Drainage For Dams
Drainage For Dams
Drainage For Dams
3217/978-3-85125-564-5-030
W. Riemer1, F. B. Samani2
1- Consultant, Trier, Germany
2- Tamavan Consulting Engineering, Tehran, Iran
Email: samanifer@gmail.com
Abstract
Drainage systems serve many tasks related to the performance of dams, auxiliary structures and reservoirs.
But for developing an appropriate design, which efficiently handles the prevailing tasks, comprehensive
information on hydrogeological parameters, hydrogeological regime and geological setting must be
elaborated. The contribution points out pitfalls experienced in this context, their potential consequences and
mentions practical precautions. Several case histories illustrate the performance of drainage systems,
successfully coping with complex conditions. But drainage systems are not only demanding in design, they
also need sustained monitoring and maintenance to assure the safety of the projects on the long term.
Keywords: Drainage, Foundation and Slope Stability, Design and Maintenance.
1. INTRODUCTION
The earliest design of gravity dams did not consider the effect of uplift pressures on their stability.
However, some masonry dams constructed at the beginning of the 20th century incorporated drains arranged
behind the upstream facing (e. g. Lister dam, Germany, 1907), similar to arrangements actually found in RCC
structures. Eventually, Casagrande (1961) stressed the importance of drainage for the stability of concrete dams
and its function in conjunction with the geology of the foundation rock. De Mello (1977, 1984) elaborated on the
subject of drainage, included embankment dams in his discussion and pointed out the relation between grout
curtain and foundation drainage. With these publications, the drainage has developed into a standard component
for all types of dams. It has proven to offer an efficient, economic and versatile tool for a wide range of purposes
as there are mainly:
control of uplift in the foundation of dams for stability against sliding and toppling
control of internal pore pressures in dams
stability of the abutments of dams, especially for concrete dams
safe handling of seepage water inside embankment dams
assistance to consolidation of foundations of dams as well as of the body of an embankment dam itself
control of uplift downstream of dams
stability of appurtenant structures (spillway, power plant, etc.) downstream of dams
control of seepage gradients in foundations and managing the hazards of internal erosion and piping
preventing deterioration of foundations by hydrochemical effects in relation with seepage
stabilizing reservoir slopes and slopes downstream of dams
preventing waterlogging downstream of dams.
Although much experience has accumulated in the application of drainage for the various purposes listed
above, the authors see the need to emphasize the importance of geological and hydrogeological investigation and
analysis to provide the basis for designing an effective and reliable system. Many geological features, geotechnical
and hydrogeological parameters are to be determined and a geotechnical-hydrogeological model must be
established. The following paragraphs, mainly from a geological point of view, discuss the methods for obtaining
the basic input, the identification of design requirements and the adjustment of designs to cope with specific site
conditions. Finally, considerations in respect of operation and maintenance of drainage systems are addressed.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
2. BASIC PARAMETERS
The permeability of soil or rock mass of foundation and abutments as well as of embankment materials
constitutes the most important parameter. Common practice in the investigation of dam sites estimates the
permeability from point tests: Lugeon or permeameter tests. These tests have limitations in accuracy, suffer from
random errors and may be misleading if statistics of the mean are applied to cases where extreme values are
decisive. The tests do not provide reliable information on the anisotropy of permeability (as for instance frequently
applies to varved soils). These conditions call for judgement in the selection of design parameters, and for cross
checking.
Permeability determined in the course of project studies will not necessarily apply to the performance of
the dam and its foundation because:
mechanical stresses exerted by the structures can raise or decrease the permeability
hydrojacking caused with the rising reservoir level can increase the permeability
internal erosion can raise the permeability of foundation soils, colmatation by sediments can create
seepage barriers
leaching of grout, of concrete and of soluble minerals from the foundation may raise the permeability.
Other aspects to be studied in relation with the design of a drainage system are:
the grainsize distribution of the foundation soils (for hydraulic efficiency of the drains, for finding
admissible gradients, for design of filters)
hydrochemical parameters (related to incrustation or corrosion).
Casagrande (1961) encourages application of flow nets in the design of drainage system and de Mello
points out that the foundation may have to be homogenized by grouting in order to meet the conditions, which
allow tracing of a flow net.
Low permeability of foundation will obstruct drainage and, accordingly, be more demanding for the
design of the system, even if little seepage water is captured. In this regard, the Tavera spillway (Dominican
Republic) suffered from an erroneous concept. A massive conglomerate with a tight argillaceous matrix forms the
foundation. During construction, the originally designed drainage system was eliminated because little seepage
was expected. Upon reservoir filling, on occasion of the inauguration ceremony, the uplift destroyed the chute.
For the Karkheh project (Iran), the assessment of the rock mass permeability posed a difficult task. Very
early in the project study, the geological investigations identified a potential problem with uplift in the alternating
horizons of conglomerate and mudstone. Lugeon tests indicated a moderate permeability for the conglomerate but
pumping tests arrived at k≈10-3m/s, which is far out of scale of the Lugeon values. Experts consulted by the
engineer questioned the representativity of the high values. In view of the uncertainty, the engineer adopted an
observational approach, providing load berms and relief wells to cope with moderate permeability and allowing
the option of substantially strengthening the defenses against high uplift pressures. Observations during reservoir
filling eventually confirmed the high permeability for the conglomerate and required remedial action.
