BLÜMEL AND SEMPRICH: SLOPE INSTABILITY – IN-SITU AND LABORATORY TESTING Slope Instability – In-situ and Laboratory Testing By Manfred Blümel and Stephan Semprich ROCK SLOPES n order to estimate the risk of slope instabili- geophysicists, and engineers is necessary to ade- Ities, representative parameters to describe the quately characterize complex geologic situations soil and rock material behaviour are needed. for engineering purposes. Apparently simple Geotechnical parameters could be separated into questions like: “What are the adequate test meth- different groups, like geophysical, mineralogical, ods, the location of the test or specimen, the hydrological, and mechanical parameters in ad- amount of tests and how can we get appropriate dition to parameters that describe the rock mass samples?” are important. structure. However, these groups should not be To achieve a realistic evaluation, specifying separated, because they are all part of a sphere of representative procedures and the testing pro- influence on the material behaviour. For in- gramme is necessary. The determination of pa- stance, a variation in the mineralogical and hy- rameters in laboratory and in-situ tests is an drological parameters influences the mechanical essential part of the characterization process. parameters. The relevant boundary conditions Because rock and the rock mass is inhomogene- and parameters for a certain project should be ous, one have to deal with wide distributions in discussed in the investigation phase. For this the parameter values and to determine which complex matter, teamwork between geologists, parameters are necessary for a given rock mass Fig. 1 Some exem- Böschungsinstabilität – plary factors of in- In-situ- und Laborversuche fluence on strength. Bild 1 Einige Ein- Mit Hilfe von In-situ- und Laborversuchen flussfaktoren auf die können boden- und felsmechanische Para- Festigkeit. meter bestimmt werden. Welche Versuchs- methoden geeignet sind, hängt von den je- weiligen Randbedingungen ab. Böschungs- untersuchungen stellen für die Versuchs- technik eine besondere Herausforderung dar, da oft stark gestörtes, verwittertes Ma- terial angetroffen wird und herkömmliche Methoden der klassischen Boden- bezie- hungsweise Felsmechanik nicht ausreichend geeignet sind. Eine umfassende Betrachtung Fig. 2 In-situ direct der jeweiligen Randbedingen ist daher be- shear test. sonders wichtig. In dieser Arbeit werden ei- Bild 2 Direkter nige Beispiele von Versuchsmethoden vorge- In-situ-Scherversuch. stellt. The mechanical parameters for soil and rock can be determined with in-situ and laborato- ry tests. The appropriate methods are de- pendent on the boundary conditions expect- ed in the field. Especially slope stability anal- ysis are a challenge for testing techniques, because one often have to deal with faulted, weathered rock masses where the classical soil and rock mechanics approaches are not sufficient enough. Therefore, a comprehen- sive view considering the relevant boundary conditions is of utmost importance. This pa- per shows some examples of advanced test- ing methods, which begin to address some of these insufficiencies. FELSBAU 21 (2003) NO. 2 39 BLÜMEL AND SEMPRICH: SLOPE INSTABILITY – IN-SITU AND LABORATORY TESTING Fig. 3 Results of Some exemplary factors that influence the direct shear tests. strength are shown in Figure 1. Each of these Bild 3 Ergebnisse der direkten Scher- influence factors can change the strength param- versuche. eter. If a non-comprehensive view is considered reasonable results we cannot be expected. To formulate the facts there are different stages, identification, classification, characterization, interpretation and verification. Quantifying the rock and rock mass behaviour will always be a challenge; appropriate testing procedures and analyses are the first step for a more realistic ROCK SLOPES evaluation. Therefore, testing and the testing Fig. 4 Gravel pit with a slope inclina- results should not be irrespective of the entire tion of 80°. project. Bild 4 Schotter- Testing methods can be divided into two grube mit einer groups, in-situ and laboratory tests, depending Böschungsneigung von 80°. on where the tests are performed. Typical in-situ tests, performed without removing samples from their natural position, are geophysical tests, pie- zometer tests, large-scale tests (e.g. in-situ shear test, embankment test), tilt test, torque vane test, penetrometer or dilatometer tests. The usual laboratory tests are direct shear tests, unconfined and confined compression tests, determination of water content, density, plasticity, mineral com- position, permeability tests, (particle) grain size analysis, thin sections, swelling tests, etc. The classical mechanical parameters for slope stability