ENGINEERING GEOLOGYAnd so geology, once considered mostly a descriptive and historical science, has in recent years taken on the aspect of an applied science. Instead of being largely speculative as perhaps it used to be, geology has become factual, quantitative, and immensely practical. It became so first in mining as an aid in the search for metals; then in the recovery of fuels and the search for oil; and now in engineering of mans structure to natures limitations and for greater in public works. (Charles P. Berkey, Pioneer Engineering Geology, 1939)Geology is the science of rocks, minerals, soils, and subsurface water, including the study of their formation, structure, and behavior. As the quotation above indicate, geology was once confined to purely academic studies, but it has since expanded into a practical science as well. Engineering geology in the branch that deals with the application of geologic principles to engineering works.Unlike geotechnical engineers, whose training is in civil engineering, engineering geologist have a background in geology. Their work includes mapping, describing, and characterizing the rock at a construction site; assessing stability issues, such as landslides; and appraising local seismicity and earthquake potentials. These two professions are complementary, and work together as a team. Nevertheless, it is important for the geologist to have some understanding of engineering, and the engineer to have some understanding of geology. Some individuals have even acquired full professional credentials in both fields. This chapter explores fundamental principles of geology and their application to geotechnical engineering, with extra emphasis on the geological origin of soils. These principles are important to geotechnical engineers because they help us understand the nature of the subsurface conditions and form much of the basis for interpreting data gathered form exploratory borings.2.1 ROCK AND SOILBoth geologist and engineers frequently divide earth materials into two broad categories rock and soil. Although this may seem to be a simple distinction, in reality it is not and has often been a source of confusion. To a geologist, rock is any naturally formed aggregate or mass of mineral matter, whether or not coherent, constituting an essential and appreciable part of the earths crust (American Geological Institute, 1976). This definition focuses on the models of origin and structure of the material. Conversely , engineers (and contractors) sometimes consider rock to be a hard, durable material that cannot be excavated without blasting, a definition based on strength and durability.Unfortunately, these two definitions sometimes produce conflicting classifications, especially in intermediate materials. For example, some materials that are rock in terms of their geologic origin are soft enough to be easily excavated with the same equipment used for soil. They may even lock like soil. Siltstone is good example. Conversely, some cemented soils, such as caliche, are hard as rock and very difficult to excavate. This difficulty in classifying some materials has often led to construction lawsuits, because contractors are typically paid more to excavate rock. It also can be a problem when piles are to be driven to rock.Therefore, it is important for both engineering geologist and geotechnical engineers to properly communicate the nature of earth materials (rock vs. soil) to other members of the design and construction teams. Our though processes tend to use the geologist definition because they help us interpret the subsurface condition, but contractors and other engineer usually interpret our comments in light of the engineers definition.Sometimes this difficulty can overcome by using the terms hard rock and soft rock, were the letter is capable of being excavated by conventional earthmoving equipment. However, this definition also can lead on confusion, and is not entirely satisfactory. In chapter 6 we will discuss more specific classification method to be use in excavation specifications.Another aspect of dividing earth materials into rock and soil is that distinction often determines the kinds of subsurface data we need to acquire, the test we will perform, end the analyses we will conduct. This is because there are important differences between these two materials, including the following (Goodman, 1990): Rocks are generally cemented; soil are rarely cemented Rocks usually have much lower porosity than soils Rocks can be found in states of decay with greatly altered properties and attributes; effects of weathering on soils are more subtle and generally less variable. Rocks masses are often discontinues; soil masses usually can be represented as continues Rocks have more complex, and generally unknown stress histories. In many rock masses, the least principal stress in vertical; in most soils the greatest principal stress is vertical.Although there are times when soil mechanics techniques can be applied to rock mechanics problems, and vice-versa, any such sharing must be done cautiously.2.2 ROCK-FORMING MINERALSMinerals are naturally formed elements or compounds with specific structures and chemical compositions. As the basic constituents of rocks, minerals control much of rocks behavior. Some minerals are very strong and resistant to deterioration, and produce rocks with similar properties, while others are much softer and