GEO-MECHANICSGEO-MECHANICS(CE2204)(CE2204)

Weathering and ErosionWeathering and Erosion

Lecture Week No 10

Nur Syazwani binti Noor RodiNur Syazwani binti Noor Rodi

LINTON UNIVERSITY COLLEGELINTON UNIVERSITY COLLEGESCHOOL OF CIVIL ENGINEERINGSCHOOL OF CIVIL ENGINEERING

Introduction

Weathering - the destructive processes by which materials at or near the Earth's surface are changed in color, texture, composition, firmness or form, with little or no transportation of the altered material.

Erosion - the mechanical or chemical destruction of the land and removal of material by moving water, ice or wind.

Rock Weathering

Weathering of rocks can be classified into:

a) Physical weathering – disintegration of rock into smaller pieces without any change in mineralogy

b) Chemical weathering – decomposition of rock minerals into new minerals and solutions

Physical Weathering

• Produces finer particles by fracturing rock• The forces involved must be strong enough to

exceed the compressional, tensional or shear strength of the rock

• 6 main processes: a) Impact & Abrasion

b) Ice Wedging

c) Salt-Crystal Growth

d) Exfoliation

e) Thermal Expansion

f) Root Wedging.

Impact & Abrasion

• Abrasion occurs when two rocks rub together or impact each other

• It occurs in many environments. Among these are:

a) Glacial environments

b) Deserts

c) Mountainous areas

d) Rivers

e) Beaches

Impact & Abrasion

Boulders rounded by impacts and abrasion

Ice Wedging – Frost Action

• As water freezes it expands (increases in volume up to 9%); the repeated growth and melting of ice in pore space or fractures can force the fracture to grow

• The initial size of the fracture is not important - over time the fracture will expand as the freeze/thaw cycle continues

• Eventually the rock is shattered due the forces generated during crystallisation and result in the formation of a talus slope, or talus cone.

Ice Wedging – Frost Action

Talus slope Talus cone

Salt-Crystal Growth• Similar to ice wedging, both result from forces created

during crystallization• Ice wedging is most prevalent in areas which

experience cyclic freezing and thawing whereas salt-crystal growth is most common in arid environment.

• Three minerals are involved in this process: Halite (salt), Calcite, Gypsum.

• As water evaporates, minerals are left behind to grow between grains in the rock.

• Can result in cavities or alcoves forming in vertical faces.

Salt-Crystal Growth

Evaporite deposits on flank of Rock

Salt-Crystal Growth

Alcove at base of rock

Salt-Crystal Growth

Surface pattern on this pedestal rock caused by salt crystallisation

Salt-Crystal Growth

Evaporite of salt deposits on Sandstone

Exfoliation - Unloading

• Rocks which form under the surface is under pressure – subjected to confining pressure

• When the pressure is released, the rock will expand

• Pressure is released as the rock is brought to the surface - tectonics, isostasy, erosion

• As it expands the outer layers break free in sheets - exfoliation.

Exfoliation - Unloading

A batholith forms at great depth – uplift and erosion expose it – release of pressure causes cracks parallel to surface

Exfoliation - Unloading

Exfoliation sheet at the outer layers of a rock mass

Exfoliation - Unloading

Exfoliation sheet at the outer layers of a rock mass

Exfoliation - Unloading

Exfoliation slab at the outer layers of a rock mass

Exfoliation - Unloading

Exfoliation slab at the outer layers of a rock mass

Thermal Expansion

• Rock has a low thermal conductivity, when rock is heated on a surface that heat does not travel inward very rapidly

• The heated exterior of the rock will expand more rapidly than the cool interior, causing the exterior to fracture - spalling

Thermal Expansion

Spalling on the surface of a rock mass

Root Wedging

• Root wedging occurs when a plant, especially trees, sink root systems into existing joints and fractures

• As the root grows it forces the fracture to expand• Relatively minor weathering force in rocks, but is

very important for soil development.

Root Wedging

Root wedging of an exfoliation slab

Root Wedging

Root wedging of an exfoliation slab

Chemical Weathering

Mechanical weathering produces smaller particles. This allows chemical weathering to progress much faster by increasing the surface area available for chemical reactions……..

