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.      

Nail penetration test for determining the uniaxial compressive strength of rock

We offer a new and practical index test method, the nail penetration test (NPT), to estimate the UCS of intact rocks, to be used as alternative to the point load test (PLT) or Schmidt rebound hammer test (SRH). The major tools used in the investigation include a gasnailer with 130 J power and its nails ranging from 25 to 60 mm in length. The study material covers 65 rock blocks of gypsum, tuff, ignimbrite, andesite, sandstone, limestone, and marble. For the NPT, five nail shots were performed on each block sample and the average value was obtained. Two to three uniaxial compression tests were carried out on each specimen. Ten impacts were applied on rock blocks by using both the L- and N-types of SRH. Regarding the PLT, either 10 axial or 10 block tests was applied on each rock type.The average nail penetration depths were correlated with the UCS, I_S(50) and rebound number for both types of the SRH. Also, the measured UCS values were compared with those obtained from the empirical relationships using the data from the NPT, PLT, and SRH. It was found that the NPT provides better estimates for UCS than the PLT or SRH. Particularly applicable to weak to very weak rocks, the NPT is capable of indirectly estimating the UCS of intact rocks up to 100 MPa. The test is proposed for use in mainly in situ applications.     

Estimating uniaxial compressive strength of rocks using genetic expression programming

The aim of this paper is to estimate the uniaxial compressive strength(UCS) of rocks with different characteristics by using genetic expression programming(GEP).For this purpose,five different types of rocks including basalt and ignimbrite(black,yellow,gray,brown) were prepared.Values of unit weight,water absorption by weight,effective porosity and UCS of rocks were determined experimentally.By using these experimental data,five different GEP models were developed for estimating the values of UCS for different rock types.Good agreement between experimental data and predicted results is obtained.     

Investigation of saturation effect on the relationship between compressive strength and Schmidt hammer rebound

Schmidt rebound values obtained by Schmidt hammer tests have recently been used to predict the uniaxial compressive strengths of both intact rocks and rough joint surfaces. The easy application procedure, short test duration and nondestructive nature of Schmidt hammer testing are the main reasons for which it is preferred by numerous investigators. To date, relationships derived between uniaxial compressive strength and Schmidt rebound values are generally valid only in dry conditions. However, while the adverse effects of saturation upon uniaxial compressive strength are known and emphasized in many studies, there has yet to be a comprehensive investigation dealing with the effects of saturation on Schmidt rebound values. Considering this deficiency, an attempt was made to investigate the effects of saturation on uniaxial compressive strength and Schmidt rebound values and to derive relationships between these two parameters considering the saturation degree. For this purpose, the uniaxial compressive strength and Schmidt rebound values of 14 rock types and one construction material were determined for dry and fully saturated conditions. According to results of laboratory tests, the adverse effects of saturation on both quantities were determined. The results showed that the levels of these adverse effects were dramatically different for the two parameters. Statistical evaluations indicated that relationships between uniaxial compressive strength and Schmidt rebound values are dependent on the degree of the saturation of intact rock material. By considering this, simple and multivariate regression analyses were performed and the resultant simple and multivariate equations were produced. In multivariate regression analysis, the degree of saturation and Schmidt rebound values are used as independent variables. All derived equations have high prediction performance and were determined to be statistically reliable.

     

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.      

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.     

Estimating the uniaxial compressive strength of pyroclastic rocks from the slake durability index

Because the preparation of standard samples may not always be possible for weak or soft rocks, the prediction of uniaxial compressive strength (UCS) from indirect methods is widely used for preliminary investigations. In this study, the possibility of predicting UCS from the slake durability index (SDI) was investigated for pyroclastic rocks. For this purpose, pyroclastic rocks were collected from 31 different locations in the Cappadocian Volcanic Province of Turkey. The UCS and SDI tests were carried out on the samples in the laboratory. The UCS values were correlated with the SDI values and a very strong exponential relation was found between the two parameters. Since some data were scattered over the UCS values of 20 MPa, the correlation plot was redrawn for above and below the UCS values of 20 MPa, respectively. Very strong linear correlations were developed for two cases. Our concluding remark is that the UCS of pyroclastic rocks can be estimated from the SDI.

     

Estimating the uniaxial compressive strength of a volcanic bimrock

Preparation of the standard sized cores from geological mixtures or fragmented rocks such as mélanges, fault rocks, coarse pyroclastic rocks, breccias and sheared serpentinites is often extremely difficult. Therefore, determination of the mechanical parameters such as cohesion, friction angle and UCS is extraordinarily difficult for these types of rocks. This paper presents the results of studies using Ankara Agglomerate to develop an empirical approach for the determination of the UCS for a volcaniclastic mixture of strong andesite blocks within weak tuff matrix, formed from the aggregation of volcanically exploded rock debris mixed with ash fall (tuff), i.e. a bock-in-matrix rock or ‘bimrock’. The uniaxial compressive strength of a volcanic bimrock can be predicted using the conceptual approach within as-yet poorly understood error limits. Therefore, the approach introduced is open to improvement depending on the number of data and cases including different type of bimrocks.     

