GIS-Based landslide susceptibility modeling: a comparison between best-first decision tree and its two ensembles (BagBFT and RFBFT)

This study aimed to explore and compare the application of current state-of-the-art machine learning techniques, including bagging (Bag) and rotation forest (RF), to assess landslide susceptibility with the base classifier best-first decision tree (BFT). The proposed two novel ensemble frameworks, BagBFT and RFBFT, and the base model BFT, were used to model landslide susceptibility in Zhashui County (China), which suffers from landslides. Firstly, we identified 169 landslides through field surveys and image interpretation. Then, a landslide inventory map was built. These 169 historical landslides were randomly classified into two groups: 70% for training data and 30% for validation data. Then, 15 landslide conditioning factors were considered for mapping landslide susceptibility. The three ensemble outputs were estimated with a receiver operating characteristic (ROC) curve and statistical tests, as well as a new approach, the improved frequency ratio accuracy. The areas under the ROC curve (AUCs) for the training data (success rate) of the three algorithms were 0.722 for BFT, 0.869 for BagBFT, and 0.895 for RFBFT. The AUCs for the validating groups (prediction rates) were 0.718, 0.834, and 0.872, respectively. The frequency ratio accuracy of the three models was 0.76163 for the BFT model, 0.92220 for the BagBFT model, and 0.92224 for the RFBFT model. Both BagBFT and RFBFT ensembles can improve the accuracy of the BFT base model, and RFBFT was relatively better. Therefore, the RFBFT model is the most effective approach for the accurate modeling of landslide susceptibility mapping (LSM). All three models can improve the identification of landslide-prone areas, enhance risk management ability, and afford more detailed information for land-use planning and policy setting.National Natural Science Foundation of China | Ref. 41977228Key Research Program of Shaanxi | Ref. 2022SF-33

Size-dependent behaviour of hard rock under triaxial loading

Understanding size effects is important to rigorously analyse the behaviour of rocks and rock masses at different scales and for different applications. A number of empirical and numerical studies have included size effects on the uniaxial compressive strength of different rocks, but only few have focussed on the triaxial compressive strength. In this study, several triaxial tests were conducted on granite samples at different confinements (from 0.2 to 15 MPa) and sizes (from 30 to 84 mm in diameter). The most relevant strength parameters were recovered including peak and residual strengths, orientation and frictional response of shear bands. Size effects were observed to be less dominant at high confining pressures for both peak and residual strengths. The resulting data were analysed in the context of size-dependent rock strength theories. For peak strength analyses, the unified size effect law (USEL) and the improved unified size law (IUSEL) were used, in which the double trends, ascending descending behaviours were observed. Size-dependent Hoek–Brown modified failure criteria based on USEL and IUSEL were fit to peak strengths exhibiting a good agreement between the models and the laboratory data. The brittle-ductile transition and the frictional behaviour of shear band under triaxial loading were also analysed. A clear brittle behaviour was particularly observed in samples with large-diameters tested at low confinements. Finally, in regards to frictional behaviour, the shear band angle found to be affected by both confinement and sample size.Ministerio de Ciencia e Innovación | Ref. PRE2019-087397Ministerio de Ciencia e Innovación | Ref. RTI2018-093563-B-I00Universidade de Vigo/CISU

Influence of microroughness on the frictional behavior and wear response of planar saw-cut rock surfaces

Saw-cut rock surfaces, classically utilized to estimate basic friction angle of discontinuities by means of tilt test and other procedures, may seem planar to the naked eye. Nevertheless, they actually present roughness at a micrometric scale. Aiming at characterizing some of these saw-cut rock surfaces and assessing the possible implications between their microscale topography and the resulting tribological behavior, the authors of this study resorted to the 3D focus-variation technique to analyze different surface-texture parameters. Tilt tests were carried out on specimens cut on three rock types, and the involved sliding surfaces were evaluated at a microscale for different testing stages (prior to any test and after two series of repeated tests). An apparently logical inverse correlation between repeated testing and friction angle has been observed, more marked for the smoother surfaces. Higher roughness at the scale of the analysis tends to produce lower friction-angle values, as otherwise observed for mismatched natural rock surfaces. In addition, saw-cut rock surfaces present systematically negative skewness and high values of kurtosis for their height distributions, indicating the occurrence of narrow and deep pits or valleys. Directional hybrid parameters and, in particular, the root mean square (RMS) of the gradient of the surface in the direction of sliding correlates rather well with the measured sliding angle. The authors concluded that the 3D focus-variation technique represents a powerful tool to assess surface-texture parameters of saw-cut rock surfaces, in addition to being useful for understanding some features of the tribological, or wear and frictional, behavior of these type of surfaces.Agencia Estatal de Investigación | Ref. RTI2018-093563-B-I0

Toppling susceptibility of a single rock block resting on a regularly rough base

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGToppling of individual rock blocks resting on an inclined surface has often been observed in nature. This instability mechanism has typically been analysed for simple block geometries, considering the contact between the block and the base where it lies is a planar surface. In this study, the authors analyse the case where this contact is a regularly rough surface. To do that, the authors resort to analytical limit equilibrium formulations, laboratory physical models and discrete element methods. All these approaches show a consistent trend of behaviour, where roughness does affect the toppling response. Regularly rough surfaces are studied in detail and a general analytical formulation able to show the potential influence of roughness on the block toppling response is derived. Additionally, some considerations are provided regarding the combined effects of rough bases and rounded corners on toppling stability. The authors show how under particular circumstances, roughness can control the potential failure mechanism of a block to produce toppling instead of sliding, and they eventually discuss the impact of rough bases on the toppling response of natural rock blocks.Agencia Estatal de Investigación | Ref. RTI2018-093563-B-I0

