Unconfined Compressive Strength Of Rock Research Articles

In situ stress magnitudes at the toe of the Nankai Trough Accretionary Prism, offshore Shikoku Island, Japan

AbstractQuantifying the orientation and magnitude of tectonic stresses is essential toward understanding deformation and faulting in subduction zones. However, constraints on in situ horizontal stress magnitudes (Shmin and SHmax) are rare. We estimate Shmin and SHmax at Ocean Drilling Program Site 808 at the toe of the Nankai accretionary prism offshore Japan, using coupled constraints from (1) the width of wellbore breakouts together with estimates of rock strength and a model describing stress redistribution at the borehole wall and (2) limits on regional differential stress defined by failure on preexisting faults. Our analysis extends from 175 to 915 m below seafloor (mbsf) and spans the active frontal thrust. For an upper bound on rock unconfined compressive strength (UCS) and assuming hydrostatic formation pore pressure, Shmin and SHmax (referenced to the seafloor) increase from 6.5 MPa at 175 mbsf to 17.4 MPa at 915 mbsf, with the stress state gradually transitioning from a thrust or strike‐slip faulting regime above 800 mbsf to a normal faulting regime below. For cases with higher formation pore pressure, horizontal stresses are slightly lower but follow a similar pattern. We show that estimated Shmin and SHmax are strongly dependent on UCS, breakout width, and friction coefficient, all of which are characterized by uncertainty. Our results suggest that the prism is near thrust failure in the upper ~300 mbsf, but far from failure below. This may be reconciled with active thrusting if thrust faults are locally weaker than the surrounding rock or if SHmax fluctuates during the seismic cycle.

An Equation to Evaluate the Unconfined Compressive Strength of Rock from Splitting Tensile Strength Test Results

The unconfined compressive strength (UCS) of rock is a basic parameter for the design of foundations resting on rock. However, it is often very difficult to retrieve intact rock core specimens for the UCS test due to mechanical breaking or natural fracturing of the rock core during the drilling process. In such cases, it becomes obligatory to correlate UCS values based on the point load strength index test results. Correlation between the UCS and the point load strength index test results has been observed to be inconsistent in many instances and hence a need for another such correlation with a relatively higher degree of accuracy to predict UCS values of rock sample has been generated. An endeavor has been made to correlate the UCS value based on the test results of a splitting tensile strength test (STS) by conducting both the tests on various rock samples. Though the UCS test provides compressive strength and the STS test provides tensile strength of a rock, a dimensional analysis technique has been used to correlate these properties. The predicted UCS results are compared with the actual lab UCS test results to validate the mathematical equation thus developed. It has been observed from comparison that this equation can be used to predict UCS values from STS results with a fair degree of accuracy. Thus the Dimensional analysis has established a provisional rule-of-thumb to correlate the UCS with STS test results and further proved that, it is an easy and sufficiently accurate analytical tool.

Evaluation of the unconfined compressive strength of rocks using nail guns

The penetration depth is one of the major index parameters for indirect determination of the unconfined compressive strength (UCS) of rock. That is, the weaker the rock, the deeper the nail penetration should be, and vice versa. However, the penetration depth depends on the strength of material, the impact energy and nail diameters of nail guns. In order for an empirical equation relating the penetration depth to the UCS to be widely used, the equation should be related to all varieties of gasnailers.The scope of this investigation is to develop an empirical equation relating the nail penetration depth (h) obtained through nail guns of different energy levels (e) and nail diameters (d) to UCS. In this context, a series of gasnailers with different energy levels and nail diameters were performed on 65 rock blocks. The results of laboratory conducted unconfined compressive test were correlated with nail penetration depth (h), nail gun energy (e) and nail diameter (d) and an empirical equation was established using multiple regression analysis. It covers a relatively wide range of energy levels and nail diameters for commercial gasnailers. The proposed equation can be used to estimate the UCS of intact rocks, thus providing the technical features needed for using commercial gasnailers.

