Securing three sided raised bed to retaining wall

[Ollie_449]

I am currently undertaking a raised bed and decking project in my back garden. The plan is for the raised bed to be comprised of softwood sleepers (2400x200x100mm) on three sides, interlocked and secured together with 250mm Timberlok screws. The rear of the raised bed will be against a high retaining wall. The image below shows a rough plan and a photograph of the site.

My original plan was to sink posts into the ground to support the front face of the raised bed. However, when I started digging, I found the footings for the brick retaining wall (what a surprise) approximately 250mm below ground level, therefore not deep enough to support the vertical posts.

My revised plan is now as follows. I purchased some sleeper anchors (http://cdn.toolstation.com/images/141020-UK/800/75498.jpg), hammered them through holes in 1200mm concrete fencepost repair spurs, and then encased them in concrete. This was repeated 4 times, with 4 anchors to support the front wall of the raised bed, and one on each end. The image below shows a cross section of this. There is also a picture showing the footings in situ.

My gut is telling me that I need to further support the raised bed due to the lateral load from the soil (assuming 1700kg m3, the bed will contain circa 2500kg of soil when wet). I was thinking that I could put some anchors into the retaining wall, to put some braces in between the wall and the sleepers, probably after the third sleeper. My questions are:

1. Am I overthinking this?
2. What sort of supports/braces should I use? (will be buried in soil - some sort of hot dipped bracket, as show in blue below?)
3. Is there any risk this could compromise the integrity of the retaining wall?

Any suggestions/advice would be greatly appreciated.

 

[matlob]

What's behind the wall?

You need to make sure you don't block the drainage holes at the base of the wall if it is earth

Where you are filling behind the sleepers you need to add gravel to help drainage and also fabric to keep the wet soil off the timber as much as possible

A perfed drainage pipe at the bottom and outlets in the timber will also help stop the wall pushing forward especially if water is coming through the brick wall via the outlets

 

[Ollie_449]

Other side of the wall is the garden of the house behind us (land is approx. 2.5m higher the other side of the wall).

I was planning on lining the back of the sleepers with pond liner. I was going to put plenty of gravel wrapped in weed proof membrane in front of the drainage holes in the brick wall to insure that they are not blocked (although in 18 months, I have never seen a drop of water come out of the drainage holes)!

I was also going to set the sleepers on a bed of gravel wrapped in weed proof membrane so water can flow out under the front, although your suggestion of a drainage pipe and outlets in the timber is a good one.

What to you reckon about additional bracing of the sleepers to the wall?

 

[lostinthelight]

What will you be planting in the bed 350mm seems very narrow in relation to 90mm depth of soil?

 

[Ollie_449]

Mostly climbers to go up trellises on the wall, maybe some grasses and some well contained bamboo. The raised bed is so high because I plan to use the front of it as the back to a bench seat.

 

[matlob]

The support will help, It will act similar to a deadman

You can also get lengths of metal bracing to join the sleepers together at the back. This will help make it one entity so any pressure is distributed along the length

 

[Ollie_449]

What do you reckon to something like this? https://www.amazon.co.uk/IB-Style-H...203350&s=gateway&sprefix=galv,aps,182&sr=8-78

Short side secured to brick with anchor bolts, long side screwed down into sleeper (on third row, as in diagram in first post).