Environmental Engineering Reference
In-Depth Information
(e)
Roller type and number of passes
It is normal to specify smooth steel drum vibrating rollers for compacting rockfill. The
roller is usually specified as having a static mass of between 10 tonnes and 15 tonnes and
a centrifugal force not less than say 240 kN “at the maximum frequency permitted by
the manufacturer for the continuous operation of the roller” (PWD of WA, 1982).
Alternatively (MMBW, 1980, for Thomson Dam) the static weight may be specified
as a weight per metre of drum (50-55 kN/m), and a centrifugal force/m of drum
(125 kN/m). The frequency was also specified (16-25 Hz) for Thomson Dam. Reference
should be made to manufacturers' specifications for rollers.
Roller trials are often specified to determine the number of passes. These are dis-
cussed below. It is usual to require at least 4 passes of the roller.
(f)
Addition of water
Water is often added to the rockfill to aid compaction and to weaken the rock-to-rock
contact points in rock-types which are weakened by wetting. The water should be
added by spraying on to the dumped rockfill before spreading. Commonly the amount
of water is specified as a percentage of the volume of the rockfill e.g. 20%.
If the rock forming the fill is strong, produces few fines and is not greatly weakened
by wetting, water may not be needed. Rolling trials can assist in assessing whether
watering is needed and the amount of rolling required to achieve good compaction.
(g)
Durability requirements
Rockfill is often required to be “hard” and “durable”. The means of measuring this
are seldom specified and in many dams such a requirement will be unobtainable, e.g.
where siltstone or sandstone is being used, which may breakdown on repeated expo-
sure to wetting and drying. It has been demonstrated (e.g. USBR, 1991 and Chapter
2, Section 2.9.1) that in some cases such breakdown only occurs on the surface of the
rockfill, and is not detrimental to the shear strength or compressibility of the rockfill as
a whole, provided the rockfill is well compacted and watered during compaction.
Hence, in general, there should be no requirements on durability for rockfill. The
exception might be for some volcanic and altered granitic rocks which can deteriorate
with time (see Sections 2.9.3 and 2.9.4.2). For rip-rap, or the outer layer of rockfill on
the downstream slope, durability under wetting and drying is important and should
be specified. The type of test will depend on the rock type proposed, but should
include testing of the substance strength and assessment of the mineral composition,
in particular the proportion of secondary minerals in the rock, and observation of
blocks of rock left exposed to the weather.
(h)
Selection and placement of rip-rap
Rip-rap on earth and rockfill dams is usually constructed by pushing the larger rocks
from the adjacent rockfill zone to the face of the embankment and finishing the face
by carefully positioning rocks with an excavator, sometimes fitted with “claws”, to satisfy
rock grading requirements for rip-rap, it will be normal to specify the grading more
closely than rockfill, e.g. requiring that at least 50% of the rock should be greater than
a certain size. Thin rip-rap layers for earthfill dams may be placed progressively as
the embankment is built or on the completed face by dumping and spreading with a
bulldozer.
(i)
Compacted density or void ratio
Many designers in the past have also had a requirement for a compacted density for
the fill, a common requirement being around 2.1-2.2 tonnes/m
3
. Alternatively a void
ratio (volume voids/volume solids) of 15-25% was required. The design is seldom
sensitive to bulk density and, again for large rockfill, it is difficult and costly to meas-
ure density or void ratio because of the large volume which has to be sampled. Hence,
it is unlikely (and unnecessary) that
insitu
density will be checked for smaller dams and
relatively few tests will be done even for large dams.
