Design, specification and construction of filters - Geotechnical Engineering of Dams - page 375

AUSTRALIAN
STANDARD SIEVES

8A

100 0

10

90

8B

D85B

20

80

70 30

7B

60 40

50 50

2A 2B

40 60

30 70

6C

20 80

7A

6A 5

10 90

6B

0 100

CLAY S I L T S A N D G R A V E L Cobbles

Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse

0.002 0.006 0.01 0.02 0.06 0.1 0.2 0.6 2 6 20 60 200

0.001 1 10 100

PARTICLE SIZE (mm)

Notes: Step 1 - Particle size distribution for Zone 2A becomes the “Base Soil” - the fine side boundary is

critical. D 85B

15% passing 0.075 mm). Step 5 -
Determine D 15Fmax from Table 9.4 . D 15Fmax less than 4D 85B less than 4 2.5 mm 10 mm
(5). Step 6 - Determine permeability criterion, D 15Fmin

2.5 mm. Go to Step 4. Step 4 - Soil is type 3 (

2% passing 0.075 mm
(6B), D 15Fmin 4D 15B 4 x 0.7 mm or 2.8 mm. However for average D 15 = 4 0.3 mm
1.2 mm accept this (6C). Step 7 - D XFmax /D XFmin less than 5 e.g. D 15Fmax = 10 mm, therefore
D 15Fmin 2 mm (7A), D 60 /D 10 less than 6, so say D 10Fmax 6.7 mm, D 60Fmax 40 mm (7B);
Follow step 7 at 60% Passing when finalising grading. Step 8 - D 10F

0.1 mm, (6A),

7 mm, so max D 90F 50

mm from Table 9.7 (8B).

Figure 9.27. Example of design of a Zone 2B filter.

9.5.2.1 Standard tests for durability
Table 9.10 shows the specified durability requirements for several dams constructed in
Australia. Also shown for comparative purposes are the requirements for fine and coarse
concrete aggregates as specified by Standards Association of Australia (1985).
These specifications can be used as a guide to selecting reasonable limits. The following
comments are offered:

(a) The Los Angeles abrasion and wet strength, wet/dry strength variation tests largely
assess the susceptibility to breakdown under the mechanical action of placement and
rolling in the dam. It can be seen that dam specifications are generally less stringent
than AS2758.1. If materials test marginally within the specification it would be advis-
able to carry out field compaction trials of the filters, to observe directly the degree of
breakdown under rolling and the size of the “broken down” product. It may be prac-
ticable to make design changes, e.g. wider or thicker filters, to accommodate the
breakdown.
(b) The Sodium Sulphate soundness test assesses the susceptibility to breakdown under
wetting and drying, with the added involvement of sodium sulphate in the solution