Firstly, sorry if this has already been answered- i had a look but couldnt see anything so if i have missed it, please feel free to point me at it!
I have an old single garage at the bottom of my garden that leads to a road running perpendicular to it, parallel to the garden. The garden is level, but built into a hill ( house built in the mid 30's) so the road is higher than the garden meaning the garage was erected on top of a 1 metre high plinth. On the other side of the garden (so opposite the road) the garden is retained by a wall, which looks about 2 metres high (hard to judge when peering over the fence!) The existing structure has around 1 metre between it's end and the retaining wall. Now, the garage is really too small to use as such, so I would love to extend it/demolish/ whatever to end up with a double garage. Unfortunately, while the actual garage building doesn't worry me, I want to make sure that the new plinth will work properly so.... do I just design the strip foundations as though i was buiding a normal building, but 1 metre higher, build the first metre with thicker blocks then infill the first metre with broken bricks etc then the floor.... Or do i need to do something funky??? oh yeah i'd describe the sub soil as firm clay, there has been on movement of the retaining wall while the garage has been there (garage built in the sixties, you can tell the wall is original) there are no trees etc nearby and I was looking to build the walls from thermalite blocks.
Any advice would be greatly appreciated,
suggest you consider the following
1] Water dispersal on the site
2] You might use the design of a new retaining wall to enhance your
garden e.g you might terrace the garden with planted steps
built in engineering brick for example which also forms the new
retaining wall/ garage foundation and which has drainage outlets or
3] Make some scaled drawings of the existing site and garage comprising
a plan and some sections incorporating services manholes drainage,
trees etc. Find North and plot on drawing.
4] Think about your overall plans for your house: Do you want to extend
at some time,if so perhaps the retaining structure can be integrated.
5] With your existing drawings and sketch proposal that you favour,
speak to a Structural Engineer and an architect.
6] Even if you just want to build a double garage, make an appointment with a structural engineer. You will,I am sure ,find that for a very modest fee, you will get advice which could avoid disaster or even a notice being
served upon you if you excavate in to the slope in a way which might put
people and /or property at risk.
7] A material you may wish to consider might be the hollow aereated concrete blocks with vertical reinforcing bars in the hollows and then filled with concrete. I would expect to use a steel reinforced concrete slab for the floors or perhaps prestressed planks such as manufactured by Bison.
In any event I would look to the Structural Engineer to guide me after a
full investigation has been made.
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