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ABSTRACT Jar test is a simple test developed to determine the reaction of weak rock material to water during a certain period of time which can be tested on irregular bulk samples. It indicates the porosity, grain interactions and density of the material. The total index observed during a 10 minutes, 15 minutes, 30 minutes and 60 minutes sequence was found to be a good indicator for the classification. This should enable the classifying of the weak rock material to be carried out based on the slaking index. The reaction of the rock matter was inevitable after immersing the sample in the jar with water. The slaking index or changes in the sample also depend on the duration of the immersion time. By the observation of the changes caused by the immersing process the rock can be classified and the description for shale and sandstone. The jar slaking test was found to be more suitable to measure durability of weak rock especially in highly weathered (grade IV) and completely weathered (grade V) states as compared to slake durability index (Id1 or Id2) which was found to be unsuitable to test the durability of such weak rock.

KEYWORDS: Jar Test, Porosity, Grain Interaction, Density, Slaking Index, Durability

INTRODUCTION The assessment of reaction of rock material to water is very essential in determining the

effects of weathering on the rock material; thus are the estimates the failure mode of the rock material. The resistance of a rock to short-term weathering is described as durability of the material. Thus, durability is an important engineering parameter, particularly for weathered rocks. This non-durable behaviour of these rocks is responsible for loss of strength especially when influenced by water. Two approaches were used in this study: slake durability and the jar test to evaluate the suitability in assessing durability of weak rock. The slake durability test aims to assess the resistance of rock material to weathering and disintegration when subjected to two standard cycles of drying and wetting (Id1 and Id2). While, the jar test was used by immersing the rock samples in water for a certain period of time and the changes of appearance will be recorded.

Durability Assessment of Weak Rock by Using Jar Slaking Test

Edy Tonnizam Mohamad

Senior Lecturer, Faculty of Civil Engineering, Universiti Teknologi Malaysia edy@utm.my

Rosli Saad School of Physics, Universiti Sains Malaysia, Penang

rosli@usm.my

Seyed Vahid Alavi Nezhad Khaili Abad Researcher, Department of Geotechnics and Transportation, Faculty of Civil

Engineering, Universiti Teknologi Malaysia v_alavi_59@yahoo.com

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TESTING PROCEDURES

i. In jar test, four samples are needed for each type of rock material. The weathering grade of rock material is classified based on Table 1.

ii. Each sample is then trimmed to a suitable size (about 40 mm).

iii. Tap water was filled into 4 separate jars.

iv. The samples were then immersed into water for a period of 10, 15, 30 and 60 minutes (Figure 3).

v. After the end of each period, the sample’s behaviour was observed and recorded.

The samples that have been tested are irregular bulk samples. A slaking index was given to each sample for the stated period by referring to the table of slaking indices. Through observation of the changes caused by the immersing process, the rock materials were then classified according to the slaking index for shale and sandstone. The slake durability test was originally developed by Franklin and Chandra (1972), recommended by the International Society for Rock Mechanics (ISRM, 1981) and standardized by the American Society for Testing and Materials (ASTM, 1990).

Figure 3: Jar Slaking Test

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Table 1: Weathering classification used to describe rock mass

(modified from Ibrahim Komoo, 1995b)

COMPARISON OF SLAKE DURABILITY AND JAR TEST RESULTS

The results of slake durability and jar slake test are presented in this section.

Slake Durability Results

Tables 2 and 3 show the results of Id1 and Id2 by weathering grades.

DESCRIPTION ZONE

MATERIAL MASS

Colour Texture

Slaking Structure Iron-rich layer

Strength (Schmidt hammer)

In water

By hand

Condition

Changes

Residual soil VI Completely

changed (homogeny)

Destroyed

dis-integrate

dis-integrate

100% destroyed

Completely changed

None

None Completely weathered

V

b Completely changed

(homogeny)

Half remains

unchanged

<25% remains

Normally exist

a

Highly weathered

IV b Completely

discoloured

unchanged

Becomes flakes or

small pieces

Becomes flakes or

small pieces

>50-75% remains Iron-rich

filling in discontinuity

May exist

none

a

Less than 25 Moderately

weathered III

Slightly discoloured

Remains as Mass

Edges can be broken

100% intact Slightly

weathered II

No changes Edges

unbroken

Discolorations along

discontinuity Exceeds 25

Unweathered I No changes

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Table 2: Summary of slake durability Id1 result for respective weathering grade

Table 3: Summary of slake durability Id2 result for respective weathering grade

The mean values for the Id1 and Id2 are shown in Table 4 and 5 respectively.

