Post on 10-May-2020
Engineering-geological investigations according
to flood protection: Saxony’s rivers in the range
of the Ore Mountains: Zwönitz
Andrea Kuhn
Institute of geotechnics, field of science: engineering geology, TU Bergakademie
Freiberg, Germany
Abstract. Flood have always been going along with men’s history. One of the
most explosive events happened from August 11th
to 15th
which was mainly
caused by a Vb-low accompanied by extreme rainfall. The sheer enormity in
Saxony amounted to 20 deaths and to a total loss of about 8.6 billion €
(Sächsisches Staatsministerium für Umwelt und Landwirtschaft 2004). The
devastating consequences led to the necessity to sharpen the flood protection up.
A conception for flood protection, therefore, was developed in 2004. In the
following, a project belonging to this conception will be introduced. This case
study deals with the judgement of stability of retaining walls that are located at the
banks of the Zwönitz as a representative river of the Ore Mountains.
Introduction
In response to flood in August 2002, a number of laws and ordinances have been
determined by provincial governments. One of the most striking measures by law
is the conception of flood protection in Saxony (Landestalsperrenverwaltung
2003). It is fixed in Saxony’s water law (2004, § 99a, b) and is used as water
provision framework plan for sharpen the flood protection up.
Among the remedy and judgement of damages, the primary issue poses a
sustainable flood protection which considers the European Water Framework
Directive (Sächsisches Staatsministerium für Umwelt und Landwirtschaft 2003).
Thus, there has to be an improved occupation of land for increasing retention.
Therefore, the extension of waters is necessary instead of constriction by civil
works. Furthermore, one strives for both simplified administrative procedures and
an intensified public relations work. In addition to that, damage maps and an
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 2
early-warn system, respectively contingency plans, have been generated for risk
management (Sächsisches Staatsministerium für Umwelt und Landwirtschaft
2003).
For scientific conception, a detailed analysis of the event is required where the
degree of the dangerous event is clarified. Taking the processes of transport into
account due to the river, the flood has to be historically conceived.
Definition of the project
The project of the conception of flood protection “HWSK 27“ included the new
building, the heightening of existing retaining walls respectively and dikes in the
range of the Zwönitz, especially between Erfenschlag and Einsiedel. For that
purpose, there are 8 individual complexes of measure along a distance of about 5
km (Seidel and Döring-Koppatz 2006).
It was intended to investigate and evaluate the situation of subsoil as well as the
contour of construction within the definite complex of measure “M 1.1” according
to DIN 4022 in those areas where heightening of walls was planned (Seidel and
Döring-Koppatz 2006).
In the course of M 1.1 (360 m) different kinds of embankment are available:
retaining walls, slope paving but also unsecured embankments.
According to the first aspect (retaining walls), an expert opinion of the subsoil
(14.11.2006) is on hand that was compiled by Seidel and Döring-Koppatz in 2006.
The administration of dams in Saxony acted as the awarding authority.
Watercourse Zwönitz: Topographical and geological setting
In Figure 1 the topograhical course of the Zwönitz is mapped: the river rises to the
south of the locality Zwönitz in the Annaberg district of the Ore Mountains and
flows in a northward direction as a water of first order (Sächsisches Wassergesetz
2004). At Chemnitz the river Chemnitz is established, due to the confluence of
Zwönitz and Würschnitz. (L 5342 Stollberg 1993).
The water of the Zwönitz is used for drinking water supply at the dam of
Einsiedel.
The area of research is located in the northern part of the Zwönitz between the
villages Erfenschlag and Einsiedel belonging to the city of Chemnitz (Seidel and
Döring-Koppatz 2006).
With respect to the geological setting, the area of research is marked by a
frequent change of rock (Seidel and Döring-Koppatz 2006). This phenomenon
appears because the region of interest was subjected to various tectonic and
metamorphic overprints, that is characteristic of the edge zone of the Ore
Mountains as a part of Saxo-Thuringia (Rothe 2006).
