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2012. Vol.3, Supplement, 39-42
Published Online December 2012 in SciRes (http://www.SciRP.org/journal/ce) DOI:10.4236/ce.2012.37B009
Copyright © 2012 SciRes.
Planning, Research and Karstological Monitoring of Expressways
Crossing Classical Karst (Slovenia)
Martin Knez, Tadej Slabe
Karst Research Institute, Scientific Research Centre of the Slovenian Academy of Sciences and Arts,
Over the last fifteen years the construction of modern expressways in Slovenia has been one of the major
construction projects aimed at connecting important parts of the country and opening them to Europe.
Almost half of Slovenia is karst and more than half of the water for the supply of the population comes
from karst aquifers. Slovenia is home to the classical karst region of Kras that gave its name for this
unique carbonate rock landscape to numerous world languages and is also the cradle of karstology. We
need to better understand this fragile karst landscape and do everything to preserve it since it is an impor-
tant part of our natural and cultural heritage.
Keywords: Expressways Planning; Karstological Monitoring; Karst Protection; Classical Karst; Slovenia
Special attention is devoted to Kras, a karst plateau rising
above the northwesternmost part of the Adriatic Sea that is
bordered on the southwest by a vast flysch area with ele vations
exceeding 600 meters. Lying between 200 and 500 meters
above sea level, the plateau covers 440 square kilometers and in
a broad sense belongs to the Outer Dinaric Alps. From the
viewpoint of the theory of tectonic plates, the plateau lies at the
northern deformed edge of the Adriatic plate and is the result of
tectonic overlapping. Only Cretaceous and Paleogenic rocks are
found here. They are characterized by exceptionally varied
limestone that mostly formed in relatively shallow sedimenta-
tion basins with lush fauna and flora. On the Kras plateau there
are no remains of the surface waters used in the past to explain
the development of the plateau. Originally, the plateau was
surrounded and covered with flysch and therefore flooded.
Vertica l pe rcol ati on wa s mini mal. The wat e r tabl e late r dropped
several hundred meters into the karst. At the contact between
the carbonate rock and flysch, surface waters created characte-
ristic and extensive contact karst. Today, all Kras rivers sink
where they flow from flysch onto limestone bedrock and flow
underground toward the springs of the Timava River in Italy.
The largest stream is the Reka River, which sinks in the
Škocjan Caves, while 65% percent of the water sinks from the
surface in a dispersed fashion. From the ecological standpoint,
Kras has one of the most vulnerable natural systems in Slove-
The low karst of the Dolenjska region is mostly covered with
a variety of alluvia under which a unique karst surface formed
with stone forests as one of its most distinctive features. The
water table is often just below the surface, and the valley sys-
tems are occasionally flooded.
Karst areas also developed in the breccias that formed from
the scree on the slopes of Mount Nanos. They lie on more or
less permeable flysch, and water flowing at the contact carved
the largest caves in this area.
For a number of years, karstologists have cooperated in the
planning and construction of expressways in the Kras region [1,
2]. In the selection of expressway and railway routes, the main
consideration is the integrity of the karst landscape and there-
fore the routes chosen avoid the more important surface karst
features (dolines, poljes, collapse dolines, karst walls) and al-
ready known caves. Special attention is devoted to the impact
of the construction and use of expressways on karst waters.
Expressways should therefore be impermeable so that runoff
water from the road is first gathered in oil collectors and then
released clean onto the karst surface.
We studied the impact of traffic routes on karst waters and
determined the contents of polluted water flowing daily from
the expressways. Small quantities of stagnant water found in
caves along the expressways contained traces of mineral oils.
During the construction of expressways we also perform
karstological monitoring. We study newly revealed karst phe-
nomena as an important part of our natural heritage and advise
on how to preserve them if the construction work allows it. At
the same time our new findings are of great help to the con-
struction companies. We have acquired a number of new find-
ings on the formation and development of the karst surface,
epikarst, and the perforation of the aquifer.