produce weaker rock.More than 2000 different minerals are present in the earths crust. They can be identified by their physical and chemical properties, by standardized test, or by examination under a microscope. Only a few of them occur in large quantities, and they form the material for most rocks. The most common minerals include :Feldspar- This is the most abundant mineral, and is an important component of many kinds of rocks. Orthoclase feldspar contain potassium (KalSi3O8) and usually range from white to pink. Plagioclase feldspar contain sodium (NaAlSi3O8), calcium (CaAlSi2O8), or both, and range from white to gray to black. Feldspar have a moderate hardness.Quartz- Also very common, quartz is another major ingredient in many kinds of rock. It is a silicate (SiO2), and usually has a translucent to milky white color. Quartz is harder than most minerals, and thus is very resistant to weathering. Chert is a type of quartz sometimes found in some sedimentary rocks. It can cause problems when used as a concrete aggregate.Ferromagnesian minerals- A class of minerals, all of which contain both iron and magnesium. This class includes pyroxene, amphibole, hornblende, and olivine. These minerals have a dark color and a moderate hardness.Iron oxides- Another class of minerals, all of which contain iron (Fe2O3). Includes limonite anf magnetite. Although less common, these minerals give a distinctive rusty color to some rocks and soils, and can act as cementing agents.Calcite- A minerals made of calcium carbonate (CaC03); usually white, pink, or gray. It is soluble in water, and thus can be transported by groundwater into cracks in rock where it precipitates out of solution. It also can precipitate in soil, becoming a cementing agent. Calcite is much softer then quartz or feldspar, and effervesces vigorously when treated with dilute hydrochloric acid.Dolomite Similar to calcite, with magnesium added. Less vigorous reaction to dilute hydrochloric acid.Mica- Translucent thin sheets or flakes. Muscovite has silvery flakes, while biotite is dark gray or black. These sheets have a very low coefficient of friction, which can produce shear failures in certain rocks, such as schist.Gypsum- A very soft mineral often occurring as a precipitate in sedimentary rocks. It is colorless to white and has economic value when found in thick deposits. For example, it is used to make drywall. Gypsum is water soluble, and thus can dissolve under the action of groundwater, which can lead to other problems.When rock breaks down into soil, as discussed later in this chapter, many of these minerals remain in their original form. For example, many sand grains are made of quartz, and thus reflect is engineering properties. Other minerals undergo chemical and physical change and take in new properties. For example, feldspar often experiences such changes, and forms clay minerals (discussed later in this chapter). Soil also can acquire other materials, including organic matter, man-made materials, and water.2.3THE GEOLOGIC CYCLEThe geologic processes acting on the earths crust are extremely slow by human standards. Even during an entire lifetime, once can expect to directly observe only a minutely small amount of progress in these processes. Therefore, geologist must rely primarily on observations of the h as it presently exists (i.e., on the result of these processes) to develop their theories.Geologic theories are organized around a framework known as the geologic cycle. This cycle, includes many processes simultaneously. The most important of these begin with the rocks begin broken down into soil, and that soil being converted back into rock.

Rocks are classified according to their place in the geologic cycle. The three major categories are igneous, sedimentary, and metamorphic, as discussed below.Igneous RockThe geologic cycle begins with magma, a molten rock deep inside the earth. This magma cools as it moves upward toward the ground surface, forming igneous rock. There are two primary types of igneous rocks: Intrusives (also called plutonic rocks) form below the ground surface, where they cool slowly, whereas extrusives (also called volcanic rocks) arrive at the ground surface in a molten state, such as through a volcano, and then cool very rapidly. Intrusives include both large bodies of rock (knows as plutons) and smaller sheets like bodies (known as sills and dikes ) that fill cracks inside other rocks. Extrusives ash, bypasses the rock stage and forms directly into sediment.Common igneous rock include :Granite An intrusive, granite is one of the most common and familiar igneous rocks. It is found over wide areas, such as the Canadian Shield and the Sierra Nevada, and in isolated domes, such as Stone Mountain in Georgia. Granite contains primarily orthoclase feldspar and quartz, with some biotite and amphibole.Basalt- A dark, dense rock; the most abundant extrusive. Very difficult for tunnel construction due to its hardness, yet the rapid cooling associated with all extrusives creates joints in basalt, and slopes made of basalt often fail along these joints.Diorite- Similar to granite, with plagioclase feldspar instead of orthoclase and little or no quartz.Andesite- A very hard extrusive.Rhyolite- The extrusive equivalent of granite.Gabbro- The intrusive equivalent of basalt. Darker in color than granite or diorite. Unweathered igneous rock generally have excellent engineering