Rock stability

Rock Stability Analysis- general

• We have to need good site investigation

• Different modes of failure

– Plane sliding

– Wedge sliding

– Polygonal sliding

– Circular sliding

– Toppling failure

• No one stability analysis is applicable to all rock slopes

Rock Stability Analysis- general

• Slopes fail on weak planes => we have to know orestimate them

• Continuum approach used in soil mechanics not usually applicable

• Kinematic analysis (stereonets) can not determine factor of stability

• Numerical analysis using FEM is not often appropriate due to modelling of discontinuits

Trigger mechanisms

• Water pressures in joints reducing normal stress >

FRICTION CAPACITY

• Less total stress capacity due to water /filling material/.

• Undercutting - natural / excavation.

• Shock - earthquake, blasting

Modes of Failure

• Plane sliding

Occurs in rocks with plane discontinuities, e.g., bedding

planes.

Plane failure

Plane failure

Factor of safety is calculated

by analysis of Plane Failure

resolving all forces acting on

the slope into components

parallel and normal to the

sliding plane.

Planar Failure Analysis

The rock mass can slide along the smooth, stepped or

undulating plane.

The shear strength for the slip surface can be:

•Mohr - Coulomb

•Hoek - Brown

•Barton – Bandis

•More-info help

Modes of Failure

• Wedge sliding

Occurs in rocks with intersecting discontinuities

forming wedges.

Wedge failure

Wedge failure analysis

The failure of

wedge/block is along

the existing joints and

is controlled by the

orientation of those

joints and friction

angle.

Stereographic Projection

Stereographic Projection

Modes of Failure

• Polygonal sliding

From observation it is known that sliding may take place

along polygonal sliding (depends on the main

discontinuities of the rock)

Polygonal failure

Poygonal failure analysis

Program admits the following assumptions:

• Motion of rock blocks is only translational

• Blocks translate along the polygonal slip surface formed either by

planar planes or planes with moderate waviness

• Rock blocks are divided by joints with known directions

• Actual deformation of rock mass inside the blocks is negligible

• Failure on the polygonal slip surface and along joints is driven by

the Mohr-coulomb failure criterion

• The same factor of safety is assumed for all joints and along the

entire polygonal slip surface

• All rock blocks are in contact (opening of joints is not allowed)

• The shear forces on the polygonal slip surface have the same

sign

Polygonal failure analysis

• The block geometry - it is necessary to ensure the

condition that all rock blocks are in contact (the opening

between joints is not allowed)

Modes of Failure

• Circular failure

Usually occurs in waste rock, heavily fractured rock and

weak rock with no identifiable structural pattern.

Circular failure

Circular failure analysis

Assumptions of the limit equilibrium methods:

• Rock mass shear strength is characterized by

cohesion and friction angle;

• Failure occurs on circular slide surface;

• A vertical tension crack occurs in the upper

surface or in the face of the slope;

• Fully saturated or dry rock mass.

Circular failure analysis

Analysis are generally done by

classical soil mechanic analysis

methods, e.g., limit equilibrium

methods (Jambu, Bishop).

GEO5 – Slope stability

Modes of Failure

• Toppling - in the GEO5 Rock Slope release 20

Occurs in rocks with columnar or block structures

separated by steeply dipping joints.

Toppling failure

Slope Support and Protection

No Support:

Scaling

Protection:

Toe buttres

Fences (at toe or on slope)

Nets (over the slope face)

Reinforcement:

Anchors

Slope Support and Protection

Concreting:

Shotcrete

Dental concrete

Toe walls

Drainage:

Surface

Deep

Re‐excavation

Slope protection with meshes reinforced

with rock bolts and shotcrete

Rock bolts

Drained rock buttres at the toe

Hand scaling

The development of Rock slope

1) Markland analysis - kinematic approach v. 19

- is used to evaluate the potential for failures due to

sliding along joints, fractures, or bedding planes

The development of Rock slope

2) Polygonal slip plane – version 19

New method of calculation – Friction cone method by

Goodman

The development of Rock slope

3) Toppling - version 20

The prime joint set dips steeply and in opposite

direction to the slope

Limit Equilibrium Analysis

Forces on base: normal and shear (Rn , Sn)

Interface forces: (Pn , Qn, Pn‐1 ,Qn‐1)

The development of Rock slope

4) Statistical measurenment

• Statistics of all the discontinuities based on the field data, usually, two kinds of map will be abtained according to these field data: rose map of dip, dip angle and strike; contour map of pole of dicontinuities.

• According to the contour map of pole of dicontinuities, we can read a few center orientations of these dicontinuities which shows the main discontinuities that we call them as center orientation. Project these discontinuities into stereographic projection map, we can tell the potential failure pattern which can help engineers to analyze the slope before any calculation.

Examples

Example 1 – Plane failure

Simple slope 60 degrees, bedding 30 degrees, tensilecrack 10m behind the end of slope, horizontal terrain.

Investigate the influence of water in tension crack, suggeststabilization (if needed) when crack is full of water(accidental situation, floods)

Example 2 – Earth wedge

Compute FS of the wedge

- without crack and water

- with tensile crack

- with planes filled half with water

Orientation of planes forming wedge

Dipa [°]

Dip directionj [°]

Shear strength f [°]

Shear strength C [kPa]

1 (plane) 60 360 30 50

2 (plane) 54 118 30 50

4 (slope face)

76 060

3 (upper surface)

15 070

5 (tension crack)

80 060

Wedge height 28m

Distance of tension crack 9m

Rock density gr 26kN/m3

Geometry 1

Geometry 2 – tension crack