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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 For this purpose inquiry in litearture, archieves, historical
documents and discernments in corresponding maps are necessary.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.
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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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Coggan, J.S., Stead, D., Eyre, J., 1998. Evaluation of techniques for quarry slope
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