properties and are good materials to build on. Intrusive rock are especially good. However, the cooling process, along with various tectonic forces within the earth, produce fractures in these rocks, especially in extrusives. The intact rock between these cracks can be very strong, but the fractures from planes of weakness. The rock can slide along these weak planes, potentially causing instability in the rock mass. The engineering properties of weathered igneous rocks are less desirable because the rock is changing into a more soil-like material.Weathering ProcessesRock exposed to the atmosphere are immediately subjected to physical, chemical, and biological breakdown trough weathering. There are many weathering processes, including : The erosive action of water, ice, and wind Chemical reactions induced by exposure to oxygen, water, and chemical Opening of cracks as a result of unloading due to erosion of overlying soil and rock Loosening through the growth of plant roots Loosening through the percolation and subsequent freezing (and expansion) of water Growth of minerals in cracks, which forces them to open further Thermal expansion and contraction from day to day and season to season Landslide and rock falls Abrasion from the downhill movement of nearby rock and soilThe rock passes through various stages of weathering, eventually being broken down into small particles, the material we call soil. These soil particle may remain in place, forming a residual soil, or they may be transported away from their parent rock through processes discussed later in this chapter, thus forming a transforming soil. Weathering processes continue even after the rock becomes a soil. As soil become older, they change due to continued weathering. The rate of change depends on many factors, including : The general climate, especially precipitation and temperature (note that climates in the past were often quite different from those today) The physical and chemical makeup of the soil The elevation and slope of the ground surface The depth to groundwater table The type and extent of flora and fauna The presence of microorganisms The drainage characteristics of the soilSedimentary RocksSoil deposits can be transformed back into rock trough the hardening process called induration or lithification, thus forming the second major category of rocks: sedimentary rock. There are two types : Clastic and carbonate.Clastic RocksClastic rocks form when deep soil deposits become hardened as a result of pressure from overlying strata and cementation through precipitation of water-soluble minerals such as calcium carbonate or iron oxide. Because of their mode of deposition, many clastic rocks are layered or stratified, which makes them quite different from massive formations. The interfaces between these layers are called bedding planes. Shale and sandstone are the most common.Often, various types of clastics rocks are interbedded. For example, a sequence might contain a 1 m thick bed of sandstone, then 5 m of siltstone, 0.5 m of claystone, and so on.Most conglomerate, breccias, sandstone, and arkose rocks generally have favorable engineering properties. Those cemented with silica or iron oxide are specially durable, but may be difficult to excavate. However, some are only weakly indurated, often cemented only with clay or other water-soluble minerals. These may behave much like a soil, and be much easier to excavate.Fine and very fine grained clastic rocks are more common, and much more problematic. Sometimes the term mudstone is used to collectively describe these rocks, but they are more precisely described as siltstone (when the rock is derived from silt), claystone (when derived from clay and slightly to mildly indurated) or shale (when derived from clay and well indurated).Some fine and very fine grained clastic rocks also are subject to slaking, which is a deterioration after excavation and exposure to the atmosphere and wetting-and-drying cycles. Rocks that exhibit strong slaking will rapidly degenerate to soil, and thus can crate problems for engineering structure built on them.CarbonatesA different type of sedimentary rocks forms when organic materials accumulate and become indurated. Because of their organic origin, they are called carbonates. Common carbonate rocks include:Limestone the most common type of carbonate rock, limestone is composed primarily of calcite (CaCO3). Most limestones formed from the accumulation of marine organisms on the bottom of the ocean, and usually extend over large areas. Some of these deposits were later uplifted by tectonic forces in the earth an now exist below land areas. For example, much of florida is underlain by limestone.Chalk similar to limestone, but much softer an more porousDolomite similar to limestone, except based on the mineral dolomite instead of calciteSome carbonate rocks also have bedding, but it is usually less distinct than in clastic rocks.Carbonate rocks especially limestone, can be issolved by long exposure to water, especially if it contains a mild solution of carbonic acid. Groundwater often gains small quantities of this acid through exposure to carbon dioxide in the ground. This process often produces karst topography, which exposes very ragged rock at the ground surface and many underground caves and passageways. In such topography, streams sometimes mysteriously disappear into the ground, only to reappear