retaining wall - weeping holes ?

T

tommied

Registered User
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I've had a retaining wall built in my back garden using
4x8x16 solid blocks layed flat.
the wall is approx 27 ft long, 4 ft high returning at the
ends.

I'm planning to drill weeping holes for drainage.

How many holes would I need for a wall of the above size.
I am not planning to install a drainage pipe etc. - assuming
that the weeping holes will provided sufficient drainage.

Thanks
Tom
 
The most important question here is: what is the nature of the materaial being retained. If it is free draining granular material, then holes every 3 ft would be fine. If it is a cohesive material then I would put a lot more. I would try one at every block (450m c/c) and possible put in two rows.

If the material at the back of the wall is such that it is likely to block the holes, then you need to consider a proper drainage solution.

But before you do all that, do you reallly need drainage. Is the water table in the retained materail high? Is the wall capable of carrying the water pressure. Is there signs of movement/cracking already?
 
"But before you do all that, do you reallly need drainage. Is the water table in the retained materail high? Is the wall capable of carrying the water pressure."

I am not an engineer; nevertheless, I feel confident in asserting that a retaining wall is not generally designed in such a way as to hold back water. Water can build up and exert enormous pressure. I have knowledge of a large building (a community centre\sports hall) having to be knocked because a retaining wall of about 4 feet did not have drainage. The water built up behind it (all the way up a hill) and one night the wall jumped into the side of the building
 
You are right MOB, if there is a build up of water the pressure can be very significant. However, if the material to be retained is at a relatively high level compared with the surrounding area or if the material is free draining, then there will probably never be a water build up.

The other point to remember is that hydrostatic pressure builds up in a non-linear way with height. So 1 ft of water will exert X pressure while 4 feet would exert 16X in total pressure. That is why you are right that 3-4 feet of water can be a big problem but 1 ft of water can easily be retained by a relatively light structure.
 
Hi tommy, what kind of money did that cost you? I ask as a garden designer has included a 28 metre curved retaining wall to about 80cm high to the rear that I'll need to build, and I want to know what kind of money am I looking at. The site slopes up behind the house so I'll also have to include drainage away from the rear of this wall. Anybody have any idea of costs on a wall like this? Finished in plaster to the visible side.

Thanks
Gubman
 
Hi Gubman

You're probably looking at around €500 to build the wall 215 block on flat (labour) based on 600 blocks at 80 90 cent a block, another 250 to scud and plaster. Excavations could be done in a day depending on complexity a jcb and driver will cost you in the region of 250 for the day.Depending on the complexity of the footings if you have to step them for your slope etc ,you may need to employ a chippy to construct some shoring 250 a day. You can get rates for block costs from suppliers say around 600 for a full load of 900 blocks ,conc rates are around 70 a sq meter for 28 m I would recommend a 300 deep strip which would work out at 7 cubic metres. You would need some sheets of steel rebar cut up for the foundations ,chippy might do this for you. Difficult to know how complex the drainage may be but if its only laying some 4" drainage pipes for runoff your only talking tens of euros
excavations can be done by jcb driver when he is cutting the footings.Add 25 % if you dont want to do it yourself and want a local contractor to organise it. Again depending on what its retaining you may need an engineer to do a few calcs to see if block on flat is sufficient.
 
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