Dr. Naseem AadilDr. Naseem AadilGeological Engineering DepartmentGeological Engineering Department

naadil@yahoo.com.au

Structure Geology

Joints

Joints

• Fractures with no movement

• Almost all rocks in the field have joints

• Usually related to tectonic forces or cooling of magma

Joints, such as those in this sandstoneJoints, such as those in this sandstone

The length of Joints are measured in feet, tens of feet, or even hundreds of feet

Weathered Jointed rocks; the distance between Joints measured in feet, or tens of feet

Attitudes; Strike & Dip & Angle of Attitudes; Strike & Dip & Angle of DipDipThe strike is the direction of a horizontal line on the surface of the plane

The dip, measured in a vertical plane at right angles to the strike of the joint,

Angle of dip is the angle between a horizontal plane and the structural plane

Attitude of JointsAttitude of Joints

Plane GHIJ is a joint that strikes north and dips 50 degrees east.

Plane ABCD is a joint that strike east and has a vertical dip. Plane BEDF is a joint that strikes north and has a vertical dip.

Classification of Classification of Joints Joints Geometric Geometric classificationclassification The joints classified on the basis of their attitude relative to the bedding that they cut

Strike Strike

Dip Dip

diagonal diagonal

Bedding Bedding

Genetic Genetic classificationclassification Joints classified genetically either as:Shear fracturesTension fractures (including extension).

Joints Related FoldingJoints Related Folding

Many joints have resulted from the same

compressive forces that produced the folds.Joints perpendicular to the axes of folds. Joints parallel to the axial planes of folds.

Ultimate causes of Ultimate causes of JointsJointsTectonic stresses, causing fracturing essentially contemporaneously with the tectonic activity.Residual stresses, due to events that happened long before the fracturing.Contraction due to shrinkage because of cooling or dissection.Surficial movements, such as downhill

movements of rocks or mountain glaciers.

Fracture Fracture Mechanics Mechanics A consideration of the genesis of joints

must be preceded by a discussion of failure by rupture. Several factors are of concern:

The nature of the deformation preceding rupture.

The physical conditions at the time of rupture.

The stresses necessary to cause rupture.The orientation of the fractures relative

to the causative stresses.

All ruptures may be classified as tension fractures and shear fractures.

Tension fractures result from stresses that tend to pull the specimen apart. When the specimen finally breaks, the two walls may move away from each other. Tension fractures may result not only from tension, but also from couples and even from compression; as will be shown later

Shear fractures result from stresses that tend to slide one part of the specimen past the adjacent part. When the specimen finally breaks, the two walls may slide past one another. Shear fractures may develop not only under compression, but also from couples and from tension.