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Abstract
Rock mass usually refers to natural geological rock mass within a certain engineering range which undergoes various geological processes and retains permanent deformation and geological structure traces under the long-term effect of ground stress.[1] In a narrow sense, rock mass is composed of rocks (rock blocks) and discontinuous/structural planes. The structure plays a major role in controlling the mechanics and deformation characteristics of rock mass. Therefore, the properties of the structural plane must be considered in the engineering classification of rock mass.
Characteristics of rock mass
Rock mass usually refers to natural geological rock mass within a certain engineering range which undergoes various geological processes and retains permanent deformation and geological structure traces under the long-term effect of ground stress.[1] In a narrow sense, rock mass is composed of rocks (rock blocks) and discontinuous/structural planes. The structure plays a major role in controlling the mechanics and deformation characteristics of rock mass. Therefore, the properties of the structural plane must be considered in the engineering classification of rock mass.
3.1 Discontinuities in rock mass
The structural plane refers to the planar geological interface formed in the rock mass during the development of geological history. It has a certain extension direction and length and is relatively thin. The tensile strength of the structural plane is close to zero.[2–3] The structural plane includes material differentiation surfaces and discontinuous surfaces such as layers, unconformities, joints, faults and schistosity.
Structural planes are classified into three types according to geological origin: primary, tectonic and secondary structural planes (Table 3.1).
Primary structural planes: Primary structural planes are formed by rock mass in the diagenetic stage. According to the different genesis of rocks, this plane can be classified into sedimentary, magmatic and metamorphic structural planes. The sedimentary rock structural plane is the geological interface formed during diagenesis, and it includes interlayer structural planes such as layers, beddings, inter-deposited sections (unconformity and disconformity surfaces) and primary weak interlayers. The magmatic structural plane is formed by magma intrusion and effusion condensation (e.g. contact surfaces between (a) immersion body and the surrounding rock and (b) dike and the original condensation joint). The metamorphic structural plane is the structural plane formed during the metamorphism of rock mass, and it includes morphological and weak schist interlayers.
Tectonic structural plane: Cleavage, joints, faults and interlayer displacements make up the tectonic structural plane which refers to the plane formed by rock mass under tectogenesis. Cleavage is a planar structure in which the rock can be split into a number of thin slices in a certain direction. The relatively small scale of joints is divided into tension and shear joints depending on the causes of the mechanics. Fault is the discontinuity in which a rock breaks under crustal stress whilst a relative displacement occurs between two fractured blocks. The direction of the displacement is generally parallel to the discontinuous surface. Most faults are formed by shearing, and tectonic rocks are generally present in fault zones. The interlayer fault zone is a structural plane commonly found in layered rock masses with an occurrence that is generally consistent with the rock formation.
Table 3.1 Classification of structural planes in rock mass[2]
|
Genetic type |
Geological type |
Main features |
Engineering geology evaluation |
|||
|
Occurrence |
Distribution |
Characteristic |
||||
|
Primary structural plane |
Sedimentary structural plane |
Bedding and level; weak intercalation; unconformity and disconformity surfaces; sedimentary discontinuity |
Generally consistent with the occurrence of rock formations; interlaminar structural plane |
Such marine structural plane formation exhibits stability; the terrestrial type is arranged in a staggered formation for easy pinching. |
Structural planes such as layers and weak interlayers are relatively flat; the unconformity and sedimentary sections are mostly composed of clastic mud and are uneven. |
Dam sliding and landslides, such as the destruction of the Austin Dam, St. Francis Dam and Malpasai Dam and the massive landslides near the Wayi Reservoir, are caused by such structural planes. |
|
Magmatic structural plane |
The contact surface between the intrusion and the surrounding rocks; the contact between the vein and the rock wall; primary condensation joints |
The rock vein is controlled by the tectonic plane whilst the primary joint is controlled by the contact surface of the rock mass. |
The contact surface extends farther and is more stable, whereas the original joints tend to be short and dense. |
The contact surface with the surrounding rock can have two different characteristics of fusion and fracture. The original joint is generally a cracked surface which is rough and uneven. |
Generally, it does not cause large-scale rock mass damage, but sometimes, it can form a slip of the rock mass if it cooperates with the structural fault, such as some local slip of the abutment. |
|
|
Metamorphic structural plane |
Schistosity |
Appearance is consistent with rock formation or structural direction. |
The film is short, and the distribution is extremely dense. The weak schist interlayer extends farther and has a fixed level. |
