Dealing with Clay Soil
Clay soil is not poor soil but actually has mineral richness just waiting to be unlocked with applications of humic acid combined with healthy organic methods.
The fact that it holds water can be a benefit at times. Please see pages 18 to 24 of my new organic month by month book for advice how to deal with soil including clay soil.
Photo by USAG-Humphreys 
One of the best ways to improve clay soil is to mulch it in fall and I even recommend manure. Though there are some people who are against this practice my experience from farming and from gardening has been that the ancient practice of mulching with manure is one of the best ways to improve soil of just about any kind. Unless you are planning to grow native plants I would recommend you find a source of clean horse manure at a ranch near you where good owners pick up after the horses daily. In this case it is unlikely that there will be any broken salt licks in with the manure. Cover the ground in fall each year with horse manure and the winter rains will wash the goodness into the ground. In a few years you will have great organic soil that drains well. The index of my new book contains several references on page 452 to manure. I’ve discussed in even more places than the index lists. If you are going to apply horse manure make sure your own and your family’s tetanus shots are up to date. You can also make a wonderful compost by layering spoiled alfalfa and manure as described on pages 35 and 36 in my book. Where you live you are close to sources of the necessary ingredients. All you need is a truck for collecting them.
Photo by USAG-Humphreys
I want to create a 25’ x 25′ Permaculture mandala garden in my Philadelphia backyard (Zone 7a), but I have poor clay soil.
Since I want to improve the soil immediately, I don’t mind spending money to have a lot of new soil delivered and dumped on top of the clay.
Would this be appropriate?
If so, then what type of soil?: Compost? mushrooms soil? or a Mix with top soil?
(Of course, each island of the mandala garden would have slightly different soil depending on what was planted there. For example, certain herbs prefer sandy, rocky alkaline soil.)
On the other hand, since I’m familiar with sheet-mulching & Hugelkultur, it may be best for long-term soil health & stability if I dug at least 12” down, put a weed barrier, then a layer of pebbles for drainage, then a layer of branches/small logs of apple & maple wood, then some old leaves and compost, then dump 6-12” of mixed soil on top, then some much?
Any advice would be appreciated.
Instead of trying to fight your clay soil, I suggest designing and building your mandala garden in rock-walled or block-walled raised beds and filling them with the best topsoil you can find. Look up photos of African Key-Hole gardens online for inspiration.