Exploring the Karst Surface and New Caves
during Expresswa y Constructio n
The removal of soil and vegetation from the karst surf a ce a nd
of course major earthworks such as the excavation of cuts and
tunnels reveal surface, epikarst, and subsoil karst features. Our
task is to study these features as part of the natural heritage,
advise on how to preserve them, and of course share our new
findings with the builders. These findings are used to overcome
The karst surface is dissected by dolines and unroofed caves.
Dolines are a sign of the current shaping of the surface by pre-
cipitation water that percolates vertically through it and passes
M. KNEZ, T. SLABE
Copyright © 2012 SciRes.
through the vadose part of the aquifer to the underground water.
Some dolines are more distinctly filled with soil than others.
There are shafts and fissures at their bottoms through which
water flows. The soil must be removed from the dolines and
their bottoms reinforced with rocks arranged in a vault-like
pattern; the mouths of shafts are often smaller than the cham-
bers beneath them. The dolines are then filled with layers of
rubble. Unroofed caves have a similar form or are more oblong.
These are old caves that appear on the surface due to the lo-
wering of the karst surface and no longer have the upper part of
their circumference. The fine-grained fill, in this case old cave
alluvia, must be removed and replaced with rocks and rubble.
Otherwise, water could gradually carry the alluvia away and
cause subsidence on the surface.
The epikarst is crisscrossed with fissures that are more dis-
tinctive in Cretaceous limestone and less so in Paleogenic li-
mestone, and many of them open at the bottoms and slopes of
dolines. In most cases they are filled with soil and their walls
are dissected with subsoil rock relief forms. Due to the lower-
ing of the karst surface, many shafts are now located just below
More than 350 caves were opened on the 70-kilometer sec-
tion of expressway built in Kras in the last few years. Relative
to the development of the aquifer, we distinguish between old
caves through which watercourses flowed when the karst aqui-
fer was surrounded and covered by flysch and shafts through
which water vertically percolates from the permeable karst
surface to the underground water. The deepest shaft found
measured 109 meters. Some old caves are empty, almost two
thirds of them are filled with alluvia, and one third are unroofed
Caves are opened when vegetation and soil is removed from
the surface, and a large number of caves were opened during
the excavation of cuts. Blasting caused their roofs to collapse,
and cross sections of passages were preserved in embankments.
The most shafts were opened at the bottoms of dolines when
the soil and alluvia were removed.
We studied all the caves, drew their plans, determined their
shape, examined the rock relief, collected samples of alluvia for
paleomagnetic and pollen analyses, and sampled flowstone for
mineralogical analyses and age determination. We extrapolated
the further extent of the caves on the basis of their shapes and
the geological conditions, which is especially useful for road
Studies that Accompanied Construction
Produced New Findings on Karst Development
The unroofed cave is a special and frequent karst form. To-
day, this significant karst surface feature is a familiar pheno-
menon, but it had not been thoroughly studied before the con-
struction of the expressway across Kras. Great attention has
been devoted to unroofed caves since the occurrence of this
phenomena turned out to be considerably higher than previous-
ly expected, and numerous articles on unroofed caves and the
construction of new expressways are now available. The shape
of an unroofed caves is the consequence of the type and shape
of the cave and the development of the karst aquifer and its
surface in various geological, geomorphological, climate, and
hydrological conditions. The distinctiveness of the surface
shape of an unroofed cave is dictated by the speed at which the
alluvium was washed out of the cave relative to the lowering of
the surrounding surface. If the speed was low, we can often see
soil and vegetation or areas of alluvium and flowstone on the
surface; where it was faster, unroofed caves on the karst surface
resemble dolines, a string of dolines, or oblong depressions.
Frequently they are an interweaving of various old forms such
as caves and recent shaping of the karst with dolines and shafts.
A large proportion of the caves were filled with alluvia, in
most cases fine-grained flysch alluvia with intervening layers of
gravel. We took alluvia samples for paleomagnetic research
from caves at Kozina and Divača and determined they origi-
nated in the Olduvai period. We therefore concluded that the
caves were filled after the Messinian crisis approximately 5.2
million years ago.
In short, unroofed caves are an increasingly recognized fea-
ture of the karst surface, an important part of the epikarst, and
an exceptional trace of the development of the karst aquifer.
