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8/15/2019 11_Engineering Geology and Soil Mechanics_Chapter 12_Common Usage of Rock and Uncemented Sediment
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From Palmström A.: RMi – a rock mass characterization system for rock engineering purposes.
PhD thesis, Oslo University, Norway, 1995, 400 p.
Chapter 2
ROCK MASSES AS CONSTRUCTION MATERIALS
"Rock masses are so variable in nature that the chance for ever finding a common set of
parameters and a common set of constitutive equations valid for all rock masses is quite
remote."
Tor L. Brekke and Terry R. Howard, 1972
A rock mass is a material quite different from other structural materials used in civil engineering. It
is heterogeneous and quite often discontinuous, but is one of the materials in the earth's crust, which
is most used in man's construction. Ideally, a rock mass is composed of a system of rock blocks and
fragments separated by discontinuities forming a material in which all elements behave in mutual
dependence as a unit (Matula and Holzer, 1978). The material is characterized by shape and
dimensions of rock blocks and fragments, by their mutual arrangement within the rock mass, as well
as by joint characteristics such as joint wall conditions and possible filling (see Fig. 2-1).
Fig. 2-1 The main features constituting a rock mass
The complicated structure of the rock mass with its defects and inhomogeneities and the wide range
of its applications cause challenges and problems in rock engineering and construction which often
involve considerations that are of relatively little or no concern in most other branches of
engineering. One of these challenges is, according to Einstein and Baecher (1982), the uncertaintiesabout geological conditions and geotechnical parameters. This is perhaps one of the most distinctive
features of engineering geology compared to other engineering fields, therefore 'engineering
judgement', adaptable design approaches, and other procedures for dealing with uncertainty or
hedging against it have been taken into use.
Important in all rock mechanics, rock engineering and design are the quality of the geo-data that
form the basis for the calculations and estimates made. This quality depends on two main features.
1. The understanding and interpretation of the geological setting of the area of interest.
2. The way the (known) rock mass at the site is described or measured.
The first feature is important mainly in the pre-construction phase and is a result of the geologicalunderstanding based on field investigations and the experienced interpretation of available results.
To a great extent this is often wholly dependent on the skill of the geologist(s) who decide how the
- a rock mass characterization system for rock engineering purposes.
CHAPTER 12
ENGINEERING GEOLOGY AND SOIL MECHANICS
Common Usage of Rock and Uncemented Sediments
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investigations should be done and how the geo-data should be combined. Thus, this process can in
many instances be said to be more an "art" than a science. The details concerning the geological part
are not dealt with further here, but the influence of the geology is discussed in Chapter 3.
The second feature is mainly connected to the present work. Brown (1986) is of the opinion that
"inadequacies in site characterization of geo-data probably present the major impediment to thedesign, construction and operation of excavations in rock. Improvements in site characterization
methodology and techniques, and in the interpretation of the data are of primary research require-
ments, not only for large rock caverns, but for all forms of rock engineering."
TABLE 2-1 BASIC ELEMENTS AND RELEVANT CONSIDERED AREAS (based on Natau, 1990)
BASIC ELEMENT SIZE RANGE STRUCTURES CONSIDERED AREA
Crystal lattice
Mineral grain
Rock material
Jointed rock
(composed of 'bricks')
Geological-tectonical
units
Geological-tectonical
large size units
Angstrom size
(10-7
mm)
m - cm
cm - 10 m
cm - 10 m
10 m - km
Several km
Micro structures
Grain structures in
rock
Massive rock
Joint pattern, rock
mass
Rock mass
volumes between
large faults
Regional plates
Electron microscope
Microscope, hand piece, test sample of rock
Hand piece, stone ornaments, building
stone, test of rock samples.
Foundations, small underground structures,
test samples of rock masses, test pits/adits
Slopes, tunnels, large underground struc-
tures, mines
(geological maps and sections)
Oil reservoirs,(general geological maps and sections)
SIZE OF SAMPLE
S T R E N G
T H
in laboratory in situ
INVESTIGATIONS
Fig. 2-2 The scale factor of rock masses and the variation in strength of the material depending on the size of the
'sample' involved. (After Janelid, 1965)
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Other special features in a rock mass and its utilization in contrast to other construction materials
are:
- the size or volume of the material involved, see Fig. 2-2 and Table 2-1,
- the structure and composition of the material,
- the many construction and utilization purposes of it, see Table 2-2, and
- the difficulties in measuring the quality of the material (see also Appendix 4).
