I had been told that a silt soil, when professionally compacted (from all the laboratory essays to the compaction with a vibratory roller), ends hard as a rock, and when hit with a metal bar it makes a metallic noise.

I find that hard to believe. Is it true? What is the common reality?

I mean the question from the viewpoint of a earth pavement construction, or as a sub base for another type of pavement.

I have interest in the answer from the viewpoint of a fast checking of the quality of the compacted soil (before any essay to check the level of compaction)

  • $\begingroup$ In geology, the end result is called "sedimentary rock" as opposed to igneous and metamorphic. $\endgroup$ Aug 25, 2017 at 13:24
  • $\begingroup$ Rocks have a whole range of strengths. A sufficiently compacted silt or overconsolidated clay strata could probably be hard enough to qualify as a soft rock from an engineering POV, but you would not be able to get it compact enough to be comparable in strength to a hard rock without adding some sort of cementitious material. Most rock has high cohesion, and silt cohesion is generally somewhat low aside from suction forces. $\endgroup$
    – Rick
    Oct 12, 2017 at 21:47
  • $\begingroup$ You mean like shale ? $\endgroup$ Nov 18, 2020 at 19:20
  • $\begingroup$ Until it gets wet and turns to soup again. What you need above all else is drainage and permeability. Without that, compaction is worthless. You'll just have endless unfixable potholes and washboard on heavy trafficked areas. I live in a silty river basin and the roads have four foot deep ditches on both sides to attempt to manage roadbed drainage, but they still move around a lot. $\endgroup$
    – Phil Sweet
    Aug 1 at 23:18

3 Answers 3


To give you an answer, I have 'Extremely sandy gravely loam' USGS calls it, or 'Glacial Rinseates'. In other words, loess and silt from two ice dam failure floods in glacial lake Missoula in Montana, that blasted it's way to the sea all the way down in Oregon. We are part of the Channeled Scablands-a mind-blowing geographical sceane of epic proportions. We also have St. Helen's ash now too. Oh my LORD this junk PACKS! It kills everything! Can you slam a metal rod against it and hear a concussion? Have not quite tried that yet, but HEAVY GARDEN TOOLS WILL BOUNCE if tossed, not thud. I am currently regrading my foundation. You don't HAVE to pack this stuff. Saturate it, let it fully dry. Repeat. Step on it if you feel like it, but not necessary. Just keep wetting it. It is one step above clay and must be ammended with sand, gypsum and organic matter. I have to put two bags of sand, two gypsum, 2 compost for a 4'×10' bed. It's THAT hard, and extremely hydrophobic as well. So I could completely see a metal bar bouncing and making a resounding BOOONG!


Yes it is.

The house we now inhabit had 60 years of light, dusty silt blown onto its roof which then got swept down the gutter drain pipes. They were occluded with leaves and as a result the silt settled out over the leaves and left sedimentary layers in which the deepest deposits contained the fossilized remains of extinct dragonflies, etc.

Each drain pipe developed a hard plug about 6 inches long near ground level which I had to dislodge by cutting the pipe open, smashing the plugs to splinters with a hammer, and then and prying out the fragments with a screwdriver.

And here's the capper! Willamette Valley silt was washed here by the Spokane Floods and so it is the same scablands stuff described by Rhodie! Hard as CEMENT.


All soil is rock. Looking at this from a geotechnical perspective rather than agrarian or geologically will clarify why.

Rock consists of up to 50% metal and that is mostly aluminium, calcium, silicon and a few other light metals in the earth's lithosphere. Rock weathers either mechanically or chemically. The processes that cause weathering are wind, rain, thermal variation and kinetic erosion due to gravity and solar energy.

Some rocks are almost devoid of metal in their composition because they are mostly carbon, hydrogen and oxygen compounds with a metal atom bond. As rock decomposes due to oxidation its density changes.

Igneous rocks create very dense, dark soils and often are highly metallic. Black granite, basalt and gabbro are good examples of such rock. These rocks decompose to metal rich souls which can be very heavy and highly compacted to produce surfaces "as hard as rock" because they are made from rock. The finer the soil particles, the greater the compaction and if treated with chemicals, they may bond together like rock and behave like bedrock structurally. To say compaction is as strong as rock is accurate but soil integrity to resist penetrative pressure requires lateral strength only available from extreme heat that made the parent material to begin with: in excess of 1750°C

In summation, so can only achieve the strength of the rock from which it consists less the pore content and frictional factors of the grains. These some may reduce structural load-bearing strength by a much as 60%. When water is added to the mix, then it may reach liquefaction and behave as a fluid instead of a solid at ambient temperatures. However, as permafrost it is another animal. Technically it is theoretically possible but in practice not. 95% compaction is considered safe in engineering applications but any more than that raises economic issues without the mechanical properties being greatly improved.

Hope this helps

  • $\begingroup$ IMO, the opening statement "All soil is rock." could be revised to say "All soil came from rock." Also, without addressing the consistency, it seems an over generalization that all silts can be compacted to the strength as good as the rock it decomposed from. Note I am not a geotechnical engineer, nor a geologist, so I could be wrong though. $\endgroup$
    – r13
    Mar 18, 2021 at 20:26
  • $\begingroup$ I distinctly remember when I did my geotechnics module for my civil engineering degree a couple years back that the lecturer, a doctor of civil engineering no less, opened the semester with that single statement. Every grain of silt is rock no matter how small. filings from a metal bar are metal no matter the size. The difference comes when there is interaction with other elements. Its strength remains identical until interactions occur such as water, wind, thermal and, biological processes. $\endgroup$
    – Rhodie
    Mar 20, 2021 at 5:30
  • $\begingroup$ @Rhodies You may have picked a single sentence out of the larger context. I suggest to talk to your classmates, who maybe now, by practice, are more qualified to address the definitions of soil and rock, and their formation. $\endgroup$
    – r13
    Mar 20, 2021 at 11:34

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