retaining wall equation??? Post your questions and find answers on all garden related topics and areas

[greggpeg]

hi i would like to know if anybody knows of a simple equation or ratio of how thick a wall has to be to contain the earth behind it. i have a small car port on my yard that i'm hoping to make bigger by chopping into my garden behind.however i'm a bit lost as to how thick the walls should be and what to make them out of...!!! thank you for any replies, gregg peg.

[ALDA]

G,

MORE INFO REQUIRED!

WHAT WILL BE THE FINISHED HEIGHT OF THE RETAINING WALL?

WHAT TYPE OF SOIL/SUB-SOIL?

IS THE LAND LEVEL OR SLOPED?

IS THERE ANY RETAINING STRUCTURE IN USE CURRENTLY AT THE DRIVE/GARDEN BOUNDRY?

A.

[greggpeg]

hi the finished height will be approximately 4 feet, the level behind is a very slight gradiant.i'm not too sure of whats at the bottom of the wall thats there at the moment but further up it's just general garden material, some larger round stones with a fair bit of soil on top. there is part of this same wall missing further round my garden and that's like a really compact,hard sand at the bottom.

[ALDA]

G,

I WOULD SUGGEST YOU BUILD OR HAVE BUILT A 9" WALL (DOUBLE BRICK WIDTH)TO YOUR REQUIRED HEIGHT.
THE FOUNDATION TRENCH FOR THIS WALL SHOULD BE DUG OUT TO A DEPTH THAT ENSURES YOU ARE INTO SUB-SOIL/CLAY OR SUB-SOIL/GRAVEL AND TO THE WIDTH OF THE CONCRETE FOUNDATION.

THEN IN THIS COMPLETED TRENCH, A CONCRETE FOUNDATION SHOULD BE POURED. 9" TO 12" DEEP AND AT LEAST TWICE THE WIDTH OF THE WALL. (2 X 9" = 18")

THE WALL SHOULD BE BUILT IN THE CENTRE OF THE CONCRETE FOUNDATION/TRENCH. USE WATER/FROST PROOF ENGINEERING BRICKS IN THIS TYPE OF RETAINING WALL AND A STRONG MORTAR MIX.
(3:1 OR 4:1 SAND TO CEMENT RATIO).

INTERNAL BUTTRESSES CAN BE BUILT INTO THE WALL STRUCTURE TO IMPROVE WALL STRENTH, PARTICULARLY IN A LONG WALL (OVER 20ft).

BACK FILL GARDEN SIDE OF WALL WITH GRAVEL TO AID DRAINAGE AND INCLUDE DRAINAGE TUBES BUILT IN THROUGH WALL AT LOW LEVEL AT APPROX. 3-4fT INTERVALS ALONG THE LENGTH OF THE WALL.

LEAVE FINISHED WALL FOR A FEW DAYS BEFORE EARTHING UP ON GARDEN SIDE.(INTRODUCE PLENTY OF GRAVEL BETWEEN EARTH AND WALL, AGAIN TO IMPROVE DRAINAGE).

HOPE THIS HELPS,

ALDA.

[greggpeg]

hi alda thanx that's put me on the right track. after i dig out and build the wall to gain the required area on the ground i'd hoped to extend the walls vertically to a decent height and turn the hardstanding into a garage, so would these walls be good enough for this purpose? provided i followed your advice word for word? and would i need to go to any other lenghs to reduce the risk of damp or water ingress through the wall? thanx once again! greggpeg

[ALDA]

G,

THIS TYPE OF RETAINING WALL IS NOT SUITABLE FOR USE AS A STRUCTURAL BUILDINGS WALL.

A SEPERATE GARAGE STRUCTURAL WALL BUILT IN FRONT OF THIS RETAINING WALL WOULD ALLOW DRAINAGE TO CONTINUE THROUGH GARDEN WALL WHERE IT COULD THEN BE CHANNELED AROUND A NEW GARAGE STRUCTURE THROUGH A SUITABLE GULLEY TO DRAIN.

IF YOU INSISTED ON USING THIS WALL AS PART OF PROPOSED GARAGE THEN, THE DRAINAGE, DAMP AND FOUNDATION ISSUES WOULD HAVE TO BE ADDRESSED EARLY ON IN THE CONSTRUCTION PROCESS AND IF NOT DONE SO TO A VERY PARTICULAR STANDARD, WILL ONLY LEAD TO PROBLEMS IN FUTURE WITH WATER INGRESS; FOR THIS REASON I WOULD SUGGEST YOU REFRAIN FROM DOING SO.

IT IS ALSO WORTH MENTIONING BUILDING CONTROL AND PLANNING APPROVAL AT THIS STAGE AS BOTH WOULD HAVE TO BE APPROACHED IN THE PROPOSED CONSTRUCTION OF A GARAGE.

A.

[greggpeg]

ok thanks again,how much gap should there be between the retaining wall and the garage wall? would it be a good idea to leave just enough to be able to get down there and sweep up?/ maintain both the retaining and garage wall? i would have to put some guard railing around the top of the retaining wall to stop my kids falling down between the walls from the higher level of the garden. oh by the way is a buttress like a big post/stake? that you set into the foundation before you brick up and make the wall?

[ALDA]

G,

YOUR SUGGESTED SPACE FOR ACCESS/MAINTENANCE IS SPOT ON AND WOULD ALSO ALLOW FOR A DRAIN.

YOUR IDEAS ON SAFETY ARE ESSENTIAL, WETHER YOU INTRODUCE RAILINGS OR FENCING ETC. IS YOUR CHOICE.

IT DEPENDS ON THE OVERAL LENGTH OF THE WALL AS TO WETHER YOU NEED TO INTRODUCE BUTTRESSES INTO WALL STRUCTURE.

BASICALLY THEY ARE BRICK PILLARS BUILT AT INTERVALS (THE ENDS OF WALLS ARE MOST VUNERABLE) INTO THE GARDEN SIDE OF THE RETAINING WALL (ALL ONE BRICK BONDED STRUCTURE) WHERE THEY WILL PROVIDE SUPPORT/REINFORCEMENT AND BE HIDDEN UNDER THE SOIL BACK FILLED AGAINST WALL.

I HAVE USED THIS TYPE OF WALLING IN MY OWN GARDEN TERRACING TO GOOD EFFECT.

CHEERS,

ALDA.

[ALDA]

G,

AN ALTERNATIVE TO A RETAINING WALL IS TO USE "GABIONS".

GABIONS ARE STEEL MESH CAGES WHICH YOU POSITION AND FILL WITH ROCK OR PEBBLES ETC. TO PROVIDE A STABLE RETAINING STRUCTURE WHICH IS FREE DRAINING.

EASY INSTALL AND QUICK.

THEY LOOK GOOD TOO.

GOOGLE "GABIONS" FOR LOTS OF INFO.

CHEERS,

ALDA.

[greggpeg]

oh right i got ya, i've seen buttresses before just didn't know what they were called, i'll look up gabions and see whats what. thanx for all your replies, it's really helped me out. no doubt i'll post again when i'm stuck next time. cheers alda! greggpeg