Explicit formulation of bearing capacity of shallow foundations on rock masses using artificial neural networks: application and supplementary studies

A major concern in the design of foundations is to achieve a precise estimation of bearing capacity of the underlying soil or rock mass. The present study proposes a new design equation for the prediction of the bearing capacity of shallow foundations on rock masses utilizing artificial neural network (ANN). The bearing capacity is formulated in terms of rock mass rating, unconfined compressive strength of rock, ratio of joint spacing to foundation width, and angle of internal friction for the rock mass. Further, a conventional calculation procedure is proposed based on the fixed connection weights and bias factors of the best ANN structure. A comprehensive database of rock socket, centrifuge rock socket, plate load, and large-scaled footing load test results is used for the model development. Sensitivity and parametric analyses are conducted and discussed. The results clearly demonstrate the acceptable performance of the derived model for estimating the bearing capacity of shallow foundations. The proposed prediction equation has a notably better performance than the traditional equations.

Review of Design Methods of the Ultimate Side Shear and Base Resistance for Rock-Socketed Pile

To determine the ultimate load capacity of drilled shaft socketed into rock under axial compression loading, it is necessary to predict both the ultimate side shear resistance and the base resistance based on field load test or/ and laboratory tests. In geotechnical engineering there are several methods proposed the relationship between rock properties (the unconfined compressive strength) and the ultimate side shears resistance and base resistance. This paper presents the review of design methods of ultimate side shear and base resistance for rock-socketed pile. These empirical functions depend on the socket type and the range of the unconfined compressive strength of rock.

절리암반에 설치된 단말뚝기초의 선단지지력에 관한 수치해석 연구

본 연구에서는 2차원 유한차분 해석을 이용하여 절리암반에 시공된 말뚝기초의 선단지지력을 분석하였다. 이를 위해 암반의 절리상태를 고려할 수 있는 Hoek-Brown 항복규준을 이용하여 다양한 조건의 수치해석이 수행되었다. 넓은 범위의 절리상태를 고려하기 위하여 GSI를 주 매개변수로 설정하였고 말뚝 직경, 암반의 일축압축강도, Hoek-Brown 상수 <TEX>$m_0$</TEX>의 매개변수 영향을 살펴보았다. 본 연구결과 매개변수들의 변화에 따른 절리 암반에 설치된 말뚝기초의 선단지지력 변화 경향을 확인하였고 이러한 결과를 토대로 매개변수 분석에 따른 선단지지력 결과를 도출하였다. This paper presents the results of a two-dimensional finite element analysis of end bearing capacity of single pile installed in fractured rock mass. A number of cases were analyzed using Hoek-Brown criterion that can consider the condition of rock joints. Considering a wide range of joint conditions in which the pile is embedded into the rock, GSI was set as a main parameter. And the effects of pile diameter, unconfined compressive strength of rock and Hoek-Brown constant <TEX>$m_0$</TEX> were considered. Based on parameter study, end bearing load factor graphs were suggested.

An intelligent approach to predict unconfined compressive strength of rock surrounding access tunnels in longwall coal mining

Unconfined compressive strength (UCS) of rocks is one of the most important parameters in rock engineering, engineering geology, and mining projects. In the laboratory determination of UCS, high-quality samples are necessary; in which preparing of core samples has several limits, as it is difficult, expensive, and time-consuming. For this, development of predictive models to determine the UCS of rocks seems to be an attractive research. In this study, an intelligent approach based on the Mamdani fuzzy model was utilized to predict UCS of rock surrounding access tunnels in longwall coal mining. To approve the capability of this approach, the obtained results are compared to the results of statistical model. A database containing 93 rock sample records, ranging from weak to very strong rock types, was used to develop and test the models. For the evaluation of models performance, determination coefficient (R 2), root mean square error, and variance account for indices were used. Based on this comparison, it was concluded that performance of fuzzy model is considerably better than statistical model. Also, the fuzzy model results indicate very close agreement for the UCS with the laboratory measurements. Furthermore, the fuzzy model sensitivity analysis shows that Schmidt hardness and porosity are the most and least effective parameters on the UCS, respectively.

