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Research proves slate waste potential for geotech structures

New research has shown that slate waste could provide a cheaper solution for construction of reinforced earth structure without compromising performance.

According to the study by Welsh Slate and Tensar International, use of slate by-product as an alternative to traditional aggregate fill in reinforced earth structures has been shown to be up to 60% more cost effective.

Egniol Environmental consultant engineer Owen Bracegirle led the project after he identified that secondary slate aggregate sourced from north Wales was not being used to its full potential.

The study set out to establish if secondary slate aggregate (13mm blinding) from Welsh Slate’s main Penrhyn Quarry was acceptable for use as the aggregate fill within a Tensar reinforced soil retaining structure.

Tensar product and technology manager for walls and slopes Craig Roberts said that was something the manufacturer had wanted to do for some time, as it had conservatively used slate fill previously on private schemes but required official test results for public projects.

“We had always been confident that slate could be used as an alternative but now we have the proof, through testing and this should give us an advantage at tender stage,” said Roberts.

The selected secondary slate aggregate used for the study, and all its associated parameters, was compared with a commonly-used primary aggregate fill (granite/limestone) to not only establish its structural performance and feasibility but most importantly, to highlight the financial benefits of its use.

The study first researched current and relevant design textbooks, online literature and design standards, such as BS 8006, to establish the design requirements and parameters of a suitable aggregate fill within a reinforced soil structure. Observation and analysis of all relevant testing results was then carried out to numerically compare slate aggregate against granite/limestone fill.

Additional site and laboratory testing was also undertaken as part of the study to address any gaps in the data. Most importantly, through research and testing, it was found that the reinforced earth design relied upon the reduction factors applied on Tensar geogrid grades.

These reduction factors are derived from the selected fill being used. It was concluded that the reduction factors RFCH (Chemical and Environmental Reduction Factor) and RFID (Installation Damage Reduction Factor) were critical in determining the feasibility of using the selected aggregate fill.

Installation damage testing was carried out at Penrhyn Quarry to ascertain the RFID for the secondary slate aggregate. This determined the selected secondary slate aggregate provided varying design reduction and safety factors over the alternative aggregates. 

These subsequent variances were inputted to two design cases. The design outputs highlighted the geogrid requirements and content for both the slate and granite/limestone design cases. The design outputs were also analysed taking aggregate costs into consideration and these highlighted that the slate aggregate not only performed successfully as a structural fill within a reinforced soil structure but also provided a commercial benefit through substantial cost savings.

Roberts cautioned that that using slate could require additional stronger layers of geogrid, as the material can cause minimal additional installation damage compared to granite and limestone, which has to be accounted for within the design.

“But the additional cost of this is more than offset by the overall structure savings,” he said.

Following the study, Tensar has already suggested slate as the aggregate fill for two major civil engineering infrastructure schemes scheduled to happen in the next couple of years.

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