Index properties of rocks

THE UNIVERSITY OF AZAD JAMMU AND KASHMIRSUBMITTED TO: PROF.YASIR SARFRAZ

SUBMITTED BY :GROUP 5TOPIC: TECHNIQUES FOR MEASUREMENT OF IN-SITU

STRESSESSESSION:2013 -2017SECTION : MORNING

INSTITUTE OF GEOLOGY

INDEX PROPERTIES OF ROCK

Because of vast range in the properties of rocks &due to certain properties that are relatively easy to measure are valuable & may be designated index properties for rock specimens.

• POROSITY• DENSITY• SONIC VELOCITY• PERMEABILITY• STRENGTH

1 Porosity identifies the relative proportion of solids and voids.2 Density adds the information about the minerals and grain

constituents. 3 The sonic velocity evaluate the degree of fissuring (planar

cracks in igneous & metamorphic rocks).4 Permeability evaluate the relative interconnection of pores.

5 Strength determine the present competency of rock fabric to bind the component together.

PorosityPorosity is the ratio or fraction b/w the proportion of void spaces & the total spaces in the rock

n= vp /vt

Vp = volume of pores

Vt = total volume

Sedimentary rocks are formed by the accumulation of grains , rock fragments & shells.

Porosity varies from 0% to 90 % with 15% as a typical value for an average sandstone

Porosity generally decreases with age & depth below the surface.

A typical cambrian sandstone has 11% porosity & cretaceous sandstone has 34 % porosity

Chalk is most porous among all the rocks its porosity is 50%.

Porosity can be determined from the following calculations

• Measured density• Measured water content after saturation in water

• Mercury content after saturation with mercury using pressure injector

• Measured solid volume & pore air volume using boyles law

DENSITYDensity is the mass per unit volume.Density = M/V

It is stated previously that the porosity of rock can be calculated from the knowledge of its weight density.Density tell about the porosity & permeability .At 20 °C in the rock the volume of water is 62.4 .If the percentage of different mineral can be estimated under a binocular microscope or from the thin section , the specific gravity of the part of the rock can be calculated as the weighted average of solid specific gravities of component grains & crystal.

G = Specific gravity of component = volume percentage in the solid part of the

rock

The relationship b/w porosity &dry density is

The dry density is related with wet density by relationship

W= water content of rock (dry weight basis)

Water content &porosity are related by

In case of mercury content

Specific gravity of mercury =13.5

SONIC VELOCITYSonic velocity evaluate the degree of fissuring( planar cracks in igneous and metamorphic rock)

SONIC VELOCITY AS AN INDEX TO DEGREE OF FISSURING:The velocity with which stress waves are transmitted. First calculate the longitudinal wave velocity that the specimen would have,if it lacked pores or fissures.If the mineral composition is known longitudinal wave velocity can be calculated as

Where the v1 is the longitudinal wave velocity in mineral constituent which have volume proportion C in the rock. Now measure the actual velocity of the longitudinal waves in the rock specimen & form the ratio

As a quality index define

Where np% is the porosity of non fissured rocks a percentage.

PERMEABILITYPermeability evaluate the relative interconnection of pores.Fluid mechanics depend upon permeability.Shale is porous ,not permeable &fluid mechanics is not involvedSandstone is porous ,permeable & fluid mechanics is involved,

STRENGTHStrength determine the present competency of rock fabric to bind the component together. Strength determination of rock usually required careful test setup & specimen preparation & result are highly sensitive to the method & style of loading.Strength index is now available using the point load test describe by “BROCH & FRANK”.

Where P is the load of rupture & D is the distance b/w the point loads

Test are done on pieces of drill core at atleast 4 times as long as the diameter

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