A Geotechnical Engineering Characterisation of Interbedded Kenny Hill Weak Rock in Malaysian Wet Tropical Environment

 

Zainab Mohamed

Assoc. Prof., Faculty of Civil Engineering, University Technology MARA, Malaysia.
e-mail: hjhzm6@yahoo.com

Abd Ghani Rafek

Assoc.Prof, Faculty of Science and Technology, National University of Malaysia.
e-mail: aghani@pkrisc.cc.ukm.my

and

Ibrahim Komoo

Professor and Director, Institute for Environment and Development, Nat. Univ. of Malaysia.
e-mail: ikomoo@hotmail.com

ABSTRACT

An attempt to quantify the geotechnical properties of interbedded Kenny Hill weak rock mass requires an integrated knowledge on the principle of rock and soil mechanics. A Kenny Hill formation is widely found within the vicinity of Kuala Lumpur, which is dominated by interbedding of sandstone and shale. An engineering field mapping was carried out to document the mode of physical deterioration of both rock materials as a unit rock mass subjected to Malaysian wet tropical environment. A systematic approach of the method of characterisation was developed for sandstone and shale that was known as a weak rock. A series of laboratory index tests were proposed for quantifying the qualitative deterioration of these two rock materials. Sandstone and shale characterisation by hardness, texture, jar slake and slake durability were carried out followed by a uniaxial compressive strength and a point load tests. From the extensive effort it can be summarised that as for weak rock, the method of geotechnical characterisation should be best carried out using test procedure that require the least sample preparation. This paper highlighted the outcome of a systematic design approach adopted to meet the objective. A comparative study showed that the geotechnical properties of these two rocks are extremely different forming a heterogeneous rock mass. Thus more than one method of testing with modified approach is required in order to obtain a higher degree of laboratory results.

Keywords: Weak rock, engineering testing, Kenny Hill formation, interbedded rock mass

 

INTRODUCTION

A Kenny Hill rock mass is a meta sedimentary rock formation, consists of interbedded sandstones and shale of Upper Silurian-Devonian age The low grade metamorphism of sandstone is known as quartzite while shale is phyllite (Mohamed et al.2004). A warm and wet tropical climate like Malaysia was known as a catalyst to the rapid deterioration of weak rock and aggressively triggers geohazards once it is exposed to the environment (Fookes 1997, Komoo 1995). Comprehensive geotechnical assessments carried out on a typical cut slope of Kenny Hill formation in Selangor showed that the interbedded rock mass is dominated by relatively thick sandstone with thin shale varies from slightly to highly weathered materials. To map out the physical deterioration and mode of weathering of the interbedded sandstone and shale was a very tedious and complicated task as no one standard method of testing was found suitable to measure the engineering properties of both rock materials. However the standard guideline was adopted for characterisation of these rocks so as the result obtained can be systematically compared to previous researchers’ findings (Mohamed et al. 2001).

Principle of classification recommended that a simple index test should be chosen due to friability of rock materials for the purpose of quantifying their engineering properties with respect to different grade of weathering. Representative rock samples of sandstone and shale were extracted base on locality for further geomechanical study in the lab. Subsequently a comparative behaviour of these two rock types was discussed by assuming that they represent the behaviour of a Kenny Hill rock mass.

FIELD INDEX

A engineering mapping and assessment were carried out on a typical cut slope of Kenny Hill formation at Jalan 8/22, Shah Alam Selangor. For the particular outcrop a slightly to highly weathered sandstone and shale were found dominating the rock mass . The mode of weathering and nature of degradation and disintegration of sandstone and shale subjected to tropical weathering were heterogeneous. Fig.1 show a typical disintegration characteristic of weathered shale and sandstone subjected to wet tropical cycles for a week. Shale tends to swell and slakes while sandstone was found to be still intact. It indicated that the state of weathering for sandstone and shale differ very much although they were exposed to the same environment and time duration.


Figure 1. Slaking of shale (left) and intact sandstone (right).

The classification approach for weathered rock as recommended by BS 5930,1999 was found to be relevant. However for interbedded rock formation more that one classification system is required. Table 1 shows the developed and recommended classification system for interbedded rock formation. It is a merging of systems established by previous researchers which was individually adopted to characterize and classify the interbedded sandstone and shale outcrop. Classification of weathered sandstone falls into six grades while shale remains as five classes. Soil to rock boundary is within grade IV for sandstone and Class D to Class C for shale.

 

Table 1. Summaries the recommended system for classification of weak rock by weathering grade for more than one rock types as a unit rock mass (after BS 5930 1999, Komoo & Mogana 1988, Santi & Higgin 1998).

A qualitative assessment of sandstone and shale were first carried out individually before their relative material behaviors were classified. The deterioration of sandstone was much easily classified as compared to shale. A series of field qualitative index test was carried out. Generally the physical properties of interbedded Kenny Hill rock mass is as summarized in Table 2 and Table 3.There are a significant differences of physical properties and slake index of sandstone and shale that explained the non uniform weathering degradation characteristic of Kenny Hill rock mass. A slake index system for sandstone was developed and proposed based on the adopted system recommended by Santi & Shakoor 1997 which was established for shale. A combination of both sandstone and shale slake index systems shall provide a good information and better understanding on the complex deterioration characteristic of Kenny hill rock mass with respect to degree of weathering, hardness and reaction when immersed in water.

