Empirical performance indices have been suggested for certain criti-
cal parameters.
(a) Hydraulic fracturing index, HFI
Hydraulic fracturing index is a measure of susceptibility to fracturing, and
is given by
HFI (2.37)
whereis the total stress and (^) wz 1 the reservoir head, determined for a
plane crossing the core. The horizontal total stress, h, normal to a trans-
verse vertical plane can be demonstrated to be critical, and HFI1 indic-
ates a risk of hydraulic fracturing (hmust be estimated or determined by
in situtests (Charles and Watts, 1987)).
(b) Settlement index, SI
Settlement index is defined as:
SI (2.38)
where∆Sis crest settlement in millimetres occurring over time interval t 1
tot 2 (years) after completion of an embankment of height H(metres).
A value of SI0.02 is considered to represent acceptable long-term
settlement behaviour in terms of secondary consolidation of fill and/or
foundation, or creep of a granular fill (Charles, 1986).
(c) Seepage index, QI
A seepage and leakage index may be defined as
QI (2.39)
whereqin litres per second is the flow through the core or water-retaining
element and Athe area of element in square metres. The coefficient kis
the maximum acceptable permeability of the core in metres per second
andithe mean hydraulic gradient across the core. A value of QI1 might
be expected if all flow is seepage, as opposed to leakage through imperfec-
tions etc. In view of the problems associated with determining qandk, the
value of QI lies principally in its use as a long-term comparative index.
q
1000 AKi∆S
1000 Hlogt 2 /t 1(^) wz 1