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Home , Brunton compass, Strike and dip

FM 5-410

CHAPTER 2

Structural

Structural geology describes the form, pat- secondary structural features. These secon- tern, origin, and internal structure of rock dary features include folds, faults, joints, and and soil masses. , a closely related schistosity. These features can be identified field, deals with structural features on a and m appeal in the field through site inves- larger regional, continental, or global scale. tigation and from remote imagery. Figure 2-1, page 2-2, shows the major plates of the earth’s crust. These plates continually Section I. Structural Features undergo movement as shown by the arrows. in Sedimentary Rocks Figure 2-2, page 2-3, is a more detailed repre- sentation of plate tectonic theory. Molten material rises to the earth’s surface at BEDDING PLANES midoceanic ridges, forcing the oceanic plates Structural features are most readily recog- to diverge. These plates, in turn, collide with nized in the sedimentary rocks. They are adjacent plates, which may or may not be of normally deposited in more or less regular similar density. If the two colliding plates are horizontal layers that accumulate on top of of approximately equal density, the plates each other in an orderly sequence. Individual will crumple, forming mountain range along deposits within the sequence are separated the convergent zone. If, on the other hand, by planar contact surfaces called bedding one of the plates is more dense than the other, planes (see Figure 1-7, page 1-9). Bedding it will be subducted, or forced below, the planes are of great importance to military en- lighter plate, creating an oceanic trench along gineers. They are planes of structural the convergent zone. Active volcanism and weakness in sedimentary rocks, and masses seismic activity can be expected in the vicinity of rock can move along them causing rock of plate boundaries. In addition, military en- slides. Since over 75 percent of the earth’s gineers must also deal with geologic features surface is made up of sedimentary rocks, that exist on a smaller scale than that of plate military engineers can expect to frequently tectonics but which are directly related to the encounter these rocks during construction. reformational processes resulting from the force and movements of . Undisturbed sedimentary rocks may be relatively uniform, continuous, and predict- The determination of geologic structure is able across a site. These types of rocks offer often made by careful study of the stratig- certain advantages to military engineers in raphy and sedimentation characteristics of completing horizontal and vertical construc- layered rocks. The primary structure or tion missions. They are relatively stable rock original form and arrangement of rock bod- bodies that allow for ease of rock excavation, ies in the earth’s crust is often altered by as they will normally support steep rock

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faces. Sedimentary rocks are frequently military engineers do not determine the sub- oriented at angles to the earth’s “horizontal” surface conditions before committing surface; therefore, movements in the earth’s resources to construction projects. Therefore, crust may tilt, , or break sedimentary where outcrops are scarce, deliberate excava- layers. Structurally deformed rocks add com- tions may be required to determine the type plexity to the site geology and may adversely and structure of subsurface materials. To affect military construction projects by con- determine the type of rock at an outcrop, the tributing to rock excavation and slope procedures discussed in Chapter 1 must be stability problems. followed. To interpret the structure of the bedrock, the military engineer must measure Vegetation and overlying soil conceal most and define the trend of the rock on the earth’s rock bodies and their structural features. surface. Outcrops are the part of a rock formation ex- posed at the earth’s surface. Such exposures, FOLDS or outcrops, commonly occur along hilltops, Rock strata react to vertical and horizontal steep slopes, streams, and existing road cuts forces by bending and crumpling. Folds are where ground cover has been excavated or undulating expressions of these forces. They eroded away (see Figure 2-3). Expensive are the most common type of deformation. delays and/or failures may result when Folds are most noticeable in layered rocks but

