I am building a house on a preconstructed Slab supported by a perimeter beam. The beam shoud have been 12" wide by 18" deep. It is only 12" deep.
The front of the house will have a solid block wall filled with grout 10' high.
Should I put some piers under the existing beam to support the wall? The soil is undisturbed decomposed granite.

Whereabouts are you located? Because the depth of frostline in your area needs to be taken into account.

Whereabouts are you located? Because the depth of frostline in your area needs to be taken into account.

I am in a mountainous area in Arizona. No frostline.

OK so there are no freezing and thawing forces to consider. I assume what you mean is the beam somehow got installed lying on its side and the slab is already poured? Trying to work around under it now sounds like a bad idea, you could get cracks. And you want to put 10 feet of wall on top of it which is a whole lot of weight. Could you simply add a second, smaller diameter metal beam right on top of the other one with your new slab sandwiched in between? That would carry the weight and give you the thickness called for in your plans. Me, I'd go back to the contractor (unless it was you) and demand he do the job right at his expense.