High Strength Rock Research Articles

Testing of rock climbing anchors in weak sandstone

Climbers sometimes rely on fixed anchors which are secured in drilled holes by mechanical expansion or with glue. Most rock climbing takes place on high strength rock and there has been considerable testing of anchors worldwide. However, there has only been limited testing of anchors in weaker rock. Over 60 anchors were tested in weak rock, failure loads of 8–48 kN were achieved. Chemically bonded anchors were the strongest, the 10 mm rings and 10 mm U-bolts were the only anchors that passed the European standard (EN 959). Older style anchors showed low strength and high variability. The relative strengths of single and twin holes anchors followed predictions based on interaction theory. The results also confirm the prediction of mixed glue/rock failure. The cyclic loading behaviour of anchors was explored, anchor with higher stiffness appeared to perform better; the U-bolts had the best fatigue performance.

Suggested Methodology for Rehabilitation of Ancient Masonry Castles and Forts on Rock Hills

Forts, including in this designation military castles, present high complexity due to their nature. Methodologies followed in the rehabilitation of forts are briefly presented, with the emphasis on characterization of materials and rock masses and on tests and numerical models developed for ancient forts. Studies concerning forts with Portuguese legacy are presented. The first case is related with rehabilitation of foundations and walls of the Guimarães Castle, Portugal. The causes of the appearance of holes or torn threads in the exposed face of the wall and the tower of the extreme southwest of the castle were assessed. The intervention performed was focused on the stabilization of the rock mass and careful sealing of discontinuities in the high strength rock mass and consolidation of the most weathered fractured rock mass. The second case is related to forts existing at Muscat, Oman. The protection of the Jalali Fort is studied in detail. During 2007, cyclone Gonu caused substantial damage to the seawall adjacent to the fort. The study addresses the redesign of the seawall.

Response of High-Strength Rock Slope to Seismic Waves in a Shaking Table Test

Studies on landslides by the 2008 Wenchuan earthquake showed that the topography is of great importance in amplifying the seismic shaking. The present study carried out experiments on rock slopes by means of a shaking table. The recorded Wenchuan earthquake waves are scaled to excite the model slopes. Measurements from accelerometers installed on free surface of the model slope simulating high‐strength rocks are analyzed, with much effort on acceleration responses to both horizontal and vertical components of seismic shaking. It is found that the amplification factor of peak horizontal acceleration (PHA) is increasing with elevation of the model slope, though the upper and lower halves of the slope exhibit different increasing patterns. The amplification factor of peak vertical acceleration (PVA) exhibits a lying S‐shaped changing trend with the elevation, indicating attenuations of PVAs at the toe and top of a slope. In addition, the XZ ‐direction shaking produces a horizontal and vertical response stronger than X ‐direction and Z ‐direction shaking alone. Both PHA and PVA increase with the excitation intensity. However, the corresponding amplification factors generally decrease, indicating the acceleration response of a slope weakens with the excitation intensity of shaking. Finally the statistic of ratio of PVA to PHA indicates that 85% of the slope height, especially the upper middle part, is likely subject to PVAs greater than or equal to 2/3 of PHA and 32% of the slope height to PVAs greater than or equal to PHA. This indicates the nonignorable role of PVA in responses of a slope to an earthquake and necessity of considering during design work.

An Experimental Investigation of Shale Mechanical Properties Through Drained and Undrained Test Mechanisms

Shale mechanical properties are evaluated from laboratory tests after a complex workflow that covers tasks from sampling to testing. Due to the heterogeneous nature of shale, it is common to obtain inconsistent test results when evaluating the mechanical properties. In practice, this variation creates errors in numerical modeling when test results differ significantly, even when samples are from a similar core specimen. This is because the fundamental models are based on the supplied test data and a gap is, therefore, always observed during calibration. Thus, the overall goal of this study was to provide additional insight regarding the organization of the non-linear model input parameters in borehole simulations and to assist other researchers involved in the rock physics-related research fields. To achieve this goal, the following parallel activities were carried out: (1) perform triaxial testing with different sample orientations, i.e., 0°, 45°, 60°, and 90°, including the Brazilian test and CT scans, to obtain a reasonably accurate description of the anisotropic properties of shale; (2) apply an accurate interpretative method to evaluate the elastic moduli of shale; (3) evaluate and quantify the mechanical properties of shale by accounting for the beddings plane, variable confinement pressures, drained and undrained test mechanisms, and cyclic versus monotonic test effects. The experimental results indicate that shale has a significant level of heterogeneity. Postfailure analysis confirmed that the failure plane coincides nicely with the weak bedding plane. The drained Poisson’s ratios were, on average, 40 % or lower than the undrained rates. The drained Young’s modulus was approximately 48 % that of the undrained value. These mechanical properties were significantly impacted by the bedding plane orientation. Based on the Brazilian test, the predicted tensile strength perpendicular to the bedding plane was 12 % lower than the value obtained using the standard isotropic correlation test. The cyclic tests provided approximately 6 % higher rock strength than those predicted by the monotonic tests.

