Help with retaining walls

[MBagshaw]

Hi folks,

 

I'm hoping someone can help me understand retaining walls/foundation walls. My wife and I are building our new home on the waterfront in Port Stephens, and the local council as recently changed its requirements for elevations for waterfront properties to take into account predicted rises in tight levels due to global warning. The block currently has an old shack on it and the land itself, and a reserved block next door, are both currently at the original land height which is about 1.8 m above high tide level (the home next door, built 15 years ago, has been raised by about 800 mm with a retaining wall along both side boundaries and the land sloped at the front (roadside) and back (waterside). We have to raise our land to a height 3.1 m above high tide level, and as the block is not huge our plan is to put a retaining wall around the boundary of the property, and have what will be a fairly steep driveway from road level to the garage and stairs and a ramp also from road level. It would seem to me the best way to do this would be to bring the retaining wall at the front of the property in line with the front of the house itself supporting the front of the slab, then have the area from there to the roadside boundary at the original 1.8 m level. I would also want to enclose the stairway from the footpath to the front door and the ramp behind a wall. The attached diagram shows some of that, but I'm having trouble with the retaining wall under the front of the house – I don't seem to be able to get it to go under the slab (in the photo I've left the retaining wall out altogether and brought it to the front of the property where I have lowered it to street height).

 

I'm thinking that, if I could get the retaining wall to go under the roadside of the slab, then perhaps I could put another retaining wall around the ramp/stair area, and another one along the right-hand side of the boundary to the footpath. I've tried many ways to do this, but I get weird affects every time I try to do it, and the retaining wall doesn't seem to want to go under the slab but rather sit in front of it, which is not what I want.

 

I have also attached the current plan. Any help will be greatly appreciated.

 

Cheers, Mark

 

Thoughtspace – single story (3).plan

[mattyt12]

Hi Mark,

 

Not really answering your query but have you checked with PSC whether they will support filling the site? Youre limited to max. 1m high retaining in the DCP, even so that is about 750t of imported fill which would likely have to be sand if its waterfront up there.

 

Not sure if you've considered the cost implications (masonry walls, fill, piering etc) but if it were me or one of my clients (of which ive been involved with a few up that way) I would be encouraging bearers and joist construction. There is scope in the DCP to allow the lesser flood level

 

"Where the proposed development facilitates ongoing
flood adaptation (e.g. where the design facilitates
building raising in the future, such as pier and beam
housing design) then Council will allow a reduced Flood
Planning Horizon level 50 years from the date of
application."
 

 

You should also be able to lower the Garage level aswell. The new minimum floor levels for habitable rooms are based off the 2100 1% AEP + 500mm freeboard (which was changed from the 2050 1% AEP), sheds or non-habitable rooms ie garages should be off the 2050 5% AEP + 500 freeboard.

 

May not be the best or correct way but if I were modelling what youre trying to achieve Ild use polyline solids or slabs to denote the fill and walls.

[MBagshaw]

Thanks Matt.

 

The requirement to raise the land level is being imposed by Council, although after a meeting with the Council planners last week it's clear there's a disagreement going on between the planners (who would like the levels lowered) and the flood engineers (who have set the height). Indications from our meeting are that we will get at least some relaxation on the height. The planners made no mention of a restriction on the height of retaining walls, and while there haven't been any other properties approved under the recently-revised LDP so far, the previous LDP already contained a requirement to raise the height of new properties along the waterfront by 2 m, so there are several existing properties that are currently raised at that level (including the one next door which was built 15 years ago). Virtually all of the ones that have been raised previously used the retaining wall with sand infill method.

 

Apart from the fact that might make our place look odd along the streetscape (one of the concerns of the planners), I wouldn't actually be overly worried about the height. Apart from anything else, any elevation of the property itself improves the view! My only real concern (apart from the fact that it would make the driveway to the garage very steep) is that I use a wheelchair and we are struggling to get ramps front and back that go even close to code (which of course we don't necessarily need to abide by given this is a private residential development). You can see what I've tried to do with the ramp at the front in the picture and on the plan.

 

I'm no expert at CA, but I'll have a play around with your suggestion on creating the front retaining wall under the building as a 3D polyline. I thought I might just be able to manually edit the automatic retaining wall that was generated along the front to drop its height under the foundation for the slab, then somehow slip it under the slab, but I don't seem to be able to get to do that.

 

Thanks for your help with this – I'm sure we will come up with a reasonable compromise all round.

 

Cheers, Mark

[glennw]

Why do you need a retaining wall at the front of the house anyway.

If you are filling inside AND outside the front wall of the house, fill the site and use a slab with concrete piers through the fill.

[mattyt12]

Good to know cheers Mark. We just completed a dwelling last month and about to submit a DA for a new one 10 doors up (basically the whole street is about 1m above the hightide mark)and theyre requesting an additional 800mm in floor heights.

[MBagshaw]

Good to know cheers Mark. We just completed a dwelling last month and about to submit a DA for a new one 10 doors up (basically the whole street is about 1m above the hightide mark)and theyre requesting an additional 800mm in floor heights.

Hi Matt. Do you mean you are working on properties just near my place?? If that's the case, what an incredible coincidence! Cheers, Mark

[Northriver]

No harsh regulations or environmental impact studies in AUS. It would take us four years for a study and could still deny the project. Our beach was visited by University of Washington students studying environmental impacts on shoreline. I was approach by one student and he recommended I tear down my bulk head retaining wall is it causes excess erosion to happen and it destroys nursery habitat for juvenile fish. I told him to write me a check for $1,000,000 ( not worth nearly that amount) and he could do what he wants.