Question

1-Type of Structures

The main ones – Bedding / Faults / Fold / Lineaments / Joints / Cleavage / Foliations

2-Why those structures given affects to the geology of a rockmass
(marks will be given for those who can show the process and followed by sketches)
3-Type of failures

Answer 1

(3) Types of failures are-

(1)Plane Failure

(2)Wedge Failure

  (3) Toppling failure

(4)Rock failure

(1)Plane Failure

The planer failure is resulted by structural discontinuities like bedding planes, faults or the interface between weathered rock and the underlying bedrock.

Planer faluire leads to sliding action along the failure surface. Hence the planar failure occurs in response to a single discontinuity.

The failure plane occurs due to following reasons-

The strikes of both the sliding plane and the slope face lie parallel to each other.

The dip angle of the failure plane should be less than the dip angle of the slope face known as daylight.

It also occurs when the dip of the sliding plane is greater than angle of friction.

(2)Wedge Failure

Wedge failure causes a rock mass to slide along two intersecting discontinuities.

Wedge failure requires dip angle of at least one joint intersection to be greater than the angle of friction of the joint surface.

The favorable lithology for occurrence t of wedge failure includes inclined bedding, foliation and well defined cleavages.

The rocks such as Shale, limestones and slate are more prone to wedge failure due to its internal form.

The wedge failure occurs due to following reasons-

The plunge line of the intersection must be greater than the friction angle of the slope.

The line of intersection must be daylight on the slope. It means that the dip angle of the intersection should be less than the dip angle of the slope.

(3)Toppling Failure

The toppling failure occurs due to steep discontinuities in the rock which leads to slippage of the layers and the outward and downward movement of a column of rock.

In this failure the column of rock rotates about a fixed point at the base of the slope.

The toppling action is due to the column center of gravity lying outside the base dimensions.

(4)Rock failure

It is a type of failure in which a rock mass of any size is detached from a steep slope or cliff where there is a presence of very little shear resistance.

When rocks gets detached it descends mostly by air and can cause significant damage.

It is caused due to change in forces acting on the rock.

It is also triggered by climatic or biologic events like rainfall. The rainfall infiltration causes increase in the pore water pressure which results in rock failures.

(1) Types of structures are -

Faults

A fault is a planar fracture or discontinuity in a volume of rock across which significant displacement as a result of rock mass movement takes place.

The large faults within the earth crust is due to the action of plate tectonic forces with the largest forming the boundaries between the plates such as subduction zones or transform faults.

Fold

A fold occurs when one or a stack of originally flat and planar surfaces such as sedimentary strata which are bent or curved as a result of permanent deformation.

Folds in rocks vary in their size from microscopic crinkles to mountain sized folds.

Bedding

The bedding plane are planar joints or fissures that split the rock along bedding planes.

Bedding plane reflect changes in depositional conditions that differentiate successive layers in stratified sediment.

Lineaments

It is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault.

The lineament appears as a fault aligned valley, a series of fault or fold aligned hills, a straight coastline or indeed a combination of these features.

Joints

A joint is a break of natural origin in the continuity of either a layer or body of rock which lacks any visible or measurable movement parallel to the plane of the fracture

Cleavage

Cleavage is a type of planar rock feature that develops as a result of deformation and metamorphism.

Foliations

Foliation refers to repetitive layering in metamorphic rocks.

Each layer can be as thin as a sheet of paper or it may over a meter in thickness.

(2)

The development of cleavage foliation involves combination of various mechanisms dependent on the rocks composition, tectonic processes and metamorphic conditions.

The magnitude and orientation of stress coupled with pressure and temperature conditions determine how a mineral is deformed.

Cleavages formed parallel to the X-Y plane of tectonic strain and are categorized based on the type of strain.

In geotechnical engineering a bedding plane often forms a discontinuity that may have a large influence on the mechanical behaviour of soil and rock masses for example in tunnel, foundation, or slope construction etc.

Lineaments are often prevalent in geological or topographic maps and can appear obvious on aerial or satellite photographs.

Foliation plane may form a discontinuity that may have a large influence on the mechanical behavior of rock masses for example tunnel, foundation, slope construction etc.