TABLE 2-2 MAIN TYPES OF WORKS CONNECTED TO ROCKS AND ROCK MASSES
TYPE ACTUAL PROCESS OR USE
Treatment of rocks
- drilling (small holes)
- boring (TBM boring, shaft reaming) *)
- blasting*)
- fragmentation*)
- crushing
- grinding
- cutting*)
Application of rocks- rock aggregate for concrete etc.
- rock fill
- building stone
Utilization of
rock masses
- in underground excavations (tunnels, caverns, shafts) *)
- in surface cuts/slopes/portals *)
Construction works
in rock masses
- excavation works
- rock support *)
- water sealing
*) Areas where the system is of particular interest.
These factors imply that other methods of data acquisition are used, and that other procedures in the
use of these data for construction purposes have been developed. Thus, the material properties of
rock masses are not measured but estimated from descriptions and indirect tests. The stress is not
applied by the engineering but is already present; the construction, however, leads to stress changes.
In the remainder of this chapter the main features of the rock mass and their effect on its behaviour
related to rock construction are briefly outlined.
2.1 ROCKS AND THEIR MAIN FEATURES
Geologists use a classification, which reflects the origin, formation and history of a rock rather than
its potential mechanical performance. The rock names are defined and used not as a result of the
strength properties, but according to the abundance, texture and types of the minerals involved, in
addition to mode of formation, degree of metamorphism, etc. According to Franklin (1970) there are
over 2000 names available for the igneous rocks that comprise about 25% of the earth's crust, in
contrast to the greater abundance of mudrocks (35%) for which only a handful of terms exist; yet
the mudrocks show a much wider variation in mechanical behaviour.
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2.1.1 Fresh rocks
Each particular rock type is characterized by its minerals, texture fabric, bonding strength and macro
and micro structure, see Fig. 2-3.
Igneous rocks tend to be massive rocks of generally high strength. Their minerals are of a denseinterfingering nature resulting in only slight, if any, directional differences in mechanical properties
of the rock. These rocks constitute few problems in rock construction when fresh.
Sedimentary rocks constitute the greatest variation in strength and behaviour. The minerals of these
rocks are usually softer and their assemblage is generally weaker than the igneous rocks. In these
rocks the minerals are not interlocking but are cemented together with inter-granular matrix
material. Sedimentary rocks usually contain bedding and lamination or other sedimentation
structures and, therefore, may exhibit significant anisotropy in physical properties depending upon
the degree of their development. Of this group, argillaceous and arenaceous rocks are usually the
most strongly anisotropic. Some of the rocks are not stable in the long term, as for example
mudrocks, which are susceptible to slaking and swelling. This group of rocks therefore creates many
problems and challenges in rock construction.
Metamorphic rocks show a great variety in structure and composition and properties. The
metamorphism have often resulted in hard minerals and high intact rock strength; however, the
preferred orientation of platy (sheet) minerals due to shearing movements results in considerable
directional differences in mechanical properties. Particularly the micaceous and chloritic schists are
generally the most outstanding with respect to anisotropy.
2.1.2 The influence from some minerals
Certain elastic and anisotropic minerals like mica, chlorite, amphiboles, and pyroxenes may highly
influence the mechanical properties of the rocks in which they occur (Selmer-Olsen, 1964). Parallel
orientation of these minerals is often found in sedimentary and regional metamorphic rocks in
which weakness planes may occur along layers of these flaky minerals. Where mica and chlorite
occur in continuous layers their effect on rock behaviour is strongly increased. Thus, mica schists
and often phyllites have strong anisotropic mechanical properties of great importance in rock
construction. Also other sheet minerals like serpentine, talc, and graphite reduce the strength of
rocks due to easy sliding along the cleavage surfaces, see Fig. 2-3.
Quartz is another important mineral in rock construction. This mineral is grade 7 in the Mohs scaleof hardness. Sharp, obtuse-angled edges of the quartz grains have an unfavourable shape regarding
drill bit and cutter wear in percussion drilling and TBM boring respectively, while the effect from
rounded quartz grains is significantly less.
Change of moisture content in swelling minerals of the smectite (montmorillonite) group can cause
significant problems related to high swelling pressures (Piteau, 1970). These minerals, occurring
either as infilling or alteration products in seams or faults, have in addition to expansion, a low
shear strength, which may contribute to rock falls and, in some cases, slides in underground
openings and cuttings. Also some rocks may show swelling properties. These rocks can be
montmorillonitic shales, altered or weathered basalts, in addition to other igneous, metamorphic
rocks, or sedimentary rocks containing anhydrite.
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Some rocks may slake (hydrate or "swell", oxidize), disintegrate or otherwise weather in response to
the change in humidity and temperature consequent on excavation. As mentioned above, an
abundant group of rocks, the mudrocks, are particularly susceptible to even moderate weathering
(Olivier, 1976). Refer to Fig. 2-3.