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 3
Fig. 1. Topographical setting of the Zwönitz: The course of the whole river is marked in
blue, the area of research in red (modified after ADAC 1997/1998).
In the bedrock phyllites or phyllite-like rocks as hornblende schists, that are
non-resistant to weathering, are present predominantly (Seidel and Döring-
Koppatz 2006). This metamorphic Palaeozoic unit is surrounded on the one hand
by northwestern accumulations of the Rotliegend-formation and on the other hand
by rocks of the mica slate formation in the southeast (Rothe 2006, Seidel and
Döring-Koppatz 2006).
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 4
According to the profile in the area of research, the solid rock -as subjacent
bed- is followed by a zone of weathering as well as deconsolidated and destructed
rocks varying in thickness up to 1 m (Seidel and Döring-Koppatz 2006).
Afterwards, either slopewash or meadow loam is located (Seidel and Döring-
Koppatz 2006). The slopewash is a silty material with sandy-pebbly constituents
from the Pleistocene. In contrast, the meadow loam is a result of Holocene
processes caused by rivers and consisting of silty-sandy alluvium (Seidel and
Döring-Koppatz 2006). The immediate roof is represented by thin topsoil (Seidel
and Döring-Koppatz 2006).
As a consequence of anthropogenic influences it is possible, that this shift
sequence is abraded, mixed or even infilled (Seidel and Döring-Koppatz 2006).
Relative to the situation of ancient mining and earthquake, a risk potential can
be excluded (after DIN 4149 Teil l A1).
Retaining walls: basics
In general, retaining walls offer the function of stability: on the one hand, they are
employed for safety of vertical spans in terrain, e.g. in cut and fill (Möller 2003).
On the other hand, they are applied in hydraulic engineering for delimiting a river
or for flood protection.
Depending on the form and constructive design retaining walls can be divided
into revetment, angular retaining and gravity retaining walls (Möller 2003).
In the following, the emphasis will be placed on the final type of retaining walls
that often are referred to as quay walls. These walls will be considered in the range
of the Zwönitz, as already described, based on the conception of flood protection.
With respect to the common architecture (Fig. 3) a trapezoidal wall section
including a perpendicular front side have prevailed (Türke 1999). In addition to
this, an adequate foundation till the sustainable bedrock is essential as well as the
backfill (Möller 2003). Backfill is defined as “the part of present soil that is
removed over the period of fabricating the retaining wall. Afterwards the material
is filled again, if the soil is capable” (Möller 2003).
The type of material for building the wall depends on the date of construction
(Vogt 1998). These days concrete is used predominantly, whereas perpend walls
in mortar bond have been applied in the past. Figure 2 gives an impression of the
frequently observed badness of retaining walls that can be at the age of about 100
years.
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 5
Fig. 2. Photographic documentation about the retaining wall’s current state of repair:
eroded and overgrown joints in ashlar masonry work. (image: Döring-Koppatz 2006)
Finally, the retaining wall (Fig. 3) has to be dimensioned with guarantee of
stability according to slide, shear failure, failure in terrain and sometimes to cant
or hydraulic shear failure (Türke 1999). This stability is represented by Fres (F4), i.e. transmitted power into the subsoil resulting from the following components of
force: first of all, the wall has a weight (FG = F1), affecting normal to the subsoil.
In addition to this, there is the thrust of the ground resulting from the backfill (Ea =
F2) and the hydrostatic pressure caused by the river (Fw = F3) and countervailing
the thrust of the ground.
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 6
Fig. 2. A retention wall’s schematic demonstration: Principal construction and system of
forces. (modified after Türke 1999)
Retaining walls: investigating the actual state for the Zwönitz-project
In contrast to the new building of retaining walls, for which expert knowledge and
numerous ordinances are available, it is complicated to judge the stability of pre-
existing and damaged retaining walls (Vogt 1998).
Below, the variety of aspects, that has to be considered in this connection, is
described as well as the investigation methods (Fig. 4) and problems that can
occur.