The structural plane is smooth and straight, and the schistosity is often closed into a hidden structural plane in the deep part of the rock. The weak schist interlayer has flaky minerals and is scaly. |
In shallow metamorphic sedimentary rocks, such as the landslides of ridges and other common slopes, the schist interlayer has an impact on the engineering and stability of underground caves. |
|
|
Tectonic structural plane |
Joint (X-shaped joint, split joint) fault (rushing fault, transverse fault); interlayer shift; feather-like fissures; cleavage |
The occurrence is related to the tectonic line, and the interlayer displacement is consistent with the stratum. |
The tensile fracture is short, the shear fracture extends farther, and the scale of the compressive fracture is huge, but sometimes, the transverse fault is cut into discontinuities. |
The tensile fracture is uneven, often with secondary filling, and is serrated; shear fracture is relatively straight with pin-shaped fissures. Compressive faults have a variety of tectonic rocks which are distributed in strips that often contain fault mud and mylonite. |
It has a great influence on the stability of rock mass. Most of the rock mass failure processes mentioned have the coordination effect of structural planes and often cause collapse and roof collapse of slopes and underground works. |
|
|
Secondary structural plane |
Unloading cracks; differentiating fissures; differentiated interlayers; mud interlayers; secondary mud layers |
It is controlled by topography and the original structural plane. |
The distribution tends to be discontinuous, lenticular, poorly ductile and mainly developed in the surface differentiation zone. |
Generally filled with muddy materials; the water–physical property is poor. |
Hazards occur on natural and artificial slopes, sometimes affecting dam foundations, dam shoulders and shallow tunnels, but they are generally treated in the foundation during construction. |
|
Secondary structural plane: The secondary structural plane refers to the structural plane formed by the external force (e.g. weathering, groundwater and unloading) after the formation of rock mass. This layer includes unloading and weathering fissures, secondary clay layers and muddy interlayers. Unloading fissures occur mostly on the surface of rock masses with free-form surface conditions with common ductility, especially on the banks of deep-cutting gorges. Weathered fissures usually develop along the original interlayers and the original structural plane. These short and dense layers have poor ductility and are limited to a certain depth. The muddy interlayer is formed by the action of water which makes the soft material muddy in the interlayer. The occurrence of the muddy interlayer is similar to that of rock formation, and the degree of muddiness varies depending on groundwater conditions.
The secondary structural plane can be divided according to the extension length of the structural plane, depth of the cut, width of the fracture zone and its own mechanical effects.[2,4,5]
Level I structural plane mainly refers to large or regional faults which generally extend from several kilometres to tens of kilometres, with the fracture bandwidth ranging from several metres to tens or even several hundred metres. The level I structural plane is a large-scale weak structural plane that constitutes an independent medium mechanical unit. Some regional large faults are often associated with modern activities that are related to the stability of the crust in the construction area and thus bring great potential damage to the construction. Construction projects should avoid level I structural planes by implementing effective measures.
Level II structural plane refers to a regional geological interface extending from a few hundred metres to several kilometres with a narrow width (several centimetres to several metres), such as interlayer displacement, unconformity and primary weak interlayer. This structural plane is a weak structural plane that can form a block-fracture boundary which controls several factors that affect project layout, such as the stability of the mountain in the construction area and the deformation and failure mode of the rock mass.
Level III structural plane refers to faults, regional joints, well-extended layers and interlayer shifts with lengths ranging from tens of metres to hundreds of metres and widths ranging from a few centimetres to about one metre. Most level III structural faces belong to hard structural planes, whereas a few belong to weak structural planes. This plane mainly affects or controls the stability of engineering rock masses; it forms block damage of different scales when combined with Levels I and II structural faces.
Level IV structural plane refers to joints, layers, secondary fractures, small faults and developed morphological and physiologic surfaces. The length is generally tens of centimetres to twenty or thirty metres (maximum of ten centimetres for small ones), and the widths range from zero to several centimetres. This plane can form the boundary of rock blocks, destroy the integrity of rock masses and affect the physical and mechanical properties of rock masses and the state of stress distribution. This level includes numerous structural planes with random distribution, thereby affecting the integrity and mechanical properties of rock masses. Aside from being the main content of rock mass classification and structural research, Level IV structural plane is also the key input in the statistical analysis and simulation of structural planes.