Structural Geology 2-3 FM 5-410 rarely occur on a scale small enough to be ob- dips up to 90 degrees. The elevation of the served in a single exposure. Their size varies beds on opposite sides of the fold may differ by considerably. Some folds are miles across, hundreds or thousands of feet. are while others may be less than an inch. Folds upfolds, and are downfolds (see Fig- are of significant importance to military en- ure 2-5c and d, respectively). They are the gineers due to the change in attitude, or most common of all fold types and are typi- position, of bedding planes within the rock cally found together in a series of fold undula- bodies (see Figure 2-4). These can lead to rock tions. Differential weathering of the rocks excavation problems and slope instability. composing synclines and anticlines tends to Folds are common in sedimentary rocks in mountainous areas where their occurrence produce linear valleys and ridges. Folds that may be inferred from ridges of durable rock dip back into the ground at one or both ends strata that are tilted at opposite angles in are said to be plunging (see Figure 2-6). nearby rock outcrops. They may also be Plunging and plunging recognized by topographic and folds are common. Upfolds that plunge in all patterns and from aerial photographs. The directions are called domes. Folds that are presence of tilted rock layers within a region bowed toward their centers are called is usually evidence of folding. basins. Domes and basins normally exhibit roughly circular outcrop patterns on geologic Types maps. There are several basic types of folds. They are— Symmetry . Folds are further classified by their sym- . metry. Examples are- Anticline. Asymmetrical (inclined). Syncline. Symmetrical (vertical). Plunging. Overturned (greatly inclined). . Recumbent (horizontal). Basin. The axial plane of a fold is the plane that A rock body that dips uniformly in one direc- bisects the fold as symmetrically as possible. tion (at least locally) is called a homocline (see The sides of the fold as divided by the axial Figure 2-5a). A rock body that exhibits local plane are called the limbs. In some folds, the steplike slopes in otherwise flat or gently in- plane is vertical or near vertical, and the fold clined rock layers is called a monocline (see is said to be symmetrical. In others, the axial Figure 2-5b). are common in plane is inclined, indicating an asym- plateau areas where beds may locally assume metrical fold. If the axial plane is greatly

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Structural Geology 2-5 FM 5-410 inclined so that the opposite limbs dip in the mapped to help determine the structure of a same direction, the fold is overturned. A rock mass. Their attitudes can complicate recumbent fold has an axial plane that has rock excavation and, if unfavorable, lead to been inclined to the point that it is horizontal. slope stability problems. Figure 2-7 shows the components of an ideal- ized fold. An axial line or fold axis is the FAULTS intersection of the axial plane and a par- Faults are fractures along which there is ticular bed. The crest of a fold is the axis line displacement of the rock parallel to the frac- along the highest point on an anticline. The ture plane; once-continuous rock bodies have trough denotes the line along the lowest part of been displaced by movement in the earth’s the fold. It is a term associated with synclines. crust (see Figure 2-8). The magnitude of the displacement may be inches, feet, or even AND SCHISTOSITY miles along the plane. Overall fault dis- is the general term describing the placement often occurs along a series of small tendency of rocks to break along parallel sur- faults. A zone of crushed and broken rock faces. Cleavage and schistosity are foliation may be produced as the walls are dragged terms applied to metamorphic rocks. past each other. This zone is called a “fault Metamorphic rocks have been altered by heat zone” (see Figure 2-9). It often contains and/or pressure due to mountain building or crushed and altered rock, or “gouge,” and an- other crustal movements. They may have a gular fragments of broken rock called pronounced cleavage, such as the metamor- “breccia.” Fault zones may consist of phic rock slate that was at one time the materials that have been altered (reduced in shale. Certain igneous strength) by both fault movement and ac- rocks may be deformed into schists or igneis- celerated weathering by water introduced ses with alignment of minerals to produce along the fault surface. Alteration of fault schistosity or gneissic foliation. The attitudes gouge to clay lowers the resistance of the of planes of cleavages and schistosity can be faulted rock mass to sliding. Recognition of

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faults is extremely important to military en- rock beds often indicates faulting, it may also gineers, as they represent potential weakness be caused by igneous intrusions and uncon- in the rock mass. Faults that cut very young formities in deposition. Faults that are not sediments may be active and create seismic visibly identifiable can be inferred by sudden () damage. changes in the characteristics of rock strata in an outcrop or borehole, by missing or repeated Recognition strata in a stratigraphic sequence, or (on a Faults are commonly recognized on rock larger scale) by the presence of long straight outcrop surfaces by the relative displacement mountain fronts thrust up along the fault. of strata on opposite sides of the fault plane Rock strata may show evidence of dragging and the presence of gouge or breccia. Slicken- along the fault. Drag is the folding of rock sides, which are polished and striated beds adjacent to the fault (see Figure 2-8 and surfaces that result from movement along the Figure 2-10, page 2-8). Faults are identifiable fault plane, may develop on the broken rock on aerial photographs by long linear traces faces in a direction parallel to the direction of (lineations) on the ground surface and by the movement. Faulting may cause a discon- offset of linear features such as strata, tinuity of structure that maybe observed at streams, fences, and roads. Straight fault rock outcrops where one rock layer suddenly traces often indicate near-vertical fault ends against a completely different layer. planes since traces are not distorted by This is often observed in road cuts, cliff faces, topographic contours. and stream beds. Although discontinuity of Terminology The of a fault plane is measured in the same manner as it is for a layer of rock. (This procedure will be described later.) The fault plane intersection with the surface is called the fault line. The fault line is drawn on geologic maps. The block above the fault plane is called the hang- ing wall; the block below the fault plane is called the footwall. In the case of a vertical fault, there would be neither a hanging wall nor a footwall. The vertical displacement along a fault is called the throw. The horizon- tal displacement is the heave. The slope on the surface produced by movement along a fault is called the . It may vary in