Quantitative Analysis of Several Influencing Factors on TBM Advance Speed

Aiming at the problem of a difference between the TBM advance speed estimated by the NTNU and CSM model, and the actual advance speed, a quantitative analysis is carried out in the paper. Combining the experimental results of the uniaxial compressive test with the rock mechanics experiment system TAW-2000 and MTS815, the paper quantitatively characterizes the main parameters including the rock compressive strength, elastic modulus, abrasiveness, and the thrust force of cutterhead, corresponding to TBM advance speed by using the method of quantitative analysis of multi-factors. Moreover, the relationship between the TBM advance speed and the parameters was established. It is indicated that higher rock strength, abrasion and a smaller thrust force of cutterhead are reasons resulting in a lower TBM advance speed.

Erosion of the Rwenzori Mountains, East African Rift, from in situ‐produced cosmogenic10Be

High relief and steep topography are thought to result in high erosion rates. In the Rwenzori Mountains of the Albert Rift, East Africa, where more than 3 km of relief have formed during uplift of the Rwenzori fault block, overall low denudation rates prevail. We measured in situ‐derived cosmogenic denudation rates of 28.2 to 131 mm/kyr in mountainous catchments, and rates of 7.8 to 17.7 mm/kyr on the adjacent low‐relief East African Plateau. These rates are roughly an order of magnitude lower than in other settings of similar relief. We present an extensive geomorphological analysis, and find that denudation rates are positively correlated with relief, hillslope gradient, and channel steepness, indicating that river incision controls erosional processes. In most upper headwater reaches above Quaternary ELA levels (>4500 m a.s.l.), glacial imprinting, inherited from several older and recent minor glaciation stages, prevails. In regions below 4500 m a.s.l., however, mild climatic conditions impede frost shattering, favor dense vegetation, and minimize bare rock areas and associated mass wasting. We conclude that erosion of the Rwenzori Mountains is significantly slower than corresponding rates in other mountains of high relief, due to a combination of factors: extremely dense mountain cloud forest vegetation, high rock strength of gneiss and amphibolite lithologies, and low internal fracturing due to the extensional tectonic setting. This specific combination, unique to this extensional tropical setting, leads to unexpected low erosion rates that cannot outpace post‐Pliocene ongoing rock uplift of the Rwenzori fault block.

Development and Application of Complementary Support Technology under the Condition of Weak and Broken Surrounding Rocks

The support of surrounding rock under the condition of deep mining has significant challenges due to high stress and low rock strength environment. Using the method of strengthening broken rock, reinforcing support force and releasing the mine-induced stress, this study has pointed out the complementary support technology which is based on the support using bolt-wire-shotcrete and is combined with steel arched yielding support. For the purpose of permanent support, the complementary support technology could combine the each support element and adequately take advantage of load-caring capacity of surrounding rock. The monitoring data of Xiaokang coal mine, Muchengjian coal mine and Yangquhe coal mine indicate that, for the various geological conditions, complementary support system has provided powerful support force to resist large deformation of weak and broken surrounding rock. It is proved that this type of support technology could reduce the mine-induced stress as well and has economical significance.

Study on the C80 High-Strength Rock Chips Concrete

Rock chips is a potential substitutable material for river sand, which can effectively solve the shortage problem of the river sand in concrete. At present most studies are about the rock chips concrete concentrate on the normal-strength concrete. The studies about rock chips as a substitution for river sand in C80 high-strength concrete were carried out. At first the basic physical properties of raw material are analyzed and the performance requirements of the concrete are confirmed. And then on the basis of the river sand concrete, the influence of the dosage of rock chips on the C80 high-strength concrete at the same water-binder ratio was studied. The particle size of rock chips larger than 4.75mm was considered as the coarse aggregate. The results showed that the C80 high-strength rock chips concrete can be prepared by increasing the dosage of superplasticizer and binder. The early age strength of high-strength rock chips concrete is higher than that of river sand concrete, while the contrary results of the long-term strength. Considering lots of fine powder in the rock chips, the coarse sand and rock chips are mixed to be used, the mixing ratio needs to be determined by experiments. In the high-strength concrete, the rock chips with good particle size distribution and with lower content of particle size larger than 4.75mm is better.

