- Safety against uplift pressure (Fig. 9.5).Ifhis the uplift pressure
head at a point under the floor, the pressure intensity is
p gh(N m^2 ). (9.3)This is to be resisted by the weight of the floor, the thickness ofwhich is tand the density (^) m(for concrete, (^) m2240 kg m^3 ). There-
fore,
(^) mgt gh
giving
hSmt
whereSmis the relative density of the floor material. Thus we can
write
h tSmt t
which gives
t(h t)/(Sm 1)h/(Sm 1) (9.4)
wherehis the pressure head (ordinate of hydraulic gradient) meas-
ured above the top of the floor. A safety factor of around 1.5 is
usually adopted, thus giving the design thickness of the concrete
floor as
t1.2h. (9.5)
The design will be economical if the greater part of the creep length
(i.e. of the impervious floor) is provided upstream of the weir where
nominal floor thickness would be sufficient.
The stilling basin area of the weir is subjected to low pressures
(owing to high velocities) which, when combined with excessive uplift
pressures, may rupture the floor if it is of insufficient thickness. Usually,
the floor is constructed in mass concrete without any joints, and with a
hard top surface to resist the scouring velocities over it.
(c) Approach slab
The provision of a concrete slab upstream of the weir section (sloping
down gradually from the 2 horizontal to 1 vertical slope of the crest