Geology Exam 3 Rose-Hulman Geo-270 Rock Mechanics

Front 1

Hydraulic Conductivity

Back 1

: A composite parameter, (m/s), is used by ground water hydrologists and soils engineers to measure the ability of a rock or soil to transmit water. The properties of the material and the fluid are combined in hydraulic conductivity because water at relatively constant values of density and viscosity is usually the only liquid encountered when dealing the upper part of the earth’s crust.
Defined as (k = intrinsic permeability, ρ = density of water, μ = viscosity of water, g = acceleration of gravity): K=k (ρg/μ)

Front 2

Porosity (%)

Back 2

Relative amount of void space, along with the type and amount of fluid occupying the space, has an important influence on the mechanical behavior of the material. The relative amount of void space is quantified by the use of the parameters porosity and void ratio.
Porosity, usually expressed as a percentage (Vv = volume of the voids, VT = total volume of a representative volume of rock):
n=(V_v)/(V_T)

Front 3

Void Ratio (Decimal)

Back 3

Principally used by soils engineers. Void ratio usually expressed as a decimal (Vs = volume of soil solids):
e=V_v/V_s

Front 4

Vertical Stress

Back 4

At shallow depths beneath the earth’s surface, the vertical and lateral (horizontal) stresses present are due to the weight of the overlying rocks, soil, and air. In most subsurface applications, the vertical stress is considered to be the maximum principal stress. When the horizon of interest is overlain by units of different unit weights, the stress components of each unit are summed to get the total vertical stress. The vertical stress, σv, acting on a horizontal plane at a depth h can be calculated as (y = unit weight of rock material and Pa is the atmospheric pressure acting on the surface above the rock):
σ=yh+P_a

Front 5

Horizontal Stress

Back 5

The minimum and intermediate principal stresses are assumed to be horizontal.

Front 6

Modulus of Elasticity

Back 6

The slope of the line relating stress and strain is an important material property called the modulus of elasticity. Specifies how much strain will occur under a given stress. It can be stated as (E =modulus of elasticity, σ = applied stress, Ɛ = strain:
E=σ/ε

Front 7

Strain

Back 7

Amount of deformation is called strain. The type and amount of strain that a particular material experiences depends on the types of stresses applied, as well as the depth and temperature. (ΔL = change of length, L = original length):
ε=∆L/L

Front 8

Compressive Strength

Back 8

Strength is defined as the level of stress at failure. Pg 225. Stresses of equal magnitude that act toward a point from opposite directions are called compressive stresses. Capacity of a material to withstand axial loads tending to reduce size.

Front 9

Shear Strength

Back 9

If the shear stress on these planes (two critical planes within the rock) exceeds the shear strength of the rock in those directions, failure of the rock will occur. Therefore the unconfined compression test indirectly determines the strength of the rock. Shear strength is directly measured in contrast to the unconfined compression test (relationship between shear stress and normal stress at failure). Strength of a material or component against the type of yield or structural failure where the material or component fails in shear.

Front 10

Tangent Modulus (Et50)

Back 10

Et50 is the modulus obtained by taking the slope of a line tangent to the stress-strain curve at 50% of the unconfined compressive strength.

Front 11

RQD (Rock Quality Designation)

Back 11

Test holes are drilled during a site investigation for an engineering project to determine subsurface rock formations (obtaining a core). RQD is defined as the percentage of core recovered in pieces 10 cm or longer in length. The index is expressed as a percentage of the total depth drilled.

Front 12

Strike

Back 12

The direction of a line formed by the intersection of the plane and the horizontal. Strike and dip measurements are often made with a Brunton compass.

Front 13

Dip

Back 13

The amount of slope of the plane. It is determined by measuring the acute angle between the horizontal and the sloping plane. Always measured in a vertical plane perpendicular to strike in the direction of maximum inclination of the rock plane.

Front 14

Anticline

Back 14

Two types of folds are recognized. Anticlines are formed by the upward bending or buckling of strata, so the fold has the shape of an arch. The sides, or limbs, of an anticline dip downward and outward from the fold crest.