Deformation and support of roadways subjected to abnormal stresses

In situ stresses, including both magnitude and orientation, play a major role in the stability of underground coal mine roadways. The orientation of the maximum horizontal stress have a significant impact on roadway stability, especially when roadway is driven perpendicular to the maximum horizontal stress direction. A numerical modeling was carried out to investigate the stress re-distribution around roadway under different horizontal stress orientations. In this study, in situ stress measurement was carried out at Changcun Colliery. There are five parallel trunk roadways servicing the S6 mining district at Changcun Colliery. The result indicated that the orientation of the maximum horizontal stress is perpendicular to the five roadways. To investigate the stress re-distribution around the five roadways group, numerical modeling was carried out using a boundary element code Examine 2D. The influence of horizontal stress direction, pillar size between the roadways, and the excavation sequence of the roadways was examined. A reinforcing system which consists of high strength rock bolts and cables was applied at the five roadways. Both the bolts and cables were installed with high pre-tension forces along the whole length of them. Field observation indicated that this reinforcing system was able effectively restrain the deformation and failure of rock mass surrounding roadways.

Geological and geotechnical properties of the medieval rock hewn churches of Lalibela, Northern Ethiopia

Lalibela is a medieval settlement in Northern Ethiopia famous for its 11 beautifully carved rock hewn churches, registered as World Heritage Site in 1978. The rock hewn churches are grouped into three based on their proximity: the Bete Medhane Alem (Church of the Holy Saviour), Bete Gabriel–Rufael (Church of St. Gabriel–Rafael) and Bete Giorgis (Church of St. George) groups. The churches are carved out of a single, massive scoriaceous basalt hill which was deposited along an East–West extending palaeovalley in the Oligo-Miocene Trap basalt of the northwestern Ethiopian plateau. The Rock Mass Rating (RMR) classification scheme was used to classify the rock mass (assuming each church as a separate rock mass) based on their uniaxial compressive strength and the spacing and conditions of discontinuities. Though most of the churches are hewn from medium to high strength rock mass, discontinuities make them vulnerable to other deteriorating agents mainly weathering, and water infiltration. Most of the rock hewn churches are affected by pre-carving cooling joints and bedding plane discontinuities, and by mostly but not necessarily post-carving tectonic and seismic induced cracks and fractures. Material loss due to deep weathering triggered by rain water infiltration and uncontrolled groundwater seepage affects most of the churches, particularly the Bete Merqorios (Church of St. Mark) and Bete Aba Libanos (Church of Father Libanos) churches. The scoriaceous basalt which is porous and permeable allows easy passage of water while the underlying basalt is impermeable, increasing the residence time of water in the porous material, causing deep weathering and subsequent loss of material in some of the churches and adjoining courtyards.

Geotechnical characteristics of large rapid rock slides

Based on the analysis of 51 case studies of large rapid rock slides, for a landslide to travel rapidly after failure there has to be a significant loss of strength on the basal surface of rupture, lateral margins, and (or) internally within the slide mass, or the factor of safety has to be maintained below 1.0 after failure by high groundwater pressures. Internally sheared compound slides and translational slides may all travel rapidly depending on their detailed geotechnical and geometric characteristics. The characteristics of these landslides that suggest an increased likelihood of rapid failure have been identified. All the rapid rock slides examined in this study involved relatively high-strength rock masses. Most cases were considered to be first-time landslides, largely involving brittleness on the basal rupture surface. However, there were some cases considered to be reactivated or active landslides on pre-sheared rupture surfaces. For this latter group, the loss of strength leading to rapid landsliding was associated with brittle internal deformation or lateral margins.

Microstructure-Mechanical Property Relationships for a Fe/Mn/Cr Rock Bolt Reinforcing Steel

The influence of low chromium additions to a 0.25C-1.5Mn semikilled steel on microstructure, and tensile and impact behaviors of high strength rock bolt reinforcing bars has been investigated. Although chromium imparted adequate tensile properties at ambient temperature (yield stress: 624 MPa; ultimate tensile stress: 819 MPa; elongation: 12.5%) by forming transformation products such as tempered martensite, lower and upper bainite, and small amounts of acicular ferrite, it increased the ductile-to-brittle transition temperature due to coarser upper bainite in the core region of bar having larger unit crack paths. The synthesized steel is considered to be effective in realizing the desired tensile properties, and suitable for application in rock bolt, as well as other reinforced concrete structures.

