Geogrid in Road Construction: Base Reinforcement and Rut Reduction

Why Roads Fail: The Subgrade Problem

Most road failures originate not in the asphalt surface layer but in the subgrade — the natural or compacted soil beneath the pavement structure. When a vehicle wheel load is applied to the pavement surface, the stress is transmitted downward through the pavement layers and ultimately to the subgrade. If the subgrade is too weak to support this stress without excessive deformation, the pavement above it deforms, cracks and eventually fails.

The California Bearing Ratio (CBR) is the standard measure of subgrade strength. A CBR of 3% or less indicates a very weak subgrade (soft clay, peat, saturated silt) that requires either deep excavation and replacement, ground improvement, or a thick granular sub-base to distribute the wheel load over a larger subgrade area. A CBR of 10–15% indicates a moderate subgrade (dense sand, gravel) that can support a normal pavement structure with a standard granular sub-base thickness.

The fundamental problem with granular sub-base on weak subgrade is lateral spreading: under repeated wheel loads, the granular material tends to spread laterally outward from beneath the loaded area, reducing the sub-base thickness and increasing the stress on the subgrade. This progressive lateral spreading is the primary mechanism of rutting in unpaved roads and the primary cause of premature failure in paved roads on weak subgrade.

How Geogrid Reinforcement Works

Geogrid reinforcement placed at the interface between the subgrade and the granular sub-base layer prevents lateral spreading of the granular material by providing a tensile restraint that confines the aggregate particles. The mechanism works in two ways: interlocking (the geogrid apertures interlock with the aggregate particles, preventing lateral movement of individual stones) and tensile membrane effect (the geogrid deflects under load and develops tensile stress that partially supports the applied load, reducing the stress transmitted to the subgrade).

The interlocking mechanism is the dominant reinforcement effect for stiff geogrids (biaxial PP or multidirectional PP) placed in granular sub-base. The geogrid apertures must be compatible with the aggregate particle size for effective interlocking — the optimal aperture size is approximately 1.0–1.5× the D50 of the aggregate. For typical crushed stone sub-base (D50 = 20–40 mm), the optimal geogrid aperture is 25–50 mm.

The tensile membrane effect becomes significant when the geogrid deflects under load — typically in unpaved roads or in the early stages of pavement loading before the sub-base has fully consolidated. For paved roads where the sub-base is well-compacted, the interlocking mechanism dominates and the tensile membrane effect is secondary.

Rut Reduction and Design Life Extension

Biaxial geogrid is used to limit lateral spread of the aggregate, which is one mechanism of rutting. Whether it reduces rutting on a given road, and whether the granular layer can be thinner, are pavement-design checks. This page does not state a rutting reduction or a thickness reduction.

On a weak subgrade the designer may check a reinforced section against an unreinforced one. Any change in design life or in granular thickness comes from that check, with the method named. This page does not state a longer life or a thinner layer.

For heavy haul roads and port yards subject to high axle loads, multidirectional geogrid (triangular aperture) provides better rut resistance than biaxial geogrid due to the more uniform aggregate confinement under omnidirectional loading. Do not treat a rut-reduction percentage from a general article as the result on a road. Ask for the trial method if a figure is offered.

The long-term performance of geogrid reinforcement depends on the creep resistance of the geogrid material under sustained load. PP geogrid has good creep resistance for road base applications where the sustained load is relatively low (the traffic load is dynamic, not sustained). For applications with high sustained loads (embankment fill, retaining wall backfill), PET geogrid with lower creep rate is preferred.

Installation Method

Geogrid installation for road base reinforcement is straightforward and does not require specialised equipment. The installation sequence is: prepare the subgrade surface (grade, compact, proof-roll to identify soft spots), unroll the geogrid panels perpendicular to the road centreline (the strong machine direction of biaxial geogrid should be oriented in the direction of traffic travel for maximum benefit), overlap adjacent panels by 300–500 mm (or connect with bodkin bars for high-tension applications), pin the geogrid to the subgrade at 1–2 m intervals using U-shaped steel pins to prevent displacement during aggregate placement, place and compact the granular sub-base in layers not exceeding 200 mm compacted thickness.

The geogrid must be kept taut during aggregate placement to ensure it remains flat and in full contact with the subgrade. Aggregate should be placed from the end of the panel and spread forward — never drive construction equipment directly onto the unprotected geogrid. A minimum cover of 150 mm of aggregate must be placed before construction vehicles travel over the geogrid.

For biaxial geogrid rolls 4–6 m wide, a single roll typically covers the full width of a two-lane road (7–8 m) with one or two panels. For wider roads, panels are overlapped at the centreline. The overlap should be in the direction of aggregate placement to prevent the leading edge of the panel from being pushed down by the aggregate.

Cost-Benefit Analysis

The cost-benefit of geogrid reinforcement in road construction depends on the subgrade CBR, the design traffic loading, the cost of granular sub-base material and the cost of the geogrid. The benefit is greatest where sub-base material is expensive (remote sites, limited local stone) and the subgrade is weak.

Do not take a thickness saving, a cubic-metre saving or a longer life from this page. Price the sub-base and the geogrid for this road, on the pavement design.

For unpaved access roads and haul roads where the primary concern is rut depth rather than pavement life, the cost-benefit is more straightforward: geogrid reinforcement reduces maintenance frequency (regrading, re-gravelling) only where the maintenance records support it. This note states no percentage and no payback period.

Shuangcheng Biaxial and Multidirectional Geogrid Specifications

Shuangcheng biaxial geogrid is manufactured from polypropylene using the biaxial stretching process. Available in tensile strength grades of 15/15 kN/m, 20/20 kN/m and 30/30 kN/m (longitudinal/transverse). Aperture size 30 × 30 mm. Roll width and length are on the quotation. Confirm the current data sheet. Suitability for a subgrade is a pavement design, not a CBR band on this page.

Shuangcheng multidirectional geogrid uses a triangular aperture geometry with ribs at 0°, 60° and 120°. Available in tensile strength grades of 20 kN/m and 30 kN/m (in each rib direction). Aperture size 40 mm triangular. Roll width 4 m, roll length 50 m. Suitable for heavy haul roads, port yards and aircraft pavements where omnidirectional loading requires uniform aggregate confinement.

Both products are supplied with carbon black UV stabiliser (≥ 2% by weight) for outdoor durability, and are chemically inert to the full range of soil pH and groundwater chemistry encountered in road construction applications.