movements in massive rock slope is needed a change in the geometry of the ...
Some releasing facts for failure mechanism are earthquakes, erosion, young ...
Classification of Massive Rock Slope and a Monitoring System for this Failure
Erik Fillinger Maxener Str. 5 01809 Dohna
Abstract This paper examines failure mechanisms of sliding and toppling rock slops, where a massive catastrophic failure follows. Processes of catastrophic failure requires a favorable structural configuration which occur more in strong, brittle rocks than in weak material. These events are use for a classification of failure mechanisms. The second part shows a monitoring system for unstable slopes. It is a new system for landslide monitoring which use the Time Domain Reflectometry (TDR). Two components are use for the TDR system: a coaxial cable and a TDR measuring device. Into the drilled borehole the coaxial cable is installed. The free space between cable and the borehole face is fill out with grout. Consequently the grout with the cable and the borehole are coupled. To get any value for the TDR measuring device an electric pulse is send from the measuring instrument through the cable and receives and analyses the reflection. For a determination of type and amplitude of movements in massive rock slope is needed a change in the geometry of the coaxial cable in surrounding rock. The result is a characteristic signature at the measuring device. The exact value determination only works when the propagation velocity of the electric pulse is known and constant. Under these circumstances the deformation can be acquired by measuring the time span between the initiation of the electric pulse and the detection of its reflection. TDR measurements are influenced by many parameters. Some important influences are the type of deformation, cable and grout. By using more TDR measurement stations a better knowledge about position, width and type of the deformation zone in landslides regions is possible. Because of a continuously value testing without staying at the investigation area you can determinate effects, like strong raining events, nearly in real time. These influences can determinate landslides events. Collecting these values a better evaluation is achievable.
Conditions and releasing facts for landslides It exists four conditions which influence landslides: geological, climatically and hydro geological, geomorphologic and “man made”. The important ones are high erosion and rain events (pic 1) but with the influence of human being. Some releasing facts for failure mechanism are earthquakes, erosion, young tectonic, groundwater, and change of vegetation, weather-beating and more. These facts are controlled from two different conditions: naturally and anthropogenic. Natural circumstances are unusually rain events, earthquakes, frostbitten, under flushing and melt water. Anthropogenic circumstances are accumulation buildings, blasting, buildings and angle changing of the slope.
Pic 1: Conditions for landslides
Mobility of rotational and translational rockslides Rotational sliding occurs in weak rocks like as shale, marl or tuff. These types are isotropic or structured so that the weak direction is horizontal or dips opposite to the slope direction. How in picture 2.
Pic 2: rotational sliding (ingenieur geologic II file) This mechanism is intrinsically self-stabilizing. The resolution is a backward rotation which diminishes the slope angle. These events are typically slow-moving and they continued over month. Only in the crest the movement is inherently higher. The failure behavior isn’t exactly the same in several slopes. Each slope consists of individual structural pattern which are demonstrate in different shearing of the weak rock masses.
Rock Collapse Rock collapse in stronger rock are characterized of shearing processes at discontinues which build “bridges”. These bridges are isolated of intact rock and consist of brittle cohesive components. The rupture surface is created of crushing, tensile and shearing. Striking forms are irregular, multi-faceted scars. Subsequent there is no systematical structural pattern and they were called rock collapse. (pic 3). Such pattern exists by intrusive material, or by strong structured rocks whose structure is adverse to sliding or toppling, but with enough ruptures. They occur on steep slopes and tend to be smaller than translational slides. When such a slope collapse began to move, it happened mostly sudden with a catastrophic ending.
Pic 3: rock collapse (ingenieur geologic II file)
Stability Number The stability number is theoretically used for a determination to define an upper limit of rock strength. It is an effort to get a define number to distinctive ductile rotational slump failure and stronger rock collapse failure.
Pic 4: stability number Ns (Paper Hungr and Evans: The Occurance and Classification of Massive Rock Slope Failure) Here, γ is the unit weight of the rock material. H is the height of the slope that means from crest to toe of the rupture surface. σc is the uniaxial compressive strength of the rock (mechanic potential). A number of known measurement values were taken from rock sliding and rock collapse events. The conclusion of these attempts is that the stability number should be considerably higher 0.25 to allow the occurrence of a slow ductile rock slump. Also seen is a continuous transition from slow rock slumps to catastrophic collapses, but without any fixed boundary.
Classification of failure mechanisms It exist two important characterization of rock slope failure for a typological classification. The first one is the role of rock structure and the second one is the mechanical property of rock mass. The table 1 can subdivide in three units. The first unit includes two none systematic structural controlled failure mechanisms. The second unit and third are systematic structural controlled. Characterized are these one of their patterns, like faults, weak layers or one or more definite discontinuity. Unit two and three displays sliding and toppling.
Tabelle 1: classification ot failure mechanisms (Hungr and Evans (2006): The Occurrence and Classification of Massive Rock Slope Failure)
Types of rockslides: Type A – rock slump This type moves generally slow or at the most rapid. It occurs in shales, mud rocks or in weathered metamorphic rocks. It is a self-stabilization rotational driving mechanism. In this system the stability number must be higher than 0,5
Pic 5: Rock slumping (ingenieur geologic II file)
Type B – rock collapse This type is characterized by shearing along random discontinuities. It occurs in strong rock like in volcanic, plutonic or low-grad metamorphic rocks and in massive sandstones or carbonates. The detachment surface is steep and irregular. The trigger is a weak brittle basement. The movement rate of the collapse is extremely rapid and the volume of material is limited.
