According to this source (which I am not saying is the best source)
How much radiation will the settlers be exposed to? - Health and Ethics - Mars One QUOTE: "Five meters of [Martian] soil will provide the same protection as the Earth's atmosphere-- equivalent to 1,000 g/cm2 of shielding."
Here is another source, possibly more definitive
Journal of Cosmology QUOTE: "From these authors, we see that attenuation to very low levels has occurred by the 3 to 4 meter depth. These models are mostly using "soil", i.e. unconsolidated regolith, as the crustal material. Rock materials may have greater ability than regolith to attenuate radiation in some cases."
Your "100 meters" ballpark estimate is almost certainly far too high. But a significant layer of soil will be required.