Answer: Based on your description, this sounds like a very unusual and difficult planting site because the plantable soil is so shallow. I also see a big problem in that it would be very wet in rainy spells due to the clay layer below, and then with such shallow soil it would dry out quickly in dry spells.
Most shrubs have a much deeper root system than eight inches. This allows them to have their roots deep in the soil where it stays naturally moist during dry spells. It also allows the root system to grow big enough to support the top growth and also anchor the plant securely in the ground. (Roots are usually about the same volume as the top growth of the plant.) Finally, the depth also insulates the roots from both excessive heat and winter cold.
With such a shallow planting depth, I do not think shrubs would thrive there. You might be able to grow some flowering annuals if you are able to water them regularly so that the soil stays slightly moist (not saturated or sopping wet) and never goes bone dry. However, even annuals generally require a moist yet well drained soil and it does not sound like this location can provide that if it becomes saturated when it rains.
Rather than try to make recommendations without fully understanding the growing conditions, I would strongly suggest you work with your county extension and/or local professionally trained and certified nursery staff to determine what might possibly thrive there or explore other options. For instance, it is possible you might be able to build a large, wide, raised bed and plant into that, or you might be able to plant further out from the house and beyond the clay layer -- but without seeing it I can't really recommend anything. I'm sorry I just can't be more specific for you.
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