Caves and Dam Safety Improving Design with Evidences from Paleoseismology and Paleohydrology

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Caves and Dam Safety
Improving Design with Evidences from Paleoseismology and
Paleohydrology
L.F. Molerio-León*
INVERSIONES GAMMA, S.A.
*Corresponding author: L.F. Molerio-León, Senior Researcher in Projects and Engineering, INVERSIONES
GAMMA, S.A., PO Box 6246, CP 10600, Habana 6, La Habana, Cuba. E-mail:
especialistaprincipal@gmail.com
Abstract
Karts regions poses enormous difficulties in dam construction and operation. The preservation of dam
safety should be firstly based in detailed and rigorous geological, hydrological and geophysical studies.
The lack of long-term data on flood frequency and seismic events should be improved incorporating
paleofloods and paleoseismological events recorded in the distribution, morphology, type of sediments
and their degree of preservation. How paleo flood hydrology and paleo seismological records can help in
this sense is are described in this contribution.
Introduction
Most dams all over the world are designed and operated to satisfy the agricultural, industrial,
recreational and domestic demand. Not a few also provide electric energy. Therefore, they are planned
to retain water with a minimum of losses and a high level of safety. Karst regions, that are extended
through almost 20% of the world emerged lands, are characterized by an extraordinary enhancement of
rock permeability that makes this purpose a hard task to accomplish. This is due to the continuous
process of rock (mainly limestone and gypsum) solution that: a) amplifies the original pores and joints
in the rocks, and in turn increase its permeability by creating wide open spaces and commonly extensive
networks of interconnected passages that could reach several kilometers; b) modified and degrades the
original geotechnical properties of the rocks weakening their resistance to stress and, in consequence,
affecting the stability of the embankment, diversion and accompanying structures, such as spillways,
buildings, waterways, tunnels, shafts, roads and railways.
The development of epigenic and hypogenic solution causes the anisotropy of the physical properties of
the karstified rocks making more difficult the understanding of the spatial distribution of the voids, and
therefore the identification and quantification of the most vulnerable sites of the embankment affecting
the foundation of the structures, the filtration losses, the internal erosion mechanisms and their response
to seismic and extreme hydrologic events.
But despite those huge engineering problems, karst has the advantage of being a morphology that reflects
and preserves most of the geologic, hydrologic, geophysical processes that have modeled the surface and
subterranean landscape through its evolution. In particular, caves in epigenic karst are an invaluable
source of useful engineering information to be considered in dam construction and operation. Caves in
hypogenic karst does not reflect exactly the past and present flow paths unless modified by epigenic
flows but provides information on the sources of a more accelerated corrosion than that due to the kinetic
mechanisms controlling karstification and speleogenesis in epigenetic environments. Seudokarst poses
a similar problem regarding safety of embankment and filtration losses but will not be considered in this
opinion.
Dam safety in karst regions is based on its proper design, construction and operation after the correct
assessment of the structural stability of the embankment foundation and the related waterworks,
(spillways, buildings, roads, tunnels) as related to:
 The mechanical properties of the rocks

