Natural Rock Mass Research Articles

Mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence

[1] Pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof. To identify how geometric and mechanical factors may influence the structural evolution of this subsidence, we used two-dimensional distinct element method numerical models. The reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic-frictional laws. Varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses. Fracturing results when bonds break, once their shear or tensile strength is exceeded. The magma reservoir was represented as a region of nonbonded low-friction particles. Withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles. Resultant gravity-driven failure and subsidence of the reservoir roof were explicitly replicated. Interaction of the roof's strength, Young's modulus, thickness/diameter ratio (T/D), and the reservoir's shape yields a variety of model structures and subsidence styles. In conceptual terms, four end-member subsidence styles developed: (1) “central sagging” favored by low strength and low T/D; (2) “central snapping” favored by high strength, low T/D, and a sill-like reservoir shape; (3) “single central block” favored by low to intermediate strength, high Young's modulus, and intermediate T/D; and (4) “multiple central blocks” favored by high strength, low Young's modulus, and high T/D. Most model realizations incorporated some combination of each style, however. The models provide a geomechanical framework for understanding natural pit crater or caldera structures, as at Nindiri (Nicaragua), Fernandina (Galapagos), Dolomieu (La Reunion), and Miyakejima (Japan).

Anisotropic damage coupled modeling of saturated porous rock

It is widely acknowledged that the natural rock mass is anisotropic and its failing type is also non-isotropic. An orthotropic elastic damaged model has been proposed in which the elastic deformation, the damaged deformation and irreversible deformation can be identified respectively. A second rank damage tensor is employed to characterize the induced damage and damage evolution related to the propagation conditions of microcracks. A specific form of the Gibbs free energy function is used to obtain the effective elastic stiffness and the limited scopes of damage parameters are suggested. The model’s parameter determination is proposed by virtue of conventional tri-axial test. Then, the proposed model is developed to simulate the coupled hydraulic mechanical responses and traction behaviors in different loading paths of porous media.

Characterizing and Locating Very Weak (−2.2 ≥ M L ≥ −3.4) Induced Seismicity in Unstable Sandstone Cliffs by Nanoseismic Monitoring

Unloaded natural rock masses are known to generate seismic signals (Green et al., 2006; Hainzl et al., 2006; Husen et al., 2007; Kraft et al., 2006). Following a 1,000 m3 mass failure into the Mediterranean Sea, centimeter-wide tensile cracks were observed to have developed on top of an unstable segment of the coastal cliff. Nanoseismic monitoring techniques (Wust-Bloch and Joswig, 2006; Joswig, 2008), which function as a seismic microscope for extremely weak seismic events, were applied to verify whether brittle failure is still generated within this unconsolidated sandstone mass and to determine whether it can be detected. Sixteen days after the initial mass failure, three small-aperture sparse arrays (Seismic Navigation Systems-SNS) were deployed on top of this 40-m high shoreline cliff. This paper analyzes dozens of spiky nanoseismic (−2.2 ≥ M L ≥ −3.4) signals recorded over one night in continuous mode (at 200 Hz) at very short slant distances (3–67 m). Waveform characterization by sonogram analysis (Joswig, 2008) shows that these spiky signals are all short in duration (>0.5 s). Most of their signal energy is concentrated in the 10–75 Hz frequency range and the waveforms display high signal similarity. The detection threshold of the data set reaches M L −3.4 at 15 m and M L −2.7 at 67 m. The spatial distribution of source signals shows 3-D clustering within 10 m from the cliff edge. The time distribution of M L magnitude does not display any decay pattern of M L over time. This corroborates an unusual event decay over time (modified Omori’s law), whereby an initial quiet period is followed by regained activity, which then fades again. The polarization of maximal waveform amplitude was used to estimate spatial stress distribution. The orientation of ellipses displaying maximal signal energy is consistent with that of tensile cracks observed in the field and agrees with rock mechanics predictions. The M L– surface rupture length relationship displayed by our data fits a constant-slope extrapolation of empirical data collected by Wells and Coppersmith (1994) for normal fault features at much larger scale. Signal characterization and location as well as the absence of direct anthropogenic noise sources near the monitoring site, all indicate that these nanoseismic signals are generated by brittle failure within the top section of the cliff. The atypical event decay over time that was observed suggests that the cliff material is undergoing post-collapse bulk strain accommodation. This feasibility study demonstrates the potential of nanoseismic monitoring in rapidly detecting, locating and analyzing brittle failure generated within unconsolidated material before total collapse occurs.