Pic 6: Rock collapse (ingenieur geologic II file)
Type C – translational rock block or wedge slide This type is characterized by a weakness planes, dipping at or near to the slope direction. It is a movement on a single or two discontinuities. The movement rate is extremely rapid and occurs in moderately-folded sedimentary rocks, in special case carbonates and sometimes in recent volcanic.
Pic 7: translational rock block or wedge slide (ingenieur geologic II file)
Type D – structurally-defined compound slide This type is characterized by defined curving discontinuities or different discontinuities. They occur in folded and faulted rock masses with a strong internal connection. During the movement of the sliding body a internal deformation starts and move extremely rapid.
Pic 8: structurally-defined compound slide (ingenieur geologic II file) Type E – block slide with toe breakout This type is characterized by the rock strength. They occur in steeply folded sedimentary rocks or foliated metamorphic rocks. That means the mechanism failure could be in weak rocks as slow slumps and in strong rocks as collapse.
Pic 9: block slide with toe breakout (ingenieur geologic II file)
Type F – compound slide This type is characterized by a controlled gently-dipping weak surface at the toe, with a steep back slope passing through intact rock in the crest area. It occur in weak, flat-lying bedded sedimentary rocks. At the crest is forms a graben. The movement is nearly like in Type A.
Pic 10: compound slide (ingenieur geologic II file) Type G – flexural toppling This type is characterized by flexural toppling and a low strength rock mass. It occurs in schist, phyllite or slate, less gneiss or sedimentary rocks. The mechanism failure is self-stabilizing like in type A.
Pic 11: flexural toppling (ingenieur geologic II file)
Type H – block toppling This type is characterized by strength rock mass with a rotational failure. The massive rock shows a steeply down slope-dipping. The slope is stress free only at the beginning is a deformation. The main stress is vertical. This mechanism failure can produce a large catastrophic slope and rock avalanches, but often not the entire rock mass collapse.
Pic 12: block toppling (ingenieur geologic II file)
Function of a TDR A TDR works like a radar system. Two components are use for the TDR system: a coaxial cable and a TDR measuring device. Into the drilled borehole the coaxial cable is installed. The free space between cable and the borehole face is filled out with grout. Consequently the grout with the cable and the borehole are coupled (pic 13).
Pic 13: installed profile of a TDR monitoring system (Hungr and Evans (2006): The Occurrence and Classification of Massive Rock Slope Failure)
To get any value for the TDR measuring device an electric pulse is send from the measuring instrument through the cable and receives and analyses the reflection. For a determination of type and amplitude of movements in massive rock slope is needed a change in the geometry of the coaxial cable in surrounding rock. The result is a characteristic signature at the measuring device (pic 13). The exact value determination only works when the propagation velocity of the electric pulse is known and constant. Under these circumstances the deformation can be acquired by measuring the time span between the initiation of the electric pulse and the detection of its reflection. TDR measurements are influenced by many parameters. Some important influences are the type of deformation (Pic 14), cable and grout.
Pic 14: two results of two different mechanism failures (Hungr and Evans (2006): The Occurrence and Classification of Massive Rock Slope Failure) In picture 14 are two different types of mechanism failure to face. A distinction between the two stress events is seen in the deformation value (pic 14 1b) and 2b)). At the shear deformation the strain rate is four times higher than at the compression. These distinctive differences can use for a determination for several mechanism failure. In this case you can do distinguish shear from compression deformation. In picture 14 1a) and 1b) you can recognize where the rupture surface lies. In picture 14 1a) the rupture lies by 302 cm from the top surface. In picture 14 2a) the rupture lies by 153 cm from the top surface. In both cases the time span between the
initiation of the electric pulse and the detection of its reflection at the rupture were the reflection coefficient fall was measured. For that measurement results you can always assign the depths value for the rupture surfaces.
Conclusions Eight types could be determined for a better understanding of massive rock slop failure. The type A, F and G occur in weak rocks and is basically ductile. These types have a limited displacement and are always slow moving. Type B and H are sometimes scale catastrophic slope failures, the detachment surface is steep and they occur in massive strong rocks. Type C and D occur in folded rock masses and the movement of the slide is extremely rapid. The movement of the block slide in Type E could be in weak material slow but in strength rocks collapse. The stability number should be considerably higher 0.25 to allow the occurrence of a slow ductile rock slump. Also seen is a continuous transition from slow rock slumps to catastrophic collapses, but without any fixed boundary. The TDR monitoring system is a really interesting method for measurement movements in unstable slopes. The measured value for the distinction of the rupture surface can readily used for a development 3D-monitoring system. It is an uncomplicated measurement with low sources of error and with more values some one could characterize the deformation types of unstable rock slopes. The only bad case is that the orientation of the slope movement couldn’t be to determine, but also this could be with more than one TDR at one unstable slope failure.
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