PREDICTION OF MECHANICAL BEHAVIOUR FROM PETROGRAPHIC CHARACTERISTICS OF LOCKHART LIMESTONE

Abstract

The present study deals with the prediction of mechanical behavior from petrographic characteristics of Paleocene Lockhart Limestone which is measured, sampled, and described from Rumli and Shah Allah Dita sections of Islamabad and Nammal Gorge section of the western Salt Range Pakistan. The limestone is predominantly nodular, fossiliferous, interbedded with clay and shale. UCS values of Lockhart Limestone samples vary from 26.63 to 86.61 MPa in the Rumli area, 21.21 to 100 MPa in Shah Allah Dita and 48.42 to 128.32 MPa in western Salt Range respectively. Point Load Test (PLT) values of western Salt Ranges (WS) ranges from 5 to 10 MPa, 2 to 7 MPa in Shah Allah Dita, and 4 to 9 MPa in the Rumli area. Mean water absorption values of SD, R, and WS common samples are 0.25%, 0.29% and 0.28% respectively. Los Angeles abrasion test values were 25.8% WS, 22.2% SD, and 17% R. Flakiness and elongation test values of SD, R, and WS aggregates were 1.6%, 3.1% and 3.4%. Based on detailed field and petrographic characteristics, the proposed localities tell that these limestones are mudstones and wackstones which are principally comprised of calcite and bioclasts. Although limestones have very little porosity it does have a negative impact on the mechanical properties (UCS) of these rocks. For Rumli the porosity ranged from 0.41% to 1.2 % with mean 0.77 %. The porosity ranged from 0.43 % to 0.87 % (average 0.69 %) and 0.62 % to 1.0 % (average 0.75) in SD and WS, respectively. Comparatively higher porosity of Rumli samples can be related to its higher bioclasts contents. In Western Salt Range area limestone sample WS1 and WS2 have lowest porosities (0.67 % and 0.62%) possessing the highest UCS (128.32 and 121.65 MPa). Samples R1 and R2 show abnormal results as these samples are highly fractured when subjected to UCS. Similarly, samples with the highest porosity (1 % and 1.2 %) have the lowest UCS values (77.3 and 86.1 MPa). Linear equation for the best fit line and regression coefficient (R²) value is determined by applying regression analysis of mechanical properties (UCS and PLT) with porosity and mineralogical characteristics of the prescribed limestone.