Soil Mechanics & Foundations
Rock Cycle, and Origin of Soil
(Das, Ch. 2)
Outline of this Lecture
1.Rock types and rock cycle
3.Weathering of rocks
1. mechanical weathering
2. chemical weathering
4.Origin of soil
What is a soil?
Soil is defined as the uncemented
aggregate of mineral grains and decayed
organic matter with liquid and/or gas in
the pores between the grains
(A) gas (mostly air);
(B) solid particles (minerals);
(C) liquid (water, contaminant
Where did soil come from?
Soils are formed by weathering of rocks.
More specifically, the mineral grains that
form the solid phase of a soil aggregate
are the product of rock weathering.
So that, we need discuss
(a) rocks and rock type;
(b) weathering of rocks;
What is a ‘rock’?
In Geology (the science studies rocks), ‘rock’ is
defined as the solid material forming the outer rocky
shell or crust of the earth. There are three major
groups of rocks by its origin:
(1) Igneous rocks: cooled from a molten state;
(2) Sedimentary rocks: deposited from fluid medium;
e.g., products of weathering of other rocks in water;
(3) Metamorphic rocks: formed from pre-existing rocks
by the action of heat and pressure.
Apparently, the igneous rock is the one far more
essential and intrinsic since the other two types are
relative secondary in origin.
The Identification Chart of the Igneous Rocks
Hi-resist to Lo-resist to
Basic Mineralogy of Rocks
Rocks are formed with minerals. What is a mineral?
1) a naturally occurring chemical element or compound;
2) formed by inorganic processes;
3) with an ordered arrangement or pattern for its atoms –
4) possesses a definite chemical composition or range of
The opposite of mineral property is amorphous, i.e., the
property of non-crystal, order-less property possessed by
glass, volcanic glass, etc.; oil or coal can neither be
regarded as minerals by their organic involvement.
Basic Mineralogy of Rocks (cont.)
So we can simply express the mineral as
mineral = composition + crystalline structure
There are more than 2000 naturally occurred minerals have
been discovered; only a bit more than 100 are common and
used in college mineralogy. However, of the 100 common
minerals only about 25 are abundant rock-forming minerals.
The main types of minerals are:
Comparison of surface and
• High temperature but • low temperature, and
constant at which highly variable
minerals reach • little or no confining
equilibrium pressure (stress)
• high confining • abundant of water
pressure (stress) • abundant of oxygen
• less water or no water
• no oxygen
Conclusion can be drawn from the comparison:
Rock at the surface will undergo changes
---- This change is called Weathering
Weathering is the physical breakdown
(disintegration) and chemical alteration
(decomposition) of rocks to form soil or loose
particles at or near Earth's surface. Weathering
causes deterioration of building materials. It also
weakens rocks, a great concern when weathered
rocks are used for foundation.
Two types of weathering
Physical disintegration or degradation of
rock pieces without a change in composition
decomposition whereby one mineral species is
changed into another through various chemical
processes. Water plays a major role, through:
1, provide oxygen,
2 provide mobility for moving ions.
Mechanical weathering always involves
fracturing--but that can occur by a whole
host of causes.
Chemical weathering tends to weaken
rock, thereby making it easier to break.
Likewise, mechanical weathering creates
additional surface area that is prone to
chemical attack. In this way, the two
processes work together.
With or without water makes distinguished
difference in rock weathering process, in
arid region, since no water or little water
presence, even the carbonate rocks (e.g.,
limestone) are not subject to solution but
persist as resistant rocks. The rock mass
may be sharp angular, and topography
Mechanical Weathering (cont.):
Mechanical weathering processes include:
1) freezing & thawing (frost wedge)
Pottery container with water in winter time,
when water is frozen and become ice it can
have 9% of relative volume change, i.e.,
dv/v = 9% = 0.09
This is a very large strain! Similarly if there is
water in rock fractures it will force the fracture
to propagate into further depth.
Mechanical Weathering (cont.):
The stress generated by frozen water is
about 550 psi, check with Table 6.1 you can
find out that this is a value comparable to
the tensile strength of most rock types. Or
only one order of magnitude less. But
remember rocks are experiencing many
freezing-thawing cycles before they are
finally breaking down.
Mechanical Weathering (cont.):
2) differential expansion and contraction
As temperature changes (in deserts or from
forest fires), not all parts of a rock or all its
minerals expand or contract by the same amount.
So when rocks are heated or cooled, the mineral
grains are subjected to differential stresses,
which may be sufficient to make the rock spall, or
break off in sheet-like pieces.
* repeated daily heating and cooling of
* heat causes expansion; cooling causes
* different minerals expand and contract
at different rates causing stresses along
The thermal expansion coefficient αL and αV
1 ∂l 1 ∂V
α L = ( )P αV = ( ) P
L ∂T V0 ∂T
characters how much a mineral change its
dimension in response to a unit degree
increase in temperature . Using a finite
Mechanical Weathering (cont.):
The linear thermal expansion coefficient αL is
on the order of 10-5/ºC, and it could be
αLx ≠ αLy ≠ αLz
For example, αmax is in the elongation axis (C-
Twin Tower, God’s Garden, Colorado
Processes of mechanical weathering
unloading: jointing, exfoliation, and sheeting
Upon removal of overburden, the elastic component of rock
deformation is recovered and the rock expands. The
unloading may occur when the overlying rocks are eroded or
rocks are removed from a quarry. The expansion caused by
unloading may be sufficient to fracture the rock. Such
naturally formed cracks are known as joints.
