DRAINS
Drains
are an important control method primarily used to drain water away from pyritic
materials to limit the contribution of water in the formation of acidic drainage.
Drains can be installed in various places at mine sites and rely on permeability
contrasts between the drain and the permeability of waste rock, tailings,
spoil, refuse and fills. There are several different types of drains including
blanket drains, chimney drains, French drains, highwall drains and porous
envelope. Selection of drain type depends on the purpose of the drain, location
of the drain and estimated flow rates.
Drains are used to
- Reduce
the pore water pressures in the downstream portion of the dam therefore
increasing the stability of the downstream slope against sliding.
- Control
any seepage that exits the downstream portion of the dam and prevent erosion
of the downstream slope: i.e. to prevent 'piping'.
The effectiveness
of the drain in reducing pore pressure depends on its location and extent.
Piping can be controlled by making grading of the pervious material to meet
the filter requirements for the embankment materials.
Figure
1, showing Piping,
http://www.dur.ac.uk/~des0www4/cal/dams/emba/seep.htm
Blanket
drains:
These are commonly used to drain water from surrounding soils or waste materials.
The depth of these drains is normally three feet. The depth of these drains
are increased significantly when need arises (i.e.), when the ground water
table has to be lowered or to drain leachate. Cedergren (1989) states that
to ensure that a blanket drain will be able to remove all the water that enters
a structural section the minimum transmissibility’s can be estimated
by Darcy’s law in the form of
kA = Q/i
Where Q is the quantity of water that needs to be removed by a drainage blanket,
‘i’ is the allowable hydraulic gradient,
k is the coefficient of permeability of the blanket and A is the cross sectional
area normal to the direction of flow
The permeability must be sufficient to allow water to drain out before damage
can occur. To prevent freezing in cold climates water that enters the system
should drain in a relatively short time. Horizontal drainage blankets are
often used for dams of moderate height. They are frequently used over the
downstream one-half or one-third of the foundation area.
Figure
2, Blanket Drain
http://www.dur.ac.uk/~des0www4/cal/dams/emba/seep.htm
Disadvantages
of blanket drains:
An earth dam embankment tends to be more pervious in the horizontal direction
than in the vertical. Occasionally, horizontal layers tend to be much more
impervious than the average material constructed into the embankment, so the
water will flow horizontally on a relatively impervious layer and discharge
on the downstream face despite the horizontal drain. Where this has occurred
the downstream slope is prone to slipping and piping. Repairs can be made
by installing pervious blankets on the downstream slopes or constructing vertical
drains to connect with the horizontal blanket. Such vertical drains are normally
composed of sand and gravel.
Toe drains: For low dams, a simple toe drain can be used successfully. Toe
drains have been installed in some of the oldest homogeneous dams in an effort
to prevent softening and erosion of the downstream toe.
Figure
3, Toe Drains,
http://www.dur.ac.uk/~des0www4/cal/dams/emba/seep.htm
Chimney
drains:
Chimney drains have been used to avoid trouble due to stratification and to
intercept water before it reaches downstream slope. Chimney drains collect
water from a backfill or valley fill and convey water through a long, high
column of coarse sand and drain runoff. Chimney drains are an attempt to prevent
horizontal flow along relatively impervious stratified layers, and to intercept
seepage water before it reaches the downstream slope. Chimney drains are often
incorporated in high homogeneous dams, which have been constructed with inclined
or vertical chimney drains.
For big dams, chimney drains have been inclined at a considerable slope, both
upstream and sometimes downstream. An upstream inclined drain can act as a
relatively thin core. In addition to controlling seepage through the dam and
increasing the stability of the downstream slope, the chimney drain is also
useful in reducing pore water pressures both during construction and following
rapid reservoir drawdown.
Figure
4, Chimney drain,
http://www.dur.ac.uk/~des0www4/cal/dams/emba/seep.htm
French Drains:
French drains collect sub-surface water from poorly drained areas and carry
it to the main drainage line, dry well and ravine. It is constructed using
materials like pea gravel or crushed rock, woven landscape fabric and perforated
drainage pipe. Advantages of French drains are low cost, easy installation.
After constructing they can be covered with a turf, making them less conspicuous.
Construction of a French Drain
- A
French drain starts with digging a trench. The depth and width of the trench
can vary, but 5 to 6 inches wide and 8 to 12 inches deep are common sizes
and usually satisfy most needs.