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height from a few feet to thousands of feet or reverse fault, the hanging wall has been dis- may be eroded away (see Figure 2-11). placed upward relative to the footwall (see Figure 2-12b). Reverse faults are frequently Types associated with compressional forces that Faults are classified by the relative direc- accompany folding. Low-angle (near-horizon- tion of movement of the rock on opposite sides tal) reverse faults are called overthrust of the fault. The major type of movement faults. Thrust faulting is common in many determines their name. These types are– mountainous regions, and overthrusting rock Normal (gravity). sheets may be displaced many kilometers Reverse (thrust). over the underlying rocks (see Figure 2-10). Strike-slip. Strike-slip faults are characterized by one block being displaced laterally with respect to Normal faults are faults along which the the other; there is little or no vertical displace- hanging wall has been displaced downward ment (see Figure 2-12c). Many faults exhibit relative to the footwall (see Figure 2-12a). both vertical and lateral displacement. Some They are common where the earth’s surface is faults show rotational movement, with one under tensional so that the rock bodies block rotated in the fault plane relative to the are pulled apart. Normal faults are also called opposite block. A block that is downthrown gravity faults and usually are characterized by between two faults to form a depression is high-angle (near-vertical) fault planes. In a called a “” (see Figure 2-13a). An upthrown block between two faults produces a “” (see Figure 2-13b). Horsts and are common in the Basin and Range Province located in the western con- tinental United States. The grabens comprise the valleys or basins between horst moun- tains.

JOINTS Rock masses that in such a way that there is little or no displacement parallel to the fractured surface are said to be jointed, and the fractures are called joints (see Figure 2-14, page 2-10). Joints influence the way the rock mass behaves when subjected to the

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stresses of construction. Joints charac- folding may cause the formation of joints. teristically form planar surfaces. They may Igneous rocks may contain joints formed as have any attitude; some are vertical, others lava cooled and contracted. In dense, ex- are horizontal, and many are inclined at trusive igneous rocks, like basalt, a form of various angles. Strike and dip are used to prismatic fracturing known as columnar measure the attitude of joints. Some joints jointing often develops as the rock cools rapidly may occur as curved surfaces. Joints vary and shrinks. Jointing may also occur when greatly in magnitude, from a few feet to overlying rock is removed by erosion, causing thousands of feet long. They commonly occur a rock mass to expand. This is known as ex- in more or less parallel fractures called foliation. The outer layers of the rock peel, sets. Joint systems are two or more related similar to the way that an onion does. joint sets or any group of joints with a charac- teristic pattern, such as a radiating or Significance concentric pattern. Because of their almost universal presence, joints are of considerable engineering impor- Formation tance, especially in excavation operations. It Joints in rock masses may result from a is desirable for joints to be spaced close number of processes, including deformation, enough to minimize secondary plugging and expansion, and contraction. In sedimentary blasting requirements without impairing the rocks, deformation during lithification or stability of excavation slopes or increasing

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the overbreakage in tunnels. The spacing of planar feature encountered is a sedimentary the joints can control the size of the material bed. Strike is defined as the trend of the line removed and can also affect drilling and of intersection formed between a horizontal blasting. The ideal condition is seldom en- plane and the bedding plane being measured countered. In quarry operations, jointing can (see Figure 2-15). The strike line direction is lead to several problems. Joints oriented ap- given as a bearing that is always in proximately at right angles to the working reference to true north. Typical strikes would face present the most unfavorable condition. thereby fall between north 0 to 90 degrees Joints oriented approximately parallel to the east or north 0 to 90 degrees west. They are working face greatly facilitate blasting opera- never expressed as being to the southeast or tions and ensure a fairly even and smooth southwest. Azimuths may be readily con- break, parallel to the face (see Figure 2-14). verted to bearings (for example, an azimuth of Joints offer channels for groundwater circula- 350 degrees would be converted to a bearing tion. In excavations below the groundwater of north 10 degrees west). table, they may greatly increase water problems. They also may exert an important The dip is the inclination of the bedding influence on weathering. plane. It is the acute angle between the bed- ding plane and a horizontal plane (see Figure 2-16). It is a vertical angle measured at right STRIKE AND DIP angles from the strike line. The dip direction The orientation of planar features is deter- is defined as the quadrant of the compass the mined by the attitude of the rock. The bed is dipping into (northeast, northwest, attitude is described in terms of the strike and southeast, or southwest). By convention, the dip of the planar feature. The most common dip angle is given in degrees followed by the

