Recommended Backfill for retaining wall

[ArfArf]

Hi,

My garden has a steep slope in parts, including an area where the previous owner has dug out soil to fit a large greenhouse, moving the soil elsewhere. This left a 2m high steep slope that I want to terrace with 1m high walls to secure the level above and gain some use of the area.

The walls will be separated a terrace level of around 1m which I will build up with backfill as the original material is no longer there. This leaves a large to fill with imported material (excluding drainage material behind the wall and at least 150mm of topsoil cover).

What material would be best to bring in for use as backfill? I initially considered type 1, but this may not be adequately permeable once compacted and could lead to waterlogging of the topsoil or a rapid rate of run-off. Would a compacted clean limestone fill of a given diameter be more suitable?

 

[KenGMac]

ArfArf, good evening.

The "Classic" material to use behind a retaining wall is a single sized gravel, but? that is advised where there is a distinct "narrow" gap between an earth bank and the rear face of the wall. the gravel is best wrapped in a "Geotextile" to prevent the gravel being clogged up with small particles of dirt and earth.

From what I understand, you will end up with [almost] free standing walls that you will be infilling against??

How about you consider? placing a [say] perforated Wavin Coil plastic land drain against the base of the wall, surrounded with single sized gravel, the gravel wrapped [meaning the geotextile encapsulates the gravel] this could be achieved by forming a [say] 300.mm trench behind the wall when excavating the bulk material out ? lead the Wavin drain to a soak away where possible.

This will allow you to backfill with T1 + have drainage.

 

[ArfArf]

From what I understand, you will end up with [almost] free standing walls that you will be infilling against??

This is exactly right for the bottom tier. So I gather that Type 1 would be fine, providing I allow a decent fall in the compacted type 1 towards the wall of and gravelled drainage zone (about 300mm). I'll probably increase the thickness of top soil too to increase attenuation.

 

[KenGMac]

ArfArf, good evening, again.

T1 should be fine in this scenario,

Suggest you consider placing geotextile under the top soil, that will keep the T1 clean and [relatively] free of small clogging particles.

Also if possible, fit a removable cap to the high end of the Wavin Coil this will amke [future] cleaning of the drain, if needed far easier.

Ken.