Physical Modelling of Permafrost Warming in Rock Slopes
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Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Physical modelling of permafrost warming in rock slopes M.C.R. Davies & O. Hamza School of Engineering, University of Dundee, Dundee, Scotland, UK C. Harris School of Earth Sciences, Cardiff University, Cardiff, Wales, UK ABSTRACT: An investigation into the influence on rock slope stability of warming permafrost has been con- ducted as part of the European PACE research programme into the effect of climate change on permafrost. This paper reports on the latest stage in this investigation. Failure mechanisms in jointed rock slopes containing ice- filled discontinuities were investigated in a series of geotechnical centrifuge model tests. As the temperature of the ice in the critical discontinuity increased, the shear strength of the joint reduced and some blocks forming the model slope became unstable. The results of the experiments help to identify mechanisms triggering failure of rock slopes in permafrost and are used to validate an analytical technique to assess the stability of rock slopes in these regions. The results of this study are also applicable for assessing the influence of anthropogenic activities on the stability of frozen rock slopes. 1 INTRODUCTION the shear strength of a simulated rock joint. In a series of laboratory direct shear box tests it was demonstrated As a result of measurement of permafrost temperatures that the shear strength of the ice-filled frozen joint is, in European Mountains – and other cold regions indeed, a function of both temperature and normal throughout the world – it is becoming clear that per- stress. However, when the results of the tests conducted mafrost is warming as a response to global climate on specimens with ice-filled joints were compared with change (Vonder Mühll et al. 1998, Vonder Mühll et al. control experiments in which the joint did not contain 2000). When assessing the stability of rock slopes ice, it was found that as the ice in the joint warms at cer- located in these cold regions in which the discontinu- tain temperatures and pressures, the ice-filled joint can ities contain ice it is generally assumed that this ice con- display less shear strength than an ice free joint. These tributes to maintaining stability (e.g. Bjerrum & Jørsbad results imply that, in slope stability assessment, if the 1968). Indeed, a reduction in rock slope stability with presence of ice in a joint is ignored (on the grounds that increase in ambient temperature has been attributed to ice will always add to shear strength and its absence rep- the loss of the stabilising influence of ice in joints when resents the most unstable conditions) then as ice in the it melts (e.g. Dramis et al. 1995). The phase change joint warms, conditions may arise where unexpected from ice to water has three potential effects. First, there failure could occur. The stability of such slopes may, is a loss of joint bonding, which is provided by ice/rock therefore, be more sensitive to changes in the thermal interlocking and “adhesion” of the ice to the rock. environment than previously envisaged. Second, the release of water may result in elevated water To test this hypothesis, Davies et al. (2001) used the pressures in the joint leading to a reduction in the effec- technique of geotechnical centrifuge modelling to tive pressure normal to the joint and, thus, a lowering of simulate the conditions of permafrost warming in its shear strength. Third, groundwater circulation may jointed rock slopes. Models of a slope with a simplified be re-established, further raising water pressures and geometry containing a single ice-filled discontinuity reducing shear strength. Although rock slope stability (i.e. a single potential sliding block) were constructed assessment techniques consider water pressures, they and permitted to warm during centrifuge operation. A do not take into account the effects of thermally induced range of slope angles and angles of inclination of the changes in ice properties. discontinuity were used in the models. The model The shear strength and stiffness properties of ice slopes were designed so that in the absence of ice in the are a function of temperature, e.g. Barnes et al. (1971), discontinuities they should have been stable (i.e. the Fish & Zaretsky (1997), and this will affect the shear angle of inclination of the discontinuity was lower capacity of a frozen rock joint. In order to quantify the than the angle of friction of the joint and, to prevent significance of such changes in material properties on the development of pore pressure in the joint as ice the stability of rock slopes containing ice-filled discon- melted, drainage paths were provided to remove water tinuities in the context of warming permafrost, Davies from the discontinuity). Nevertheless, in each case et al. (2000) investigated the influence of temperature on slope failure occurred whilst a large proportion of the 169 joint (typically approximately 40% of its length) still contained ice. The results of the experiments con- 4 3 firmed the hypothesis arising from the results of the W laboratory shear tests and also enabled the validation of a method to assess quantitatively the reduction in stabil- h 2 ity of slopes containing ice-filled discontinuities with H warming permafrost. 