Estimation of uniaxial compressive strength from point load strength,Schmidt hardness and P-wave velocity

Uniaxial compressive strength is considered one of the most important parameters in the characterization of rock material in rock engineering practice. The study investigated correlations between uniaxial compressive strength and point load index, P-wave velocity and Schmidt hardness rebound number together with the effects of core diameter size. A total of 150 core samples at five different diameters (54, 48, 42, 30 and 21 mm) were obtained from sandstone, limestone and cement mortar. Ten saturated samples at each diameter (length:diameter ratio 2:1) were prepared from each of the three materials. The best correlations were found between uniaxial compressive strength and point load or Schmidt hammer values. The closest relationship was observed for the 48 mm diameter cores.      

Assessment of relationships between drilling rate index and mechanical properties of rocks

This paper examines the relationships between drilling rate index (DRI) and some mechanical properties of rocks in order to evaluate the effect of strength and indexes of rock on rock drillability. For this purpose, some index properties (in situ Schmidt rebound hardness (SRH), Shore scleroscope hardness (SSH), and point load strength (PLS)) and strength properties (uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS)) values of 32 sedimentary, igneous and metamorphic rock samples were determined. The relationships between DRI and both strength and indexes properties were evaluated using regression analysis and statistical methods. As a result, decreasing linear relationships were found between DRI and uniaxial compressive strength, Schmidt rebound hardness, Shore scleroscope hardness, diametral and axial point load strength.     

Predicting uniaxial compressive strength,modulus of elasticity and index properties of rocks using the Schmidt hammer

The Schmidt hammer test is a non-destructive method which can be used in both laboratory and field to provide a quick and relatively inexpensive measure of rock hardness. The study investigated the relationship between the Schmidt hardness and modulus of elasticity, uniaxial compressive strength and index properties of nine types of rock including travertine, limestone, dolomitic limestone and schist. The empirical equations developed indicated the Schmidt hardness rebound values have a reliable relationship with the uniaxial compressive strength of rock (r = 0.92). Comparing the results with those reported by other researchers, it is concluded that no single relationship can be considered reliable for all rock types. Whilst the equations developed in this study may be useful at a preliminary stage of design, they should be used with caution and only for the specified rock types.      

Correlating uniaxial compressive strength with schmidt hardness,point load index,Young's modulus,and mineralogy of gabbros and basalts (northern Greece)

An attempt has been made to correlate the uniaxial compressive strength and Young's modulus of gabbros and basalts with Schmidt hammer rebound number, the point load strength index, I_s(50) and the degree of weathering. Sixty three samples of gabbro and thirty of basalt from the ophiolitic comlex of Pindos zone (Northern Greece) have been collected by core drilling and tested accordingly. The results have been processed using techniques from the statistical software SPSS. Some of the equations produced show relatively high correlation coefficients, all significant at a significance level higher than 95%. The equations establish reliable prediction models for the uniaxial compressive strength and modulus of elasticity of the above rock types by means of simple tests which can be carried out in the field.     

Dominant rock properties affecting the penetration rate of percussive drills

Percussive blast hole drills were observed in eight rock types at an open pit mine and three motorway sites. The net penetration rates of the drills were calculated from the performance measurements. Rock samples were collected from the drilling locations and the physical and mechanical properties of the rocks were determined both in the field and in the laboratory. The penetration rates were correlated with the rock properties. The uniaxial compressive strength, the Brazilian tensile strength, the point load strength and the Schmidt hammer value exhibit strong correlations with the penetration rate. Impact strength shows a fairly good correlation with penetration rate. Weak correlations between penetration rate and both elastic modulus and natural density were found. Any significant correlation between penetration rate and P-wave velocity was not found.It was concluded that, among the rock properties adopted in this study, the uniaxial compressive strength, the Brazilian tensile strength, the point load strength and the Schmidt hammer value are the dominant rock properties effecting the penetration rate of percussive drills. Theoretical specific energy as defined by different research workers is proved also to be well correlated with penetration rate of percussive drills which verifies basic theoretical works on the subject. In addition, the point load and the Schmidt hammer test can practically be used in the field as a predictive tool for the estimation of penetration rate.     

