Research

Road Reinforcement Field Trial, KIWA Institute, NL, Van Gurp and Westera (2008).

NPA geocell pavement reduction factor exceeded the test values for geogrids by 30-40% in a controlled field trial of stiffness and deformation for road bases with 7 leading geogrids.

This controlled field trial, conducted by KIWA (formely KOAC-NPC), the leading road research and standards institute in Netherlands in 2008, compared the reinforcement factor of various geogrids in base courses of pavement structures and ballast beds. NPA Geocells were the only geocell among the tested geogrids from 7 leading geogrid manufacturers, and the only product tested that could accommodate inferior aggregate material as road base infill. The results showed that Neoloy Geocells had the highest Road Base Thickness Reduction Factor of any tested product (0.72), and substantially exceeded the values for geogrids.

Challenge – Comparing reinforcement products

CROW Publication 157 “Thin asphalt roads” highlights the importance of comparing results from matching projects, when measuring Life Extension Factor (LVF), Road base Thickness Reduction Factor (FRF) and similar variables. However, most studies did not have the requisite scope to compare the performance and response characteristics of multiple products.

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Neoloy Geocells made from different materials, with different cell heights and diameters were tested in a test box. The test result show that a single layer increases the bearing capacity of the soil and reduces the vertical stresses on the soft subgrade.

Study includes lab plate box test and in-situ asphalt pavements (Road K-23).

Results show that Neoloy decreased vertical stresses by 30-, reduced deflections on the asphalt service by15- and the back-calculated layer modulus increased by 10-.

Geocells consist of a series of interconnected single cells which are connected at their joints and forming a honeycomb structure. The geocells are expanded at the construction site and filled with soil. The cell walls completely encase the infill material and provide an all -around confinement to the soil. During vertical loading hoop stresses within the cell walls and earth resistance in the adjacent cells are mobilized which increase the stiffness and the load-deformation behavior of the soil. 1g model tests on a scale of 1:1 were carried out to evaluate the influence of a geocell layer on the load-deformation behavior of th e soil. Geocells made from different materials, with different cell heights and different cell diameters were tested in a test box with inside dimensions of 2m length, 2m height and 2m width. To simulate soft subgrade material an artificial mixed soil called “Glyben” was used. The test results show that a geocell layer increases the bearing capacity of the soil and reduces the vertical stresses on the soft subgrade. To verify these results geocell reinforced and unreinforced in-situ test fields were carried out within different asphalt paved road constructions. Vehicle crossing tests and falling weight deflectometer measurements show that the stresses beneath the geocell layer were reduced about 30 percent, the deflections on asphalt surface were reduced about 15 percent and the back calculated layer modulus were increased about 10 percent compared to the unreinforced test section.

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Large-scale model static load tests in test box and two in-situ field test asphalt road constructions were carried out to evaluate the influence of a geocell layer on the load deformation behavior of the soil.

Geocells increase bearing capacity of infill soil x 1.1-1.7x.

Vertical stresses were reduced in soft subgrade by ~30% and deflections on the surface were reduced by ~ 25% .

The performance of 40 cm Neoloy reinforced layer with medium grade infill was equal to a 70 cm unreinforced high quality aggregate layer.

Geocells consist of a series of interconnected single cells that are manufactured from different types of polymers. The geocells are expanded at the construction site and filled with soil. The cell walls completely encase the infill material and provide all-round confinement to the soil. During vertical loading, hoop stresses within the cell walls and earth resistance in the adjacent cells are mobilized which increases the stiffness and the load-deformation behaviour of the soil. Thus the soil-geocell layers acts as a stiff mat and distribute the vertical traffic loads over a much larger area of the subgrade soil. Large scale static load tests were carried out to evaluate the influence of a geocell layer on the load-deformation behaviour of the soil. The test results show that a geocell layer increases the bearing capacity of the infill materials up to three times compared to an unreinforced soil. The vertical stresses on the soft subgrade, measured by eight earth pressure cells, where also reduced about 30 percent. To verify the results of model tests in-situ field test where carried out within different road constructions. Earth pressure cells where installed on the subgrade to measure the influence of the geocell layers on the stress distribution. After finishing the road construction vehicle crossing tests with a 40-ton truck were carried out while the stresses on the subgrade where measured. Compared to an unreinforced test section the stresses beneath the geocell layer were reduced by about 30 percent. In addition to vertical stress measurements, falling weight deflectometer (FWD) measurements were conducted in reinforced and unreinforced test sections. The results show that the deflections measured in geocell reinforced test section were significantly smaller than in the unreinforced section. Back calculated layer modules were significantly higher in the geocell reinforced section compared to an unreinforced section.

