HOW TO DESCRIBE ROCK MASS CHARACTERISTICS ON SITE?

DESCRIPTION OF ROCK MASS CHARACTERISTICS

Standardized geologic mapping and logging procedures should be used for describing rock mass characteristics. The types of information collected will depend on site access, extent of rock outcrops, and the criticality of the proposed structure to be constructed on or in the rock mass. A method proposed by the International Society of Rock Mechanics (ISRM) provides standardized quantitative and qualitative information on rock masses. This and other rock mass classification systems are described in ASTM D5878. To introduce and explain the ISRM method, several figures and tables were prepared, primarily to provide a standardized definition of terms. Fig-1 provides an illustration of a rock mass and the 13 parameters that are included in a detailed rock description. These 13 parameters can be divided into 5 categories such as

  1. Rock material description
  2. Discontinuity description
  3. Infilling
  4. Rock mass description
  5. Ground water

Rock Material Description

1. Rock Type

The rock type is defined by the origin of the rock (i.e., sedimentary, metamorphic, igneous), color (including whether light or dark minerals predominate), texture or fabric ranging from crystalline, granular, or glassy, and grain size ranging from boulders to silt/clay size particles.

2. Wall Strength

The compressive strength of the rock forming the walls of discontinuities will influence shear strength and deformability. Rock compressive strength categories and grade vary from extremely strong (> 250 MPa grade R6) to extremely weak (0.25 to 1 MPa grade R0) (see Table-1).

Table-1 (Rock Material Strength)
Grade Description Field Identification Range of Uniaxial Compressive Strength (MPa)
R0 Extremely weak rock Indented by thumbnail 0.25 – 1.0
R1 Very weak rock Crumbles under firm blows with point of geological hammer; can be peeled by a pocket knife 1.0 – 5.0
R2 Weak rock Can be peeled by a pocket knife with difficulty; shallow indentations made by firm blow with point of geological hammer 5.0 – 25
R3 Medium strong rock Cannot be scraped or peeled with a pocket knife; specimen can be fractured with single firm blow of geological hammer 25 – 50
R4 Strong rock Specimen requires more than one blow of geological hammer to cause fracture 50 – 100
R5 Very strong rock Specimen requires many blows of geological hammer to cause fracture 100 – 250
R6 Extremely strong rock Specimen can only be chipped with geological hammer > 250

3. Weathering

Reduction of rock strength due to weathering will reduce the shear strength of discontinuities as well as reduce the shear strength of the rock mass due to the reduced strength of the intact rock. Weathering categories and grades are summarized in Table-2.

Table-2 (Weathering Grades)
Term Description Grade
Fresh No visible sign of rock material weathering; slight discoloration on major discontinuity surfaces is possible I
Slightly weathered Discoloration indicates weathering of rock material and discontinuity surfaces. All the rock material may be discolored by weathering and the external surface may be somewhat weaker than in its fresh condition. II
Moderately weathered Less than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as core stones. III
Highly weathered More than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as core stones. IV
Completely weathered All rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact. V
Residual soil All rock material is converted to soil. The mass structure and material fabric are destroyed but the apparent structure remains intact. There may be a large change in volume, but the soil has not been significantly transported. VI

 

Discontinuity Description

4. Discontinuity Type

The discontinuity type range from smooth tension joints of limited length to faults containing several centimeters of clay gouge and lengths of many kilometers. Discontinuity types include faults, bedding, foliation, joints, cleavage, and schistosity.

5. Discontinuity Orientation

The orientation of discontinuities is expressed as the dip and dip direction of the surface. Alternatively, the discontinuity can be represented by strike and dip. The dip of the discontinuity is the maximum angle of the plane to the horizontal (angle ψ in Fig-1) and the dip direction is the direction of the horizontal trace of the line of dip, measured clockwise from north (angle α in Fig-1).

Fig-1- Illustration of 13 parameters used for describing rock mass characteristics
Fig-1- Illustration of 13 parameters used for describing rock mass characteristics

6. Roughness

Roughness should be measured in the field on exposed surfaces with lengths of at least 2 m. The degree of roughness can be quantified in terms of the Joint Roughness Coefficient (JRC) as described below. Wall roughness is an important component of shear strength, especially in the case of undisplaced and interlocked features (e.g., unfilled joints).

Fig-2 Range of JRC values
Fig-2 Range of JRC values

The roughness of the fracture surface is defined by the joint roughness coefficient, JRC. Barton carried out direct shear tests on a large number of natural discontinuities and calculated JRC values corresponding to the surface roughness of the different shear test specimens. From these tests a set of typical roughness profiles with specified JRC values were prepared (Fig-2). By comparing a fracture surface with these standard profiles, the JRC value can be evaluated.

7. Aperture

Aperture is the perpendicular distance separating the adjacent rock walls of an open discontinuity (thereby distinguishing it from the width of a filled discontinuity), in which the space is air or is water filled. Categories of aperture range from cavernous (> 1 m) to very tight (< 0.1 mm).

Infilling

8. Infilling Type and Width

Infilling is the term for material separating the adjacent walls of discontinuities such as fault gouge; the perpendicular distance between adjacent rock walls is termed the width of the filled discontinuity. Filled discontinuities can demonstrate a wide range of behavior and thus their affect on shear strength and deformability can vary widely.

Rock Mass Description

9. Spacing Discontinuity

Spacing can be mapped in rock faces and in drill core; spacing categories range from extremely wide (> 6000 mm) to very narrow (< 6 mm). The spacing of individual discontinuities has a strong influence on the mass permeability and seepage characteristics of the rock mass.

10. Persistence

Persistence is the measure of the continuous length or area of the discontinuity; persistence categories range from very high (> 20 m) to very low (< 1 m). This parameter is used to define the size of blocks and the length of potential sliding surfaces. Persistence is important in the evaluation of tension crack development behind the crest of a slope.

11. Number of Sets

The number of sets of discontinuities that intersect one another will influence the extent to which the rock mass can deform without failure of the intact rock. As the number of sets increases and the block sizes reduce, the greater the likelihood for blocks to rotate, translate, and crush under applied loads.

12. Block Size and Shape

The block size and shape are determined from the discontinuity spacing, persistence, and number of sets. Block shapes include blocky, tabular, shattered and columnar, while block size ranges from very large (> 8 m3) to very small (< 0.0002 m3).

Ground Water

13. Seepage

Observations of the seepage from discontinuities should be provided. Seepage quantities in unfilled discontinuities range from very tight and dry to continuous flow. Seepage quantities in filled discontinuities range from dry in heavily consolidated infillings to filling materials that are washed out completely and very high water pressures are experienced.

Example

Example Description-1

Using the terms described in table 36 and the first category in figure 93, a typical rock material description would be as follows (Wyllie, 1999):

slightly weathered, crystalline, gray, fine grained, medium strong basalt

Example Description-2

An example of a rock mass description using table 36 and the four remaining categories in figure 93 would be as follows:

Columnar jointed basalt with vertical columns and one set of horizontal joints, spacing of vertical joints is very wide, spacing of horizontal joints wide, joint lengths are 3 to 5 m vertically, and 0.5 to 1 m horizontally; the discontinuity infilling is very soft clay with widths of 2 to 5 mm. The vertical columnar joints are smooth, while the horizontal joints are rough. No seepage observed.

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