Characterisation and Classification of the Physical Deterioration of Tropically Weathered Kenny Hill Rock for Civil Works
Assoc.Prof, Faculty of Civil Engineering. UiTM
Abd Ghani Rafek
Assoc. Prof, Faculty Science and Technology. UKM
Professor, Faculty Science and Technology. UKM
An appreciation to the concept of rock weathering is vital for engineer in order for him to achieve sustainability in design and construction of geotechnical structure in tropical environment. The study is to characterize and classify the physical deterioration of tropically weathered Kenny hill weak rock. The objective is to demonstrate the significant influence of tropical rock weathering in civil engineering works. A Kenny Hill formation is one of the typical sedimentary rock formation widely found within the vicinity of Kuala Lumpur and Klang valley, which is dominated by interbedding of sandstone, siltstone and shale. A study has been carried out to quantify the mode of physical deterioration of this rock materials as a unit rock mass. An engineering field mapping and series of laboratory index tests were conducted to quantify deterioration of the rock materials. Sandstone and shale characterisation by hardness, texture, jar slake and slake durability were carried out followed by a point load test. From the comprehensive study it can be summarised that the method of engineering characterisation of tropically weathered sandstone and shale are best to be carried out using test procedures that required the least sample preparation. Results showed the outcome of a systematic research design approach has successfully quantified the physical deterioration of tropically weathered Kenny Hill weak rock .Comparatively , it was found that the extreme differences in the physical deterioration and durability of sandstone and shale characteristics has resulted to the complex geotechnical problems frequently encountered by civil engineer working in tropically weathered Kenny Hill formation.
Keywords: Kenny Hill formation, Weak rock , Tropical Weathering, Physical deterioration.
The tropical climate such as Malaysia is known to produce the unique weathering profiles and heterogeneous physical deterioration of rock mass (Komoo 1995, Fookes 1997, Zainab 2004 ). The weathering profiles differ with rock types. The large scale infrastructures development such as North-South Highway, the Lebuhraya Pantai Timur and Simpang Pulai – Ladang Blue Valley, Cameron Highland had day lighted the complex geological features and their respective geotechnical problems in sedimentary formation. The Kenny Hill rock formation (KHF ) is a typical meta sedimentary rock formation, consists of interbedded sandstone, siltstone 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). To date, the geotechnical works in Kenny Hill formation has been much simplified , due to lack of understanding of its engineering characteristics . It is reported that many cut slopes from this formation displayed the complex physical deterioration characteristics.
The key to an optimum engineering design of geotechnical structure, its performance and stability in interbedded sedimentary formation lays on the reliable characterisation and classification methods, hence its respective results. The characterisation and classification of rock mass by weathering grade requires a sound knowledge on the principle of engineering characterisation and classification of rock and soil. However, the conventional approach of site investigation and characterisation techniques has failed to quantify the in-situ engineering properties of sedimentary weak rock. As such, most of structural design in Kenny Hill formation had been simplified for example, a bore pile for foundation of massive structure and instability of cut slopes despite a its high factor of safety.
A comprehensive and systematic geotechnical assessment has been carried out on a typical cut slope of Kenny Hill formation in , Shah Alam Selangor. It has an example of typical interbedded rock mass which is dominated by relatively thick sandstone and shale with thin layer of siltstone varied from slightly to highly weathered materials (Zainab 2004 ).This study only focussed on the sandstone and shale that dominated a Kenny Hill rock mass, hence siltstone properties shall be predicted. To quantify and model the physical deterioration and mode of weathering of the interbedded sandstone and shale were very tedious and complicated task as no one standard method of testing was found to be 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).
The principle of classification recommended that a simple index test should be adopted for characterization and classification of rock mass by weathering grade (Zainab 2004 ). The representative samples of sandstone and shale dominating Kenny Hill formation has been extracted base on locality for further geomechanical study in the laboratory. Subsequently a comparative behaviour of these two rock types was measured and discussed by assuming that they represent the behaviour of a Kenny Hill rock formation.
