**Answer:**

The CBR (California Bearing Ratio) method is a method used to evaluate the strength of soils and base materials for pavement design. It measures the ratio of the bearing capacity of a soil or base material to that of a standard crushed stone base material. The higher the CBR value, the stronger the material.

In the CBR method, the thickness of the pavement is determined based on the CBR value of the base course. The base course is the layer of material that is placed directly on top of the subgrade (the natural soil or prepared foundation). It is designed to provide support and distribute the load from the pavement surface to the subgrade.

The thickness of the base course is determined based on the CBR value of the subgrade. The CBR value of the subgrade is a measure of its strength and ability to support the load from the pavement. The higher the CBR value of the subgrade, the thicker the base course needs to be.

Once the thickness of the base course is determined, the thickness of the surface course can be determined based on the design requirements, traffic loads, and other factors. The surface course is the top layer of the pavement and is designed to provide a smooth and durable riding surface.

While the CBR value of the subgrade and the surface course may affect the overall pavement design, the total thickness of the pavement is primarily determined by the CBR value of the base course. This is because the base course is the layer that directly supports the load from the pavement and distributes it to the subgrade. If the base course is not thick enough or does not have a high enough CBR value, it can lead to pavement failures such as rutting, cracking, and structural distress.

In summary, the total thickness of the pavement by the CBR method depends on the CBR value of the base course. The CBR value of the subgrade and the surface course may also affect the design, but the base course is the critical layer that supports the load and determines the overall thickness of the pavement.