MINERAL SIZE
TEXTURE
MINERALCOMPOSITION
SWELLINGMINERALS -smectite -montmorillonite -anhydrite
ALTERATIONor
WEATHERING
SLAKINGROCKS
ALTEREDor
WEATHEREDROCKS
SCHISTOSEROCK
rocks withstrongly
anisotropicproperties
FLAKYMINERALS -mica -chlorite -talc
HOMOGENEOUSand
LAYERED ROCKS
SWELLINGROCKS
rocks with reduced strength and durabi l i ty
HYDRATIZATIONof mudrocks etc.
fresh rocks
common rock features
some special processes actinginfluence from some minerals
rocks withisotropicor slightly
anisotropic properties
Fig. 2-3 The main variables influencing rock properties and behaviour
2.1.3 The effect of alteration and weathering
The processes of alteration and weathering with deterioration of the rock material have reducing
effect on the strength and deformation properties of rocks, and may completely change the
mechanical properties and behaviour of rocks (refer to Fig. 2-3). For most rocks, except for the
weaker types, these processes are likely to have great influence on engineering behaviour of rock
masses. Hence, the description and characterization of rock masses should pay particular attention
to such features.
Rocks are frequently weathered near the surface, and are sometimes altered by hydrothermal
processes. Both processes generally first affect the walls of the discontinuities1
1. Mechanical disintegration or breakdown, by which the rock loses its coherence, but has littleeffect upon the change in the composition of the rock material. The results of this process are:
. The main results of
rock weathering and alteration are:
- The opening up of joints.
- The formation of new joints by rock fracture, the opening up of grain boundaries.
- The fracture or cleavage of individual mineral grains.
1 In this work, the following terms have been applied for the various types of discontinuities:
Joints - Minor and medium sized discontinuities, including fissures, cracks, fractures,
breaks, etc.; also some minor seams are included in this group.
Seams - Filled discontinuities, including shears; they are also named 'singularities'.Weakness zones - Including faults, crushed zones and zones of weak rocks surrounded by stronger
rocks.
The characteristics of these features are further described in Appendices 1 and 2.
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2. Chemical decomposition, which involves rock decay accompanied by marked changes in
chemical and mineralogical composition results in:
- Discoloration of the rock.
- Decomposition of complex silicate minerals (feldspar, amphibole, pyroxene, etc) eventually
producing clay minerals; some minerals, notably quartz, resist this action and may 'survive'
unchanged.
- Leaching or solution of calcite, anhydrite and salt minerals.
The disintegration leads mainly to a greater number of joints in rock masses located in the upper
zone of weathering, while decomposition influences the joint condition as well as the rock material.
2.1.4 Geological names and mechanical properties of rocks
Rocks that differ in mineral composition, porosity, cementation, consolidation, texture and
structural anisotropy can be expected to have different strength and deformation properties.Geological nomenclature of rocks emphasizes mainly solid constituents, whereas from the
engineer's point of view, pores, defects and anisotropy are of greater mechanical significance
(Franklin, 1970). For each type of rocks the mechanical properties vary within the same rock name.
Petrological data can, however, make an important contribution towards the prediction of
mechanical performance, provided that one looks beyond the rock names to the observations on
which they are based. It is, therefore, important to retain the names for the different rock types, for
these in themselves give relative indications of their inherent properties (Piteau, 1970).
2.2 DISCONTINUITIES IN ROCK
Any structural or geological feature that changes or alters the homogeneity of a rock mass can be
considered as a discontinuity. Discontinuities constitute a tremendous range, from structures which
are sometimes thousands of meters in extent down to - per definition - mm size, see Fig. 2-4.
0.01 0.1 1 10 100
fissures
cracks
partings
bedding planes
joints
1000 10 000
faults
LENGTH (m)
seams / shears
JOINTSROCK DEFECTS WEAKNESS ZONES
Fig. 2-4 The main types of discontinuities according to size. The size range (length) used for joints in this work is
indicated.
The different types, such as faults, dykes, bedding planes, tension cracks, etc. have completely
different engineering significance (Piteau, 1970). The roughness, nature of their contacts, degree
and nature of weathering, type and amount of gouge and susceptibility to ground water flow willvary greatly from one type of discontinuity to another since their cause, age and history of develop-
ment are fundamentally different. The effect on rock masses due to these localised discontinuities
,
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from the 'host' rock. The problems related to weakness zones may, therefore, depend on several
factors which may all interplay in the final behaviour.
