Landscaping & Design/retaining wall


retaining wall posts
retaining wall posts  
QUESTION: If i have a 15inch high retaining wall, 80 feet long and I put the posts 32inches apart and 30inches in the ground will it be properly supported?  What I am using are 8'/6-6.5posts, 2high.  I was going to fill it with dirt to make it level with the fence.

ANSWER: Hello:

You would not need posts to build such a retaining wall. Stack the posts on their side and drive rebar through the posts into the ground to a depth of 18 inches. Place cable anchors (deadmen) or use posts for anchors.

If for some reason you desire posts,they do not need to be placed 2 feet into the ground for such a short structure. Eighteen inches is sufficient. Two vertical posts for every horizontal post would be sufficient,in other words,the posts can be spaced about 8 feet apart.

Mark Harshman

---------- FOLLOW-UP ----------

QUESTION: Thanks for the answer.  You more than answered my questions.  Looking at the internet and asking for other opinions, if you ask 40 people you get 40 different answers.  Thanks again.

Always looking down the road if it will last 20-30 years how can I make it last longer.  Not that I care, I'll be gone by then.

What do you think of products like liquid rubber sealant ( (Ames' Blue Max Liquid Rubber) or something similar for the posts in the ground up to ground level?  I know you can't do the whole post so moisture can escape.  Or doing it along the posts on the inside (I even thought of pond liner) and underneath that touch the soil along with pea gravel or similar between dirt and post?

I just hate the thought of something I build giving out in my lifetime knowing I should have done it differently or better.  

Thanks for the help!!!


While the rubber may offer protection from water it does not offer protection from insects and that is a function of the preservatives used in pressure or surface treated woods and so if you go with the rubber and you are using a wood susceptible to rot,apply a preservative first.

If you are using a wood relatively unsusceptible to rot,such as oak,you may want to forego the expense of rubber or preservatives. Unless you are in an area of very low rainfall applying rubber or any impermeable material behind the wall will create problems because it will create hydrostatic pressure against the wall that the wood may not be able to withstand and will impede drainage from the soil which will negatively impact your plants.

Permeable landscape fabric is ok against the face of the wall to keep soil from leaching out and gravel can be placed in back of the fabric to aid drainage away from the face of the wall.

Mark Harshman  

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Design of landscape structures and design of planting arrangements in the landscape. I usually do not answer questions of a horticultural nature or about growing plants or gardening unless they are related to design. I will answer these questions at my discretion. I will answer maintenance questions at my discretion,usually when they are related to design. I am a landscape designer. I am not a contractor or a gardener. Although I will describe how something is built I do not provide detailed installation or construction instructions. Here are example categories of problems I can help you with: Design of residential walks,low walls,low retaining walls,fences,drainage systems,arbors,plant selection,xeriscapes (low water use landscapes),rain garden design,rain water harvesting,permeable paving,sustainable drainage. No advise on an architectural or engineering level. I DO NOT PERFORM ACTUAL DESIGN WORK THROUGH THIS SITE. I only provide suggestions and advice. For more comprehensive help check out my website at:


Landscape concept plan for a 30,000 square feet housing site. Involved site recon by pace measurements and photographs and a site analysis and a site analysis report and recommendations and a preliminary cost estimate. Design elements included canopy trees,shrub cover,accent flowers,trash receptacles,benches and planters. Landscape concept plan for a 7,000 square feet site for a community center. Involved site recon by tape measurements and photographs,a site analysis and the creation of 3 conceptual designs for the site. Work was done through a local professional design center. Design elements included tree garden,playground,sitting area,lawn,cooking area,lighting and concrete and brick paving. Landscape concept plan for a 3,000 square feet housing site. Involved site recon by tape measurements and photographs and a site analysis. Concluded in 3 concept designs for the site,shown at a formal presentation,a site analysis and recommendations. Work was done through a community volunteer program. Design elements included decks,a playground and fencing. Briefly as draftsman for landscape designer/gardener. Conceptual landscape design for residence. Involved site recon by tape and pace measurements and site analysis. Design included covered patio area,ground cover and flower beds. Two years experience as draftsman for transportation department in engineering firm and for engineering consultants to phone company. Duties consisted of plotting topography. Survey crew member. General assistant to surveyor. Work was done for U.S. Army and County Highway Department. No instrument work. Duties included tape measurement,holding graduated survey rod and plumb bob for sighting and clearing brush. Marketed Landscape Design Services. Brochure writing,promotional material and marketing graphics.

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From Civil Engineer: I am very impressed with your work and with the obvious thought you have put into the marketing of your venture. It was especially good to see a print of that job we discussed on the phone. I wish you much luck with the venture as I do with your plans for school. Im darn proud of you buddy. And by the way,when I get my license,maybe you will consider me for a consulting job down the road. I hope to get together with Rick sometime soon and I would really like to show him your stuff if its ok with you. From all these questions you ask I get the feeling that you are genuinely interested and have put alot of thought into the idea and process of landscape architecture and if you could possibly afford the cost I would urge you to take some classes in the areas you are most interested in. The subject is not so simple as it appears,mixing science and engineering with art and a certain amount of religion to create a comfortable and pleasing balance between man and nature. A thoughtful soul like yours can do this but it can be a lifes work getting it right.

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