At the Colbún project (Chile), a heterogeneous sequence of soils formed part of the reservoir rim. The
low groundwater level in the reservoir rim left a substantial part of the sequence in dry, a conditions which further
complicated the determination of the permeability. The design anticipated a permeability of 10-4m/s and a seepage
of 0.3m³/s (Noguera et al, 1988). With rising reservoir, the seepage increased drastically and emergency measures
had to be taken to prevent hydraulic failure of the reservoir rim. At full reservoir, with the strengthened drainage
system, underseepage approaches 10m³/s, about 30 times of the initially expected value. In another case,
permeameter tests in the soil deposits forming the foundation for part of the Middle Marsyangdi dam (Nepal)
suggested the existence of highly permeable layers and raised the call for the construction of a diaphragm. But the
high permeability was incompatible with conditions established by geological field surveys, the diaphragm was
rejected and observations during reservoir filling and operation justified the decision (Rissler et al., 2015). Finely
stratified sediments can be notably anisotropic in permeability, a condition which common permeability tests
cannot quantify accurately and reliably. But the anisotropy significantly affects uplift and seepage gradients in
such type of foundation and, accordingly, has to be considered in the design of the foundation and its drainage
system. The study of the Naga Hammadi barrage on the Nile in Egypt encountered typical problems in the
determination of hydrogeological parameters: permeameter tests indicated k=5x10-5 m/s, estimates based on grain
size distribution rendered an average of 6x10-4 m/s and interference pumping tests found k=10 -3 m/s for the sand
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
and a ratio horizontal/vertical permeability >10 (Guth et al., 1988). Figure 1 demonstrates the important effect of
the anisotropy on the flow net. Mitigating the risk of hydraulic failure downstream of the barrage, the design
provided a diaphragm on the upstream side and a downstream apron with sheetpiles on the downstream side.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
Drainage layers constitute a standard element in homogeneous and zoned embankments. But there are
also cases with less common design features. For instance, impervious upstream facing allows adopting steep
slopes, under condition that the body of the dam drains freely. Drains will have to be provided, if this condition
is not granted. For instance, the compaction test of basalt for the Mohale dam produced fines potentially
obstructing drainage. Therefore, a drainage layer with dolerite was placed in the valley floor, which proved useful
when the concrete face cracked, drastically increasing the seepage. Foreclosing even more serious cracking at the
Paute Mazar dam (Ecuador), 175 m high in a narrow valley, required intensive compaction of the rockfill, which
left a layer of low permeability fines on the top of each lift. A central chimney drain was added to the design and
proved useful when seepage developed. A similar drain in the Porce III CFRD (Columbia) permitted steep slopes
constructed with problematic schist. If gravel with a notable proportion of fines supplies the fill for a concrete
face dam, the incorporation of a drainage system acquires particular importance. The internal drainage performed
satisfactorily on the Caracoles dam (Argentina) and was also incorporated into the body of the Misicuni dam
(Bolivia). The Bigge dam (Germany) displays a special design. With the experience of the catastrophic damage
during World War 2 of the Möhne masonry dam, an embankment with an internal reinforcement of the crest and
with an inner buffer zone was adopted. The buffer zone serves to contain excessive seepage in the event that the
upstream facing would be damaged. However, drains in the valley floor will relieve internal pressure against the
facing when the lake is drained.
Figure 3: Drains in dams with upstream facing, Mohale (ICOLD, 2009), Paute Mazar
(Cruz et al., 2009), Bigge (Heitfeld, 1973)
Figure 4 Left: drains relieving pressure in cracks of the Piedra del Aguila gravity dam
(Hidroeléctrica Piedra del Águila, Argentina). Note goose necks on drains. Right:
vertical drains in Platanovrissi RCC intercepting lift joints
It is common practice to provide drains in concrete dams to collect seepage from block joints and in
RCC also from lift joints (Figure 4). But there is an apparently paradoxical application of drains to reduce seepage
from cracks in the concrete. Thermal shrinkage is a frequent cause of such cracks. When the reservoir rises, the
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
hydraulic pressure jacks the cracks open and seepage increases with the third power of the crack width. If this
happens, drains drilled into the crack will relieve the jacking pressure and reduce the seepage (Figure 4).
The design of the drainage system requires hydrogeological parameters, geotechnical characteristics and
a hydrogeological model. Developing the model may need exploratory works covering an area substantially
extending beyond the foundation area of the dam. For the Karkheh dam, early stages of geological explorations
had correctly defined the hydrogeological model, indicating the risk of uplift. But with the uncertainty regarding
the permeability, the final requirements for drainage and buttress fills were quantified concurrent with reservoir
impounding. At the Colbún dam, in addition to the uncertainty in respect of the permeability, an unforeseen
geological feature aggravated the problem. A lateral moraine, buried by fluvial and volcanoclastic soils, formed
a hydrogeological barrier downstream of the dam, creating dangerous upward gradients (see Figure 6). An
emergency action, constructing counterweight berms, 8 km of drainage ditches and 150 relief wells prevented the
failure of the dam and the reservoir rim (Noguera, 1988).
