Lecture Notes: Geology for Civil Engineers PDF

CE 25: Geology for Civil Engineers Topic 1 General Geology Einstine M. Opiso Department of Civil Engineering Central Mindanao University 1 Lecture notes for Week 1 Learning Contents  Geology in C...

CE 25: Geology for Civil Engineers Topic 1 General Geology Einstine M. Opiso Department of Civil Engineering Central Mindanao University 1 Lecture notes for Week 1 Learning Contents  Geology in Civil Engineering  Branches of Geology  Earth Structure and Composition 2  Geology in Civil Engineering practice What is Geology? Is a broad, interdisciplinary science with a rich vocabulary.  Geology in Civil Engineering practice What is Geology? Geology is the study of earth, the materials of which it is made, the structure of those materials and the effects of the natural forces acting upon them. It is important to civil engineering because all work performed by civil engineers involves earth and its features.  Geology in Civil Engineering practice Importance of Geology to Civil Engineering? Civil Engineers utilizes geological data, techniques and principles to the study of rock and soil surficial materials and ground water. This is essential for the proper location, planning, design, construction, operation and maintenance of engineering structure. Rock, soil, water and the interaction among these constituents, as well as with engineering materials and structures. 5  Geology in Civil Engineering practice Serve civil engineering to provide information in 3 most important areas: Resources for construction; aggregates, fills and borrows. Finding stable foundations; Mitigation of geological hazards; Identify problems, evaluate the costs, provide information to mitigate the problem 6  Geology and Civil Engineering Relationship Civil engineering works are carried out either on site or within the site. For this reason, erosional and geological process which cause the stability of the rocks and ground and their changes are important for civil engineering  Major Branches of Geology Physical geology: It is concerned with the work of natural processes which bring about changes upon the earth’s surface. Petrology: The discussion of different kinds of rocks is known as petrology. Mineralogy: The study of minerals, its composition & properties is called mineralogy. Structural Geology: It includes the study of the structures of the rocks in the earth’s crust. Stratigraphy: Stratigraphy is a branch of geology which studies rock layers (strata) and layering (stratification).  Major Branches of Geology Palaeontology: It deals with the study of fossils. Historical Geology: The study of Stratigraphy and paleontology is included under historical geology. Economic Geology: It deals with the study of minerals of economic importance or is concerned with earth materials that can be used for economic and/or industrial purposes. ” Mining Geology: It is concerned with the study of application of geology to mining engineering. Engineering geology: It includes the study of application of geology to civil engineering.  Other Branches of Geology Sedimentology: It encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Geochemistry: It is a branch that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. Petroleum Geology: The study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. Hydrogeology: It deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquifers).  Other Branches of Geology Environmental Geology: The study of the interactions between humans and their geologic environment: rocks, water, air, soil, life. Geophysics: It is concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geomorphology: It deals with the study of minerals of economic importance or is concerned with earth materials that can be used for economic and/or industrial purposes. ” Geochronology: the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Volcanology and seismology: It includes the study of volcanoes and earthquakes respectively. Earth’s Internal Structure This drilling ship samples sediment and rock from the deep ocean floor. It can only sample materials well within the upper crust of the earth, however, barely scratching the surface of the earth's interior  History of the Earth’s Interior  The Earth is thought to have formed some 4.6 billion years ago.  It is thought to have formed from a disk of particles and grains that condensed and then were pulled together by gravitational attraction until it became massive enough to eventually become planet sized.  History of the Earth’s Interior In the early years the Earth was bombarded by fragments that were left over from the formation of new planets and satellites. – This bombardment heated up the Earth’s surface, liquefying the surface to hot, molten lava. – Eventually this magma cooled and formed igneous rocks.  History of the Earth’s Interior A second heating of the Earth occurred from the inside as uranium, thorium, and other isotopes began to decay. – As the rate of nuclear decay began to slow down, the outer layer (the crust) slowly cooled. – Today the inside is still molten and the crust is cool and hard. The center of the Earth is an extreme place. Pressure estimates are 3.5 million atmospheres. Temperature estimates are 6,000OC (11,000OF)  Interior of earth To the engineer interested in earthquake effects, the earth is a sphere having the layered structure of a boiled egg. It has a crust (the shell), a mantle (the egg white), and a core (the yolk.) Crust: Continental crust (25-40 km) Oceanic crust (~6 km) Mantle Upper mantle (650 km) Lower mantle (2235 km) Core Outer core: liquid (2270 km) Inner core: solid (1216 km)  Structure of the Earth Crust - variable thickness and composition and made up aluminosilicates minerals Mantle - made up of a rock called peridotite Core - made up of Iron (Fe) and small amount of Nickel (Ni) Structure of the Earth Pressure, Temperature, Composition  Layers of the Earth  It is important to note that there has been, so far, no drill that has penetrated the surface of the earth more than a few kilometers.  