Rock strength determination from scratch tests

This paper provides compelling experimental evidence that the unconfined compressive strength of rocks can be reliably assessed from scratch tests performed with a sharp cutter, and at depth of cut small enough to prevent any significant chipping of the rock. The paper describes the model used to interpret the experimental results, the test methodology, and the Rock Strength Device that was developed to perform scratch tests under kinematically controlled conditions. It concludes with a summary of an extensive experimental campaign involving the testing of several hundred rocks to compare strength data from conventional uniaxial compression experiments and from scratch tests.

A comparative study of generalized regression neural network approach and adaptive neuro-fuzzy inference systems for prediction of unconfined compressive strength of rocks

The engineering properties of rocks play a significant role in planning and designing of mining and civil engineering projects. A laboratory database of mechanical and engineering properties of rocks is always required for site characterization and mineral exploitation. Due to discontinuous and variable nature of rock masses, it is difficult to obtain all physicomechanical properties of rocks precisely. Prediction of unconfined compressive strength from seismic wave velocities (Compressional wave, Shear wave) and density of rock using generalized regression neural network (GRNN) and adaptive neuro-fuzzy inference systems (ANFIS) can be appropriate and alternate methods to minimize the time and cost of tests. GRNN and ANFIS models were trained with 41 data sets using conjugate gradient descent algorithms and hybrid learning algorithm, respectively. Performance of both the models was examined with 15 testing data sets. In the present study, obtained network performance indices such as correlation coefficient, mean absolute percentage error, root mean square error and variance account for indicate high performance of predictive capability of GRNN system and closer to actual data over the ANFIS.

Prediction of unconfined compressive strength of rock surrounding a roadway using artificial neural network

The unconfined compressive strength (UCS) of rocks is an important design parameter in rock engineering and geotechnics, which is required and determined for rock mechanical studies in mining and civil projects. This parameter is usually determined through a laboratory UCS test. Since the preparation of high-quality samples is difficult, expensive and time consuming for laboratory tests, development of predictive models for determining the mechanical properties of rocks seems to be essential in rock engineering. In this study, an attempt was made to develop an artificial neural network (ANN) and multivariable regression analysis (MVRA) models in order to predict UCS of rock surrounding a roadway. For this, a database of laboratory tests was prepared, which includes rock type, Schmidt hardness, density and porosity as input parameters and UCS as output parameter. To make a database (including 93 datasets), different rock samples, ranging from weak to very strong types, are used. To compare the performance of developed models, determination coefficient (R2), variance account for (VAF), mean absolute error (Ea) and mean relative error (Er) indices between predicted and measured values were calculated. Based on this comparison, it was concluded that performance of the ANN model is considerably better than the MVRA model. Further, a sensitivity analysis shows that rock density and Schmidt hardness were recognized as the most effective parameters, whereas porosity was considered as the least effective input parameter on the ANN model output (UCS) in this study.

Axial bearing capacity of socketed single cast-in-place piles

In this paper, a comparison is made of the axial bearing capacities estimated with pile load tests and empirical methods for seven rock-socketed single cast-in-place piles constructed in Turkey. The unconfined compressive strength of rock, obtained from pressuremeter tests, is used in the empirical correlations. It is commonly assumed that axial loads applied at the top of a socketed pile are transferred to the sides of the socket until a certain displacement is reached and that the end bearing capacity contributes only after this threshold displacement is exceeded. In practice, however, due to typically small displacements occurring in rock sockets, most, if not all, of the axial capacity is estimated to derive from the side shear. The limit displacement up to which the side frictional capacity of a socketed pile governs and the end bearing capacity starts mobilizing is examined, and no such threshold value is observed based on the findings of this study. Nevertheless, the bearing capacities obtained from the empirical correlations agree reasonably well with those calculated from pile load tests, when a systematic approach for estimating the threshold value from pile load tests is utilized and the unconfined compressive strength of socketed rocks can be estimated within reasonable accuracy applying actual field conditions.