Figure 6: Seepage model for the drainage divide dam of Colbún reservoir, left expected
flow pattern, right with flow forced upward by low permeability moraine
On the contrary, for the Middle Marsyangdi project, a hydrogeological model indicated high seepage
gradients, implying a risk of erosion and suffusion in the soil foundation of the dam, and the contractor claimed
the need for a diaphragm and drainage system. However, the critically high exit gradients were an artefact, due to
incorrect placement of a barrier boundary in the FE model shortly downstream of the toe of the dam, which does
not exist in reality, as meanwhile confirmed in nearly 10 years of operation.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
A complex geological structure of the foundation rock frequently also produces intricate hydrogeological
conditions, difficult to unravel and requiring extensive treatment. The Thissavros dam, Greece, illustrates such
setting (Anastassopoulos et al., 2004). Faults in the metamorphic rock create compartments differing in
hydrogeological regime and requiring individual treatment. Although the dam partially rests on a slide mass, the
drainage system has secured the stability of the abutments. Budget limitations prevented the downstream
extension of the drainage system in the right valley flank and eventually a voluminous rockslide occurred there
(communication O. Papageorgiou).
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
shale by Na. The process constitutes a double hazard: increasing permeability by dissolution and activating the
dispersivity of the smectite by exchanging Ca for Na. In fact, boils appeared downstream of the dam and seepage
water left efflorescence of minerals leached from the shale (Figure 9). To halt the deterioration of the shale layer,
a line of relief wells was proposed, dropping the head in the vuggy limestone just enough to keep the piezometric
level below the ground surface.
Figure 10: Daule Peripa reservoir rim dike, relief wells showing erosion and settlement
cracks.
An 18 km long auxiliary dam was constructed on the reservoir rim of the Daule Peripa reservoir. It rests
on a complex series of volcanoclastic and fluvial soils. At depressions in the reservoir rim, drainage wells are
arranged, intended to relieve potentially hazardous uplift pressures. With a few exceptions, the drainage system
has performed satisfactorily for more than 20 years. Very few wells have indications of erosion and/or settlements
(Figure 10). In these places remedial measures will have to be taken.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
Figure 12: Geological and hydrogeological conditions at Cairnmuir Slide. Mainly the
drainage of the sub-basal confined aquifer stabilized the slide. Hatching marks
groundwater below main failure plane.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
Routine checks verify the efficiency of the drain holes to determine the need for flushing or replacement. An
independent panel periodically reviews the conditions of the monitoring system and the performance of the
landslides.
Figure 13: Drainage system in buried valley under left abutment of Piedra del Águila
Dam
On the left abutment of Piedra del Águila Dam, Argentina, the stabilization measures include drainage
by gravity as well as by pumped tube wells (Figure 13). The area is controlled by video, instrumentation data are
automatically transmitted to the operation center of the powerplant (with two independent lines), the well field
has a multiple of the normally required capacity, there are two lines of power supply and additional emergency
power supply. ORSEP, the Argentinian authority in charge of dam safety, has developed plans for emergency
action and imposes periodic safety inspection by independent engineers.
Also drainage systems less complex than the above mentioned two cases should be monitored and
checked for need of maintenance. Common problems are incrustation of drain holes, typically by calcium
carbonate or by oxides of iron and manganese, due to loss of dissolved gases, to oxidation, sometimes assisted by
biological processes. Goose necks on drain pipes (Figure 4) help prevent incrustation. Alternatively, corrosion
may attack the installations. H2S, which forms with eutrophication, acts particularly aggressive.
Figure 14: Device for collecting sediments from drainage water (design A. Pujol), severe
corrosion of piezometer panel by aggressive water, incrustation with carbonate and
manganese precipitate.
8. CONCLUSIONS
Properly designed drainage systems efficiently cope with many tasks related to the stability of dams,
their foundations, associated structures and reservoir slopes. Drainage also serves for preventing damage to
foundations by erosion or leaching. However, developing the appropriate design, requires information on
hydrogeological and geological conditions, which in turn calls for specific geological and hydrogeological
explorations and studies, sometimes covering areas substantially extending beyond the foundation of a dam.
Reliance on conventional point permeability tests has led to mistakes.
If the safety of a dam depends on the correct functioning of the drainage system, sustained monitoring
and capacity for maintenance have to be provided and plans for emergency action must exist.
Ample experience evidences the satisfactory performance of drainage system even under highly complex
geological and hydrogeological conditions and high risk settings. On the other hand, disregard in the design or
neglect in operation have caused incidents.
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Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD 2017) DOI:10.3217/978-3-85125-564-5-030
9. ACKNOWLEDGMENT
The authors wish to express their gratitude to the many professionals with whom they had the privilege
of working on hydro projects.
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