ᐅ Retaining Wall Made of Masonry Panels – How Is Proper Backfilling Done?

Hello everyone,

I have a question regarding the proper construction of a retaining wall made of wall panels between 105 and 130 cm (41 and 51 inches) high. My concern is about drainage capability and frost resistance.

What can happen if the retaining wall is not backfilled frost-proof with a permeable sand-gravel mix, but instead existing sand (medium sand, partly fine sandy, silty) is used? Will the wall then have a shorter lifespan? Or are we being overly cautious?

The contracted landscaping contractor has a firm opinion about proper workmanship and wants to set the wall panels directly into the concrete in one go. He also believes that due to the existing sandy soil, there is no need for a special gravel-sand mix to backfill the retaining wall in a way that ensures drainage and frost resistance.

We are laypeople and what we can do is research online: both manufacturer specifications and “internet experts” recommend backfilling with a permeable, coarse-grained gravel or gravel-sand mix. Additionally, this layer should be separated from the surrounding soil to prevent mixing of layers.

The landscaping contractor plans to start right after the weekend on Monday. I would like to know what requirements we should set for the contractor and what I should discuss with him on Monday morning.

o.s. schrieb:

We are amateurs, and what we can do is Google: Both manufacturer specifications and "internet experts" claim,

Are you aware that you also ended up here on the internet?
What is this actually about: L-shaped blocks – should the height measurements refer to the clear height on the downhill side of the height difference – is the "concrete" in which the wall panels are placed supposed to be a foundation...?

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You basically have several issues with poor drainage:
- Wet soil is heavier and exerts more pressure
- The cohesion in wet soil is reduced, which also increases earth pressure
- When freezing occurs, water expands and creates additional pressure

How severe these issues are depends mainly on your soil type. If you expect waterlogging, I would definitely not cut corners on drainage.

11ant schrieb:

But you do realize that you have also ended up on the internet here, right?

Good point. 😉

What is this about exactly: L-shaped blocks – are the height measurements supposed to indicate the free height on the downhill side of the height difference – is the "concrete" in which the wall panels are set meant to be a foundation ... ... ...?

1. The planned height difference between our property and the neighbor’s ranges from 35 cm to 100 cm (14 inches to 39 inches).
2. The top of the wall should be level along its entire length; the land slopes down at both the front and back.
3. The L-shaped blocks will be installed "with C20/25 concrete," according to the quote.
Is this explanation sufficient?

WilderSueden schrieb:

You basically have several problems with poor drainage:
- wet soil is heavy and exerts more pressure
- wet soil has lower cohesion, which also increases earth pressure
- water expands when it freezes and creates pressure

How severe these issues are mainly depends on your soil. As soon as you expect waterlogging, I would definitely not skimp on drainage.

WilderSueden, thanks a lot for your reply late in the evening!

The question is, at what point exactly is drainage considered "poor," and how can I tell if the soil is good enough?

Honestly, I assumed that with sandy soil, there is no need to install a drainage pipe at the base of the L-shaped retaining walls. While this is regularly required for public projects, I guess it is more of an exception for private projects. I mainly wanted to know what quality the backfill soil should have.

We have 7-meter (23 feet) L-shaped concrete blocks between 1.2m and 3m (4 feet and 10 feet) in height, which are even backfilled with cellular concrete, and a drainage system is installed within them. However, our soil here consists of clay and forest soil.

I would say that drainage is always necessary for soil, but be aware that this will add to your costs.