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Table 4: Mean value of Id1 for respective weathering grade

Table 5: Mean value of Id2 for respective weathering grade

Slake Durability Index Id1 and Id2 are determined and presented in the graphs as shown in Figures 4 and 5 respectively. The durability index Id1 and Id2 generally shows increase of deterioration percentage with increase of weathering grade. As for Id1, some materials may be difficult to distinguish whether in grade II, III or IVa by just depending on values from 1 cycle. The values for grade II in the range of 94 percent – 99 percent with a mean value of 97 percent while the value for grade III, are in range of 90 percent to 99 percent with a mean value of 94 percent. By using Id1 as the parameter, the division of grades are clear for grade IVa and IVb. Values for Grade IVa are lower in ranges of 60 percent to 98 percent with a mean value of 80 percent. There seems to be a gap of values of more than 20 percent between grade IVa and IVb, where the values for IVb are in ranges 10 percent to 40 percent with a mean value of 31 percent. Id1 values cannot be used to distinguish grade Va and Vb materials. Grade Va to Vb has 0 percent value which means that these samples are totally destroyed in the test. Figure 4 shows the result of Id1 that is graphically illustrated. It shows a rapid decrease in values from grade IVa to IVb. This shows that shale materials in grade Va and Vb could not even retain their structure in the first cycle of slake durability test.

As samples are further tested in the second cycles (Id2), results show a clearer division for samples in grade II, III and IVa. However, samples in grade IVb, Va and Vb will further

Vol. 16 [2011] , Bund. O 1326 destroyed in the second cycle. For both Id1 and Id2, the alluvium samples in Desa Tebrau for grade IVa show a difference in the result for its different grain size. The coarse material shows lower values as compared to the finer ones. This signifies that coarser material is destroyed faster than the finer grain size.

Figure 4: Box-plot of Id1 Versus Weathering Grade

The Id2 results are shown graphically in Figure 5. From the boxplot, we can see that grade IVb to Vb materials could not survive the second cycle of the test and has a zero value. The percentage value can only be obtained until grade IVa. Values for grade II are in the range of 90 percent to 95 percent with a mean value of 93 percent; grade III is in the range of 80 percent to 93 percent with a mean value of 86 percent and grade IVa is in the range of 30 percent to 90 percent with a mean value of 56 percent.

It was found that slake durability test is primarily influenced by rock properties which allow ingress of water into the rock material. The presence of clay minerals enhanced rock susceptibility to slaking as can be seen in the higher weathering grades materials.

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Figure 5: Box-plot of Id2 Versus Weathering Grade

Jar Slaking Results

The objective of Jar test is to observe the reactions of the rock material to water in terms of weathering (Santi, 1998 a,b). The test is done by immersing the samples. The samples are placed in jars filled with tap water for a period of 10 minutes, 15 minutes, 30 minutes and 60 minutes. Slaking index is given to each sample based on the behaviour of samples after each period of time. This test can be carried out at the site or in the laboratory.

The rock samples for the test were classified as their respective weathering grade as what have been determined at the site. The rock samples were immersed in water and their slaking behaviours were noted. For each observation, an index was given based on Figures 1 and 2. Test results for jar slaking are presented in Figure 6 until Figure 11. Figure 6 and 7 shows the jar slaking index for Bukit Indah and Mersing sandstone respectively. Grade II samples were found intact for the first 10 minutes. After 30 minutes of immersion, the samples deteriorated to index number 5 and few samples were still intact as index 6. At the end of the 60 minutes immersion, the samples showed index 4 as the lowest index. As for grade III sandstone, the samples were observed to be of index 5 after 10 min of immersion in water. As the immersion was prolonged for another 20 minutes, the samples broke down to index no 4. At 60 minutes, the samples were observed to show number 2 as the lowest index.