Weakness zones and faults show numerous variations in their structures and compositions, see Fig.
2-6. In cases where the zones or faults are composed mainly of joints and seams they may be
characterized by the same descriptions as for jointing. In other cases it may be necessary tocharacterize them by special descriptions and measurements or tests, as further described in
Appendix 2. The fact that faults and weakness zones of significant size can have a major impact
upon the stability as well as on the excavation process of an underground opening necessitates that
special attention, follow-up and investigations often are necessary to predict and avoid such events.
A B C
D
E
Fig. 2-6 Sketches of some types of weakness zones. A - C are from ISRM (1978) and D - E from Selmer-Olsen
(1950).
2.2.2 Joints and their main features
Joints are the most commonly developed of all structures in the earth's crust, since they are found in
all competent rocks exposed at the surface. Yet, despite the fact that they are so common and have
been studied widely, they are perhaps the most difficult of all structures to analyse. The analytical
difficulty is caused by the number of fundamental characteristics of these structures. There is,
however, abundant field evidence that demonstrates that joints may develop at practically all ages inthe history of rocks (Price, 1981).
A joint can be open or closed. Closed joints may be nearly invisible. Yet they constitute surfaces
along which there is no resistance against separation. In quarries the spacing of joints determines the
largest size of blocks of sound rock which can be obtained. Therefore, joints and joint systems have
attracted the attention of builders ever since cut stones have been used.
A joint is composed of several characteristics. In addition to length and continuity of the joint the
main are:
- roughness and strength of the joint wall surface,
- waviness or planarity of joint wall,- alteration or coating of the joint wall, and
- possible filling. Refer to Fig. 2-7.
,
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All these parameters influence on the shear strength of the joint (Brekke and Howard, 1972; Price,
1981; Hoek and Brown, 1980; Barton et al., 1974; Barton and Choubey, 1977; Bieniawski, 1984;
Turk and Dearman 1985; and several other authors). They also determine the amount of water that
can flow through the joint.
c o n d i t i o n
o f j o i n t
w a l l s u r f
a c e :
- s m o o t h n
e s s
- p o s s i b l e
c o a t i n g
- p o s s i b l e
a l t e r a t i o n
o f w a l l r o c
k
joint thickness andpossible filling material
waviness or undulationof joint wall
l e n g t h a n d
c o n t i n u i t y o
f t h e j o i n t
Fig. 2-7 Sketch showing the main features of a joint.
The distance between the two matching joint walls controls the extent to which these can interlock.
In the absence of interlocking, the properties of the filling of the joint determine the shear strength
of the joint. As separation decreases, the asperities of the rock wall gradually become more
interlocked, and the rock wall properties are the main contributor to the shear strength.
2.2.3 The main jointing characteristics
By jointing is meant the pattern and frequency or density of joints. Field studies of several workers
have shown that the joints preferentially are found in certain directions. One to three prominent sets
and one or more minor sets may occur; in addition several individual or random joints are often
present.
The joints delineate blocks. Their dimensions and shapes are determined by the joint spacings, by
the number of joint sets and by random joints. ISRM (1978), Barton (1990) and several other
authors state that the block size is as an extremely important parameter in rock mass behaviour. A
number of scale effects in rock engineering can be explained by this feature including compressive
strength, deformation modulus, shear strength, etc.
Different methods are used for measuring the jointing density. The most common are:
- Joint spacing, either in surfaces or in drill cores or scan lines.
- Density of joints, either in surfaces, or in bore holes or scan lines.
- Block size, in surfaces, and
- Rock quality designation (RQD), in drill cores.
They are further outlined in Appendix 3, where also correlation equations between them have been
developed.
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2.2.4 The rock mass
Discontinuities ranging in lengths from less than a decimetre to several kilometres divide the
bedrocks into units, volumes or blocks of different scales (Fig. 2-8):
1. The regional pattern or first order fault blocks are bounded by the larger weakness zones or
faults (see Fig. 2-5).2. The second order blocks formed by singularities, i.e. small weakness zones or seams.
3. The third order blocks formed by normal joints.
4. The small joints in the appearance of bedding or schistosity partings form the smallest
pattern or fragments, which are of interest for engineering purposes.
5. The microcracks are responsible for making up small fragments or grains in the rock. These
discontinuities are, however, mostly considered a rock property and are therefore generally
included in the strength characterization of the rock material.
Based on this it has been found useful for engineering geological and design purposes to divide theground into:
- "The detailed jointing" formed mainly by the third and fourth order blocks or units, and
- "The coarse pattern of weakness zones" formed by the first order blocks or units by faults and
weakness zones.
