CE 240 Soil Mechanics & Foundations Lecture 2. Rock Cycle, and Origin of Soil (Das, Ch. 2)

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CE 240 Soil Mechanics & Foundations Lecture 2 Rock Cycle, and Origin of Soil (Das, Ch. 2)

Outline of this Lecture 1.Rock types and rock cycle 2.Basic Mineralogy 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 liquid, etc.);

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.

Hi-resist to weathering Lo-resist to weathering The Identification Chart of the Igneous Rocks (Bowen reaction)

Rock Cycles

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 crystalline structure; 4) possesses a definite chemical composition or range of compositions. 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: metallic minerals; nonmetallic minerals; carbonate minerals; sulfate minerals; sulfide minerals; silicate minerals; oxide minerals; clay minerals.

Comparison of surface and subsurface conditions Subsurface High temperature but constant at which minerals reach equilibrium high confining pressure (stress) less water or no water no oxygen Surface low temperature, and highly variable little or no confining pressure (stress) abundant of water abundant of 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 Mechanical weathering: Physical disintegration or degradation of rock pieces without a change in composition --size reduction Chemical weathering: 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.

mechanical weathering: 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 developed.

Mechanical Weathering (cont.): Mechanical weathering processes include: 1) freezing & thawing (frost wedge) Example: 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. σ

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.

Thermal expansion * repeated daily heating and cooling of rock; * heat causes expansion; cooling causes contraction. * different minerals expand and contract at different rates causing stresses along mineral boundaries.

The thermal expansion coefficient α L and α V α L = 1 l ( ) L T P characters how much a mineral change its dimension in response to a unit degree increase in temperature. Using a finite expression L=Lα L T α V 1 = V V ( T 0 ) P

Mechanical Weathering (cont.): The linear thermal expansion coefficient α L is on the order of 10-5 /ºC, and it could be anisotropic, i.e., α Lx α Ly α Lz For example, α max is in the elongation axis (Caxis).

Differential weathering

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

See also T&L Figure 5.8 Chapter 5: 2) Mechanical weathering Unloading Another sheeting example

Vertical columns from magma cooling & shrinking columnar jointing

Glacially polished basalt columns (end view) Columnar jointing in basalt

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 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 weathering.

Chemical weathering rate depends on 1. Temperature 2. Amount of surface area 3. Availability of water or natural acid Thus, rocks in tropical environment experience most severe chemical weathering.

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 to 9 ph > 9 or < 4 occurs in extreme environments

Chemical Weathering of silicate minerals by carbonic acid feldspar + water + carbonic acid = clay minerals + dissolved ions 2KAlSi 3 O 8 + H 2 O + 2H 2 CO 3 = Al 2 Si 2 O 5 (OH) 4 + 2K + + 4SiO 2(aq) + 2HCO 3 -

Why is rainwater naturally acidic? Rainwater contains dissolved CO 2 from atmosphere. Dissolved CO 2 reacts with water to form carbonic acid (H 2 CO 3 ) CO 2 + H 2 O H 2 CO 3 Carbonic acid dissociates to produce hydrogen ion (H + ) and bicarbonate H 2 CO 3 H + + HCO 3 -

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 water i), halite; ii), calcite * Limestone and marble contain calcite and are soluble in acidic water.

Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution. Note that the urn and tops of ledges are heavily weathered, but the inscriptions are somewhat sheltered and remain legible. Photo taken in one New Orleans graveyard.

A 16th-century monastery in Mexico shows the ravages of weathering mostly from wind and wind-driven rain. The rock is volcanic tuff.

Karst landscape of Guilin, China, caused dissolved Carbonate rocks.

The other two kinds of chemical weathering Oxidation Oxidation - Oxygen combines with iron-bearing silicate minerals causing "rusting". Iron oxides are produced that are red, orange, or brown in color. Hydrolysis Hydration-reaction between mineral and water.

Transport of weathering products Glacial soils formed by transportation and deposition of glaciers; Alluvial soils transported by running water and deposited along streams; Lacustrine soils Marine soils formed by deposition in quiet lakes; formed by deposition in the sea; Aeolian soils transported and deposited by wind; Colluvial soils 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

Summary 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.

Readings: Ch. 2