Predicting the physico-mechanical properties of rocks from electrical impedance spectroscopy measurements

Electrical resistivity values of eight different samples cored from a fault breccia were measured using an impedance analyser. The uniaxial compressive strength, elastic modulus, point load strength, Schmidt hammer value, P-wave velocity, density and porosity values of the samples were determined in the laboratory. Electrical resistivity values were correlated with the corresponding physico-mechanical properties using the method of least-squares regression and the derived equations were checked by the t and F-test. A strong logarithmic relation between uniaxial compressive strength and resistivity was found. The relation between elastic modulus and resistivity is significant and follows a logarithmic function. Density was linearly, and porosity was exponentially correlated with resistivity.It may be concluded that electrical resistivity can be used as a representative measure of rock properties, particularly for characterizing rocks for which regularly shaped specimen are difficult to obtain. However, the effect of different rock types on the correlations must be further investigated. Additionally, the effect of certain minerals on the rock's resistivity must be taken into account, especially when testing dry or partially saturated rocks.     

Evaluation of simple methods for assessing the uniaxial compressive strength of rock

Published data on 48 different rocks are used to evaluate the correlations between the uniaxial compressive strength (UCS) values and the corresponding results of point load, Schmidt hammer, sound velocity and impact strength tests. The variability of test results for each test and each rock type was evaluated by calculating the coefficient of variation. Using the method of least squares regression, the UCS values were correlated with the other test values. Also, the test methods were evaluated by plotting the estimated values of compressive strength vs. the measured values of compressive strength for each test. The results indicate that the least variability is shown in the impact strength test. So, among the test methods included in this study, the impact strength test is the most reproducible test; but the variability of test results for the other test methods is within acceptable limits for most engineering purposes. Strong linear relations between the point load strength index values and the UCS values were found for the coal measure rocks and the other rocks included in this study. The Schmidt hammer and the sound velocity tests exhibit significant non-linear correlations with the compressive strength of rock. In the sound velocity test, the data points are scattered at higher strength values. There is no clear relation between the impact strength values and the compressive strength values for the coal measure rocks. A weak non-linear correlation was found between the impact strength values and the compressive strength values for the other rocks. All test methods evaluated in this study, except the impact strength, provide reliable estimate of the compressive strength of rock. However, the prediction equations derived by different researchers are dependent on rock types and test conditions, as they are in this study.     

A field index to determine the strength characteristics of crushed aggregate

The suitability of a simple technique to determine the strength characteristics of aggregates is reviewed. The results indicate that the strength characteristics of aggregates – in terms of crushing, impact and abrasion values – can be estimated and predicted from simple and quick tests for rock strength such as the Schmidt hammer and point load. The rocks and aggregates used in this work were of igneous, sedimentary and metamorphic origin and of different types, ages and degrees of weathering. More than 110 rocks and aggregate samples were tested according to British Standard and ASTM. The laboratory tests included the Schmidt hammer and point load for rock material while the crushing impact and Los Angeles abrasion test were undertaken on the aggregates. Regression analyses were performed and empirical relationships between the strength of rocks (in terms of their compressive strength and point load indices) and aggregates (in terms of their crushing, impact and abrasion values) were developed. Relatively strong relationships were obtained between the compressive strength of the rock and the crushing and impact value of the aggregate, while moderately strong relationships were obtained for the Los Angeles abrasion values. Electronic Publication     

Estimation of rock modulus: For intact rocks with an artificial neural network and for rock masses with a new empirical equation

The elastic modulus of intact rock is used for many rock engineering projects, such as tunnels, slopes, and foundations, but due to the requirements of high-quality core samples and associated sophisticated test equipment, instead the use of empirical models to obtain this parameter has been an attractive research topic. In the rock mechanics literature, some empirical relations exist between the elastic modulus of intact rock and other rock properties, such as the uniaxial compressive strength (σ_ci), unit weight (γ), Schmidt hammer rebound number, point load index and petrographic composition. However, the past use of specific rock types is the main limitation of the existing empirical equations. In other words, they are not open to the general purpose use. To eliminate this deficiency, a total of 529 datasets, including uniaxial compressive strength, unit weight and elastic modulus of intact rock (E_i), were collected via an extensive literature review. In addition to these datasets, a further total of 80 datasets was obtained from laboratory tests performed on greywacke and agglomerate core samples for this study. To prepare a chart for the prediction of the elastic modulus of intact rock, an artificial neural network was constructed using the large database. In addition, after a brief overview of existing empirical equations, a new empirical equation, which considers RMR and the elastic modulus of intact rock (E_i) as input parameters, is also proposed using worldwide data.     