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dip direction quadrant (for example, 30 direction of the strike is designated as it is for degrees northeast). inclined beds. The direction of the dip is a short line crossing the strike line at a right The strike and dip measurements are angle extending on both sides of the strike taken in the field on rock outcrops with a line (see Figure 2-18b, page 2-12). For standard . The Brunton horizontal beds, the direction of strike is rep- compass is graduated in degrees and has a resented by crossed lines which indicate that bull’s-eye level for determining the horizontal the rock strikes in every direction. The dip is plane when measuring the strike direction. represented by a circle encompassing the The strike is determined by aligning the com- crossed lines. The circle implies that there is pass along the strike direction and reading no dip direction and the dip angle is zero (see the value directly from the compass. In- Figure 2-18c, page 2-12). These basic symbols cluded with the Brunton compass is a are commonly used to convey attitudes of clinometer to measure the dip angle. This sedimentary rocks (see Figure 2-19, page angle is measured by placing the edge of the 2-13). Similar symbols are used to convey at- compass on the dipping surface at right titudes of other types of planar features, such angles to the strike direction and reading the as folds, faults, foliation, and jointing in other acute angle indicated by the clinometer (see rock bodies. Figure 2-17, page 2-12). Strike and dip symbols are used on geologic Section II. Geologic Maps maps and overlays to convey structural orien- tation. Basic symbols include those for TYPES inclined, vertical, and horizontal beds (see Geologic maps show the distribution of Figure 2-18, page 2-12). For inclined beds, geologic features and materials at the earth’s the direction of strike is designated as a long surface. Most are prepared over topographic line that is oriented in reference to the map base maps using aerial photography and field grid lines in exactly the same compass direc- survey data. From a knowledge of geologic tion as it was measured. The direction of the processes, the user of a geologic map can draw dip is represented by a short line that is al- many inferences as to the geologic relation- ways drawn perpendicular to the strike line ships beneath the surface and also much of and in the direction of the dip. The angle of the geologic history of an area. In engineer- the dip is written next to the symbol (see Fig- ing practice, geologic maps are important ure 2-18a, page 2-12). For vertical beds, the guides to the location of construction

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materials and the evaluation of foundation, at some shallow mapping depth (often one excavation, and ground water conditions. meter) so that minor residual soils and deposits do not mask the essential features of The following geologic maps are used for engineering concern. military planning and operations: Bedrock or aerial maps. Special Purpose Maps Surficial maps. These maps show selected aspects of the Special purpose maps. geology of a region to more effectively present information of special geologic, military, or Bedrock or Aerial Maps engineering interest. Special purpose maps are These maps show the distribution of rock often prepared to show the distribution of— units as they would appear at the earth’s sur- Engineering hazards. face if all unconsolidated materials were Construction materials. removed. Symbols on such maps usually Foundation conditions. show the age of the rock unit as well as major Excavation conditions. structural details, such as faults, fold axes, Groundwater conditions. and the attitudes of planar rock units or fea- Trafficability conditions. tures. Thick deposits of alluvium (material Agricultural soils. deposited by running water) may also be Surface-water conditions. shown. Very detailed, large-scale geologic maps Surficial Maps may show individual rock bodies, but the These maps show the distribution of uncon- smallest unit normally mapped is the forma- solidated surface materials and exposed tion. A formation is a reasonably extensive, bedrock. Surface materials are usually dif- distinctive series of rocks deposited during a ferentiated according to their physical and/or particular portion of geologic time (see Table chemical characteristics. To increase their 2-1, page 2-15). A formation may consist of a usefulness as an engineering tool, most surfi- single rock type or a continuous series of cial maps show the distribution of materials related rocks. Generally, formations are

