Lecture#3 (Mechanical Weathering and its types).pdf

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1. ENGINEERING GEOLOGY
CE-226
Lecture 03: Mechanical Weathering and its types

2. Topics to be covered in this Lecture
1.Weathering
2.Processes of mechanical weathering
3.Processes of chemical weathering
4.Resistance to weathering
5.Soil profiles
6.Controls of soil profile development
7.Soil hazards: expansive soil

3. WEATHERING
•What is weathering?
•Weatheringis 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.
1.Mechanical weatheringis accomplished by physical forces that break rock
into smaller and smaller pieces without changing the rock's mineral composition.
2.Chemical weatheringinvolves breaking down rock components and internal
structure and forming new compounds.
•Whereas weathering breaks rocks apart, erosionremoves rock debris by mobile
agents such as water, wind, or ice.

5. Stephen Marshak
Contrasts between
fresh and weathered
granite.

6. MECHANICAL WEATHERING
•Mechanical weathering: unloading
•Upon removal of overburden (unloading), the elastic component of rock
deformation is recovered and the rock expands, e.g. 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 called, joints.

7. MECHANICAL WEATHERING

8. MECHANICAL WEATHERING
•Physical weathering occurs everywhere, but is especially
predominantly in areas of the Earth that are
•either very hot (e.g., deserts) or
•very cold (e.g., mountains, tundra).
•In hot areas, alternations between hot and cold conditions
causes rock to expand and contract.

9. MECHANICAL WEATHERING
•Following are the basic types of mechanical weathering
1.Exfoliation
2.Frost Wedging
3.Salt Wedging
4.Root Wedging
5.Expansion & Contraction

10. TYPES OF MECHANICAL WEATHERING
1.Exfoliation
The unloading of large plutons may split into sheets that are
parallel to the mountain face, a process called exfoliation. It is
also known as sheetingif the expansion occurs in granite to
form rock slabs.

11. The exposure of once-deep rocks cause them to crack. Here, the pluton
develops exfoliation and vertical joints, while the sedimentary rock layers
developed mostly vertical joints. (W.W. Norton)

12. Exfoliation joints in the Sierra Nevadas. (Martin Miller)

15. Vertical joints in sedimentary rock (Brazil).

16. •Sheeting of in granite in Olmstead Point, Yosemite National Park, CA.
Sheeting occurs as erosion removes the overlying rock cover and
reduces the confining pressure. The bedrock expands, and large
fractures develop parallel to the surface. Frost wedging may later
enlarge the fractures.

17. TYPES OF MECHANICAL WEATHERING
2.Frost Wedging (Ice Wedging)
•Liquid water expand by 9% in volume when freezing. So one of
the most effective mechanical weathering processes is the
wedging action of repeated cycles of freezing and thawingof
water in rock fractures.
•Conditions for frost wedging include moisture, rock fracture or
weakness planes, and temperature fluctuation around the
freezing point.
•A product of frost wedging is talus slopemade of angular rock
pieces piling up at the base of steep cliffs.

18. Joints in a rock are a pathway for
water –they can enhance
mechanical weathering

19. Frost Wedging:
rock breakdown
caused by
expansion of ice
in cracks and
joints

20. •Shattered rocks are common in cold and alpine
environments where repeated freeze-thaw cycles
gradually pry rocks apart

21. •An illustration of frost wedging. (Tarbuck and Lutgens).

22. •Talus slopes near Banff, Canada. (Hamblin and Christiasen).

23. TYPES OF MECHANICAL WEATHERING
3.Salt Wedging
In arid climates, dissolved salt precipitates in the small pores and joints
between grains, wedging them apart on a small scale.
Water evaporates from the saltwater leaving behind salts.
These salts crystallize and expand, breaking apart rocks.

25. TYPES OF MECHANICAL WEATHERING
4.Root Wedging
On both a large and small scale, plants and fungi invade joints and the
spaces between grains and wedge them apart. In cold climates, they
act in concert with frost.

27. TYPES OF MECHANICAL WEATHERING
5.Thermal Expansion And Contraction
As temperature changes, 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.

28. Thermal expansion due to the extreme range of temperatures
can shatter rocks in desert environments. Repeated swelling
and shrinking of minerals with different expansion rates will
also shatter rocks.
These stones were
once rounded stream
gravels; however,
long exposure in a
hot desert climate
disintegrated them.