TROPICALLY WEATHERED KENNY HILL FORMATION
An engineering mapping and assessment were carried out on a typical cut slope of Kenny Hill formation at Jalan 8/22, Shah Alam Selangor. The tropical weathering characteristics was initially identify and classify by adopting the geological classification systems recommended (BS 5930 1999, Santi & Higgin 1998 and Komoo & Mogana 1988). Firstly, the identification, characterization and classification by weathering grade were carried out separately for sandstone and shale. Siltstone was not considered in this study due its relatively small quantity and limitation. It is expected that physical properties of siltstone can be predicted once sandstone and shale has been quantify. Geologically siltstone lies between sandstone and shale.
This particular outcrop has a slightly to highly weathered sandstone and shale , 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. Figure 1 and Figure 2 show a typical disintegration characteristic of weathered shale and sandstone subjected to wet tropical weathering. Shale tends to swell and slakes while sandstone was found to be still intact. Figure 2 showed flakes of shale covering sandstone underneath. It indicated that the mode of tropical weathering and disintegration of sandstone and shale differ very much although they were exposed to the same environment and time duration.
The classification approach for weathered rock as recommended by BS 5930,1999 was found to be relatively relevant. The qualitative assessment of sandstone and shale were first carried out individually by conducting a series of field qualitative index test. before their relative material behaviors were classified. The deterioration of sandstone was found to be much easily classified as compared to shale. However for interbedded rock formation more that one classification system is required. Table 1 shows the developed and recommended classification system for sandstone and shale by degree of weathering for this study It is a merging of two systems proposed by previous researchers which have been individually adopted to characterize and classify the interbedded sandstone and shale outcrop. Classification of clastic sandstone constituted of six weathering grades while shale remains as five classes of weathering grade. Soil to rock boundary is found to be within grade IV for sandstone and Class D to Class C for shale. Therefore the summarized system of classification by weathering grade recommended for Kenny Hill can be used to other similar types of rock formation.
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)
Figure 1. Slaking of shale (left) and intact sandstone (right).
Figure 2. Inclined bedding of shale and sandstone cut slope, showing shale flakes covering sandstone underneath.
PHYSICAL METHOD OF CHARACTERISATION BY WEATHERING GRADE
For sedimentary weak rock, the material sampling is the main problem for engineering testing purposes. Present of lamination and friability of the rock material as degree of weathering increases causes little advancement in laboratory research work. Sandstone and shale as a rock mass are hard and stiff however any robust sampling technique using water easily destroyed the sample. Mostly bore log records showed that sampling by wash boring technique on interbedded rock formation was unsuccessful. Figure 3, highlighted the correlation of SPT N-value to the percentage of sample recovery with respect of depth of sampling. The in-situ strength (N-value) of tropically weathered Kenny Hill formation with depth indicated that as depth increases, strength increases. 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 caused failure in retrieving cylindrical samples. Therefore mapping of weathering profile of sandstone and shale were carried out in-situ. A laboratory study was carried out on randomly classified weathered sandstones (BP2, BP3,BP4 and BP5a ) and weathered shale (S2, S2a and S5) and the most practical approach of quantifying the physical deterioration of tropically weathered sandstone and shale were proposed. The physical and strength properties of rock material are the mandatory parameters for rock mass classification in civil engineering work.
Figure 3. A typical record of N-value with depth and percentage recovery from ILKAP, Bangi. Selangor
The following methods of characterization and classification of physical properties of sandstone and shale by weathering grade constituting Kenny Hill formation were carried out as follow:
Hand Specimen Hardness
A hand specimen hardness test is carried out to determine the reliability and suitability of the simple method of assessment commonly done geologist, yet to be understood and appreciated by geotechnical engineer. This test method shall quantify the relative hardness of weathered and weak rock. A geological hammer was used to extract out rock lumps (3x-4x size of your tumb) from rock mass. The sample hardness was measured by the ability to break the rock lump into two using fingers or strength of one hand or both hand (Komoo 1989 ). Unfortunately the test method depend very much on the fingures strength of the personel and also his or her skill to generally conclude the material hardness. Despite, it was fairly a conservative approach, the technique able to guide engineer to differenciate and classify between soil, rock and weak rock. From repeated exercises and familiarity, the rating of hardness by hand specimen method was concluded as in Table 2, hence rating the physical hardness of shale and sandstone by weathering grade respectively.