Structural Geology 2-13 FM 5-410 named after the locality where they were first to show the value (in degrees) of the dip defined. Formations may be grouped by age, angle. The number is omitted on repre- structure, or lithology for mapping purposes. sentations of both horizontal and vertical beds, because the values of the dips are auto- SYMBOLS matically acknowledged to be 0 and 90 de- Symbols are used to identify various fea- grees, respectively. Figure 2-22, page 2-19, tures on a geologic map. Some of those shows the placement of strike and dip sym- features are— bols on a geologic map with respect to the loca- tion and orientation of a sedimentary rockbed. Formations (see Figure 2-21, page 2-18). Contacts (see Figure 2-21, page 2-18). Attitudes (see Figure 2-21, page 2-18). Fault Lines and Fold Axes Fault lines and fold axes (see Figure Heavy black lines, which may be solid, 2-21, page 2-18). dashed, or dotted (as described for contacts), show fault lines and fold axes. The direc- Cross sections (see Figure 2-23, page tion of movement along faults is shown by 2-19). arrows or by the use of symbols to indicate up thrown and down thrown sides. The Formations arrows accompanying fold axes indicate Letters, colors, or symbolic patterns are the dip direction of the limbs and/or the used to distinguish formations or rock units plunge direction of the fold. on a geologic map. These designators should be defined in a legend on the map. Letter sym- Cross Sections bols usually consist of a capital letter Cross sections show the distribution of indicating the period of deposition of the for- geologic features and materials in a vertical mation with subsequent letters (usually plane along a line on a map. Cross sections lower case) that stand for the formal name of are prepared in much the same way as the unit, (see Table 2-1). Maps prepared by the topographic profiles using map, field, and US Geological Survey and many other agen- borehole data. Geologic sections accompany cies use tints of yellow and orange for many geologic maps to clarify subsurface Cenozoic rocks, tints of green for Mesozoic relationships, Like geologic maps, geologic rocks, tints of blue and purple for Paleozoic sections are often highly interpretive, espe- rocks, and tints of red for Precambrian rocks, cially where data is limited and structures Symbolic patterns for various rock types are are complex or concealed by overburden. given in Figure 2-20, page 2-17. Maps and sections use similar symbols and conventions. Because of the wealth of data that can be shown, geologic maps and sections Contacts are the two most important means of record- A thin, solid line shows contacts or boun- ing and communicating geologic information. daries between rock units if the boundaries are accurately located. A dashed line is used OUTCROP PATTERNS for an approximate location and a dotted line An outcrop is that part of a rock formation if the cointact, is covered or concealed. Ques- that is exposed at the earth’s surface. Out- tionable or gradational contacts are shown by crops are located where there is no existing a dashed or dotted line with question marks. soil cover or where the soil has been removed, leaving the rock beneath it exposed. Outcrops Attitudes may indicate both the type and the structure Strike and dip symbols describe planes of of the local bedrock. Major types of structural stratification, faulting, and jointing. These features can be easily recognized on geologic symbols consist of a strike line long enough so maps because of the distinctive patterns they that its bearing can be determined from the produce. Figures 2-24 through 2-31, pages map, a dip mark to indicate the dip direction 2-20 through 2-21, show basic examples of of the plane being represented, and a number common structural patterns.

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Each illustration contains a block diagram illustrations include some of the following showing a particular structural feature along structural features: with its topographic expression. The outcrop Horizontal strata. pattern of each rock unit shown on the block diagram is projected to a horizontal plane, Inclined strata. resulting in the production of a geologic map Domes. that is also shown. This allows the reader to Basins. readily relate the structure shown on the Plunging folds. block diagram to the map pattern. Structural Faults. details can be added to basic maps using Intrusive rocks. the symbols in Figure 2-21, page 2-18. The Surficial deposits.

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Horizontal Strata the contours. Escarpments and gentle slopes Dendritic (branching or treelike) drainage generally develop on resistant and nonresis- patterns typically develop on horizontal tant beds, respectively, producing variations strata and cut canyons or valleys in which in the width of the map outcrop pattern. The progressively older rock units are exposed at upper and lower contacts are close together depth (see Figure 2-24, page 2-20). The result on steep cliffs; on gentle slopes of the same is that the map patterns of horizontal strata formation, the contacts are further apart. parallel stream valleys, producing dendritic The map width of the outcrops of horizontal pattern on the geologic map. Although all beds does not indicate the thickness of the maps do not show topographic contour lines, strata. Gently dipping beds develop the same the contacts of horizontal rock units parallel basic outcrop pattern as horizontal beds.

