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February 12, 2016

What Soils are Best for Building Retaining Walls? Working the With On-site Soils

 The soils used behind and below a wall are a critical part of the total wall structure.  A reinforced retaining wall contains four basic building materials – blocks, wall rock, geogrid reinforcement and soil.  Understanding the properties and characteristics of soils is the key to building better walls.  Different soil types will compact differently and require different amounts of reinforcement.  Knowing the type of soil you are working with is important to building a quality retaining wall.

In general, granular soils are better to build with than clay soils.  Sand and gravel will compact better, drain better and often will require less reinforcement.  Sands and gravels also have better design properties and this can greatly impact the pressure a retaining wall will deal with.  Soils are typically defined by an internal strength characteristic known as a friction angle.  This angle is roughly equivalent to the natural angle a pile of this soil would make if it were an oven dry pile of individual particles.  Dry clay (with particles not clumped together) will form a much flatter pile naturally than gravel would.  This means clay would also apply more pressure to your retaining wall.


Clay Soil
Sandy Soil


If the on-site soils are of a very low quality, you should remove them and replace them with better soils.  Using stronger soils will reduce reinforcement requirements, allow faster compaction, and have better long-term performance.  Heavy clays and organic soils are both unsuitable for use in the geogrid reinforced zone and should be removed and replaced.  If you are unsure about what soils you are working with and you have concerns that they are unsuitable you can always check with a qualified geotechnical engineer to obtain an accurate soil classification.

February 5, 2016

Building a Retaining Wall - Excavating and laying out your wall

The site had been cleared of all trees and bushes so we could start excavating and laying out the base (Check out how we did this here.)  There was a lot of earth to move so a skid steer was rented for a long weekend of fun.  It was worth every penny in the time and energy it saved.  If we had to dig it by hand we would still be digging.  The utility company had already been contacted and they gave us the all-clear for digging.  The initial soil was mainly black dirt but once we dug deeper we found mostly sand.  It is important to not build your wall on top of black dirt because it consists of mostly organic material.  Organic material will decompose and your wall will settle.  The excavated soil was carefully piled in the most out of the way location in the yard.  We will use this soil at the very end of the project as either backfill or top soil for planting and having it out of the way was important for site management.





The shape of the wall was laid out using a 100 ft (30.5 m) garden hose paying close attention to our desired symmetrical shape along the back property line.  We learned the garden hose trick on allanblock.com.  My wife and I worked together on the layout so when we finally hit on the right set of curves, we painted the edges for the trench to guide us when digging. 

January 29, 2016

Preparing your Backyard for Retaining Wall Construction

Preparing your Site for Construction


Our backyard is very small and we needed to deal with several challenges in site preparation before starting construction.  We did not have easy access for equipment without involving my neighbor’s yard.  I talked to them and explained the magnitude of the project and they were happy to help out.  We decided to use the space between the two houses.  It was just big enough for a large skid steer to maneuver through but without this access it would have meant all hand labor. 

Allan Block has a great resource page for helping you ask the right question when getting started with your site preparation.  I used these helpful points to help me evaluate my site for property lines and utility locations and even how to discuss the project with my neighbors. 

Site Demolition


The entire slope where the walls were going was covered with overgrown brush, 4 large trees, old landscape timber edging and an old chain link fence.  Before any excavation could begin the site needed to be cleared.  Planning ahead for the demolition ultimately saved time and money.  I planned on removing the brush, the fence and the timbers myself, but not the trees.  So I contracted with a tree remover to take the trees down and haul the brush away.  I had all the brush piled in the front yard so the day the tree remover came it was ready to go.  What he and his crew did in 3 hours, I may still be working on.


The next things to go were all the stumps.  The machine this guy brought was a 4-wheel drive stump eating monster!  In less than three hours, 4 large stumps and about 75 small ones were chewed up into chips.  The end result was a clean canvas to start excavating.


Now that the hill was clear it was time to excavate.  The rented skid steer was delivered the next day and construction began.

