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GEOLOGICAL ENGINEERING INVESTIGATION OF ROCK SLOPES

AT THE EXAMPLE OF THE WEIßERITZTAL RAILWAY BETWEEN

HAINSBERG AND SEIFERSDORF IN SAXONIA GERMANY 2006

BENJAMIN PAUL1

1TU BERGAKADEMIE FREIBERG

Abstract:

The following paper describes the cause, course and result of geological engineering

investigations of rock slopes. For that the appearances, reasons and trigger of typical rock

failures on the one hand and the possibilities of slope stabilization and akin countermeasures,

as well as the advantages and disadvantages of them, on the other hand are mentioned. By the

geological engineering analysis only kinematic and limit equilibrium analysis are looked at in

detail. All that is shown on the basis of an example. This example is the geological

investigation of the Weißeritz valley. Reason of this investigation is the reconstruction of the

railway, which was destroyed by a high water event.

Introduction:

The geological engineering investigation of rock slopes are a very important fact for the

stability and because of that for the safety of the population. Especially in the industrial

countries with a high population density it is important to know about the rock slope

stabilities along roads, railways, construction pits or footpaths, because the density of human

interventions in the natural rock slopes is much more abundant than in less industrialized

countries. The correct method for stabilization of a rock slope not only depends on the

geological structures but also it is an expense factor. Therefore it is really important to

investigate the geology in and around the rock slope, so that a safe and economic solution can

be found.

Preperatory work

The first step after getting the task is to collect information about the local and regional

geology of the investigation area. For this purpose inquiry in litearture, archieves, historical

documents and discernments in corresponding maps are necessary.

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The next step is to get access to all of your investigation areas therefore you have to do a lot

of paper work with the corresponding authorities. If the investigation area is in private

property and the owner won´t give the permission, there is no chance to gain data in the

affected area. Soon as all formalities are done it is possibly to do the geological engineering

fieldwork and mapping.

Typical Rock Slope Failures

After Kliche (1999) there are four general modes of slope failure: planar failure (Fig.1),

rotational failure (Fig.2), wedge failure (Fig.3) and toppling failure (Fig.4), which have the

following properties. In planar failure the mass progresses out or down and out along a more

or less planar or gently undulating surface. The movement is commonly controlled

structurally by (1) surface weakness, such as faults, joints, bedding planes and variations in

shear strength between layers of bedded deposits or (2) the contact between firm bedrock and

overlying weathered rock. Conditions which have to be fulfilled are shown in table 1.

Table 1. – Conditions for appearence of planar failure

number conditions

1 The strike of the slope doesn´t differ more than ± 20° from the strike of the

weakness plane.

2 The toe of the failure plane has to cross the slope between his toe and crest.

3 The dip of the failure plane must be less than the dip of the slope face, and the

internal angle of friction for the discontinuity must be less than the dip of the

discontinuity (Hoek and Bray 1981)

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Figure 1. – The planar failure mode (Kliche 1999)

The most common examples of rotational failures are little- deformed slumps, which are

slides along a surface of rupture that is curved concavely upward. In slumps, the movement is

more or less rotational about an axis that is parallel to the slope (Figure 2). In the head area,

the movement may be almost wholly downward, forming a near- vertical scarp and have little

apparent rotation; however, the top surface of the slide commonly tilts backward away from

the preexisting slope face, thus indicating rotation. A purely circular failure surface on a

rotational failure is quite rare because frequently the shape of the failure surface is controlled

by the presence of preexisting distcontinuities, such as faults, joints, bedding, shear zones, etc.

The influence of such discontinuities must be considered when a slope stability analysis of

rotational failure is being conducted.

Rotational failures occur most frequently in homogeneous materials, such as constructed

embankments, fills, and highly fractured or jointed rock slopes.

Figure 2. – The rotational failure mode (Kliche 1999

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The possibility of wedge failure exists where two discontinuities strike obliquely across the

slope face and their line of intersection daylights in the slope face (Figure 3.). The wedge of

rock resting of these discontinuities will slide down the line of intersection provided that (1)

the inclination of the line of intersection is significantly greater than the angle of internal

friction along the discontinuities, and (2) the plunge of the line of intersection daylights

between the toe and the crest of the slope.