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However, the contacts of gently dipping apply the “rule of Vs” as explained above to strata, if traced far enough up a , cross interpret the direction of dip. topographic contours and form a large V- shaped pattern that points in the direction Basins the beds dip, assuming that the beds do not, Eroded structural basins form an outcrop dip in the direction of the stream gradient, pattern very similar to that of an eroded dome but at a smaller angle. (see Figure 2-27, page 2-20). However, two major features serve to distinguish them: Inclined Strata younger rocks outcrop in the center of a basin When a sequence of rocks is tilted and cut and, if the structure has been dissected by off by erosion, the outcrop pattern ap- stream erosion, the outcrop Vs normally point pears as bands that, on a regional basis, are toward the center of a basin, whereas they roughly parallel. Where dipping strata cross usually point away from the center of a dome. a valley, they produce a V-shaped outcrop pattern that points in the direction of dip, ex- Plunging Folds cept in cases where the beds dip in the Folding is found in complex mountain direction of the stream gradient at smaller ranges and sometimes in lowlands and angles than the gradient. The size of the V is plateaus. When folds erode, the oldest rocks inversely proportional to the degree of dip. outcrop in the center of the anticlines (or up- Low-angle dip (large V) (see front folds) and the youngest rocks outcrop in the part of Figure 2-25, page 2-20). center of the synclines (or downfolds). The High-angle dip (small V). axes of folded beds are horizontal in some Vertical dip (no V) (see back part of folds, but they are usually inclined. In this Figure 2-25, page 2-20). case, the fold is said to plunge. Plunging folds form a characteristic zigzag outcrop pattern Other relationships that are basic to the in- when eroded (see Figure 2-28, page 2-21). A terpretation of geologic maps are also shown plunging anticline forms a V-shaped pattern in Figure 2-25, page 2-20. For example, they with the apex (or nose) of the V pointing in the show that older beds dip toward younger direction of the plunge. Plunging synclines beds unless the sequence has been over- form a similar pattern, but the limbs of the turned (as by folding or faulting). Maps also fold open in the direction of the plunge. show that outcrop width depends on the thickness of the beds, the dip of the beds (low Faults dip, maximum width), and the slope of the Fault patterns on geologic maps are distinc- topography (steep slope, minimum width). tive in that they abruptly offset structures and terminate contacts (see Figure 2-29, Domes page 2-21). They are expressed on the geologic Eroded dome-shaped structures form a map by heavy lines in order to be readily roughly circular outcrop pattern with beds distinguished. Some common types are– dipping away from a central area in which the Normal. oldest rocks outcrop (see Figure 2-26, page Reverse. 2-20). These structures range from small fea- Thrust. tures only a few meters across to great Upwarps covering areas of hundreds or Normal and Reverse (see A and B, respec- thousands of square kilometers. tively, in Figure 2-29, page 2-21). Both normal and reverse fault planes generally dip at a Drainage patterns are helpful in interpret- high angle, so outcrop patterns are relatively ing a domal structure. Radial drainage straight. Older rocks are usually exposed on patterns tend to form on domes. Streams cut- the upthrown block. It is thus possible to ting across the resistant beds permit one to determine the relative movement on most