Laboratory Simulation of Drill Bit Dynamics Using a Model-Based Servohydraulic Controller

Drilling costs are significantly influenced by bit performance when drilling in offshore formations. Retrieving and replacing damaged downhole tools is an extraordinarily expensive and time-intensive process, easily costing several hundred thousand dollars of offshore rig time plus the cost of damaged components. Dynamic behavior of the drill string can be particularly problematic when drilling high strength rock, where the risk of bit failure increases dramatically. Many of these dysfunctions arise due to the interaction between the forces developed at the bit-rock interface and the modes of vibration of the drill string. Although existing testing facilities are adequate for characterizing bit performance in various formations and operating conditions, they lack the necessary drill string attributes to characterize the interaction between the bit and the bottom hole assembly (BHA). A facility that includes drill string compliance and yet allows real-rock/bit interaction would provide an advanced practical understanding of the influence of drill string dynamics on bit life and performance. Such a facility can be used to develop new bit designs and cutter materials, qualify downhole component reliability, and thus mitigate the harmful effects of vibration. It can also serve as a platform for investigating process-related parameters, which influence drilling performance and bit-induced vibration to develop improved practices for drilling operators. The development of an advanced laboratory simulation capability is being pursued to allow the dynamic properties of a BHA to be reproduced in the laboratory. This simulated BHA is used to support an actual drill bit while conducting drilling tests in representative rocks in the laboratory. The advanced system can be used to model the response of more complex representations of a drill string with multiple modes of vibration. Application of the system to field drilling data is also addressed.

Effect of Rock Trenching Vibrations on Nearby Structures

Trenching to install deep gravity sewers and drains in medium-to high-strength rock requires large and heavy rock trenchers that produce high levels of vibrations that may affect the structural integrity of nearby utilities and buildings. Although not every vibration causes damage, owners often believe that their structures have been harmed by rock excavation. The resulting disputes can waste a great deal of time and money. In an effort to reduce structural damage and associated disputes, this paper provides guidelines for the safe distance between rock trenchers and nearby buildings and underground structures. Vibration data at different distances from the trencher centerline were collected from five trenching projects in Northwest Ohio. The data analysis suggests a moderate relationship between the vibration level and the distance from the source of vibration. In addition, the risk of damage to nearby structures dissipates significantly and quickly as the distance from the point of excavation increases. Rock trenching should not take place closer than 1.50 m for buried structures and 1.00 m for residential buildings. Beyond this safe distance, damage to nearby structures should not take place.

Prediction of radial bit cutting force in high-strength rocks using multiple linear regression analysis

Prediction of radial bit cutting force in high-strength rocks using multiple linear regression analysis

The relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks

Closure pressures measured during injection tests such as mini-fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude. This paper presents stress, strength and image log data from the Australian Cooper Basin, which suggests that in reservoirs with high in situ stress, high tensile strength and weak geological fabrics, interpreted closure pressures may be significantly greater than the minimum principal stress. Closure pressures interpreted from mini-frac injection tests in the Cooper Basin, suggest the minimum principal stress varies from 12.4–27.2 MPa/km (0.55–1.2 psi/ft). To better understand the reasons for this variation in closure pressure, image logs and mini-frac data from 13 treatment zones, and core from seven of these treatment zones, were analysed. The analysis revealed that treatment zones with high measured closure pressures (⩾18.1 MPa/km; 0.8 psi/ft), high treating pressures (>31.6 MPa/km; 1.4 psi/ft) and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength (>7 MPa; 1015 psi) and geological fabrics (planes of weakness) including natural fractures. Conversely, treatment zones with lower measured closure stress (⩽19 MPa/km; 0.84 psi/ft) and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and/or no geological fabrics. We suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the Cooper Basin where they are associated with hydraulic fracture complexity. Rather, they reflect the normal stress incident on pre-existing weaknesses that are exploited by hydraulic fluid during the mini-frac injection.

