Environmental Engineering Reference
In-Depth Information
strengths when tested in saturated conditions (than when tested dry), water should prob-
ably be added routinely for the upstream shell (Zone 3A). (3) For rocks with questionably
high contents of earth and sand size particles, water should nearly always be used. For
dirty rock, the water softens the fines so that the larger rocks can be forced into contact
with each other by the vibrating roller.” Cooke (1993) relates the need for adding water
to the water absorption capacity of the rock, with little benefit being achieved for low
(
2%) water absorption.
As pointed out by Sherard and Cooke (1987), it is not intended that the application of
water will wash fines into the voids, and hence the use of a high pressure nozzle is not nec-
essary. Adding water in the truck prior to dumping on the surface is practicable and eco-
nomical (Varty et al., 1985).
Some indication of likely rockfill behaviour will be obtained from consideration of rock
type, unconfined compressive strength and jointing in the rock in the quarry.
The best guide to rockfill behaviour in the investigation phase of a project will be to con-
struct test fills, and possibly carrying out large diameter plate bearing tests on the compacted
fill. This, coupled with measurement of grading and void ratio of the resulting rockfill and
consideration of the height of the dam, shape of the valley and comparison with other proj-
ects, should enable a reasonable guide to expected rockfill modulus. The method for esti-
mating the modulus is described in Section 15.2.4.
15.2.4
Estimation of the modulus of rockfill
Hunter (2003), Hunter and Fell (2002, 2003c) developed a method for estimating the
secant modulus of rockfill during construction (E
rc
) and pseudo modulus on first filling
(E
rf
) based on analysis of monitoring data, mostly for CFRF.
15.2.4.1
Estimation of the secant modulus E
rc
Hunter and Fell (2002, 2003c) recommended that the following steps be followed:
(a) Determine the representative secant modulus at the end of construction E
rc
from the
D
80
size (size for which 80% is finer) and unconfined compressive strength of the rock
in the rockfill using
Figure 15.11
.
The representative secant modulus at the end of construction, E
rc,
is for Zone 3A
type rockfill, i.e. placed in layers 0.9 m to 1.2 m thick, compacted with 4 to 6 passes
of a 10 tonnes smooth drum vibratory roller and water added, and applies to the
stresses:
- 1400 kPa for the very high strength, well-compacted rockfills;
-800 kPa for the medium to high strength, well-compacted rockfills;
- 1500 kPa for the well-compacted gravels.
The D
80
size should be obtained from construction records, rolling trials or estimates
based on particle size from samples from test pitting into the existing rockfill.
(b) For Zone 3B or other thicker layer rockfills; apply a correction factor of up to 0.5 (for
rockfill placed in layers up to 2 m thick) to obtain a representative E
rc
. This correction
factor is based on the ratio of E
rc
(of Zone 3A to Zone 3B) from six field cases. There
is not sufficient data to be prescriptive regarding this correction factor. As a guide,
a correction factor of 0.5 would apply to rockfill placed in 2 m layers without the
addition of water and compacted with 4 to 6 passes of a 10 tonne vibratory roller
(i.e. reasonably compacted rockfill). A correction factor of 0.75 would apply for
rockfill placed in 1.5 m to 1.6 m layers with the addition of water and compacted with
4 to 6 passes of a 10 tonne vibratory roller (i.e. reasonably to well compacted
rockfill).