Structural Geology 2-22 FM 5-410 high-angle faults from map relations that of the stock, but the age relation with alone. Linear streams, offsets, linear scarps, the other dikes is not indicated. straight valleys, linear-trending springs or ponds, and omitted or repeated strata are Surficial Deposits common indications of faulting (see para- Surficial deposits are recent accumulations graph on recognition of faults, page 2-7). of various types of sediment or volcanic debris on the surface of the landscape (see Figure Thrust (see C in Figure 2-29, page 2-21). 2-31, page 2-21). The primary types are— Thrust faults are reverse faults that dip at low angles (less than 15 degrees) and have Windblown sand and loess. strati graphic displacements, commonly Stream channel and floodplain deposits. measured in kilometers (see Figure 2-10, page Landslide deposits. 2-8). The trace of the thrust commonly forms Glacial deposits. Vs where it intersects the valleys. The Vs Present beaches and other shoreline point in the direction of the fault plane dip, sediments. except in cases where the fault plane dips in the direction of the stream gradient, but a Section III. Engineering smaller angle. Erosion may form windows Considerations (fensters) through the thrust sheet so that un- derlying rocks are exposed or produce isolation remnants (klippen) above the un- ROCK DISTRIBUTION derlying rocks. Hachure symbols are used to Geologic structure controls the distribution designate the overthrust block that usually of rock bodies and features along and beneath contains the oldest rocks. the earth’s surface. The presence and orienta- tion of such features as bedding, folding, faulting, and unconformities must be deter- Intrusions mined before construction begins. Otherwise, Larger igneous intrusions, such as foundation, excavation, and groundwater con- batholiths and stocks, are typically discor- ditions cannot be properly evaluated. dant and appear on geologic maps as elliptical or roughly circular areas that cut across the ROCK FRAGMENTATION contacts of surrounding formations (see Fig- Rocks tend to fracture along existing zones ure 2-30, page 2-21). Smaller discordant of weakness. The presence and spacing of intrusions, such as dikes, are usually tabular bedding, foliation, and joint planes can con- and appear on geologic maps as straight, trol the size and shape of rock fragments usually short, bands. However, some dikes produced in quarries and other excavations. are lenticular and appear as such on the map. Operational and production costs may be Concordant intrusions, such as sills and lac- prohibitive if rock fragments are too large, too coliths, have contacts that parallel those of small, too slabby, or too irregular for ag- the surrounding formations (see Figure 1-5, gregate requirements. Advantageous joint or page 1-7). bedding spacings can significantly reduce ex- cavation and aggregate production costs. The relative age of igneous bodies can be recognized from crosscutting relationships. Many weak, thinly bedded, or highly frac- The younger intrusions cut the older ones. tured rocks can be excavated without blasting With this in mind, it is clear from the relation- by using ripping devices drawn by heavy ships in Figure 2-30, page 2-21, that the crawler tractors. When ripping is used to elliptical stock is the oldest intrusion, the break up and loosen rock for removal, the northeast trending the next oldest, and work should proceed in the direction of the the northwest trending dike the youngest. dip. This prevents the ripping devices from The age of the small discontinuous dikes near riding up the dip surfaces and out of the rock the western part of the map is younger than mass (see Figure 2-32, page 2-24).

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Most rock must be drilled and blasted for (1 foot horizontal to 2 feet vertical) removal. Where joints or bedding planes in- are usually feasible in such cases un- cline across the axis of the drill hole, drill bits less weak rocks underlie the excava- tend to follow these planes, causing the holes tion sidewalls. to be misaligned; or, more often, the bits to Some rocks may slide on surfaces that bind, stick, or break off in the holes (see dip between about 18 degrees and 35 de- Figure 2-33). Open fractures and layers of grees toward an excavation, particularly weak rock greatly reduce blasting effective- if the surfaces are wet, clayey, ness by allowing the force of the blast to smooth, and continuous. Side slopes escape before the surrounding rock has been of 1:1 or flatter may be required to properly fragmented. Such situations re- stabilize such surfaces. It may be quire special drilling and blasting necessary to remove the hazardous techniques that generally lower the ef- rock entirely. Where excessive ex- ficiency of quarrying operations. cavations must be avoided for economic, environmental, or other reasons, ar- tificial supports or drainage works may be employed to stabilize the rock. Unless rock surfaces are discon- tinuous or very rough and uneven,

ROCK SLIDES AND SLUMPS Massive rock slides may occur where un- confined rock masses overlie inclined bedding, foliation, fault, or joint surfaces (see Figure 2-34). The risk of such slides is generally greatest over smooth, continuous, water- or clay-lubricated surfaces that dip steeply toward natural or man-made excava- tions. The following general observations may assist in evaluating hazards (see Figure 2-35): Most rocks are stable above surfaces that dip less than about 18 degrees toward an excavation. Excavation slopes with horizontal to vertical ratios of 1:2

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rocks are weak or the planes of weak- ness are closely spaced.

WEAK ROCKS Weak rocks, such as shales, may or crush under the weight of overlying rock and allow excavation sidewalls to slump or cave in. Such failures can be prevented by instal- ling artificial supports or by using flattened or terraced side slopes to reduce the load on the potential failure zone.