Table 2. Physical properties of weathered shale
Rebound Hardness Test
The rebound hardness test is to measures the in-situ rock surface hardness due to tropical weathering. The rock surfaces was randomly selected by observations, differentiated by their physically classified hand hardness. The surfaces was first to be cleaned from fractures, dirt or roots. The impact tests were repeated ten times for a particular rock surface,spaced by about one to two centimeter. The readings were recorded, irrespectively of weather the rock surface fracture or not due to the impact load. The test was further repeated on the similar rock surfaces to conclusively quantify their hardness.
Two types of rebound hardness hammers were used to determine the most reliable and representative results for metasedimentary rock mass. The rebound hammer, model NR with impact load of 2.207 Nm and the ELE model, specifically EL35-1480 with higher impact energy were used to determine the rock mass surface hardness. Both types were differentiated only by their lower reading and no obvious difference for higher readings. The NR record minimum reading of 5, while ELE model give reading of 10, when test was repeatedly carried on the same rock surfaces. The ELE impact load caused the rock surface to crack due to its higher spring stiffness. Trial test on concrete slab for both model found to gave same result as compared to weak rock surfaces. Therefore NR rebound hammer model was selected to be more suitable for quantifying the deterioration in hardness of sandstone and shale due to tropical weathering in this study.
The orientation of the hammer was made perpendicularly downward to the rock surface during testing, consistently. For each surface, rebound impact test was repeated ten times with average of 1 cm spacing. It was observed that rebound hardness readings are consistent for relatively fresh and slightly weathered rock surfaces, i.e harder surface. As for highly weathered rock surface the range of rebound hardness readings is quite large. The lithology and mode of tropical weathering of sandstone and shale very much influenced the result obtained. For moderately and highly weathered rock, rebound impact test caused the surfaces to fractures and broken, hence this behavior shall be considered as its natural respond to the test carried out. It can be said that the used of rebound hammer for the characterization and classification of rock surface hardness by its respond to degree of impact sound, induced cracks, dented surface and rebound number shall conclusively measure the physical deterioration of rock surfaces hardness due to tropical weathering.
The characterization and classification by rebound hardness for slightly weathered to unweathered sandstone can be as high as more than 30. As the degree of weathering increases, the impact energy was being absorbed resulted to no rebound value and dented surface. Where as shale which is geologically weaker than sandstone does not showed distinct differences. These phenomena indicated the amount of energy required to break the individual material. However to break a unit rock mass consist of these materials, the optimum energy require is yet to be determine on in-situ . Similarly as the material weakened due to weathering, less impact energy is required or else excessive energy will cause the material to over break or fracture unnecessarily. The classification of sandstone and shale by weathering grade using rebound hammer test is as summarized in Table 2 and Table 4.
Jar Slake test
The test is to measure the response of rock material when soaked in water. The test helps to simulate the possible deterioration pattern of weathered sandstone and shale in tropical environment. Basically the samples must have similar weathering characteristics, no oxide content or obvious fissures. Present of oxide was only considered for highly weathered rock samples. The rock samples which were classified by weathering grade earlier using Rebound hammer was then randomly selected in a group of six for further testing . Each sample group was immersed in glass jar contained water. A trial test was first carried out by using rain water, tap water and ground water that easily obtained at the field . No significant difference in their response was observed, hence for simplicity, tap water was used consistently for this test.
Groups of slightly weathered sandstone (BP2), moderately weathered (BP3) to highly weathered sandstone (BP4, BP5s) and slightly weathered shale (S2) to moderately weathered (S5) were classified. Six glass jars contained tap water was prepared at every one time of testing A six randomly selected samples of sizes 3 cm to 4 cm each , were slowly immersed in the glass jar. The individual material reaction by weathering group was recorded immediately and subsequently after ten minutes and thirty minutes. The jar slake rating system developed by Santi 1997 for shale has been adopted for both weathered shale and sandstone in this study. It is for the purpose of consistency in testing technique and procedure, to determine the slaking index. It was found that the slaking behavior of weathered shale was similar to Santi however sandstone did not. The reaction of shale and sandstone samples and their respective slake index by weathering grade were then summarized in Table 2.
