Fences Down Under

Fence Systems
By Joe Kowalski
Apr 16, 2009 - 8:43:02 AM

  We received this question from West Perth, Australia. How close can you excavate for a new wall near existing fence posts?

From: Frederick 
Sent: Thursday, April 16, 2009 4:00 AM
To: JoeK@RetainingWallExpert.com
Subject: wall question

Hey guys, I just found your website and its great.
I need to build a retaining wall in my back yard but the neighbouring fence is five feet higher than the ground level, how do I build the retaining wall without the fence falling down?
thanks guys
Frederick in West Perth WA 6005
We'd be glad to help with your wall project.  By the way, I've got some relatives in Australia, but they live near Adelaide!  My wife visited a number of years ago, and my father-in-law usually travels there about every 2 years, and spends a few weeks traveling from Perth to Sydney.
Anyway, back to your wall question.  First, feel free to send a few digital photos of the area, if my description is inaccurate.  I'm guessing that you have a slight hill, with your property being on the lower elevation and your neighbour being on the higher elevation.  Next, you plan to make an excavation into that hillside, to create a nearly level area.  By doing this, a nearly vertical cut slope (about 5 feet high) will be made close to the property line.  You are concerned that the neighbour's fence posts will be undermined and his fence will collapse when you make the excavation.
Basically, you are correct in your thought process about potentially losing the fence.  The factors that will determine if the fence will fall include: (i) the soil type (i.e., clay or sand); (ii) the depth of the posts supporting the fence; (iii) the height and type of fence; and, (iv) how close the excavation is made to the posts supporting the fence.
Taking these in order:
  1. If you have dry, sandy soil then all bets are off - you'll probably lose the fence posts as the excavation gets closer and closer to the posts because the sand will simply fall away.  However, if you have stiff clay soils, then these soils can often be cut nearly vertically without falling, but only for a very short time.  Beware: This cut face can be a safety issue, because the collapsing soils from a clayey bank weigh thousands of pounds!
  2. If the fence posts are deeper, this is better for you, because there is more soil surrounding and supporting them for the temporary condition when they are nearly exposed.
  3. If the fence is a 3-4 foot high open fence (e.g., chain link, or split rail) then the wind will only minimally affect the fence and if it is windy while the excavation is exposed this will be a minor issue.  On the other hand, if the fence is high (several feet) and a "closed" type of fence like wood, then the wind applies a lot of load sideways and could tip the fence over while the excavation is made, before the new wall is built to protect the posts.
  4. The farther away you make the excavation from the fence posts, the better.
You'll have to weigh the costs and benefits and determine just how close you can get to the fence posts.  Is it better for you to keep the new wall 3-5 feet from the property line, or is it better to simply replace the fence during construction ... of course with your neighbour's permission beforehand!    Let me know how it works out!
Best regards,
Joe Kowalski