Engineering-geological investigations according to flood ... Kuhn.pdf · Engineering-geological investigations according to flood protection: Saxony’s rivers in the range of the

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


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).



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




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.



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.



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.



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).



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)



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


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


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).



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).

References

Landestalsperrenverwaltung des Freistaates Sachsen (2003) Hochwasserschutzkonzepte:

Eine integrierte Strategie für Sachsen



Möller G (2003) Geotechnik kompakt Grundbau. Bauwerk Verlag GmbH: 235-258

Rothe P (2006) Die Geologie Deutschlands. Primus Verlag: 98-100

Sächsisches Staatsministerium für Umwelt und Landwirtschaft (2004) Ereignisanalyse

Hochwasser August 2002 in den Osterzgebirgsflüssen

Sächsisches Staatsministerium für Umwelt und Landwirtschaft (2004) Das neue Sächsische

Wassergesetz: Regelungen für einen zukunftsweisenden Hochwasserschutz und den

guten Zustand der Wässer

Sächsisches Staatsministerium für Umwelt und Landwirtschaft (2003) Hochwasserschutz in

Sachsen: Wie weiter nach der großen Flut?

Seidel H, Döring-Koppatz I (2006) Baugrundgutachten: Geotechnischer Bericht zu den

Baugrund- und Gründungsverhältnissen nach DIN 4022 (unveröffentlichtes

Dokument): 1-25

Türke H (1999) Statik im Erdbau. Ernst&Sohn: 15-26

Vogt L (1998) Untersuchungen zum Tragverhalten und Verbesserung der Standsicherheit

von Stützmauern. Institut für Geotechnik Technische Universität Dresden Mitteilungen

Heft 6: 1-10

Voth B (1982) Erdarbeiten für Bauführer, Schachtmeister und Poliere. Bauverlag GmbH:

99-100

ADAC (1997/1998) Reiseatlas Deutschland 1:200.000: 126

DIN 18196 Erd- und Grundbau- Bodenklassifikation für bautechnische Zwecke (2006)

DIN 4022 Benennen und Beschreiben von Boden und Fels (1987)

DIN 4030 Beurteilung betonangreifender Wässer, Böden und Gase (2006)

DIN 4149 Teil 1 A1 Bauten in deutschen Erdbebengebieten- Lastannahmen, Bemessung

und Lastausführung üblicher Hausbauten (2005)

LAGA M 20 Länderarbeitsgemeinschaft Abfall (2004)

L 5342 (1993) Topgrafische Karte Stollberg 1:50.000

Sächsisches Staatsministerium für Umwelt und Landwirtschaft (2004) Sächsisches

Wassergesetz: § 99 a, b

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