A
3
5 0 0 - 1 5 0 0 m
5 0 - 1 5 0 m
5 - 5 0 m
2
1
3
2
2
3
2
2
A
3
4
4
45
B
B
5
Fig. 2-8 Simplified model of various dimensions units or blocks formed by discontinuities of different size (after
Pusch and Morfeldt, 1993).
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This corresponds with the division suggested by Selmer-Olsen (1964). The rock blocks in the
detailed jointing pattern including the rock fragments or pieces caused by the small joints/fissures is
a main feature in the rock mass characterization developed herein.
2.3 ROCK MASS CHARACTERIZATION FOR DESIGN AND CONSTRUCTION
PURPOSES
An important issue in rock mass description and characterization is to select parameters of greatest
significance for the actual type of design or construction. There is no single parameter or index,
which can fully designate the properties of jointed rock mass. Various parameters have different
significance and only if combined can they describe a rock mass satisfactorily (Bieniawski, 1984).
Testing of rock masses in situ has brought out very clearly the enormous variations that exist in the
mechanical behaviour of a rock mass from place to place. According to Lama and Vutukuri (1978)
the engineering properties of a rock mass depend far more on the system of geological
discontinuities within the rock mass than of the strength of the rock itself. Further, the strength of a
rock mass is often governed by the interlocking bonds of the unit "elements" forming the rock mass.
Terzaghi (1946) also concludes that, from an engineering point of view, a knowledge of the type
and frequency of the rock discontinuities may be much more important than of the types of rock
which will be encountered. Similarly, Piteau (1970) has stressed the importance of distinguishing
between the behaviour of the rock and the rock mass, especially for hard rocks. Thus, characterizing
a discontinuity system in a way that describes the variability of its geometric parameters constitutes
an essential step in dealing with stability problems in discontinuous rock masses (Tsoutrelis et al.,
1990).
This does not mean that the properties of the intact rock material should be disregarded in the
characterization. After all, if discontinuities are widely spaced, or if the intact rock is weak, the
properties of the intact rock may strongly influence the gross behaviour of the rock mass. The rock
material is also important if the joints are discontinuous. In addition, the rock description will
inform the reader about the geology and the type of material at the site. Although rock properties in
many cases are overruled by discontinuities, it should be brought to mind that the properties of the
rocks highly determine the formation and development of discontinuities.
Therefore, an adequate and reliable estimation of the nature of the rock is often a primary
requirement. For some engineering or rock mechanics purposes the mechanical characterization of
rock material alone can be used, namely for drillability, crushability, aggregates for concrete, asphalt
etc. Also, in assessment for the use of fullface boring machines (TBM), rock properties likecompressive strength, hardness, anisotropy are among the more important parameters.
Kirkaldie (1988) mentions a total of 28 parameters present in rock masses which may influence the
strength, deformability, permeability or stability behaviour of rock masses: 10 rock material
properties, 10 properties of discontinuities and 8 hydrogeological properties. Because it is often
difficult or impossible in a general characterization to include the many variables in such a complex
natural material, it is necessary to develop suitable systems or models in which the complicated
reality of the rock mass can be simplified by selecting only a certain number of representative
parameters. For this purpose several classification and design systems have been developed, of
which some are shown in Table 2-3 for information. Further, Table 2-4 indicates the main rock
mass and ground features and which of these that have been applied and combined in the various
systems.
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From Table 2-4 it is seen that the following parameters are most frequently applied in design and
classification systems:
- the rock material (rock type, geological name, weathering and alteration, strength);
- the degree of jointing (joint spacing, block size, RQD); and
- in situ stresses.
Also such features as:
- orientation of main discontinuities or joint set;
- joint conditions;
- block shape or jointing pattern;
- faults and weakness zones; and
- excavation features (dimension, orientation, etc.)
have been considered as important parameters in rock masses.
