Slab Failure Research Articles

Modeling hard rock failure induced by structural planes around deep circular tunnels

Construction of tunnels is often associated with fault or structural features that could affect tunnel stability during the construction phase and service life. Therefore, failure characteristics of hard rock around circular tunnels induced by pre-existing structural features under high in-situ stresses have been the subject of various studies. In the current study, a combined finite element approach, namely ELFEN, has been used for better reflection of entire failure process (including crack initiation, propagation and coalescence) and intrinsic properties of hard rock mass, thus rock heterogeneity, around circular tunnels during excavation unloading process. Parametric analysis which consider the dip angle, location (exposure or not), frictional coefficient of structural planes and lateral pressure coefficient was conducted in detail to reveal the mechanical responses of circular tunnel induced by structural plane under unloading condition. Numerical results indicate that the failure intensity of rock tunnel is a function of both dip angles and frictional coefficients of structural planes. The most critical dip angles of structural planes leading to failure in tunnel rocks largely depend on the frictional coefficient. Also, the results indicate that the released strain energy for the case of exposed structural plane is higher than those not intersecting the tunnel, leading to more violent rock failure for the former. With the increase of the lateral pressure coefficient, the failure intensity and damage extent around the tunnel is aggravated, especially for the roof and floor of the tunnel. Rock failure can be categorized as slabbing failure near excavation boundary and shear slip failure, controlled by structural plane. Progressive slabbing failure induced by excavation unloading may activate internal structural planes and the extensive release of energy caused by shear and slip failure may in turn further induce the slabbing failure. Rockburst is more prone to be triggered under such condition.

Resolving the crustal composition paradox by 3.8 billion years of slab failure magmatism and collisional recycling of continental crust

In the standard paradigm, continental crust is formed mainly by arc magmatism, but because the compositions of magma rising from the mantle are basaltic and continental crust is estimated to contain about 60% SiO2 and much less MgO than basalt, the two do not match. To resolve this paradox, most researchers argue that large amounts of magmatic fractionation produce residual cumulates at the base of the crust, which because arcs are inferred to have magmatically thickened crust, form eclogites that ultimately founder and sink into the mantle. Not only are there problems with the contrasting bulk compositions, but the standard model also fails because prior to collision most modern arcs do not have thick crust, as documented by their eruption close to sea level, and in cases of ancient arc sequences, their intercalation with marine sedimentary rocks.Our study of Cretaceous batholiths in the North American Cordillera resolves the crustal composition paradox because we find that most are not arc-derived as commonly believed; but instead formed during the waning stages of collision and consequent slab failure. Because the batholiths typically have silica contents >60% and are derived directly from the mantle, we argue that they are the missing link in the formation of continental crust.Slab failure magmas worldwide are compositionally similar to tonalite-trondhjemite-granodiorite suites as old as 3.8 Ga, which points to their collective formation by slab failure and long-lived plate tectonics. Our model also provides (1) an alternative solution to interpret compiled detrital zircon arrays, because episodic peaks that coincide with periods of supercontinent amalgamation are easily interpreted to represent collisions with formation of new crust by slab failure; and (2) that models of early whole-earth differentiation are more reasonable than those invoking progressive growth of continental crust.

Full-Scale Linear Cutting Tests in Chongqing Sandstone to Study the Influence of Confining Stress on Rock Cutting Forces by TBM Disc Cutter

In order to study the influence of confining stress on rock cutting forces by tunnel boring machine (TBM) disc cutter, full-scale linear cutting tests are conducted in Chongqing Sandstone (uniaxial compressive strength 60.76 MPa) using five equal biaxial confining stressed conditions, i.e. 0-0, 5-5, 10-10, 15-15 and 20-20 MPa; disc cutter normal force, rolling force, cutting coefficient and normalized resultant force are analysed. It is found that confining stress can greatly affect disc cutter resultant force, its proportion in normal and rolling directions and its acting point for the hard Chongqing Sandstone and the confining stress range used in this study. For every confining stressed condition, as cutter penetration depth increases, disc cutter normal force increases with decreasing speed, rolling force and cutting coefficient both increase linearly, and acting point of the disc cutter resultant force moves downward at some extent firstly and then upward back to its initial position. For same cutter penetration depth, as confining stress increases, disc cutter normal force, rolling force, cutting coefficient and normalized resultant force all increase at some extent firstly and then decrease rapidly to very small values (quite smaller than those obtained under the non-stressed condition) after some certain confining stress thresholds. The influence of confining stress on rock cutting by TBM disc cutter can be generally divided into three stages as confining stress increases, i.e. strengthening effect stage, damaging effect stage and rupturing effect stage. In the former two stages (under low confining stress), rock remains intact and rock cutting forces are higher than those obtained under the non-stressed condition, and thus rock cutting by TBM disc cutter is restrained; in the last stage (under high confining stress), rock becomes non-intact and rock slabbing failure is induced by confining stress before disc cutting, and thus rock cutting by TBM disc cutter is facilitated. Meanwhile, some critical values of confining stress and cutter penetration depth are identified to represent the changes of rock cutting state. This study provides better understanding of the influence of confining stress on disc cutter performance and can guide to optimize the TBM operation under stressed condition.