Correlation between natural slope angle and rock mass strength rating in the Betic Cordillera, Granada, Spain

This study proposes a method to determine zones gentler or steeper than the equilibrium slope angle based on rock mass strength (RMS) and to regionalize the parameters. The method was developed using GIS and applied to rock masses of the Alpujarride Complex (Granada, Spain). 41% of the natural slopes studied have angles lower than the equilibrium due to dissolution processes and the presence of alluvial or colluvial deposits, while 28% of those slopes have angles higher than the equilibrium due to rapid erosion in tectonically active zones. Cross correlation analysis between maps showing degrees of slope equilibrium and landslide inventories suggest that long-term geomorphic changes have induced slope angles higher than the equilibrium which are associated with rock falls and rock slides.

A numerical study on the effect of heterogeneous/anisotropic nature of rock masses on displacement behavior of tunnel

The structural anisotropy and heterogeneity of rock mass, caused by discontinuities and weak zones, have a great influence on the deformation behavior of tunnel. Tunnel construction in these complex ground conditions is very difficult. No matter how excellent a geological investigation is, local uncertainties of rock mass conditions still remain. Under these uncertain circumstances, an accurate forecast of the ground conditions ahead of the advancing tunnel face is indispensable to safe and economic tunnel construction. This paper presents the effect of anisotropy and heterogeneity of the rock masses to be excavated by numerical analysis. The influences of distance from weak zone, the size or dimension, the different stiffness and the orientation of weak zones are analysed by 2-D and 3- D finite element analysis. The tunnel behavior is largely influenced by thickness as well as orientation of weak zones. By analysing these numerical results, the tunnel behavior due to excavation can be well understood and the prediction of rock mass condition ahead of tunnel face can be possible. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.

Stress dependent mechanical properties and bounds of poisson's ratio for fractured rock masses investigated by a DFN-DEM technique

The equivalent elastic modulus is a parameter for controlling the deformation behavior of fractured rock masses in the equivalent continuum approach. The confining stress, whose effect on the equivalent elastic modulus is of great importance, is the fundamental stress environment of natural rock masses. This paper employs an analytical approach to obtain the equivalent elastic modulus of fractured rock masses containing random discrete fractures (RDFs) or regular fracture sets (RFSs) while considering the confining stress. The proposed analytical solution considers not only the elastic properties of the intact rocks and fractures, but also the geometrical structure of the fractures and the confining stress. The performance of the analytical solution is verified by comparing it with the results of numerical tests obtained using the three-dimensional distinct element code (3DEC), leading to a reasonably good agreement. The analytical solution quantitatively demonstrates that the equivalent elastic modulus increases substantially with an increase in confining stress, i.e. it is characterized by stress-dependency. Further, a sensitivity analysis of the variables in the analytical solution is conducted using a global sensitivity analysis approach, i.e. the extended Fourier amplitude sensitivity test (EFAST). The variations in the sensitivity indices for different ranges and distribution types of the variables are investigated. The results provide an in-depth understanding of the influence of the variables on the equivalent elastic modulus from different perspectives.