Figure 3. – The wedge failure mode (Kliche 1999)

Toppling failure occurs when the weight vector of a block of rock resting on a inclined plane

falls outside the base of the block. This type of failure may occur in undercutting beds.

(Figure 4.)

Figure 4. – The toppling failure mode (Kliche 1999)

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Reasons for Rock slope failures

Usually it isn´t one cause that leads to a slope failure but a addition of different causes. After

Varnes (1978) there are two groups of reasons which trigger slope failures (Table 2.): (1)

factors that contribute to increased shear stress and (2) factors that contribute to low or

reduced shear strength.

Table 2. – Slope failure causes (Varnes 1978)

Factors that contribute to increased shear stresses.

Factors that contribute to low or reduced shear strength.

factors examples factors examples removal of lateral support

erosion by streams or rivers, wave action on lakes, glaciers

changes in shear strength due to weathering

softening of fissured clays, hydration or dehydration of clay minerals

addition of surcharge to the slope

weight of rain, hail, snow or water

changes in intergranular forces due to water content

caused by: rapid drawdown of a lake or reservoir, rapid changes in the elevation of the water table

transistory earth stresses

vibrations of earthquakes, traffic, etc.

changes in structure

caused by remolding clays upon disturbance, by the fissuring of shales and precons. Clays

removing of underlying support

road construction, sqeezing out of underlying material

miscellanious causes

weakening of a slope due to progressive creep or due to actions of roots

lateral pressure

water in pore spaces, cavern or cavities, freezing of water

Basics of the geological engineering slope model

There are three main groups of geological enginnering models: (1) Kinematic and Limit

Equilibrium Back- Analysis, (2) Continuum & Discontinuum Numerical Methods and (3)

Hybrid Finite-/ Discrete Element with Fracture (Table 3 shows the conventional methods of

analysis). In practice the Kinematic and Limit Equilibrium Back- Analysis is used mostly,

cause it is a simple, fast and cheep method which describes the slope under certain conditions

sufficient. But in case of very high fractured rock slopes these methods work not reliable and

aren´t exactly enough (Stead et. al. 2005). For the application of the finite element method

however high numerical costs and accurate measurements of the parameters of the

geomaterials are required, which are often difficult to obtain. This make the use of the finite

element method less attractive for current applications (Yang et al. 2006).

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Table 3. – Conventional methods of analysis (modified after Coggan et al., 1998)

The requirement for the stability investigation of a slope is the recognition of geometric

conditions. Therefore you have to do an intensive mapping cause these are the primary data

for the slope model. Special importance comes to the fracture cleavage, the vegetation and the

hydrogeological conditions which influences the slope. Focal points in field investigation are:

(1) determination of depth, shape and character of the failure zone (grade of weathering,

permeability and colour of the mountains), (2) investigation in change of hydrogeological

circumstances, (3) registration of morphologic appearances (extent of the affected plane,

difference in altitude between the highest and lowest point, highness, shape and surface of the

slope, shape of the surface, wideness, depth and declination of crevices and direction of

slickenside striation) and (4) mapping of the vegetation (kind, condition and irregularities). In

the case of the Weißeritz Railway the ground- water table is at the level of the Weißeritz river.

Also there were only small water escapes in the investigation area. But important for failure

are the upper areas where surface water penetrates into crevices and joints and causes an

increasing of pressure and so a decreasing of the slope stability. These appearances are

located at all investigation parts of the Weißeritz railway. The seen fracture cleavages and the

slopes are plotted in a stereogram. Exactly information about the formation stand are critically

because the high fracturing and the different extend of weathering. The formation stand

fluctuates from c´= 0,1 MN/m² till 2,8 MN/m² at a cohesion φ´ from 23° till 48°.

Through partly small but also very wide opened joints which can be filled with loam the shear

strength is partly decreased very much. The distance of the joints ranges from a few cm till

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more than 2 m and the joints have got openings between mm and dm- scale. Especially in the

upper areas the pressure of roots lead to removal of stones, blocks and boulders along the

direction of the cleavage. All rock slopes in the investigation area are stable in the global

meaning, that means the hole slope is stable. But some parts of the slopes aren´t stable so that

they need to be stabilized. Geodynamic processes which forces the destabilization of the

slopes in parts of the investigation area are root pressure, penetration of surface water and the

increasing effect of frost explosion in the fractured rocks. Also it´s important to know

something about the seismic setting of the area. In case of the Weißeritz valley there is no

relevant seismic activity but at other locations it often can be a trigger for slope failures.

