ᐅ Retaining Wall Made of Masonry Panels – How Is Proper Backfilling Done?
Created on: 18 Feb 2022 22:03
O
o.s.
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.
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.
W
WilderSueden19 Feb 2022 11:43o.s. schrieb:
The question is, at what point exactly is drainage considered "poor," and how can I tell if the soil is good enough? There are different ways to find out. You might have a soil report including a permeability coefficient (kf-value). Medium-grained sand is usually around 10^-3. Alternatively, you can test it yourself. Dig a 40x40cm (16x16 inch) hole in the subsoil, pour in a 10-liter (2.6 gallon) bucket of water, and see how long it takes to drain. Repeat this several times.
o.s. schrieb:
Honestly, I assumed that with sandy soil, you wouldn’t need to install a drainage pipe at the base of the L-shaped concrete blocks. Although this is regularly required for public projects, I guess it’s more of an exception in private builds. With our soil, there is no way around proper drainage. That’s why I’m also planning to use a small retaining wall since they are fundamentally water-permeable.
o.s. schrieb:
What I mainly wanted to know was the quality the backfill soil should have. It needs to be compactable and permeable to water.
tomtom79 schrieb:
(...)
I would say that drainage is always required with soil, insist on it but be aware that you will have to pay for it as well. Thanks for your advice! I will insist on backfill that allows for infiltration. Of course, that costs something.
What about the drain pipe? Is it a nice-to-have?
WilderSueden schrieb:
There are several options for this. You might have a soil report with a permeability coefficient (kf value). Medium-grained sand is usually around 10^-3.
Alternatively, you can simply test it yourself. Dig a 40x40cm (16x16 inch) hole in the subsoil, pour a 10 liter (2.6 gallon) bucket of water in, and observe how long it takes to drain. Repeat this several times.
For our soil, a proper drainage system is essential. That’s why I’m planning a small retaining wall since they are generally water-permeable.
The soil must be compactable and provide proper drainage. Excerpts from the infiltration test report with kf values. The dynamic cone penetration tests 1 and 2 are located near the planned wall site. As a non-expert, I would say: natural soil, partially heavily silty, more fine sandy than medium sandy => special backfill with coarse sand or gravel is required, right?
W
WilderSueden22 Feb 2022 08:38Yes, backfilling should definitely be done, and with that infiltration rate, I would also generally recommend installing drainage. A rule of thumb for differentiating between permeable and impermeable soils is a hydraulic conductivity (kf) of 10^-6, and your value is slightly worse than that.
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