Prediction of end-bearing capacity of rock-socketed shafts considering rock quality designation (RQD)

Existing empirical methods for determining the end-bearing capacity, qmax, use empirical relations between qmax and the unconfined compressive strength of intact rock, σc. As rock-socketed shafts are supported by the rock mass, not just the intact rock, one should consider not only the intact rock properties, but also the influence of discontinuities on rock mass properties when determining qmax. Although semi-empirical and analytical methods have been developed that can consider the effect of discontinuities, they are more complicated than the empirical relations and require information about discontinuities that is often not available or difficult to obtain in engineering practice. In this paper, an empirical relation between qmax and the unconfined compressive strength of rock mass, σcm, is developed. The new empirical relation explicitly considers the effect of discontinuities represented by rock quality designation (RQD), which is the parameter normally obtained in engineering practice. The accuracy of the expression for estimating σcm based on RQD is verified by comparing its estimation values with those from the existing empirical expressions based on rock mass classification. Two examples are presented to show the application of the newly developed empirical relation between qmax and σcm.

Calibration of Methods For Designing Large-Diameter Bored Piles: Ultimate Limit State

The development of limit-state design codes for large-diameter bored piles requires a careful calibration of existing design methods. Due to various sources of uncertainty in understanding the geology and construction details, estimation of design loads, and design analysis models and their parameters, the estimated pile capacity values from several design methods for large-diameter bored piles vary widely and the corresponding reliability levels are not clearly known. This paper aims at evaluating the performance of six design methods for bored piles in soils and four design methods for bored piles socketed in rocks based on a database of 62 full-scale load tests on large-diameter bored piles in Hong Kong. These methods include a descriptive method, the β-method, and several empirical correlations with blow count from the standard penetration test or unconfined compressive strength of rock. The capability of these design methods to estimate individual resistance components and the total capacity is calibrated separately. Construction effects, particularly the use of permanent liners, pile-shaft grouting, and the time delay between the end of pile excavation and pile concreting, are explicitly considered. The performance of each design method is indicated by the first and second moments of the model bias factor, which can be used as a basis for developing partial factors for limit state design.

A new testing method for indirect determination of the unconfined compressive strength of rocks

Abstract A new testing method for the indirect determination of unconfined compressive strength (UCS) of rock core samples is presented. As known, there exist several methods for indirect estimation of UCS, such as point load index ( I s ), Schmidt hammer, sonic velocity, block punch strength test, etc. Although the point load index testing method is widely used to estimate UCS, there are many problems and limitations related to this method as reported in the recent literature. The “core strangle test” (CST) proposed in this paper overcomes some of these deficiencies and limitations. The principle of this test depends on the “strangle” type of loading a core along a circle perpendicular to the core axis. In the first stage of this study, blocks of different types of rocks having the strength in a range from weak to strong were collected and cored for UCS, point load index and CST tests. These tests were then conducted and relationships between UCS with I s (50) and CST were empirically explained and discussed.

Relationship between Unconfined Compressive Strength and Degree of Saturation for Selected Sandstones

