Written Test 2 Rock Mechanics

Question 1

The stability of a rock construction at greater depths is then determined by

Answer 1

– the stress condition in the rock mass
– the geometry of the excavation
– the strength of the rock mass

Question 2

• Primary stresses are

Answer 2

– Exists naturally in the undisturbed rock mass and depends on its geological history
• Also called virgin stress state or in situ stresses state

Question 3

• Secondary stresses

Answer 3

– Occurs when an excavation is created and depends on the primary stresses
and the geometry of the excavation

Question 4

when does the Failure in rock occurs

Answer 4

– Occur when the secondary stresses are greater than the strength of the rock

Question 5

What is Stress

Answer 5

– is the amount of force 
acting on a rock unit to 
change its shape and/or 
volume (concentration of 
force)
– stress is a fictitious 
quantity!

Question 6

What is Strain

Answer 6

– is the change in shape
and/or volume of a rock
unit caused by stress

Question 7

. How many stress components, and which stress components (in the xyz-system), are required to describe the stress state in a general case , both for 2d and 3d ?

Answer 7

2d : three , 2 normal , 1 shear

3d : six , 3 normal , 3 shear ,

Question 8

What is principal stress

Answer 8

Normal stress acting on a free shear surface , can be determined at any point of the body , 2 in 2d and 3 in 3d , where segma 1 is bigger than segma 2 and segma 2 is bigger than segma 3

Question 9

Invariant ?

Answer 9

set of equations , A quantity independent of the coordinate system

Question 10

Mohr’s stress circle ?

Answer 10

It is a graphical representation of the stress state

– Normal stress () as x-axis and shear stress (

Question 11

what is – Plane state of stress

Answer 11

One of the principal stresses = 0

Question 12

what is – Plane state of strain

Answer 12

• One of the principal strains = 0

Question 13

what is Hydrostatic state of stress (Spherical state of stress) ?

Answer 13

• All directions in space are principal stress directions: sigma1 = sigma2= sigma 3

Question 14

what is – Cylindrical state of stress

Answer 14

All directions in the plane are principal stress directions , either sigma 1 equals sigma 2 or sigma 2 equals sigma 3

Question 15

what are the factors effecting primary stress

Answer 15

• Erosion and melting of land ice
– Previously loaded by sedimentary rock types, sediments, or ice
– Plastic deformation of the earth crust
– Erosion and melting of land ice – imply an increase of
• Residual stresses
– Inhomogeneous physical or chemical processes in a limited part of a
material volume
– May generate locally “frozen-in” stress state

Question 16

Condition and assumptions to calculues secondary stresses ?

Answer 16

• Plane state of strain (tunnel&raquo_space; diameter)
• Linearly elastic and isotropic rock
• Excavation oriented parallel to the direction of on of the
principal stresses