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Rock Mechanics ROCK MECHANICS INTACT ROCK STRENGTH

Measured in MPa (mega Pascal) . or Mega Newton’s(MN) • Point Load • Schmidt Hammer • UCS

Dependent on Rock Type Crystalline V Clastic Crystalline tend to have greater strength Weathering Graded on a Numeric scale (IIV), more weathered the weaker the rock. Linked to time and climate Water Generally speaking if water is present potential for weaker rock strength esp mudstoneshales

Structural Geology

Rocks are subjected to stress (Force over area) resulting in strain(deformation)

Product of strain is Folding (ductile bending of the rock) Faulting( brittle ) breaking and movement of the rock

ROCK MASS CHARACTERISTICS

Depends on the amount, type of discontinuities on the overall rock mass Faults Decreasing size Fractures Joints Cracks

Measured using spacing and frequency and condition of discontinuities. Method used:Fracture indices:- number of fractures over a given length (if mm). RQD:Rock quality designation (give idea how intact rock is from a core sample. RMR (Rock mass ratings) Variety of method, very detailed and look at :Discontinuities spacing Quality of fractures (eg open, closed) Water RQD Weathering

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Rock Mechanics

ROCK MASS STRENGTH Strength:-

Number of Cars

1 MegaPascal = 1 MegaNewton = 100 tonnes= 1 ton = 10KN (car = 15 kN) TASK1 convert rock strength in to equivalent number of cars. A scale of strength based on UNIAXIAL COMPRESSIVE STRENGTHS POINT LOAD apparatus can also be used in the field to give an estimate of strength. The strength refers to the solid parts of the rock mass, the OVERALL strength will be affected by JOINTS, FRACTURES and BEDDING PLANES. It is for this reason that a number of Systems are used to classified rock mass rather than just rock strength alone. Rock Mass Properties:A rock mass is divided into a number of units, each unit has a characteristic property. For each unit the following parameter can be determined.: • • • • • • •

Uniaxial compressive strength RQD(rock quality designation) weathering characteristics joint and fracture spacing fracture openness orientation of fractures groundwater conditions.

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Rock Mechanics TASK 2 Rock Strength Testing (watch the video) http://grow.arizona.edu/SPT--BrowseResources.php?ParentId=236 TASK 3 Virtual Rock Strength Testing (click on the following link) http://grow.arizona.edu/Grow--GrowResources.php?ResourceId=144 Ďƒ = F/A

Uniaxial compressive strength Briefly outline the procedures Specimens from drill cores are prepared by cutting them to the specified length. The specimens are loaded axially up to failure or any other prescribed level whereby the specimen is deformed.

Schmidt hammer The Schmidt Hammer was developed in 1948 for non-destructive testing of concrete hardness, and later to estimate rock strength. It consists of a springloaded mass that is released against a plunger when the hammer is pressed onto a hard surface. The plunger impacts the surface and the mass recoils; the rebound value of the mass is measured either by a sliding pointer or electronically. Hammer rebound readings are considered consistent and reproducible. Such fast, non-destructive and in-situ evaluations of rock mechanical parameters reduce the expenses for sample collection and laboratory testing. Consequently, the mechanical parameters can be determined in dense arrays of field measurements that reflect the real inherent inhomogenity of rock masses. Try out Virtual Rock Mass Strength http://grow.arizona.edu/Grow-GrowResources.php?ResourceId=0&name=slide7&StartId=145 Plot results. TASK 4 Convert your (mean)UCS results into Hammer Rebound and Point load Rock Type Granite

Rebound Result

UCS (MPa)

Point Load

Sandstone Basalt Shale

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Rock Mechanics

Empirical relations between hammer rebound values and the measured uniaxial compressive strength. Heavy line is the best-fit correlation (eq. 2 in text); horizontal error bars indicate standard deviations of the hammer rebound measurements. Weathering (Access the following website) Access the following websites:http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html http://www.gpc.edu/~pgore/geology/historical_lab/weathering.php TASK 5 Complete the table below by place listing types of weathering and and example. Type of Weathering Physical/Mechanical Disintegrate the rock

Example Freeze Thaw Frost Shattering Thermal Expansion

Comment Leads to the disintegration of the rock into smaller fragment.

Chemical Decomposition

Acid Rain Hydrolysis Oxidation

Leads to the decomposition of the rock to form clays.

Biological

Eg Tree Roots

Tree roots, growing in cracks and joints eventually prise fragments of rock away.

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Rock Mechanics TASK 6 Study the following pictures/samples and classify the degree of weathering using Table Weathering Grade Description

Fresh Grade I

Residual Soil Grade VI

Slightly Moderately II/III

Highly Weathered Grade IV

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Rock Mechanics

ENGINEERING CLASSIFICATION OF WEATHERED ROCK Weathering Grade Description Grade VI

Description Soil

V

Completely Weathered

IV

Highly weathered

III

Moderately weathered Slightly weathered

II I

Fresh Rock

Lithology Some organic content, no original structure Decomposed soil, some remnant structures Partly changed to soil Soil>rock core stones Partly changed to soil Rock> soil Increased fractures, and mineral staining Clean Rock

Foundations Unsuitable Assess by soil testing Variable and unreliable Good for most small structures. Good for anything but large dams. Sound

Weathering Grade and Rock Properties Grade of Weathering Granite unconfined compressive strength MPa

I 250

II 150

III 5100

IV 2-15

V

Triassic sandstone:- UCS MPa Carboniferous Sandstone RQD % Chalk:-SPT N Value Chalk Safe Bearing Pressure KPa

30 80 >35 100 0 400

15 70 30 750

5 50 22 400

2 20 17 200

<1 0 <15 75

250

150

50

Triassic Mudstone KPa

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Rock Mechanics