analysis are cohesion, friction, dilatancy, shear stiffness (hardening and softening), (joint) compressive strength, joint asperity geometry (roughness coefficient). Because the failure mechanisms are a very complex process and influenced by the boundary characteristics, as to properly convey lab behaviour to the real mentioned before, it is often necessary to leave situation. standardized procedures. Fig. 5 Large scale The shear strength is the main parameter for This paper shows some examples of tests to in-situ test for waste slope stability analysis and can be described by determine the mechanical parameters for slope removal at Fischer Deponie, Theresien- either a linear or non-linear failure criterion. One stability problems. feld, Austria. must recognize that strength is not an independ- Bild 5 In-situ-Groß- ent parameter. Strength can only be described if In-situ tests of quaternary soil versuch für die Räu- the boundary conditions are defined. This is also mung der Fischer Deponie, Theresien- valid for a differentiation between the peak and The first example describes test procedures used feld, Österreich. residual strength. to investigate the shear strength of quaternary soil. The most relevant strength parameters, the cohesion and friction angle, of fine-grained soil such as clay or silt are very often investigated by direct shear tests or triaxial tests. These tests are carried out on undisturbed soil samples in a laboratory. For cohesive soils, sampling and test preparation is typically unproblematic. However, retrieving high quality, undisturbed samples in cohesionless materials is very difficult and often requires additional techniques such as freezing. Besides, coarse soils, such as gravels, require relatively large sample dimensions and respec- tively the test equipment. With respect to economical reasons investiga- tions of the shear strength of coarse soils are mostly performed on disturbed probes, which consist only of the soils fine grain portion because the coarse grains are removed by sieving previ- ously. Consequently the expressiveness of results 40 FELSBAU 21 (2003) NO. 2 BLÜMEL AND SEMPRICH: SLOPE INSTABILITY – IN-SITU AND LABORATORY TESTING UrgentSubscription Technical journals with competence ROCK SLOPES Verlag Glückauf Essen P.O.Box 18 56 20 D-45206 Essen Montebruchstraße 2 D-45219 Essen Phone +49 (0) 20 54 / 92 41 21 Telefax +49 (0) 20 54 / 92 41 29 E-Mail [email protected] Internet www.vge.de I would like to subscribe to the international edition of Felsbau · Rock and Soil Engineering. Journal for ❑ at an annual subcription Engineering price of 65.50 EUR Geology, ❑ I like to order the Rock and Soil Engineering Geomechanics 2002 CD-ROM at a price of 50.00 EUR and Tunnelling ❑ I would like to familiarize myself with Felsbau · Rock and Soil Engineering. Please send me a specimen copy free of charge. Edited by Austrian Society for Geomechanics ❑ I am interested in advertising in (ÖGG) Felsbau · Rock and Soil Engineering. Published in English Please send me your media information. thrice annually International subscription 65.50 EUR Date, Signature (airmail, overseas) The leading Austro-German technical journal contains contri- butions from the field of inter- national rock and loose rock structures and reports on current tunnelling projects from surveying via planning to construction work. ORDER COUPON Please fax to: +49 (0) 20 54 / 92 41 29 41 / 92 54 20 (0) Please +49 fax to: My address FELSBAU 21 (2003) NO. 2 41 BLÜMEL AND SEMPRICH: SLOPE INSTABILITY – IN-SITU AND LABORATORY TESTING The results of six in-situ shear tests performed with different normal stresses are shown in Figure 2 3. Based on these results, a cohesion ck = 30 kN/m ϕ ° and a friction angle k = 40 have been derived. These figures have been used for stability analyses of slopes for several gravel pits (Figure 4). A similar task appears at waste disposal site, the „Fischer Deponie“, located 20 km east from the above mentioned construction site with the same subsoil conditions. At the Fischer Deponie slopes Fig. 6 Rock mechanic with heights up to 20 m have to be designed. laboratory equipment. ROCK SLOPES Bild 6 Felsmechanik However, for the first phase of stability analyses no Laborausstattung. site-specific strength data from shear tests either in the laboratory or in-situ were provided. Conse- from laboratory shear tests of coarse grain soils is quently, strength parameters were chosen with 3 ϕ ° limited and in important cases in-situ shear tests conservative values: ck = 26,5 kN/m and k = 35 . are necessary. For a second phase of analyses investigating Figure 2 shows an in-situ shear test carried out more economical solutions undisturbed site specif- on a construction site 50 km south of Vienna using ic shear strength