Front 15

Syncline

Back 15

The opposite of an anticline is a syncline, in which the central portion is bent downward to form a trough. The limbs of a syncline dip toward the center of the trough.

Front 16

Fold Axis

Back 16

The line connecting the points of maximum curvature is called the axis. The axial plane is a plane containing the axis that divides the fold into two equal sections. The axis of a fold or group of folds may be approximately horizontal or titled, in which case the structure is classified as a plunging fold

Front 17

Symmetrical Folds

Back 17

The limbs dip in opposite directions in about the same amount.

Front 18

Asymmetrical Folds

Back 18

Dips of opposing limbs are not equivalent.

Front 19

Monocline

Back 19

A steplike bend in strata without two well-defined limbs is known as a monocline. Not all folds consist of two limbs.

Front 20

Joint

Back 20

Fractures in which there has been no movement parallel to the failure plane. Although joints are produced by unloading and cooling of lava, our main concern here is for joints caused by tectonic stresses. These fractures commonly occur in sets consisting of numerous parallel planes spaced throughout a rock mass at regular intervals. One or more joint sets, each having a distinctive orientation, are often present. Joints can be produced by tensional, shearing, and compressional stresses. If structures other than joints are found in an area, the joints may be related to the stresses that are responsible for those structures. Rock-slope stability is also controlled by the spacing and orientation of joints. The point of intersection of two or more joints is a prime location for high well yields in such areas. Contaminated groundwater can migrate rapidly along joints and faults.

Front 21

Fault

Back 21

Fractures along which movement of rock masses has occurred parallel to the fault plane. Faults are classified according to the type of movement, or slip, that has taken place. Two categories below.

Front 22

Dip-Slip Faults (definition, types, types defined)

Back 22

The relative movement of rock masses is in the direction of the dip of the fault plane, include normal faults, reverse faults, and thrust faults. Thrust fault is similar to reverse fault except that fault plane is inclined at a lower angle. The amount of slip can range from fractions of a centimeter to thousands of meters. Sometimes occur in pairs.

Front 23

Strike-Slip faults (definition, types, types defined)

Back 23

Characterized by the lateral movement of fault blocks along the strike of the fault plane. These faults are either right-lateral or left-lateral types, depending on the relative movement of fault blocks. The two varieties can be distinguished by facing the fault from either side and noting the direction, either left or right, toward the continuation across the fault of a linear feature such as a stream, fence, or road that passed straight across the fault prior to displacement. (San Andreas)

Front 24

Hanging Wall

Back 24

Always the block above the fault plane. If the hanging wall has moved downward with respect to the footwall, the fault is a normal fault, and relative movement in the opposite sense is characteristic of reverse faults.

Front 25

Foot Wall

Back 25

Always the block below the plane.

Front 26

Horst

Back 26

The structure formed when the central block between two dip-slip faults is up-thrown is known as a horst.

Front 27

Why does characterizing rock mass properties require use of results from laboratory tests on samples along with characteristics of field discontinuities?

Back 27

The properties are determined by laboratory tests on rock samples. These samples are necessarily small, intact specimens taken from large bodies of rock at a field location. Although the test results obtained from intact samples are useful for comparison of properties between various rock types, the strength values cannot be directly applied to the overall rock mass in the field situation. The reason for this apparent discrepancy is that the behavior of a rock mass under load in the field is partially controlled by the strength developed along discontinuities in the rock and by the weathering characteristics, rather than by the strength of the intact portions of the rock itself. Discontinuities are present in almost every type of rock and they act to lower the strength of the rock mass.

Front 28

How might the rock discontinuities identified in Table 7.6 contribute to a rock mass failure?

Back 28

Discontinuities are present in almost every type of rock and they act to lower the strength of the rock mass. Some of the types of rock discontinuities are shown in the table; those discontinuities identify different discontinuity features for each rock type. For example, sedimentary rock discontinuities include bedding planes, mud cracks, ripple marks, etc. Failure in this situation occurs when part of the rock mass breaks away along a discontinuity and moves downslope when there is a steep slope.

Front 29

Be able to explain and/or compare any of figures 7.28-7.31 in the same way as the discussion in the text (have a good understanding of the figures and the text that goes with the figures)

Back 29

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