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

Project Snapshot

Good roads depend on what happens below the surface far more than on what you can see. On a road and infrastructure project in Tanzania, the ground beneath the alignment was the main obstacle, and the solution began with a layer of fabric most drivers will never know is there. 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 explains what the geotextile does, why it was chosen, and how it protects the road for the long term.

Why Geosynthetics Are Used on Roads in Emerging Markets

Across much of East Africa, road builders face the same difficulty that Indian engineers know well. The natural ground is often soft, fine and moisture-sensitive, and good crushed aggregate has to be quarried and hauled over long distances at high cost. Geosynthetics change the equation by improving the ground in place and making the imported aggregate work harder. A thin, engineered fabric placed at the right level allows a thinner, longer-lasting pavement to be built with less material, which is why geotextiles have become standard practice on road projects in developing regions. The International Geosynthetics Society and agencies such as the FHWA treat separation and filtration geotextiles as established road-building technology.

What a 150 GSM Non-Woven Geotextile Is and What It Does

A non-woven geotextile is a permeable fabric made from polypropylene fibres that are needle-punched into a felt-like sheet. It is not a reinforcement grid and it is not a waterproof membrane. Its strengths are separation, filtration and drainage, and the 150 GSM grade, which refers to a mass of 150 grams per square metre, is a light to medium weight well 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. This matters more than it first appears, because 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 prevents the slow loss of fines that weakens a formation. Its third role was drainage, giving pore water a path to escape rather than building pressure in a saturated subgrade. Together these functions protect the investment in the granular layers and extend the life of the road.

Why a Geotextile Was Chosen Over Conventional Methods

Without a geotextile, the usual way to deal with a weak, wet subgrade is to dig it out and replace it, or to build an extra-thick granular layer to offset the contamination that will inevitably occur. Both waste material and time, and neither addresses the real problem, which is the boundary between the soft soil and the good aggregate. Excavation and replacement move large volumes of earth and demand a great deal of imported fill, while an over-thick aggregate layer still deteriorates once the subgrade soil starts to migrate into it.

A 150 GSM non-woven geotextile addresses that boundary directly and at low cost. It is light, quick to lay, and manufactured to a known and repeatable specification, so its behaviour is predictable in a way a site-built solution is not. Because it keeps the granular layer clean, it allows a thinner and more reliable pavement section, which reduces the quantity of aggregate that has to be quarried and transported. On a project where good stone is scarce and haulage is expensive, that saving is significant, and it is achieved without compromising the durability of the road.

The Engineering Challenges on Site

Several ground conditions made this project a good candidate for a separation geotextile. Much of the region is underlain by fine, moisture-sensitive soils, and in many parts of Tanzania that includes expansive black cotton soil, which swells when wet and shrinks when dry and offers very poor support to a pavement. A subgrade like this is weak, deforms under load, and pumps badly when water is present. The climate adds to the difficulty, with heavy seasonal rainfall that saturates the formation and drives the pore-water pressures that cause pumping and rutting. On top of this, the road had to carry heavy commercial and construction traffic, which concentrates loads and accelerates any weakness at the subgrade level. Finally, like most infrastructure work, the project ran to a schedule that left little room for the slow, material-heavy methods that a weak subgrade would otherwise demand.

How the Geotextile Improves Road Performance

The contribution of the 150 GSM geotextile is best understood through what it prevents. By maintaining a clean separation between the subgrade and the sub-base, it preserves the load-carrying capacity of the granular layer, so wheel loads continue to be spread properly to the soil below instead of punching through a contaminated, weakened base. By filtering and draining, it keeps the subgrade from softening further and relieves the water pressure that causes pumping, which is one of the main mechanisms of early road failure on wet ground. The combined effect is a firmer, more stable formation, better control of rutting and differential movement, and a longer service life for the pavement. In practical terms, the road behaves as if it were built on better ground than it actually sits on, which is exactly the outcome a separation geotextile is designed to deliver.

The Technical Basis

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

Installation and Site Practice

The performance of any separation geotextile depends on careful installation. The subgrade is first trimmed and cleared of sharp objects that could puncture the fabric. The geotextile is rolled out across the formation and overlapped at the joints by the specified amount, with the overlap increased on softer ground where panels can move apart. The granular sub-base is then spread by pushing material forward over the fabric rather than driving directly on the exposed sheet, and aggregate is not dropped from a height that could tear it. Because polypropylene loses strength under prolonged sunlight, the fabric is covered promptly rather than left exposed. The most common mistakes are simple ones, namely insufficient overlap, driving plant directly on the uncovered geotextile, and using the wrong opening size for the soil, and all were guarded against here.

Long-Term Performance

Polypropylene non-woven geotextiles are well suited to a long service life in this role. The polymer resists the chemical and biological conditions of soil and water and is unaffected by the mild acidity and salts found in most ground. Under the repeated wetting, drying and loading that a road experiences, the fabric continues to separate and filter without breaking down, and provided its opening size is matched to the soil it keeps working without clogging. It is stabilised against ultraviolet exposure for the short period before it is covered, and once buried it is protected for the design life of the road. Because it is a permanent buried layer with no moving parts, it needs no maintenance, and its main lifecycle benefit is that it avoids the recurring repair and early reconstruction that a contaminated, pumping subgrade would otherwise force.

Where Non-Woven Geotextiles Are Used

The same separation, filtration and drainage functions apply far beyond a single road. Non-woven geotextiles serve as separators and filters in highways, rural roads and haul roads, in railway formations, behind retaining walls and in subsurface and edge drains, in the filter layers of dams and canals, beneath river and coastal erosion protection, and in landfill drainage. Anywhere water must pass while soil is held in place, or two dissimilar materials must be kept apart, the non-woven geotextile is a dependable and economical solution.

About Ocean Non Wovens

The supply of 15,350 square metres of geotextile to this Tanzania project reflects the reach and reliability Ocean Non Wovens brings to infrastructure work. As a leading Indian manufacturer and supplier of non-woven and woven geotextiles, geocells, geogrids, geobags and HDPE liners, the company produces materials tested to international specifications and delivers them at the volumes and timelines large civil works require, both across India and to export markets abroad. Ocean Non Wovens supports highways and roads, airports, railways, industrial and environmental developments, water resources and defence infrastructure, supplying geosynthetics engineered for demanding ground and difficult climates. For separation, filtration, reinforcement, drainage or containment on your next project, Ocean Non Wovens can supply the correct grade for the soil and loading, backed by technical support to match the product to the design.