8.1 Rock Mass
The large volume of rock intersected by the discontinuities is known as rock mass. The rock mass is considered in between two discontinuities. In civil engineering practice, rock mass is considered as construction site where as rock is considered as construction materials. Rock mass as construction site provides foundation for building, reservoir, base for road alignment etc.
Rock mass = Intact rock +discontinuity
The following parameters are related to the intact rock. The rock mass is classified on its outcrop on the basis of following characteristics of intact rocks.
Rock mass rating system
The following six parameters are used to classify a rock mass using the RMR system
1. Uniaxial compressive strength of rock material
2. Rock Quality Designation (RQD)
3. Spacing of discontinuities
4. Condition of discontinuities
5. Groundwater conditions
6. Orientation of discontinuities
Each of the six parameters is assigned a value corresponding to the characteristics of the rock. These values are derived from field surveys. The sum of the six parameters is the "RMR value", which lies between 0 and 100.
RMR Rock quality
0 – 20 Very poor
21 - 40 Poor
41 - 60 Fair
61 - 80 Good
81 - 100 Very good
Characteristics of Discontinuities in Rock Mass
Discontinuities are the structural features of rock, which are developed due to the existence of different stresses on the periphery of the earth. It separates the two rock masses to each other. The classification of rock mass is based on the characteristics of intact rocks as well as the discontinuities.
Bieniawski’s Geo-mechanics classification system provides a general rock mass rating (RMR) increasing with rock quality from 0 to 100. The following parameters are considered-
Rock type:
The rocks are classified on the basis of the characteristics of intact rock as well as the discontinuities present.
Cleavage/foliation plane, bedding plane
Materials
Particle size
Textures, etc.
Orientation:
Orientation of discontinuities is the attitude of the discontinuities. Depending upon the slope of the discontinuity rock has different strength at different direction. If the direction of the discontinuity is in the same direction as the rock mass, it is an unfavorable condition. But if the discontinuity is in an opposite direction to the rock mass, it is a favorable condition.
Intact Rock Strength:
The strength of the intact rock is tested by Schmidt Hammer rebound test. Schmidt Hammer is the instrument used to test bearing capacity of site rock mass by rebound test. There are two ways of testing by Schmidt Hammer. One is uni-axial test and the other is tri-axial test.
Spacing:
It is the perpendicular distance between the two adjacent discontinuities of the same set. The space between the discontinuities set in the same direction also causes variation in the strength of the rock. The rock material in between the discontinuity is intact material. The volume of the intact material governs the strength of the rock.
Aperture:
Aperture is the open spacing present in the rock due to discontinuity present in it. The crack due to any means like alkaline water has high tendency to dissolve calcite material may get widened up. Depending upon space it is classified as widely open (>1cm), open (2mm-1cm), close (<2mm), tight (<1mm). The apertures wide and open cause the mechanical discontinuity as no stress is transferred all over the rock. However if the open discontinuity is filled with any other material then strength is transferred. The open and close aperture if is filled by any other material then the strength and the stability of the rock increases depending upon the material type filling the aperture.
Roughness:
It is one of the characteristics of the discontinuity surface. In rough discontinuity surfaces due to low friction shear strength is high. It is generally of two types - rough planar (rough surface with a plane flow) and rough wavy (rough surface with a wave like flow).
Seepage:
Seepage is the flow of water under gravitational forces in a permeable medium. Flow of water takes place from a point of high head to a point of low head. The flow is generally laminar. A flow line represents the path taken by a water particle.
Infilling materials:
These are the materials filled in the open apertures of discontinuities. If there is no fill material it is called clean material, if the rock has mineralized discontinuity (rock fragment) the strength may be considerably high. If the rock is powdered material (e.g., soil) and mineralized both, the rock may be either cohesive or non-cohesive. Tensile strength of soil is low.
8.2 Rocks-Slope Stability Analysis
The term rock slope stability may be defined as the resistance of inclined surface to failure by sliding or collapsing.
The main objectives of slope stability analysis are
· Finding endangered areas, investigation of potential failure mechanisms, determination of the slope sensitivity to different triggering mechanisms, designing of optimal slopes with regard to safety, reliability and economics, designing possible remedial measures, e.g. barriers and stabilization
· Successful design of the slope requires geological information and site characteristics, e.g. properties of soil/rock mass, slope geometry, groundwater conditions, alternation of materials by faulting, joint or discontinuity systems, movements and tension in joints, earthquake activity etc
· Choice of correct analysis technique depends on both site conditions and the potential mode of failure, with careful consideration being given to the varying strengths, weaknesses and limitations inherent in each methodology
Failure mechanism in rock-slope:
- Plane Failure
- Toppling Failure
- Wedge Failure
8.2.1. Plane Failure
Condition for plane failure
· dip direction of hill slope and discontinuity plane is same
· dip amount of hill slope is greater than discontinuity plane
8.2.2. Toppling Failure
Condition for toppling failure
· dip direction for hill slope is opposite from discontinuity plane
· dip amount of hill slope is greater than discontinuity plane
8.2.3. Wedge Failure
Condition for wedge failure
· dip direction for hill slope and line of intersection of two discontinuity plane are same
· dip amount of hill slope is greater than line of intersection of two discontinuity plane
