Question

the wall of a dam is broad at its base

Answer 1

The wall of a dam is broad at its base. The reason behind this is, pressure exerted by the liquid increases with increase in depth.

The dam contains huge amount of water and as we go deep the pressure exerted by such a huge amount of water increases to very high.

In order to release this pressure the area of contact must me more and hence the walls are made broad at its base. 

Answer 2

There are two reasons for which the base of the dams are broader.

1. In order to provide the stability to the structure, and

2. The bottom of the dam must be strong enough to withstand the increased water pressure.

Thus, to deal with the high water pressure, the base is built stronger.