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Rock Notes

The building structures pages have been added over the six months to Dec. 2012.     They are very much work-in-progress and I will be updating them on a regular basis over the next six months.



Introduction

A rock is a solid cumulative of minerals located in the earth's lithosphere.    Rocks are generally classified into igneous, sedimentary and metamorphic rocks.     Rocks consist of mixtures of minerals, fragments of other rocks and organic matter.    These are classified on the basis of their mineral, chemical and textural composition.    Rocks are essentially mixtures and therefore it is not possible to determine detailed physical and chemical properties.

Rocks are extremely important in terms of their properties of stability and strength as a geological material on which construction foundations are made and historically as building material from which important and monumental buildings are made.   

Stone used for wall , building, and bridge construction over history until recent times is generally cut from natural rocks.     The building stones are normally extracted by surface quarrying, drilled and split using diamond saws or iron wedges, and then shaped and polished according to their requirements.

Traditional stone masonry is rarely used today, because stone is expensive to quarry, cut and transport, and the building process is labour and skill-intensive.    Instead, most modern stonework utilises a veneer of stone ( thin, flat pieces ) glued against a wall of concrete blocks. This is known as veneered stone or stone cladding.

Over the years an advantage of stone made from durable rock is that it is sustainable.   Over time constructions have collapsed or fallen into disuse because of wars natural disasters, or economic considerations.     The stone walls have then become available for construction of other local buildings, or bridges.



Igneous Rocks

Igneous rocks are created when molten magma cools down and is split into two main types. These are mainly plutonic and volcanic rocks. The upper 10 miles of the earth's crust comprises almost entirely of igneous rocks. Plutonic or intrusive rocks come into being when magma cools and crystallizes gradually within the earth's crust.    Granite is a typical plutonic rock. Volcanic or extrusive rocks results from magma reaching the surface either as lava or as fragmental missiles.     Basalt and pumice are examples of volcanic / extrusive rocks. There are more than 700 types of igneous rocks. The two important factors used for the classification of igneous rocks are the particle size, and the mineral composition of the rock.

Sedimentary Rocks

Sedimentary rock formation results from the sedimentation of material in the earth's surface and within collections of water .     They are extensive in their spread over the earth's crust, covering 70% to 80% of the land area, but they comprise only about 5% of the earth's total crust.    The study of sedimentary rock provides much information as to the evolution of life on earth.    Sedimentary rocks alone can contain fossils.     Sedimentary rocks are deposited in layers as in strata, referred to as bedding.    Sedimentary rocks are classified as clastic, chemical precipitate or biochemical. Limestone and sandstone are examples of clastic formations.

They also are important sources of coal, drinking water and fossil fuels.    Many construction-related activities are based on the geotechnic properties of sedimentary rock.

Sedimentary rock can be mechanically formed from materials (gravels, sand, silt and clay) suspended in flowing water.    The suspended materials are then deposited and consolidated.    The mechanically formed sedimentary rocks are of three types:

Rudaceous rocks which is the cementing together of boulders, for example, conglomerate.
Arenaceous rocks for example sandstone,
Argillaceous rocks which clay rocks for example shale

Organically formed Sedimentary rock consists of accumulated animals and plants remains. These include

Calcarious rocks, lime stone
Carbonaceous rocks, coal

Chemically formed sedimentary rocks are formed by precipitation and accumulation of soluble constituents.

These include Carbonate rocks, Limestone, dolomite, Sulphate rocks, Gypsum, Chloride rocks, salt


Metamorphic Rocks

Metamorphic rocks form when rocks undergo metamorphosis/changes due to heat and pressure. Igneous and sedimentary rocks mainly undergo this change and become metamorphic rocks. Gneiss, marble, slate, schist and quartzite are some of the different types of metamorphic rocks. The major types of metamorphism are contact and regional. The former takes place when magma is injected into the surrounding solid rocks while, the latter is primarily associated with large masses of rock spread over a very wide area. This regional metamorphosis is caused mainly by temperature and pressure.


General

Although rocks have been classified into igneous rocks, sedimentary rocks and metamorphic rocks, there are seldom distinct regions of continuous masses of one type within the earths crust.     As a result of seismic activity, surface motion, weathering , movement of surface water etc, over millions of years, igneous rock masses can include sedimentary rock and/or metamorphic rock and in the same way sedimentary rocks and metamorphic rocks are also seldom without inclusions of other rock types.    Rock masses are also generally discontinuous in nature including voids which contain soil , fluids, and gases on a microscopic and macroscopic scale.



Application of rocks to construction..

