Webinar Q & A Transcript – Lessons learnt from geogrid reinforced wall failures

1. Does the geogrid/reinforcement installation increases the safety factor. If yes, by how much will it be increased?

Yes geogrid increases Factor of Safety (FOS). Reinforcement length is very dependent on design height and design load. The geogrid strength and length have a directly proportional effect on the FOS.

2. What is the 75mm max particle size based on – just an arbitrary number?

Transport for NSW (TfNSW, previously RMS) QA SPECIFICATION R57-DESIGN OF REINFORCED SOIL WALLS, Table R57.10; and Transport and Main Roads Specifications MRTS06 Reinforced Soil Structures, Table 7-2(b).

3. What types of soil are suitable to be used in the design of reinforced soil walls?

Soil with high friction, typically a granular material which can interact and interlock with geogrid reinforcement. For on site soil aggregates, we recommend that large shear box tests be used to determine friction angle of that material.

4. What was the RMS standard you referenced in your presentation?

RMS R57 free to download from website. https://roads-waterways.transport.nsw.gov.au/business-industry/partners-suppliers/documents/specifications/r057.pdf

5. What type of I&M do you use?

Strain gauges, pressures cell and settlement cells. We also have survey target prisms on wall faces.

6. Is pumice sand (high friction angle material) an appropriate material to use as a backfill since when compacted the fine content increase?

If compaction material pulversises, so is not typically used in the soil reinforced zone.

7. How much damage is considered acceptable?

There is actually not a standard value for acceptable damage in the guidelines/specifications. Technically the less damage the better and one should try to keep the damage on the geosynthetic to minimum. GRI Standard Practice GG4-Standard Practice for Determination of the Long-Term Design Strength of Geogrids, has suggested installation damage reduction factor of 1.4-1.5 as upper-bound values (in lieu of testing). Of course the specific installation damage test with site specific soil material or similar prepresentative soil is recommended for projects. If the suggested material in the standards is selected as reinforced fill material, the installation damage should be in the acceptable range. If a different soil (e.g. course rock) is being used, the specific field installation damage test is required to determine/verify the installation damage reduction factor.

8. What can we do if the clay soil must be used for reinforcement structure?

For rigid facings not advised as you cannot compact clay and swell percentage will affect long term serviceability as wall movement and deflections will highly occur. If flexible wrap around facings with greening options are being investigated, there are several design and construction considerations to be followed. Please contact us for more information. It’s very important to have good drainage in place and limit wall heights for these walls.

9. In what application would you use a gravel chimney drain vs geodrain?

Geocomposite drains are factory produced strip filter or sheet drains. They will have geotextile on both sides of a core and will vary in thickness from 5mm to 40mm. They are easily installed and provide drainage capacity in a relatively thin, light weight composite. They will typically drain to a collector/slotted pipe which connects cirectly to stormwater or weep holes through the wall face. These advantages normal trump thick and difficult to lay gravel layers on site.

10. Jeroen had a slide showing a wraparound that was damaged, is this considered a major issue? Was the damage repaired? What was the fix?

Non-conformance was issued and was replaced.

If you have any further questions or would like further information please contact: amir@globalsynthetics.com.au

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