Prediction of Unconfined Compressive Strength of Limestone Rock Samples Using L-Type Schmidt Hammer

Transcription

1 Prediction of Unconfined Compressive Strength of Limestone Rock Samples Using L-Type Schmidt Hammer Dr. Ramli Nazir Associate Professor, Faculty of Civil Engineering, Department of Geotechnics and Transportation, Universiti Teknologi Malaysia (Malaysia) Ehsan Momeni PhD Student, Faculty of Civil Engineering, Department of Geotechnics and Transportation, Universiti Teknologi Malaysia (Malaysia) Danial Jahed Armaghani PhD Student, Faculty of Civil Engineering, Department of Geotechnics and Transportation, Universiti Teknologi Malaysia (Malaysia) Mohd For Mohd Amin Associate Professor, Faculty of Civil Engineering, Department of Geotechnics and Transportation, Universiti Teknologi Malaysia (Malaysia) ABSTRACT Unconfined Compressive Strength (UCS) of the rock is one of the most crucial parameters in the analysis of geotechnical problems such as tunnel design, rock blasting and mechanical rock excavation. However, proper determination of UCS through direct experimental test is not always possible as it is difficult to obtain sufficient numberof rock samples from highly to moderate weathered rocks. In addition, Unconfined Compression Test (UCT) is time consuming and expensive. Indirect method of UCS estimation such as Schmidt Rebound Hammer Test which is a quick and inexpensive test can be used for predicting the unconfined compressive strength. This study investigates different correlations between UCS and Schmidt hammer. Hence a total number of 60 laboratory tests (UCT, Dry density, and Schmidt hammer) on 20 limestone core samples were conducted. Findings show a good agreement between the results of experimental studies and previous well established literatures. A new correlation with high accuracy level for UCS prediction of limestone rocks is proposed and the reliability of one the most widely used correlation in Malaysia (Miller`s correlation) for UCS prediction using Rebound hammer is examined. Results show the degree of reliability of Miller`s correlation is high enough and it is in close agreement with the new proposed correlation. However, being only a function of Rebound number (R L ) is the advantage of the new proposed correlation over Miller`s correlation. KEYWORDS: Unconfined Compressive Strength, Rebound Number, Schmidt hammer, Dry density, Limestone

2 Vol. 18 [2013], Bund. I 1768 INTRODUCTION One of the important engineering properties of the rock, which plays an essential role in designing various geotechnical applications such as tunnel and dam is Unconfined Compressive Strength (UCS) of the core samples (Bieniawski 1974). However, UCS test is an expensive and time consuming test which is not always practical especially in case of highly weathered rock samples. This is mainly because it is difficult to obtain sufficient number of proper rock samples in this situation. (Cargill and Shakoor, 1990; Gokceoglu and Zorlu, 2004; Ceryan et al., 2012). Due to the aforementioned problems, the use of indirect method of UCS determination has become popular especially because of its ease of use and quick procedure. In indirect methods such as point load index and Schmidt hammer simple index, parameters and physical properties are considered for UCS prediction (Kahraman 2001; Chang et al. 2006).Schmidt (1951) introduced a special kind of hammer for non-destructive testing of concrete hardness. The application of Schmidt hammer later on was extended to rock strength estimation (Aydin and Basu, 2005; Cargill and Shakoor, 1990). In order to have comparable and reliable data, the test is standardized by ISRM (1978) and ASTM (2001). This paper presents a review of the most famous correlations between Schmidt hammer and UCS in addition to a new correlation obtained through a set of experimental tests in Geotechnical Laboratory of University Teknologi Malaysia (UTM). LITERATURE REVIEW There are many published works which focused on obtaining a correlation between UCS and other physical properties of the rock samples (Inoue and omi, 1970; Broch and Miller, 1972; Sheorey et al. 1984; Shakoor and Bonelli, 1991; Romana, 1999; Katza et al. 2000; Guney et al. 2005; Vasconcelos, 2008; Sharma and Sing, 2008). Table 1 presents a list of previous researches conducted for UCS prediction using Schmidt hammer on different type of rocks. As shown in table 1, the empirical relationship between UCS and Rebound number (R L ) are in the form of linear, exponential, and power functions. Although most of the correlations are defined in a way that R L is the only independent variable for UCS prediction, there are some reported case that UCS is correlated with rebound number and dry density ( ) of rock samples. Table 1: UCS correlations with Schmidt hammer References Correlation R Rock Type Kilic and Teymen (2008) Different rock types Cobanoglu and Selik (2008) Limestone, sandstone, cement mortar Yasar and Erdogan (2004) Carbonates, sandstone, basalt Tugrul and Zarif (1999) Granite Sachpazis (1990) different carbonates Xu et al. (1990) Mica-schist Cargill and Shakoor (1990) Carbonates O Rourke (1989) Sandstone, siltstone, limestone, anhydride Ghose and Chakraborti (1986) Coal Haramy and DeMarco (1985) Coal Singh et al.(1983) Sandstone, siltstone, mudstone, seatearth Aufmuth (1973) different lithologies

