Retaining wall design questions

[Sam45]

Hi I just need some advice. We are building two retaining walls as the neighbours garden is sliding and pushed our wall down. We are on a hill and our garden is lower than the neighbours.

One wall will be the original double brick wall with pillars (16m x 1m and then 1 metre high pillars with wood panels) on our border and then there will be a reinforced hollow concrete block wall filled with concrete on the neighbours side of the garden ( up to his soil level), to ‘retain’ his garden and stop his garden sliding down and pushing our wall down. Does anyone know how much gap needs to be left in between the two walls? 

[Mr Punter]

Any gap would just be there for maintenance or ease of construction.  You don't need 2 walls.

[Conor]

"Depends"

 

Need more details. key thing will be how the digging of the new footings would interact with the existing wall and how you will be managing draining.

What is the existing retaining arrangement? What condition in the existing wall in? Photos? Who is designing this? Are you able to excavate the new one without damaging the existing?

 

Edit: Sounds like the existing wall will be removed and you are building a new retaining wall and then a facing wall? You'd normally just build it as one wall and face the block wall with bricks.

Edited July 23 by Conor

[Sam45]

See pictures. The existing brick wall is on the border and is our wall on our land. We are excavating the existing wall. Then rebuilding as original. Then there will be a reinforced hollow concrete filled wall on the neighbours side up to his soil level as he needs to stop his garden from pushing our wall down again. His garden is 1 metre higher than ours. There will be a joint concrete foundation which will cross into the neighbours garden plus ours to build both walls on. Planning the foundation to be 800mm deep and about about 750mm wide. 

[Sam45]

Here are some more pictures. The neighbours garden is sliding towards our wall. The soil is built up with old rubbish from when they knocked down the old house and dig a basement in the new house. They also rebuilt their patio right against our wall. There will is approximately 15metres by 1 metre high and then pillars which are 550mm x 1 metre. 
 

[ProDave]

The main thing here is you have cooperation from the neighbour who is allowing a footing straddling the boundary and a second wall on his side.  i.e. it has every potential to be a proper job done well to last into the future.

 

It is when a neighbour refuses to take part or take an interest that you are forced to do the best from your side which might well be a poor solution.

 

If only all neighbours would share a problem like this then the solutions would be a lot better.

[Marvin]

Hi @Sam45

 

In my humble opinion, I would not go for a two wall design.  If you did, the only thing holding the wall up would be the strength of the strongest designed wall. The other wall would only be for show. The ones I have build retaining the high ground to about 3 feet high have the wall becoming thicker on the ground side the further below the level of the high ground it goes.

 

As @ProDave comments, the best design is for the thickening of the wall to be under your neighbours ground.

[Sam45]

45 minutes ago, ProDave said:

The main thing here is you have cooperation from the neighbour who is allowing a footing straddling the boundary and a second wall on his side.  i.e. it has every potential to be a proper job done well to last into the future.

 

It is when a neighbour refuses to take part or take an interest that you are forced to do the best from your side which might well be a poor solution.

 

If only all neighbours would share a problem like this then the solutions would be a lot better.

The neighbour can’t refuse to take part as it is his garden which is sliding downhill and pushed our wall down. His garden weight has increased as it has been ‘built up’ from the old original house that was knocked down’s level without considering the effect on our wall. Also he has built his patio right against our wall which is pushing our wall down. His whole garden patio plus soil area is dropping vertically and moving downhill horizontally so to be frank he would be responsible for ‘ securing’ his garden so it does not damage other people’s property. Hence the second wall on the bright side to retain his garden and stop it sliding. 
Our brick wall is on our borderline originally built like that so we wanted it built the same. Otherwise our borderline will change. 

[Sam45]

14 minutes ago, Marvin said:

Hi @Sam45

 

In my humble opinion, I would not go for a two wall design.  If you did, the only thing holding the wall up would be the strength of the strongest designed wall. The other wall would only be for show. The ones I have build retaining the high ground to about 3 feet high have the wall becoming thicker on the ground side the further below the level of the high ground it goes.

 

As @ProDave comments, the best design is for the thickening of the wall to be under your neighbours ground.

So you mean an extension of our brick wall which thickens on the neighbours garden side ( under his soil) as it goes deeper into the soil.

Problem is this would not be reinforced as the neighbours reinforced hollow brick will be. Also a brick wall built this way would mean a) it’s our whole wall going into his border which I am not sure he would agree b) the brick wall can’t be reinforced ( and the horizontal pressure from the neighbours garden against our wall, especially when the soil is wet from heavy rain and water coming downhill generally as we live on a hill, is high. Our double brick wall has cracked from the middle to to horizontal pressure c) we might have problems accessing the neighbours garden in the future for repairs. d) the neighbours only want about 215 mm of the side of their garden dug to accommodate a hollow brick wall and drainage gravel behind it. 

[Redbeard]

It needs to be designed by a SE, I think. If the walls are entirely separate, and not adding strength to each other, I have a feeling 225mm will not be enough. My 1200 - 1500 wall is 600+ thick at the bottom IIRC.

[Iceverge]

Are you sure and certain that you entirely own the wall? 

 

How about the existing footings?  Surely they belong to your neighbours on their side of the wall. 

 

In any case it's a technically simple(ish) problem to solve. I'd remove the existing wall and footings, pour a reinforced concrete wall tied into a L or T shaped foundation. Then face it with whatever you want on your side.

 

The tricky point is deciding who will pay for it. My suggestion is shake hands on 50/50 and stay well away from any legal disputes which will cost much more.

 

 

 

 

 

[Marvin]

Hi @Sam45

 

Yes there are many choices of this type of wall called retaining walls. Its worth searching "retaining wall design" and go to images.

 

You will see there are many designs. You may also see solid brick walls that get wider the deeper they go...

 

Designs that allow for drainage either through weep holes or drainage to foundations or back filled with drainage material depending on the soil type, slope and existing drainage ground water levels and so on are also indicated.

 

As you say the wall needs to be attached to the base, however, again, there are many choices but basically, depending on the design the bigger the heel or toe of the foundations the less chance of failure.

 

Whilst the mix of views may seem confusing, the answer, as always is in the maths and someone competent to calculate it and come up with the design that will achieve it. 

 

@Mr Punter, @Conor, @ProDave, @Redbeard and @Iceverge have all come up with good information.

 

I wonder when the neighbours paving was completed and if the sloping land was levelled at the time bringing the earth higher on the failed wall. Any builder would know that the wall without backing would fail....

 

Best of Luck

 

Marvin