This is how I would work out the volume

A = surface area

t = thickness (0.01m)

a = angle between vertical and radius line

R = Radius of circle ( 60m)

r = horizontal radius = Rsina

s = distance along surface in meriden direction

circufrence of a ring circle = 2*pie*r = 2*pie*Rsina

Surface area is the just the sum of all the horintal circufrences ie

(with limits s and 0)

However easier to transform into angles to work out

ds = Rda (using angle*Radius = chord length)

0 = 0

s = pie/6

(with limits pie/6 and 0)

Area = 2*pie*R^2* ( 1- sqrt(3)/2)

Mass = density*A*t

Work it out and see if its right?