elsewhere.Sometimes the rocks is covered with soil, so the surface expressions of karst topography may be hidden. Nevertheless, the underground caverns remain, and sometimes the groun above caves into them. This creates a sinkhole, such as the one in figure 2.6. This caving process can be triggered by the lowering of the groundwater table, which often occurs when wells are installed for water supply purposes.In areas underlain by carbonate rock, especially limestone, geotechnical engineers are concerned about the formation of sinkholes beneath large and important structures. We use exploratory borings, geophysical methods, and other techniques (see Chapter 3) to locate hidden underground caverns, then either avoid building above these features, or fill them with grout.METAMORPHIC ROCKSBoth igneous and sedimentary rocks can be subjected to intense heat and pressure while deep in the earth s crust. These conditions produce more dramatic changes in the minerals within the rock, thus forming the third type of rock metamorphic rock. The metamorphic processes generally improve the engineering behavior of these rocks by increasing their hardness and strength. Nevertheless, some metamorphic rocks still can be problematic.Some metamorphic rocks are foliated, which means they have oriented grains similar to bedding planes in sedimentary rocks. The foliations are important because the shear strength is less for stresses acting parallel to the foliations. Other metamorphic rocks are nonfoliated and have no such orientations.Common metamorphic rocks include:Foliated rocks:Slate devired principally from shale, dense, can be readily split into thin sheets parallel to the foliation (such sheets are used to make chalkboards)Schist a strongly foliated rock with a large mica content, this type of foliation is called schistosity, prone to sliding along foliation planes.Gneiss derived from granite and similar rocks, contains banded foliationsNonfoliated rocks:Quartzite composed principally or entirely of quartz, derive from sandstone, very strong and hard.Marble derived from limestone of dolomite, used for decorative purposes and for statuesUnweathered nonfoliated rocks generally provide excellent support for engineering works, and are similar to instrusive igneous rocks in their quality. However, some foliated rocks are prone to slippage along the foliation planes. Schist is the most notable in this regard because of its strong foliation and the presence of mica. The 1928 failure of St. Francis Dam in California (Rogers, 1995) has been partially attributes to shearing in schist, and the 1959 failure of Malpasset am in France (Goodman, 1993) to shearing in schistose gneiss.Metamorphic rocks also are subject to weathering, thus forming weathered rock, residual soils, and transported soils and beginning the geologic cycle anew.All sedimentary rocks formed in horizontal or near horizontal layers, and these layers often reflect alternating cycles of deposition. This process produces parallel bedding planes as shown in figure 2.5. The shear strength along these planes is typically much less than across them , a condition we call anisotropic strength. When these rocks were uplifted by tectonic forces in the earth, the bedding planes usually were roteted to a different angle, as shown in figure 2.7. Because the rock could shear much more easily along these planes, their orientation is important. Many landslides have occurred on slopes with unfavorable bedding orientations. Therefore, engineering geologists and geotechnical engineers are very careful to compare the attitudes of bedding planes with the orientation of proposed slopes.Some metamorphic rocks have similar planes of weakness. They are called schistosity and are mapped in a similar way.FOLDSTectonic forces also distort rock masses. When horizontal compressive forces are present, the rock distorts into a wavy pattern called folds as shown in Figure 2.8. Sometimes these folds are gradual, other times they are very abrupt. When folds are oriented concave downward they are called anticlines, when concave upward they are called synclines.FracturesFractures are cracks in a rock mass. Their orientation is very important because the shear strength along these fractures is less than that of the intact rock mass, so they form potential failure surfaces. There are three types of fractures: joints, shear zones, and faults.Joints are fractures that have not experienced any shear movements. They can be the result of cooling (in the case of igneous rocks), tensile tectonic stresses, or tensile stresses from lateral movement of adjacent rock. Joints usually occur ar fairly regular spacings, and a group of such joints is called a set.Shear zones are fractures that have experienced a small shear displacement, perhaps a few centimeters. They are caused by various stresses in the ground, and do not appear in sets as joints do. Shear zones often are conduits for groundwater.Faults are similar to shear zones, except they have experienced much greater shear displacements. Although there is no standard for distinguishing the two, many geologists would begin using the term fault when the shear displacemen exceeds about 1 m. such movements are normally associated with earthquakes, as discussed in Chapter 20.Faults are classified according to their geometry and direction of movement, as shown in figure 2.9. Dip-slip faults are those whose movement