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
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
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.
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
Sandstone Basalt Shale
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.
Acid Rain Hydrolysis Oxidation
Leads to the decomposition of the rock to form clays.
Eg Tree Roots
Tree roots, growing in cracks and joints eventually prise fragments of rock away.
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
ENGINEERING CLASSIFICATION OF WEATHERED ROCK Weathering Grade Description Grade VI
Moderately weathered Slightly weathered
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
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
Triassic Mudstone KPa