ASSESSMENT OF DURABILITY OF KHEWRA SANDSTONE OF CAMBRIAN AGE, SALT RANGE, PAKISTAN.
ABSTRACT: The Khewra Sandstone Formation (KSSF) of Eocene age is outcropped at a number of locations in the Salt Range of Pakistan. Due to good strength and blocky nature of the sandstone it is very frequently being used as construction material for building and aggregate. However along road side sections such as Kalar Kahar section of Motorway and Khewra-Choha Sadden Shah road and link road to Shahwal village, slope stability and landslides problems are very common in this formation. To determine the causes of slope failure in Khewra Sandstone and to protect the mass wasting and landslides in the mountainous area of Salt Range, the engineering properties are evaluated for estimation of the durability of the formation. The results indicate that the durability of Khewra Sandstone Formation is controlled by the alternate geology, degree of discontinuities and their orientation with respect to the slope face in the outcropped sections.
The durability of the formation on the basis of field characterization and slake durability tests is estimated as medium high to high with 2nd cycle range from 83.6 to 98.4 %.
Keywords: Khewra Sandstone Formation, geology, slake durability, wedge failure.
The Khewra Sandstone is being used as construction material such as stone walls, roof slabs and decoration tiles. It is also useful material for construction of retaining walls to control landslides and aggregate for pavements. In the outcropped areas, the sandstone is inter-bedded with shale layers. The overall behavior due to presence of alternate shale beds and orientation of discontinuities has resulted in slope stability problems along many sections.
Shah (2009) described the geology of the Khewra Sandstone as "The formation consists predominantly of purple to brown, yellowish-brown, fine grained sandstone. The lowermost part of formation is red, flaggy shale. The sandstone is mostly thick bedded to massive. Sedimentary features like ripple marks; mud cracks etc. are common in the formation". (Khan, et al 2012) characterized the formation as, "The porosity values determined with the Helium Porosimeter for the Khewra Sandstone rangingfrom18.76 % to 21.07 % are representative of potential petroleum reservoir".
Duodenal: Researchers have investigated the durability of sedimentary formations including sandstone, mudstone and clays (Dick and Shakoor, 1997; Gupta and Ahmad, 2007; Dhakal et al 2012). The durability of sedimentary formations is controlled by a number of factors including clay minerals, uniaxial compressive strength and orientation of discontinuities along exposed face. (Ayakwah et al., 2009) investigated the "Effects of weathering and alteration on point load and slake durability indices of Questa mine materials, New Mexico". (Koly and Kayabali 2006) determined the effect of aggregate shape and sutface roughness on the slake durability index using the fractal dimension approach. Topal and (Doyuran 1997) studied the durability of Capadocian tuff by using, "the average pore diameter, saturation coefficient, wet to dry strength ratio, stastic rock durability index, index of rock durability and slake durability index of the tuff are used for the durability assessment of rock and of the fairy chimneys".
In this study the durability of the Khewra Sandstone Formation which is extensively exposed in Salt Range (Fig 1) has been assessed with reference to three aspects, alternate geology, discontinuities and their orientation and slakes durability tests.
The procedure adopted for investigation of durability of Khewra Sandstone Formation of Cambrian age is given as follows
The physical characterization of the formation shows that it is composed of medium grained light brown to yellowish brown sandstone with thin layers of shale. Along the exposed sections the degree of weathering is also variable over horizontal spreading. The sampling was made from different locations from upper and lower horizon so that it covers vertical variation due to depositional environment. Scan line survey was carried out over a number of sections to measure joints, their orientation and degree of persistence. Rock samples were collected each 1-2 cubic feet, from Khewra Sandstone Formation of Cambrian age from Khewra gorge, Khewra- Choha Sadden Shah link road to Shahwal village and Motorway.
For the estimation of slake durability the proposed method by Franklin and Chandra (1971) was adopted. To fulfill the instrument requirements, the samples were crushed to obtain representative volumes for determination of slake durability. Representative sample of 500 grams comprising of rock lumps with average weight of 50 grams per fragment were used for tests. The lumps were converted to roughly spherical shape by removing sharp edges as suggested by ISRM. (1981) The samples were placed in slake durability apparatus with 20 revolutions per minute to obtain total 200 revolutions during 10 minutes slaking. The slaking fluid was used as tap water at 200 C. on completion of 1stcycle the sample was placed in an oven at 1050 C temperature for 3 hours before start of 2nd cycle.
RESULTS AND DISCUSSION
The alternate layers of shale within the KSSF play an important role in overall durability of the formation. The most of the rock falls in the KSSF along link road to Shahwal village and motorway are due to the fast weathering of shale layers which lead toppling or wedge failure of the overlying bed of hard sandstone as shown in the Fig 2.
Another important factor controlling the durability of the formation is inherited effect of tectonic activity in the area. For example the formation is very fragile along few sections such as along link road of Khewra Choha Sadden Shah to Shahwal village. The degree of fracturing along motorway is comparatively less; however its bedding planes are directly dipping towards road causing plane failure at a number of locations. Slumps are also very common due to fast erosion of soft layers of shale causing rock falls of over lying strata(Fig 3). Hence our results are in good agreement with reported findings of Dick and Shakoor (1997) who evaluated the durability classification with respect to slope instability for mud rocks and proposed potential failure through slumps for medium durable formations. However along the Khewra Gorge the outcropped sections with less fracturing are more durable and form vertical cliffs where it is dipping contradictory to the gorge.
The role of discontinuities on durability of rock mass is well established. In KSSF the primary discontinuities and bedding planes are dissected by a number of oblique joints. The persistent joint sets are responsible for transmitting water to the lower
Table 1. Estimation of slake durability of the Khewra Sandstone from various outcropped locations
Sr.###Identity No.###Location###1st cycle###2nd cycle###Remarks
1###KSSD-01###Lower part, right side of Khewra###98.2###96.8###Sampled from intact block 20
###Gorge###feet above gorge floor
2###KSSD-02###Lower bed of Khewra sandstone###99.2###98.4###Sampling from Khewra Gorge
###from Khewra Gorge
3###KSSD-03###Top bed of formation###95###87###Nearby a gully
4###KSSD-04###Link road to Shahwal village.###96.8###94###-
5###KSSD-05###Link road to Shahwal village.###92.8###83.6###Sampled from landslide section.
The results of slake durability testing are given in Table 1 which indicates that the 1st cycle durability range is 92.8 to 99.2 % and for 2nd cycle it varies between 83.6 to 98.4 %. Hence by comparing the results with suggested ranges for slake durability by ISRM. (1981) the formation durability can be classified as medium high to high. The slake durability values are higher for Khewra Gorge and Motorway compared to link road to Shahwal village. The low durability along link road is probably due to axial plane where more fracturing took place through which rainfall water seeps to lower horizons causing further weathering of the formation.
Conclusion and Recommendation: On the basis of field investigations and tests performed in the laboratory the following conclusions are made.
* The durability of Khewra Sandstone Formation is medium high to high.
* The durability of the formation is controlled by the degree of discontinuities in outcropped sections and the persistence of joints.
* The alternate shale beds with in the Khewra Sandstone Formation are also responsible of failure in the slopes.
* Retaining walls with drain holes at the toe of the critical areas and improvement in surface drainage is suggested.
Acknowledgement: The University of Engineering and Technology Lahore is greatly acknowledged for providing necessary facilities for field visits and laboratory testing.
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Department of Geological Engineering UET Lahore
Department of Mining Engineering UET Lahore
Corresponding Author E-mail: email@example.com)
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|Publication:||Pakistan Journal of Science|
|Date:||Dec 31, 2013|
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