Free Face Research Articles

Field Experiment on Soaking Characteristics of Collapsible Loess

In collapsible loess area, migration of soil moisture often causes the temporal discontinuity and spatial nonuniformity of collapsibility, which leads to great damage for infrastructures. Therefore, the research on water infiltration is the key to solving the problem of collapsibility. The aim of this paper is to investigate the spatiotemporal evolution of infiltration characteristics of collapsible loess. A field soaking experiment was conducted on collapsible loess in western China, in which a soaking pool with diameter of 15 m was built. Time-Domain-Reflectometry (TDR) system and soil sampling were employed to measure the water content within the depth of 12 m. Then the saturation isograms were drawn for visualization of the process of infiltration. Also, a pilot tunnel was excavated to investigate how the free face can affect the infiltration behaviors. The experimental results revealed the characteristics of infiltration in both horizontal and vertical directions. Moreover, the response of free face on infiltration behaviors was also found. These findings of research could provide the data for the infiltration laws of unsaturated loess and thereby provide the basis for integrated treatment of collapsible loess.

Prediction Model of Collapse Risk Based on Information Entropy and Distance Discriminant Analysis Method

The prediction and risk classification of collapse is an important issue in the process of highway construction in mountainous regions. Based on the principles of information entropy and Mahalanobis distance discriminant analysis, we have produced a collapse hazard prediction model. We used the entropy measure method to reduce the influence indexes of the collapse activity and extracted the nine main indexes affecting collapse activity as the discriminant factors of the distance discriminant analysis model (i.e., slope shape, aspect, gradient, and height, along with exposure of the structural face, stratum lithology, relationship between weakness face and free face, vegetation cover rate, and degree of rock weathering). We employ postearthquake collapse data in relation to construction of the Yingxiu-Wolong highway, Hanchuan County, China, as training samples for analysis. The results were analyzed using the back substitution estimation method, showing high accuracy and no errors, and were the same as the prediction result of uncertainty measure. Results show that the classification model based on information entropy and distance discriminant analysis achieves the purpose of index optimization and has excellent performance, high prediction accuracy, and a zero false-positive rate. The model can be used as a tool for future evaluation of collapse risk.

A Distribution-Free Description of Fragmentation by Blasting Based on Dimensional Analysis

A model for fragmentation in bench blasting is developed from dimensional analysis adapted from asteroid collision theory, to which two factors have been added: one describing the discontinuities spacing and orientation and another the delay between successive contiguous shots. The formulae are calibrated by nonlinear fits to 169 bench blasts in different sites and rock types, bench geometries and delay times, for which the blast design data and the size distributions of the muckpile obtained by sieving were available. Percentile sizes of the fragments distribution are obtained as the product of a rock mass structural factor, a rock strength-to-explosive energy ratio, a bench shape factor, a scale factor or characteristic size and a function of the in-row delay. The rock structure is described by means of the joints’ mean spacing and orientation with respect to the free face. The strength property chosen is the strain energy at rupture that, together with the explosive energy density, forms a combined rock strength/explosive energy factor. The model is applicable from 5 to 100 percentile sizes, with all parameters determined from the fits significant to a 0.05 level. The expected error of the prediction is below 25% at any percentile. These errors are half to one-third of the errors expected with the best prediction models available to date.

A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa – Part I: stratigraphic and structural features

Who would have thought that the southern part of ‘stable, old South Africa’ was in fact ‘not-so-stable’, and that there has been spectacular tectonic activity in the Cape Fold Belt as recent as the early Holocene, about 10,327 ± 755 years ago? Large-scale surface rupture, with at least 2 m of vertical geomorphic offset, has been recorded along the Kango Fault over a lateral distance of at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the western end of the Baviaanskloof in the Eastern Cape. Although the Kango Fault scarp was reported in the mid-1970’s and late-1980’s, detailed palaeoseismic investigation was not undertaken until recently, when the chief energy supplier for South Africa, Eskom, rekindled investigations to establish a number of nuclear power plants along the South African coastline. This paper reports the results of the first palaeoseismic trench excavated across the Kango Fault during the initial phase of the nations’ current New Build programme. The location and date of the most recent event (MRE) is reported, as well as the style of faulting and local stress direction. These palaeoseismic data are derived from carefully-mapped logs of an 82 m long, 5 m deep trench, supported by 12 optically-stimulated luminescence (OSL) dates of stratigraphic horizons exposed in the trench. Various features of the stratigraphy, pedology and structure in the trench sidewall are described in detail, particularly those leading to the interpretation of the MRE. Development of a shear fabric in the cobble- to small boulder-sized alluvium at the main fault zone indicates that interpretation of local neotectonic fault style and stress direction should be made at least 4 to 5 m below the free face of the fault scarp. Retro-deformation of the trench log clearly defines the MRE and distinguishes apparent vertical displacement from the actual displacement of 2.0 ± 0.06 m. It also reveals that 1.9 m of surface extension occurred during the rupture, as well as the depositional processes that subsequently buried the 32 m wide graben formed at the site.