FAULT ZONES Fault zones are often filled with crushed and broken rock material. When these materials are water-soaked, they may most unconfined rocks will slide over weaken and cause the fault zone to become surfaces steeper than about 35 unstable. Such zones are extremely hazard- degrees. Excavation side slopes ous when encountered in tunneling and deep should be cut back to the dipping sur- excavations because they frequently slump or face or slightly beyond to assure cave in. Artificial supports are usually re- stability against sliding. quired to stabilize such materials. Rocks along planes of weakness that dip almost vertically toward or away GROUNDWATER from excavation sidewalls should be Water entering the ground percolates cut on horizontal to vertical ratios of downward, through open fractures and per- 1:4 or 1:2 to prevent toppling failures. meable rocks, until it reaches a subsurface This is particularly important if the zone below which all void spaces are filled

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with water. Where such ground water is in- horizontal sedimentary rock layers, the direc- tersected in an excavation, such as a road cut tion of quarrying should be chosen based on or tunnel, drainage problems may occur; rock the most prominent joint set or other discon- slides triggered by the weakening and/or tinuity. lubricating of the rock mass may result. In addition, water trapped under hydrostatic ROCK DEFORMATION pressure in fault zones, joints, and permeable rock bodies can cause sudden flooding Rocks may behave as elastic, plastic, or vis- problems when released during excavation. cous solids under stress. Heavy loads, such as Permeable rock zones may also permit water dams, massive fills, tall buildings, or bridge to escape from canals and reservoirs. How- piers, may cause underlying rocks to com- ever, if properly evaluated, the structural press, shear, or squeeze laterally. Particular conditions that produce ground water problems exist where rocks of different problems can also provide potential supplies strength underlie a site. For example, where of groundwater or subsurface drainage for en- weak shale and stronger limestone support gineering projects. different parts of the same structure, the structure may tile or crack due to uneven set- ROAD CUT ALIGNMENT tlement. The most advantageous alignment for road cuts is generally at right angles (perpen- The removal of confining stresses during dicular) to the strike of the major planes of excavation may cause rocks to expand or weakness in the rock (usually the bedding). squeeze into the excavated area. Such This allows the rock surfaces to dip along the problems seldom cause more than an increase cut rather than into it (see Figure 2-36a and in excavation or maintenance costs for roads, airfields, and railroads; however, they may 2-36 b). Where roads must be aligned parallel cause serious damage to dams, buildings, to the strike of the major planes of weakness, canals, and tunnels where deformation can- the most stable alignment is one in which the not be tolerated. Weak clays and shales major planes of weakness dip away from the (especially shales) are the most excavation; however, some overhang should common cause of such problems. Other rocks be expected (see Figure 2-36c). can also cause trouble if they are weathered or if they have been under great stress. To QUARRY FACES neutralize the effects of rock flow or rebound, Quarries should normally be developed in the following may be required: the direction of strike so that the quarry face Additional excavation. itself is perpendicular to strike (see Figure Artificial supports or hold-downs. 2-37a, page 2-28). This particular orientation Compensating loads. is especially important where rocks are steeply Adjustment periods before construc- inclined, because it allows for the optimiza- tion. tion of drilling and blasting efforts by creating a vertical or near-vertical rock face after each (FAULT MOVEMENTS) blast. If necessary, quarries maybe worked Movement along active faults produces perpendicular to the strike direction in in- powerful ground vibrations and rock dis- stances where the rocks are not steeply placements that can seriously disrupt inclined, but drilling and blasting will prove engineering works. Unless proven otherwise to be more difficult. In addition, if the rocks by geological or historical evidence, all faults dip away from the excavation, overhang and that disrupt recent geologic deposits should oversized rocks can be expected (See Figure 2- be considered active. Many areas suffer 37b, page 2-28). If the rocks dip toward the earthquakes as a result of deep-seated faults excavation, problems with slope instability that do not appear at the earth’s surface. and toeing may result (See Figure 2-37c, page Consequently, seismic hazards must be 2-28). In massive bodies and thoroughly investigated before any major

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structure is undertaken. Power lines, dams, vertical structures must be designed to ac- canals, tunnels, bridges, and pipelines across commodate the lateral movements and active faults must be designed to accom- vibrations associated with earthquakes modate earth movements without failure, where seismic hazards exist. Expect in- Buildings and airfields should be located creased seismic risk in marginal areas of away from known active fault zones. All continental plates (see Figure 2-1, page 2-2).

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