Allan Block Retaining wall excavation: How much soil removed and why?

Offering reliable landscaping services to homeowners in British Columbia since 2013

Picture of Sam Maerz

Sam Maerz

Owner/Foreman

A major component to building an Allan Block retaining walls is the excavation and disposal of the onsite soils. Soil is excavated from under and behind the retaining wall. This material is called the spoils, and must be removed from the wall area, moved to the road, and loaded into dump trucks.

retaining-wall-excavation

When building a retaining wall in the front yard, larger more efficient machines can be used, but when working in the backyard of a home, narrow construction equipment must be used.

excavator-loading-dumper

 Back 40 Landscaping has a 39” excavator, and two 31” wide track dumpers. These machines are very compact, and can move a lot of material in a short time.

excavator-loading-track-dumpers

These machines excavate and move the soils to the front yard where our large loader can load our company owned dump truck and trailer.

On projects that are large, and open enough to move the excavated soils to the front yard fast enough, we hire a full-size dump truck to truck the soils away.

Unfortunately, dump trucks charge by the hour, and most residential sites are too small to efficiently move this material to the front yard fast enough to be economical.

How much soil is excavated for an Allan Block retaining wall project?

Retaining wall projects built to best practises produce a lot of soil.

Every project we have done, the client is in disbelief with how much soil is removed.

Having the right knowledge, and equipment makes it a breeze to do, but it still is a significant amount of work. The retaining wall design will ultimately determine how much soil is affected.

Here are the 2 most typical scenarios

Typically retaining walls are classified by either Gravity or Reinforced.

Gravity retaining walls are always under 4’ in height, and have no ground slope above or below the wall.

Gravity walls cannot have any additional weight surcharge above them such as a building, parking area, pool, etc.

If building multiple gravity walls that are tiered, the walls must be spaced apart a distance of 2 X the height of the lower wall. I.E if the lower wall is 4’ than there needs to be an 8’ wide tier in between the two walls.

allan-block-gravity-retaining-wall

A typical excavation zone for a gravity wall is about 30” wide at the bottom and 36” wide at the top of the excavation cut. This allows for 6” of gravel in front of the wall, 12” for the block, and a minimum of 12” for clear draining gravel behind the wall.

Retaining wall block can be straight vertical, but are stronger the more they angle back into the soil, this is why the cut is wider at the top of the excavation.

The amount of soil excavated for a gravity wall is approximately 30” wide X the height of the wall plus 10” (6” base rock, and 4” buried block), X the length of the wall.

Once that cubic yardage has been calculated, times it by 1.4-1.5 to account for the soil uncompacting, and the additional material removed from the top of the excavation cut.

Reinforced Allan Block retaining walls use the same blocks, but are built with reinforcements to make them stronger.

Some examples of reinforcements include: Burying more of the bottom of the wall in the ground, adding more gravel behind the wall, a better, or bigger drainage pipe system, and adding Geogrid (a type of structural netting into the gravel/soil behind the wall).

A reinforced retaining wall will require at least 70% more excavated soil. This is because of the wall being buried more, and the geogrid required.

Typically, the grid will have to extend behind the wall a minimum of 70% the wall height.

For a 10’ tall wall this is 7’ of soil removed to instal the geogrid.

geogrid-retaining-wall

What exactly does geogrid do?

Geogrid is a structural netting that combines the weight of the soil behind the wall together with the weight of the blocks.

Instead of the retaining wall just being the blocks, which are 12” deep, geogrid makes the blocks and the 7’ wide soil mass one.

This is now a 10’ high cube that is 8’ deep. A 10’x 8’ cube is going to resist the weight and pressures of the soils above or behind it much better than a 10’ high by 1’ deep wall.

geogrid-retaining-wall

If you notice in the typical reinforced drawing above there is a soil area called “infill soil”. This is the additional material that has to be removed to install the geogrid layers.

On a large site, this material can be the excavated soils that were removed to instal the grid. The challenge residential retaining walls face is there is no place to store this quantity of soil to reuse.

When you take into account the effort to reuse this soil properly, it doesn’t make financial sense.

Excavated soils uncompact, so in order to reuse it the soil has to be watered to a tested moisture content, and heavy equipment has to compact it until it reaches 98% compaction.

This takes time, and requires the geotechnical engineer to be testing it throughout the project. When you have to do this for every row of blocks you add it quickly becomes impractical.

In this case uncompacted  soil will not only not help the retaining wall retain the soil behind it, but will help to overturn the retaining wall.

Properly compacted soil stays put, while uncompacted soil sluffs and will sluff towards the retaining wall, adding pressure to it instead of helping it.

Compaction is one of the most critical steps to building a strong retaining wall.

allan-block-retaining-wall

What we do  instead is remove the material from the site, and replace it with ¾” clear wall rock.

This material requires little water to compact, and achieves 95% compaction just by placing it behind the wall.

A few passes with the compactor, and it’s at the required 98% compaction without an engineer onsite to test it.

The choice to reuse the infill soil or replace it with ¾” clear is site and project specific.

If there are significant savings from reusing the soil, and not having to buy gravel, it should be considered. If the costs are close, then ¾” gravel should be used. This will provide more drainage through it which will reduce the weight of the surrounding soils the wall is retaining, and provide a much cleaner site, and working area.

gravel-infill-allan-block-retaining-wall

Passion, education, and experience

As you can tell, I am very passionate about not only building retaining walls to best practises, but educating you along the way.

So, what makes me qualified to do so?

Well, first off I see failed retaining walls all around my hometown of Abbotsford. These walls have not failed because of the block, but have failed due to inexperienced, low-quality installations.

Unfortunately, the customers hiring these contractors were left with a subpar installation based on what the contractor chose to do.

Here are Back 40 Landscaping our mission is to “Challenge the Status Quo”. We are able to this through education like this article, and are backed by schooling and experience.

I am a trained, certified and an experienced Allan Block retaining wall builder, and CMHA Segmented Retaining wall builder, and inspector.