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ESTABLISHMENT OF CORRELATION BETWEEN LOS ANGELES ABRASION LOSS AND STRENGTH DETERMINED THROUGH POINT LOAD INDEX AND SCHMIDT REBOUND HAMMER.

Byline: Muhammad Zahid Jamil and Muhammad Saleem Khan

ABSTRACT

Unconfined compressive strength is one of the most important parameter which is frequently

used by the experts for design criteria in all engineering projects. The estimation of unconfine d compressive strength (UCS) has a costly and time consuming procedure. Therefore it is required to explore the estimation of UCS through indirect methods such as Los Angeles abrasion test which is relatively simple and fast method. This study has been performed on 29 rock samples belonging to three different formations of Cambrian age sampled from Salt Range Pakistan. The results indicate that there is a strong inverse linear correlation between LA abrasion values and strength determined from point load te st and a significant correlation exists between LA abrasion values and strength estimated from Schmidt rebound hammer.

Key Words: Rock formation Los Angeles Abrasion Test Schmidt rebound hammer point load index linear correlation

INTRODUCTION

The strength and quality of rock mass can be determined by numerous methods. These methods of strength evaluation give the idea of behavior of rock mass under loading and deformation conditions. To obtain direct strength of rock is sometime difficult and time consuming hence there are other indirect methods which can be as good as direct methods such as unconfined compressive strength [1]. Los Angeles abrasion test measures degradation of rock aggregate and this test can be used to evaluate the quality and strength of rock aggregate. The objective of this study is to correlate the LA abrasion values with other indirect methods such as point load index and Schmidt rebound hammer. As these test are easy and relatively simple so the established correlation will be helpful for determination of rock strength through this method. These simple methods of estimating the strength of rock are useful for the estimation purposes especially for the initial studies therefore these methods save both time and money.

Correlation of Los Angeles abrasion loss with different engineering properties of rocks including Schmidt hammer point load index UCS unit weight and porosity had been studied by many researchers. Kazi and Al-Mansour [2] based on study of igneous rocks investigated the abrasion characteristics of rock. They observed that fine grained rocks have low abrasion loss as compared to coarse grained rocks Ballivy and Dayre [3] concluded that an inverse relationship exists between LAA loss and UCS. Strong correlation was obtained among the LA abrasion value UCS and point load index of different rock types by Al-Harthi [4]. Kahraman and Toroman [5] derived an equation to estimate LA abrasion loss from crushability index. Ugur et al [6] evaluated correlation among LA abrasion values Schmidt hardness and UCS and point load index. It was found that good relationship was obtained when LA abrasion values were divided by p-wave velocity.

They obtained a inverse relationship between LA abrasion values and measured properties. Kahraman and Fener [7] worked on 35 different rock types (igneous metamorphic and sedimentary rocks) to

predict the LA abrasion loss of rock aggregate from UCS and found a good correlation between these parameters as It was studied that LA abrasion loss can be predicted more accurately from point load index as compare to Schmidt hammer". Cargill and Shakoor [8] studied the results of UCS and corresponding values of point load index Schmidt hammer and LA abrasion values. They obtained a linear relationship between UCS and LA abrasion values when log-log scale was used. They reported that Schmidt hammer hardness depends upon types of rocks. Shakoor and Brown [9] obtained a useful equation among LA abrasion loss dry density and UCS based on multiple linear regression analyses for carbonate rocks. Shalabi et al [10] developed good relationship between UCS and different types of hardness (Schmidt hardness abrasion hardness). This study has been performed in Department of Geological Engineering UET

Lahore as a part of MSc research of principal author registration # 2011-MS-GS-14 and partially being published in this paper.

METHODOLOGY

Representative samples of sandstone and marl belonging to three geological formations were collected from Salt Range Pakistan. Total 29 samples were tested for this study

For Schmidt rebound hammer testing L type hammer was used with impact energy of 0.735 Nm. Rebound number was calculated from the scale located on the side of hammer. The unconfined compressive strength was calculated from the graph against the unit weight of rock specimen and Rebound number. For Point load test the rock specimen inserted in the conical platens of machine. The load was applied by gradually increasing until the sample break and failure load was recorded and result was calculated according to standard method. The test sample for Los Angeles abrasion was of

grading A for which 12 steel balls were used and the total mass of the aggregate was 5000 + 10 g. Material coarser than the 1.70-mm (No. 12) sieve was determined to the nearest 1 g. Loss was calculated as a percentage of the original mass of the test sample.

RESULTS AND DISCUSSION

The results of LAA values were correlated with point load strength index and strength estimated from Schmidt rebound hammer through the method of least square regression analyses. The suitable equation of correlation with highest correlation coefficient was determined. These correlations can be used to predict the indirect strength through LAA values.

The results of mean value of Los Angeles abrasion test carried out on 29 rock samples of three different rock formations are listed in Table 1. Baghanwala formation has greater value of standard deviation due to variability in sample while khewra Sandstone and Sahawal Marl member has low value of standard deviation.

The mean values of strength estimated from Schmidt hammer are shown in Table 2. These values have narrow ranges and have low values of standard deviation. The values for 95% confidence interval were also computed to check the validity of the derived equations.

