What is the maximum allowable slope for excavations less than 20 feet made in solid rock?

For excavations made in solid rock, the maximum allowable slope can indeed reach a vertical angle, which is defined as 90 degrees. This is applicable because solid rock generally provides a strong and stable wall that is less likely to collapse when compared to other materials like soil or loose aggregates. With rock, operators can safely achieve a vertical cut without significant risk, provided proper safety precautions are adhered to and the conditions of the excavation are consistently monitored.

Other slopes, such as 45 degrees or 60 degrees, are generally more applicable to excavations in less stable materials, where the potential for collapsing is greater. A slope of 15 degrees would be far too shallow for rock, allowing for excessive lateral pressure and potential instability. Therefore, the maximum allowable slope is indeed vertical, making it the optimal and safest choice in solid rock conditions.