Horizontal cracking is sometimes assumed to occur at the upstream
face if zu (computed without uplift) falls below a predetermined
minimum value:zu min (3.38)wherekdis a drainage factor (kd0.4 if drains are effective; kd1.0 if
drains are not present, or blocked), tis the tensile strength of the con-
crete across a horizontal joint surface and Ftis a factor of safety, scaled to
load combination (Ft3.0 for NLC, 2.0 for ULC and 1.0 for ELC).
Cracking is normally permissible only under the extreme load combi-
nation, except in the case of old dams. The crack is assumed to propagate
to the point where zpw. Stability and stresses are then reassessed for
the uncracked thickness of the section, and the dam is considered safe if
the resulting stresses remain within specified maxima and sliding stability
remains adequate (USBR, 1976, 1987).3.2.6 Upstream face flareThe upstream face of a gravity profile is frequently modified by the intro-
duction of a significant flare, as shown in Fig. 3.8. A flare is advantageous
in that it serves to lengthen the base contact, and so considerably enhances
the contact area available at foundation level to distribute stress and resist
sliding. The influence of a generous flare can also prove significant in rela-
tion to providing an adequate margin of safety against seismic load. For
profiles having a considerable crest width in relation to their height, thekd (^) wz t
Ft
148 CONCRETE DAM ENGINEERING
Fig. 3.8 Upstream flare diagram