Determining the age of the alluvia helps us understand the
oldest periods of karstification and has proven that the oldest
caves in Kras are much older than earlier karstologists thought.
Planning Road Construction
Before construction starts we verify the accuracy of known
data about caves in the field and add possible new measure-
ments and explanations of their development. To throw light on
the situation we present existing data on perforation of the
aquifer and elaborate prognosis subsurface maps with special
emphasis on the anticipated lithological and tectonic change s in
rock composition and structure. Before the start of construction
work we try to present the perforation of the karst as accurately
as possible. We can determine the position of underground
caves by drilling, and along with measurement indicators we
also determine the type of potential fill material (flowstone,
alluvia). To a certain extent we can anticipate the shape, type,
and occurrence of caves in the vicinity using our knowledge of
the known surface and underground features.
Perforation (Figure 1) puts a special stamp on the construc-
tion of expressways in the Kras region. In addition to its varied
development, Slovenia’s karst is marked by tectonic and litho-
stratigraphic diversity and it is therefore difficult to determine
in advance where caves will occur. As a rule, caves occur more
frequently along the contacts of flysch with limestone. The
perforation of the karst aquifer is therefore determined primari-
ly on the basis of good and comprehensive knowledge of the
karst and continuous intensive work in planning and construct-
ing the expressways.
When planning expressways, the link between surface and
underground karst features requires the karstological evaluation
of the karst surface as well as the karst underground, the hy-
drological situation, and the presented variables. On all the
expressway construction sites in Kras we encountered numer-
ous karst phenomena including dolines, filled and empty caves,
and sections of old and current drainage systems through the
karst. The lowering of the karst surface exposed many karst
caves that are now visible in the Kras region. In recent years,
we have focused on unroofed caves “discovered” during the
construction of expressways. We are certain that a quality
karstological study of the area where a road is planned enables
the better selection of a route and is one of the basic starting
points for planning expressway construction in this unique and
We begin by assembling published literature, archives, and
M. KNEZ, T. SLABE
Copyright © 2012 SciRes.
various unpublished studies to learn about the surface karst
features, and thus identify dolines, collapse dolines, and other
morphological features in particular. Through a field survey we
establish the starting points for mapping the areas of the se-
lected route. In the field, we evaluate different types of rock
from the karstological aspect. On theme maps we present the
known entrances to underground caves and supplement them
with potential new entrances. We anticipate the branching of
underground cave systems on the basis of surface mapping and
explanations of the development of morphologically identified
unroofed caves visible in the relief. On the basis of surface
mapping we also consider possibilities for dumping waste ma-
terial if nec es sary.
We know from experience that during construction every
route crossing Kras will sooner or later encounter underground
caves or parts of cave systems. To a certain degree we can pre-
dict the shape and type of caves using our knowledge of surface
and underground phenomena. We trace the caves in the wider
area of the traffic route, determine their type, position, and role
in the aquifer, their shape, rock relief, the alluvia and flowstone
found in them, and present them on suitable maps. To make the
maps easier to read, we present the previous data on the perfo-
ration of the aquifer and elaborate predictions with special em-
phasis on anticipated lithological and tectonic changes in the
During the construction many caves of different shape and size open
and dictate further appropriate building works.
Due to the special characteristics of carbonate rock, karst
rivers and stream that sink in the studied area easily find direct
routes into the underground (karst aquifer); it only takes them
an hour to percolate through 100-meter thick rock beds. Al-
though the flysch rock beds in Kras found in permanent direct
contact with carbonate rock are often presented as exclusively
impermea ble beds, it must be empha sized that the fly sch (often
in thin beds) is only an isolated lens lying on permeable car-
bonate rock. Furthermore, it must also be observed that a
smaller number of underground conductive channels do occur
in flysch and that precipitation water collecting on flysch runs
off onto carbonate rock. We therefore undertake hydrological
mapping in the field. For this purpose we delineate and define
the basic characteristics of hydrogeological units in the wider
route area, identify hydrological objects (captured and uncap-
tured springs, surface streams, water caves, boreholes, measur-
ing stations, etc.), and establish the physical and chemical
properties of springs. When necessary we perform tracing ex-
periments during low and high waters, primarily to determine
the direction and velocity of underground flows in the wider
area of the traffic route. With the results of field mapping and
tracking experiments, we elaborate and upgrade the existing
hydrogeological maps, build a database on the state of the en-
vironment, and assess the impact of the construction on karst
The basic guidelines for planning traffic routes include:
-the selection of a route shall be based on a comprehensive
assessment of the karst with emphasis on local features;
-the selected traffic route shall avoid specific exceptional
-the conservation of karst aquifers shall be one of the priority
goals of planning.
