THE PROBLEM

The project required road widening at horizontal curves, stabilisation of the road side and construction of a bus lay-by. The original pavement design to achieve the target modulus required by the Polish National Standard was based on replacing or improving existing organic soils, which was proving to be expensive.

THE SOLUTION

A Tensar Mechanically Stabilised Layer (Tensar MSL™) incorporating Tensar TriAx® geogrids was designed to achieve the required design modulus for the road class, whilst omitting the need for expensive ground replacement operations.

PROJECT DESCRIPTION

The standard of National Road No 62 between Kruszwica-Gocanowo in Poland was to be raised in order to carry traffic loads expected for Polish National Road Classification NRC 4.  The proposed reconstruction included widening to 7 m at the horizontal curves, stabilisation of the road side and construction of a bus lay-by.

Local soil conditions were found to be variable and typically of low strength.  Natural soils in the area were covered by layers of uncontrolled “made ground” with high moisture content.  A sub-grade modulus of 25 MPa was measured for use in the design assessment.

To comply with the Polish National Standard NRC 4, a modulus of 120 MPa based on an EV2 assessment was required at the top of the granular sub-base.  To achieve this target value, Tensar’s designers proposed a Tensar MSLincorporating Tensar TriAx® geogrids to reduce the construction thickness, whilst achieving the required surface modulus.

By adopting Tensar proposals and using the experience of Tensar’s long established distributor in Poland, significant cost reductions have been demonstrated compared with the original soil replacement technique.

The Tensar TriAx® geogrids were simply rolled out on the subgrade and suitable granular fill placed and compacted on top.  Some areas required a geotextile to enhance filter stability due to the local ground conditions and aggregate grading, but operational savings were achieved whilst maintaining design targets.