January 22, 2016

Planning and Estimating your Segmental Retaining Wall

Our backyard wall was going to be a big project for a small backyard.  We needed to have a staging plan for all the material, but before that we needed to estimate the project and figure our budget.  The first step in building a retaining wall is to make a plan.  A good plan will lead to a good project.    To start, figure out how much grade change you are working with.  In our case we had around 7 ft.  We decided to cover this height with two terraced walls.  But how tall should each wall be?  We decided the lower wall should be about 3 ft tall for easy access for planting and gardening and the upper wall should cover the rest of the grade change and be as close to the property line as possible. 



Now that we knew the rough wall height and wall length (property line length) we used the incredibly easy AB Estimating Tool that we downloaded from allanblock.com.  When using this great tool, I had to remember to add a little to the wall height for the buried block.  You can also use this estimate to calculate the amount of wall rock and infill soil you will need.  Don’t forget to consider equipment rental such as a skid steer, plate compactor, diamond saw and maybe even a wet saw for cutting caps at the end.  It all adds up and if you plan ahead you will limit the surprises to your budget during the process.



How about staging the material?  I think a lot of do-it-yourselfers like me, forget that with a project of this magnitude you need to store/stage a lot of material.  Our project had 23 pallets of Allan Block, 3 truckloads of washed wall rock and 1 truck load of sand, drain piping and geogrid.  We also had to have a place to store the spoil, or the earth that we removed during the excavation of the hillside.  This was a huge pile that was there from day one until the end of the project when it was used as topsoil backfill.  Because we had a good plan for all the materials, the neighbors were surprised we had space to actually build the wall!  

January 15, 2016

Can I Use a Simple Gravity Wall?

The question often comes up whether a wall can be built by just stacking block using the installation recommendations found on the Allan Block website, or if geogrid reinforcement is necessary behind the wall.  Every time this question is asked, four equally important issues need to be addressed:  how tall is the retaining wall, what is going to be above the wall, what type of soil will be behind the wall, and what type of block is being used?

To help simplify this decision, take a look at the Maximum Gravity Wall Height Chart on the Allan Block website.  This handy chart can give you an idea of the maximum wall height for a variety of situations.  The AB Commercial Wall Manual also describes the fundamental principles of gravity walls and how to build them using Allan Block retaining wall blocks.


How tall is the retaining wall?
This is an important question to answer because the taller the segmental retaining wall (SRW), the more soil behind the wall is going to be trying to push the wall forward or tip it over.  This is called the wall’s external stability.  In order for a gravity wall to work, the weight of the block and crushed stone within the block must be greater than the forces pushing against the wall.

What is going to be above the wall?
It is intuitive that a landscape wall with a level planter bed of flowers above it would have less force pushing on the wall than if you were building a retaining wall to hold up your driveway, but what about a slope above the wall?  When your wall has a slope above it, there is more weight above the wall trying to push the wall over.  What another wall above, forming a terrace?  True, you might only have two 3-foot (0.9 m) walls, but that top wall of the terrace is going to be pushing on the bottom wall.  In summary, any time you have a surcharge (some type of additional weight) above the wall, your overall gravity wall height will be reduced.
 
What type of soil is behind the wall?
Clay soils are going to push against the wall more than granular sandy soils.  This is because a typical clay soil has a lower internal angle of friction than a sandy soil. Use the AB Commercial Wall Manual  to learn more about soils and their influence on the wall.

Which Allan Block retaining wall block is being used?
A wall made from AB Stones will tilt into the hill more than a wall made from AB Classic blocks.  This is because the AB Stones have a setback of roughly 12-degrees, while the AB Classic has a setback of about 6-degrees.  Therefore, under the same conditions, a gravity wall constructed from AB Stones will have greater stability than a wall built using the AB Classic.


Next time you are planning your landscape or retaining wall project, keep these four questions in mind.  Then, reference the Allan Block website to assist in designing a gravity retaining wall that you can enjoy for a lifetime.