Table.3 shows the result obtained from point load test. The values of confidence interval are also listed. The results of LAA values and strength estimated from point load test were plotted. It was observed that a strong linear relationship with high value of correlation coefficient (R2 = 0.85). The plot and regression line is shown in Figure 1. The following correlation equation was obtained

y = -0.8607x + 84.769 R2 = 0.85 (1)

where y = point load strength index x = LAA value (%)

The results of LA abrasion values and strength estimated from Schmidt Hammer were plotted.

A good inverse relationships exits between them with higher value of correlation coefficient (R2 = 0.76).The scatter plot and regression line for selected rocks is shown in Figure 2. The correlation is given below:

y= -0.5782x +55.812 R2 = 0.76 (2)

where y = Strength estimated from Schmidt hammer

(MPa) x = LA Abrasion values (%)

It was observed that all the values predicted from the derived equations of point load strength and Schmidt hammer strength range between the computed values of 95% confidence interval which indicates the validity of predicted equations. The predicted values are listed in Table 4. The predicted value of Schmidt hammer strength and point load strength were plotted against the corresponding measured values shown in Figure 3 and Figure 4 respectively .These predicted values were very close to the actual values which show accuracy of derived equations.

CONCLUSION AND RECOMMENDATIONS

This study reveals that there is a strong inverse linear correlation between LA abrasion loss and strength determined by point load index and significant correlation exists between LA abrasion loss and Schmidt rebound hammer strength. It was observed that Khewra sandstone behaves differently; it has narrow range of LA abrasion loss i.e low standard deviation value whereas it has greater standard deviation value of point load strength. It has higher point load strength compare to strength estimated by Schmidt hammer which may be due to depositional environment and orientation of the sample while taking readings through point load test and Schmidt hammer test.

Table 1. Test results of Los Angeles Abrasion test performed on samples of 3 rock formations

###Maximum Minimum###St. Deviation###Co-efficient of

###Rock Formation###LA abrasion value (%)###(MPa)###Variance (%)

###Khewra Sandstone###52.4###57###50###2.45###4.69

###Baghanwala Formation###33###45###23###11.21###33.98

###Sahawal Marl Member###71.3###76###65###3.59###5.03

Table 2. Test results of Schmidt rebound hammer Test performed on samples of 3 rock formations

###UCS estimated from###Co-efficient of###95% Confidence

###Rock Formation###Schmidt Hammer (MPa)###St. Deviation (MPa)###Variance (%)###Intervals (MPa)

###Khewra Sandstone###32.5###2.99###9.20###30.64 to 34.36

###Baghanwala Formation###33.4###4.36###13.06###30.69 to 36.11

###Sahawal Marl Member###10.4###1.83###17.67###9.26 to 11.54

Table 3. Results of Point load test performed on samples of 3 rock formations

###St. Deviation###Co-efficient of###95% Confidence

###Rock Formation###Point load index (MPa)###(MPa)###Variance (%)###Intervals (MPa)

###Khewra Sandstone###38.6###11.29###29.24###31.60 to 45.60

###Baghanwala Formation###56.1###10.82###19.29###49.38 to 62.81

###Sahawal Marl Member###24.7###5.98###24.22###20.98 to 28.4

Table 4. Validity of derived equations for prediction of strength through point load index and Schmidt hammer

###y = -0.8607x + 84.769###y = -0.5792x + 55.812

###Sample No.###x = LA abrasion Loss###where y = Is(50) (MPa)###where y = Strength from

###(%)###Schmidt hammer (MPa)

###Khewra Sandstone

###1###57###35.69###22.79

###2###55###37.41###23.95

###3###54###38.28###24.53

###4###54###38.28###24.53

###5###52###40.00###25.69

###6###50###41.72###26.85

###7###50###41.72###26.85

###8###50###41.72###26.85

###9###51###40.86###26.27

###10###51###40.86###26.27

###Baghanwala Formation

###1###28###60.66###39.59

###2###28###60.66###39.59

###3###27###61.52###40.17

###4###24###64.10###41.91

###5###24###64.10###41.91

###6###23###64.96###42.49

###7###48###43.44###28.01

###8###50###41.72###26.85

###9###45###46.02###29.74

###Sahawal Marl Member

###1###76###19.34###11.79

###2###72###22.78###14.16

###3###72###22.72###14.10

###4###70###24.50###15.26

###5###68###26.22###16.42

###6###65###28.81###18.16

###7###68###26.22###16.42

###8###72###22.78###14.10

###9###76###19.34###11.79

###10###74###21.06###12.95

ACKNOWLEDGEMENT

The University of Engineering and Technology Lahore is greatly acknowledged for providing necessary facilities for field visits and laboratory testing to complete this study.

REFERENCES

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9. Shakoor A. and Brown C.L. Development of a quantitative relationship between unconfined compressive strength and los angeles abrasion loss for carbonate rocks". Bulletin of Engineering Geology and the Environment.. 53. pp. 97-103.1996.

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Publication:Science International
Article Type:Report
Geographic Code:9PAKI
Date:Jun 30, 2014
Words:2037
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