I meant that the Barrows are only made of a pile of earth, without reinforcements inside of it.

...you know, now that you bring it up, I actually don't know if burial mounds like this have supports inside of them.

One Google search later...

Yep, you're absolutely right. From what I can tell, burial mounds are usually built up around a wooden or stone tomb. So maybe not reinforcements, per se, but a solid core around which the earth is packed.