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Figure 6: Test Results of Jar Slaking Index for Bukit Indah Sandstone

Figure 7: Test Results of Jar Slaking Index for Mersing Sandstone

Grade IVa samples showed index numbers 2, 3 and 4 for the first 10 minutes of immersion. The range of slaking index for sandstone grade IVa after 30 minutes was observed to be 2, 3 or 4. At the end of the test (60 minutes) all samples showed index of number 1. For grade IVb samples, they were observed to show index number of 3 or 4 after 10 minutes of immersion in water. All the samples broke down to index number 1 after 30 minutes. Fine and coarse grained materials from Desa Tebrau (grade IVa) showed index number of 4 after 10 minutes of immersion in water (Figure 8). The coarser grained materials broke down faster than the finer grained ones due to the higher porosity that was detected after 15 minutes. Subsequently, the coarse grained materials showed index numbers of 1 and 2 after 60 minutes while the fine grained showed index number of 2 and 3. As for grade Va samples, it showed index number 2 after 10 minutes and all samples

Vol. 16 [2011] , Bund. O 1329 showed index number 1 after 15 minutes. The same results were also observed for Kempas materials as shown in Figure 9. As for the grade Vb sandstone, the materials broke down to the lowest index of number 1 after 10 minutes.

Figure 8: Test Results of Jar Slaking Index for Desa Tebrau

Figure 9: Test Results of Jar Slaking Index for Kempas

Figures 10 and 11 show the jar test result for shale. Grade II shale showed index number 6 after 10 minutes of immersion in water for both Bukit Indah and Mersing materials. The index dropped to number 5 after 15 minutes and maintained at the same index after 30 minutes immersed in water. At the end of 60 minutes, the materials showed index number 4. As for grade III shale, samples from Bukit Indah and Mersing showed index number 5 after 10 minutes. After 15 minutes, they showed index number 4 and after 60 minutes the samples finalized at index number 2, 3 and 4. The grade IVa materials showed a lower index when observed after 10 minutes which was index number 4. In 15 minutes, the materials broke down to index number 2, 3 or 4 depending on the porosity of the samples and their strength. Higher porosity samples with lower strength broke down faster than high strength lower porosity samples. After 30 minutes,

Vol. 16 [2011] , Bund. O 1330 samples showed index number 2 and 3. At the end of the test period (60 minutes), the samples showed index number 1. Grade IVb samples showed index number 3 after 10 minutes and subsequently after 15 minutes immersion, the samples broke down to index number 1. The shale of grade IVb broke down faster as compared to sandstone in the same grade as the shale has clay constituents that swelled when immersed in water. The findings in the rate of deterioration in shale were also reported by Santi (1997). Samples Va showed index number 2 after 10 minutes and completely broke down to index number 1 in 30 minutes. Samples Vb showed index number 1 after 10 minutes of immersion. Table 6 and 7 show the summary of the results and the lowest index observed during the study respectively.

Figure 10: Test Results of Jar Slaking Index for Bukit Indah Shale

Figure 11: Test Results of Jar Slaking Index for Mersing Shale

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Table 6: Summary of jar test results for respective weathering grade

Table 7: Summary of the lowest index observed

The indices were used to produce the total jar slake index by adding the index observed in 10, 15, 30 and 60 minutes. The result of the jar slake index with regard to the weathering grade is shown by box-plot in Figure 12. The boxplot showed the decrease of total jar slake index with the increase of weathering grade and the mean value for respective weathering grade is shown in Table 8. Figure 13 (a) to (f) show the typical slaking indices observed during the test.

Table 8: Mean value of jar slaking index for respective weathering grade

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Figure 12: Boxplot of slaking index versus weathering grade

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Figure 13: Typical jar slake result: (a) index no. 6 (sample R8LN6R2S- 15 min), (b)- index no 5 (sample RL1L5- 15 min), (c)- index no. 4 (B1L3- 10 min), (d)- index no. 3 (B7L2- 10 min),

(e)-index no. 2 (sample B1L3- 30 min), (f)- index no 1 (sample B6L2- 10 min)

CONCLUSIONS

These modes are limited to the degrading of rock material caused by weathering process before forming residual soil. As stated, shale has different reactions with water apart from sandstone. Shale usually turns into flakes and sandstone would break into smaller pieces. It was found that slake durability test is primarily influenced by rock properties which allow ingress of water into the rock material. The presence of clay minerals enhanced rock susceptibility to slaking as can be seen in the higher weathering grades materials.