New technology and equipment for non-explosive formation of free face in deep open pit mines

The described technology and equipment enable eliminating drilling-and-blasting in open pit mining. The authors also discuss usability of transfer bins with vibrating discharge in combination with intermodal transport and adaptability of vibratory machines to dump truck-aided stockpiling.

Mapping of vulnerability of flooded area in arid region. Case study: area of Ghardaïa-Algeria

The issue of natural hazards in general and flooding in particular is a hot topic which marks memorable actions in the world and specifically in Algeria. Managing this risk is becoming more of a necessity that must include all stakeholders and all possible means available. The objective of this work is the vulnerability mapping of flood areas in the arid region Oued M’zab-region Ghardaia. It’s about implement predictive models and flood control. This allows the development of a risk prevention plan to flooding. To achieve this goal we evaluated the hazard and vulnerability. The evaluation of the risk passes by a study of the mode of rising. This study leads to the determination of the synthetic hygrogram which are taken into account under the boundary conditions upstream of a hydraulic model of the nonpermanent flows on free face. The results obtained for one period of return given constitute the risk of the floods relating to these hydrograms. The vulnerability is represented by a rate which translates the objective of protection against a probable risk. Two types of models are tested on a section of Oued M’zab-region where the data are available. Initially, are considered hydrological models (HEC-HMS 1D). In the second place of the hydraulic models (HEC-RAS 1D). After comparison of the results obtained by various simulations, the vulnerable zones are thus charted.

3D identification and stability analysis of key surface blocks of rock slope

Complicated geological structures make it difficult to analyze the stability of rock slopes, such as faults, weak intercalated layers or joint fissures. Based on 3D geological modeling and surface block identifying methods, an integrated methodology framework was proposed and realized to analyze the stability of surface blocks in rock slopes. The surface blocks cut by geological structures, fissures or free faces could be identified subjected to the four principles of closure, completeness, uniqueness and validity. The factor of safety(FOS)of single key block was calculated by the limit equilibrium method. If there were two or more connected blocks, they were defined as a block-group. The FOS of a block-group was computed by the Sarma method. The proposed approach was applied to an actual rock slope of a hydropower project, and some possible instable blocks were demonstrated and analyzed visually. The obtained results on the key blocks or block-groups provide essential information for determining potential instable region of rock slopes and designing effective support scheme in advance.

Free-face-Assisted Rock Breaking Method Based on the Multi-stage Tunnel Boring Machine (TBM) Cutterhead

In order to improve the rock breaking efficiency of hard rock tunnel boring, many innovative rock breaking methods have been proposed (e.g., the water jet cutting, the high-power laser cutting, the impact-rotary drilling, and the undercutting method). However, most of the methods are not applicable to TBMs due to some structural reasons. Aiming on this problem, a free-face-assisted rock breaking method based on the multi-stage TBM cutterhead has been proposed. Series of proof-of-concept tests includes (1) the static compression test with vertical free face and (2) the rotary cutting tests in different free surface conditions were designed and carried out. The results show that the rock breaking force and efficiency can be significantly reduced and improved, respectively, with the assistance of the free face, due to the failure of the rock close to the free face is tensile-dominated failure. The influencing distance of the free face in the radial direction is at least 330 mm which covers about 5 disk cutters. Finally, the general structure of a small two-stage cutterhead (4 m in diameter) was tentatively designed in order to provide a possible approach to apply the free-face effect to TBMs.

Insights on surface spalling of rock

Surface spalling is a complex failure phenomenon that features crack propagation and detachment of thin pieces of rock near free surfaces, particularly in brittle rock around underground excavations when large in situ stresses are involved. A surface instability apparatus was used to study failure of rock close to a free surface, and damage evolution was monitored by digital image correlation (DIC). Lateral displacement at the free face was used as the feedback signal to control the post-peak response of the specimen. DIC was implemented in order to obtain the incremental displacement fields during the spalling process. Displacement fields were computed in the early stage of loading as well as close to the peak stress. Fracture from the spalling phenomenon was revealed by incremental lateral displacement contours. The axial and lateral displacements suggested that the displacement gradient was uniform in both directions at early loading stages and as the load increased, the free-face effect started to influence the displacements, especially the lateral displacement field. A numerical approach, based on the discrete element method, was developed and validated from element testing. Damage evolution and localization observed in numerical simulations were similar to those observed in experiments. By performing simulations in two- and three-dimensions, it was revealed that the intermediate principal stress and platen–rock interfaces have important effects on simulation of surface spalling.