- Grading
is a critical consideration — you must ensure that enough slope exist
for the water to actually flow, and flow in the right direction. It might
be adequate to check very short stretches of drain with a level to ensure
that a slope exists to carry water in the desired direction. However, you
should take whatever measures are necessary, including a survey and grading,
if needed, to ensure that you have at least a 0.5 percent slope. A 1 or
2 percent grade is better.
- Add
gravel to the trench to within a few inches of the surface. Gravel for this
use is typically 0.5 to 1 inch in size.
- On
top of the gravel, lay at least 3 or 4 inches of coarse sand. This provides
a medium in which turf can grow so that the trench will not be visible.
But remember that the sand must be coarse or it won't allow water to properly
drain through.
Turf
may be seeded into the sand or simply allowed to grow in from the adjacent
stand, if the turf is a spreading type. Or, you can lay sod over the sand.
However, if you do this, be sure to wash the soil from the sod roots before
laying it so that you don't contaminate the sand with finer soil.
Figure 5, French Drain, http://www.createalandscape.com/Houston%20Drainage.htm
Figure 6, French Drain,
http://handbooks.btcv.org.uk/handbooks/content/chapter/828
Highwall
Drains:
Highwall drains collect ground water that enters a mine site before it comes
in contact with mine spoils and distributes it through the site rapidly with
minimal contact with the spoil. In this way the groundwater bypasses potentially
acid-forming materials. Highwall drains can reduce the potential for acid
mine drainage on sites with marginal overburden quality or can reduce the
quantity of acid mine generation.
The design and installation of a highwall drain system must be customized
to each site. Design parameters that must be considered include:
- Where
to place the drains
- What
materials to use
- How
to construct the drain system
- The
transport medium (i.e., pipe or rock)
- Protection
of the drain to ensure that it is not crushed during backfilling.
It is
crucial that the drain systems are designed in a way that allows all groundwater
to be collected where it enters a mine site; this may be at the highwall,
endwall, or even the lowwall. It is equally important to ensure that drains
are constructed in a matter that allows for positive drainage.
Drains
can be constructed in three different methods:
The first
drain construction technique initiates with the excavation of a small channel
in the pit floor with a backhoe or similar equipment to a depth just sufficient
(about 1 ft (0.3 m)) to capture groundwater from the highwall. A pipe (4 or
6 in (10-15 cm)) is then placed in the bottom of the channel and covered with
gravel or coarse-grained material. Then, to prevent infiltration of sediment,
which could plug the pipe, filter fabric is installed over the ditch.
The second
method is to install a pipe at the low spot of each pit and allow water to
naturally flow into it. This method does not include any disturbance of the
underclay. In the one instance where this method was used, an inert 2 ft (0.6
m) compacted clay seal was placed on the pit floor under and on either side
of the pipe; this permitted groundwater flow along the top of the inert clay
rather than on the acidic underclays.
The third
procedure is generally the same as the first but does not use pipes. Using
this approach, groundwater flows into a channel along the highwall (constructed
similar to Method 1) and flows down-dip through a porous gravel (or on-site
rock) medium.
Factors
that should be considered for sites where drains are proposed
- All
drain outlets should be designed with a "water trap" near the
outlet to prohibit oxygen from entering the site through the drains.
- Minimally,
the discharge from drains should be monitored quarterly for quantity and
quality. This will give an indication of how much groundwater is being intercepted
and whether or not the intercepted water is being influenced by mine spoil.
- Reclamation
should be conducted rapidly since sites with highwall drains often have
marginal overburden quality (near neutral or slightly acidic).
- Dual
highwall drains may be useful for large sites with significant infiltration
from precipitation.
Porous
Envelope: No Data
References:
http://handbooks.btcv.org.uk/handbooks/content/chapter/828,
accessed December 2003
http://www.createalandscape.com/Houston%20Drainage.htm,
accessed December 2003
http://www.dur.ac.uk/~des0www4/cal/dams/emba/seep.htm,
accessed December 2003
Harry R. Cedergren, 1989, Seepage, Drainage and FlowNets, Wiley- Interscience
Publishers, pp 465
http://www.dep.state.pa.us/dep/deputate/minres/districts/cmdp/chap16.html,
accessed January 2004
University
of Nevada