is primarily along the dip. It is a normal faults if the overhanging block is moving downward, or a reserve fault if it is moving upward. A reserve fault with a very small dip angle is called a thrust fault. Conversely, strike-slip faults are those whose movement is primarily along the strike. They can be either right-lateral or left-lateral depending on the relative motion of the two sides. Some faults experience both dip-slip and strike-slip movements. The fault trace is the intersection of the fault and the ground surface.The term discontinuity is often used in this context to include bedding planes, schitosity, joints, shear zones, faults, and all other similar defects in rock. Because the orientation of these features is one of the most important engineering aspects of the rock mass, extensive analytical methods have been developed to systematically evaluate discontinuity data gathered in the field (Priest, 1993).Strike and DripWhen developing geologic maps, we are interested in both the presence of certain geologic structures and their orientation in space. For example, a rock mass may be unstable if it has joints oriented in a certain direction, but much more stable if they are oriented in a different direction. For similar reasons, we also are interested in the orientation of faults, bedding planes, and other geologic structures.Many of these structures are roughly planar, at least for short distances and therefore may be described by defining the orientation of this plane in space. We express this orientation using the strike and dip, as shown in Figure 2.10.The strike is the compass direction of the intersection of the plane and the horizontal and is expressed as a bearing from true north. For example, if a fault has a strike of N30W then the intersection of the fault plane traces a line oriented 30 west of true north. The dip is the angle between the geologic surface and the horizontal an is measured in a vertical plane oriented perpendicular to the strike. The dip also needs a direction. For example, a fault with a N30W strike might have a dip of 20 northeasterity. When expressed together, this data is called an attitude, and may be written in condensed form as N30W, 20NE. Although the strike direction is exact, the dip direction is omly approximate. In this case, there are only two possibilities for the dip direction. NE or SW, so the purpose of this direction is simply to distinguish between these two possibilities. The exactdip direction is 90 from the strike.Attitudes are usually measured in the field using a bruton compass, as shown in Figure 2.11. this device includes both a compass and a level and thus can measure both strikes and dips. The measured attitudes are them recorded graphically on geologic maps using the symbol shown in Figure 2.12. This symbol may be modified to indicate the type of structure being identified.Sometimes we need to know the dip as it would appear in a vertical plane other than the one perpendicular to the strike. Figure 2.13 shows such a plane. For example, we may have drawn a cross-section that is oriented perpendicular to a slope, but at some angle other than 90 from the strike, and need to know the dip angle as it appears in that cross-section. This dip is called the apparent dip and may be computed using:Tan a = tan . sin Where:a: apparent dip: dip: horizontal angle between strike and the vertical plane on which the apparent dip is to be computedExample 2.1Compute the apparent dip of the bedding planes as they would appear in the central portion of section B-B in Figure 2.12.Solution:Base analysis on the 17 measured attitude. The angle between its strike and section B-B is 65. Therefore using equation 2.1:Tan a = tan . sin = tan 17 . sin 65a= 15 Thus, the bedding plane will appear to be flatter than it really is.2.5 SOIL FORMATION, TRANSPORT, AND DEPOSITIONGeotechnical engineers work with both rock and soil, and need to be familiar with both. Nevertheless, we facus more of our energies on the engineering behavior of soil because: More civil engineering projects are built on soil Soil, being generally weaker and more compressible than rock, is more often a source of problems.Therefore, we are especially interested in those portions of the geologic cycle that produce and transport soils. A clear understanding of these processes helps geotechnical engineers interpret data gained from exploratory borings, and thus supports the very important function of engineering judgement. This discussion focuses on the inorganic components within a soil. Organic soils and their origins are discussed in Chapter 4.Residual SoilsWhen the rock weathering process is faster than the transport processes induced by water, wind, and gravity, much of the resulting soil remains in place. It is known as a residual soil and typically retains many of the characteristics of the parent rock. The transition with depth from soil to weathered rock to intact rock is typically gradual with no distinct boundaries.In tropical regions, residual soil layers can be very thick, sometimes extending for hundreds of meters before reaching unweathered bedrock. Cooler and more arid regions normally have much thinner layers and often no residual soil at all.The soil type depends on the character of the parent cock. For example, decomposed granite (or simply DG) is a sandy residual soil obtained from granitic rocks. DG is commonly used in construction