TABLE 2-3 SOME OF THE MAIN DESIGN AND CLASSIFICATION SYSTEMS IN USE
Name of classification Form and Type*) Main applications Reference
The Terzaghi rock loadclassification system
Lauffer's stand-up timeclassification
The new Austriantunnelling method(NATM)
Rock classification forrock mechanical purposes
The unified classificationof soils and rocks
The rock qualitydesignation (RQD)
The size-strengthclassification
The rock structure rating(RSR) classification
The rock mass rating(RMR) classification
The NGI Q classification
systemThe typologicalclassification
The unified rockclassification system
Basic geotechnicalclassification (BGD)
Descriptive and behaviouristic formFunctional type
Descriptive formGeneral type
Descriptive and behaviouristic formTunnelling concept
Descriptive formGeneral type
Descriptive formGeneral type
Numerical formGeneral type
Numerical formFunctional type
Numerical formFunctional type
Numerical formFunctional type
Numerical form
Functional typeDescriptive formGeneral type
Descriptive formGeneral type
Descriptive formGeneral type
For design of steel support intunnels
For input in tunnelling design
For excavation and design inincompetent (overstressed)ground
For input in rock mechanics
Based on particles and blocks forcommunication
Based on core logging; used inother classification systems
Based on rock strength and blockdiameter; used mainly in mining
For design of (steel) support intunnels
For use in tunnel, mine andfoundation design
For design of support in
underground excavationsFor use in communication
For use in communication
For general use
Terzaghi, 1946
Lauffer, 1958
Rabcewicz, Müllerand Pacher,1958 - 64
Patching and Coates,1968
Deere et al., 1969
Deere et al., 1967
Franklin, 1975
Wickham et al., 1972
Bieniawski, 1973
Barton et al., 1974
Matula and Holzer,1978
Williamson, 1980
International Societyfor Rock Mechanics(ISRM), 1981
*) Definition of the following expressions:
Descriptive form: the input to the system is mainly based on descriptions
Numerical form: the input parameters are given numerical ratings according to their character
Behaviouristic form: the input is based on the behaviour of the rock mass in a tunnel
General type: the system is worked out to serve as a general characterization Functional type: the system is structured for a special application (for example for rock support)
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As for most other construction materials, there is also in rock engineering and construction a need
for a strength specification of the material, i.e. the rock mass. The strength of other construction
materials can be determined from the process of refining or ensured during production of the
material. In rock construction, however, the material already exists, the task is to evaluate the
strength properties it possesses (and not to produce them).
The considerations outlined above have been important in the development of the present system for
rock mass characterization.
TABLE 2-4 APPLICATION OF ROCK MASS AND GROUND PARAMETERS IN VARIOUS DESIGN AND
CLASSIFICATION SYSTEMS
CLASSIFICATION SYSTEM NO. 1 2 3 4 5 6 7 8 9 10 11 12 13
ROCK- Origin, name, or type- Weathering- Anisotropy
o : x
"x x x
+ x
ROCK PROPERTIES- Unit weight- Porosity- Rock hardness- Strength- Deformability- Swelling
: : : :
o :
xo
xx x
+ x+
+ x x+
JOINT CONDITIONS- Joint size/length- Joint separation- Joint wall smoothness- Joint waviness
- Joint filling
o o
xx
x ox : x
x x
DEGREE OF JOINTING- Block size- Joint spacing/frequency- RQD- Number of joint sets
o : : x
o xx
xx
x xxx
JOINTING GEOMETRY OR STRUCTURE- Joint orientation with respect to excavation- Jointing pattern- Continuity- Structure (fold, fault)
: : : :
o o
x +
+
x
EXTERNAL FEATURES- Water conditions- Rock stress conditions- Blasting damage- Excavation dimensions
o :
:
x xx +
+x x
xx
x
CLASSIFICATION SYSTEM NO. 1 2 3 4 5 6 7 8 9 10 11 12 13
Legend:
x well defined input o very roughly defined or included
: included, but not defined " partly included (in other parameters)
+ used as additional information (in RMR as adjusted value)
Classification system no.:
1 Terzaghi (1946) 5 Deere et al. (1969) 8 Wickham et al. (1972) 11 Matual and Holzer (1978)
2 Lauffer (1958) 6 RQD (1966) 9 RMR (1973) 12 Williamson (1980)
3 NATM (1957-64) 7 Franklin (1970, 1975) 10 Q-system (1974) 13 BGD (1981)
4 Coates and Patching (1968)
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Go to Page 31 Uses of Soil (Uncemented sediment)
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two
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;
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Weathering of Granite in Hong Kong
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Chapter 14 Structural Geology I
Self-assessment Exercises
1. Describe how folds are formed.
2. With the aid of a labeled diagram, show the characteristics of folds.3. Describe how faults are formed.
4. With the aid of labeled diagrams, distinguish between a normal fault
and a reversed fault.
5. Describe the following terms:
Syncline and anticline
Punch or pitch
Dome and basin
Horst and graben
Symmetrical fold and unsymmetrical fold
Step faulting
6. What are the differences between faults and joints?
7. What are the effects of folding on rocks?
8. What are the effects of joints on rocks?
;
;,
Back to Page 6 DISCONTINUITIES IN ROCK
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Uses of Soil (Uncemented sediment)
Soil or uncemented sediment plays a significant part of the construction industry. It is
used as a foundation for homes and buildings. Soil compaction is used to increase the
density of the soil and ensure its stability. Compacting the soil also prevents soil
settlement and reduces water seepage. The strength of soil is measured before a
construction project to determine how easily the soil changes shape and whether it is
capable of maintaining under the weight of a building. Working on the wrong type of soil
may lead to cracks in the foundation, leaks and floods.