Preserving as Many Karst Caves as Possible
The shafts were easiest to preserve and concrete plates were
used to close the smaller entrances. It was similarly possible to
preserve old caves with solid circumferences but caves located
in fractured rock or opened during blasting had to be filled.
Rock walls were used to close caves crossed by road cuts with
entrances on embankments. Their circumferences were frac-
tured to such an extent that they were unsuitable for visiting,
and water could wash clay from caves filled with alluvia and
deposit it on the roads. One well preserved cave was left open
for travelers crossing the border with Italy to visit. The most
interesting and best preserved caves were completely secured
and made accessible for visiting even though they were loca ted
under the expressway or even wound around a tunnel. They are
accessible via concrete culverts closed at the roadside and in the
tunnel with a door (Figure 2).
We also studied the consequences of blasting in caves, which
will help us in road construction and the preservation of karst
features in future.
Protecting the Karst during Expressway
Construction and Use
Experience acquired tracing waters and accidental spillages
of various substances on the karst surface drew attention to the
great perforation of the karst aquifer, which the number of
caves newly discovered during construction confirmed. Low
permeability occurs only on individual relatively small patches
M. KNEZ, T. SLABE
Copyright © 2012 SciRes.
Preservation of caves. a. In ro ad cuts th e caves are hidden behind rocky
scarps. b. The caves lying below the road with narrow mouth and if
their rim is not too much damaged by blasting a re covered by concrete
lids. c. In the side of the tunnel pipe there is a special door leading to
the caves. Below traffic belt caves are connected with large concrete
pipes. d. Karst openings (bottom of dolines, tops of shafts) are often
reinforced by arches of big rocks poured over by concrete.
such as the bottoms of dolines heavily covered with soil
washed usually from a larger surrounding area, dolines on the
Paleogene limestone of the Divača valley system that are
transformed into ponds during rainy periods, and smaller
patches of clay that filled old caves. Maximum precautions
must therefore be employed during both the construction and
use of roads. Daily traffic leaves numerous environmentally
harmful substances on road surfaces, and mineral oils were
found in stagnant waters in caves located near traffic routes.
Due to these findings and the persistence of karstologists, ex-
pressways are made to be impermeable. Pipes and gutters
along the roads lead to wastewater collectors. Untreated water
should never reach the permeable karst surface and the specifi-
cations for drainage systems must meet this requirement. The
existing wastewater collectors are often too small and abundant
precipitation can easily wash the sediments from them.
It is clear that the cooperation of karstologists in the con-
struction of expressways in the Kras region has brought posi-
tive results. It is important that karstologists participate in the
planning and construction of expressways and later that they
monitor the impact of the expressways on the environment, that
is, throughout the entire process of encroachment on the vul-
nerable karst landscape. This logical cooperation helps preserve
natural heritage and increase our basic knowledge about the
creation and development of karst and about the construction of
expressways in this unique environment. There are many types
of karst and each requires a unique approach, which calls for
permanent and continuous cooperation between road builders
and karstologists. Over the last ten years, the cooperation be-
tween the planners and builders of expressways and karstolo-
gists has resulted in knowledge used in the planning and im-
plementation of other encroachments in karst areas.
M. Knez and T. Slabe, Kraški pojavi razkriti med gradnjo slovenskih
avtocest = Karst features discovered during motorway construction
in Slovenia. Ljubljana, Slovenia: ZRC Publishing, 2007.
D. Culver et al., Karstology and development challenges on karst 2,
Construction, touris m ecology, protection . Ljubljana, Slovenia: ZRC