With the following equation (Figure 5.) it is possible to calculate the stability of a block

against sliding including the pressure of joint water.

Figure 5. – calculation of stability of a block against sliding

η = R/S = N*tanφ / T = [(GN – WN) * tanφ] / GT + WT

N: weight of the sliding block vertical to the sliding plane [kN]

GT: weight of the sliding block parallel to the sliding plane [kN]

WN: pressure of joint water vertical to the sliding plane [kN]

WT: pressure of joint water parallel to the sliding plane [kN]

R: holding forces [kN]

S: pushing forces

N: resulting force parallel to the sliding plane [kN]

T: resulting force vertical to the sliding plane [kN]

G: weight of the sliding block [kN]

υ: inclination of the sliding plane (°)

φ: angle of friction along the sliding plane (°)

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The geological slope model and failure mechanism in the investigation area

First it is important to know of what kind of rock the investigation area is build up and are the

slopes pure rock slopes. Now for the development of the slope model you take the data from

the mapping and put it into a stereogram and compare it with the conditions for appearance of

typical rock slope failures such as planar, wedge or toppling failure. In the case of agreement

the slope isn´t stable and measures for stabilization has to be acted. In the case of the

Weißeritz valley a big amount of slopes show hints of failures like planar failure and

especially wedge failure in future. Because the high dip of most of the slopes in this area it is

highly probable that planar and wedge failure will appear. Toppling failure is only expected at

slopes with a dip of more than 90 degrees, which exists also at many locations. The next and

the last step in the geological engineering investigation is the suggestion of stabilization

measures.

General recommendation for safety and rock removal measures

To realize the stability of rock slopes there are a lot of available opportunities. The choose of

these depends on several factors like size of slope, joint blocks, vegetation, water influences

and fracturing. Enduring measures are e.g.: elimination and drainage of waterflow, fast

installation of heavy concrete bodies at the bottom of the slope, removal of rock material in

the upper slope, construction of retaining walls and other supporting structures. After Kliche

(1999) slope stabilization techniques can be divided into six general categories: grading,

controlled blasting, mechanical stabilization, structural stabilization, vegetative stabilization

and water control. But there are also other methods like the removal of endangered blocks,

avoiding of breaking-up through removal of trees, other vegetation and the installation of fang

ditches and fang embankments. In case of rock slopes mechanical and structural stabilization

is provided. Mechanical stabilization methods of slope treatment are those that alter or protect

the slope face to reduce erosion, prevent rockfall or reduce ravelling. In general it´s all about

nets which encases the slope or parts of it. There are two main groups: protective blankets and

geotextiles and wire net or mesh. Protective blankets are often combined with seeds and

fertilizer to protect the slope from erosion till the vegetation gain a foothold. After

Christopher and Holtz (1985, p.27) geotextiles are “any permeable textile material used with

foundation, soil, rock, earth or any other geotechnical engineering- related material, as an

integral part of a man- made project, structure or system”.

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On the other side there are two kinds of wire nets used to span the slopes: welded wire fabric

and chain- linked mesh (Fig. 6). A typical welded wire mesh application would be to use

mesh with a 100- mm by 100- mm or 150- mm by 150- mm opening and a wire size from 9

till 4 gauge (Seegmiller 1982). Comparatively the chain- linked mesh is stronger and more

flexible than the welded wire fabric because of the construction and chain- linked meshes are

normally galvanized so that they´re more weatherproofed. The nets hold the loose or

endangered rock blocks in their current position and avoid that they leave the slope and fall

down. To realize this the chain- linked mesh has to be strong enough and spanned very close

to the slope. So it is necessary to know about the approximately mass and size of the

concerned blocks. From time to time it is possible that detached rock material is gathering

behind the net so that it should be periodically cleaned up to avoid a destruction of the net.

