Question

why dams are provided with broad base and why a truck has two tyres on each side

Answer 1

Pressure and depth are directly proportional to each other. Pressure increases with depth, that is why the dam are built broad at the base and narrow at the top.
Trucks have two broad tyres on each side, so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields and trucks can carry weights equally.