Research Article| February 01, 2009 Relationship between Unconfined Compressive Strength and Degree of Saturation for Selected Sandstones ABDUL SHAKOOR; ABDUL SHAKOOR 1Department of Geology, Kent State University, Kent, OH 44242 Search for other works by this author on: GSW Google Scholar EDWARD H. BAREFIELD EDWARD H. BAREFIELD 2Michael Baker Jr., Inc., 4301 Dutch Ridge Road, Beaver, PA 15009 Search for other works by this author on: GSW Google Scholar Author and Article Information ABDUL SHAKOOR 1Department of Geology, Kent State University, Kent, OH 44242 EDWARD H. BAREFIELD 2Michael Baker Jr., Inc., 4301 Dutch Ridge Road, Beaver, PA 15009 Publisher: Association of Environmental & Engineering Geologists First Online: 02 Mar 2017 Online ISSN: 1558-9161 Print ISSN: 1078-7275 © 2007 Association of Engineering Geologists Environmental & Engineering Geoscience (2009) 15 (1): 29–40. https://doi.org/10.2113/gseegeosci.15.1.29 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ABDUL SHAKOOR, EDWARD H. BAREFIELD; Relationship between Unconfined Compressive Strength and Degree of Saturation for Selected Sandstones. Environmental & Engineering Geoscience 2009;; 15 (1): 29–40. doi: https://doi.org/10.2113/gseegeosci.15.1.29 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Unconfined compressive strength of rocks is known to vary with changes in moisture content. However, previous studies report inconsistent results about this subject. The purpose of this study was to investigate the relationship between unconfined compressive strength and varying degrees of saturation for sandstone rocks and explain it in terms of index engineering properties and petrographic characteristics. Eighteen NX-size (2.125 inches or 5.4 cm in diameter) cores were prepared from each of the nine sandstones sampled from central Ohio through central Pennsylvania. Laboratory tests were conducted to determine absorption, dry density, specific gravity, and porosity values for each core. Cores were then tested for unconfined compressive strength at 0 percent, 20 percent, 40 percent, 60 percent, 80 percent, and 100 percent saturation. Statistical analyses were performed to determine the relationship between unconfined compressive strength and degree of saturation. The results indicate significant trends of unconfined compressive strength reduction with increasing degree of saturation and strength reductions of up to 71.6 percent between dry and saturated states. The trends are more consistent for high-strength sandstones than those for medium- to low-strength sandstones, which display less predictable behavior. Some sandstones show a majority of strength reduction occurring by 20 percent saturation, and minimal to indiscernible strength reductions at higher saturation levels. Based on these results, prediction equations were developed for sandstones, which were grouped into four classes on the basis of their absorption values. The input parameters required for the prediction equations include degree of saturation and one or more of the index engineering properties. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A ROP-Management Process in Qatar North Field

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 105521, "Implementation of ROP-Management Process in Qatar North Field," by S.M. Remmert, SPE, and J.W. Witt, SPE, RasGas Co. Ltd., and F.E. Dupriest, SPE, ExxonMobil Development Co., prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20–22 February. In March 2005, the operator implemented a rate-of-penetration (ROP) -management process in Qatar's North field. The ROP-management process uses real-time, customized surveillance technology to maximize continuously both drill-bit-cutter efficiency and transmission of energy to the bit. To date, the development pro-gram has been accelerated by 1 year, and USD 54 million has been saved while drilling 470,000 ft of hole. More than 440 personnel have been trained in mechanical specific energy (MSE) analysis, and 50 new field drilling records have been set. Introduction The Qatar operator currently conducts drilling operations at eight platform locations across six very-large producing blocks. The majority of wells are drilled in batch mode by section. Over the years, the operator gained extensive field knowledge and developed many efficient operating practices. Thus, the introduction of any new ROP-management process would be examined against solid baseline operating performance. At the same time, 2005 to 2006 has been a period of transition, as the company increased rig count from three to nine. The addition of many new support personnel in a multicultural working environment presented a significant challenge in terms of maintaining or improving drilling efficiency. In the midst of these challenges, the decision was made to pilot test the ROP-management process, with its first application in such an extensive carbonate-field environment. The focus of the program was fourfold:Conduct extensive customized field training in MSE analysis,implement a standard surveillance program,introduce new practices in a phased manner concurrent with measurement of energy efficiency, andcommunicate learning systematically across all rigs using MSE curves as the basis for discussion. Field Background Aside from one vertical data well at each platform, all wells drilled are 55-to-65° S-shaped directional wells with an abbreviated drop section into the Khuff reservoir. Although plat-forms are miles apart, many wells are quite similar in terms of lithology, hole size, casing configuration, and casing-setting points. An important reference data set when discussing MSE is rock unconfined compressive strength (UCS). The MSE recorded while drilling should vary as the bit traverses rock of various strengths. However, it should vary only by the amount of change in rock strength. In field operations, a baseline MSE is established, and any increase above this that exceeds the change in rock strength is likely to be an indication of bit dysfunction. Learning Cycle To begin the learning cycle, the operator analyzed historical information with hindcast MSE curves (i.e., curves generated from offset wells using existing data). Intervals of significant energy waste (high MSE) were identified and became opportunities for improvement. MSE increases beyond baseline were discussed in relation to change in rock strength. The actual energy waste visible even on the best offset was significant.

