To determine the value of the rigidity factor for design purposes when the tyre pressure is greater than 0.7 MPa, we need to understand the concept of rigidity factor in the context of highway engineering.

The rigidity factor is an adjustment factor used in the design of flexible pavements. It accounts for the effect of tyre pressure on the distribution of load on the pavement surface. A higher tyre pressure tends to focus the load over a smaller area, which means that the flexibility or rigidity of the pavement plays a more significant role in stress distribution.

According to standard highway engineering principles:

• If the tyre pressure is greater than 0.7 MPa, the pavement tends to behave more rigidly due to the concentrated load.
• In such cases, the rigidity factor is typically adjusted to be less than 1 because it signifies that the pavement material deforms less under a concentrated load, distributing the stresses more efficiently.

This is because, at higher tyre pressures, the role of compaction and material integrity in distributing stress is emphasized, making the pavement appear more rigid than it would under lower pressure conditions.

Thus, for tyre pressures greater than 0.7 MPa, the correct answer is less than 1.

Let's evaluate other options:

• Greater than 1 - This would imply the pavement is behaving overly flexible, which is not usually the case at high pressures.
• Equal to 1 - This would suggest no adjustment needed, typically applicable when tyre pressures are within a standard range.
• Zero - This would suggest no rigidity, which is impractical for any functioning pavement.

Therefore, in conclusion, the rigidity factor for a tyre pressure greater than 0.7 MPa should be less than 1.