The role of vegetation on gully erosion stabilization at a severely degraded landscape: A case study from Calhoun Experimental Critical Zone Observatory

Gully erosion was evidence of land degradation in the southern Piedmont, site of the Calhoun Critical Zone Observatory (CCZO), during the cotton farming era. Understanding of the underlying gully erosion processes is essential to develop gully erosion models that could be useful in assessing the effectiveness of remedial and soil erosion control measures such as gully backfilling, revegetation, and terracing.Development and validation of process-based gully erosion models is difficult because observations of the formation and progression of gullies are limited. In this study, analytic formulations of the two dominant gullying processes, namely, plunge pool erosion and slab failure, are utilized to simulate the gullying processes in the 4-km2 Holcombe's Branch watershed. In order to calibrate parameters of the gully erosion model, gully features (e.g., depth and area) extracted from a high-resolution LiDAR map are used. After the calibration, the gully model is able to delineate the spatial extent of gullies whose statistics are in close agreement with the gullies extracted from the LiDAR DEM. Several simulations with the calibrated model are explored to evaluate the effectiveness of various gully remedial measures, such as backfilling and revegetation. The results show that in the short-term, the reshaping of the topographical surface by backfilling and compacting gullies is effective in slowing down the growth of gullies (e.g., backfilling decreased the spatial extent of gullies by 21–46% and decreased the average depth of gullies by up to 9%). Revegetation, however, is a more effective approach to stabilizing gullies that would otherwise expand if no gully remedial measures are implemented. Analyses of our simulations show that the gully stabilization effect of revegetation varies over a wide range, i.e., leading to 23–69% reduction of the spatial extent of gullies and up to 45% reduction in the depth of gullies, depending on the selection of plant species and management practices.

Modeling of failure characteristics of rectangular hard rock influenced by sample height-to-width ratios: A finite/discrete element approach

In the present study, a combined finite/element approach is used to analyze failure characteristics of a rectangular hard rock with different sample height-to-width ratios based on the uniaxial compression test. The objective of this paper is to illustrate the transitional mechanisms from shear fracture to extension failure of brittle rocks and the occurrence conditions of surface-parallel slabbing. Numerical results show that failure modes have experienced the shear, tension with shear and surface-parallel slabbing failure with the decrease of sample height-to-width ratios. It is seen from the simulation results that the shear band in larger height-to-width ratios is composed of an amount of extension meso-cracks that are parallel to the loading direction, and the slabbing failure tends to be the result of the accessible propagation of extension meso-cracks due to short sample height, which validate the essence of extension failure in hard rocks. The good agreement between simulation and laboratory testing also shows the practicability and advancement of a combined finite/discrete element approach.

Lessons Learned from Foundation and Slab Failures on Expansive Soils

AbstractFour case histories are presented to define lessons that can be learned from the investigation of the failure of slabs, footings, and deep foundation elements caused by heaving of expansive...

Modelling the Slab Failure of an Open Structure Acted by External Blast Loads

Abstract The explosion of bombs near buildings generally yields severe damages to the structures. Explosion resistant standards and requirements are constantly being developed and upgraded. This paper focuses on the damages which occur toa RC slab due to blast action. The numerical model replicates a ¼ scale experiment. The analysis is conducted using a software based on the recently developed Applied Element Method. This numerical method is able to model accurately all the structural behavior stages up to failure. The results are compared to experimental data available in the literature. The analysis reveals that the slab failure due to uplift pressures may be avoided by some simple reinforcing details, as they are listed in the Romanian National Annex – accidental loads of the Eurocode EN 1991-1-7.