Typically, large plutons (bodies of igneous rock) or
metamorphic bodies split into sheets that are parallel to the
mountain face, a process known as exfoliation. It is also
known as sheeting if the expansion from unloading occurs in
granite to form rock slabs.
joints are parallel cracks in which rocks on either
side are not offset; Sheeting rock layers peel like
layers of an onion
Chapter 5: 2) Mechanical weathering Unloading
Another “sheeting” example
See also T&L Figure 5.8
Vertical columns from
magma cooling &
Columnar jointing in basalt
Glacially polished basalt columns
Spheroidal Weathering. Granite illustrates weathering forms quite well.
Chemical weathering attacks to granite along joints and makes rounded
boulders (Alabama Hills near Lone Pine) .
Chemical weathering=chemical processes that
dissolve and decay earth materials;
Chemical weathering needs the rock exposes to
air and water. Mechanical weathering could
enhance chemical weathering by disintegration,
i.e., increase the surface area of rock blocks and
debris and this will greatly accelerates chemical
Chemical weathering rate depends on
2. Amount of surface area
3. Availability of water or natural acid
Thus, rocks in tropical environment
experience most severe chemical
Because of its dipolar nature
water is able to dissolve many
chemical compounds. In
addition to the solution effect,
water aids decomposition
through acid action,
oxidation, and hydrolysis.
Acidity of Natural Waters
Water is a good solvent.
Acidic water is better!
pH of most natural
waters ranges from 4
pH > 9 or < 4 occurs in
Chemical Weathering of silicate
minerals by carbonic acid
feldspar + water + carbonic acid = clay
minerals + dissolved ions
2KAlSi3O8 + H2O + 2H2CO3 =
Al2Si2O5(OH)4 + 2K+ + 4SiO2(aq) + 2HCO3-
Why is rainwater naturally acidic?
Rainwater contains dissolved CO2 from
Dissolved CO2 reacts with water to form
carbonic acid (H2CO3)
CO2 + H2O ⇔ H2CO3
Carbonic acid dissociates to produce
hydrogen ion (H+) and bicarbonate
H2CO3 ⇔ H+ + HCO3-
Chemical weathering (cont.)
Common chemical weathering processes are
solution (dissolution), oxidation, and hydrolysis.
Rock reacts with water, gases and
solutions (may be acidic); will add or
remove elements from minerals.
Solution (or dissolution)
* Several common minerals dissolve in
i), halite; ii), calcite
* Limestone and marble contain calcite
and are soluble in acidic water.
and carvings are
that the urn and
tops of ledges are
but the inscriptions
Photo taken in one
monastery in Mexico
shows the ravages of
weathering mostly from
wind and wind-driven
rain. The rock is volcanic
Karst landscape of Guilin, China, caused
dissolved Carbonate rocks.
The other two kinds of chemical weathering
Oxidation - Oxygen combines with iron-bearing
silicate minerals causing "rusting". Iron oxides
are produced that are red, orange, or brown in
Hydration-reaction between mineral and water.
Transport of weathering products
formed by transportation and deposition of glaciers;
transported by running water and deposited along
formed by deposition in quiet lakes;
formed by deposition in the sea;
transported and deposited by wind;
formed by movement of soil from its original place by
gravity (e.g., landslides).
Typical Soil Profile
This photo is an outcrop of a glacial till deposit. Glacial till is a heterogeneous mixture
of clay to boulder size particles deposited within or beneath glacial ice. The till type on
this photo is a dense or basal till with lenses of looser, sandy material (sandy till), the
soil type mapped in this area is the Montauk series [the solum (the A and B horizons)
has been removed on this photo. photo location: Fearing Hill, Wareham, MA].
A photo of a glacial
fluvial deposit (the
topsoil and subsoil of a
Hinckley soil has been
removed) from a gravel
pit. This photo shows
the horizontal stratified
layers of sand and
gravel on the top of the
photo called the topset
beds or delta plain. The
inclined or dipping
layers of fine and
coarse sand (visible on
left part of photo) are
called the foreset beds
or delta slope. The
foreset beds were
deposited into a glacial
lake, the contact of the
topset and foreset beds
(delta plain/delta slope)
marks the former water
level of the lake. town
Soil come from weathering of rocks.
Mechanical weathering is accomplished by
physical forces that break rock into smaller
and smaller pieces without changing the
rock's mineral composition.
Chemical weathering involves breaking down
rock components and internal structure and
forming new compounds.
Whereas weathering breaks rocks apart,
erosion removes rock debris by mobile agents
such as water, wind, or ice.