Just like other construction materials soils has its own scientific analysis with regards to
its abilities on dealing with forces. Being the oldest construction and probably
engineering material soil is one of the most complex fields in civil engineering to the
point that when it comes to the factor of safety in design whatever has direct contact with
soils, e.g. foundations, or soil based constructions, e.g. embankments, requires a
significantly higher safety factor compare with other construction materials, i.e. the
uncertainty in soil analysis and design is higher. This is most likely resulted from the way
soil originates.
Usage of soil as the main element of construction goes back to the first civilization when
Sumerian built Ur, first city in the history, on south of Mesopotamia near the mouth ofEuphrates River. They used bricks to build their first houses and earlier they built
embankments and dams to direct the water for irrigation. The Western history of
recognition the soil as a main element goes back to Romans, in the first century B.C.,
when their engineers used the trial and error experiences to construct foundations.
After all today soil and rock are still one of the most important materials used in
construction. It is used or on its natural state or with improvements, such as compaction,
reinforcement and etc., as the main component such as in dams, embankments and
highways or as supporter element in every construction, i.e. foundation support.
Soil material is also a critical component in the mining and construction industries. Soil
serves as a foundation for most construction projects. The movement of massive volumes
of soil can be involved in surface mining, road building and dam construction as well as
reclamation.
Soil material is extensively used in earthfilling works or the use of natural or screened
soils as road construction materials as well as in projects involving slopes, tunnels,
foundations, etc).
ENGINEERING GEOLOGY AND SOIL MECHANICS
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1. Earthwork
Filling materials must be available and compaction must be properly performed to
prevent settlement.
Earthworks fill material may consist of soil, rock, or inert construction and demolition
material.
Fill material shall be capable of being compacted to form stable areas of fill.
Earthwork fill materials when deposited are normally loose and bulked. It is therefore
necessary to compact the materials so as to prevent softening, dislodgment and settlement
of the earth.
Fill material shall be compacted in layers to a stable condition. The thickness of each
layer shall be 150 mm to 300 mm which depends of the capacity of the compaction plant
used. The amount of compaction attained is measured by “dry density” of the fill.
Generally, the fill material shall be compacted to obtain a relative compaction of at least
95% of the “maximum dry density” of that material.
2. Founding materials for footings and foundations
Should possess adequate Bearing capacity
3. Reclamation
With proper fill treatment
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3. Reclamation
3.1 Purpose of Reclamation
Reclamation may generally be carried out:
To provide land for essential major transport infrastructure.
To provide land for housing, community facilities and public open spaces.
To provide land for port and industrial uses.
To eliminate areas of badly polluted water and improve hydraulic conditions by
rearranging the coastline.
3.2 Reclamation Method
3.2.1 UDrained Method
The Udrained methodU leaves the soft marine deposit in place, and the consolidation is
usually accelerated by the use of vertical drains and sometimes with surcharge
preloading. Drained reclamation is usually carried out in the following sequence:
USequence of Drained Reclamation
Reclamation is the process of depositing materials either in the sea or in
low-lying swampy areas in such a way that useful areas of land are
formed. Almost any type of material can be used for reclamation,
depending on the use to which the land is to be put. This will range from
agricultural land and land for light industrial uses, which can utilise
materials which have low load-bearing capacities, to land for the
construction of dock and harbour installations and power stations, which
will require high quality incompressible materials.
The main phases of reclamation are:
1. Site establishment and mobilisation; 2. Dredging of a stockpile;
3. Construction of sea walls or bunds; 4. Pumping sand behind sea
walls or bunds; and 5. Stabilisation of surface.
Factors affecting the operation of reclamation are Location of site,
T e of material and Trans ort of materials
3.2.2 Fully dredged method
3.2.3 Partial dredged method
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By Marine Plant By Land Plant
UInstallation of vertical drains
a. ULaying of geotextile on the seabedU -
Geotextile may be laid on the seabed to separate the fill from the underlying soft
marine deposits, preventing migration of fines. It also enhances the stability of
the underlying marine deposits in supporting the loading of the reclamation fill.
b. UDeposition of blanket layer
This blanket should consist of free draining granular material of about 2 m thick.