Level V structural plane, which is also called the microstructure plane, comprises hidden joints, microlayers, micro-fissures and undeveloped cleavages and textures. The small size, poor continuity and random distribution of this layer reduces the strength of rock mass, mainly affecting the mechanical properties of rocks. If the distribution is dense, rocks can become loose media under weathering.
3.2 Description of rock mass
A rock mass is often described according to strength, color, structure, degree of weathering, name, structural plane and other geological information. Prior to the description, the rock mass should be first divided into units with consistent engineering characteristics. Rock type, weathering degree and structural plane characteristics are usually adopted as the basis for the boundary selection of a rock mass unit. Information about rock mass characteristics should contain the description of geological structures, characteristics of discontinuities and weathering characteristics.[6]
3.2.1 Structure of rock mass
Structure of rock mass refers to the macroscale structure. In geology, construction generally encompasses faults and folds which are often described as large-scale rock structures. Engineers are often concerned about the structural features of small rock masses. Sedimentary rocks are also referred to as ‘layered’, ‘thin-plated’ and ‘blocky’. For magmatic (and volcanic clastic rocks) and metamorphic rocks, other terms include ‘blocky’ and ‘flow-like’, ‘leaf-shaped’, ‘band-shaped’ and ‘cleavage-shaped’. Construction spacing can be classified as very thick, thick, moderate, thin and very thin.
3.2.2 Characteristics of structural plane
A comprehensive description of the structural plane contains information about occurrence, spacing, continuity, roughness, aperture, filling characteristics and permeability. The survey of structural planes can be divided into two levels: (1) subjective surveys, which only describe the structural planes exerting a significant impact on the project, and (2) objective surveys, which describe all structural planes that intersect the baseline or are located within the defined area.
1 Occurrence
The appearance of a structural plane can be expressed by the dip and dip angle. A dip is represented by a clockwise angle from the true north direction of the structural plane’s tilting direction, whereas a dip angle is expressed by the maximum inclination angle of the structural plane measured in the horizontal direction. Dips and dip angles are commonly measured using compasses and inclinometers. The general expression of the appearance is ‘dip direction∠dip angle’.
2 Spacing
The spacing of structural planes refers to the average spacing in the normal direction of a set of structural planes. The description standards recommended by the International Society of Rock Mechanics are shown in Table 3.2.
Table 3.2 Description of spacing between structural planes[7]
|
Spacing |
Descriptive term |
|
<20 mm |
Extremely close-spaced |
|
20–60 mm |
Very close-spaced |
|
60–200 mm |
Close-spaced |
|
200–600 mm |
Medium-spaced |
|
600–2000 mm |
Wide-spaced |
|
2000–6000 mm |
Very wide-spaced |
|
>6000 mm |
Extremely wide-spaced |
3 Continuity
Continuity, which is basically the size of a structural plane, is a crucial description of structural planes. Obtaining the quantity of the three-dimensional shape of a structural plane is difficult. Continuity can only be approximated by measuring the length of the structural plane’s traces (i.e. the rock mass exposed to the surface). In the description, specifying whether the structural plane terminates inside the rock or in other structural planes is necessary.
4 Roughness
The roughness of a structural plane has two parts: corrugation and unevenness. Corrugation, or the fluctuation of structural planes within large-scale ranges (tens of metres long), can be evaluated using wavelength and amplitude. Unevenness refers to the surface irregularities of a structural plane within small-scale ranges (few centimetres to a few metres), as described by Barton’s 10-level standard.[8]
5 Aperture
The degree of aperture is the vertical distance between the rock walls on both sides of an open structural plane. It is usually caused by tensile stress, rinsing and dissolution of filler materials or shearing of a substantially rough structural plane. This characteristic determines the shear strength and hydraulic conductivity of a structural plane. The classification for aperture degrees is shown in Table 3.3.
6 Characteristics of filling materials
This characteristic describes the filling materials within structural planes. In most cases, parts of the filling materials of a structural plane contains foreign materials which are formed because of the intense weathering in the structural plane. The mechanical strength of the filling material is generally weaker than that of the parent rock. Typical filling materials are soil, weathered or decomposed rock minerals (e.g. quartz, calcite, manganese or kaolin) and breccia formed at the faults or shear zones.
