indicated in Table 2.5. The major advantages of rockfill as an embankment
construction material are high frictional shear strength, allowing the con-
struction of much steeper face slopes than earthfill, and relatively high per-
meability, eliminating problems associated with construction or seepage
porewater pressures.
The disadvantages of rockfill lie in the difficulties of controlling the
grading of crushed rock, e.g. from excavations and tunnels, and in the con-
struction and post-construction settlements, which are relatively high. This
can result in interface problems where rockfill shoulders are adjacent to a
compressible clay core (Section 2.7.2).
The quality and suitability of rockfill is discussed in Penman and
Charles (1975) and ICOLD (1993b). An exhaustive study of the engin-
eering characteristics of compacted rockfill and of the special large-scale
test techniques required is reported in Marsal (1973).
The shear strength of compacted rockfills is defined by a curved
failure envelope with the form (De Mello, 1977)fA()b. (2.40)Illustrative values of parameters Aandb, from data presented in Charles
and Watts (1980), and with fandin kN/m^2 , areA3.0 (poor quality slate)→6.8 (sandstone),b0.67 (sandstone)→0.81 (basalt).The stability of slopes in compacted rockfill can be determined using limit-
equilibrium methods, including those introduced in Section 2.7, and compa-
rable analyses for wedge-type failure surfaces. Rapid parametric stability
studies can be conducted using the dimensionless stability numbers, F(Fel-
lenius analysis) and B(Bishop analysis) developed in Charles and Soares
(1984) and based on the relationship of equation (2.40).
Rockfill settlement is associated with particle contact crushing and is
greatly increased by saturation; it can therefore be accelerated during con-
struction operations. The construction settlement occurring at crest level,(^) r1, can be estimated from the approximate relationships
(^) r10.001H3/2(m) (2.41)
and
(^) r1 (^) rgH^2 /Er (2.42)
whereEris the modulus of deformation and (^) ris the density of the rockfill.
Values of Erare typically of the order of 20–50 MN m^2.