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 The local organization of the groundwater flow systems and the increase of water flow velocity
(massive turbulent flows);
the fast erosion of unconsolidated deposits in caverns and joints; the
great kinetic energy of underground flows; propagation of hydraulic pressure at large distances
(piston effect); and the enormous hydraulic pressures created in periods of full aquifer
saturation, including water-hammer and air-hammer effects due to rapid fluctuation of the
water level
 The geochemical hydrodynamics of the interaction between surface and groundwater
 The geologic structure of the embankment zones as well as of the reservoir; rock anisotropy and
heterogeneity
 Seismic response, particularly in the case of diffuse seismicity, low to medium seismic activity
and/or active neotectonics. Reservoir induced earthquakes associated to filling and dewatering
of the reservoirs should also have to be accounted.
 Assimilation and modulation of extreme hydrologic events (particularly in the case of old dams
that were designed, constructed and operated with short hydrologic data that needs to be
updated)
 The prevention of seepage, piping and internal erosion from dam foundations and abutments as
well as lateral drainage to contiguous valleys
 The prevention of slope movements in the reservoir area (landslides, creeping, solifluction)
Caves as sources of geological, hydrological and geophysical information
Caves as indicators of karst development was some time ago proposed by Curl (1966) based on the trend
that there is an observable dependence in nature of both the length distribution and what he defined as
the karst constant upon a nonlinear relation of the basic karst factors.
Epigenetic caves are, as stated by Poulson and White (1969) caves are truncated fragments of the larger
conduits of the karst drainage net and must be interpreted according to their hydrologic role. This means
that the reflect evidences of the evolution of the surface and groundwater dynamics. This includes floods
and paleofloods, changes in the flow patterns, abandonment and dewatering of the flow paths in
correspondence to climate changes. Therefore, each cave possesses a unique geomorphic history and
depositional environment (Springer, 2002) that provide detailed information of the hydrologic history
of the region at a surprisingly very small scale.
Paleofloods in caves
Long-term hydrological and hydrometeorological data are sometimes difficult to obtain for a particular
basin. Extrapolation of data from other basins to a particular engineering project increase uncertainties
in computations of the flood-frequency analysis and of the magnitudes and types of floods (an even
droughts). Paleoflood studies then become useful in the analysis of flood hazards. In the assessment of
dam safety, as stated by Baker, Webb and House (2002), the most important utility lies on the retrofitting
of dams to pass the Probable Maximum Flood (PMF), v.gr. the most severe combination of hydrologic and
meteorological conditions that are considered reasonably possible for a drainage under study. The PMF
appears to be unreasonably large-in comparison to the paleoflood record. Therefore, as they pointed out,
it is important top account that the cost of retrofitting dams to pass the PMF is extremely high prompting
some observers to question whether dams can or should be retrofitted.
Cave sediments and their relation to surface sediments, the evolution of cave levels, their lateral and
vertical facial variation, the reexcavation of cave fluvial terraces, the growth of speleothems are all
indictors of the variations of flow regimes in the drainage basin (Figure 1). Therefore, they provide
information and data in flood hazard analysis, both in terms of traditional flood-frequency analysis and
the potential use of flood history within the context of flood-hazard assessment and reduce the
uncertainty in estimates of long return-period floods, use of this technique offers substantial societal
benefit (Baker, Webb and House, 2002). Overgrowth of speleothems is often an indicator of climate
changes and, hence, of the paleo hydrology of the territory.

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Figure 1. Abandoned fluvial terrace at the Salon de Los Gigantes (Giant´s Hall) at the Majaguas-
Cantera Cave System, Pinar del Río, Cuba. Well-defined paleoflood markers are identified at this
place, at several meters above the present course of the river. Alluvial sediments show important
vertical variations indicating different paleoflood stages. In the upper part of the photo, the
remains of heavy fluvial erosional patterns can be observed.
Paleoseismology in caves
Paleoseismology is the study of the ground effects of past earthquakes as preserved in the geologic and
geomorphic record. Possibly the main goal of paleoseismology is the identification of capable faults and
seismogenic structures.
Paleoseismological evidence constitutes an appropriate tool to extend the
earthquake catalogue beyond the instrumental and historical records in areas of low or medium
seismicity where seismogenic structures are not well defined. Evidences of these events are named
speleoseismites and remain recorded in speleothems, cave breakdown, terrigenous cave sediments. One
of the most important features is the deviation of the growing axis of the stalagmites that can be also
dated as it is shown in Figure 2.
The most reliable information is derived from speleothems. Damaged (fractured and/or collapsed
speleothems) are a direct indicator of the peak ground acceleration of given earthquakes. The
measurement of the natural frequency of speleothems provides quantitative information of the of the
vulnerability to vibrations which is important in the definition of the dynamic amplification phenomena
of the seismic motion. The comparison of their fundamental natural yields reliable information of when
they are higher or not than the range of seismic excitation.
On the other hand, unbroken speleothems as
are testimonies that no event greater than a certain level has occurred in the region.
References
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American Geophysical Union. Water Science and Application Volume 5:1-19
Curl, R. L. (1966): Caves as a measure of karst. J. Geol., 74 (5), part 2: 798-830

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Figure 2. Arrows shows the deviations in the verticality in the growing axis of the stalagmite
CDANAS-01 (Dos Anas Cave, Majaguas Cantera Cave System, Pinar del Río, Cuba) due to seismic
oscilations. 14C absolute dates derived from Pajon et al., 2006