foundation for retaining wall.

[laurence peddle]

Hi. I'm terracing my garden, which slopes steeply up from the house.The diggers have just removed the heavy clay soil from the area in front of the house. This leaves a rectangle 10 x 9 metres, the back of which is a clay cliff 6 feet high, in front of which I have to build a retaining wall. I'll probably use hollow concrete blocks with rebars through them and in the footings, with a land drain behind the wall on gravel, with gravel above it. I've been told the concrete foundation should be at least a foot thick because of the clay and the height of the wall. My problem is that the cliff is already collapsing here and there and I don'r want to make it worse by digging a foot deep trench in front of the cliff, to make it almost 7 feet high. There will, however, be a raised bed in front of the wall along its whole length apart from the steps in the middle. The raised bed will be about two feet high and two feet deep. My question is whether I could build the footings in the shallow trench already in place and raise them above the ground level and patio level, to make them a foot thick. They will be hidden and covered by the raised bed. Is this feasible if I use rebars in the footings to strengthen them?

 

[freddiemercurystwin]

Seeing as that beasty is a killer if it fails it really needs designing by a structural engineer.

 

[r896neo]

I firmly agree with freddy. A wall that size will need designed by an engineer. The foundation needs designed as part of it also

 

[RonnyRaygun]

As others have said, you need it designed by an engineer. It feels like I've spent most this year designing retaining walls and I can tell you that as long as your clay soil is stiff enough, you will get away with a 225mm thick base with A393 mesh in the top and bottom.

However, it won't work with a single skin of hollowblocks - near the base for a few courses at least you would need to be 440mm thick. Hollowblocks are more difficult to work with as they have to be physically lifted over the rebar, can be difficult to fill without leaving voids, and have no distribution steel to help spread the load throughout the wall.

With the above in mind, reinforced cavity is - in my opinion - the better option. It is still vital, though, that workmanship is of a high standard, and as above, you need it properly designed by an SE. There are cases of non-engineered retaining walls overturning and killing people.

 

[bernardgreen]

A curved wall is much stronger than a straight wall so if possible design it with a curve towards the higher ground. Look at large dams where the water pressure on the curved wall presses the stones / bricks tighter together. The ends of the wall need to be stronger to take this sideways pressure but the rest of the wall need not be so strong.

 

[laurence peddle]

Thanks RonneyRaygun. I'll make it a foot thick.

 

[laurence peddle]

Thanks, bernardgreen. I hadn't thought of curving it.

 

[cAtLeYx]

It would be best to carryout the works systematically, i.e. excavated and cast the foundations in bays leaving reinforcement through the formwork to allow the next base to be tied in. You really don't want to excavate the whole area and then cast foundations. Also note curing period prior to excavating next bay. You may, subject to Structural Engineer, be able to excavate and cast say bays 1, 3, 5, and then return to bays 2, 4, 6

 

[laurence peddle]

Thanks cAtLeYx. Yes, I wondered about doing it in sections if I use rebars. Thanks again