Deepen the road

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A friend contracted a small project to build an asphalt road about one kilometer long by the river. The specific method is as follows: After digging the foundation pit, fill it with 50 cm thick gravel, add 300 mm stone powder, and then lay 20 cm of asphalt sand. The key is that the tar sand is laid relatively late, and it may take three or four months before it can be laid. The geology is relatively soft near the river. If the excavation is carried out according to the design, the ground will settle. However, the top of the pavement has a certain elevation, so more tar sand will eventually be needed to make up for it. Therefore, the best approach is to leave a certain margin when initially excavating, for example, excavating ten centimeters or twenty centimeters less, so that after settlement, he does not need to add more tar sand to make up for it. I would like to ask, how much margin should be left? Do you have any relevant experience?