The jar slake test method was found to be suitable to measure the disintegration of rock materials due to change in weathering grade and their response to moisture content. Its slaking index can be easily rated when the slaking characteristics was similar. When it is not, the samples were rated by their dominant slaking characteristic. It was observed that slaking or disintegration characteristics of shale and sandstones in water vary with respect to weathering grade and mineral constituents. The advantages of the jar slake test result was that it clearly showed the reaction of shale and sandstone significantly with respect to moisture content, thus differentiating their respective potential engineering problems in wet climate (Figure 1 and 2 ) . Subsequently, the jar slake test on slightly to highly weathered sandstone distinctively showed the relative degradation in material density and an increase in porosity with respect to increase in weathering grade.
Table 3. Slaking index of sandstone
A slake index system for sandstone was developed from this study (Table 3) and proposed based on the adopted system recommended by Santi & Shakoor 1997 which has been 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 3. Slaking index of sandstone
A slake durability test was carried out for sandstones and shale samples which have been group by weathering grade. The test method was recommended to quantify the resistance of rock mass to degradation due to tropical weathering. The test was carried out by putting four groups of 10 rock lumps each into four separate drums which has been immersed in water. The drums were rotated at constant speed for 20 minutes. The weight of dry samples before and after test was recorded. The result obtained after each test is considered as the durability of samples against slaking after a cycle. The test was repeated subsequently to determine the cumulative percentage of samples resistance to disintegration .The study on the slake durability resistance of sandstone and shale was analyzed after first cycles (K1)and fifth cycles (K5) and the results are discussed accordingly. Table 4. summarized the results of the weathered rock samples resistance to slake durability. The slake durability index was further correlate to point load strength for the purpose of classification of sandstone and shale durability with respect to strength.
Table 4. Physical properties of weathered sandstone and shale representative of Kenny hill rock mass
The slake durability index (Isd) and point load index (Isp50) of sandstone from slightly weathered (BP2) to moderately weathered (BP3) and highly weathered (BP5) after first cycle (BP-K1) and fifth cycle (BP-K5) are plotted in Figure 4. Similarly to slightly weathered shale (S) to highly weathered shale (S5a).The graph mapped two separate profiles of sandstone and shale resistance to slake durability after first cycle and fifth cycles. The disintegration resistance for every sample groups of sandstone i.e BP2, BP3 and BP5 due to tropical weathering after 5 cycles is as high as 53% . Their respective rate of disintegration decreases gradually from K1 to K5 and almost the same quantity indicated by the parallel lines.
However shale does not has similar trend and non-uniform. Shale tends to flake into smaller thin pieces which did not able to pass through the 2 mm wire mesh drum. ). The slake durability of slightly weathered shale (S2) is higher than highly weathered shale (S5a). At the sametime the resistance index of S5a is exceeding 40% from K1 to K5 which is higher than S2. As a unit rock mass the results obviously explained and justified the complex mode of disintegration as observed in-situ (Figure 2 ).
Figure 4. Correlation between slake durability and uniaxial compressive strength of weathered sandstone and shale
Thin section study of Sandstone and Shale
The selected samples of sandstone and shale by weathering grade were prepared for rock thin section study. The objective is to determine the contributing factor to the change in engineering behavior of KHF with respect to the change in rock quality i.e rock material structure and texture by weathering grade. It was observed that sandstone and shale had obvious differences in their texture, structure and mineral content. These studies choose to use quartz grain as its measuring index. The rock samples thin section study was carried out by quantifying the quarts grains characteristics by their respective shape, saiz and percentage by volume as follow:
Quartz shape - subrounded or subangular
Quartz size – medium grain (> 0.25mm), fine grain (<0.25 mm ) and very fine also named as matrix in tis study (<0.125 mm )
Percentage volume of quartz
A secondary data was also recorded such as degree of fracturing, quartz oxide and quartz fragments. However these features were comparatively less significant therefore was not considered for further analysis. Figure 5(a) to (c) showed a typical texture of sandstone range from interlocking quartz grains (BP3 ) to well cemented (BP4 )and loosely cemented matrix (BP5a ).