Geophysical studies of rock masses : Savich, A I; Mikhailov, A D; Koptev, V I; Iijin, M M Proc 5th Congress of the International Society for Rock Mechanics, Melbourne, 10–15 April 1983V1, PA19–A30. Publ Rotterdam: A. A. Balkema, 1983

The basic results and methods of geophysical studies of rock masses in the USSR are presented. Three main objectives of studies are considered: studies of the structure, properties and the state of natural rock masses, studies of change of their properties as a result of mining and construction work and studies of the dynamics of deformation processes going on in the foundation rock of the structures in operation. The number of the covering abstract of the congress is TRIS no. 385148. (TRRL)

Earthquake precursory effects due to pore fluid stabilization of a weakening fault zone

We report the analysis of two mechanisms by which pore fluids could partially stabilize the earthquake rupture process in natural rock masses. These mechanisms are based on dilatancy strengthening and on the increase of elastic stiffness for undrained as opposed to drained conditions. Both are studied in relation to an inclusion model in which a zone of strain weakening material, possibly representing a highly stressed seismic gap zone, is embedded in nominally elastic surroundings subjected to steadily increasing tectonic stress. Owing to the coupling between deformation and pore fluid diffusion, the inclusion does not exhibit an abrupt rupture instability; rather, a period of self‐driven precursory creep occurs which ultimately accelerates to dynamic instability. The precursory time scale is reported for a wide range of constitutive parameters, including fluid diffusivity, ratio of undrained to drained stiffness, and factors expressive of strain softening and dilatancy. Our conclusions are that the precursory times for a spherical inclusion of 1‐km radius are of the order of 15–240 days for a range of constitutive parameters that we suggest are representative. The predicted times are shorter by a factor of approximately 10 for a flattened ellipsoidal inclusion that we analyze with an 18 : 1 aspect ratio. It is suggested that perhaps only toward the latter part of the precursory period are the effects of accelerating inclusion strain detectable in terms of surface deformation or alteration of transport or seismic properties.

Shear strengths of fissures in ledge rock before and after grouting

1. Cement grouting of fissured ledge rocks leads to a marked reduction in slip deformation due to an inclined load. The effect of cementation is most marked in badly fractured rocks, the slip of which in the natural state is characterized by an elongated horizontal-displacement graph. 2. Cement grouting leads to an increase in the shear strength of hard rocks. Cementation is most effective in badly fractured rocks without fine earth filling, owing to the efficient filling of the cracks by the cement solution. (We must, however, remember that such conditions are not always found in natural rock masses.) 3. The use of cement solutions based on dispersed cement (colloidal cement or dry-vibromilled cement) permits cement grouting to be more effective than with ordinary cement. 4. The presence of badly broken sections in a rock foundation is not necessarily a reason for reduced slip resistance parameters or for removal of much of the rock, and in such cases we should always consider whether it is advantageous to use cement grouting to improve the quality of the rock.

Laboratory experiments on the magnetization of rocks

Specimens of natural rock masses collected by H.M. Geological Survey were tested for magnetic susceptibility and permanent magnetization before and after various artificial treatments. After cooling in the earth's field, cut cubes were found to be magnetized with an intensity much greater than that of the natural rock, and the values decayed very little with time. When artificially magnetized in the cold, the cubes were only affected by fields above a certain value, and the decay was often considerable. Curves are given showing the decay with time and the demagnetization of the heated cubes by increasing fields. The susceptibility of natural rocks increases with the field, in some cases reaching a maximum between 50 and 100 C.G.S. These results are compared with data by J. G. Koenigsberger.

Laboratory determinations of the magnetic properties of certain igneous rocks

Although the methods of magnetic survey in the field are now well estab­lished, there have been relatively few direct determinations of the magnetic properties of the rocks concerned. Conclusive evidence has now been obtained that some of the natural rock-masses are permanently magnetized ; this effect is superimposed upon the magnetization induced by the earth’s field, and in certain rocks it appears that even a small permanent magnetization may seriously modify the magnetic profiles. Experiments were therefore under­taken with the general object of linking up the evidence provided by field surveys with that obtainable in the laboratory. The material was chosen with a view to providing an experimental basis for the interpretation of the magnetic surveys that have been completed by the Geological Survey during the past few years. The collection and prepara­tion of the specimens were undertaken by the Survey, and the laboratory measurements have been carried out by Professor E. F. Herroun, to whom the author has been greatly indebted for criticism and advice throughout this work.