HOW TO DESCRIBE ROCK MASS CHARACTERISTICS ON SITE?

Standardized geologic mapping and logging procedures should be used to describe rock mass characteristics. The type of information collected will depend on the accessibility of the site, the extent of the rock outcrops and the severity of the proposed structure to be constructed on or over the rock mass. A method proposed by the International Society of Rock Mechanics (ISRM) provides standardized quantitative and qualitative information on rock mass. This and other rock mass classification systems are described in ASTM D 5878. To introduce and explain the ISRM method, a number of figures and tables were produced, mainly to provide a standardized definition of the terms. Figure 1 provides an illustration of a rock mass and 13 Parameters Which are included in a detailed rock description. These 13 parameters can be divided into 5 categories such as

  1. rock material description
  2. continuation statement
  3. Fill
  4. rock mass description
  5. ground water

rock material description

1. rock type

Rock type is defined by the rock’s origin (i.e., sedimentary, metamorphic, igneous), color (including light or dark minerals), crystalline, granular, or glassy to texture or fabric, and stone to grain size. . For silt/clay sized particles.

2. wall strength

The compressive strength of the rock forming the walls of the discontinuity will affect the shear strength and deformation. Rock compressive strength categories and grades vary from extremely strong (>250 MPa grade.) R6) extremely weak (0.25 to 1 MPa grade.) R0) (See Table 1,

Table-1 (Strength of Rock Material)
grade Description area identification Range of axial compressive strength (MPa)
R0 very weak rock indent by thumbnail 0.25 – 1.0
R1 very weak rock The geodesics crumble under strong blows from the tip of the hammer; Can be peeled with pocket knife 1.0 – 5.0
R2 weak rock can barely be peeled off with a pocket knife; shallow indentation 5.0 – 25
R3 medium strong rock Cannot be scraped or peeled with a pocket knife; The specimen can be fractured with a single firm strike of the geological hammer. 25 – 50
R4 strong rock More than one stroke of the geological hammer is required to cause the specimen to fracture. 50 – 100
R5 very strong rock Specimens require multiple blows of a geological hammer to cause fracture 100 – 250
R6 very strong rock The sample can only be chipped with a geological hammer >250

3. weathering

A decrease in the strength of the rock due to weathering will reduce the shear strength of the discontinuity as well as reduce the shear strength of the rock mass due to a decrease in the strength of the intact rock. Weathering categories and grades are summarized in Table 2,

Table-2 (Weather Grade)
Duration Description grade
fresh no visible sign of rock material weathering; Minor discoloration possible on major disassembly surfaces I
little weathering Discoloration indicates weathering of rock material and discontinuity surfaces. All rock material may be discolored by weathering and the outer surface may be somewhat weaker than in its fresh state. Second
moderate weathering Less than half the rock material in a soil is decomposed and/or decomposed. Fresh or discolored rock exists either as a discontinuous structure or as core stones. third
excessive weathering More than half of the rock material is decomposed and/or decomposed into a soil. Fresh or discolored rock exists either as a discontinuous structure or as core stones. IV
fully weathered All rock material decomposes and/or decomposes into soil. The original mass composition is still largely intact. v
residual soil All rock material is converted into soil. The mass structure and material fabric are destroyed but the apparent structure remains intact. There may have been a large change in volume, but the transport of the soil has not occurred significantly. Sixth

continuation statement

4. discontinuity type

Dissection types range from smooth stress joints of limited length to faults with a soil gauge of several centimeters and a length of several kilometers. Types of imbalance include doshas, ​​beds, leaves, joints, cracks and schisms.

5. discontinuity orientation

The orientation of discontinuity is expressed as the dip and dip direction of the surface. Alternatively, discontinuity can be represented by strike and dip. The dip of discontinuity is the maximum angle of the plane with the horizontal (angle .) I In Figure 1) and the direction of dip is the direction of the horizontal mark of the line of dip measured clockwise from north (angle .) α In Figure 1,

Figure 1 - Illustration of 13 parameters used to describe rock mass characteristics
Figure 1 – Illustration of 13 parameters used to describe rock mass characteristics

6. rudeness

Roughness should be measured in open ground with a length of at least 2 m. The amount of roughness can be quantified in terms of the joint roughness coefficient (JRC) described below. Wall roughness is an important component of shear strength, especially in the case of non-displaced and interconnected features (eg, unfilled joints).

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

The roughness of the fracture surface is defined by Joint Roughness Coefficient, JRC. Barton performed direct shear tests on a large number of natural discontinuities and calculated JRC values ​​corresponding to the surface roughness of various shear test specimens. From these tests a set of specific roughness profiles with specified JRC values ​​was produced (line drawing number 2) By comparing a fracture surface with these standard profiles, the JRC value can be evaluated.

7. hole

The aperture is the vertical distance separating the adjacent rock walls of an open discontinuity (whereby it is separated by the width of a filled fissure), in which the space is air or waterlogged. The ranges of aperture range from cavernous (>1 m) to very tight (<0.1 mm).

Fill

8. Fill type and width

Infilling is the term for the material that separates adjacent walls such as fault gouges; The vertical distance between adjacent rock walls is called the width of the filled discontinuity. Filled discontinuities can exhibit a wide range of behavior and thus their effect on shear strength and deformation can vary widely.

rock mass description

9. spacing discontinuity

Spacing can be mapped to rock faces and drill cores; Spacing ranges range from extremely wide (>6000 mm) to very narrow (<6 mm). The distance between the individual gaps has a strong effect on the mass permeability and leakage characteristics of the rock mass.

10. stubbornness

Persistence is the measure of a constant length or region of discontinuity; The hardiness ranges range from very high (>20 m) to very low (<1 m). This parameter is used to define the size of the blocks and the length of the possible sliding surfaces. Persistence is important in the evaluation of stress crack development behind slope crests.

1 1 number of sets

The number of sets of fissures that intersect each other will affect the extent to which a mass of rock can deform without failure of the intact rock. As the number of sets increases and the block size decreases, the greater the potential for the block to rotate, translate and crush under the applied load.

12. block size and shape

The size and shape of the block is determined by the disassembly spacing, persistence, and number of sets. Block sizes include blocky, tabular, shatter and columnar, while block sizes range from very large (>8 m3) to very small (<0.0002 m3).

ground water

13. seepage

Observations of leakage from dissection should be provided. The amount of seepage in unfilled discontinuities ranges from very tight and dry to a continuous flow. The amount of seepage into filled blockages ranges from dry to heavily consolidated fills that tend to fill completely washed material and experience very high water pressure.

Example

Example Statement-1


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

Slightly weathered, crystalline, grey, fine-grained, moderately strong basalt

Example Statement-2

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

Columnar joint basalt with a set of vertical columns and horizontal joints, the spacing of the vertical joints is very wide, the spacing of the horizontal joints is wide, the joint length is 3 to 5 m vertically, and 0.5 to 1 m horizontally; There is very soft soil with a width of 2 to 5 mm. Vertical column joints are smooth, while horizontal joints are rough. No leakage was observed.


Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.