GEOLOGICAL

WORKS OF
NATURAL AGENCIES
Rocks exposed at the surface of the Earth
are subject to direct or indirect attacks of
a number of natural agencies.
* These agencies are ceaselessly operating, season after
season, and year after year, thus they are responsible
for modifying the physical features existing in the
surface.
*Moreover, the work of these natural agencies may either
destructive or constructive in nature in relation to the existing
landform of an area at any given point of time.
Destructive:
Rivers carve out valleys by eroding rocks, bit
by bit, from the mountains; these valleys are
constantly enlarged and deepened by them in
association with other natural agencies.
Constructive:
Rivers deposit their load at appropriate places
on the land, along the sea-shore and even at the
sea bottom, building up such features as
alluvial plains, flood plains, deltas, etc.,
 Alluvial Plains Flood Plains
An alluvial plain is a largely flat landform A floodplain is a generally flat area of land
created by the deposition of sediment over a next to a river or stream.
long period of time by one or more rivers It is the Yellow River winds through the
coming from highland regions, from which plains of Sichuan, China. Many rivers are
alluvial soil forms. surrounded by plains, or broad areas of flat
land.
1. GEOLOGICAL
WORK
OF THE
ATMOSPHERE
(ROCK
WEATHERING)
Weathering is defined as a natural process of
mechanical disintegration and or chemical
decomposition of the rocks of the crust of the
earth by certain physical and chemical agencies
of the atmosphere.
Before and after photos of concrete sculpture Compassion by Una Hanbury, which has sat between
Wilson and Clarendon Blvd since 1969
Weathered animal statue from Parks of New York City
 TYPES OF
WEATHERING
MECHANICAL/PHYSICAL
WEATHERING

CHEMICAL WEATHERING
 MECHANICAL
WEATHERING
Mechanical weathering is a natural
process of disintegration of rocks into
smaller fragments and particles through
essentially physical processes without a
change in their composition.
SOME COMMON MECHANICAL
WEATHERING PROCESSES:

1. FROST ACTION
Frost action is an effective form of
mechanical weathering. When water
trickles down into fractures and
pores of rock, then freezes, its
volume increases by almost 10
percent.
2. THERMAL EFFECTS

Temperature changes can also contribute

to mechanical weathering in a process
called thermal stress. Changes in
temperature cause rock to expand (with
heat) and contract (with cold). As this
happens over and over again, the
structure of the rock weakens. Over time,
it crumbles.
3. UNLOADING

In a process called unloading, overlying

materials are removed. The underlying rocks,
released from overlying pressure, can then
expand. As the rock surface expands, it
becomes vulnerable to fracturing in a process
called sheeting.
CHEMICAL
WEATHERING
Chemical weathering is essentially a process of
chemical reactions between the surfaces of rocks and
atmospheric gases in the direction of establishing a
chemical equilibrium. The end product of chemical
weathering has a different chemical composition and
poor physical constitution as compared to the parent
rock.
 CHEMICAL WEATHERING
COMMON PROCESSES

• Solution
• Carbonation
• Hydration
• Oxidation and Reduction.
Solution weathering- is the process by
which certain minerals are dissolved by
acidic solutions. For example, calcite in
limestone is dissolved easily by carbonic
acid.
Carbonation - is another type of chemical
weathering. Carbonation is the mixing of
water with carbon dioxide to make carbonic
acid. This type of weathering is important in
the formation of caves.
Hydration - is a form of chemical
weathering in which the chemical bonds of
the mineral are changed as it interacts with
water. One instance of hydration occurs as
the mineral anhydrite reacts with
groundwater.
Oxidation: In the process of oxidation,
rock breakdown occurs due to the
disturbance caused by the addition of
oxygen.
Reduction: When oxidised minerals are
placed in an environment where oxygen is
absent, reduction takes place.
MECHANICAL VS. CHEMICAL
WEATHERING
MECHANICAL CHEMICAL

Mechanical weathering is the physical Chemical weathering involves the

breakdown of rocks into smaller pieces without breakdown of rocks due to chemical
changing their chemical composition. reactions.