Figure 6. – chain- link wire mesh

In the structural stabilization there are more approved methods like shotcrete (sprayed

concrete), rock bolts, rock anchors, rock dowels, buttresses and retaining walls. Shotcrete is

used to fill the space between rock bodies and weathered material and bind it together.

Generally for rock slope stabilization the material is applied in one 50- to 75-mm layer

(Brawner 1994). A disadvantage of the shotcrete is the low tension strength, which can be

countervailed with the installation of a wire net, and the weathering over a period of years.

Rock bolts, anchors and dowels are used to tie together the rock mass so that the stability of a

rock cut or slope is maintained. Rock bolts are commonly used to reinforce the surface or

near-surface rock of the excavation, and rock anchors are used for supporting deepseated

instability modes in which sliding or separation on a discontinuity is possible. Rock dowels

are commonly used to provide support for steeply dipping rock formations. They also can be

used to anchor wire mesh, to pin wire mesh to the face of a highwall, to hold strapping in

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place, or to anchor restraining nets or cables. Buttresses and retaining walls play a tangential

role in the rock slope stabilization. They were of prime importance by the stabilization of soil-

like slopes and so they can be neglected in this paper. The same goes for the vegetative

stabilization. These methods are most successful when minor or shallow instability is

involved, as is usually the case for soil slopes or highly fractured rock slopes (Buss et al.

1995). A much more important technique is the control of the water conflux. Because Water

decreases the stability of the rock slope it is necessary to avoid a water conflux or drain the

area around the slope. There are two primary sources where water can come from: (1) surface

water and (2) groundwater. Grading and shaping are major considerations in the control of

surface water. Surface water can be controlled through a combination of topographic shaping

and runoff control structures (Glover et al. 1978). Methods which belong to the topographic

shaping are manipulating the gradient, length and shape of the slope. Runoff control structures

include dikes, waterways, diversion ditches, diversion swales, and chutes or flumes (Glover

1978). They got the advantage that they avoid the infiltration of water in crevices, fractured

zones and the appropriated endangered areas. Also the use of shotcrete and sodium silicate is

a possibility to close such spaces. Controlling groundwater is an effective means of increasing

the stability of a slope. The purpose of subsurface drainage, i.e., groundwater control, is to

lower the water table and, therefore, the water pressure to a level below that of the potential

failure surfaces. Methods of subsurface drainage include drain holes, pumped wells, and

drainage galleries.

General recommendation for safety and rock removal measures in the Weißeritz valley

In the case of the Weißeritz valley control of groundwater isn´t useful cause the groundwater

table is below the affected rock slopes and their failure planes. The existing steep slope and

the overhangs represent a high danger for rock slides in the investigation area. Thereby sizes

of jointed rocks of more than 1 cubic meter are possible. For realization of the safety along

the railway and the footpaths there are some measures which are summarized in Table 4.

Thereby is to pay attention that all measures are attuned with the nature conservation agency.

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Table 4. – recommendated measures for stabilization of the rock slopes at the Weißeritz

Nr. Recommendations

1 removal of loose rock material; because the geological structur flattening of the slope

isn´t useful

2 anchorage and covering of the slopes with close spaced wire nets; the anchors at the

same time should subserve as attachments

3 placing of concrete seals as thrust bearing and protection of erosion

4 erection of catch fences for protection falling jointed rocks from higher slope areas

5 sealing of joints to avoid water infiltration; maybe draining wells

6 no removal of the vegetation, only in the critically areas should be cutted back

References

Brawner, C.O., 1994. Rockfall Hazard Mitigation Methods Participant Workbook.

NHI Course No. 13219. FHWA-SA-93-085. McLean, Va.: U.S. Department of

Transportation. Federal Highway Institute

Buss, K. , R. Prellwitz, and M.A. Reinhart. 1995. Highway Rock Slope Reclamation

and Stabilization Black Hills Region, South Dakota Part II, Guidelines. Report SD94-

09-G. Pierre, S.D.: South Dakota Department of Transportation.

Christopher, B.R. and R.D. Holtz. 1985. Geotxtile Engineering Manual. FHWA- TS-

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Institute

Coggan, J.S., Stead, D., Eyre, J., 1998. Evaluation of techniques for quarry slope

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