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This may not belong here as it relates more to hardscaping, but let's see what you think:

A job we are working on requires a retaining wall of approximately 8'. "We" plan to build it out of mafia block (see below). A slope will intersect the face of wall perpendicularly, and cover it diagonally from one corner to the other (other diagram).

The plan is to create a substantial footing out of 3/4" crushed stone, and to have a 3' wide drainage space behind the wall - also filled with 3/4" crushed stone.

Soil is very sandy and well draining.

  • Is this crazy?
  • What should we be concerned about?
  • Is this something that should get an engineer's stamp? (We're in NY)

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That Idiot
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3 Answers3

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Some of the usual suspects are not mentioned in your plan including:

  • landscape fabric between the gravel back fill behind the wall and the soil
  • four inch drain pipe with sleeve at the inside base of the wall so when water cannot drain in the winter due to frozen ground it accumulates in drain pipe instead of pushing the wall
  • a slight incline into the bank
  • drain holes on the bottom row to allow water to seep out
  • "Deadmen" or bricks that go into the bank at three to four foot intervals
  • cap stone to keep water from draining down inside walls
  • small ditch at top parallel to wall to drain water away

This picture from extension.umn.edu clarifies what I mention

enter image description here

kevinskio
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    Drainage and footing below grade... Two very important items left out on pre-econolypse of 2007 walls built in the subdivision above us. Water has washed down behind and flushed the soil off from under the block, allowing it to be undermined. One really good torrential "Pineapple Express" winter will lead to the wall collapsing. It's amazing what could be gotten away with during the height of the building boom. Retaining walls need more stringent adherence to code and geoengineering than the actual buildings they will landslide onto. – Fiasco Labs Oct 15 '15 at 02:47
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As an engineer who has designed numerous retaining walls it is obvious that non engineers are doing internet research and don't fully understand what they are looking at.

The wall in questions can be properly designed in several way to use so called "mafia" blocks. The wall must resist the horizontal force from the soil and any live loads on top. The overturning forces on the wall also needs to be resisted. Finally the internal stresses within the wall need to be low enough to prevent failure of the block or separation at the seams.

Another engineering consideration is the existing soils type and conditions at the location. Is the soil cohesive, non- cohesive or rock. What is the maximum load the soil can take before failure or excessive movement / settlement.

It is very important to either prevent water from getting trapped behind the wall or account for it in the design.

In most localities, a wall of this height requires a professional engineers stamp for approval.

Jeff S. PE. of Connecticut
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I just need to add that any wall above 4' has to be engineered. Is there anyway that you could terrace this? Is there enough room to install 2, 4' walls or better yet, 3' walls? Perhaps the rules are different where you live but 8' walls will need extra attention to drainage, dead men and need to know what is happening above this wall? Is it getting charged with water from properties above? Use crushed 5/8- for base, 4" drain pipe (covered with landscape fabric or purchased integral, pipe already covered by mesh to keep small particles from clogging up the drain pipe). Use 1 -1,1/2" drain rock to allow flow of water towards drainpipe (rounded aggregate not crushed). If you get caught with too high of a wall not permitted nor engineered...well, they'll make you take it out. The section Kevinsky provided is great. Make one similar to take to get permitted.

stormy
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