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The test results indicate that the strength, durability and density of rock materials deteriorate with the increase of weathering grade. As strength of rock material is a function of several properties including the hardness of the mineral constituents, degree of compactness, texture and inter-granular bonding material, their inter relation can be expected. Petrographic analysis revealed that feldspar and biotite minerals decomposed to clay as weathering takes place. Sandstone and shale have different orders of slaking index. These differences are caused by the variable amount of pores and particles in each grade of rock material.

Various tests were adopted in this study to determine the material properties. However, it was found that certain tests were only suitable to be adopted for certain weathering grades. Id1 were found suitable to be used to test grade II, III and IVa materials. This was because weaker samples of grade IVb, Va and Vb easily broke down during the first cycle of slake durability test. Thus, Id1 cannot be measured. The Id2 was found only suitable to measure stronger rock materials in grades II and III, which can sustain the first cycle (Id1). On the other hand, jar slaking test can be carried out in the field for fast identification of the material properties. The jar slaking test was found to be suitable to measure weak rock materials especially in grade IV and V. The total index observed during the sequence: 10 minutes, 15 minutes, 30 minutes and 60 minutes, was found to be a good indicator for the classification.

ACKNOWLEDGEMENTS Authors would like to extent sincere gratitude and appreciation to Research Management

Centre, UTM and the Government of Malaysia for the research grant and making the study a success.

REFERENCES 1. Franklin, J.A. and A. Chandra (1972) The Slake-Durability Test. Int. J. Rock. Mech. Min. Sci.

9: 325- 341.

2. Edy Tonnizam Mohamad, Mohamed Fauzi Md Isa, Mohd For Mohd Amin, Ibrahim Komoo, Nurly Gofar & Rosli Saad (2011) Effect of Moisture Content on the Strength of Various Weathering Grades of Granite, Electronic Journal of Geotechnical Engineering, Vol.16, Bund. H, pp. 863-886

3. Ibrahim Komoo (1995) Syarahan Perdana Geologi Kejuruteraan Perspektif Rantau Tropika Lembap,: Universiti Kebangsaan Malaysia., Universiti Kebangsaan Malaysia.

4. Santi, P.M. (1995) Assessing the Strength and Durability Properties of Shales in Keefer, D.K., Ho, C.L.ed. Landslides Under Static and Dynamic Condition- Analysis, Monitoring and Mitigation, ASCE Geotechnical Special Publication. 52 : 37-55.

5. Santi, P.M. (1997) Comparison of Weak and Weathered Rock Classification Systems, Santi, P.M., and Shakoor, A. (eds). Characterization of Weak and Weathered Rock Masses: Association of Engineering Geologist Special Publication, v. 9 : 139-159.

6. Santi, P.M. (1998a) Refined field methods for identifying, describing and testing shale and weak rock., Proceedings of the 29th Ohio River Valley Soil Seminar: Louisville, Kentucky. U.S.A.

Vol. 16 [2011] , Bund. O 1335 7. Santi, P.M. (1998b) Improving the Jar Slake, Slake Index, and Slake Durability Tests for

Shales, Environmental & Engineering Geoscience, Vol. IV, No. 3, Fall 1998 : 385-396.

8. Wood, L.E. and Deo, P. (1975) A Suggested System for Classifying Shale Materials for Embankment. Bulletin of the Association of Engineering Geologists. Vol. 12, no. 1 : 39-55.

9. Zainab Mohamed (2004) Pencirian Kejuruteraan Batuan Sedimen Terluluhawa Untuk Kerja Kejuruteraan. Universiti Kebangsaan Malaysia: PhD Thesis, Unpublished.

10. Zainab Mohamed, Ibrahim Komoo, Abd Ghani Rafek and Nurul Aini Osman (2004) A comparative study for strength determination of weak rock. In Fatt, I.D.C.S. ed., Proceedings of the Malaysian Geotechnical Conference 2004 : 255-260.

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