Acoustic Plate Mode sensing in liquids based on free and electrically shorted plate surfaces

The sensing behavior to liquids for Acoustic Plate Modes (APMs) propagating along 64°Y, 90°X LiNbO3 plate was investigated vs. two electric boundary conditions. The changes in the APMs phase velocity and attenuation were measured upon exposure to different liquids wetting one of the surfaces of the plate, either free or electrically shorted by a thin conductive Al layer. The experimental data confirm that the presence of a metallic layer covering one of the plate surfaces affects the viscosity and temperature sensitivity of the device. The differences between the sensor response for various liquids, with free or metalized faces, are interpreted in terms of the APM polarization.

Evolution of fracture permeability with respect to fluid/rock interactions under thermohydromechanical conditions: development of experimental reactive percolation tests

The evolution of fracture permeability is crucial as regards the lifespan of the deep fractured geothermal exchanger of an Enhanced Geothermal System site. The objective in developing reactive percolation tests in fractures under thermohydromechanical conditions is to improve our understanding of fluid/rock interactions and their role in the evolution of fracture permeability. This article describes the test apparatus and the experimental protocol developed to meet this objective. The data from a test on a sample of fractured granite are interpreted with a view to characterising the phenomena that occurred during the reactive percolation and their impact on the behaviour of the fracture and its permeability. The test showed that the free face type dissolution of some minerals led, through a deepening of existing channels on the fracture walls, to an increase of the fracture’s permeability by one order of magnitude and to a change in its hydromechanical behaviour.

Upper-bound and lower-bound solutions for the axisymmetric compression of a concrete plug

Deep radioactive waste deposits require specific solutions to close underground galleries and avoid any radioactive migration in the environment. One investigated solution consists of a swelling clay plug that induces a swelling pressure of approximately 7MPa on the walls of the galleries. The clay plug is confined by a concrete plug to maintain and drive the swelling pressure towards the walls of the galleries. The concrete plug must therefore be designed to endure such a pressure. This paper addresses the mechanical strength of a cylindrical concrete plug with a ring-shaped tooth, a free face and a uniform compressive stress over the other face. Two experimental tests over 1/43 reduced samples are first performed and show two collapse mechanisms: dome collapse and tooth-shear collapse. Upper- and lower-bound solutions are investigated based on the Mohr–Coulomb criterion from nonlinear criteria of concrete. The choices of parameters for the friction angle (φ) and cohesion (C) are discussed in relation to the uniaxial compressive and tensile strength of the concrete and the magnitude of the stresses in the model. The results of the simplified analytical, numerical and experimental approaches are then compared, with good agreement found between the approaches.

Fullerene and corannulene derivatives acting as insulators of Cl(-) and BeH2.

The capacity of corannulene and its benzo-derivatives CxH10 (x = 20-60) as prototypes of non-planar π-aromatic systems which mimic the end of carbon nanotubes to act as insulators between BeH2 and Cl(-) chemical entities has been explored by means of M06-2x/cc-pVDZ and M06-2x/6-311+G(d) calculations. For the sake of completeness, the set investigated includes also fullerene, C60. All these aromatic derivatives lead to stable binary complexes either with BeH2 or halogen (Cl(-)) anions. For BeH2, however, only the complexes in which the interaction involves the convex face of the aromatic system are stable. No significant changes are observed when the binding energies (BEs) of the BeH2 complexes are compared to those involving planar aromatic compounds, but the ones involving Cl(-) with the concave face of the aromatic moiety can be very large, because its curvature favors many contacts of the anion with the carbon atoms of the π-aromatic system. The formation of these binary complexes changes to a large extent the electrostatic potential on the free face of the aromatic system leading to a mutual reinforcement of both interactions, the beryllium bond and the interaction with Cl(-), when the ternary complexes are formed. As a result, the BEs for the triads are larger than the sum of the BEs of the corresponding binary complexes and the distances between the aromatic subunit and BeH2 or Cl(-) become shorter in the triads than in the binary complexes. A MBIE analysis also indicates that the enhanced stability of ternary complexes arises mainly from the reinforcement of the beryllium bonds as well as from the three-body terms. An exploration of all the minima for BeH2:C60H10:Cl(-) shows that BeH2 binds preferentially to the peripheral aromatic rings than those in the more curved region.