Experimental Investigation on Shear Connectors in Steel-concrete Composite Deck Slabs

Background/Objectives: The steel-concrete composite construction has become an effective construction practice in the recent years. The cold form profiled sheet acts a platform for during the construction phase and acts as an external reinforcement after the construction. In steel-concrete composite construction, the shear connectors transfer the longitudinal shear force across the steel flange/concrete interface. The ability to transfer longitudinal shear force by shear connectors mainly depends on the strength of concrete against longitudinal cracking and mechanical properties of the shear connectors. This research investigates shear connector's capacity; the experimental investigation is involved using push-out test for two sets with different shear connectors framing to the profile steel sheet. Methods/Statistical Analysis: Set-W consists of three composite slab specimens with welded stud shear connectors, while set-B consists of three composite slab specimens with bolted shear connectors. Trapezoidal profile steel sheet of thickness 1 mm with rectangular embossments is used. Several key parameters of the composite slab are examined and measured, including slippage capacity, load carrying capacity, composite slab ductility and failure mechanism. Findings: the welded shear connectors provided a better performance in resisting slippage and providing a welded set higher load carrying capacity, however, the bolted shear connector specimens set showed more ductile behavior as compared to of specimens.

Failure of wall–slab joint in unreinforced masonry building

Investigations on buildings severely damaged due to earthquakes or explosions have indicated that unreinforced masonry buildings with simply supported precast concrete floor slabs exhibit deficiencies in resistance to progressive collapse, compared to unreinforced masonry buildings with continuous cast in situ concrete floor slabs. The collapse mechanisms observed in the two types of unreinforced masonry buildings are closely related to wall–slab joint failure. The purpose of this study is to investigate the failure behavior of wall–slab joints and the effect on the collapse of the two types of unreinforced masonry buildings. Six wall–slab joint specimens and eight grooved wall specimens, induced by partial failure of wall–slab joints, were tested under monotonic vertical and horizontal loading. Numerical models were then developed, verified, and used to perform a parametric study. It was found that the wall–slab joints failed in various modes, that is, slab failure, wall failure, and slab pullout failure. The grooved wall could fail in bending or in compression. Analyses indicated that the collapse of unreinforced masonry buildings with simply supported precast concrete floor slabs develops in both vertical and horizontal directions. However, the collapse of unreinforced masonry buildings with continuous cast in situ concrete floor slabs is prone to develop only in the vertical direction, resulting in improved progressive collapse resistance.

Monitoring and behavior of unsaturated volcanic pyroclastic in the Metropolitan Area of San Salvador, El Salvador.

Field monitoring and laboratory results are presented for an unsaturated volcanic pyroclastic. The pyroclastic belongs to the latest plinian eruption of the Ilopango Caldera in the Metropolitan Area of San Salvador, and is constantly affected by intense erosion, collapse, slab failure, sand/silt/debris flowslide and debris avalanche during the rainy season or earthquakes. Being the flowslides more common but with smaller volume. During the research, preliminary results of rain threshold were obtained of flowslides, this was recorded with the TMS3 (a moisture sensor device using time domain transmission) installed in some slopes. TMS3 has been used before in biology, ecology and soil sciences, and for the first time was used for engineering geology in this research. This device uses electromagnetic waves to obtain moisture content of the soil and a calibration curve is necessary. With the behavior observed during this project is possible to conclude that not only climatic factors as rain quantity, temperature and evaporation are important into landslide susceptibility but also information of suction–moisture content, seepage, topography, weathering, ground deformation, vibrations, cracks, vegetation/roots and the presence of crust covering the surface are necessary to research in each site. Results of the field monitoring indicates that the presence of biological soil crusts a complex mosaic of soil, green algae, lichens, mosses, micro-fungi, cyanobacteria and other bacteria covering the slopes surface can protect somehow the steep slopes reducing the runoff process and mass wasting processes. The results obtained during the assessment will help explaining the mass wasting problems occurring in some pyroclastic soils and its possible use in mitigation works and early warning system.

Shear band propagation analysis of submarine slope stability

The paper summarises recent developments in the shear band propagation approach, which enables simplified submarine slope stability analysis to account for catastrophic and progressive failure. This approach covers a wide variety of potential failure mechanisms, such as slab failures, spreadings, ploughings and run-outs, and provides analytical energy balance criteria for predicting their occurrence. The simple form of the resulting criteria enabled their incorporation into deterministic and probabilistic slope stability analysis of offshore developments within the geographical information system. In contrast to conventional limiting equilibrium approaches used in such analysis, the shear band propagation approach is capable of explaining enormous dimensions of observed palaeo-landslides and predicting annual probabilities of failure that are orders of magnitude higher, as validated by reconstructed historical frequencies.

