CHAPTER 2
LITERATURE REVIEW
2.1
Introduction.
In assessing rock slope, the most important field data is the geological
information. The geological data being referred are type of discontinuities and their
properties in term of geometrical and as well as the weathering state of the rock.
Discontinuities or weak planes are faults, dykes, bedding planes, cleavage, tension
joints or shear joints. Geometrical properties of the discontinuities are dip direction
or strike and dip angle.
2.2
Types of discontinuities.
In rock mechanics, discontinuity or weak plane is commonly used term for
rock defects. Bedding planes in sedimentary rocks, cleavages and schistosities in
metamorphic rock are typical examples of fabric defects. Folds, faults and joints are
structural defects (Giani,1992). Discontinuities are divided into two scales either
large-scale discontinuities or small scale discontinuities.
Large-scale discontinuities are bedding plane, folds, faults and joints.
Bedding planes represent interruptions in the course of sedimentary rock grain
deposition, which are therefore separated by beds or strata. Folds are caused by a
bend in the strata of layered rock. Faults are fractures or fractured zones along which
5
there has been an appreciable shear displacement. Joints are fractures in rock along
which there has been little or no displacement or very slight movement perpendicular
to the joint surface.
Small-scale discontinuities are cleavage planes and schistosities. Cleavage
planes are generated under the influence of tensile stresses, which determine rock
splitting along definite parallel planes. Cleavage is also associated with changes in
rock fabric and large folding. Schistosities are the varieties of foliations that occur in
the coarser-grained metamorphic rocks and are usually the results of the parallel
arrangement of platy and ellipsoidal grain within the rock substance.
2.3
Geometrical term of geological data.
The geometrical term of geological data is shown in Figure 2.1. Dip is the
maximum inclination of a structural discontinuity plane to the horizontal, defined by
the angle ψ in the figure. Angle ψ should be expressed in degrees as a two digit
number, e.g. 05 or 55 (00 – 90). Dip direction or dip azimuth is the direction of
the horizontal trace of the line of dip, measured clockwise from north as indicated by
the angle α in the margin sketch. Angle α should be expressed as a three digit
number, e.g. 010 or 105 (000 – 360). Strike is the trace of the intersection of an
obliquely inclined plane with horizontal reference plane and it is right angles to the
dip and dip direction of the oblique plane. Dip direction and dip data will be used on
the stereo-net projection analysis.
6
Figure 2.1 : Definition of geometrical terms (Hoek and Brown, 1980).
2.4
Graphical techniques for data presentation.
One of the most important aspects of rock slope analysis is the systematic
collection and presentation of geological data in such a way that it can easily be
evaluated and incorporated into stability analyses. Most familiar graphical
presentation of geological data is used of spherical projections. There are two types
of spherical projections; equal area and equal angle projection.
Figure 2.2 : (a) Equal area projection (b) equal angle projection (Hoek and
Brown, 1980).
7
The equal area projection, also known as the Lambert or Schmidt projection,
is generated by the method shown in Figure 2.2(a). A point A on the surface of the
sphere is projected to the point B by swinging it in an arc centered at the point of
contact of the sphere and a horizontal surface upon which it stands. If this process is
repeated for a number of points, defined by the intersection of equally spaced
longitude and latitude circles on the surface of the sphere, an equal area net will be
generated. This net has a larger diameter than the sphere and, in order to reduce its
diameter to that of the sphere; the radius of each point on the net is reduced by 1/√2.
The equal angle projection, also known as a Stereographic or Wulff
projection, is obtained by the method illustrated in the Figure 2.2(b). The projection
C of a point A on the surface of the sphere is defined by the point at which the
horizontal plane passing through the centre of the sphere is pierced by a line from A
to the zenith of the sphere. The zenith is the point at which the sphere is pierced by
its vertical axis.
Both types of projection are used for the analysis of structural geology data.
In general, geologist prefers the equal area projection because, as the name implies,
the net is divided into units of equal area and this permits the statistical interpretation
of structural data. The engineers tend to prefer the equal angle projection because
geometrical constructions required for the solution of engineering problems are
simpler and more accurate on this projection than for the equal area projection.