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.     

A new approach for applying the in-situ Schmidt hammer test on a coal face

The indirect Schmidt hammer test is used to determine the mechanical behavior of rock materials. The method has so far been widely applied to intact rock, and therefore researchers have developed procedures for its use on rock cores and blocks. However, there is currently little information about using the Schmidt hammer for in-situ testing. The purpose of this study was therefore to evaluate the application of the Schmidt hammer test in-situ on a coal face. In this study, a measurement area of 72 m² was selected in the center of a 330 m² longwall face. The main measurement area was divided into a grid system, with each grid section having an area of 0.375 m². The tests were conducted as a single hammer impact at the center of each grid section. One thousand nine hundred and sixty-five records were constructed using the data obtained from 10 face positions, and these were used for statistical analyses. The statistical results suggested that the total number of tests carried out on the main measurement area was more than the minimum required. An equation was then developed that not only gives the appropriate test number, but also provides information about the grid system. Hence, a new approach is proposed for determining the optimum grid system and test number for in-situ testing of coal faces.     

复杂层状结构岩石抗剪强度试验方法研究

岩石的抗剪强度是边坡稳定性、地基强度和稳定性评价中的一项非常重要的指标,不同类型的岩体,岩石的抗剪强度特性是不同的,其中复杂层状结构岩石的抗剪强度是岩土工程研究的重点内容之一.工程中经常采用的岩石剪切试验方法包括室内试验和现场原位试验,由于试验约束条件不同,试验结果存在一定的差异.本文以某核电工程为例,该项目分别采用了...     

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

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.     

高强橡胶混凝土单轴受压力学性能的试验研究

对高强橡胶混凝土进行了单轴受压试验,得到了含不同掺量、不同粒径的高强橡胶混凝土的应力-应变曲线.分析了橡胶掺量、胶粉粒径对高强橡胶混凝土的单轴抗压强度、极限应变、初始弹性模量和破坏形态的影响.研究结果表明:橡胶掺量对高强橡胶混凝土的单轴抗压强度、极限应变、初始弹性模量和破坏形态的影响都比较明显;胶粉粒径对高强橡胶混凝土的强度、初始弹性模量和破坏形态有一定的影响,对极限应变的影响不明显.     

The effect of length to diameter ratio of test specimens on the uniaxial compressive strength of rock

One of the parameters which affect the uniaxial compressive strength (UCS) of rock materials is the length to diameter ratio (L/D) of test cores. ASTM recommends a ratio of between 2 and 2.5, and ISRM suggests 2.5–3:1. Research has shown that high UCS values are obtained for L/D ratios <2, a very slight difference in values between 2 and 2.5, and they remain effectively constant with a L/D ratio >2.5:1. In this study, the shape effect on the UCS of seven rocks was investigated by testing dry cores with L/D ratios from 1 to 2.5:1. Based on the results, a decrease in UCS with increasing L/D ratios up to 2.5:1 was measured for all the rocks except the tuff. By omitting the data from the tuff sample, a correction formula was determined for the rocks tested in this study and results obtained from the literature. However, further work is required on different types of rock to verify or modify this formula.     

A comparative study between simplified and nonlinear dynamic methods for estimating liquefaction potential

This paper estimated the liquefaction potential of a saturated soil deposit subjected to a horizontal seismic excitation at its base using the total stress approach.A comparative analysis between the simplified and the nonlinear dynamic methods was used to verify to what extent the simplified method could be reliable.In order to generalise the reliability of the simplified method for any value of the maximum acceleration for the used earthquakes,a correction for the maximum acceleration less than 0.3 g was proposed based on the comparison of safety factor values determined by the dynamic method illustrated by the equivalent linear model with lumped masses and the simplified method for a given profile of soil subjected to 38 earthquakes.The nonlinear behaviour of soil was represented by two hyperbolic models:Hardin and Drnevich,and Masing.To determine the cyclic resistance ratio(CRR),the cone penetration test(CPT) based method,the standard penetration test(SPT) based method,and the shear wave velocity based method were used.The safety factor was calculated as the ratio of CRR/CSR,where CSR represents the cyclic stress ratio.The results of the proposed correction have given smaller values of the safety factor compared to the nonlinear dynamic methods for the maximum acceleration less than 0.3 g.In other words,by considering this correction,the most unfavourable case is always given by the modified simplified method.