Anisotropy effect on strengths of metamorphic rocks

This paper aims to study the effect of anisotropy on strengths of several metamorphic rocks of southern(Cine) submassif of Menderes metamorphic massif in southwest Turkey. Four different metamorphic rocks including foliated phyllite, schist, gneiss and marble(calcschist) were selected and examined.Discontinuity surveys were made along lines for each rock and evaluated with DIPS program. L-type Schmidt hammer was applied in the directions parallel and perpendicular to foliation during the field study. Several hand samples and rock blocks were collected during the field study for measurements of dry and saturated densities, dry and saturated unit weights and porosity, and for petrographic analysis and strength determination in laboratory. L-and N-type Schmidt hammers were applied in the directions perpendicular(anisotropy angle of 0°) and parallel(anisotropy angle of 90) to the foliation on selected blocks of phyllite, schist, gneiss and marble(calcschist). The phyllite and schist have higher porosity and lower density values than the other rocks. However, coarse crystalline gneiss and marble(calcschist) have higher rebound values and strengths, and they are classified as strong-very strong rocks. Generally, the rebound values in the direction perpendicular to the foliation are slightly higher than that in the direction parallel to foliation. Rebound values of N-type Schmidt hammer are higher than the L-type values except for phyllite. Sometimes, the rebound values of laboratory and field applications gave different results. This may result from variable local conditions such as minerals differentiation,discontinuities, water content, weathering degree and thickness of foliated structure.     

Predicting the sawability of carbonate rocks using multiple curvilinear regression analysis

Performance measurements of large-diameter circular saws were conducted on 13 different carbonate rocks in marble factories located in Turkey. Rock samples were collected from the factories for laboratory tests. Cohesion, friction angle, uniaxial compressive strength, tensile strength, Schmidt hammer value, point load strength, impact strength, Los Angeles abrasion loss and P-wave velocity values were determined in the laboratory. Slab production and rock properties were evaluated using multiple curvilinear regression analysis and estimation models were developed. Advancing rate of saw, saw diameter, depth of cut, tensile strength and impact strength were included for the best model. Alternative and more universal models were developed, including each rock property in turn together with the rotational speed of the saw. The developed models were verified by statistical tests.It was concluded that the slab production of carbonate rocks using large-diameter circular saws can reliably be estimated using one of the developed models. The models which include Schmidt hammer value, point load strength, impact strength and P-wave velocity have practical and economical advantages for the stone industry.     

Physical and mechanical characterization of phyllites and metagreywackes in central Portugal

This work defines the physical and mechanical characteristics of phyllites and metagreywackes belonging to a schist–greywacke complex in central Portugal. The index properties, point load strength, uniaxial compressive strength, slake durability and Schmidt rebound hardness were determined. In general, the metagreywackes gave a higher strength than phyllites with the same weathering degree. Possible correlations and estimation models were established and compared with the equations obtained by different researchers.     

Correlation between Schmidt hardness,uniaxial compressive strength and Young’s modulus for andesites,basalts and tuffs

Volcanic rocks from the Bodrum Peninsula have been studied to establish a relationship between the uniaxial compressive strength (UCS), Youngs modulus (E) and Schmidt hammer rebound number (N). Test results obtained on andesites, basalts and tuffs have been assessed statistically and the parameters obtained correlated. Regression equations were established between N, E and UCS.The study suggests the following empirical relationships: UCS=2.75 N–36.83, E=0.17 UCS+0.28 and E=0.47 N–6.25. The relationships are somewhat different from the correlations suggested by previous studies and should therefore be used only for andesites, basalts and tuffs with degrees of weathering and mineralogical structure similar to those used in the present study. The results obtained using solely the Schmidt hammer test for estimating mechanical properties of rocks are less accurate than when a full suite of laboratory tests is carried out but it is hoped that these empirical equations will help geotechnical engineers making practical decisions at a preliminary site investigation stage.     

Determination of mechanical properties of rocks using simple methods

Statistical equations have been determined for estimating the mechanical properties of rocks using non-destructive and indirect test methods. Nineteen different rock types were tested to obtain the relationships between Shore hardness, point load index, sound velocity, Schmidt hardness and porosity and uniaxial compressive strength, indirect tensile strength and abrasion resistance. Results of regression analyses showed satisfactory correlations.      