The mineral type, particle size and strength of the filling material should be comprehensively described. The width (maximum, minimum and average), moisture and permeability of the filling material should also be stated.
Table 3.3 Classification of aperture
|
Descriptive term |
Aperture distance between discontinuity walls |
|
Wide |
>200 mm |
|
Moderately wide |
60–200 mm |
|
Moderately narrow |
20–60 mm |
|
Narrow |
6–20 mm |
|
Very narrow |
2–6 mm |
|
Extremely narrow |
0–2 mm |
|
Tight |
0 mm |
7 Permeability
The permeability assessment of structural planes is important in engineering practice. The structural plane’s permeability is often described as dry, wet (without flowing water) or with existing flow. For the case of existing flow, the amount of flow and flow rate should be recorded. The observation date should also be noted to distinguish dry and rainy seasons.
8 Fracture state
Core fracture can be described using the following parameters: total core recovery, solid recovery, rock quality index and fracture index. ‘Solid core’ is the key term in assessing the fracture state. It refers to the core between two natural structural planes. The entire core is not necessarily cylindrical, but at least one complete circular cross section should exist along the axial direction.
Total core recovery is the percentage of the recovered core taken from a certain footage per roundtrip regardless of whether the cross section is complete, partially complete or incomplete.
The rock quality designation (RQD) is the percentage of the total length of the solid core (for lengths greater than 100 mm) per footage per round trip.
The fracture index (FI) refers to the number of structural planes that can be clearly identified per metre of the core. It is measured over the length of the core with a uniform fracture distribution. If the fracture frequency of the core taken from a certain footage per round trip is significantly changed, the fracture index of each part should be separately calculated. When the core is fragmented, FI is described as ‘incomplete’.
9 Rock mass weathering
From the surface to deeper grounds, the weathering intensity of a rock declines. By determining the weathering zone of the rock mass, the rock mass is classified according to the volumetric proportion of the weathering zone. However, the contour of the weathering zone cannot be accurately determined because the three-dimensional features of the rock mass are difficult to obtain. In practice, only the volume percentage can be estimated.
10 Additional information
When describing rock masses, other information that can help engineers to understand the properties of the rock mass should be recorded. Examples are the geometry of the pores in the carbonate rock, surrounding structural plane, groundwater condition and permeability characteristics.
3.3 Engineering classification of rock mass
In engineering, the purpose of rock mass classification is to comprehensively analyse the geological conditions that affect the stability and the physical and mechanical properties of rock masses. The classification divides rock masses into several categories with different degrees of stability to guide planning, design and construction. Two examples of internationally applied engineering classification systems of rock masses are the rock mass rating (RMR) system proposed by Bieniawski (1974, 1984) and the tunnelling quality index (Q) system proposed by Barton et al. (1974).
3.3.1 RMR system
The RMR classification system, also known as the geo-mechanical classification, was proposed by Professor Z.T. Bieniawski (1973) in South Africa. This system is mainly designed for tunnels, mining and foundation construction. The system is developed on the basis of rock strengths (Table 3.4), drill core quality (Table 3.5), joint fracture spacing (Table 3.6), joint characteristics (Table 3.7) and groundwater conditions (Table 3.8). RMR increments corresponding to each parameter are summed to determine the total RMR.