Frequency comparison of blast-induced vibration per delay for the full-face millisecond delay blasting in underground opening excavation

Responses and damage to the structures subjected to blast-induced vibration are highly dependent on vibration frequency. Understanding the vibration frequency characteristics and its formation mechanism is essential for the safe and economic design of mining and construction blasts. When millisecond delay blasting is used in excavation, a global frequency value of all of the delays of vibrations fails to present a true picture of the vibration frequency characteristics. In the present study, comparisons of frequency characteristics for the blast-induced vibration per delay were first conducted via an underground opening case excavated by the full-face millisecond delay blasting sequence. The results show that if the blasthole geometry and charge structure are kept the same in each blast delay, the frequencies of single-delay vibration signals will decrease as the dimensions of the equivalent blasting vibration sources increase. However, the cutting blast used for the headmost holes inside is an exception, and its vibration frequency is lower than that of the breaking blast outside. It is the authors’ belief that this counter-intuitive phenomenon is caused by the free faces created by the cutting blast for the breaking blast. A small-scale blasting test and the related numerical simulation were subsequently performed to demonstrate this belief. It is found that the vibration frequency from the holes in the free-face blast is higher than that in the confined blast. In terms vibration frequency, therefore, the vibration from the breaking blast is less harmful to structures compared to the cutting blast for a certain velocity, and decreasing the burden of breaking blastholes is beneficial to reduce the vibrational damage of structures.

On corner singularities in Reissner‐Mindlin's theory of elastic plates

AbstractIn the framework of linear elasticity, singularities occur in domains with non‐smooth boundaries. Particularly in Fracture Mechanics, the local stress field near stress concentrations is of interest. In this work, singularities at re‐entrant corners or sharp notches in Reissner‐Mindlin plates are studied. Therefore, an asymptotic solution of the governing system of partial differential equations is obtained by using a complex potential approach which allows for an efficient calculation of the singularity exponent λ. The effect of the notch opening angle and the boundary conditions on the singularity exponent is discussed. The results show, that it can be distinguished between singularities for symmetric and antisymmetric loading and between singularities of the bending moments and the transverse shear forces. Also, stronger singularities than the classical crack tip singularity with free crack faces are observed. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The forced vibration of the system consisting of an elastic plate, compressible viscous fluid and rigid wall

The forced vibration of the system consisting of a pre-stressed elastic plate, barotropic compressible Newtonian viscous fluid and rigid wall is considered. The space between the plate and rigid wall is filled by the fluid. It is assumed that the forced vibration is caused by the lineally-located time-harmonic force acting on the free face plane of the plate. The motion of the plate is written by utilizing the exact equations of elastodynamics, but the motion of the compressible viscous fluid is described by the linearized Navier-Stokes equations. Moreover, it is assumed that the velocities and stresses of the constituents are continuous on the contact plane between the plate and fluid, and that the impermeability conditions on the rigid wall are satisfied. The dimensionless parameters which characterize the compressibility and viscosity of the fluid as well as the elasticity constants of the plate are introduced. Plane strain state in the plate and two-dimensional plane flow of the fluid are considered. Numerical results on the interface normal stress and velocities are presented. The influence of the problem parameters is also discussed, including the fluid viscosity and compressibility, thickness of the plate and fluid depth as well as the excitation frequency. In this discussion the focus is on the influence of the fluid depth on the studied quantities. This is the parameter through which the main difference arises between the present and previous works by the authors.

Pathways of geomorphic evolution of sandstone escarpments in the Góry Stołowe tableland (SW Poland) — Insights from LiDAR-based high-resolution DEM

The tableland of the Stołowe Mountains (SW Poland), with its prominent mesas and sandstone-capped escarpments, belongs to the most spectacular geomorphic landscapes of Central Europe. While the gross morphological features of the area have long been recognized, the evolutionary pathways of densely forested and poorly accessible escarpment slopes remained poorly understood. In this paper we use LiDAR data to shed a new light on landform inventories within the escarpments, their spatial patterns and, using process–from–form reasoning, on the longer-term evolution of the escarpments. Four sites, two on each major escarpment, have been subject to detailed analysis which involved examination of shaded relief, slope, plan and profile curvature and topographic wetness index. In each case, the 1×1m model was used, while for the most complex site at Mt. Szczeliniec Wielki the results were compared with the 5×5m model to check the impact of model resolution on geomorphic interpretation. Despite some loss of information involved in model re-interpolation to the coarser scale, the main features of escarpment morphology could still be recognized. On the other hand, automatic landform classification based on the calculation of Topographic Position Index from the 10×10m model and performed for the entire tableland failed to reveal differences between various sections of the escarpments, detectable on finer models. The analysis of spatial patterns of minor landforms within the escarpments, identified on LiDAR-derived models shows that no single pathway of escarpment evolution exists. Both the upper slopes (in sandstone caprock) and the mid-slopes (in weaker rocks) show signs of instability and these are not necessarily coupled. Large-scale caprock failures do occur but seem rare and localized. Sandstone free faces are rather subject to continuous slow retreat by detachment of individual joint-bound blocks. Another zone of instability occurs well below the caprock and the dominant processes are shallow landslips initiated within weak, deformable rocks.

