Ivan Gratchev

Back analysis of a natural jointed rock slope based on the photogrammetry method

In 2013, a potentially unstable jointed rock slope above a road in the Gold Coast area, Australia, partially failed after a rainfall event. In this study, the rock failure was back analyzed to evaluate the presence of key blocks through block analysis based on photogrammetry surveys. Friction angles of the joint sets at the point of failure were also investigated by means of a parametric study using a 3D distinct element method (DEM). The photogrammetry method was employed to obtain measurements of the orientation of the joint sets of the slope and the shape and size of rock blocks at the inaccessible slope. The joint roughness coefficient (JRC) obtained from the 3D photogrammetry model was utilized to estimate friction angles and the strength of the joint set. Safety factors of the rock mass were computed by both analytical and numerical approaches. Parametric analyses, using a DEM model, assessed the critical friction angle of the joint sets and also demonstrated the failure mechanism of the blocks. The result of this study indicates that the obtained safety factors are in agreement with the block analysis and the results from the numerical analysis performed by the distinct element code “3DEC.”

Strength of Rock-Like Specimens with Pre-existing Cracks of Different Length and Width

Discontinuities play a key role in the failure process of rock and rock mass. Small fractures can decrease the strength of rock while large-scaled joints can have a significant effect on the overall stability of rock mass (Eberhardt et al. 2004). Discontinuities are often separated by intact rocks, which are known as “rock bridges”. These rock bridges contribute to the stability of jointed rock mass by providing a strength reserve (Gehle and Kutter 2003) that needs to be broken first before failure can take place. In recent years, a number of experimental studies have been performed to investigate the mechanism of crack propagation in rock-like material under uniaxial (Shen et al. 1995; Wong and Chau 1998; Sagong and Bobet 2002; Xu et al. 2013; Yin et al. 2014; Cao et al. 2015), biaxial (Bobet and Einstein 1998), and shear (Gehle and Kutter 2003) stress conditions. It was revealed that under load conditions, wing cracks first appear at the tips of pre-existing cracks (flaw) while shear cracks typically lead to coalescence and failure. The aforementioned studies also indicated that the process of coalescence was rather complex and depended on the rock material and geometry of pre-existing cracks. While the mechanism of crack propagation is relatively well understood, the extent to which pre-existing cracks can affect the strength characteristics of rocks still remains unclear. Ramamurthy and Arora (1994), Yang et al. (1998) and Park and Bobet (2009) noted that the characteristics of joints such as their number and orientation can affect the strength of rock. Shen (1995) and Park and Bobet (2009) reported that the joint roughness and the friction of the filled material (Shen 1995; Park and Bobet 2009) can also influence the strength of rock and rock mass. These joint characteristics are considered in the joint strength parameter, which is commonly used in engineering practice (Ramamurthy and Arora 1994; Zhang 2010). Unfortunately, much less attention has been given to the length and width of joints, which are parameters that can also affect the strength of rock and rock mass. This study seeks to address this issue by investigating the effects of pre-existing cracks of different width and length on the unconfined compressive strength (UCS) of rock-like material of various strengths. This technical note presents and discusses the obtained results.

Cyclic Shear Strength of Soil with Different Pore Fluids

AbstractThe effects of pore water chemistry on the cyclic behavior of fine-grained soil were examined by performing undrained cyclic ring-shear tests on a natural soil treated with solutions of sulfuric acid (H2SO4), sodium hydroxide (NaOH), and sodium chloride (NaCl). In addition, different amounts of the chemicals were used to clarify the influence that ion concentrations in the pore fluid can have on the cyclic shear strength of soil. The results of laboratory tests indicated that cyclic shear strength was sensitive to changes in the pore fluid composition, and changes in cyclic behavior of soil became more pronounced as the concentration of each chemical increased.