3 Vol. 18 [2013], Bund. I 1769 EXPERIMENTAL TESTS Schmidt hammer test Schmidt hammer test or rebound test is a simple index test to determine surface strength of rock sample. The test does not involve destruction of sample (non-destructive) and it can be repeated on the same sample. Index value obtained is rebound number (R), which is an indicator of surface hardness of the sample. The value of R can be converted to compressive strength using the following formula (Miller, 1965). Log 10 JCS = ( )(R) (1) where, JCS is joint compressive strength (MPa) and is unit weight of rock (kn/m 3 ). JCS also indicates compressive strength of sample surface being tested. Miller`s correlation is one of the most widely used correlations for UCS prediction in Malaysia.For fresh rock (weathering grade I) sample, JCS is approximately equal to the UCS of the internal material. In other words, when rock is not weathered, its surface compressive strength is approximately equal to its material strength. In this test program, when index test does not involve destruction of sample (e.g. rebound hammer test), the sample is reused for other strength tests (e.g. uniaxial compression test). Such procedure will help reduce effect of sample variation on test data. Uniaxial Compression Test Uniaxial compression test evaluates compressive strength of rock sample under uniaxial loading. The compressive strength (UCS) of rock sample is determined by loading rock sample under load-controlled (e.g.10 kn/s) condition or under strain-controlled (e.g. 0.1 % strain/s) condition. The mode of loading under strain-controlled condition represents loading conditions more accurately in the field and enables complete stress-strain curve of sample to be obtained. Depending on monitored parameters during test, several fundamental properties of rock can be determined from this test. In its simplest form, UCT is used to determine uniaxial/ultimate compressive strength (UCS). Stillborg (1986) listedtypical values for compressive strength (UCS) for various rocks. TEST PROCEDURE A number of 60 laboratory tests on limestone core samples including 20 physical property tests i.e. dry density test, 20 index strength tests i.e. Schmidt s hammer test and 20 direct strength testsi.e UCT were conducted in Geotechnical Laboratory of University Teknologi Malaysia. The procedures suggested by ISRM (1981) were considered for the tests.more detail of tests procedures are given in the following paragraphs. Dry Density Test Dry density is determined on all prepared core samples for the UCT. The dry mass of samples was obtained after they have been dried at room temperature for several days. In order to obtain the volume, the diameter (D) and height (H) of core samples was measured using electronic vernier caliper with the accuracy of 0.01 mm (Figure 1). Dry mass of sample was determined using digital balance of 0.1 gm accuracy.