Resistivity imaging technique to investigate the subsurface strata conditions due to blasting in underground coal mines in India

ABSTRACTProduction from underground coal mining in India has been decreasing since the last three decades. Blasting‐off‐the‐solid, in Indian underground mines, has been a widely practised method for winning coal while developing mine galleries in bord and pillar system. The blast holes are drilled in solid coal without free face in blasting‐off‐the‐solid method of mining and are blasted with permitted types of explosives and delay detonators. Blasting‐off‐the‐solid will play an important role in underground coal production. The mines that were planned earlier to be away from residential areas are now approaching to those areas. The coal reserves are being exploited very fast, and mine owners cannot ignore to take the coal beneath the structures/houses. In this paper, resistivity imaging technique has been used to document the change of subsurface structures/features between the surface and underground face, where blasting is carried out to extract the coal in underground coal mines. Resistivity imaging survey conducted at three mining sites revealed that there was no significant change in subsurface strata conducted during pre‐ and post‐blasting conditions. The slight change in resistivity values up to 35 Ωm up to a few metres of peripheral area of the blasting face in the mine may be either due to the effect of blast vibration and/or same void created by blasting or increase in porosity of overlying subsurface strata.

Boulder aprons indicate long-term gradual and non-catastrophic evolution of cliffed escarpments, Stołowe Mts, Poland

Caprock-crowned escarpments are characteristic geomorphic features of the sandstone tableland of the Stołowe Mountains (SW Poland). Their mid- and lower slopes truncate weaker sedimentary formations but are littered with sandstone boulders of various size, often more than 5m long, which form nearly continuous aprons. A model of escarpment retreat by rock fall has been widely accepted in literature but in the Stołowe Mountains it has never been tested against field evidence; in addition no rock fall events except one minor fall in 1921 have been recorded in historical times. In this paper we erected five hypotheses of how escarpments may have evolved through time to result in widespread boulder presence on the slopes. Mapping boulder extent and slope morphology, run-out distance simulations using Conefall software, and rock strength determinations using the Schmidt hammer carried out on boulders along eight representative slope profiles provide data about characteristics of the boulder covers. Systematic decrease in intact rock strength with an increasing distance from the sandstone cliffs suggests that the boulder aprons are diachronic and record long-term retreat of escarpments. There exists no conclusive evidence of widespread boulder downslope movement, although in a few localities shallow landslides have rafted boulders far away from the cliff lines. Modelling exercise shows that the actual extent of boulders is 2–3 times bigger than suggested by Conefall predictions. These findings, coupled with observations along the cliff lines themselves, lead us to propose a model of free face in situ disintegration as the most likely hypothesis to explain block detachment and release from rock faces. As cliff lines recede and the slope surface below is lowered, remnant boulders occupy a position increasingly further away from escarpment rims, but experience little actual movement. This model complements the existing models of tableland evolution which tend to emphasize catastrophic mass movement processes on cliffed escarpments.

Recent Activity of the Badu‐Longwei Segment, Guguan‐Xiangong Fault in Southern Liupanshan, Constrained by Rhythmic Sediment Lithology and Geomorphic Characteristics

AbstractThis study's objective was to investigate the Guguan‐Xiangong Fault, which lies in the southern Liupanshan area, through satellite image interpretation and field observations. Guguan‐Xiangong Fault is divided into five subsegments; among these, the Badu‐Longwei segment has been the most recently active. The geomorphic features of the Badu‐Longwei segment are clearly displayed, including multiple high fault scarps with fresh bedrock free faces. There is significant evidence for Holocene activity of the three fault sections, located in Renhuashu, Tianjiagou, and Xinjiecun respectively. The three sections feature distinct episodic deposition and fault scratches. Based on 14C‐dating and field observations on the three fault sections, two or more paleoearthquakes across the Badu‐Longwei fault segment are ascertained, between 5874±116 and 5430±140 a BP, and after 2037±83 a BP respectively. The Badu‐Longwei segment of the Guguan‐Xiangong Fault is preliminarily extrapolated as the seismogenic structure of the 600 A.D. Qin‐Long earthquake.