Chemical Weathering

• The process of weathering by which chemical reactions transform rocks and minerals into new chemical combinations that are stable under prevailing conditions at or near the Earth's surface

• 5 main processes: a) Hydrolysis

b) Hydration

c) Oxidation

d) Solution/Dissolution

e) Ion Exchange

Hydrolysis

• Hydrolysis is a decomposition reaction involving water and a silicate mineral

• H+ ions or OH- ions are taken in by a mineral, altering the mineral's chemistry

• Not only does the chemistry change - the crystal structure changes - this can weaken the rock structure

Hydration

• Hydration is also a reaction involving water and a silicate mineral

• H2O is taken in by certain minerals

• Hydration will also add water to a clay mineral which will cause the mineral to expand

• Grus1 is formed when this reaction takes place in granite

1 Grus is an accumulation of angular, coarse-grained fragments (sand and gravel) resulting from the granular disintegration of crystalline rocks Grus sand, when cemented into a sandstone, will form an arkose

Oxidation

• Oxidation occurs when an element combines with an oxygen ion, O-2

• The most commonly observed is the oxidation of Fe+2 (iron)combines with O-2 (oxygen) to form Fe2O3 (iron oxide)

• This process is better known as 'rusting'. • Many rocks have a reddish colour due to the

presence of hematite

Oxidation

A freshly broken rock shows chemical weathering (mostly oxidation) progressing inward

Solution/Dissolution

• This process involves the transformation of minerals containing Ca, Mg, K, Na, and Fe into carbonate minerals (calcite) by reactions with CO2 (carbon dioxide) and H2O (water)

• Both CO2 and H2O are readily available in the atmosphere, surface water, and groundwater

• Carbonic acid is a weak acid but over long periods of time can dissolve large quantities of calcite/limestone/marble

Solution/Dissolution

The solution effects by the sea on the limestone at the base of the pillar

 

Ion Exchange

• Ion exchange is the substitution of ions in solution for ions in minerals

• Water contains ions of H+, OH-, Ca+, Na+, K+, Cl-

• Most minerals have the capability to exchange ions - most common in the clay minerals which attract ions

• Which ions are exchanged will depend on the chemistry and environment

Weathering of Coastline

The occurrence of various rock types & weathering actions create different coastal features e.g. (1) cliff,

(2)cove, (3)sea-cave, (4)sea arch, (5) stack

Deep weathering profile in tropical climate (Dengkil, Selangor)

Deep weathering profile in tropical climate (Dengkil, Selangor)

Fracture plane due to weathering

Joints lead to disintegration of rock into smaller blocks; they also act as channels for agents of chemical weathering to

penetrate deep into the rock body.

Disintegration of rock without any chemical change on its mineral composition due to physical weathering

Disintegration of rock without any chemical change on its mineral composition – physical weathering.

Physical disintegration of jointed granite

Oxidising environments can be recognized by theirreddish-brown colour. Oxygen-poor environments can be

recognized because they are green-grey

Engineering Classification of Weathering

APPROACH 1: FACTUAL DESCRIPTION OF WREATHING (MANDATORY)

Standard descriptions should always include comments on the degree and nature of any weathering effects at material or mass scales. This may allow subsequent classification and provide information for separating rock into zones of like character. Typical indications of weathering include

a)Changes in colourb)Reduction in strengthc)Changes in fracture stated)Presence, character and extent of weathering products

These features should be described using standard terminology, quantified as appropriate, together with non-standard English descriptions as necessary to describe the results of weathering. At the mass scale the distribution and proportions of the variously weathered materials (e.g. core stones vs matrix) should be recorded

Engineering Classification of Weathering

Engineering Classification of Weathering

Engineering Classification of Weathering

0 1 2 3 A N Y W H E R E 0 1 0 3 1 1 E L 3

1 1 1 0 0 1 1 25m high. Signs of instability.Some wedge failure have occurred.

1 1 9 3 3 2

LIMESTONE

2 3 3 5

0 0 0 0 0 1 5 3 5 2 - 3 Base of slope

2 0 1 3 0 2 0 2 5 1 - 2 Berm level9 0 3 2 0 3 0 9 5 5 - 7 Centre line of slope

Schmidt Hammer