I agree with your client, CLAY and CHALK are 2 different materials. CHALK, even structureless does not expand

get a copy of Engineering In Chalk Ciria C574

I understand what you've said but CIRIA (extract below)refers to frost action and potential heave

Full disclosure, I've never worked in the UK or with Chalk specifically although I did live / work in Canada in some 'frosty' soils.

Even if an intact sample of the chalk itself isn't particularly expansive, how does it weather / fracture? If there are alot of cracks / gaps / voids in the chalk at a macro level that are unevenly distributed, water could sit in those, freeze, and cause differential expansion / movement of the overall mass even if an intact sample isn't particularly expansive. Water freezing on its own is still enough to push things around. (volume change of ~10% just from water freezing and expanding IRC?) Eg. refer to this paper and some of the images in it: http://sro.sussex.ac.uk/id/eprint/75546/1/Acepted%... Maybe that's what the CIRIA guide is written that way.

I don't know what standard practice in the UK is. In Canada we just try and make the footings deeper than the frost depth (conventionally this is easy because at least in Alberta the houses all have basements).

Also found a couple of other articles (maybe you can find more) : https://pure.port.ac.uk/ws/portalfiles/portal/5798... , https://www.issmge.org/uploads/publications/1/41/1...

Also, the bit about supersaturation is pretty standard. We call it ' spring break up ' on the prairies. It freezes from the top down, and it thaws from the top down, and when it thaws from the top down you end up with melted snow or melted water sitting on top of the still-frozen bit underneath not draining. Though I suppose there wouldn't be much melted snow in England? But maybe water from spring rain?

I think the omission of any discussion of heave from expansive minerals is proof that Chalk is not susceptible to it. Its minerals are calcite, not expansive illite, montmorilite etc.

again heave from frost is a different issue

EireChch - the articles I linked seem to support that notion (both intact and re-worked chalk don't seem to be expansive). However from looking at the photos in the one article it does look like something that might be prone to differential frost heave.

I guess I'd like to konw what OP means by this: "as the chalk at the site (no superficial) is matrix dominated (Dm)."

If the matrix ("Dm") is expansive then maybe it is worth thinking about (I guess there would have to be some depositional / formation process where the chalk cracks and the gaps are filled in with clay particles suscpetible to shrink swell heave?).

In the UK chalk is graded from A to Dm, Dm is deconstructed chalk so like clayey sand kina. Its hard to describe.

there are no fractures etc in grade Dm as its already mush!