The subgrade shall be prepared by excavating or filling to the required elevation of the bottom of the concrete. The subgrade shall be well-drained and cleaned of all sod and organic material. Where necessary for the construction of ramps and drive approaches, the curb shall be cut for its full depth. Where necessary to cut or remove tree roots, adequate precautions shall be taken so as to protect exposed roots and preserve existing trees. Any material found to be unsuitable, such as muck, peat, marl, soft clay, and other such materials subject to frost heaving, differential frost action, or unable to provide adequate support for surface improvements, shall be excavated and removed from the job site and replaced with granular base. A minimum of two inches of granular material compacted in place is required as a base for sidewalk concrete.