Table 3.4 Rating of rock strength in RMR
|
Point load strength (MPa) |
Uniaxial Compressive Strength (MPa) |
Rating |
|
>10 |
>250 |
15 |
|
4–10 |
100–250 |
12 |
|
2–4 |
50–100 |
7 |
|
1–2 |
25–50 |
4 |
|
Not applicable |
10–25 |
2 |
|
3–10 |
1 |
|
|
<3 |
0 |
Table 3.5 RQD ratings
|
RQD (%) |
90–100 |
75–90 |
50–75 |
25–50 |
<25 |
|
Rating |
20 |
17 |
13 |
8 |
3 |
Table 3.6 RMR rankings of joint spacing (most influential set)
|
Joint spacing (m) |
>2.0 |
0.6–2.0 |
0.2–0.6 |
0.06–0.2 |
<0.06 |
|
Rating |
20 |
15 |
10 |
8 |
5 |
Table 3.7 Ratings of joints in RMR
|
Description |
Rating |
|
Very rough surfaces of limited extent; hard wall rock |
30 |
|
Slightly rough surfaces; aperture less than 1 mm; hard wall rock |
25 |
|
Slightly rough surfaces; aperture less than 1 mm; soft wall rock |
20 |
|
Smooth surfaces, l–5 mm-thick gouge filling or aperture of 1–5 mm; joints extend more than several metres |
10 |
|
Open joints filled with more than 5 mm of gouge or open by more than 5 mm; joints extend more than several metres |
0 |
Table 3.8 Rating of groundwater condition in RMR
|
Inflow per 10 m tunnel length (L·min−1) |
Joint water pressure divided by major principal stress |
General condition |
Rating |
|
None |
0 |
Completely dry |
15 |
|
<10 |
<0.1 |
Damp |
10 |
|
10–25 |
0.1–0.2 |
Wet |
7 |
|
25–125 |
0.2–0.5 |
Dripping |
4 |
|
>125 |
>0.5 |
Flowing |
0 |
Table 3.9 Modification of RMR values for joint orientations
|
Assessment of influence of orientation on the work |
Rating increment for tunnels |
Rating increment for foundations |
|
Very favourable |
0 |
0 |
|
Favourable |
−2 |
−2 |
|
Fair |
−5 |
−7 |
|
Unfavourable |
−10 |
−15 |
|
Very unfavourable |
−12 |
−25 |
Table 3.10 RMR classification of rock mass
|
RMR |
100–81 |
80–61 |
60–41 |
40–21 |
21< |
|
Class |
I |
II |
III |
IV |
V |
|
Description |
Very good rock |
Good rock |
Fair rock |
Poor rock |
Very poor rock |
Bieniawski (1973) recommended adjusting the sum of the first five ratings to account for favourable or unfavourable orientations because the orientation of joints relative to the work can affect the behavior of the rock.[9] Table 3.9 illustrates the adjustments to account for the adverse effects of joints and fissures on the stability of rock masses. The modified RMR places the rock in one of the five categories defined in Table 3.10.
3.3.2 Q system
The Q system is a tunnel excavation quality classification system proposed by the Norwegian Institute of Geotechnical Engineering (Barton et al., 1974). The classification parameters of the Q system are similar to those of the RMR system. The Q value can be expressed as
Table 3.11 Values of J n [10]
|
Joint set number |
Jn |
|
A. Massive, no or few joints |
0.5–1 |
|
B. One joint set |
2 |
|
C. One joint set plus random joints |
3 |
|
D. Two joint sets |
4 |
|
E. Two joint sets plus random joints |
6 |
|
F. Three joint sets |
9 |
|
G. Three joint sets plus random joints |
12 |
|
H. Four or more joint sets, random, heavily jointed, ‘sugar-cube’, etc. |
15 |
|
J. Crushed rock, earth-like |
20 |
Table 3.12 Values of J r [10]
|
Joint roughness number |
Jr |
|
(a) Rock–wall contact | |
|
(b) Rock–wall contact before 10 cm sear | |
|
A. Discontinuous joints |
4 |
|
B. Rough or irregular, undulating |
3 |
|
C. Smooth, undulating |
2 |
|
D. Slicken-sided, undulating |
1.5 |
|
E. Rough or irregular, planar |
1.5 |
|
F. Smooth, planar |
1.0 |
|
G. Slicken-sided, planar |
0.5 |
|
(c) No rock–wall contact when sheared | |
|
H. Zone containing clay minerals thick enough to prevent rock–wall contact |
1.0 |
|
J. Sandy, gravely or crushed zone thick enough to prevent rock–wall contact |
1.0 |
Table 3.13 Values of J a [10]
|
Joint alteration number |
φr (°) |
Ja |
|
(a) Rock–wall contact (no mineral fillings, only coatings) | ||
|
A. Tightly healed, hard, non-softening and impermeable filling (i.e. quartz or epidote) |
- |
0.75 |
|
B. Unaltered joint walls, surface staining only |
25–35 |
1.0 |
|
C. Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc. |
25–30 |
2.0 |
|
D. Silty- or sandy-clay coatings, small clay fraction (non-softening) |
20–25 |
3.0 |
|
