In this paper, the classical work of Spangler is revisited to derive a modi
fied Iowa formula for estimating vertical deflection of flexible pipe. Duri
ng the derivation process, the approach suggested by Spangler is faithfully
followed. No new assumptions or empirical elements are introduced beyond t
hose originated by Spangler. Once the vertical deflection formula is establ
ished, its reliability is verified against field deflection measurement dat
a supplied by Spangler. Then, a few parametric studies are conducted using
the horizontal and vertical deflection modified Iowa formulas to gain furth
er insights into the pipe-soil interaction problem as revealed by the work
of Spangler. Finally, the paper concludes by offering a summary and then po
inting out a few key issues that need to be remembered when applying the cl
assical approach pioneered by Spangler.