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A comprehensive performance test was conducted on high-speed rail track which suffered from geometry track degradation and substructure problems. As part of a US Federal Railway Administration (FHA) program to promote innovative railway technologies, Amtrak (US National Railroad Corporation) used PRS Neoloy® Geocells to add support and stiffness in a complete rebuild of a section of high-volume rail corridor with severe ballast fouling from problematic soils, which carries 2,200 high-speed commuter and freight trains daily. The project and impact of the Neoloy Geocells was fully monitored and evaluated on a regular basis by the Harsco Rail Consulting Group, Senior Researchers at the Universities of Delaware and Colombia, as well as by the FHA and Amtrak, to assess the impact of NPA (Novel Polymeric Alloy) Geocells on the track geometry performance.

The authors of the report (Palese, Harsough, Zarembski, Thompson, Pagano and Ling) presented their findings at the 2017 AREMA (American Railway Engineering and Maintenance-of-Way Association Conference. They concluded that Neoloy Geocells improved subgrade pressure distribution considerably and reduced the rate of track geometry degradation. In particular, Neoloy Geocells reduced pressure at the ballast/subgrade interface by 50% and reduced track maintenance cycles by a factor of more than 6x. This typically reduces maintenance cycles from every 3-4 months to once every 3 years, representing a significant return on investment for a site experiencing track geometry degradation, requiring frequent resurfacing.

Results – Significantly reduced pressure, more stable track geometry

In the section of track reinforced with Neoloy Geocells, recorded pressure data magnitude was roughly 50% of the pressures in the non-reinforced rebuilt section of track. Data plotted in the 6-month period following the track rebuild, shows that reinforced section was stabilized, experiencing slight degradation, while the unreinforced section suffered from a high rate of degradation.Graph of Track Quality Index (TQI) before and after installed of Neoloy Geocell Reinforcement.

Before the rebuild and geocell material installation, a TQI degradation rate of 0.0265 inches/month was calculated. After rebuild with the geocell material, a rate of 0.004 inches/month was calculated, constituting a reduction in track geometry degradation rate of 6.7x. Consequently, while the track previously required maintenance every 3-4 months, with Neoloy Geocell reinforcement maintenance could be extended to once every 3 years. Results suggested that a site experiencing track geometry degradation and undergoing resurfacing at least once annually, would be a candidate for Neoloy Geocell installation with a positive return on investment.

Assuming a surfacing cost of $12,000/per mile, and approximately $25,000/mile in train delay costs, using an estimate of a 50-year NPA Geocell life span, it was calculated that a return on investment of approximately 112% could be expected by using Neoloy Geocells.

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Geocells were originally used to reinforce base layers over soft cells. But this study, published in Geosynthetics India, provides academic proof that NPA Geocells are also highly effective in upper base layers. Building on studies by Han, Pokharel and others, which demonstrate that NPA Geocell-reinforced bases have a significantly higher performance than unreinforced bases, this study shows that an NPA-Geocell reinforced layer has a modulus improvement factor of 2.75. Field plate load tests and a demonstration project, revealed that NPA Geocells significantly reduce construction and maintenance costs.

Validation – Field plate load testing

A field study was carried out on samples from an access road at the Govind dairy in Phaltan. Plate load tests were carried out in December 2010 to compare the modulus of NPA Geocell reinforced samples and unreinforced samples.

The test found that the NPA Geocell reinforced soil layer has a modulus improvement factor of 2.75, confirming similar results in other research.

In addition, the following visual observations were noted:

The unreinforced road sections showed extensive undulations, while the NPA Geocell reinforced road sections had maintained a perfect level surface

The unreinforced road sections displayed severe rutting, requiring frequent maintenance

Validation – Demonstration project

A further test was carried out on a 0.5 kilometre demonstration section on the Cross Israel Highway 6, as part of a road widening project. Plate load testing, comparing an NPA Geocell reinforced sample with an unreinforced sample, was designed specifically to investigate reducing the cost of infill and asphalt. The test found a modulus improvement factor of 2.92 on the NPA Geocell reinforced sample