E. Softening or low friction clay mineral coatings (i.e. kaolinite or mica, chlorite, talc, gypsum, graphite and small quantities of swelling clays) |
8–16 |
4.0 |
|
(b) Rock–wall contact before 10 cm shear (thin mineral fillings) | ||
|
F. Sandy particles, clay-free disintegrated rock, etc. |
25–30 |
4.0 |
|
G. Strongly over-consolidated non-softening clay mineral fillings (continuous, 5 mm thickness) |
16–24 |
6.0 |
|
H. Medium or low over-consolidation, softening, clay mineral fillings (continuous, but <5 mm thickness) |
8–12 |
8 |
|
J. Swelling clay fillings, i.e. montmorillonite (continuous, but >5 mm thickness). Value of Ja depends on the percentage of swelling clay-sized particles, access to water, etc. |
6–12 |
8–12 |
|
(c) No rock–wall contact when sheared (thick mineral fillings) | ||
|
KLM. Zones or bands of disintegrated or crushed rock and clay (see G, H and J for description of clay conditions) |
6–24 |
6, 8 or 8–12 |
|
N. Zones or bands of silty- or sandy-clay, small clay fraction (non-softening) |
- |
5.0 |
|
OPR. Thick, continuous zones or bands of clay (see G, H and J for description of clay conditions) |
6–24 |
10, 13 or 13–20 |
Table 3.14 Value of J w [10]
|
Joint water reduction factor |
Water pressure (MPa) |
Jw |
|
A. Dry excavations or minor inflow (i.e. <5 L/min locally) |
<0.1 |
1. |
|
B. Medium inflow or pressure, occasional outwash of joint fillings |
0.1–0.25 |
0.66 |
|
C. Large inflow or high pressure in competent rock with unfilled joints |
0.25–1 |
0.5 |
|
D. Large inflow or high pressure, considerable outwash of joint fillings |
0.25–1 |
0.33 |
|
E. Exceptionally high inflow or water pressure at blasting, decaying with time |
>1 |
0.2–0.1 |
|
F. Exceptionally high inflow or water pressure continuing without noticeable decay |
>1 |
0.1–0.05 |
Table 3.15 Value of SRF [10]
|
Stress reduction factor |
SRF |
||
|
(a) Weakness zones intersecting excavation which may cause loosening of rock mass when tunnel is excavated | |||
|
A. Multiple occurrences of weakness zones containing clay or chemically disintegrated rock, very loose surrounding rock (any depth) |
10.0 |
||
|
B. Single weakness zones containing clay or chemically disintegrated rock (depth of excavation ≤50 m) |
5.0 |
||
|
C. Single weakness zones containing clay or chemically disintegrated rock (depth of excavation >50 m) |
2.5 |
||
|
D. Multiple shear zones in competent rock (clay-free), loose surrounding rock (any depth) |
7.5 |
||
|
E. Single shear zones in competent rock (clay-free) (depth of excavation <50 m) |
5.0 |
||
|
F. Single shear zones in competent rock (clay-free) (depth of excavation >50 m) |
2.5 |
||
|
G. Loose, open joints, heavily jointed or ‘sugar cube’, etc. (any depth) |
5.0 |
||
|
(b) Competent rock, rock stress problems | |||
|
σc/σ 1 |
σθ/σc |
SRF |
|
|
H. Low stress, near surface, open joints |
>200 |
<0.01 |
2.5 |
|
J. Medium stress, favourable stress condition |
200–10 |
0.01–0.3 |
1.0 |
|
K. High stress, very tight structure, usually favourable to stability, may be unfavourable for wall stability |
10–5 |
0.3–0.4 |
0.5–2.0 |
|
L. Moderate slabbing after >1 h in massive rock |
5–3 |
0.5–0.65 |
5–50 |
|
M. Slabbing and rock burst after a few minutes in massive rock |
3–2 |
0.65–1 |
50–200 |
|
N. Heavy rock burst (strain-burst) and immediate dynamic deformations in massive rock |
<2 |
>1 |
200–400 |
|
(c) Squeezing rock: plastic flow of incompetent rock under the influence of high rock pressure |
σθ/σc |
SRF |
|
|
O. Mild squeezing rock pressure |
1–5 |
5–10 |
|
|
P. Heavy squeezing rock pressure |
>5 |
10–20 |
|
|
(d) Swelling rock: chemical swelling activity depending on presence of water | |||
|
R. Mild swelling rock pressure |
5–10 |
||
|
S. Heavy swelling rock pressure |
10–15 |
||
The first term of Equation 3.1 is the measure of the joint block sizes, the second factor expresses the shear strength of the block surfaces, and the last factor evaluates the important environmental conditions that influence the behavior of the rock mass. The value of Q ranges from 0.001 to 1000 whilst the quality of the surrounding rock ranges from low to high and can be divided into nine grades (Table 3.16).
