Textural Implications in Assessment of Physico-Mechanical behaviour of Metavolcanic Rocks from Dir Upper, north western Pakistan

Abstract

The Dir-Utror meta-volcanics from the south western portion of the Kohistan arc in northern Pakistan are analyzed in term of their petrography, physico-mechanical properties. Field observations and petrography show the collected representative samples to be fine-grained meta-andesites (FMA), coarse-grained meta-andesites (CMA) and agglomerate (AG). The relationship between petrography and physico-mechanical properties has been investigated which inferred the grain size to be the major factor, alongside grains’ shape, arrangement and size distribution as well as degree of mineral alteration significantly affecting the mechanical behavior of rocks. The CMA yield more strength (98 MPa) than FMA (93 MPa) due to its lesser degree of mineral alteration, inequigranular texture, lack of preferred mineral alignment, relatively low porosity and water absorption. The lower strength of agglomerate (57 MPa) corresponds to abundance of soft minerals (calcite), exotic rock fragments and coarse-grained texture. Based on physico-mechanical properties including specific gravity, bulk density, aggregate impact value, Los Angeles abrasion value and unconfined compressive strength (UCS), these rocks fall within permissible range to be utilized for multiple engineering purposes including dimension stones and foundation materials for other civil structures. However, petrographic investigations reveal excessive amount of reactive silica in these rocks making them prone to alkali-silica reactivity in concrete works with ordinary Portland cement (OPC). Hence these rocks are not recommended for use as sole aggregate material or low-alkali cement is recommended, if used.