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<br />Appendix A
<br />Character and Behavior of Earth Materials
<br />
<br />The earth's surface is characterized by great variety of form
<br />and features. This derives from a combination of different
<br />physical properties, behavior, cl imate, and earth processes.
<br />These conditions, in turn, Int luence our I ives in many ways,
<br />Including construction projects, resource development, and
<br />recreational acti "Ities.
<br />
<br />In general, earth materials may be divided into three groups:
<br />1) crystal I ine rock, 2) sedimentary rock, and 3)
<br />unconso I i dated mater I a I (so i I ) .
<br />
<br />Crystal I Tne rocks, I ike granite, basalt, gneiss, and schist,
<br />are usually hard, erode slowly, are difficult to excavate,
<br />yield water only from fractures, may contain metallic mineral
<br />resources, and make a-xcellent foundations. Crystal I ine rocks
<br />frequently form the core of mountain ranges. They are rocks
<br />which have formed by crystal I ization either from a molten
<br />I iquld or under conditions of high temperature pressure.
<br />Crystalline rocks, If not exposed at the earth's surface, are
<br />a I ways found at some depth because they form the
<br />I'basement"_-and maJorlty--of the earth's crust.
<br />
<br />Sedimentary rocks are those rocks which have been formed by
<br />deposits in water (fresh or salt) or on land (by wind, Ice,
<br />or gravity) and have since been solidified by processes such
<br />as cementation, canso I idatlon, or precipitation. Sedimentary
<br />rocks I nc I ude sandstones, cong 1 omerates, and limestone, wh' ch
<br />can behave much like cr ysta I Ii ne rocks in the I r strength,
<br />hardness, and erodabi Ilty. Shale, coal, and evaporites In
<br />contrast are frequently soft, weak, and erode quickly. The
<br />var i ab III ty of phys i ca I propert I as is the on I y reasonab I e
<br />genera Ii zat i on wh i ch can be made. The I r propert I es and
<br />effect on man are dependent on such things as mineralogy,
<br />grain size, and consol idatlon, water content, bedding, and
<br />jointing. Many of our minerai and energy resources come from
<br />sedimentary rock Including coal, oi I shale, 011, gas, cement,
<br />clay, salt, uranium, and some metals. ~ajor ground water
<br />resources are found in the sandstones and II mestones of
<br />Colorado.
<br />
<br />In general, sandstones, conglomerates, and limestones form
<br />ridges, mesas, and mountains, whi Ie shale, evaporites, and
<br />other weak sedimentary rocks form valleys and low areas.
<br />
<br />Unconso I I dated mater I a I 5 are the youngest
<br />volcanics) in Colorado and, as the name
<br />enough to be shove I ed by hand. They
<br />
<br />(except for recent
<br />implies, are weak
<br />are composed of
<br />
<br />fragments of the crysta II j ne or sed I mentary rocks from wh I ch
<br />they are derived. They are classified by origin (alluvial.
<br />fluvial, colluvial, glacial, eolian, residual, ,landslide, or
<br />debris fan) or by type and particles size distribution (clay,
<br />s i It, sand, pebb I e, bou I der, organ i c, and comb I nat I on of
<br />these) .
<br />
<br />".'.\
<br />
<br />I nc I uded in these unconso 1 I dated mater I a I s are usa I Is."
<br />Different professionals and disciplines use 11501 I" to mean
<br />different things: To many engineers, soils are any material
<br />wh i ch can be excavated wi th norma I power equ 1 pment j to the
<br />agricultl,lral community, soil is any natural medium that wi II
<br />support the growth of land plants; to geologists and
<br />engineering geologists. sol Is may be either slm! lar to that
<br />of agronomists or may be all unconsolidated material above
<br />bedrock. These differences become even more confusing when
<br />soils are described and classified by different systems.
<br />These systems may be based on the origin of the soi I
<br />material, the 5011 forming process, or the physical
<br />properties. The engineering geologist and soi Is engineer
<br />wi II define physical and chemical properties such as size,
<br />gradation, organic content, specific gravity, moisture,
<br />optimum water content, permeabi I ity, elasticity, plasticity,
<br />cohesion, shearing strength, soluble salts, consolidation,
<br />swell, and frost susceptibi Ilty. In construction, it is
<br />these physical and chemical properties which, when confined
<br />with the geologic processes, are the key to sound
<br />geotechn I ca I deve I opment.
<br />
<br />These unconsol idated materials then, form the thin skin which
<br />mantles most of the crystal I ine and sedimentary rocks In
<br />Colorado. They are the soils, stream sands and gravels,
<br />debris fans, slope wash, and wind blown sands and silts.
<br />They are the major source of aggregate for concrete and
<br />conta in the most eas II y access I b I e ground water. They are
<br />Indeed the source of our food. They al so, when not
<br />understood, can cause the worst harm to man's work as they
<br />rapidly erode, deposit, swell, shrink, collapse, and move
<br />downslope.
<br />
<br />The character and behavior of earth materials to an
<br />eng! neer I ng geo I og I st I s a natura I outgr.owth of the
<br />combination of geologic and engineering knowledge.
<br />Understanding their geologic origin and history provides an
<br />exce II ent framework from wh i ch to pred i ct character and
<br />future behavior of earth materials as they interact with man
<br />and continuing natural processes. The complexity and
<br />varlabi I ity of the natural world leads tp a necessary degree
<br />of uncertainty when extrapolating detailed information from a
<br />small s.ample to the side of a mountain or a square mile of
<br />the plains. Geologic understanding can help to bridge this
<br />gap between the deta II ed phys i ca I parameters and the rea I
<br />world of dynamic earth processes and heterogeneity.
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