Table 2. Slaking index of sandstone

 

Table 3. Physical properties of weathered shale

Quantitative index test such as rebound hardness on to the rock mass surface followed by hand sample hardness and discoloration, macro-texture and jar slake tests were carried out to classify each material. It was found that besides these test, lamination characteristic is equally significant criteria for classifying the degree of weathering of these weak rock. Table 4 summaries the qualitative classification of sandstone and shale as a unit rock mass. There is an obvious progressive degradation of physical properties of sandstone as compared to shale. Their classification with respect to slake durability also was not consistent. The disintegration of both rock materials when being subjected to 5 cycles of slake durability test is as high as 53% for sandstone. However shale does not show a clear trend because the mode of disintegration is not similar. Shale tends to slake as smaller thin pieces which does not able to pass through the 2 mm wire mesh drum.

 

Table 4. Physical properties of weathered sandstone and shale representative of Kenny hill rock mass

The consistent field and laboratory testing techniques adopted to characterised and classify the physical properties of sandstone and shale with respect tropical weathering has shown that either same techniques will result in more reliable data in favour of sandstone with shale will not give a good result or shale need a modifies approach of testing to best determine and measure its true physical properties.

Sampling

For weak rock, material sampling is the main problem for engineering testing purposes. Present of lamination and friability of the rock material as degree of weathering increases had caused little advancement in laboratory work so far. Sandstone and shale as a rock mass are hard and stiff however any robust sampling technique using water easily destroyed the sample. Most borelog records reviewed so far showed that sampling techniques by wash boring on similar types of interbedded rock formation was unsuccessful. Fig. 3, summarises the correlation of SPT N-value to the percentage of recovery and depth of sampling. The in-situ strength (N-value) of Kenny Hill formation with depth showed that as depth increases, N-value increases due to confinement effect. However the percentage of sample recovery is independent of N-value instead depends on the rock types. The structural bedding and lamination in weathered sandstone and shale had caused failure in retrieving cylindrical samples. Hence the laboratory design work was carried out to overcome this problem and propose the most practical approach of determining the strength properties of weak rocks. The strength property of rock material is a vital parameter for rock mass classification and engineering design purposes.

 


Figure 2. A plot of N-value with depth and percentage recovery for ILKAP, Bangi.

 

LABORATORY DESIGN

A major hiccup when dealing with laboratory testing of weak rock material was that most of the techniques available were tailored for soil and hard rock materials. Therefore these equipments could not readily be used. Hence some modifications and upgrading of standard soil and rock testing equipments and testing procedures need to be carried out but bearing in mind that it must not deviated very much from the fundamental of engineering measurement.

A comprehensive laboratory study has been carried to develop the best, most reliable and repetitive methods of strength index test for highly weathered sandstone (Grade 111 to IV) and shale representing a Kenny Hill rock mass. Uniaxial compressive strength has been the most popular strength test for geotechnical design, unfortunately a point load test seem not known by many engineers in Malaysia. This could be due to the fact that most empirical geotechnical design is base on uniaxial compressive strength. As for weak rock it was found that point load test has greater advantages because of flexibility in sample sizes and shape.

Fig. 4 shows the proposed laboratory design work carried out in order to determine the strength of weathered sandstone and shale representative of Kenny Hill rock mass. The method of strength test was chosen to suit the sample condition. As weak rock is fragile and prone to break along its lamination, thus coring of block samples was minimised and favoured for prismatic shape samples. The rock material grade of weathering and percentage of samples successfully prepared for testing were also highlighted. Figure 4 shows that as degree of weathering increases the percentage by volume of samples obtained for testing decreases. However this does not correspond to shale due to the present of weak laminations. The sequence of strength test recommended was uniaxial compressive strength followed by point load strength of various sizes. Tailoring the testing technique by the material condition has made the strength test of weathered sandstone and shale rock materials successful with higher degree of efficiency.

 


Figure 3. Design for laboratory strength test of weathered sandstone and shale.

 

Strength index

Point load strength

A point load strength test was found to be the best option for testing weak and weathered sandstone and shale rock materials. The prismatic samples were manually cut using trimming saw. The cut samples must have at least two sides parallel with a control of thickness not less than 40 mm. The idea is to assure the mode of sample failure will be in such a way it breaks into two pieces with failure surfaces must be more or less uniformly rectangular. Unfortunately the portable point load testing equipment is not reliable for testing weak rock. Most of the samples break during clamping between the cone tips. An improved equipment set up and procedure were made. Using the same cones, they were fitted to a servo control UTM machine. Trial tests were first carried out to determine the suitable loading rate, so as the samples should break within the recommended failure time of 10-60 seconds. A loading rate of 0.6mm/sec for sandstone and 0.3 mm/sec for shale were best selected to test all the rock samples in the direction perpendicular and parallel to sample lamination. The advantages of this new approach were that besides recording failure load, a load-displacement profile and mode of failure can also be determined.