4 Vol. 18 [2013], Bund. I Figure 1: Digital vernier (left) and Electronic balance (right) Schmidt hammer test procedure Rebound hammer used in this study was Schmidt hammer L-type (MATEST of Italy). Sample cradle was used to hold the core sample rigidly in position during test (see Figure 2). For each sample, 20 rebound tests were undertaken at different point on the core surface. Data collected is rebound number, R. From the 20 readings collected, the average R value was calculated from 50 % of the highest readings. Value of R can be used to calculate compressive strength of sample surface tested (equation 1). Figure 2: Schmidt Hammer L-type Uniaxial compression test The compression tests were conducted on Tinius Olsen (USA) Universal Testing Machine (UTM).This 3000 kn capacity machine is able to apply compressive load at constant strain rate on the sample. All compression tests were conducted at strain rate at of 0.5mm/mm/s. Strain gauges were used to measure deformation (strains) of the sample, while TML 500 kn load cell (see Figure 3) monitored applied load (stress) throughout the test. Both these strain and stress transducers were wired to data logging system. The stress-strain data was continuously logged into a computer and the stress at failure was considered as the UCS of core sample.

5 Vol. 18 [2013], Bund. I 1771 Figure 3: 500 kn TML load cell Result and Discussion The results of 60 laboratory tests (dry density, Schmidt hammer and UCS) are tabulated in Table 2. The general trends of the findings show good agreement with previous well established studies (Daly et al., 1966; Bickel and Kuesel, 1982; Kahraman, 2001; Aydin and Basu, 2005). In Table 2, apart from obtained UCS in the laboratory, the UCS was predicted according to Miller`s correlation to investigate the reliability of this correlation. No. Sample Type Table 2: Rock characterization results Dry Density UCS: Miller`s (kg/m 3 R ) L correlation(mpa) UCS:Obtained in Laboratory(MPa) 1 Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone

6 Vol. 18 [2013], Bund. I 1772 The accuracy of Miller`s correlation is shown in Figure 4. As it is shown in this figure, it can be seen that the value of R 2 is about 0.93, showing high reliability of Miller`s equation for prediction of UCS. However, it is worthy to note that the reliability of this correlation can be reduced significantly if the rock samples are categorized among moderate or highly weathered rocks. This is mainly due to the fact that Schmidt hammer measure the strength on the surface of the rock. Figure 4: Reliability of Miller`s correlation Miller`s correlation was based on both rebound number (R L ) and unit weight of the rock sample. Figure 5 presents a developed correlation which is obtained from the results of the laboratory tests. The proposed correlation is only based on rebound number. As shown in this figure, the reliability and accuracy of the new correlation is very close to Miller`s correlation. However, in the new correlation, there is no need to determine unit weight. The R 2 of the new correlation is almost The unremarkable error shows high accuracy of the new correlation. The formula for the new correlation is as follows: UCS (MPa) = 12.83e R L (2)

7 Vol. 18 [2013], Bund. I 1773 Figure 5: Developed correlation between Rebound number (R L ) and UCS Since the laboratory tests were conducted on limestone samples, the use of new correlation for this specific type of rock is recommended as it will increase the reliability and the degree of accuracy of the correlation. Nevertheless, as mentioned earlier, the weathering grade can affect the reliability of the correlation inversely. In fact, as the degree of weathering grade in rock samples increases, the reliability of Schmidt hammer correlations including both Miller`s correlation and new developed correlation decreases. Hence, the use of Schmidt hammer correlations in weathered rock samples should be done with caution. CONCLUSIONS The reliability of one of the most widely used Schmidt hammer correlation in Malaysia for prediction of unconfined compressive strength of rock samples was evaluated by obtaining the dry density of 20 different rock samples and conducting both Schmidt hammer tests and UCT on the prepared core samples. It was observed that the degree of reliability of Miller`s correlation is high enough. Based on the results of the experimental tests, a new correlation with almost same degree of accuracy and reliability was introduced. In comparison to Miller`s equation, the new proposed correlation has the advantage of being a function of only one independent variable i.e rebound number. However, like other Schmidt hammer correlations, the use of new developed correlation for UCS prediction in case of highly weathered rock is not recommended due to improper UCS prediction. On the other hand, the use of developed correlation for limestone rocks is recommended as the laboratory tests were conducted on limestone core samples. ACKNOWLEDGEMENTS The second and third authors would like to thank University Teknologi Malaysia for its financial support via allocating International Doctoral Fellowship.

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