Development of a fiber optic stress sensor

To define the behaviour of rocks under stress, certain material properties are required to characterise deformation and failure. The laboratory unconfined compressive strength (UCS) and triaxial behaviour forms the basis of many failure criteria and pillar strength equations. It has been well established that the UCS of rocks is greatly influenced by sample size and structural discontinuities. This is particularly true for coal which has a microscale fracture pattern (cleat) throughout its fabric. The aim of this study was to investigate the relationship between strength and structure and relate this to field performance. Results from laboratory strength tests with petrographic and thermal analysis were collated with other test data on the coal. Samples were selected from a thick, sub bituminous coal seam in the Waikato Coal Measures. This coal is a relatively weak compared to strength data from published sources and it has a defined cleat system. The coal was found to vary in composition throughout the seam section with strength differences partially attributed to material variations as well as preparation and structural controls. The thick seams are mined by a modified room and pillar method which leaves slender yield pillars on retreat. These pillars are essentially large-scale UCS samples. In the field the geometry of the pillars was recorded on completion of mining and the cleat was mapped where possible. It was found that the laboratory results and the historical case study information correlated well with pillar strength formulae. However when compared to the slender yield pillars it did not predict performance. Numerical modelling also failed to represent the observed behaviour. This suggests a different mechanism is involved which is possibly structurally controlled, where the intact strength of the coal appears to be less important than the structure and geometry of the excavation.

Structural effects on the strength of New Zealand coal

To define the behaviour of rocks under stress, certain material properties are required to characterise deformation and failure. The laboratory unconfined compressive strength (UCS) and triaxial behaviour forms the basis of many failure criteria and pillar strength equations. It has been well established that the UCS of rocks is greatly influenced by sample size and structural discontinuities. This is particularly true for coal which has a microscale fracture pattern (cleat) throughout its fabric. The aim of this study was to investigate the relationship between strength and structure and relate this to field performance. Results from laboratory strength tests with petrographic and thermal analysis were collated with other test data on the coal. Samples were selected from a thick, sub bituminous coal seam in the Waikato Coal Measures. This coal is a relatively weak compared to strength data from published sources and it has a defined cleat system. The coal was found to vary in composition throughout the seam section with strength differences partially attributed to material variations as well as preparation and structural controls. The thick seams are mined by a modified room and pillar method which leaves slender yield pillars on retreat. These pillars are essentially large-scale UCS samples. In the field the geometry of the pillars was recorded on completion of mining and the cleat was mapped where possible. It was found that the laboratory results and the historical case study information correlated well with pillar strength formulae. However when compared to the slender yield pillars it did not predict performance. Numerical modelling also failed to represent the observed behaviour. This suggests a different mechanism is involved which is possibly structurally controlled, where the intact strength of the coal appears to be less important than the structure and geometry of the excavation.

Estimating aggregate properties from the unconfined compressive strength of rock : G. West, Quarterly Journal of Engineering Geology, 27(3), 1994, pp 275–276

Estimating aggregate properties from the unconfined compressive strength of rock : G. West, Quarterly Journal of Engineering Geology, 27(3), 1994, pp 275–276

Estimating aggregate properties from the unconfined compressive strenght of rock

Abstract A method is given for estimating the aggregate strength properties from the unconfined compressive strength of rock. It is envisaged that this will prove useful to those who may wish to infer aggregate properties from rock strength data in site investigation reports, in the absence of aggregate test results.