PETROGRAPHIC ANALYSIS

Petrography is the only reliable tool that adjudges suitability of geological materials for civil structures particularly as a coarse or fine aggregate. Alkali Silica Reaction (ASR) and Alkali Carbonate Reaction (ACR) prevalent in the natural material or having been developed subsequently can be discerned by petrography alone. Though deleterious minerals are not very commonly present in abundance in nature, yet their presence in unsuitable geologic environ leads to development of signals causing weakness of structural members.

It is thus, in fitness of the things that all geological materials must be tested petrographically before being brought into use. In this era, all mega civil engineering projects are required to submit petrographic report along with DPR.

To meet this demand of petrography work, ATES Laboratories, has come in a big way with establishment of High resolution microscope inter faced with computer and attached digital camera for study and documentation. A team of geologists, with long standing experience in petrography, have been deputed to accomplish the task.

To achieve this, it is equally important that the sampling procedure for rocks/fine aggregates/silt must be clearly understood and followed scrupulously. This is highly important to decipher petrographic observations into meaningful interpretations.

Guidelines for collection of rock/ fine aggregate/Silt samples

• Rock samples: Sample, by definition, represents the population. It is therefore, necessary to collect physically similar entities in one sample. Rock sample should be preferably of 3”x4”x2” size which allows the microstructures/ textures of the rock to be observed megascopically. Samples should be legibly labeled and location of the sample must be mentioned. If possible, the regional geological name of the rock must be mentioned.
• Fine aggregates: Sample should consist of size fractions ranging between 75μ to 300μ and should have at least 100 grams dry weight.
• Silt (suspended sediments) samples: These samples are analyzed petrographically with a purpose to determine the mineral content, their hardness, granulometric characters (size and shape) of mineral grains, their hardness, specific gravity vis-à-vis weight per cent in the given sample. Silt samples are collected from the sediments laden waters which have to pass through water conducting system including turbines. One thousand liter of water is collected and kept overnight and decanted to collect silt that should have at least 100 grams dry weight. These samples must be collected periodically every fortnight during pre-monsoon, monsoon and post monsoon periods.