as a high quality fill material. Shales, which are sedimentary rocks that consist largely of clay minerals, weather to form clayey residual soils.Saprolite is a general term for residual soils that are not extensively weathered and still retain much of the structure of the parent cock. Some have used the term rotten rock to describe saprolite. They typically include small concretations (harder, less weathered fragments) surrounded by more weathere material. Extensive saprolite deposits exist in the Piedmont area of the eastern United States (the zone between the Appalachian Mountains and the coastal plain)(Smith, 1987).Laterite is a residual soil found in tropical regions. This type of soil is cemente with iron dioxides, which gives it a high dry strength.The engineering properties of residual soils range from poor to good, and generally improve with depth.Glacial SoilsMuch of the eatrhs land area was once covered with huge masses of ice called glaciers. In North America, glaciers once extended as far south as the Ohio River. In Europe, glaciers once existed as far south as Germany. Many of these areas are now heavily populated, so the geologic remains of glaciation have much practical significance.Glaciers had a dramatic effect on the landscape and created a category of soils called glacial soils. Glacial ice was not stationary; it moved along the ground, often grinding down some areas and filling in others. In some locations, glaciers reamed out valleys, leaving long lakes, such as the Finger Lakes of upstate New York. The Great Lakes also have been attributed to glacial action. Glaciers grind down the rock and soil, and transport thesc materials over long distances, even hundreds of kilometers, so the resulting deposits often contain a mixture of materials from many different sources. These deposits also can have a wide range of hardness and particle size, and are among the most complex and heterogeneous of all soils. The term drift encompasses all glacial soils, which then can be devided into three categories: till, glaciofluvial, and glaciolacustrine.Till is soil deposited directly by the glacier. It typically contains a wide variety of particle sizes, ranging from clay to gravel. Soil that was bulldozed by the glacier, then deposited in ridges or mounds is called ablation till. These ridges and mounds are called moraines and are loose and easy to excavate. In contrast, soil caught beneath the glacier, called lodgement till, has been heavily consolidated under the weight of the ice. Because of these heavy consolidation pressures and the wide range of particle sizes, lodgement till has a very high unit weight and often is nearly as strong as concrete. Lodgement till is sometimes called hardpan. It provides excellent supportife structural foundation, but is very difficult to excavate.Geotechnical site assessments need to carefully distinguish between ablation till and lodgement till. Both engineers and contractors need to be aware of the difference and plan accordingly. For example, construction of the St. Lawrence Seaway along the U.S.-Canada border during the 1950s. encountered extensive deposits of lodgement till that caused significant problems and dalays. This problem was especially acute on the Cornwall Canal section of the seaway, causing one contractor to go bankrupt, another to default, and a third to file a $5.5 million claim on a $6.5 million contract (Legget and Hatheway, 1988).When the glaciers melted, they generated large quantities of runoff. This water eroded much of the till and deposited it downstream, forming glaciofluvial soils (or outwash). Because of the sorting action of the water, these deposits are generally more uniform than till, and many of them are excellent sources of sand and gravel for use as concrete aggregates.The fine-grained portions of the till often remained suspended in the runoff water until reaching a lake or the ocean, where it finally settled to the bottom. These are called glaciolacustrine soils and glaciomarine soils. Sometimes silts and clays were deposited in alternating layers according to the seasons, thus forming a banded soil called varved clay. The individual layers in varved clays are typically only a few millimeters thick, and often are separated by organic strata. These soils are soft and compressible, and thus are especially prone to problems with shear failure and excessive settlement.Glaciolacustrine soil that formed in seawater are especially problematic because they have a higt sensitivity (they lose shear strength when disturbed, as discussed in Chapter 13), and thus are prone to disastrous landslides. Such deposits are found in the Ottawa and St. Lawrence river valleys in eastern Canada (know as Champlain, Laurentian or Leda clays) and in southern Scandinavia. Soils in the Chicago area are good example of glacial deposits, and are typical of conditions in the Great Lakes region (Chung and Finno, 1992). The bedrock in this area consists of a marine dolomite that was overridden by successive advances and retreats of continental glaciers. At times this area was under ancient Lake Chicago, which varied in elevation from 18 m above to 30 m below the present level of Lake Michigan. These glacier left both lodgement till and moraines, glaciolacustrine clays (deposited in the ancient lake), and glaciofluvial deposits in the riverbottoms.Alluvial SoilsAlluvial soils (also known as fluvial soils