Recurrent rockfall phenomena affecting the sea-cliffs of the Campania shoreline

By means of the illustration of two case-histories regarding sea-cliffs located at Agropoli and Palinuro (southern Italy), an overview of the most widely common and recurrent rockfall failures affecting the Campania shoreline is shown. At Agropoli, wedge failures are the most frequent episodic phenomena favoured by the geo-structural and geomechanical layout of the rock mass, and subordinately the assailing force of waves. A probabilistic approach for the slope safety factor calculation, that uses kinematic and kinetic instability assessment of wedge failures, was performed. In order to design suitable supports for the sea-cliff protection, also a wedge probability failure zonation of the cliff was proposed. With reference to the Palinuro case-history, the geostructural layout and geomechanical characterization of a sea arch affected by toppling and slab failures were performed, in order to propose actions through cement-based injection grouts and rock bolting able to reinforce the structure.

Concrete slab analyses with field-assigned non-uniform support conditions

Intelligent compaction (IC) technologies have been used to locate and identify areas of soft, weak or non-uniform subgrade support, which can be linked to pavement performance to set realistic limits on allowable non-uniformity as identified by IC. Based on field data, finite element analyses were performed to investigate the impact of non-uniformities on concrete slab tensile stresses. Non-uniform support cases were modelled by deterministically and randomly assigning field-correlated moduli of subgrade reaction (k-values) to specific spatial areas in the foundation layer. Stresses depended on the size of the non-uniform area, the stiffness change between adjacent non-uniform areas, load location and configuration, and temperature differential. Cases with randomly assigned k-values showed as much as a 39% increase in peak slab tensile stresses relative to a uniform support condition, which could lead to premature slab failures. The ability of IC to locate foundation non-uniformities could increase the reliability of concrete pavements.

Failure properties of rocks in true triaxial unloading compressive test

Slabbing failure often occurs in the surround rock near a deep underground excavation. The mechanism of slabbing failure is still unclear. In order to reveal the influence of the intermediate principal stress (σ2) on slabbing failure, true triaxial unloading compressive test was carried out based on the stress path of the underground engineering excavation, i.e., unloading the minimum principal stress (σ3), keeping σ2, increasing the maximum principal stress (σ1). The initiation and the propagation of slabbing fracture in rock specimens were identified by examining the acoustic emission (AE) and the infrared radiation characterization. The test results show that the failure modes of the granite and red sandstone specimens are changed from shear to slabbing with the increase of σ2. The AE characteristic of rock specimen under low σ2 is swarm type which is the main shock type under high σ2. The infrared radiation properties of rock specimen under different σ2 are also different. The temperature change area is just along the shear fracture such as the uniaxial compression. With the increase of σ2, the temperature change area is planar of rock specimen which proofs that the failure mode of rock specimen turns into slabbing.

Strengthening of flat slabs with FRP fan for punching shear

The results of an experimental program on two-way flat slabs with central loading are presented in this paper. All the slabs were designed according to ACI 318-08 code provisions. One slab served as control without any modification while three slabs were strengthened in different ways with an innovative technique of using FRP fans after casting to increase shear capacity and provide sufficient anchorage. For strengthening in each case, 8, 16 and 24 strengtheners were used. The results of the experiments showed that the presented strengthening method is capable of enhancing both maximum loading capacity and deformation capacity as well as avoiding brittle failures that may occur under vertical loading. The dominant failure mode for flat slabs strengthened with FRP fans was deboning of FRP fans due to the small depth of slabs and simultaneously with separation of end anchorage from the concrete surface. It was shown that this method of strengthening not only can increase the shear capacity of the slab up to a high value around twice of that of controlled slab but also may change the slab failure mode from shear to shear–flexural and also flexural. The test results were also compared with the codes predictions to show the efficiency of the presented strengthening. The comparison showed that the ACI 318 code predictions are more conservative than those of JSCE code for results of this study.

Experimental Study on Flexural Capacity of Prestressed RC Hollow Slabs Strengthened by CFRP

Based on the experiment of prestressed concrete hollow slab flexural capacity, it obtained CFRP prestressed concrete hollow slab Ultimate bearing capacity, failure mode and load - displacement curve, and analyzed the influence of Mechanical performance and reinforcement effect caused by the paste way and paste amount of CFRP. Based on the test results to determine the most optimal paste way and paste amount of CFRP for the different damage (slight damage, moderate damage and severe damage) prestressed concrete hollow slab to meet their safety requirements.

