness at subgrade level, and the programme left little room for slow, material-heavy ground treatment.

How the geotextile improves road performance

With the subgrade and sub-base kept apart, the granular layer keeps its load-carrying capacity and continues to spread wheel loads to the soil below instead of punching through a contaminated base. Filtration and drainage stop the subgrade softening further and relieve the water pressure that causes pumping, one of the main mechanisms of early failure on wet ground. The result is a firmer formation, better control of rutting and differential movement, and a longer pavement life.

The technical basis

Geotextile filtration balances two opposing requirements. The fabric must retain the soil, so its openings, described by the apparent opening size (AOS) and measured to ASTM D4751, must be small enough to hold back the particles. It must also stay more permeable than the soil, a property described as permittivity and measured to ASTM D4491, so water passes without pressure building up behind it. A fabric that is too fine can clog and one that is too open lets soil through, so the grade is matched to the soil. Separation also depends on the fabric surviving aggregate placement and traffic, which is why grab tensile strength (ASTM D4632) and puncture resistance (ASTM D4833) are specified. For road separation over a fine-grained subgrade, a 150 GSM non-woven fabric gives a sound balance of these properties.

Installation and site practice

A separation geotextile only performs if it is installed carefully. The subgrade is trimmed and cleared of sharp objects. The fabric is rolled out across the formation and overlapped at the joints by the specified amount, with larger overlaps on softer ground where panels can move apart. The granular sub-base is then pushed forward over the fabric rather than driven onto the exposed sheet, and aggregate is not dropped from a height that could tear it. Polypropylene loses strength under prolonged sunlight, so the fabric is covered promptly. The usual mistakes are simple ones: insufficient overlap, driving plant on the uncovered geotextile, and using the wrong opening size for the soil.

Long-term performance

Polypropylene resists the chemical and biological conditions found in most soils and groundwater, including mild acidity and salts. Under the repeated wetting, drying and loading of a road, the fabric keeps separating and filtering, and if its opening size suits the soil it does not clog. UV stabilisation covers the short period before it is buried. As a permanent buried layer it needs no maintenance; its lifecycle benefit is avoiding the repeated repair and early reconstruction that a contaminated, pumping subgrade would force.

Where non-woven geotextiles are used

The same separation, filtration and drainage functions apply well beyond roads: highways, rural and haul roads, railway formations, behind retaining walls, in subsurface and edge drains, in the filter layers of dams and canals, beneath river and coastal erosion protection, and in landfill drainage. Wherever water must pass while soil stays in place, or two dissimilar materials must be kept apart, a non-woven geotextile is an economical answer.

About Ocean Non Wovens

Ocean Non Wovens supplied 15,350 square metres of geotextile to this Tanzania project. The company is an Indian manufacturer and supplier of non-woven and woven geotextiles, geocells, geogrids, geobags and HDPE liners, supplying projects across India and in export markets. Its materials go into highways and roads, airports, railways, industrial and environmental developments, water resources and defence infrastructure. For separation, filtration, reinforcement, drainage or containment on your next project, Ocean Non Wovens can recommend the grade for your soil and loading and support the match between product and design.

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