The basic idea in building a road or parking area for all-weather use by vehicles is to prepare a suitable sub-grade or foundation, provide necessary drainage, and construct a pavement that will:
Have sufficient total thickness and internal strength to carry expected traffic loads,
Have adequate compaction to prevent the penetration or internal accumulation of moisture, and
Have a top surface that is smooth, skid resistant, and resistant to wear, distortion, and deterioration by weather and deicing chemicals.
The sub-grade ultimately carries all traffic loads, Therefore, the structural function of a pavement is to support a wheel load on the pavement surface and transfer and spread that load to the sub-grade without exceeding either the strength of the sub-grade or the internal strength of the pavement itself.
The next figure shows wheel load,( W ), being transmitted to the pavement surface through the tire at an approximately uniform vertical pressure, ( Po ), the pavement then spreads the wheel load to the sub-grade so that the maximum pressure on the sub-grade is only ( P1 ). By proper selection of pavement materials and with adequate pavement thickness, ( P1 ) will be small enough to be easily supported by the sub-grade.