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 7
Preparing methods
In the run-up to the investigations, the circumstances in terrain have to be
conceived by means of topographical, geological and other maps (Vogt 1998).
Besides, various essential information according to the line system have to be
gathered, such as water, effluent, electrical power supply, phone, gas etc. Beyond,
a competent knowledge of ground-water level and rainfall is assumed, among
information on the retaining wall from the historic point of view (Vogt 1998).
Finally, several consents are needed e.g. for entering private area.
Investigation methods for stability judgement of retaining walls
Fig. 4. Schematic demonstration of a retention wall: Investigation methods for stability
judgement. (modified after Türke 1999)
In the beginning, the engineering structure was visually characterised and
documented by photographs (Fig. 2).
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 8
For investigating the subsoil ramming core soundings (RKS) were used in the
backfill area up to the bottom of sounding or a maximum of depth (ca. 5 m below
top ground surface). From this sampled loose rock, a composite sample was
compiled in order to test the environmental stress after LAGA (Seidel and Döring-
Koppatz 2006).
To gather information on the stability of retaining walls, one has to prove the
thickness of the walls by horizontal core holes.
Angular core holes (60°), in contrast, provided a basis for investigating the
wall’s foundation depth.
Furthermore, the aggressiveness of concrete, that means the chemism of water,
was checked on a sample of the river after DIN 4030 (Seidel and Döring-Koppatz
2006).
Results
Investigation of the subsoil, that was realised by 15 ramming core soundings after
DIN 18196, refers to the chapter “Geological setting”. Thus, deconsolidated and
destructed rocks or weathered phyllites, followed by pebbly accumulations due to
the river’s transport processes, were exposed depending on the outcrop. Because
of the fluvial influence, both slopewash and meadow loam occur in the range of
floodplain. Subsequently, there was filling, resulting from transferred river
accumulation, meadow loam, topsoil or disturbed rock in the range of the wall’s
backfill, that reached till the Zwönitz’ riverbed. The covering layer of the profile
is represented by topsoil (Seidel and Döring-Koppatz).
The judgement of base and contour of structure is based on visual descriptions
and masonry holes (Fig. 3). Therefore, the sections of masonry could be
predominantly characterised as “in a good state of repair” (Seidel and Döring-
Koppatz 2006). However, alluvial material was accumulated at the foot of the
walls. Besides, the joints of the ashlar masonry work (rubble wall with granites or
phyllites in mortar bond) show weathering, overgrowth by roots and moss cover
(Fig. 2).
Via 3 horizontal core holes it was possible to circumstantiate an average wall
thickness (ca. 0.8 m) and the wall material (concrete) according to Seidel and
Döring Koppatz. The wall’s superior part is adopted well cony-shaped, otherwise
righted (Seidel and Döring-Koppatz 2006). In the course of investigating a wall’s
thickness, difficulties can occur because in some cases the back of the retaining
wall is hardly distinguishable from the backfill (Vogt 1998).
On the visible side the retaining walls simply possess a natural stone facing
(Fig. 2), that in part was not able to resist the dangerous current and the increased
water level whilst flood (Sächsisches Staatsministerium für Umwelt und
Landwirtschaft 2004). That way, the porous and sanding composite material got
eroded causing undercuttings and scourings of retaining walls as well as head
walls (Seidel and Döring-Koppatz 2006)
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 9
Besides, it was possible to detect open spaces filled with gravel or fragments of
concrete as well as deconsolidated lean concrete in the intern wall structures. With
respect to this observation deficits in the wall’s stability could be concluded
(Seidel and Döring-Koppatz 2006).
Considering the results that were extracted from the 3 angular core holes a
distinction is necessary: On the one hand, a low material strength of the lean
concrete basement was reasoned (ca. 1.1 m of foundation depth) because of the
basement’s destruction while drilling (Seidel and Döring-Koppatz 2006). On the
other hand, the ashlar masonry work including phyllites in mortar bond was
investigated, whose horizon of foundation was assumed to consist of fluvial
gravel. For this case, both frost and scour inalterability adopted (Seidel and
Döring-Koppatz 2006).
