Physical Properties of RocksPhysical properties of rocks are of interest and utility in many fields of work. The performance of rock under particular condition depends upon its physical and mechanical properties. These properties describe the rock material and help to classify them. These are directly related to the strength of the rock and greatly concerned in engineering and other fields.

Some important physical properties of rocks are given below.

1. Porosity

2. Density

3. Permeability

4. Moisture content

5. Degree of saturation

These are explained one by one below.

1. Porosity:

Porosity of rock, indicated by a dimensionless number n, is a fraction expressing the proportion of void space to the total space in the rock. It can be shown by formula as

n ¿VpVt × 100

Where

n = number of pores

Vp = volume of pores

Vt = total volume

1.1 Porosity dependenceFollowing are the important factors that affect the rock porosity.

I. Shape of mineral grain

Rock sample consisting of spherical grains have more porosity as compare to those consisting of angular

grains.

II. PACKING The more closely packed the particles the lower the porosity.

III. SORTING

If all particles are the same size they are sorted. If the particles are different sizes they are Unsorted (poorly sorted). The more sorted the particles the higher the porosity.

IV. . Weathering

Due to weathering in the rock mass and micro fractures the porosity of the rock may be higher.

V. Cementing material:

The porosity is decreased in the presence of cementing material because it fills the spaces between grains hence reducing the porosity.

VI. Age of the rock

With the passage of time compaction and cementation of rock increases and porosity decreases.

1.2 Porosity present in rocks In sedimentary rocks, formed by the accumulation of grains, rock fragments, or shells the

porosity varies from 0 to as much as 90% with 15% as a typical value for average sandstone. These rocks are more porous than any other rock type because there is more open space between the individual sediment grains than between the minerals in the crystallized rocks. In these rocks porosity generally decreases with age, and with the depth below the earth surface.

E. g. Limestone has porosity 0.6% to 31%.

Igneous rocks at the surface tend to expand because of the release of pressure from where it came from beneath the earth. The expansion of the rock may produce cracks and pores so it becomes porous. These rocks can hold as much water in its cracks as sedimentary rocks can hold between their grains.

E.g. Granite has porosity o.4% to 1.5%.

In metamorphic rocks micro voids appear during the recrystallization process. This is because natural crystal grows and re-arrange or re-pack causing pores to develop between the grains. Porosities of metamorphic rocks can vary considerably depending on the protolith and other factors such as levels of heat, pressure and friction present during the complete process.

Marble has porosity between o.5percent to 2%. Slate has porosity between o.4percent to 5%.

one thing must be mention here that the rock is not favorable for engineering purposes if it is highly porous.

Types of porosity

Primary and secondary porosity

Intact rock contains pores and cracks between and within grains and crystals. This voids space is called primary porosity. Other voids, in the form of joints, faults, and blast-induced fractured, together form secondary porosity.

Lab measurement of Porosity:

Physical method of measuring porosity upon measuring the three phases, solid, water, air. Weight of solid and pore water are simply measure by weighing the specimen at natural water content, drying at standard 105oC and then reweighing to determine dry weight and water loss. The volume is easily measured if the specimen has regular geometry. Volume of irregular sample can be measured by saturating the rock in vacuum or by coating pieces with thin film of plastic or wax. Then the saturated specimen is placed on measuring cylinder and volume is determined either from water displacement in measuring cylinder, or as the weight or volume of water overflow. In another method, the coated rock is weighted first in air then suspended in water. The difference in weight is equal to the volume displaced.

These described methods measure only the interconnected porosity, not the whole porosity of the rock sample. Total porosity is measured by crushing the rock sample into fine powder then measuring the volume of powder by fluid displacement in a density bottle. The total volume of the pores can be calculated as the difference between the volume of the specimen and that of the crushed particles.

Application of porosity:

i. Determination of rock porosity is used to infer the mechanical behavior and strength of the rock.ii. Determination of porosity is used to study reservoir rock and hydrocarbon potential in the

petroleum industry.iii. It is used to determine the subsurface water aquifer and their yield potential.

2. Density

Density is defined as the mass per unit volume. It is denoted by “ρ”. Rock density is also called bulk density.

ρ = mass/volume =m/V

Where as

ρ = density of sample

m= mass of sample

V= volume of sample

Density may be characterized into dry density, bulk density and saturated density.

Density often has units of grams per cubic centimeter (g/cm3).

Density may be characterized into dry density, bulk density and saturated density.

Dry density is the mass per unit volume when the rock mass is completely dry i.e. void contain only air.

Bulk density refers to mass per unit volume in normal conditions, which means that the rock mass

may contain some liquid and some air in pores.

Saturated density refers to mass per unit volume when rock mass is fully saturated.

Density varies significantly among different rock types because of differences in mineralogy and porosity. Knowledge of the distribution of underground rock densities can assist in interpreting subsurface geologic structure and rock type.

2.1 Dependence

2.1.1 Confining pressure

At great depths below Earth’s surface there is extremely high pressure from the overlying rocks.

Minerals are compressed

• Volume decreases.

• Density increases

2.1.2 Temperature

As the temperature of most substances increases

– Atoms move faster and spread apart.

Expansion increases the volume which decreases the density because the mass remains the

same.

Fig 2.2: in the right material is in colder state and left in warmer state.

2.1.3 Shape and Size

If the temperature of a material remains constant, the size and shape will not affect its density.

The mass and volume change proportionately.

Fig 2.3 All of these Aluminum objects have the same density

Lab. Measurement

Rock density can be measured in the lab using core sample and for or this purpose we calculate the mass

and volume of the sample.

Measuring the Mass:Measure the mass of your samples using the triple beam balance provided. You will make this measurement three times and calculate the average.