The thin section study to characterize and classify the sandstone by weathering grade has been carried out in the following manner:
i) Size of Quartz
Result showed that sample group BP2 is dominated by grains size more than 0.25 mm which has been classified as medium grained sandstone. It can be observed that from samples BP2 to BP5, the percentage of medium grains and fine grains reduces and percentage by volume of matrix increases as weathering grade increases. Hence it can be concluded that quartz index can be used as a weathering index.
ii) Percentage of Quartz to Matrix
Classification by the percentage of quartz to matrix found to be that sandstone BP2 has higher quartz to matrix content as compared to BP3.On the other hand BP4 and BP5 has higher percentage of matrix . Disintegration of quartz grains from medium size to fine grained is believed to be caused by stress released due to weathering process. In other word slightly to moderately weathered sandstone (BP2 and BP3 respectively), quartz dominated rock minerals composition. Where as moderately to highly weathered sandstone (BP4 and BP5 respectively), matrix dominated minerals composition of these group sandstone. Conclusively it can be said that the BP2 cemented texture changed gradually to matrix texture in sample BP3, BP4 and BP5 due to tropical weathering.
In summary the results showed that the size of quartz grains range from about 0.62 mm (coarse grained ) to 0.25 mm (medium/fine grained) (Pettijohn,1975) . The quartz grains bigger than 0.125 mm has been measured individually by point counting method. The matrix is a cement material constitute of very fine quartz grains less than 0.125 mm and other fines to clay size minerals. Summary of quartz grains in weathered sandstone is as tabulated in Table 5.
Figure 5. Thin section study of sandstone showing texture degradation by weathering grade
Table 5. Summary of classification by quartz grains for sandstone
iii) Quartz characteristics in weathered sandstone
Quartz as rock forming mineral for sandstone found to be embedded or cemented in matrix. Figure 5a showed also the present of rock fragment. Rock fragment has been reported to be caused by the metamorphism of quartz grains. Generally rock fragments were observed in slightly to moderately weathered sandstone. Metamorphic sandstone has crystal like texture that provides higher stress resistance. Highly fractured quartz also named as matrix in this study is considered as cemented material that filled up the void in between mineral grains. It was highly found in highly weathered sandstone groups (BP4 and BP5). It is expected that fine quartz grains as matrix material provide less stress resistance.
A study on micro-texture of sandstone and shale by their respective weathering grade under 40X magnification clearly showed a significant changes in the texture of these rocks due to tropical weathering. Although quartz mineral known for its highest resistance against tropical weathering , result verified that tropical weathering has induced a very high internal stresses that caused quartz grains to self disintegrate from medium grained slightly weathered sandstone to fine matrix texture in highly weathered sandstone (Figures 5 a to Figure 5 c). It presents the physical deterioration of sandstone from a close interlocking of quartz minerals (Figure 5a ), to an increase in quartz to matrix ratio (Figure 5c).
Unfortunately similar study on shale found to be no obvious physical changes of shale matrix micro texture can be printed. Instead it was observed that mica has changed to sirisite due believed to be caused by chemical weathering.
In summary the physical 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.
STRENGTH CHARACTERIZATION TEST
The main problem when conducting laboratory testing for sedimentary rock material (name as weak rock) is that most of the techniques available has been tailored for soil and hard rock materials. Therefore commonly available laboratory equipments would not readily be used. Some modifications and upgrading of standard soil and rock testing equipments and testing procedures has been carried out but bearing in mind that it has not deviated from the fundamental of engineering measurement.
A comprehensive laboratory study has been carried to improvised the best technique , most reliable and repetitive method of strength index test for weathered sandstone and shale representing a Kenny Hill rock mass. The uniaxial compressive strength is the most popular strength test for geotechnical design, where as point load strength seem not popular among most engineers in Malaysia. This could be due to the fact that most empirical geotechnical design is based on uniaxial compressive strength. As for weak rock it was found that point load strength test has greater advantages due to its flexibility on sample sizes and shapes.
Figure 6 shows the proposed laboratory design work carried with respect to percentage of sampling for the purpose of determining 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 and limitation. Since these weak rocks were 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 with respect to percentage of samples successfully prepared for uniaxial compressive strength and point load strength tests were also highlighted. It can be concluded that as the degree of weathering increases the percentage by volume of samples obtained for strength tests reduces. However this does not correspond to shale due to the inherited weak laminations structure. The respective uniaxial compressive strength and point load strength test was carried out by tailoring to the testing techniques , sample sizes and sample conditions. Hence it has allowed for the objective to determine the strength deterioration of rock samples due to tropical weathering been successful achieved with higher degree of reliability.