or alluvium) are those transported to their present position by rivers and streams. These soils are very common, and a very large number of engineering structures are built on them. Alluvium often contains extensive groundwater aquifers, so it also is important in the development of water supply wells and in geoenvironmental engineering.When the river or stream is flowing rapidly, the sits and clays remain in suspension and are carried downstream; only sands, gravel, and boulders are deposited. However, when the water flows more slowly, more of the finer soils also are deposited. Rivers flow rapidly during periods of heavy rainfall or snowmelt, and slowly during periods of heavy rainfall or snowmelt, and slowly during periods of drought, so alluvial soils often contain alternating horizontal layers of different soil types.The water also slows when the stream reaches the foot of a canyon, and tends to deposit much of its soil load there. Large boulders are sometimes carried by water, especially in steep terrain. Sometime such boulders are subsequently covered with finer soils and become obscured. However, they can cause extensive problems when engineers attempt to drill exploratory borings or contractor try to make excavations or drive pile foundations.Rivers in relatively flat terrain move much more slowly and often change course, creating complex alluvial deposits. Some of these are called braided stream deposits and meander belt deposits. In addition, the deposition characteristics at a given location can change with time, so one type of alluvial soil is often underlain by other types.In arid areas, evaporation draws most of the water our of soil, leaving any dissolved chemical behind. The resulting deposits of calcium carbonate, calcium sulfate, and other substances often act as cementing agents, converting the alluvial into a very hard material called caliche. These deposits are common in the southwestern states, and can be very troublesome to contractors who need to excavate through them.Most alluvial soils have moderately good engineering properties, and typically provide fair to good support for buildings and other structures. Lacustrine and Merine Soils Lacustrine soils are those deposited beneath Lakes. These deposits may still be underwater, or may now be exposed due to the lowering of the lake water level, such as the glaciolacustrine soils in Chicago. Most lacustrine soils are primarily silt and clay. Their suitability for foundation support ranges from poor to average.Marine soils also were deposited underwater, except they formed in the ocean. Deltas are a special type of marine deposit formed where rivers meet larger bodies of water, and gradually build up to the water surface. Examples include the Mississippi River Delta and the Nile River Delta. This mode of deposition creates a very flat terrain, so the water flows very slowly. The resulting oil deposits are primarily silts and clays, and are very soft. Because of their deposition mode, most lacustrine and marine soils are very uniform and consistent. Thus, although their engineering properties are often poor, they may be more predictable than other more erratic soils.Some sands also accumulate as marine deposits, especially in areas where rivers discharge into the sea at a steeper gradient. This sands is moved and sorted by the waves and currents, and some of it is deposited back on shore as beach sands. These sands typically are very poorly graded (i.e., they have a narrow range of particle sizes), have well-rounded particles, and are very loose. Beach deposits typically move parallel to the shoreline, and this movement can be interrupted by the construction of jetties and other harbor improvements. As a result, sand can accumulate on one side of the jetty, and be almost nonexistent on the other side. Changes in sea level elevations can leave beach deposits oriented along previous shorelines.Deeper marine deposits are more uniform and often contain organic material from marine organisms. Those that have a large organic content are called oozes, one of the most descriptive of all soil names. The construction of offshore oil drilling platforms requires exploration and assessment of these soils.Some lacustrine and marine soils have been covered with fill. This is especially common in urban areas adjacent to bays, such as Boston and san Francisco. The demand for real estate in these areas often leads to reclaiming such land, as shown in figure 2.23. However, this reclaimed land is often a difficult place to build upon, because the underlying lacustrine and marine deposits are weak and compressible. Sometimes these soils have special names, such as Boston Blue Clay and San Francisco Bay Mud.Figure 2.23 when the puritans first settled in Boston, Massachusetts, the land area was as shown by the black zone in this map. It was connected to the mainland via a narrow isthmus. Since then, the city has been extended by placing fill in the adjacent water, thus forming the shoreline as it now exists.Aeolian SoilsAeolian Soils (also known as eolian soils) are those deposited by wind. This mode of transport generally produces very poorly graded soils (i.e., a narrow range of particle sizes) because of the strong sorting power of wind. The soils also are usually very loose, and thus have only fair engineering properties.There are three primary modes of wind-induced soil transport (see Figure 