Each layer in a pavement can only be as durable as the layers below it. If a layer below is weak, it will require the layer itself to be stiffer or thicker to compensate provided that extra stiffness does not make it too brittle. The ultimate case is the substrate on which the pavement rests and that substrate can vary from solid rock to peat or loose sand. If the substrate is considered to be too weak, it will be dug out and replaced by a firmer foundation that will take the expected loads.