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Comment by samtho | Hacker News Reader

The problem with dry stone construction is that you cannot determine the performance characteristics required for, say, a building foundation. Not all boulders are created equal, but a professional concrete pour, mixed and installed to the manufacturer’s specification is going to have a guaranteed maximum strength- something random boulders will not have.

What we can do is create our own concrete mix using Portland cement (the “glue”) and locally sourced materials to bulk it up such as gravel, aggregates, sand, etc.

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So the question becomes, how do existing concrete manufacturers determine whether locally sourced materials ("random rocks") are suitable?

In traditional dry stone construction, strength of each rock is determined by sound and by sight. It seems crazy, but nowadays this should be feasible with a neural net. If you want to get fancy (or achieve high reliability) add ultrasound in the grippers or other sensor modalities.

Or... you target a market other than building foundations? :) I suspect different groups could pursue both paths.

> how do existing concrete manufacturers determine whether locally sourced materials ("random rocks") are suitable?

They test individual samples from each pour on a calibrated press. For large buildings and critical pours, they'll keep & test multiple samples that undergo accelerated aging to make sure they're up to spec for the anticipated lifetime of the building (or until the concrete fully stabilizes over a few months / years).

So could catagorization of local stone based on acoustic and visual indicators, combined with random sampling and strength testing provide the assurance needed that a wall will have the required structural properties?

The point is that the concrete manufacturers pulverize and homogenize the aggregate. This makes random sampling acceptable.

If you haven't pulverized and mixed the inputs, a random sample is merely a random sample. It isn't meaningful unless it comes from a high-entropy source.

So are you saying we couldn’t estimate the structural properties of autonomous built dry walls from local stone? Or that the error bars would be unacceptably large? Or just that this approach would only work with certain stone in certain locations?

I think the problem is more that you cannot at all guarantee a specific performance of the structure ahead of time, because you can't know what the properties of the local rocks will be before you are on site and have measured them all.

Geotechnical engineering is all about taking samples and using wide enough safety margins. And in the vast majority of cases, the end result is a building that doesn't fall down.

Sure, you might have been really unlucky that all your test samples of soil and rock were really strong granite, but the exact spot you put the main structural pile, unknown to you, was all mud and silt... But the chances are so vanishingly unlikely we accept it.

Doesn't that apply to wood too? Yet we use wood to build houses almost everywhere.

People rarely build entirely using found wood on site, at the very least each tree to be used gets selected before you bother felling it. But the overwhelmingly most common use of wood is in the form of processed product: using quality controlled lumber of standardised size.

The interesting thing here is you can feasibly measure them all.

Not ahead of time, but as you build.

Sure, but then it's somewhat up to luck precisely what properties you get. This variation might me manageable if you design enough redundancy into the design, but by necessity you will need more margins than if you used a more controlled material.

A Deep Learning Based Method for the Non-Destructive Measuring of Rock Strength through Hammering Sound Appl. Sci. 2019, 9(17), 3484; https://doi.org/10.3390/app9173484

Acoustic inverse problems are solved routinely in seismology and exploration geophysics. Seeing trough a rock the cracks and internal structure using acoustics seems solvable.

Modern concrete only lasts a hundred years or so. Stone walls last much longer when correctly constructed.

Presumably the placement algorithm understands compression strength of the local rocks, soil heaving, etc.

So, software addresses the initial problems you describe at construction, and geology/materials solve the problem of needing to inspect it every decade or so, and replace it in 50-100 years.

> Not all boulders are created equal, but a professional concrete pour, mixed and installed to the manufacturer’s specification is going to have a guaranteed maximum strength- something random boulders will not have.

Random boulders surely have some guaranteed maximum strength. Now minimum strength is the real problem.

Why not do a stones-wall like in the article, but add cement as glue between the rocks?

The wall will then dam up water or crack instead of shifting slightly, I read in another comment.

I did this for a landscaping project because I was not confident enough that it will stay ..lasted ten plus years and then it started cracking. I am now left with unsightly gray blotches and worried the rest might tumble..

It was/is gorgeous..I did three sections ..mostly for three very large kidney shaped raised beds..moss has grown over the stone and I also tried sticking thyme and mint in some of the suitable gaps..

I guess it’s diff if it’s enclosed and filled with soil. I wish i had the confidence and skill to attempt an actual wall.

I used to drive past a house here where the guy took like three years to complete a 80 ft long dry wall and he planted all kinds of beautiful flowering plants ..it was stunning and when it was done, the flowers and fruit trees popped up with their colours too ..it took forever but he didn’t care..you could tell that it was a labour of love and he was really enjoying it.

That is what I assumed when I first started drywalling. However, in the wild, I can't see such a solution working. Unlike the walls of a house, walls bedded on soil move all the time. In the ever-moving domain of the Welsh hillsides, cement glue would crack within a year.

Ideally, drystone walls are composed of two skins. The way the stones are arranged tends these skins to each incline inwards slightly. In this way, the wall as a whole acquires cohesion through physics.

Several reasons… Primarily, because the ground underneath shifts overtime, particularly in area, subject to a freeze and fall cycle. But you also have to solve for water flow - if you build a solid stone wall with concrete, you also need to add drainage on the backside of the wall. With dry, stacked stone, you can often get away without a complex drainage solution

With concrete you have outsourced, not solved, that problem

There's a lot of drystone walls here, in Aotearoa, but not of buildings.

There is a related technique for buildings, building from rubble, using lime morter.

I live in a building partly built that way

It’s more difficult but still possible. Mountain passes in Switzerland are often built on dry walls

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