Mechanical weathering is caused by physical Chemical reactions between water, air, and
forces such as temperature changes, wind rock minerals cause chemical weathering.
erosion, and water erosion

Mechanical weathering results in physical Chemical weathering results in chemical

changes to the rock changes to the rock, including forming new
minerals.
ROLE
OF PLANTS
AND
ORGANISMS
• Plants
Hydrogen ions (H+) are known to be released
at the roots of plants during their growth and
metabolism. These ions are capable of
replacing K+, Ca++ and Mg++ ions from the
minerals and rocks surrounding the root
system and make them available for use in
plant growth.
• Animals
PRAIRIE
Animals that tunnel DOGS

underground, such as moles

and prairie dogs, also work
to break apart rock and soil.
Other animals dig and
trample rock aboveground,
causing rock to slowly
crumble.
MOLES
• Humans
Weathering is a natural process, but human
activities can speed it up. For example,
certain kinds of air pollution increase the rate
of weathering like burning coal, natural gas,
and petroleum releases chemicals such as
nitrogen oxide and sulfur dioxide into the
atmosphere.
 FACTORS AFFECTING WEATHERING

* Nature of * Physical
* Climate Environment
Rocks
01 Nature of Rocks
• Rocks vary in chemical composition and
physical constitution. Some rocks are easily
affected by weathering processes in a particular
environment whereas others may get only
slightly affected and still others may remain
totally unaffected under the same conditions.
• For instance, the Granite
and Sandstone. If granite and
sandstones exposed to
atmosphere simultaneously in
the same or adjoining areas
having hot and humid climate, Granite

the sandstone will resist

weathering to a great extent
because they are made up
mainly of quartz which is Sandstone
highly weathering resistant
Climate Factors 02
The process of weathering is
intimately related to the climatic
conditions prevailing in an area. Same
types of rocks exposed in three or
more types of climates may show
entirely different trends of
weathering.
 • In totally dry and cold
• Cold and humid
climates, neither chemical
conditions favour
nor mechanical
both chemical and
weathering may be quite
mechanical types of
conspicuous (due to
weathering.
absence of moisture)
03 Physical Environment
• The topography of the area where rocks are
directly exposed to the atmosphere also affects
the rate of weathering to a good extent. Rock
forming bare cliffs, mountain slopes devoid of
vegetation and valley sides are more prone to
weathering than same rocks exposed in level
lands in similar climates and/or under vegetable
cover.
 03
Resistance to Weathering
● Mineral constituents of a rock show
remarkable variation in their
susceptibility to weathering. Some
minerals get decomposed and
disintegrated easily whereas others may
remain intact for considerable length of
time with little or no alteration.

FOR MOST COMMON ROCK FORMING MINERALS,

RESISTANCE TO WEATHERING INCREASES IN THE
FOLLOWING ORDER:

• Dark Coloured Minerals: Olivine, Augite, Biotite

• Light Coloured Minerals: Calcic felspars, Sodic

felspars, Potash felspars, Potash mica
PRODUCTS OF
WEATHERING
1. ELUVIUM

• It is the end product of weathering

that happens to lie over and above
the parent rock. It may consist of
fragmentary material resulting
from rock disintegration or fine
powdered material resulting from
chemical decomposition or a
mixture of both.
2. DELUVIUM
• It is that category of end
product of weathering that
has been moved to some
distance after its formation
due to weathering
processes. It is invariably
associated with weathering
of slopes and forms heaps of
various thickness and grade
at the base of slopes.
Engineering Considerations
• Engineering Projects are built either on soil or on rocks. Soil, as we know, is the
ultimate end product of weathering of rocks. As such, for a better understanding of the
engineering properties of soils, the engineer will benefit a lot if he has clear knowledge
and background of these soils.

• Similarly, when foundations are to be carried down to the bed rock, the depth of
weathered cover, degree of weathering and most important of all, the trend of weathering
in that area have very important bearing on the ultimate safety of the project.
• For the construction engineer, it is always necessary to find out (a) to what extent the area
for a proposed project has already been physically deteriorated by cumulative effects of
weathering processes operative in that area, (b) what would be the likely effect of the
weathering on the construction materials proposed to be used in the project.

• This will also help in selecting the right type of construction material that will be more
durable against the weathering processes typical of that area as well as best
recommending use of some special type of stones in major constructions (such as
marble. limestone, granites, etc.)
ENGINEERING CLASSIFICATION OF WEATHERED ROCK
MASSES
Weathering of wall rock (rock constituting discontinuity surfaces) is classified in
accordance with the recommendations of The International Society for Rock
Mechanics (ISRM) Committee on rock classification:

1. Unweathered/fresh: No visible signs of weathering can be observed. The

rock is fresh, and crystals appear bright.

2. Slightly weathered rock: Discontinuities are stained or discoloured and

may contain a thin filling of altered material. Discolouration may extend
into the rock from the discontinuity surfaces to a distance of up to 20% of
the discontinuity spacing.

3. Moderately weathered rock: Slight discolouration extends from

discontinuity planes for greater than 20% of the discontinuity spacing.
Discontinuities may contain filling of altered material. Partial opening of
grain boundaries may be observed.
4. Highly weathered rock: Discolouration extends throughout the rock and
the rock material is partly friable (easily crumbled). The original texture of
the rock has mainly been preserved, but separation of the grains has
occurred.

5. Completely weathered rock: The rock is totally discoloured and

decomposed and in a friable condition. The external appearance is that of
soil.