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What foundation and mortar is best for a rock wall around a garden bed?

Our house has a half-finished rock wall (400mm high and 400mm wide) around a garden bed, picture below. I'm looking to finish the wall and am keen for some advice.

1) Can anyone please help identify the existing rock? It looks like bluestone?

2) What foundation is best for this size rock wall? My current thinking is digging down 200mm, then adding 150mm of drainage gravel and 25mm of screenings (compacting each layer), and laying the largest rocks on the bottom (partially below the soil line). Is there anything else I should do or do differently?

3) I've been tossing up whether or not to do a dry stack technique, but the existing wall looks like it has some type of mortar between the rocks (particular the top layer and also visible from the back of the wall between each layer). If I do use a mortar, what is best for this type of rock? I've read about Type S and Type M mixes, lime mortar mixes and now am a bit confused.

Thanks! Simon

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @SimonBl. It's terrific to have you join us, and many thanks for your question about building stone wall.

For the foundation, what you’ve described is a solid approach. You could also add a thin bedding layer of mortar, say around 100mm, across the base and bed your lowest stones into that. This helps anchor them without being too visible at the front.

From what you can see in the existing wall, it looks like the previous builder may have just added dollops of mortar at the back of the stones rather than filling all the joints. This locks the stones together while keeping the front looking natural. A standard mortar mix, like a Type S or general-purpose masonry mix, will work fine for this type of wall. Lime-based mortars are also an option, but for a garden bed wall, standard mortar is perfectly suitable.

It does look like it could be blue stone, possibly second grade. Matching your new stones to the existing ones in colour and texture will help the finished wall look consistent. Checking out blue stone options would be a good starting point.

Please let me know if you have any questions.

Mitchell

Simon_B

https://www.workshop.bunnings.com.au/discussion/comment/60096#Comment_60096

Thanks Mitchell! I'm still learning, so this may be a simple question, but can you please explain the composition of:

- standard mortar mix - Type S

- general-purpose masonry mix

- lime-based mortars (standard ratio of components)?

Thanks!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/92533#Comment_92533

We're all still learning @Simon_B - that never stops!

Standard mortar mixes (like what we stock in-store) are usually a blend of cement, sand and a small amount of lime, so the composition is typically one part cement, one part lime and around six parts sand. This gives you a balanced mix with good workability and moderate strength for general use.

Type S mortar has the same basic ingredients, but the proportions are different. It contains more cement and less lime compared with a standard mix, which lifts its compressive strength. A typical Type S ratio is one part cement, half to one part lime and four and a half to five parts sand.

Lime-based mortars shift the balance the other way. They still contain cement, but in a smaller proportion, and rely more heavily on lime for workability and flexibility. A common ratio is one part cement, two parts lime and nine parts sand. This produces a softer, more breathable mortar that suits older or more delicate masonry.

Mitchell

Simon_B

https://www.workshop.bunnings.com.au/discussion/comment/112379#Comment_112379

Too easy, thanks for providing that detail - it has definitely helped me get my head around it all!

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