Question 150874
volume for a cylinder is {{{V= pi*r^2*h}}}
Think of a cylinder as a whole bunch of circles stacked on top of each other. So the Volume is just the area of each thin circle times the height of the stack.

In this case, the shape is only a half circle. So the formula we need is
{{{V = ((pi*r^2)/2)*h}}}

Our radius is 12 feet. Our height is 1/3 foot. And the answer is in cubic yards. 
So we will need to convert everything to yards
12 feet = 4 yards
4 inches = 1/9 yard
{{{V = ((pi*4^2)/2)*(1/9)}}}
{{{V = ((pi*16)/2)*(1/9)}}}
{{{V = ((pi*16)/18)}}}
{{{V = 2.79}}} cubic yards
Concrete folks sell by the "full yard", so you need 3 yards