Figure 6. Design for laboratory strength test of weathered sandstone and shale.
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 cut and must have at least two sides parallel with a control of thickness not less than 40 mm. The idea was to assured 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 were broken 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 consisted 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.
The strength result for irregular samples was corrected to the equivalent point load strength of 50 mm diameter (Isp50) perpendicular to lamination as shown in Table 6. Table 7 summarizes the point load strength conducted parallel to lamination orientation (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 significant 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 8).
Table 6. Summary of Isp and Isp50 for sandstone and shale perpendicular to sample lamination
Table 7. Summary of Is and Is50 of sandstone and shale parallel to sample lamination
Table 8. of strength anisotropy (Ia50 ) of sandstone and shale with respect to weathering
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.
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 (ISRM 1981) was only possible for slightly weathered sandstone. For slightly (BP2) to highly weathered sandstone (BP3,BP5a) and shale (S2) range of samples sizes obtained ranged from 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/(= 2.0 (sc2) in order to rationalise size error (Mohamed et al. 2004). Table 9 summarises the sc2 for sandstone and shale rock samples.The calculated result of uniaxial compressive strength (sc) was transformed to sc2 .
A strength reduction profiles is obvious where highly weathered sandstone (BP5b) has the lowest value 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 by their average value.
Figure 7 and Figure 8 show the strength envelope of sandstone and shale base on point load strength and uniaxial compressive strength respectively. Both graphical plots clearly classify and verify the equivalent strength of shale falls within the zone of highly weathered sandstone irrespective of its grade of weathering. Therefore shale is relatively weaker than sandstone and it is anticipated that the difference in strength increases as quality of sandstone is increases. Hence the composite rock mass consist of these two materials matrices shall has a heterogeneous mechanical respond when being subjected to loadings.
A point load strength test was much preferred to solve sampling problem for weak and highly weathered sandstone and shale. While a uniaxial compressive strength test was only be carried out for slightly to moderately weathered sandstone that can be cored into cylindrical shape. Thus a strength index of slightly to highly weathered sandstone and shale were measured and analyzed accordingly.
Strength Envelope For Kenny Hill Rock Mass
The reductions in strength of these two types of weak rock were plotted. Figure 7 summarises the strength envelope from point load test while Figure 8, 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 strength and durability against degree 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 Figure 9 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 7. A point load strength envelope for shale and sandstone
Figure 8. A uniaxial compressive strength envelope of shale and sandstone
Figure 9. Strength envelope for Kenny Hill rock mass.
The systematic characterization and classification research approach had produced a significant evident on physical deterioration due to tropical weathering, a case study in Kenny hill formation. A consistent field and laboratory testing techniques and procedures has able to characterised and classify the physical properties of sandstone and shale with respect weathering grade. However it was found that the same testing method resulted to a more reliable data in favour of sandstone as similar test on shale has not produce definite result due to its intrinsic properties. Hence characterisation and classification of shale need a modified approach of testing to best determine and measure its actual physical properties and behaviour in tropical environment for specific engineering purposes.
The study has successfully documented the deterioration of sandstone and shale by weathering grade range from rock-like state to soillike state. The comprehensive approch was conducted guided by the mode of weathering of Kenny Haill rock mass. The challenging task was to be able to quantify the physical changes of sandstone and shale due to tropical weathering by the most practical manner to the engineer yet produce a meaningful and acceptable result.
Conclusively the above method of engineering characterizations and classification of sandstone and shale by weathering grade proved to be the best , cheapest and the most practical techniques for classification of weak sedimentary rock, specifically Kenny Hill formation, thus is fully recommended to be adopted by geotechnical engineer in their site investigation work.
This paper only highlighted and discuss a sub topic of a comprehensive research funded by Ministry of Science, Technology and Innovation Malaysia (IRPA 09-02-01-0006EA006 ) and is gratefully acknowledge. Further findings will be published in the near future. The author would like to thank CSL Sdn.Bhd and The Institute of Research and Consultancy UiTM for their support.
|© 2007 ejge|