Numerical modelling of the western Hodgkinson Province, northeast Queensland: implications for gold mineralisation

Results of coupled mechanical and fluid-flow modelling provide insights into some of the factors that controlled gold mineralisation and deformation in the Hodgkinson Province in northeastern Queensland. These results aid in resolving why the north – south-striking segment of the terrane-bounding Palmerville Fault is barren, while many north – south- to northwest – southeast-trending, second-order fault zones throughout the Hodgkinson Province are spatially associated with gold deposits. The simplified, regional-scale model geometry used in this study permitted variation of boundary conditions, material properties and stress regimes. This included variation of lithological properties, the absence of fault zones in some models, and the application of plane strain or transpression. The model outcomes illustrate the importance of a listric fault geometry for focusing deformation and fluid flow in zones of relatively high permeability and/or low rock strength. Also, the orientation of fault zones with respect to the dominant far-field compressive stress regime is shown to be a key factor controlling deformation and fluid flow. In addition, fault bends and terminations are shown to influence significantly the distribution of deformation and fluid flow in our models. The results provide an improved understanding of first-order factors that may have controlled localisation of deformation and fluid flow in the regional geological architecture of the Hodgkinson Province. Focused fluid flow arising from localised dilation and permeability increase plays an important role in the formation of ore deposits; hence the outputs of the modelling may be of significant value for future exploration in the Hodgkinson Province and analogous regions elsewhere. The study illustrates the usefulness of numerical modelling as a tool for testing multiple scenarios, leading to improved conceptual understanding of geological systems.

Geomechanical aspects of CO2 sequestration in a deep saline reservoir in the Ohio River Valley region

The Ohio River Valley CO2 Storage Project is an ongoing characterization of deep saline formations being considered as potential sites for geological CO2 sequestration. We completed a geomechanical analysis of the Rose Run Sandstone, a potential injection zone, and its adjacent formations at the American Electric Power's 1.3-GW Mountaineer Power Plant in New Haven, West Virginia. The results of this analysis were then applied to three investigations used to evaluate the feasibility of anthropogenic CO2 sequestration in the potential injection zone. First, we incorporated the results of the geomechanical analysis with a geostatistical aquifer model in CO2 injection-flow simulations to test the effects of introducing a hydraulic fracture to increase injectivity. We observed a nearly fourfold increase of injection rate caused by the introduction of a hydraulic fracture in the injection zone. The flow simulations predict that a single vertical well with a hydraulic fracture could inject a maximum of 300–400 kt of CO2/yr. In the second investigation, we determined that horizontal injection wells at the Mountaineer site are feasible because the high rock strength ensures that such wells would be stable in the local stress state. The third investigation used the geomechanical analysis results to evaluate the potential for injection-induced seismicity. If preexisting, but undetected, nearly vertical faults striking north-northeast or east-northeast are present, the increased pore pressure from CO2 injection would raise their reactivation potential. Geomechanical analysis of potential CO2 sequestration sites provides critical information required to evaluate its sequestration potential and associated risks.

Geotechnical Properties of Certain Iraqi Carbonate Rocks

Petrography and some geotechnical ail properth properties of certain Iraqi carbonate rocks belonging to (8) geologic formations from (10) locations, were studied in lab. The influence of petrography and water saturation on the engineering properties was investigated, Correlation between different physical and mechanical properties was achieved. Finally a proposed engineering classification based on (bulkidensity, compressional wave velocity. dynamic modulus of elasticity. uniaxial comparative strength and brazillian tensile strength) for the studied rocks was established it is concluded that ti e studied carbonate rocks can be !divided thy groups according to shay physical and mechanical propertiest they are r u strength, medium to high strength (4.1- low to medium strength) rocks.

Prediction of hollows in abandoned underground workings at shallow depth

Composite statistical analysis of the lithological composition of the rock mass above underground workings in coal seams and experimental work (laboratory mechanical tests and boring for the determination of hollows) in Donetsk city (Ukraine) have been used to develop a method for predicting the presence of hollows. The empirical criteria K1−K2>0 and K3≤2.5 which relate to the physical characteristics of the overlying strata were found to predict the existence of such hollows in abandoned underground workings at shallow depth. The values of K1−K2 and K3 depend on the thicknesses of the different rock layers and the uniaxial compressive strength of the immediate roof over underground openings. The different layers e.g. sandstones, argillites, aleurolites and alluvium in the rock mass are shown to influence the existence of hollows in abandoned workings. Large thicknesses of sandstones in the rock mass or relatively high uniaxial compressive strength rock in the immediate roof contribute to the existence of hollows in abandoned workings. On the other hand, sandstones of small thickness, weak alluvium, argillites and aleurolites only give additional weight on immediate roof of the opening and allow collapse of the rock mass. The method of prediction for hollows was developed from the back-analysis of data from 41 boreholes which were drilled from the ground surface above underground openings.