GEOLOGICAL ENGINEERING INVESTIGATION OF ROCK SLOPES AT THE EXAMPLE OF THE WEIßERITZTAL RAILWAY BETWEEN HAINSBERG AND SEIFERSDORF IN SAXONIA GERMANY

Size: px

Start display at page:

Download "GEOLOGICAL ENGINEERING INVESTIGATION OF ROCK SLOPES AT THE EXAMPLE OF THE WEIßERITZTAL RAILWAY BETWEEN HAINSBERG AND SEIFERSDORF IN SAXONIA GERMANY"

Sharleen Gaines
6 years ago
Views:

1 GEOLOGICAL ENGINEERING INVESTIGATION OF ROCK SLOPES AT THE EXAMPLE OF THE WEIßERITZTAL RAILWAY BETWEEN HAINSBERG AND SEIFERSDORF IN SAXONIA GERMANY 2006 BENJAMIN PAUL 1 1 TU 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 For this purpose inquiry in litearture, archieves, historical documents and discernments in corresponding maps are necessary

2 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) - 2 -

3 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

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.).

4 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) - 4 -

5 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 examples factors examples removal of erosion by streams or rivers, changes in lateral wave action on lakes, shear strength support glaciers due to weathering addition of weight of rain, hail, snow or changes in surcharge to water intergranular the slope forces due to transistory earth stresses removing of underlying support lateral pressure vibrations of earthquakes, traffic, etc. road construction, sqeezing out of underlying material water in pore spaces, cavern or cavities, freezing of water Factors that contribute to low or reduced shear strength. water content changes in structure miscellanious causes softening of fissured clays, hydration or dehydration of clay minerals caused by: rapid drawdown of a lake or reservoir, rapid changes in the elevation of the water table caused by remolding clays upon disturbance, by the fissuring of shales and precons. Clays weakening of a slope due to progressive creep or due to actions of roots 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)

6 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 -6-

7 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 = [(G N W N ) * tanφ] / G T + W T N: weight of the sliding block vertical to the sliding plane [kn] G T : weight of the sliding block parallel to the sliding plane [kn] W N : pressure of joint water vertical to the sliding plane [kn] W T : 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 ( ) - 7 -

8 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

9 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 - 9 -

10 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 soillike 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

11 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., Rockfall Hazard Mitigation Methods Participant Workbook. NHI Course No FHWA-SA McLean, Va.: U.S. Department of Transportation. Federal Highway Institute Buss, K., R. Prellwitz, and M.A. Reinhart 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 Geotxtile Engineering Manual. FHWA- TS Washington, D.C.: Federal Highway Administration, National Highway Institute Coggan, J.S., Stead, D., Eyre, J., Evaluation of techniques for quarry slope stability assessment. Trans. Inst. Min. Metall., Sect. B: Appl. Earth Sci. 107, B 139- B

12 Glover, F., M. Augustine and M. Clar Grading and Shaping of Erosion Control and Rapid Vegetative Establishment in Humid Regions. In Reclamation of Drastically Disturbed Lands. Edited by F.W. Schaller and P. Sutton. Madison, Wis.: American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. Hoek, E., and J.W.Bray Rock Slope Enineering. London: Institute of Mining and Metallurgy Kliche, C.A Rock SLope Stability, Society for Mining, Metallurgy, and Exploration, Inc. (SME) Pregl, O Böschungen. Selbstverlag des Institutes für Geotechnik und Verkehrswesen. Universität für Bodenkultur Wien Stead, D., E. Eberhardt, J.S. Coggan Developements in the Characterization of complex rock slope deformation and failure using numerical modelling techniques. Engineering Geology vol. 83 no p Seegmiller, B.L Artificial Support of Rock Slopes. In Stability in Surface Mining, Vol. 3. Edited by C.O. Brawner. New York: Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers. Varnes, D.J Slope Movement Types and Processes. In Landslides, Analysis & Control. Edited by R.L.Schuster and R.L. Krizek. Special Report 176. Washington D.C.: Transportation Research Board, Commission on Socioltechnical Systems, National Research Council, National Academy of Sciences. Yang, Xiaou-Li, Zou, Jin-Feng Stability factors for rock slopes subjected to pore water pressure based on the Hoek- Brown failure criterion. International Journal of Rock Mechanics & Mining Sciences vol. 43, no.4, p

13 Yang, Z.F., L.Q. Zhang, Y.L. Shang, Q.L. Zeng, L.H. Li Assessment of the degree of reinforcement demand (DRD) for rock slope projects- principles and a case example application. International Journal of Rock Mechanics & Mining Sciences vol. 43, no. 7, p

CHAPTER FIVE 5.0 STABILITY OF CUT SLOPES IN THE STUDY AREA. them limited by a thick canopy of vegetation and steep slope angles.

CHAPTER FIVE 5.0 STABILITY OF CUT SLOPES IN THE STUDY AREA 5.1. Introduction Ukay Perdana area is a developing community with continuous building activities and road construction. There are thus only left