Table 3.16 Rock mass classification in Q system[10]
|
Q |
Rock Mass Quality |
|
0.001–0.01 |
Exceptionally poor |
|
0.01–0.1 |
Extremely poor |
|
0.1–1 |
Very poor |
|
1–4 |
Poor |
|
4–10 |
Fair |
|
10–40 |
Good |
|
40–100 |
Very good |
|
100–400 |
Extremely good |
|
400–1000 |
Exceptionally good |
The parameters of the Q system can be converted into RMR parameters and vice versa using the connecting relationship proposed by Barton (1993):
3.4 Strength theory of rock mass
The theory of rock mass strength establishes the criterion for judging rock failure by analysing the stress state of rock mass failure. The empirical models for judging structural plane damage include the joint roughness coefficient–joint compressive strength (JRC-JCS) model, Patton’s law and the Gerrard shear formula.[11] This work focuses on the JRC-JCS model, which is the most widely used model in engineering. Models for determining the overall strength of the rock mass include the Hoek-Brown, Bieniawski and Balmer strength criteria, but this study only discusses the Hoek-Brown strength criterion.
3.4.1 JRC-JCS model
The JRC-JCS model is a formula proposed by Barton (1977) for the shear strength of structural planes on the basis of numerous structural plane shear tests.
1 Determination of JCS
The method for determining JCS depends on the weathering degree of the structural plane. For non-weathered or slightly weathered rock masses, the strength of the structural wall rock is approximately equal to that of the rock mass. JCS can adopt the uniaxial compressive strength or point load strength of the rock sample. If the structural wall rock is weathered, then the rebound value R can be measured by the L-type rebound hammer. In addition, the gravity of rock γ is also measured. The JCS is calculated according to the calculation in Figure 3.1 and Equation (3.4).
2 Determination of ϕb
Barton’s (1977) research showed that the basic friction angle of the rock mass is mostly between 25° and 35°. The basic friction angle can be determined using the tilt test of the flat surface of the structural plane’s wall rock. The method involves obtaining the test rock block from structural plane wall, cutting the block into two halves, removing the rock powder and combining them after air drying. For each type of rock, more than 10 test pieces must be examined. When tilt test data are not available, the residual friction angle of the structural plane (ϕb = 30°) can be adopted.
3 Determination of JRC
Barton suggested that the value of JRC can be determined using the standard contour curve comparison method and direct shear test back calculation. The standard contour curve comparison method is adopted to artificially compare the contour curve of the surface of the structural plane with the standard contour curve to obtain the value of JRC (Figure 3.2). The direct shear test inverse algorithm calculates the value of JRC by performing the direct shear test to obtain the peak shear strength and the basic friction angle.
Figure 3.1 Relationship between JCS, Schmidt hardness and rock density[2]
where ϕ p is the peak shear angle (ϕ p = arctan[τ p/σ n ]).
The common methods used to determine the value of JRC includes fractal theory, statistical parameter method, straight line and trace length ratio method and straight and corrected straight edge method.[12] The following discussions introduce fractal theory.