Results – Lower thickness, less maintenance

This study demonstrated that not only do NPA Geocells display high tensile strength, resistance to creep, dimensional stability, but that they can be deployed to great effect in the upper base layers of heavy-duty paved roads. Due to the increased modulus, the following results for the demonstration project were recorded:

Asphalt thickness could be reduced from 200 mm to 160 mm

High cost crushed stone could be replaced with lower quality, cheaper subbase material

Total subbase layer thickness could be reduced by 20 mm

One complete deep scraping and overlaying of asphaltic layers could be saved over a 20-year design life

Benefits – Lower costs, better for environment

Using NPA Geocells was found to increase strength and bearing capacity, enabling a reduction in layer thickness of asphalt, base and subbase layers. The demonstration project showed that savings could therefore be made:

8% reduction in construction costs

50% reduction in maintenance costs over a 20-year period

Additional savings on manpower, equipment and logistics

Greater use of local materials and manpower, reducing the carbon footprint

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This controlled field trial, conducted by KIWA (formely KOAC-NPC), the leading road research and standards institute in Netherlands in 2008, compared the reinforcement factor of various geogrids in base courses of pavement structures and ballast beds. NPA Geocells were the only geocell among the tested geogrids from 7 leading geogrid manufacturers, and the only product tested that could accommodate inferior aggregate material as road base infill. The results showed that Neoloy Geocells had the highest Road Base Thickness Reduction Factor of any tested product (0.72), and substantially exceeded the values for geogrids.

Challenge – Comparing reinforcement products

CROW Publication 157 “Thin asphalt roads” highlights the importance of comparing results from matching projects, when measuring Life Extension Factor (LVF), Road base Thickness Reduction Factor (FRF) and similar variables. However, most studies did not have the requisite scope to compare the performance and response characteristics of multiple products.

Validation – Loading tests on multiple geosynthetics

Tests were conducted in the Netherlands on experimental pavement sections, constructed in large hangars (1,250 sq. m) with each geosynthetic installed on 55 cm clay subgrade (CBR=1.4%). The pavement was divided into 30 test sections (each 4×4 sq. m) of varying reinforcing geosynthetics, and including 7 unreinforced control sections. Each sample consisted of a road base, subbase and subgrade. It should be noted that the NPA Geocell sample was the only one tested using a recycled construction waste infill material.

Stiffness and deformation of the test sections were tested by FWD loading. Deformation was measured using 2 contact displacement transducers, while a penetrologger measured cone resistance of the clay subbase.

Results – Road base thickness reduction factors exceeds limits

The comparative test results recorded a road base reduction factor (RF) for NPA Geocells of 0.73. This figure was off the charts, with the highest published RF for geogrids of 0.5 set as the maximum limit. NPA Geocells recorded a mean RF of 0.43, higher than all other products tested. In addition, the tests also demonstrated that Neoloy Geocells cause a significant reduction in deformation.

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This article by Prof. Leshchinsky (et al) summarizes testing of Neoloy earth retention walls on the large shake table at the National Seismic Research Institute in Japan. The tests on a variety of wall types replicated seismic activity similar to a severe earthquake. The study concluded that novel polymeric alloy geocells – more suitable for long-term use than HDPE geocells – can be used successfully to form gravity walls as well as reinforcement layers even when subjected to a very high seismic load beyond that of the Kobe earthquake.

The Editors of Geosynthetics Magazine noted that this article departs from policy by focusing on a specific product….”in an effort to offer a guideline, an example, of how product development for the geosynthetics industry can be done effectively (to)…advance the geosynthetics industry into the 21st century with much success.”

Challenge

Ideally, the design of any structure subjected to earthquakes should be based on tolerable recoverable and/or permanent displacements, but this approach is difficult to implement for many reasons. The objective of this study was to quantify a reasonable reduction factor (RF) on the as peak ground acceleration (PGA) for geocell retention structures. The RF can then be integrated with limit equilibrium (LE) stability analysis to conduct seismic and static design.

Conclusions

The study resulted in recommended seismic reduction factors (0.3-0.4) that are used in the design of gravity and reinforced walls. Tests 1 and 2 show that gravity walls made of geocell can perform well under seismic loading. Such gravity systems may be economical for walls up to 3-4m high. Tests 3 and 4 show that a reinforced system, made entirely of geocell and soil, can be effective and likely economical.

Prof. Leshchinsky noted, however, that geocells made from HDPE are unsuitable for long-term applications, and guidelines were given to PRS to further the development of its Neoloy based geocell for demanding applications requiring long-term performance.

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