03 Guests conferences - Conférences invitées


Professor Derek FORD (McMaster University, Canada)



Lecture:
Karst Groundwater:
an overview.



















Expect the Unexpected! Groundwater Flow in Karstified Carbonate Aquifers


All carbonate rock formations that display some measure of integrated groundwater flow through solutionally enlarged channels (conduits) are potentially triple porosity aquifers, the most complex type of water supply aquifer. The complexity arises both from antecedent conditioning of the host rock and from current factors of recharge and discharge determined by topographic, geomorphic and climatic controls. The important antecedent factors include variation in depositional facies (much greater than in any other sedimentary rock), transformations at all stages of diagenesis including dolomitisation and pressure solution, the imposition of tectonic structures, hypogene (thermal water, oil and gas) intrusion, and inheritance of paleokarst forms. Because of these variables the effective porosity of the matrix in significant carbonate aquifers is known to range from zero to forty per cent or more. The range of effective porosity in fractures is similar.

From the analytic standpoint we may divide modern karst aquifers into two categories:
  1. (those in which at least some of the solutional conduits are large enough and otherwise accessible for us to physically explore and map them;
  2. those which are not, and thus an be approached only as ‘black boxes’ or ‘grey boxes’ (in Systems Analysis terminology).
During the past one hundred years or so a total of several thousand of kilometres of cave conduits have been discovered and mapped, from every climatic and topographic setting (mountain, plateau, plain, coast) except the high Arctic and the Antarctic. From this sample it is found that in any one generation the conduits that develop tend to exhibit branchwork (dendritic) or network (maze) patterns, with the former perhaps predominating below the epikarst at the global scale. Over successive generations the shift of spring elevations up or down in response to aggradation or erosion may produce much more complex 3D patterns that can be activated during flood stages to produce very complicated flow responses. Nevertheless, the basic principles of solution conduit genesis are now quite well understood: they will be illustrated with examples from the author’s field experience.

Cave exploration above and below current water tables has also permitted sampling of calcite and aragonite karst precipitates (speleothems). Amenable to highly precise dating

over the range of Quaternary time and beyond, speleothem stable isotope and other records are now yielding data on the nature and regulators of groundwater recharge and flow through the rock matrix and fractures that is of unprecedented quality and detail.

Where the aquifer is not accessible to human exploration there are many alternative ways to study it. These can include defining and mapping the recharge points such as stream sinks or dolines; groundwater tracing from them or from wells, using dyes or other tracers: applying remote geophysical mapping techniques such as electrical conductivity to detect voids or potential water courses; intruding into the aquifer with observation boreholes drilled for rock core studies, injection and drawdown tests, dye tracing, etc; locating the springs, measuring their discharge, temperature, water chemical and turbid responses, even their yield of phreatophytes and other specialised underground fauna, over time. Integrating some or all of these different sources of data into aquifer computer models such as the popular MODFLOW (with added conduit packages) can add much insight. The author and former students have experience with many of these techniques applied to practical problems in North America: some successes and shortcomings will be noted in a concluding review.

Professor Petar MILANOVIC (Beograd, Serbia)




Lecture:



Karst Groundwater,
impacts on infrastructure and construction.





For engineers the term ‘karst’ suggests – problem or failure. A number of case histories show that in karstified rocks the groundwater impact on infrastructure and constructions had as consequences serious problems, and, sometimes - failures.
Construction of dams, reservoirs, tunnels including large underground and surface structures in karst is a risky task. In many cases, in spite of detailed investigations, the problem is deeply hidden in the karst underground. Large structures may fail despite extensive investigations and remedial works. The most common destructive influence of groundwater is consequence of the:
  • huge solution capacity (particularly in evaporites),
  • the rapid erosion of unconsolidated deposits in caverns and joints,
  • the great kinetic energy of underground flows,
  • 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 levels.
Despite extensive investigations the destructive impacts are, mostly, unpredictable in space and time. In many cases these destructive processes takes time to become established but final effects appear abruptly, causing considerable damages or failures. The most common consequences of these impacts are subsidence at the ground surface as well as at the bottom of reservoirs, water seepage from reservoirs, break-in of groundwater under high pressure during underground excavation, destruction of surface remediation structures, destruction of tunnel lining, degradation of grout curtains, induced seismicity, decreasing of downstream spring discharges, endangerment of underground species, various other environmental consequences, and the creation of many other unpredictable and unexpected problems.

Successful remediation solutions require serious and comprehensive investigations including long period monitoring of groundwater regimes and (in many cases) remedial works during lifetime of the structure. Modifications and adaptations of structures during construction are very common in karst, not the exception. Persistent, time-consuming and expensive remedial works during lifetime structure are also not the exception, - but, rather, they are the rule.

A number of case histories illustrating those topics are presented in the lecture.
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