May 16, 20203 min read

Engineering Geology: Rock descriptions and Rock families explained

Updated: Aug 16, 2020

Introduction

Rocks descriptions are made by examination of samples recovered from boreholes or from material that is found on the side (in-situs material). Rocks can be divided into three categories based on the general characteristics of the formation such as mineralogy and grain size of the rock.

Geological science terms are used to describe the geological aspects of the rocks that have no direct significance to engineering characteristics but features such as the names of the rock type can often indicate a range of useful details on an engineering and geological perspective.

Visual identification of rocks outcrop can help the configuration of the engineering performance (strength, compressibility, etc.) of the construction site bedrock as well as a full history description of the earth’s crust. A complete rock description is commonly divided into three parts.

Rock Description feutures

a) Materials Characteristics

Strength: (Uniaxial compressive strength measurements)
Structure: (The structure of the rock is concerned the textural features and lithology of the rock in terms of thickness and Spacing)
Colour: (Colour of the rock described in words)
Texture: (Described according to geological terminology)
Grain size: (Grain size refers to the average dimension of the minerals or rock fragments dominating the rock’s behaviour.
Rock name (in capitals, e.g. “GRANITE”)

b) General information

Additional information and minor constituents: (mineralogy)
Geological formation.

c) Mass Characteristics

State of weathering.
Discontinuities
Fracture state.

Principal Rock Types (Rock family)

Igneous rocks:

• derived from the cooling of molten material (magma/lava).

• mainly crystalline.

• common types are granite, basalt, dolerite.

• Petrological classification of igneous rocks can be described in: - Mineral assemblage: minerals as quartz, feldspar, biotite are formed in igneous rocks - Grain size: 2mm-5mm (crystals)

Sedimentary rocks:

• rocks derived from the breakdown of all other types of rocks including pre-existing sedimentary rocks.

• mainly fragmentary

• common types are sandstone, shale and limestone.

• The two groups of sedimentary rocks can be described as:

- Clastic sediments: accumulations of rock

- Organic and chemical sediments: dead plants or chemically precipitated minerals.

Metamorphic rocks:

• derived from all other rocks through solid-state changes in response to changes in temperature and/or pressure.

• often foliated or banded.

• common types are slate, marble, schist and gneiss.”

• There are three types of classification of metamorphic rocks:

- Dynamic metamorphism: intense stresses locally that tend to deform and fracture the rock.

- Regional metamorphism: effects of increasing temperature and pressure.

- Contact metamorphism: heating of the rock in a body of intruded igneous magma.

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Example of full rock description

Conglomerate

Conglomerate, is a sedimentary bedded rock that is composed of fragments and pre-existing rocks, minerals. It contains rounded to subangular gravel-size clasts that are larger than 2mm in diameter. Between the clast, most commonly smaller particle-like quartz cement is filled that binds the rock together in a finer matrix. In this particular rock, grain-size may or may not vary in composition and therefore the type of research done to classify the rock is of big importance.

Due to that, conglomerate rocks are clastic (=composed of fragments) there is a variety of compositions. Classification of the rock can be done by analysing the difference in the lithology of the gravel-size clasts and the matrix with which the rock is made.

Clasts created by weathering conditions that washed away by water can be mineral particles or metamorphic or igneous rock fragments. The matrix that fills the space between the clast is a variety of slit, sand, mud or chemical cement. Depending on the clasts- matrix composition colour and strength can be obtained.

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[1] Encyclopedia Britannica. (2018). Geology | science. [online] Available at: https://www.britannica.com/science/geology

[2] Flexiblelearning.auckland.ac.nz. (2018). Geology - rocks and minerals. [online] Available at: https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/schist.html

[3] Geology.com. (2018). Geology and Earth Science News, Articles, Photos, Maps and More. [online] Available at: https://geology.com

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