基于回弹法测试岩石表面强度影响因素分析

为了测试集料加工宕口岩石的力学性能,应用回弹仪测试不同类型岩石的表面强度,对比各类岩石在非饱和及饱和状态下的回弹强度和抗压强度等。研究结果表明利用回弹仪测试岩石的表面强度方法简便、有效;岩石在非饱和及饱和状态下岩石的回弹强度差异较小;岩石具有明显软弱面时回弹强度会较小;岩石结构致密回弹值较大;岩石的测试区域的选取对结果具有明显的影响。     

Correlating schmidt hardness with compressive strength and young’s modulus of carbonate rocks

Schmidt hammer has increasingly been used world-wide as an index test for a quick rock strength and deformability characterization. The reason is mainly due to its rapidity and easyness in execution, non destructiveness, simplicity, portability and low cost. Twentynine different types of Carbonate rocks from Greek territory and four ones from England have been collected and tested. The tests include the determination of Schmidt hammer rebound hardness, (N) number, Tangent Young’s modulus, (Et), and Uniaxial compressive strength (U.C.S.). Finally, these parameters were correlated and regression equations, of high practical value, were estanlished among N, Et and U.C.S., all presenting high coefficients of determination (R²).     

彭灌杂岩单轴抗压强度和弹性模量的多元线性回归预测模型

完整岩块的单轴抗压强度(UCS)和弹性模量(E)是岩石力学和地质工程中极其重要的指标,高质量的岩芯试件是获取UCS和E值的关键因素,但由于龙门山地区地形复杂及震后交通不便,并不总能从彭灌杂岩体中获取高质量岩芯试件。因此,为了快速获取彭灌杂岩体的UCS和E值,沿岷江和渔子溪采集了53组彭灌杂岩岩块,并于室内作了一系列物理力学测试。以实测UCS和E值为因变量,以岩块的密度、Schmidt回弹硬度及地质强度指数为自变量,建立了彭灌杂岩体UCS和E值的多元线性回归预测模型。经F-检验、t-检验及其回归系数方差膨胀因子(VIF)、Durbin-Watson统计量的计算结果表明,回归模型的自变量选取得当,对因变量UCS和E具有影响,回归模型具有适用性。同时,UCS和E回归模型调整后判定系数R珔2的计算结果表明,所建立的彭灌杂岩体多元线性回归模型具有较高的预测性能,可用于彭灌杂岩体UCS和E的现场快速确定。     

Variation of Schmidt hammer values with imbrication direction in clastic sedimentary rocks

In this study, Schmidt hammer rebound (HR) values of clast-supported conglomerates, matrix-supported conglomerates and normally graded gravelly sandstones were measured to investigate the effect of such anisotropy on strength. The highest Schmidt HR values were obtained from the gravelly sandstones, with an average value of 39, decreasing with gravel content. In clast-supported conglomerates, the average HR values were 34 and 38, parallel and perpendicular to the palaeocurrent direction, respectively. Similar measurements were also made over the matrix-supported conglomerates. Average HR values of matrix-supported conglomerates were determined as 37 and 41 parallel and perpendicular to the palaeocurrent direction, respectively. In the clast-supported conglomerates, average HR value on perpendicular palaeocurrent side was 12% higher than those parallel to palaeocurrent. This ratio was 9% for matrix-supported conglomerates. Preferred orientation or clast imbrication creates more gravelly and less matrix covered area parallel to the palaeocurrent than the other directions. This visible difference in the gravel–matrix ratio is considered to be the basis of the strength anisotropy in clastic sedimentary rocks. This study demonstrates the potential of the Schmidt hammer for quantifying the degree of anisotropy in such rocks.     

Point-load strength: An index for classification of rock material

Point-load strength (Is) as a measure for the determination of rock strength and for estimating uniaxial (unconfined) compressive strength (UCS) are described and both put together and used for rock strength classification of brittle and hard rocks. The estimated point-load strength values of specimens of varying sizes and also the values corrected to a standard thickness of 50 mm, and the resultant point-load strength values (Is-50) have been used to estimate the uniaxial (unconfined) compressive strength which correlates well with actual recorded uniaxial (unconfined) compression test results. Using graphical and mathematical relationships between the observed and estimated UCS and Is values, a conversion factor of 16 is obtained for estimating uniaxial (unconfined) compressive strength values from point load strength results. A nomogram for computing point-load strength index and a system for the classification of rock material are presented.