Performance and damages of reinforced concrete buildings during the October 23 and November 9, 2011 Van, Turkey, earthquakes

In this paper, it aimed to investigate the performance and damages of reinforced concrete buildings during October 23 (Erciş) and November 9 (Edremit), 2011 Van earthquakes in Turkey. A total of 28,000 buildings are damaged or collapsed in the city center and surrounding villages after the Erciş earthquake. This number increased to 35,000 after the Edremit earthquake. Large proportion of non-engineering reinforced concrete buildings completely collapsed or were damaged heavily. Most of the reinforced concrete buildings in the affected area are not designed and constructed in accordance with Turkish Earthquake Resistant Design Code. The cracking and failure patterns of the reinforced concrete buildings are examined. Seismic code requirements are discussed and compared with observed details. It is seen that the damages are due to several reasons such as site effect, location, and length of the fault, the poor construction quality, the poor concrete strength quality and unribbed reinforcement steel, poor detailing in beam column joints, strong beam–weak columns, soft stories, weak stories, inadequate transverse reinforcement, existence of short lap splices and incorrect end hook angle, short columns, weak walls, inadequate safe distance between buildings, unconfined gable walls, concrete slab failures, insulation materials, broken staircase slab and cracks at the corners of the windows and doors. Also, it is emerged that 26% of the buildings have no building license, 66% of the buildings' ground floor is used as a shop, 36% of the buildings have no static project, building (static) projects of 57% of the structures are insufficient, 57% of the buildings are not constructed in accordance with their static project, 74% of the static projects have no detail drawings and hoops densifications, the majority of the buildings have not got sufficient compressive strength according to the related earthquake code, aggregate dimensions are larger than maximum aggregate diameter for concrete, 60% of the building was constructed using un-ribbed reinforcement steel, and the majority of the buildings (85%) have not got any soil report. In addition to these reasons, the two earthquakes hit the reinforced concrete buildings within 17 days, causing progressive damage.

Field-scale assessment of effective dilation angle and peak shear displacement for a footwall slab failure surface

When designing high steep footwall slopes, an accurate assessment of the dilational shear strength component is necessary. Bedding surface profiles taken from a detailed digital terrain model of a slip surface exposed by a large slab-type footwall failure were used to characterize the geometric roughness and dilation angles in the sliding direction. The dilation angles predicted from best-fit dilatancy curves are sensitive to the base distance or shear displacement. The dilation angle for a given shear displacement (or base distance) is not sensitive to the profile length if it exceeds the typical in situ block size. For the case history, a 2D finite element analysis was performed to back-calculate the effective dilation angle assuming no cohesion was present and the basic friction angle was 25°. This analysis suggested that a dilation angle of 12° was required to create a condition of incipient slab failure. Using dilatancy curve equations, it is possible to convert dilation angles back-calculated from finite element stability analyses into shear displacements required to mobilize the peak strength. The shear displacement required to mobilize a dilation angle of 12° is estimated to be 0.02–0.15 m for the slab failure that was studied. Analysis of profile geometries to estimate the shear displacement needed to reach peak shear strength will improve the interpretation of slope displacement monitoring data.

Rockfall monitoring by Terrestrial Laser Scanning – case study of the basaltic rock face at Castellfollit de la Roca (Catalonia, Spain)

Abstract. This case study deals with a rock face monitoring in urban areas using a Terrestrial Laser Scanner. The pilot study area is an almost vertical, fifty meter high cliff, on top of which the village of Castellfollit de la Roca is located. Rockfall activity is currently causing a retreat of the rock face, which may endanger the houses located at its edge. TLS datasets consist of high density 3-D point clouds acquired from five stations, nine times in a time span of 22 months (from March 2006 to January 2008). The change detection, i.e. rockfalls, was performed through a sequential comparison of datasets. Two types of mass movement were detected in the monitoring period: (a) detachment of single basaltic columns, with magnitudes below 1.5 m3 and (b) detachment of groups of columns, with magnitudes of 1.5 to 150 m3. Furthermore, the historical record revealed (c) the occurrence of slab failures with magnitudes higher than 150 m3. Displacements of a likely slab failure were measured, suggesting an apparent stationary stage. Even failures are clearly episodic, our results, together with the study of the historical record, enabled us to estimate a mean detachment of material from 46 to 91.5 m3 year−1. The application of TLS considerably improved our understanding of rockfall phenomena in the study area.

Comparison of limit-equilibrium, numerical and physical models of wall slope stability

We review commonly observed failure mechanisms associated with footwall slopes, and examine the basic concepts underlying the limit equilibrium approach assessing footwall slope stability. The failure mechanisms described are those, where failure follows pre-existing joints or occurs in intact rock, namely, fully and partially joint-controlled bilinear and ploughing slab failures. Numerical models implemented using UDEC illustrate the code’s potential for analysing these failure mechanisms. Physical models representing the studied instability mechanisms were studied in a tilt-test set-up in the laboratory, with the empirical results comparing fairly well with the theoretical approaches.