In the course of testing the accepting base pressure the following was detected:
areas of non-cohesive material caused comparatively high base pressures (2 – 3
cm) decaying slowly. Whereas low and rapidly decaying sole pressures (1.5 cm)
represented pebbly and a rather secure subsoil (Seidel and Döring-Koppatz 2006).
It has to be mentioned, that all base pressures are decayed at present (Seidel and
Döring-Koppatz 2006).
According to the hydrogeological circumstances, the groundwater was
ascertained roughly in the range of the Zwönitz. Hence, it is to be assumed that the
aquifer layer, which is dominated by fluvial gravel, is geared to the river’s
delivery in a seasonable fluctuating way (Seidel and Döring-Koppatz 2006).
For evaluating the aggressiveness of concrete according to the river’s water
after DIN 4030 the following parameters had to be analysed: pH-value, carbonic
acid dissolving lime [mg/l], magnesium [mg/l], sulphate [mg/l] and ammonium
[mg/l]. The results classify the river’s water as “non-concrete-aggressive” (Seidel
and Döring-Koppatz 2006).
Finally, the composite sample was used to determine the environmental stress
on the backfill (Seidel and Döring-Koppatz 2006): Due to the minor heightened
concentrations [µg/l] of arsenic, copper, nickel and zinc the solid material has to
be categorised as Z 1.1 (after LAGA). This causes a restricted exposed integration
“considering certain modification of utilisation” (LAGA). However, one has to
surrender the integration of material in areas of drinking water protection or flood.
As a result, the spoil of the Zwönitz either have to be locally determined in a more
intensive way or the spoil’s reusableness is impossible (Seidel and Döring-
Koppatz 2006).
Engineering-geological investigations according to flood protection: Saxony’s rivers in the
range of the Ore Mountains: Zwönitz 10
Consequences and constructional advices
The whole area of research offers both scour protection and frost-safety (Seidel
and Döring-Koppatz 2006) although in many real cases people are confronted
with scourings.
According to the detected state of base, retaining walls that will be built in the
future, have to take the depth of foundation of about 1 m below the base of the
Zwönitz into account (Seidel and Döring-Koppatz 2006). The predominantly
pebbly subsoil provides a good bearing capacity. However, local regions
containing a mixture of silt, sand or meadow loam have to be excavated or even
exchanged because of the low bearing capacity (Seidel and Döring-Koppatz
2006).
The heightening of the walls, that was projected, is practicable if the concrete
will be tested and if the accepting base pressure will be considered (Seidel and
Döring-Koppatz 2006). The new building of retaining walls (after the break of the
former ones) or the heightening of the constructions, is recommended for periods
with little rainfall because of the temporarily river’s bording (dewatering) during
construction work (Seidel and Döring-Koppatz 2006).
Furthermore, the protection of embankment is to preserve as well as the
consideration of the effects on adjacent buildings. That can be realised for
example by using low-vibration technologies (Seidel and Döring-Koppatz 2006).
Perspective
There is no denying that the consequences of flood in 2002 have to be corrected in
the long run, although 6.2 billion € of the reported total loss in Saxony were
eligible until 31st July 2003 (Sächsisches Staatsministerium für Umwelt und
Landwirtschaft 2004). Besides, the conception of flood protection has already
been achieving numerous prosperities by work on specific projects, as introduced.
In addition, instructive damage maps have been generating, including a
sophisticated occupation of land whereas the degree of dangerous event is based
on the probability and intensity of flood (Sächsisches Staatsministerium für
Umwelt und Landwirtschaft 2004).
Ultimately, urgent need for research is still existing, e.g. according to a
hydrologic applicable prediction of rainfall, especially for both regional and local
supply (Sächsisches Staatsministerium für Umwelt und Landwirtschaft 2004).
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