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October 2, 2015

Planting Around Black Walnut Trees

Planting a garden and shrubs near a Black Walnut tree is extremely challenging. Finding out some facts about the toxins produce

d by this type of trees is important to ensure proper preparation and success for your garden.

Black Walnut trees along with hickories produce a chemical called Juglone. This chemical is very toxic to plants. Its highest concentration is in the buds, leaves, stems, nut hulls and roots of the tree. The actual concentration in each tree part varies with the season. In spring, Juglone is high in the growing leaves. In the summer, it is high in the roots, and it increases in the hulls of the fruit as they mature.

The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree grows. 

In most cases, the damage caused by Black Walnuts to other plants is a combination of the presence of Juglone in the soil, and the competition for light, water and nutrients. Cutting the tree down will provide immediate relief for light water and other nutrients, but the Juglone can stay in the soil for a long time. In some cases couple of years.

General tips for planting around black walnuts:
Allan Block Raised Planter (AB Fieldstone)

  • Before anything talk to your local landscape center and ask to have an expert evaluate the soil in your yard to see where the toxin is most concentrated.
  • Locate your garden away from the drip line of the black walnuts.
  • Create and plant in raised beds to reduce root contact. This will require lining the bed to reduce root contact using weed fabric and filling the raised bed with new topsoil.
  • Improve soil drainage with organic matter additions.
  • Prevent leaves, hulls, and stems from decomposing near planting areas.
  • Avoid using mulch containing walnut bark, wood, hulls, and leaves.
  • Consider planting shrubs that are Juglone tolerant around the tree, you can find a whole list online. 

September 25, 2015

Don’t Underestimate the Importance of Your Soil

The soil on your site is a critical aspect to your retaining wall project.  Among the ways your soils can affect your overall project cost are:

  • Geogrid Requirements
  • Increased Effort to Compact
  • Cost of Importing Additional Material and Removing Existing Soils

Clay Soil
A wall built in clay will have an increased need for geogrid when compared to the same wall build on a sandy site.  This is because clay will “push” on the wall more than sand.  This geogrid will be used to create a mass of soil large enough to overcome these forces.

Sandy Soil
Clays and silts resist compacting due to the shape of each grain of soil.  They to trap water more and make proper compaction more labor intensive.  It is important to achieve proper compaction to reduce future settlement and to increase the stability of the reinforced soil mass.  To achieve the required compaction, you may need to compact more frequently with smaller lifts of soil, or use specialized compaction equipment.

Organic Soil

When your site does not meet the minimum requirements suggested on the planning your retaining wall  page on the Allan Block website, it may be required to remove the existing soil from the site and use select fill for the area within the geogrid.  You may also find it necessary to increase the width of your base to compensate for poor soils under the wall. 

September 18, 2015

When Do I Need Geogrid?

 A common question we get is do I need geogrid for my wall?  Well, the answer depends on a number of factors. 

    • Wall Height: What is the height of your wall?  If you are only going to have 2 or 3 courses of block then geogrid is likely not needed, but as your wall starts getting taller, geogrid may be required.

    • Soil Type: What type of dirt do you have?  Different soils have different strength characteristics.  What an engineer looks at is the shear capacity of the soil (also known as the friction angle).  In common terms this is an approximation of how easily the individual particles of soil will slide past each other.  This value helps an engineer determine how much pressure the soil will apply to the back of the wall.  Unfortunately this is where things start to get less clear.  Soils can vary greatly in their friction angle and therefore the pressures can also vary by a large margin.  In general, clays are typically the weakest soil and exert more pressure on a wall than gravel would.  

    • Surcharges/Load above the Wall: Is there anything extra the wall needs to support? Loads above the wall can be sidewalks, roadways or a steep slope above the wall.  These can also apply pressure that the wall must support.  

    • Other Things to Consider:  wall set back, block weight, and soil weight that are also important and play a role in the answer.

    So what is the answer then?  Because all of the above variables come into play, there isn’t a single definitive answer for every job site. But there are some tables and charts to help you get to the answers you need.   You can find these charts in the Allan Block Installation Manuals (walls OVER 6 ft. 1.8m or walls UNDER 6 ft. 1.8 m) or at   

    These charts can be used as a reference chart but it is not intended for use for final design purposes.  If the city is requiring a design for your permit you should always be working with a qualified design professional.  This design professional can take a look at the particular conditions for your wall and provide you with the final design.

    September 11, 2015

    What is Geogrid?

    Geogrids are flexible, synthetic meshes which are manufactured specifically for slope stabilization and earth retention. These “grids” are available in a variety of materials, sizes and strengths. They can be made of high tensile strength plastics or woven polyester yarns and are typically packaged at the factory in rolls.

    What does all that really mean, and how would you use it in your retaining wall?  To answer that, refer to the How Does Grid Work page on the Allan Block website.

    Here is a simple demonstration of how geogrid helps your retaining and landscape walls.  I started with two 6 inch (154 mm) diameter columns of compacted sand around 8 inches (200 mm) tall.  The one on the right would not support a small block weighing about 22 lbs (10 kg) and simply fell into a big pile.  The one on the left held over 75 lbs (35 kg) before I stopped because I thought the point was made.  So, what was the difference between the two columns of sand? 

     Both were from the same bucket of sand with just enough water to make the sand stick together a little.  Both were compacted in about 2 inch (50 mm) lifts.  The difference – “geogrid” was added to the column of sand on the left.  For this demonstration, household screen was used to simulate geogrid and was placed after compacting each layer of sand.

    If designed properly, and the installer follows the recommended geogrid spacing and compaction criteria, you can think of this mass of soil within the geogrid area as a single large block of soil.  This entire block of soil is going to resist the forces that are trying to tip the wall over, or slide it forward; much like a large block of concrete would act under the same conditions.

    September 4, 2015

    Retaining Wall Enemy #1

    What is the number one cause of wall failure? Water

    Ok…so maybe I should actually say improper water management is the #1 enemy and not just water.  Working as an engineer for Allan Block for almost 14 years I have been able to witness what to do and, obviously, what not to do when it comes to wall stability.  

    Whenever, I see a wall failing or not performing properly the first question that pops into my mind is “Where is the water coming from and where is it going?” Finding the answer will often lead to discovering the wall problem.


    The best thing for any wall is to handle the surface water that is pot initially above the wall.  Would you drain your entire backyard to the back side of your house?  Of course not, because this would lead to a very wet basement.  With any structure, you want to divert the water around it and not directly it over it.  This includes segmental retaining walls.  

    The water should not be allowed to simply flow over the wall.  Simple berms and swales can be incorporated in the wall design and layout to prevent this from happening.  There is more information on our website, about proper water management, which would keep the enemy at bay.