Section 3: Field Description of Rock Mass in Engineering

Practice.

Practical 4: Systematic Description of Rocks and Rock Discontinuity.

Introduction

• Rock Mass should be described first as a rock material, and its includes: – Colour.– Texture + grain size.– Structure (fabric).– Weathering state.– Rock group name and specific geological name.– Engineering properties + strength.

• The shear strength of a rock mass and its deformability are very much influence by discontinuity pattern, it geometry and how it is developed.

• Important factor in rock stability:– Number of set of discontinuities which intersect.– Number of set also effect the degree of overbreak.

Introduction (cont…)

• The description of jointed rock masses base on:– Incidence of discontinuity.– Geometry of discontinuity.– Surface of discontinuity.– Flow of water in discontinuity.

Introduction (cont….)

Discontinuities

• Discontinuities are fracture and plane of weakness in the rock mass.

• Tensile strength reaches minimum value across discontinuities.

• Joint frequently appear as parallel sets of joint planes (called joint set).

• Single faults may be encountered in a rock mass, but seldom more than one.

• The data needs for each discontinuity:– Continuous or not.– Spacing (Table 4.1 pg 83)– Nature of discontinuity.– Attitude of discontinuity (horizontal, vertical,

inclined).

Discontinuities (cont…)

Discontinuities (cont…)

Discontinuities (cont…)

• Terminology: – Type of discontinuity:• Fault, vein, joint, fracture, cleavages, bedding plane,

etc.

– Number of discontinuity set. – Location and orientation – scan line + strike & dip.– Spacing between adjacent discontinuities.– Aperture of discontinuity surfaces (Table 4.2 pg

84)– Infilling – clay, quartz, etc.

Discontinuities (cont…)

• Terminology (cont…):– Persistence or the extent – continuous or not.– Nature of discontinuity surface – Table 4.3 pg 84.– Discontinuity spacing in 3-D – Table 4.4 pg 85 and

Figure 4.4 pg 86.• Blocky• Tabular• Columnar

– Weathered and alteration conditions – Table 4.5 pg 86.• Weathering grade I, II, III, IV, V or VI.

Discontinuities (cont…)

Discontinuities (cont…)

Discontinuity Survey

Direct Discontinuity Survey Using Scan Line

2 types of Discontinuity Survey: Direct and Drill holes.

Streographic Representation

Pic: University of Maryland, Department of Geology

One of Kajang Housing Development area: Ehsan from SEADPRI UKM

• Plane failure: 1 plane failure occur at the discontinuity line of______

outside the slope face in the direction of _____N

• Wedge failure: 1 wedge failure occur at the intersection of 2 discontinuity

line of line _______and line _______ outside the slope face in the direction of ________N.

• Toppling failure: 1 toppling failure occur at the discontinuity line of _________

inside the slope face in the direction of _______N.

Failure:

• Draw the streonet: (Try exercise 4.2 pg 105)1. Draw the circle.2. Indicate the north pole.3. Draw the slope face: given 358o/45o.4. Mark the strike / facing direction = 358o.5. Rotate the tracing paper:

If direction ≥ 180o, rotate it to west position (270o). If direction ≤ 180o, rotate it to east position (90o).

6. Count from outside into inside: for angle 45o.7. Draw the line from 0o to 180o along that side, using solid

line.8. Rotate the tracing paper back to datum: North.

Streographic Representation (cont..)

9. Draw the discontinuity data set: given 87o/80o.10. Mark the strike / dip direction = 87o.11. Rotate the tracing paper:

If direction ≥ 180o, rotate it to west position (270o). If direction ≤ 180o, rotate it to east position (90o).

12. Count from outside into inside: for angle 80o.13. Draw the line from 0o to 180o along that side, using

discontinuous line.14. Rotate the tracing paper back to datum: North.15. Repeat procedure 9 to 14 for the rest.

Streographic Representation (cont..)

Points to remember:• Never write on the projection itself; always

write on the tracing paper.• Use pencil, not pen.• Make notes on the tracing paper as you go.• When rotating, always use the normal to

plane.

Streographic Representation (cont..)

Exercise 1: Slope Stability• Slope Face = 162/68. Please identify the possible

failure.• Discontinuity readings are:

162/58 68/86 322/69 123/57 300/56

98/76 106/80 126/54 93/72 183/74

216/48 16/62 100/57 64/70 78/56

93/56 165/65 81/56 284/28 108/58

144/72 182/74 162/60 337/87 154/88

176/84 264/82 178/68 274/32 128/38

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