Difference between Marl and Chalk 1

[IngeIvan]

Hello,

I recently had a site with two different geotechnical reports (same site, two reports from different years) and the subsurface conditions were similar (depth of clays, bearing stratum, etc), but one of them used Chalk for the bearing stratum, and the other used Marl". Both were found at the same depths.

Are these two types of soil basically the same?? can they be used interchangeably? Or what is the difference?

Thanks!

 

[fattdad]

I really don't know, but. . .

In my world, chalk is a rock and marl is a soil - a soil composed of calcium carbonate.

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

Chalk is a rock but can be encountered in different Classes in the UK, from intact blocky Chalk to Chalk performing like fine grained soil. CIRIA report C574 (Engineering in Chalk) is the gospel in the UK for working with Chalk.

What type of tests were undertaken? For a Clay i would expect atterbergs, triaxials, odeometer. For a Chalk , depending on the grade (i.e. intact or structureless) it could range from UCS, NMC, saturated moisture content, atterbergs, PSD, to name a few.

Clay will obviously consolidate and this will be the primary cause of settlement, however for Chalk, consolidation settlement is relatively small and occurs quickly.

 

[Mad Mike]

Geologist in the house- a marl is a lime-rich mudstone derivative i.e. a blend of lime and mud; chalk on the other hand is a biogenic limestone, usually almost pure lime derived from the skeletons of cretins on the ancient sea floor.

So the lime content is the main difference- in respect of bearing capacity the terms could safely be used interchangeably.

All the best,
Mike

 

[maxim22]

In UK, "marl/marly" might be used for all grey/non-white chalk or even calcareous glacial clay by non-geologists. The term may be accurate according to what the geology actually is (on the map and in specimen), or written as a loose description of what a person saw having some knowledge.

 

[fattdad]

To the OP: There's no chalk in Virginia (USA). We do have marl in our Coastal Plain. I like MadMike's explanation and it does make sense.

Unrelated to all this, I reflect on a rhetorical question, "What is the RQD of chalk?" I ask this question, because I'd expect if we had it in Virginia, the drillers would just drill through it with the auger or tricone, try an SPT and then make some claim of some sort. Who'd ever see a coring attempt!

Then I reflect on the other question, "If we actually had core, would it score an RQD?" According to the ASTM (the standards we follow mostly in the USA), rock core that can be broken by hand is excluded.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

I like MadMike's description as well. One significant distinction, particularly in driven piles, is the ability for marl to remold after driving. I don't think the same happens with chalk. This is significant because in many of the coastal plains marls, driven piles that seem to have insufficient capacity during driving will usually reach capacity if they are left alone for a few days and upon retapping, the capacity is much higher.

 

[racookpe1978]

One significant distinction, particularly in driven piles, is the ability for marl to remold after driving. I don't think the same happens with chalk. This is significant because in many of the coastal plains marls, driven piles that seem to have insufficient capacity during driving will usually reach capacity if they are left alone for a few days and upon retapping, the capacity is much higher.

Is that gradual movement back towards the pile the "remolding" you mentioned?

 

[Ron]

@racookpe1978....yes

 

[EireChch]

@Ron - remoulding of chalk is possible too. Driven CIP piles and CFA piles perform better that standard driven concrete or Steel piles

 

[Ron]

Thanks, EireChch......I have no experience with chalk, but have dealt with limestone marls as they are common in the Southeastern US coastal areas.

 

[geomane]

Here is an answer I recently received regarding chalk vs marl,

Generally, chalk is thought of a fine-grained sedimentary rock that is composed of a high percent (say 90%) of pelagic carbonate (forams, coccoliths, etc.). There can be a small component of siliceous microfossils, clay, and framework silicate silts.

Marl is usually though of as an impure chalk with a much higher content of clay and silt grains. The marls in the Alabama Coastal Plain, for example, are usually about one-third pelagic carbonate, one-third clay, and one-third silt (quartz and feldspars, mainly).

Chalk is oceanic, whereas marl may be oceanic or lacustrine. Chalk is most common in the Cretaceous System, whereas marl has no restrictions in the time scale.

The engineering properties of chalk and marl have some similarities, but also some differences (mainly due to clay content of the marl).

I have to say that there is some confusion in the literature about these two terms and that can be frustrating. Some early writers did not make much distinction between chalk and marl, for example, in the Alabama Coastal Plain. There is no agreed petrologic boundary between chalk and marl, as far as I know.