Measuring the Volume:

The volume of cores and irregularly shaped objects can be determined by measuring the amount of water it displaces. Fill a graduated cylinder about half way with water. Note the level of the water. Submerge your sample and mark the new water level. As the volume of water or any liquid is measured in liter (and 1liter= 1000ml=1000cc) so we can easily interchange different units.

2.3 Density applications in the geosciences

Isostasy - determining how high continents will sit on the mantle

Plate tectonics - mechanisms that drive plate tectonics

Minerals - determining the name of a mineral through its density

Rocks - determining the name and composition of a rock by its density

The hypsometric curve - examining the causes of elevation variation on Earth

Oceanography - some ocean currents and ocean circulation is controlled by density.

3. Permeability The permeability of a rock is a measure of the ease with which the rock will permit the passage of fluids.

Although a rock may be very porous, it is not necessarily very permeable. Sandstone is typically porous

and permeable. Shale is porous but has a lower permeability because the finer grain size creates smaller

pore spaces. Igneous rocks tend to have low porosity and low permeability unless they are highly

fractured by tectonic processes.

The permeability has following three types

Absolute permeability is the permeability of the porous medium if a single fluid is flowing.

Effective permeability is the permeability of a fluid if another fluid is present.

Relative permeability is the effective permeability divided by the absolute permeability.

Fig 3.1: illustration of pore and pore channels in a rock.

These two figures have the same porosity (same pore space).

In the figure to the right the pore channels are closed and the permeability is zero.

3.1 Permeability dependence:3.1.1. Packing:

In Tighter grain packing fluids cannot pass through the medium easily which means lesser permeability

and Looser Packing will allow more water to pass through, which means higher permeability.3.1.2. Grain size:

In smaller grain rocks fluids face difficulties to flow which means lesser permeability than coarse grains

medium which has higher permeability.3.1.3. Host Area:

According to the Darcy law (mentioned below) the fluids can easily pass through the medium which has

high area of cross section than those which has less area of cross section.

Fig 3.2: showing rapid, moderate and slow drainage through different materials having different co-efficient of permeability.

3.2 The Darcy Law

Henry Darcy (1803-1858) was a Hydraulic Engineer.

His law is a foundation stone for several fields of study including ground-water hydrology, soil physics,

and petroleum engineering.

“This law states that net flow of fluids is directly proportional to the hydraulic conductivity

and area of cross section of the host medium.”

Darcy’s law can be shown by following equation.

Q=KiA

K= QiA

Where Q is net flow

K is co-efficient of permeability

I is the hydraulic gradient

A is the area of cross section

Units of K are cm/s

From the above formula it can be seen that higher the area of cross section or gradient, more fluid can

pass through that medium.

K is different for different rocks that can be seen from table below.

Tg

Table 3 shows different material having different co-efficient of permeability.

3.3 MEASURING LIQUID PERMEABILITY

To measure the permeability in the lab , Measure inlet and outlet pressures (P1 and P2) at several

different flow rates.

   

Graph ratio of flow rate to area (q/A) versus the pressure function (P1 - P2) / L

    For laminar flow, data follow a straight line with slope of k/μ

    At very high flow rates, turbulent flow is indicated by a deviation from straight line.

Fig 3.3: Finding permeability with liquid or high rate gas flow

3.4 Use of Permeability

Knowledge of the permeability properties of soil is necessary to:

Estimating the quantity of underground seepage.

Solving problems involving pumping seepage water from construction excavation.

Stability analyses of earth structures and earth retaining walls subjected to seepage

forces.

Permeability influences the rate of settlement of a saturated soil under load.

The design of earth dams is very much based upon the permeability of the soils used.

The stability of slopes and retaining structures can be greatly affected by the

permeability of the soils involved.

Filters made of soils are designed based upon their permeability.

4.  Moisture contentWater content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture) and rock. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 (completely dry) to the value of the materials' porosity at saturation.

The moisture content of a rock is defined as “the ratio of weight of water in the voids to the weight of the dry solids in the sample”. m = Ww/Ws

Where m = the moisture content Ww = the weight of water Ws = the weight of solid (dry sample mass)

Natural moisture content of a rock sample is the moisture content of the sample when taken from ground due to excavation or boring.

4.1 Measurement We can also calculate moisture content by equation.

The moisture content is determined by noting the loss in weight of the sample after drying it for 2 hours at

a temperature ranging from 105 0C to 1100C. An excess natural moisture content gives an indication that

the rock is more porous, making it of lesser strength. When confining pressure is increased around the

rock mass, there is a decrease in moisture content making a rock mass stronger and vice-versa. That is

why, due to an excavation near a rock mass, confining stress is decreased which may cause the natural

moisture content to increase resulting in a decrease in strength. Duncan has given a relation b/w the

moisture content and the bearing capacity which has reverse relation. Bearing capacity decreases with

increase in natural moisture content.

4.2 Uses of moisture content

In soil science, hydrology and agricultural sciences, water content has an important role for groundwater recharge, agriculture, and soil chemistry.

5. Degree of saturationDegree of saturation is defined as the volume of water in the void to the total volume of void in the rock sample. S = Vw/Vv

Where S = the degree of saturation Vw = the volume of water Vv = the volume of void

The rock mass having higher porosity has higher degree of saturation if it lies below water table.

Degree of saturation is one of the most important factors influencing rock strength. Considerable research

has been carried out to investigate rock strength under both dry and water saturated conditions. According

to these results, the petro physical properties of rocks decrease with increasing moisture and this can

result in an increase in the mechanical compliance in some cases. In several cases, the strength decrease is

remarkable after only 1% water saturation.