An asphalt-surfaced car park and truck access lane was required to be constructed over swamp land. This area contained tree root vegetation in saturated peat. The penetrometer-device CBR values at this two hectare site were generally less than 1%, i.e. very soft. An alternative to deep sand replacement was needed.


The Tensar distributor was contacted by the contractor for a solution to the construction of a pavement over weak soils. A design CBR of 2% was considered at this preliminary stage and three road types having different design ESA’s were analysed. The original design considered a deep excavation and replacement with Woodhill sand. Using Tensar software, it was possible to demonstrate that the incorporation of Tensar geogrid within the pavement structure would be an economical alternative for the contractor to develop further.


The subsequent approach taken by the contractor for this site having a highly variability in foundation conditions was to measure deflections from a previously constructed Tensar stabilised haul road on the site, with its known construction and layer thickness. The results were then compared with theoretical deflections, using the known depth of structure.

The back-analysis indicated a design subgrade CBR of 1% and less, and this was used for forward analysis and design of pavements. Deflection profiles, as successive pavement layers were constructed, were compared with targets derived from analysis (CIRCLY).

A performance-basis for design was developed by the Contractor. The theoretical targets of deflection that were established for the site, were reached and exceeded during construction.

The use of geotextiles and Tensar biaxial geogrid provided a number of benefits. Firstly they allowed for the construction of a haul road over very weak peat for heavy construction equipment. Secondly the separation function of the geotextile ensured that the design depth could be maintained during the fill placement and compaction, thereby preventing loss of expensive imported fill into the soft subgrade. Thirdly, the use of a mechanical stabilisation layer, incorporating Tensar biaxial geogrids, minimised construction time as this technique is less weather-dependent than chemical stabilisation or deep excavation methods. Lastly the performance of Tensar biaxial geogrid, which have been extensively tested overseas, was verified through locally accepted test methods.

By monitoring pavement construction layer by layer, a high level of quality control was achieved by the contractor.This straight forward measurable approach was also easily understood by the field crews.