Almost all information about the internal structure of the earth is inferred from observed characteristics and propagation (travel rates and reflections) of seismic waves.  Magnetic and gravitational observations also help complete the picture.  Layers of the Earth The earth is divided into three main layers: Inner core, outer core, mantle and crust. The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The Crust The crust is the thin layer of solid, brittle material that covers the Earth. There are some differences in the crust depending on where on the surface you are. The crust under the ocean is much thinner than the crust under the continents. Seismic waves move faster through the oceanic crust that through the continental crust. The material that makes up the crust is called sial This is due to the fact that it is mostly made up of rocks containing silicon and aluminum. The oceanic crust is called sima as it is made up mostly of rocks containing silicon and magnesium. The Crust – There is a sharp boundary between the crust and the mantle that is called the Mohorovicic discontinuity or Moho for short. This is an area of increased velocity of seismic waves as the material is denser in the mantle (due to higher proportion of ferromagnesium materials and the crust is higher in silicates). – There are differences in the material that makes up the continental crust and the oceanic crust. The continental crust is at least 3.8 billion years old, while the oceanic crust is 200 million years in the oldest parts. Continental crust is made mostly of less dense (2.7 g/cm3) granite type rock, while the oceanic crust is made of more dense (3.0g/cm3) basaltic rock.  THE MOHO The Moho, or the Mohorovicic Discontinuity, refers to a zone or a thin shell below the crust of the earth that varies in thickness from 1 to 3 km. Continental crust is less dense, granite-type rock, while the oceanic crust is more dense, basaltic rock. Both types of crust behave as if they were floating on the mantle, which is more dense than either type of crust. The Mantle The mantle is the middle part of the Earth’s interior. 2,870 km thick between the crust and the core. At about 400 and 700 km the pressure and temperature of the mantle increase and change the structure of the olivine minerals found. – above 400 km the typical tetrahedral silicate olivines are found with one silicon surrounded by 4 oxygen atoms. – At 400 km, the increase pressure and temperature result in a structure that collapses on itself and produces a silicate that is more dense than that found in the upper 400 km. – At 700 km the structure is changed again, this time to a silicon atom surrounded by 6 oxygen atoms. – 700 km is the boundary between the upper mantle and the lower mantle. No earthquakes occur in the lower mantle.  THE MANTLE The mantle can be thought of having three different layers. The separation is made because of different deformational properties in the mantle inferred from seismic wave measurements. (1) The upper layer is stiff. It is presumed that if the entire mantle had been as stiff, the outer shell of the earth would have been static. This stiff layer of the mantle and the overlying crust are referred to as the lithosphere. The lithosphere is approximately 80- km thick  THE MANTLE (2) Beneath the lithosphere is a soft layer of mantle called the asthenosphere. Its thickness is inferred to be several times that of the lithosphere. One may think of this as a film of lubricant although film is not exactly the word for something so thick. It is assumed that the lithosphere, protruding (meaning: extending beyond) parts and all, can glide over the asthenosphere with little distortion of the lithosphere  THE MANTLE (3) The mesosphere is the lowest layer of the mantle. Considering the vagueness in defining the lower boundary of the asthenosphere it would be expected that the thickness and material properties of the mesosphere are not well known. It is expected to have a stiffness somewhere between those of the lithosphere and the asthenosphere.  The earth's interior, showing the weak, plastic layer called the asthenosphere.  The rigid, solid layer above the asthenosphere is called the lithosphere. The lithosphere is broken into plates that move on the upper mantle like giant tabular ice sheets floating on water.  This arrangement is the foundation for plate tectonics, which explains many changes that occur on the earth's surface such as earthquakes, volcanoes, and mountain building. Seismic wave velocities increase at depths of about 400 km and 700 km (about 250 mi and 430 mi). This finding agrees closely with laboratory studies of changes in the character of mantle materials that would occur at these depths from increases in temperature and pressure.  THE CORE At a depth of approximately 2900 km, there is a large reduction (on the order of 40%) in the measured velocity of seismic waves. The boundary between the mantle and the core is assumed to be at this depth. Because no S-wave has been observed to travel through the material below this boundary for a thickness of approximately 2300 km, it has been inferred that the core comprises two layers. The 2300-km thick outer layer which is in a molten state and an 1100-km thick inner layer which is solid.  Earth’s Core – There is also an S-wave shadow zone that is larger than the P-wave shadow zone. – S-waves are not recorded in the entire region more than 103O away from the epicenter. – There appear to be 2 parts to the core. The inner core with a radius of about 1,200 km (750 mi) The inner core appears to be solid The outer core has a radius of about 3,470 km (2,160 mi) The core begins at a depth of about 2.900 km (1,800 mi)  Earth’s Core – An earthquake will send out P-waves over the entire globe, except for an area between 103O and 142O of arc from the earthquake. – This is called the P-wave shadow zone, as no P-waves are received here. – P-waves appear to be refracted by the core, which leaves a shadow. – The liquid outer layer versus the solid inner layer is rationalized by recognizing that the melting point of the material increases (with pressure) at a faster rate than the temperature as the center of the earth is approached. END