This granular layer works with the vertical drains to enable drainage from the
clayey deposits. It also acts as a capping layer to spread the load from the fill
during the filling operation.
c. UInstallation of vertical drains U (also known as wick drain or band drain)
The vertical drain was band-shaped with a plastic core enclosed by a non-woven
geotextile filter jacket. It functioned as a passage for water flow, to accelerate
the dissipation of pore water pressure during the consolidation of the marine
deposit layer. The band drains were installed in a triangular grid pattern with
1.5 m c/c spacing.
To commence the acceleration of consolidation earlier, the band drains are
usually installed over water using special marine plant just after laying of the
sand blanket. The vertical drains can also be installed after reclamation using
land plant.
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UInstallation of Wick Drain UWick Drain
d. UControlled thin-layer placement
Controlled even placement of thin layers of fill on the reclamation site is
necessary to avoid shear failure of the underlying marine deposits and the
formation of mud waves. An initial thickness of no more than one meter of fill
is usually required with subsequent layers increased as appropriate. Placement
can be by bottom-dump barges, hydraulic filling, grabbing or end tipping.
UHydraulic Filling
End Tipping
Bottom-dump Barge
Grabbing
,
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3.2.2 UFully Dredged Method
In Ufully dredged methodU, all marine and alluvial clays or silts are removed by
dredging and replaced with fill.
UPros:
The method is relatively simple.
Settlement of the reclamation fill is more quickly and more predictable.
UCons:
Can be expensive where thick layers of soft deposits exist.
Causing mud waves during dredging
Disposal of dredged sediments, particularly for contaminated mud may be
problematic.
This method is generally discouraged unless there is strong justification.
3.2.3 UPartial Dredged Method
The U partial dredged methodU involves partial removal of marine or alluvial
deposits, leaving the lower, stiffer or stronger deposits in place.
The remaining marine deposits shall be treated as that in the drained method.
It fact it is the combination of the drained method and fully dredged method, so
it combines and neutralizes both the pros and cons of the two methods.
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Suction hopper dredger
3.3 Fill Materials
Fill materials for reclamation includes public fill, marine sand fill and crushed rock,
but public fill and marine sand fill are the most commonly used types of fill in local
conditions.
3.3.1 UPublic Fill
Public fill is the inert portion of construction and demolition material from
private and public developments and demolition sites.
Because of the shortage of areas to accommodate the public fill generated by the
construction industry, priority should be given to its use. It is also the government policy to maximize the use of public fill in reclamation
projects.
3.3.2 UMarine Sand Fill
Sophisticate dredgers are use to
obtain the sand from a marine
borrow area.
Since the mobilization costs are high, the size of the project must be large
enough to justify the use of sophisticated dredgers.
Plant such as trailing suction hopper dredgers may dredge marine sand fill very
fast and at relatively low costs, particularly when the borrow area is close to the
reclaimed site.
These dredgers can deposit marine sand in the reclamation by bottom dumping
or by hydraulic pumping.
The rate of formation of reclamation can be very rapid compared to the use of
other types of fill.
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3.3.3 URock Fill
Crushed rock from local land sources should not normally be used for
reclamation. It should be used as foundation materials or processed to produce
aggregate products, as far as possible.
In case a works project involving large quantities of rock excavation and
removal, the surplus rock material can be used for reclamation.
Where crushed rock over 250 mm is used, it should be placed in areas where no
building development will take place, to avoid impeding piling or excavation
works in the future.
3.4 Fill Treatment
Fill treatment processes are to speed up the consolidation of the reclaimed area
in order to reduce the long term settlement.
It shall be noted that the settlement is contributed from both the existing marine
deposits and the newly reclaimed materials.
3.4.1 USurcharge Preloading
Surcharge preloading can be used to accelerate settlement of fill that would
otherwise occur more slowly.
Monitoring of the consolidation of the fill will be carried out periodically.
The surcharge should only be removed when the required settlement or increase
in strength has been achieved.
USurcharge Pre-loadingVertical Drains
Drainage
La er
Vertical
Drain
,
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3.4.3 UVibro-compaction
The vibro-compaction method is used to compact a thick layer of fill, particular
in reclamation.
It is used for granular soils, in particular sand.
This method is very similar to the stone column method except that no
additional granular material will be used to fill the borehole; instead the original
fill material is pushed back into the borehole.
The vibroflot is penetrated into the fill and retracted in a controlled motion such
that a dense column of fill is formed.
The compaction is carried out in a triangular grid pattern of 2.5 m to 4 m c/c
spacing.
Effective compaction depth can be up to 35 m.
Vibro-compaction is only applicable to granular materials of certain grading
properties.
Vibro-compaction cannot effectively compact the surface few metres of fill and
therefore separate compaction of the surface layer will be required.
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