2.24): Suspension occurs when wind lifts individual silt particles to high altitudes and transports them for great distances. This process can create large dust storms, such as those that occurred in Oklahoma and surrounding states during the dust bowl drought of the 1930s. Saltation ( from the latin saltatio-to dance) is the intermediate process where soil particles become temporarily airborne, the fall back to earth. Upon landing, the particle bounces or dislodges another particle, the initiating another ... This motion occurs in fine sands, and typical bounce distances are on the order of 4 m. Particles moving by saltation do not gain much altitude; generally no more than 1 m. Creep occurs in particles too large to become airborne, such as medium to coarse sands. This mode consists of rolling and sliding along the ground surface.There are no distinct boundaries between these processes , so intermediate modes of transport also occur.Figure 2.24 modes of Aeolian transport.Aeolian sands can form horizontal strata, which often are interbedded with soils, or they can form irregular hills called sand dunes. These dunes are among the most striking Aeolian deposits, and are found along some beaches in some desert areas. Sand dunes tend to migrate downwind, and thus can be a threat, as shown in figure 2.25. Migrations of 3 m/yr are not unusual, but this rate can be slowed or halted by appropriate vegetation on the dune.Figure 2.25 this same dune near the beach in .., California a slowly migrating as the right and has partially buried the Aeolian silts often form deep deposits called loess. Such deposits are often found downwind of deserts and glacial outwash deposits. Extensive loess deposits are present in the Midwestern states.Because of its deposition mode, loess typically has a very high porosity. It is fairly strong when dry, but becomes weak when wetted. As a result, it can be stable when cut to a steep slope (where water infiltration is minimal), yet unstable when the slope is flatter and water is able to enter the soil. Figure 2.26 shows a near-vertical cut slope in loess.Figure 2.26 the slope in the center of this photograph is a cut made in a loess deposits near the Mississippi River in Tennessee. Notice how it is stable in spite of being near vertical.Nearly all Aeolian soils are very prone to erosion, and often have deep gullies. Good erosion control measures are especially important un these soils.Colluvial Soils A colluvial soils is one transported downslope by grafity, as shown in Figure 2.27. there are two types of downslope movement, slow and rapid. Both types occur only on or near sloping ground.Slow movement, which is typically on the order of millimeters per year, is called creep. It occurs because of grafity-induced downslope shear stresses, the expansion and contraction of clays, frost action, and other processes. Creep typically extends to depths of 0.3 to 3 m, with the greatest displacements occurring at the ground surface. In spite of the name, this process is entirely different from the creep process in Aeolian soils.Such slow movements might first appear to be inconsequential, but in time they can produce significant distortions in structures founded on such soils. Foundations that extend through creeping soils to firm ground below may be subjected to significant downslope forces from these soils, and need to be designed accordingly. In addition, the engineering properties of the soil deteriorate as it moves downhill, thus producing a material that is inferior to the parent soils.Rapid downslope movements, such as landslides or mudflows, are more dramatic events which we will discuss in Chapter 14. Although these rapid movements can occur in any type of soil, the product is considered to be a colluvial soil.Although colluvial soils occur naturally, construction activities sometimes accelerate their formation. For example, making an excavation at the toe of a slope may change a slow creep condition into a landslide.Figure 2.27 colluvial soils: a) slowly formed by creep; b) rapidly formed by landslides or mudflows.SummaryMajor points1. Engineering geology is profession closely related to geotechnical engineering. It deals with the application of geologic principles to engineering works, and is especially useful at sites where rock is at or near the ground surface.2. It is important for geologists to have some understanding of engineering, and for engineers to have some understanding of geology.3. Earth materials may be divided into two broad categories, rock and soil. Unfortunately, everyone doesnt agree on how to distinguish between the .., especially in intermediate materials.4. Minerals are naturally formed elements or compounds with specific structures and chemical compositions. They are the basic constituents of rocks and soils.5. The earths crust is always changing through a process called the geologic cycle. Although this process is very slow, we must understand it to properly interpret geologic profiles.6. There are three major categories of rock: igneous, sedimentary, and metamorphic.7. Properly identifying the configuration and orientation of rock formations is at least as important as identifying the rock types contained in them. This study is called structural geology.8. Soils are formed through several different geologic processes. Understanding these processes gives us insight into the engineering behavior if these soils.