A total of 384 rock samples has been successfully tested which composed of moderately weathered sandstone (BP3)to highly weathered (BP4,BP5b) and slightly weathered shale (S2) to highly weathered shale (S5 and S5a). The experimental point load strength varies with samples sizes and heterogeneity. However the strength is corrected to the equivalent point load strength of 50 mm diameter (Isp50) perpendicular to lamination as shown in Table 5. Table 6 summarizes the point load strength in parallel direction (Iss50). A study on the effect of sizes on the point load strength of sandstone and shale prevailed that the influence of size factor was not as great as the effect of weathering. It can be summarized that the point load strength test successfully concluded a strength reduction of almost consistent irrespective of orientation of sample lamination for sandstone (Table 7).On the other hand shale materials had not showed a clear trend. The data has validated the field observation studies where tropical weathering has also caused a progressive disintegration of lamination structure besides material strength for sandstone but cannot be easily differentiated for shale.

 

Table 5. Summary of Isp and Isp50 for sandstone and shale perpendicular to sample lamination

 

Table 6. Summary of Is and Is50 of sandstone and shale parallel to sample lamination

Uniaxial compressive strength

Sample preparation for uniaxial compressive strength test is almost impossible for weak rock. To retrieve good cylindrical samples with diameter to length ratio of 2.5 is only possible for slightly weathered sandstone. For slightly (BP2) to highly weathered sandstone (BP3, BP5a) and shale (S2) range of samples sizes obtained is within 1.0 to 2.5. The cored samples were best selected to represent intact samples with minimum ratio of 1.0.

A total of 72 samples of weathered sandstone and 8 samples of weathered shale has been successfully tested . A standard procedure of testing was carried out according to ISRM(1981) except that the loading rate adopted was 0.66 mm/sec. The results were analysed and standardized to the equivalent uniaxial compressive strength of size L/Ć = 2.0 (sc2) in order to rationalise size error (Zainab et al. 2004). Table 8 summarises the sc2 for sandstone and shale rock samples.The calculated result of uniaxial compressive strength (sc) was transformed to sc2.

 

Table 7. Summary of strength anisotrophy (Ia50) of sandstone and shale with respect to weathering

A strength reduction profiles is obvious where highly weathered sandstone (BP5b) has the lowest (high end) of about 32 MPa. However group BP2s and BP5a can be group together base on range of sc2 while group BP3 has lower strength than BP2g. Sequence of strength deterioration representing degree of material weathering can be said as BP2a, BP2s, BP2g, BP3, BP5a, and finally BP5b. As for shale the strength reduction was best analyzed according to its equivalent strength (c2 determined from point load strength test. A strength correlation of sc2 and Isp for S2 shale sample was first derived . The empirical equation was used to calculate the equivalent sc2 for S5 and S5a using their respective Isp values. Result showed a reduction in strength, sc2 from S2’ to S5’ as high as 40% . As for S5a it is much lower than S2 by 18% but of similar strength with S5 if referred to their average value.

 

Table 8. Summary of sc2 for weathered sandstone and shale samples from Kenny Hill rock mass

Strength envelope for Kenny Hill rock mass

The reductions in strength of these two types of weak rock were plotted. Fig. 5 summarises the strength envelope from point load test while Fig. 6, shows the respective uniaxial compressive strength envelope base on group of samples .Both figures show that the relative reduction in strength with respect to weathering for sandstone is very obvious as compared to shale. The differences in material properties are well reflected where comparatively slightly to highly weathered shale is much lower even compared to highly weathered sandstone. Thus it indicated that the materials genesis have significant influence to the durability against robustness of weathering and sampling technique.

From a comprehensive field and laboratory study on the Kenny Hill rock mass and rock material, the progressive strength reduction due to weathering has been successfully quantified. The projected strength reduction profile of the weak rock mass is as shown in Fig. 5 below. The strength reduction and deterioration was plotted from rock state on the right hand side of the vertical axis to soil like state on the left hand side. Sandstone rock state has a much higher strength while its soil like state is also much lower than shale. The overall interbedded rock material strength lies between sandstone as the upper limit and shale as the lower limit.

 


Figure 4. A point load strength envelope

 


Figure 5. A uniaxial compressive strength envelope.

 


Figure 6. Strength envelope of Kenny Hill rock mass

 

CONCLUSION

A systematic approach and the proposed laboratory design work have produced reliable results. The techniques proved to be the best method, cheapest and the most practical for testing weak rock. By adopting the design approach recommended the strength of weak or highly weathered rock material can still be measured with higher accuracy. Therefore it is recommended that for engineering testing of weak and weathered rock, rock lumps and pieces normally obtain from wash boring sampling technique should not be wasted instead tested to determine it actual properties as a true design parameter.

ACKNOWLEDGEMENT

The authors would like to thank CSL Sdn.Bhd, for providing the borehole records. This research project is funded by Ministry of Science, Technology and Innovation Malaysia (IRPA 09-02-01-0006EA006) and is gratefully acknowledged.

REFERENCES

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