1 The study presented in this paper extends the investi- β gation of Davies et al. (2001) by considering a more complex slope geometry. The motivation for this is α that whilst the earlier study demonstrated clearly the fundamental mechanisms associated with the warming of ice-filled discontinuities, the simple geometry used for the investigation might not represent the wider range Figure 1. Geometry of centrifuge model containing a sin- of field conditions. In particular, these are generally gle ice-filled discontinuity inclined at an angle, b, with three more complex because rock slopes frequently contain vertical joints (blocks forming slope are numbered 1 to 4). more than one set of discontinuities. When surface tem- peratures increase, heat is conducted into the slope LVDT resulting in the warming and eventual melting of ice along the discontinuity on which failure might occur. If as this process occurs the joint system permits clo- sure along a length of the potential shear plane there will be a resulting increase in shear capacity that may be sufficient to avoid slope failure. This potentially + more complex mechanism was investigated in geo- + technical centrifuge model tests. 440 mm + 2 CENTRIFUGE MODEL TESTING 2.1 Model design 803 mm + Thermocouple location The stress-strain behaviour of rock joints is stress level dependent and can be highly non-linear. Therefore, to Figure 2. Cross-section of centrifuge model showing con- simulate the constitutive behaviour of such joints cor- struction and instrumentation. rectly it is necessary for prototype stress levels to be reproduced in a model (e.g. Schofield 1980). This slopes would be expected to be stable. The model slope, may be achieved by subjecting a 1/N scale model to of height H ϭ 0.365 m, was constructed in a centrifuge an elevated gravity field, N times the acceleration due strong box of internal dimensions 0.803 m long, 0.5 m to gravity (g) obtained by rotating the model in a large high and 0.6 m wide (Fig. 2). The prototype dimensions centrifuge. Centrifuge modelling laws indicate that of the model were, therefore, H ϭ 43.8 m and h ϭ 29 m. the time for conductive and advective heat transfer As can be seen from Figures 1 and 2, the potential slid- and diffusion processes – in the experiments reported ing zone contained three vertical discontinuities and herein the rates of temperature rise and pore pressure four potentially sliding blocks. Two model tests were dissipation, respectively – both scale as 1/N2. An expla- conducted for this geometry, one in which the discon- nation of the appropriate centrifuge modelling laws tinuity inclined at b ϭ 35° was ice-filled (the “frozen” for simulating cryogenic processes has been given else- model) and the other, which was a control experiment, where (e.g. Harris et al. 2000). in which the discontinuity contained no ice. Plane strain models were used to represent rock slopes using a model scale of 1/120 (i.e. at a centrifuge acceleration, N.g, of 120 g). The geometry selected 2.2 Configuration, preparation and for the models is shown in Figure 1, where the slope instrumentation of the models angle a ϭ 60° and the inclination of the discontinuity b ϭ 35°. The angle b was less than the angle of inter- To ensure consistency in joint roughness between mod- nal friction for the joint, which was obtained in a series els, Davies et al. (2001) developed a technique for con- of direct shear tests. Thus in the absence of ice the structing slopes from concrete having similar physical 170 properties to granite. This involved casting blocks form- acquisition system they were accelerated to the test ing the slope in moulds of the appropriate geometry and acceleration of 120 g. As expected, the control slope internal roughness. In the study reported herein, a simi- remained stable following acceleration to 120 g. This lar process was used to form the slopes. The only differ- provided confirmation that shear stresses acting on ence was that the faces of the blocks, which formed the the inclined plane were not sufficient to shear off part vertical joints, were cast against a flat steel plate, whilst of the concrete asperities; which might result in a sig- the faces forming the inclined discontinuity were cast nificant reduction in the angle of friction of the joint. against a sheet of profiled steel.