Question
We are constructing a concrete retainer wall for a fire pit area in our yard, the walls are 48" tall and we are hoping to berm soil up to the outside of these for landscaping.  How much slope is recommended?  We have clay soils like much of western Washington and will add french drains into the design to help with drainage.  I'd like to be able to cover this area with shrubs, groundcovers and a few small trees(less than 25ft in 10 years).

Answer
Hello,

The steepness of the berm should not exceed the angle of repose of the soil.  The angle of repose,also known as angle of rest,is the natural angle the soil material will slump to when poured in a pile.    As a general guide here are some angles of repose for various soil types:

stiff clay: 63 degrees
firm clay: 56 degrees
granular soil (dry):  45 degrees
granular soil (wet)   34 degrees
saturated granular soil:  26 degrees
sand:  33 degrees
dry silt:  25 to 40 degrees

As a general rule,do not exceed an angle of 30 degrees for the clay you have or for any other soil type. If possible,backfill the area behind the wall out to eight feet with a better draining soil. You should take extra steps to increase drainage in clay.   While you will want a steeper slope for clay (to better shed water),you do not want to foster sliding movement against the wall. Sliding movement increases with an increase in degree of slope.   Since clay retains more water than other soil types,hydrostatic pressure will also be increased behind the wall.

A swale at the top edge of the retaining wall would also be helpful if a suitable outlet for the water can be found.   You will also need an outlet for water in the form of weep holes in the wall or a drainage pipe embedded in gravel behind the wall. Good luck. Write back if you have further questions.

Mark  Harshman
http://www.mahdrafting.com
Email: [email protected]

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