Structural Pavement Design with Geocells made of Novel Polymeric Alloy (Kief), Geosynthetics 2015
Case study in St Petersburg shows how Neoloy® Tough-Cells were incorporated in ME design method, and how they reduce pavement thickness by more than 25%, double design life and carry 3x traffic
A case study in E18 motorway in St. Petersburg by Kief (2015) demonstrated how NPA Tough-Cells (PRS Neoloy Tough-Cells) – the only cell guaranteed to maintain its volume for the pavement design life – are incorporated into the Mechanistic-Empirical (ME) Structural Pavement Design method for structural pavements. Stress monitoring by plate load tests verified that the NPA Tough-Cell confinement mechanism improves granular sub-base/base material modulus by a factor of 2.45 to over 1000 MPa on a sand sub-base (MIF – Modulus Improvement Factor). This reduced vertical stress by almost 50% and allowed a 27% reduction in the pavement thickness. Consequently, an NPA Tough-Cell reinforced pavement can sustain almost three times more traffic and almost double design life, compared to a conventional equivalent.
Although the use of geocells in geotechnical applications dates back to the early 1980’s, two major drawbacks prevented their widespread adoption in structural pavements:
- The ability to maintain structural stability for the pavement design life, and
- A lack of integration into the Mechanistic-Empirical (ME) Structural Pavement Design method.
In recent years extensive research and comprehensive field tests were conducted on geocell technology. One result of these studies was the development of a novel polymeric alloy (NPA), which significantly decreases large strains associated with HDPE-based Soft-Cells that prevented their implementation in flexible pavements below the asphaltic layer.
Two major innovative mechanisms are associated with NPA Tough-Cells:
- Beam Effect – the ability to create a semi-rigid platform as a result of the confinement of unbound granular material
- Significant vertical stress reduction to the under-laying layers.
The development of novel polymeric alloy (NPA) “Tough-Cells” and their enhanced reinforcement mechanisms enable a new use for geocells in today’s highway infrastructure.
In parallel, design methodologies had to be developed to quantify the contribution of the NPA Tough-Cell reinforcement to the pavement structure, to be used, for example in ME pavement design. A Modulus Improvement Factor (MIF) was developed to calculate the increased modulus (stiffness) contribution of the NPA Tough-Cell reinforcement.
Case Study – Western High-Speed Diameter (ZSD)
A proof-of-concept case study was carried out on the St Petersburg E18 Motorway, featuring a conventional pavement design section and an NPA Tough-Cell reinforced section that designed to be structurally equivalent. Pressure cells were installed on the sand infill surface of the two pavement sections to verify the design and MIF assumptions. The testing used dynamic plate loading to assess the vertical stresses in the pressure cells and the accumulated plastic deformations in both sections.
Semi-cyclical plate load testing on surface of granular layer measuring vertical stresses in pressure cells
Results – Modulus improvement, reduced asphalt thickness
- The MIF for the NPA Tough-Cell reinforced section of up to 2.45, demonstrated that NPA Tough-Cell reinforcement can upgrade granular sub-base/base material modulus to a modulus suitable for a stabilized granular material
- The accumulated plastic strain at the NPA Tough-Cell confined surface was 2.25 mm in comparison to 3.04 mm at the conventional solution
- The NPA Tough-Cell reinforced solution showed a reduction of almost 50% in the vertical stress of the asphalt layer
Benefits – More traffic, reduced asphalt thickness, less maintenance
The implications of the results above verify the following significant advantages to utilizing NPA Tough-Cells in structural pavements:
- Decreased accumulated plastic strain allows for a much higher number of load applications and consequently prolonged time between maintenance
- Reduction in asphalt layer thickness can almost double design life from 20 to 38 years
- NPA Tough-Cell reinforced pavement can sustain 2.9 times more traffic in comparison to conventional solutions