Environmental Engineering Reference
In-Depth Information
-Variably cemented and permeable ironstone zone;
- Mottled zone and underlying kaolin rich zone. This is commonly medium to high
permeability due to relict joints and root holes as shown in
Figure 10.4
. Many of
these features are near vertical and not readily intercepted by vertical boreholes.
The mass permeability of the clay will commonly be of the order of 10
6
to
10
5
m/sec, i.e. the permeability of sand;
- The quartz rich zone overlying weathered bedrock is often a mixture of clean sand
and clayey sand of high permeability. In some cases it may be cemented as silcrete,
but will still be highly permeable;
- The upper bedrock is often fractured and has moderate permeability;
- The depth of weathering may be up to 30 m, e.g. in the Darling Ranges of Western
Australia (Gordon, 1984) and in the subtropical monsoonal climate areas of Weipa,
North Queensland and Ranger and Jabiluka, Northern Territory. The weathering profile
usually does not follow the topography but rises more gradually in the abutments as
shown in
Figure 10.3
.
At Weipa, Ranger and Jabiluka, with low relief, it is the authors'
experience that the depth of weathering is related to rock type rather than topography;
- The water table fluctuates markedly between end of wet season and end of dry sea-
son, often by as much as 10 m or 20 m.
10.3
DETAILS OF SOME MEASURES FOR PORE PRESSURE AND
SEEPAGE FLOW CONTROL
10.3.1
Horizontal and vertical drains
Seepage beneath a dam on a permeable soil (or permeable weathered rock) foundation
should be allowed to exit in a controlled manner into a horizontal drain. The horizontal
drain may consist of a single layer of Zone 2A filter, or 3 layers of Zone 2A and 2B as
shown in
Figure 10.2
. In both cases Zone 2A should be designed to act as a filter to con-
trol erosion of the soil from the foundation into the drain. The filter design criteria
detailed in Chapter 9 should be used.
The 3 layer drain incorporates the layer of high permeability Zone 2B to ensure that the
drain has sufficient discharge capacity.
It is good practice (Cedergren, 1972, USBR, 1987) to design the horizontal drain to
have sufficient capacity to discharge the flow entering the drain from the dam foundation
and from the vertical drain without the phreatic surface rising into the low permeability
fill (see Figure 10.5).
If the horizontal drain has insufficient capacity, the phreatic surface will rise into
the downstream low permeability fill as shown in
Figure 10.6
, reducing the stability
Figure 10.5.
Earth dam with internal drain designed to prevent the phreatic surface from rising above
the top of the drain.
