Creeping thymes are very tough plants that do not require fancy soil or excessive watering. I am having some difficulty with understanding how your thyme is growing over a concrete garage pad - it will creep over the edges and can actually cover up some real estate this way but it obviously won't grow on the concrete. Shallow soil is fine as long as it is well draining. Soil that is too moisture retentive will promote root rots, which can be fatal to thymes. Like most Mediterranean herbs, they like tough conditions - full sun, lean, well-draining soil and minimal watering once established.

Oh,sorry for confusion. The top of the garage (underground basement) is the courtyard at center of property, from which all rooms have doors entering into at first level, and windows on 2nd level.. what can i do to ensure that the soil is well draining. can i mix sand into it? or layer gravel on bottom.