# Forum Home Renovation Retaining Walls  Treated Pine Retaining Wall

## Art Martin

I've got a old bush rock wall that runs about 45 degress up a slope about a metre high, looks structurally sound, but very ugly. I'm putting up a 1 metre high treated pine retaining wall in front of it, then backfilling to create some extra space on top.  
My question is, will the forces and loads be significantly less on the sleeper wall because the rock wall is already there? In other words, because the sleeper wall will only be retaining the soil between the rock wall and the vertical sleeper wall, I could get away smaller footings. It seems logical, but just want to be sure. 
Was going to do 150*100posts , 600mm deep footings concreted 300x300 diameter. Seem reasonable, but if the rock wall wasnt there I'd do 1 metre deep footings. 
Seem ok?

----------


## Master Splinter

Will the forces be less?  Not really, no.   
The retained earth that will act on (any) retaining wall is generally taken as the earth in the triangular section defined by the base of the wall and a 45 degree slope (the natural angle of repose) up to the finished height.  The angle varies a bit with the material (90 degrees for 'chiseled out of solid igneous rock' to 25ish degrees for fine sand). 
Download technical data sheet 9 from here for the full details:  Finlayson's - Products - Technical Brochures 
As for footing depth...since you can't get more than one 1.6 meter upright out of a 2.4 meter sleeper, I'd go the extra depth - it's not like its costing you anything extra (and the deeper holes mean that you can easily justify hiring a motorised post hole digger rather than doing them by hand!!)

----------


## Art Martin

Thanks for the advice Master Splinter. :2thumbsup:  
I think I'll go the full metre down for peace of mind. I'll also be filling mainly with ash rather than soil, so the water drains and the forces on the wall are lowered...

----------