Assessment of rock slope stability using remote sensing technique in the Gold Coast area, Australia

Rock falls and landslides along major roads cause significant damage to infrastructure in the Gold Coast area, Australia. Current methods of hazard assessment, which mostly include field mapping and data collection for site characterization, are inherently labor intensive and subject to bias due to safety issues and time constrains. However, many of these problems have recently been addressed through the development and deployment of digital imaging technology based on photogrammetry. This method involves the use of high-resolution digital stereo-photographs, from which a three-dimensional image of the slope can be constructed. Such images can highlight the surface texture of slopes and identify potentially unstable zones, thus providing engineers with valuable information regarding the slope design. Photogrammetry is still a relatively new remote sensing technique in Australia and has mostly been used in the mining industry. Little has been done to study the feasibility of its application in civil engineering to solve geotechnical problems related to the stability of natural slopes and road cuts. This paper presents the results of a pilot study aimed at assessing the stability of rock slopes in the Gold Coast area. Field surveys including photogrammetry were performed to study the geological settings of the sites, and characterize the slopes topography and type of discontinuities. Based on the 3-D models, the potentially unstable zones were identified, and slope stability analysis of those areas was performed. The obtained results indicated that photogrammetry can be a helpful tool in assessing geohazard related to slope stability problems.

Rock slope stability problems in Gold Coast area, Australia

The Tamborine Mountain area of the Gold Coast, Australia often faces rock slope failures during rainy seasons. To have a better understanding of the factors that may affect the slope stability, several rock slope sites examined in detail to assess the effect of discontinuities formed in the rock masses, and degree of weathering of the rocks on the overall stability of slopes. Different methods of slope stability analysis such as kinematic approach and slope mass rating, were applied to investigate the potential failure mode and predict the behavior of rock slopes in the future. This paper will present results of this work and make comparisons between different methods of rock slope stability analysis.

A Photogrammetric Approach for Stability Analysis of Weathered Rock Slopes

The geological strength index (GSI) system is dependent on the rock block volumes and the joint surface conditions. The weathering degree of rock slopes and their strength properties also depend on these characteristics. This study thus focuses on the use of the GSI system and the Hoek–Brown strength criterion to estimate the engineering parameters for weathered rock masses. Photogrammetric methods based on 3D surface models are used to obtain reliable data on the joint sets in rock slopes, instead of general site investigation using labor-intensive techniques. Photogrammetric surveys were conducted on weathered rock slopes in Gold Coast, Australia to obtain the joint spacing, orientation and roughness. The 3D models are then used to estimate the block sizes the joint roughness coefficients (JRC). The block volumes and JRC values were then used to estimate GSI values. Then parametric studies using the finite element method is conducted to investigate the stability of the slope using the GSI values.

Feasibility of using fly ash, lime, and bentonite to neutralize acidity of pore fluids

Acidic groundwater resulting from the poorly planned use of acid sulfate soils has become a major environmental issue in coastal Australia over the last several years. Use of permeable reactive barriers (PRBs) designed to generate alkalinity by promoting sulfate reduction has recently become popular as an alternative solution to this problem. However, recent studies have also revealed that the long-term performance of such PRBs can be significantly undermined by chemical precipitation and clogging of pore space, which would decrease the buffer capacity and hydraulic conductivity of the reactive material. This study seeks to explore the feasibility of using bentonite in addition to lime and fly ash to form mixtures with a high buffer capacity and permeability that would enable groundwater flow through PRBs over a substantial period of time. A series of laboratory experiments, including buffer capacity and leaching tests, were performed on different mixtures of fly ash with lime and bentonite using acidic fluids of low pH. It was found that the ability of such mixtures to neutralize acidic fluids was mostly controlled by the content of lime. Laboratory data also showed that an addition of bentonite to lime—fly ash mixtures could decrease the buffer capacity of soil. Compaction tests indicated that the presence of bentonite would increase the dry density of mixtures at the optimum moisture content. A series of hydraulic conductivity tests were carried out to study changes in the coefficient of permeability of lime—fly ash mixtures with different contents of bentonite permeated with acidic liquids. The obtained results revealed that the coefficient of permeability of the specimens tended to increase over a period of time, likely due to the changes in the diffuse double layer of bentonite particles.