Convergence-confinement curve analysis of excavation stress and strain resulting from blast-induced damage

Using the convergence-confinement curves methodology, we analysed excavation behaviour for a range of rock masses of different geotechnical qualities, taking into account blast-induced damage. The novelty of the research is (i) we include blast-induced damage to the rock mass in ground reaction curve construction, and (ii) we analyse results for 54 rock mass and rock geotechnical quality scenarios. The research, an application of a previously developed methodology (González-Cao et al., 2013), provides practical guidelines for the preliminary design phase for an excavation resulting from blasting. Our main conclusions are (i) that rock mass quality has a greater bearing on the plastic radius and excavation maximum displacement than blast-induced damage, and (ii) that the plastic radius and maximum displacement around an excavation increase with the level of blast-induced damage, most especially for poor quality rock masses. This would justify the need to limit blast-induced damage in poor quality rock masses.Ministerio de Economía y Competitividad | Ref. BIA2014-53368

Study of size effects on the peak and residual strength of intact and artificially fissured granite samples

There are not many studies on jointed rock specimens, which can be considered small scale rock mass analogs. On the other hand, the scale effects in the mechanical properties of such samples have seldom been studied. With the aim of continuing previous research on intact granite rocks, the authors have carried out sets of 25 stress-strain triaxial compressive tests on 1 sub-vertical and 2 sub-horizontal 38 mm, 54 mm and 84 mm diameter jointed granite specimens at various confinements. Peak and residual strength values were obtained and compared to those recovered form intact rock samples. Results suggest that peak strength follows similar trends with scale to those observed on intact rock, even if lower strength values are logically recorded. Regarding residual strength, the obtained results are in line with those observed trends for standard size samples, showing a similar trend for all cases independently of scale, even if we observe larger variability for jointed samples. The authors have also compared the values fitting the generalized Hoek-Brown criterion for rock masses to better understand the behavior in relation to sample size. So scale effects clearly appear on jointed rock peak strength of jointed sample; even if residual strength seems hardly affected by scale.Agencia Estatal de Investigación | Ref. RTI2018-093563-B-I0

Validity of continuous-failure-state unloading triaxial tests as a means to estimate the residual strength of rocks

The residual strength of rocks and rock masses is an important parameter to be constrained for analysis and design purposes in many rock engineering applications. A residual strength envelope in principal stress space is typically developed using residual strength data obtained from compression tests on many different specimens of the same rock type. In this study, we examined the potential for use of the continuous-failure-state testing concept as a means to constrain the residual strength envelope using a limited number of specimens. Specifically, cylindrical specimens of three rock types (granodiorite, diabase, and Stanstead granite) were unloaded at the residual state such that a full residual strength envelope for each individual specimen was obtained. Using a residual strength model that introduces a single new strength parameter (the residual strength index, or RSI), the results of the continuous-failure-state unloading tests were compared to conventionally obtained residual strength envelopes. Overall, the continuous-failure-state residual strength data were found to be consistent with the conventional residual strength data. However, it was identified that the primary factor limiting an accurate characterization of the residual strength for a given rock type is not the amount of data for a given specimen, but the variety of specimens available to characterize the inherent variability of the rock unit of interest. Accordingly, the use of continuous-failure-state testing for estimation of the residual strength of a rock unit is only recommended when the number of specimens available for testing is very limited (i.e. < 5)

Residual strength of granitic rocks

When carrying out compressive tests on rock specimens, a stress plateau is typically attained following peak strength within a strain range a few times larger than the strain at peak strength. This stress level is commonly known as residual strength. A large database of these tests on granitic rock specimens has been compiled and analyzed in detail with the aim of improving understanding of the residual strength of this type of rock. It is observed that physical scale does not significantly change this residual strength. This stress threshold is also shown to be independent of the initial level of jointing in the rock volume. The implication of these findings is that residual strength derived from rock specimen testing can be potentially extended to rock mass scale, at least for granitic rocks. Three one-parameter residual strength models have been used to fit laboratory test residual strength data of granitic rocks, and all of them are shown to reasonably approximate the actual data. Additionally, the residual laboratory strength parameters of all the varied studied granitic rocks for varying scale and pre-jointing tend not to be very different, covering a limited range of values. Ultimately, the potential errors in simple excavation analysis that might result from a priori assumptions regarding the residual strength of granitic rock masses are quantified.Universidade de VigoCISUGAgencia Estatal de Investigación | Ref. RTI2018-093563-B-I0

Stability of wall slopes: limit equilibrium, numerical andphysical models

We reviewcommonlyobservedfailuremechanismsassociatedwithfootwallslopes,andexaminethe basicconceptsunderlyingthelimitequilibriumapproachassessingfootwallslopestability.Thefailure mechanismsdescribedarethose,wherefailurefollowspre-existingjointsoroccursinintactrock, namely,fullyandpartiallyjoint-controlledbilinearandploughingslabfailures.Numericalmodels implementedusingUDECillustratethecode’spotentialforanalysingthesefailuremechanisms. Physicalmodelsrepresentingthestudiedinstabilitymechanismswerestudiedinatilt-testset-upin the laboratory,withtheempiricalresultscomparingfairlywellwiththetheoreticalapproaches