We have several Civil engineers aboard, I will defer to their opinions on the matter. However, the OP's point was to compare the current earth work to the creation of a logistics parking lot. We know for certain (from WuWa's video records) that no such fill was used to create either the logistics yard on the East side of Phase 1, or the staging area East of Phase 2 which is currently in use for construction materials. IIRC, the stamping presses go in first at a new Gigafactory, and they need substantial foundations.
http://straits-engineers.com › pub › publication_20 | PDF (4 pages)
by R Mohamad · Cited by 1 — The
engineering properties of importance are the maximum past pressures σvc', compression and recompression indices Cc and Ccr, the coefficient of consolidation ...
OK, I am a civil engineer and will try to explain in a simple way (and give you some understanding how since ages we have dealt with this here in The Netherlands).
In regions near the coast of a country very often, over a thousands of years, the rivers and lakes have brought layers of clay and compressed layers of plants (estuary land).
The western part of The Netherlands has this characteristic. Deeper down, say 15 meters till sometimes more than 30 meters, are sand layers that are solid.
This is why houses in Amsterdam have since long past been built on wooden poles, to bring the weight of the houses to these solid sand layers. They would have sunk into the soft ground without them. If you could look under Amsterdam you would see a huge forest of wooden poles.
To build a complete huge warehouse or road on poles would of course be very expensive. So you use them only for large concentrated loads (under heavy machinery, columns of buildings, etc.
How to obtain an even settlement under other parts of the building or roads/parking lots (like roads in the western part of my country)? It's too expensive to dig the soft layers out completely, you would just create a lake that you cannot easily drain and fill with sand afterwards.
The answer is prestressing. These soft layers contain a lot of water that has to pressed out of it, in order to make them more solid. Pressing water out of the pores of clay is difficult however and takes a long time. By putting the weight of meters of sand on top the pressing out of water is accelerated, but still takes a long time (many months).
The land in Shanghai has the characteristics as described above. We will see proof of the prestressing when we will see measuring sticks on top of them. By regularly measuring how these sticks go slowly down in time because of the weight of the sand on top, it can be calculated how much time is needed for the soil to have been compacted sufficiently for the purpose ahead. It can be accelerated by pressing vertically draining into the layers, but that is a question of extra cost against shorter compression time.
For us of importance: I don't think they will be building something there in 2021, it is preparing the land months ahead.