There are huge variations within each of these rock types, caused by specific mineralogy and geology conditions, and while any stone can be used for building, they each have constraints that make them more or less suitable for different purposes.    Granite, sandstone and limestone can all be used for building walls, but slate is only suitable for roofs and floors. Some types of granite can contain mineral salts that cause spalling, where the outer face of stone falls off; slate can contain harmful minerals that break down on exposure to the atmosphere causing stone damage; and sandstone can be too porous and fragile for load-bearing structures.    An knowledge of the rock material is required to understand how it can be used in a building, what its limitations are, and how it will weather over time.

Limestone

Limestone has been used widely in the UK for construction of important buildings and monuments.    Portland stone strongly resist weathering but it can be readily worked (cut and carved) by masons, and this is one of the reasons why Portland Stone is so favoured as a monumental and architectural stone.    [Portland stone ( a white -grey limestone) has been used in the construction of St. Paul's Cathedral (1666), Buckingham Palace, the Palace of Westminster(1347), The British Museum (1753), Somerset House (1792), the General Post Office (1829), the Bank of England, the Mansion House and the National Gallery.    This scale of utilisation is reflected in many major cities in the UK. Often building are made from other materials such as reinforced concrete and are faced with Portland stone.

Sandstone

Sandstone is a sedimentary rock and is also readily worked and some sandstone (but not all) have good weathering properties although not nearly as good and Portland stone and granite.    Sandstone in widely used in the UK for construction of buildings including Stonehenge, Chester Cathedral, Tintern Abbey Shrewsbury Cathedral.

Granite

Granite is and Igneous rock which has extremely good weathering properties because it is very hard.    This hardness makes it relatively difficult to work.     Granite has been used in a large number of important buildings in the UK including Truro Cathedral, London Tower Bridge, Parts of St Pauls Cathedral, Nelson's Column.    Granite has been mostly mined in the South West of England (Devon and Cornwall and in Aberdeenshire.    Aberdeen is called the Granite city.   

Marble

Marble is a granular limestone that has been recrystallised under the influence of heat pressure and aqueous solutions.    There are a wide varieties of mables providing many pleasing surfaces when polished.  It is a very soft decorative rock which can be worked and polished to great effect.   Marble is also strong and durable and can withstand weathering    Marbles are used principally for buildings , monuments, interior decorations, statues , table surfaces and novelties.    Many of the famous italian statues are made from marble.   Notable marble building include the Taj Mahal in India , the leaning tower of Pisa in Italy, the Parthenon in Greece.


Application of rocks to Geotechnics /Foundations..

The factors that are important when considering erection structures on rock foundations are listed below

.. Faults, joints, stratigraphy
.. Ground Water levels, springs, surface water, or other effects of ground-water regime.
.. Potential cavities due to mines , tunnels and other caves etc.
.. Potential problems due to dissolving , swellin, and/or erosion
.. Potential rock slope instability.
.. Utility service lines e.g gas water , sewage etc.

It is most important when considering a rock site for construction that detailed information is available on the local and adjacent areas.    Often there is is significant information available on the local ground geology from national ground survey organisations such as the British Geological Survey. (BGS).    The preliminary geotechncial reports should be based on obtaining detailed local information using boreholes.    In fact the BGS (see link below) provides stores data on completed boreholes, mines, underwater springs, wells etc.

The rock structure is an important consideration for geotechnics an the following factors are very important.

Note the factors are each listed from best condition to worst

.. Thickness of Bedding.
- Massive: 1m thick or greater.
- Thick bedded: beds from 0,2 to 1m thick.
- Medium bedded: beds from 100mm to 300mm thick.
- Thin bedded: 100mm. thick or less.

... Degree of Fracturing (Jointing).
- Unfractured: fracture spacing - 2m or more.
- Slightly fractured: fracture spacing - 0,6m to 2m.
- Moderately fractured: fracture spacing - 200mm to 600mm.
- Highly fractured: fracture spacing - 50mm. to 200mm.
- Intensely fractured: fracture spacing - 50mm. or less.

.. Dip of Bed or Fracture.
- Flat: 0 to 20 degrees.
- Dipping: 20 to 45 degrees.
- Steeply dipping: 45 to 90 degrees.

Basic table of rock types

Namesimple Description
Rock SaltSedimentary Rock
SandstoneSedimentary Rock-clastic
LimestoneSedimentary Rock-organic
CoalSedimentary Rock-organic
EvaporitesSedimentary Rock-chemical
TravertineSedimentary Rock-chemical
GraniteIgneous rock -intrusive
BasaltIgneous rock -volcanic
PumiceIgneous rock -volcanic
Marble Metamorphic
Slate Metamorphic
Useful relevant Links
  1. The Geological Cycle - Rock formation... Very clear and relevant notes.
  2. Rock Foundations... An American pdf document with lots of detailed relevant information, 121 pages
  3. British Geological Survey... A site providing a vast quantity of information on the geology or the UK and worldwide


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Last Updated 28/10/2011