Final answer:
Unsupported excavations can create unbalanced stress, leading to collapse. Thermal stress, due to thermal expansion or contraction, can have destructive or useful effects. Earthquakes can induce resonance in buildings, causing selective damage depending on their resonant frequency.
Step-by-step explanation:
An unsupported excavation can indeed create an unbalanced stress, leading to the failure of the excavation's walls or collapse. This is because when the material is removed from the earth, the balance of forces that maintained the integrity of the soil or rock is disrupted. The excavation destabilizes the material that remains, causing it to move or collapse into the created void, especially in the presence of additional external factors like water saturation, earthquakes, or added loads.
Thermal stress is another example of stress that can be induced in materials; it results from thermal expansion or contraction and can have both destructive and useful effects. For example, it can cause gasoline tanks to rupture due to expansion or be utilized in manufacturing processes to fit parts together. Similarly, thermal stress can explain the weathering of rocks and pavement due to the expansion of ice when water freezes.
In the case of earthquakes, the concept of resonance and standing waves can lead to differential damage in buildings based on their resonant frequency, which may match the frequency of the earth's vibrations during the event. This can cause some buildings to collapse while others remain intact, creating constructive and destructive interference patterns affecting the distribution of stress and damage.