The Mechanism of Liquefaction of Clayey Soils (M124)

An experimental study on the liquefaction of clayey soils was conducted under ICL Project M124 “The influence of clay mineralogy and ground water chemistry on the mechanism of landslides” in order to better understand the mechanism of this phenomenon. The first section of this study deals with artificial mixtures of sand with different clays while the second is concerned with natural soils collected from landslides. The results from the first section are presented in this article. The investigation was conducted by means of a ring-shear apparatus and a scanning electron microscope (SEM). The results obtained for artificial mixtures enabled us to draw a line between liquefiable and non-liquefiable clayey soils and to define a criterion to estimate their liquefaction potential. In addition, the influence of clay content and clay mineralogy on the cyclic behavior of clayey soil was studied. It was found that an increase in clay content as well as the presence of bentonite clay raised the soil resistance to liquefaction. The analysis of microstructures of bentonite-sand mixtures along with the results from ring-shear tests revealed that the soil microstructure is the key factor in determining the dynamic properties of soil. For example, in the microstruc-tures of soils vulnerable to liquefaction, the clay matter was observed to form “clay bridges” between sand grains that were easily destroyed during cyclic loading. In the microstructures of soils resistant to liquefaction, the clay matter seemed to form a matrix that prevented sand grains from liquefaction. The influence of pore water chemistry on the liquefaction potential of artificial mixtures was also studied. It was found that the presence of ions in pore water changed the microstructure of clayey soil, thus making it more vulnerable to liquefaction.

The Application of Normal Stress Reduction Function in Tilt Tests for Different Block Shapes

This paper focuses on the influence of the shapes of rock cores, which control the sliding or toppling behaviours in tilt tests for the estimation of rock joint roughness coefficients (JRC). When the JRC values are estimated by performing tilt tests, the values are directly proportional to the basic friction of the rock material and the applied normal stress on the sliding planes. Normal stress obviously varies with the shape of the sliding block, and the basic friction angle is also affected by the sample shapes in tilt tests. In this study, the shapes of core blocks are classified into three representative shapes and those are created using plaster. Using the various shaped artificial cores, a set of tilt tests is carried out to identify the shape influences on the normal stress and the basic friction angle in tilt tests. The test results propose a normal stress reduction function to estimate the normal stress for tilt tests according to the sample shapes based on Barton’s empirical equation. The proposed normal stress reduction functions are verified by tilt tests using artificial plaster joints and real rock joint sets. The plaster joint sets are well matched and cast in detailed printed moulds using a 3D printing technique. With the application of the functions, the obtained JRC values from the tilt tests using the plaster samples and the natural rock samples are distributed within a reasonable JRC range when compared with the measured values.

Calibration of restitution coefficients using rockfall simulations based on 3D photogrammetry model: a case study

This paper presents a case study of an excavated rock slope to assess the coefficients of restitution using the results from field rockfall tests and corresponding rockfall simulations based on 3D photogrammetric slope surface models. The results from the field rockfall tests showed that rockfall trajectories were controlled by the orientations of joints at the initial point of the rockfalls. The lateral dispersion ratio (DH/L) of the trajectories was influenced by the shape index (γ) of falling rocks as well. The directions of rockfall trajectories, which were obtained from the 3D simulation based on the 3D photogrammetric surface models, were in agreement with the field rockfall tests. Throughout 2D analysis, which was performed using predefined trajectories from the 3D simulations, the coefficients of restitution were successfully calibrated using elapsed times and distances obtained from the field rockfall tests. Based on the accurate 3D photogrammetric slope model, the back-calculated coefficients of restitution from the 3D and 2D simulations combined with field rockfall tests provided consistent results between the numerical analyses and the field experimental data.