Joints are measured with different lengths r. The method for measurement is shown in Figure 3.3. If the rock joint has fractal self-similarity, the relationship between the fractal dimension D of the rock joint and the number of measured sizes N can be expressed as
Figure 3.2 Standard joint profiles and the JRC values[4]
Figure 3.3 Fractal method for measuring joint[4]
By transforming r, D and different values of N can be obtained. Li (2015) calculated D using contour curves and their corresponding JRC values and obtained the following empirical formula:[13]
In addition, the JRC-JCS model considers the size effects, which can be corrected as
3.4.2 Hoek-Brown criterion
The Hoek-Brown strength criterion is a rock failure criterion proposed by Hoek and Brown (1988) for underground hard rock excavation. The criterion is based on numerous experiments and studies on the properties of jointed rock masses. The mechanical parameters are provided to the rock mass by reducing the mechanical parameters of the intact rock, which can be expressed as
Table 3.17 Gsi[14]
Table 3.18 Values of m i of various rocks[14]
|
Rock type |
Class |
Group |
Texture |
|||
|
Coarse |
Medium |
Fine |
Very fine |
|||
|
Sedimentary |
Clastic |
Conglomerate (21±3)* Breccia (19±5) |
Sandstone 17±4 |
Siltstone 7±2 Greywacke (18±3) |
Claystone 4±2 Shale (6±2) Marl (7±2) |
|
|
Non-Clastic |
Carbonate |
Crystalline Limestone (12±3) |
Sparitic Limestone (10±2) |
Micritic Limestones (9±2) |
Dolomite (9±3) |
|
|
Evaporite |
Gypsum 8±2 |
Anhydrite 12±2 | ||||
|
Organic |
Chalk 7±2 |
|||||
|
Metamorphic |
Non-foliated |
Marble 9±3 |
Hornfel (19±4) Metasandstone (19±3) |
Quartzite 20±3 | ||
|
Slightly foliated |
Migmatite (29±3) |
Amphibolite 26±6 |
Gneiss 28±5 | |||
|
Foliated |
Schist 12±3 |
Phyllite (7±3) |
Slate 7±4 |
|||
|
Igneous |
Plutonic |
Light |
Granite 32±3 Granodiorite (29±3) |
Diorite 25±5 | ||
|
Dark |
Gabbro 27±3 Norite 20±5 |
Dolerite (16±5) | ||||
|
Hypabyssal |
Porphyria (20±5) |
Diabase (15±5) |
Peridotite (25±5) |
|||
|
Volcanic |
Lava |
Rhyolite (25±5) Andesite 25±5 |
Dacite (25±3) Basalt (25±5) |
Obsidian (19±3) |
||
|
Pyroclastic |
Agglomerate (19±3) |
Breccia (19±5) |
Tuff (13±5) | |||
Table 3.19 Values of D[15]
|
Picture of Rock Masses |
Description of Rock Masses |
Reference value D |
|---|---|---|
|
|
Well-controlled blasting or TBM excavation produces minimal disturbances. |
D=0 |
|
|
Mechanical or manual excavation of rock mass (no blasting) produces minimal disturbance on the surrounding rock mass. The squeezing problem causes a significant floor arching that is severely disturbed unless a temporary transition (as shown in the picture) is available. |
D=0 D=0.5 (No conversion) |
|
|
Poor blasting in hard rock tunnels produces severe local damage (the tunnel profile is extended by 2 or 3 m) and rock masses. |
D=0.8 |
|
|
Small-scale blasting causes severe rock damage in geotechnical slope engineering, especially when controlled blasting is adopted (as shown on the left). However, stress relief can cause certain damage. |
D=0.7 (Good blasting) D=1.0 (Poor blasting) |
|
|
Vast open pit slopes are subject to severe production blasting, and the stresses on the overburden are released, causing disturbances. Some soft rocks can be excavated with little disturbance by crushing and overturning. |
D=1.0 (Production blasting) D=0.7 (Machine excavating) |
where GSI stands for geological strength index and mi is the empirical parameter of the rock dimension (mi = 1) which reflects the softness and hardness of the rock. The range of the mi values is from 0.001 (highly fractured) to 25 (hard and complete rock). Moreover, s reflects the degree of rock fragmentation, with values ranging from 0 to 1 (1 represents a complete rock). D is the disturbance parameter that considers the blasting effect and stress release, and its values range from 0.0 (field undisturbed rock mass) to 1 (disturbed rock mass).
The Hoek-Brown strength criterion can be determined if the rock uniaxial compressive strength (σci), GSI, mi and D are known. σci can be determined through the rock uniaxial compression test, whereas the GSI can be derived from Table 3.17 on the basis of the rock mass structure grade and the surface condition grade (SCR). The range of mi can be obtained from Table 3.18. However, exact mi values must be obtained through laboratory tests, such as uniaxial compression and conventional triaxial compression tests, uniaxial compression and direct tensile tests, uniaxial compression and indirect tensile tests and uniaxial compression and acoustic emission tests. Table 3.19 presents the values for D.
References
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