Figure 2.3 shows a sphere with one quarter removed and with meridional and polar
nets projected onto the exposed vertical and horizontal faces.
2.5
Stereographic projection of a plane and its pole.
Imagine a sphere, which is free to move in space so that it can be centered on
an inclined plane as illustrated in Figure 2.4. The intersection of the plane and the
surface of sphere is a great circle, which shaded in the figure. A line, passing
through the center of the sphere in a direction perpendicular to the plane, pierces the
8
sphere at two diametrically opposite points, which are called the poles of the great
circle representing the plane
Figure 2.5, shows the method of construction of stereographic projection of
great circle and its pole and Figure 2.6 shows the appearance of this projection.
Figure 2.7 shows the dip, dip direction and strike conventions used in conjunction
with the lower reference hemisphere stereographic projection. Note that dip
direction is always measured clockwise from north and that the strike line is at 90˚ to
the dip direction of a plane.
Figure 2.3 : Sectioned isometric view of a sphere showing the relationship
between the meridional and polar nets (Hoek and Brown, 1980).
9
Figure 2.4 : Great circle and its poles, which defined the inclination and
orientation of an inclined plane (Hoek and Brown, 1980).
Figure 2.5 : Stereographic projection of a great circle and its pole onto the
horizontal plane of the lower reference hemisphere (Hoek and Brown, 1980).
10
Figure 2.6 : Stereographic projection of a great circle and its pole (Hoek and
Brown, 1980).
Figure 2.7 : Definition of terms used in conjunctions with the lower reference
hemisphere stereographic projection (Hoek and Brown, 1980).
11
2.6
Mode of rock slope failure.
Mode of rock slope failure depending on the discontinuity those appear at the
slope. Modes of failure are plane failure, wedge failure, circular failure and toppling.
Modes of failure are showing in Figure 2.8.
Normally, there are three types of failures that occurs on rock with
weathering grade I to III; plane failure, wedge failure and toppling failure. Plane
failure occurs when geological discontinuity, such as a bedding plane, strikes parallel
to the slope face and dips into the excavation at an angle greater than the angle of
friction. Wedge failure occurs when two discontinuities strike obliquely across the
slope face and their line of intersection daylights in the slope face, the wedge of rock
resting on these discontinuities will slide down the line of intersection, provided that
the inclination of this line is significantly greater than the angle of friction. Toppling
failure when form of columnar separated by steeply dipping discontinuities.
Fully weathered or moderate weathered rock, which the weathering grade are
grade IV to VI normally behaviors like the soils. Therefore, modes of this failure
like to be soils slope like circular failure. Circular failure occurs when the material is
very weak, as in a soil slope or when the rock mass is very heavily jointed or broken,
as in a waste rock dump, the failure will be defined by a single discontinuity surface
but will tend to follow a circular failure path.
Hoek and Bray (1981) classified the rock slope failure into two categories;
either factors of safety can be calculated or cannot be calculated. Modes of rock
slope failure that can be calculated the factor of safety are plane failure, wedge
failure and circular failure. Factor of safety can be defined as the ratio of the total
force available to resist sliding to the total force tending to induce sliding. The
critical state is when the factor of safety (FOS) equal to 1. The most suitable FOS
that taking of the factors that effecting to the rock slope stability like presence of
water, fractured, and method of excavation is greater than 1.5 (Hoek, 2000).
12
Figure 2.8 : Main types of slope failure and stereo plots of structural conditions
likely to give rise to these failures (Hoek and Bray, 1981).
13
2.7
Additional factors influencing slope stability
The geometric boundaries imposed by orientation, spacing and continuity of
the joints, as well as the free surface boundaries imposed by the excavation, defined
the modes of potential failure. However, failure itself is frequently initiated by
additional factors not related to geometry. These factors include erosion,
groundwater, temperature, in-situ stress, and earthquake-induced loading (US Army
Corps of Engineer, 1994).
a) Erosion
Two aspects of erosion need to be considered. The first is large scale erosion,
such as river erosion at the based of a cliff. The second is relatively localized
erosion caused by groundwater or surface runoff. In the first type, erosion
changes geometry of the potentially unstable rock mass. The removal of
material at the toe of a potential slide reduces the restraining force that may
be stabilizing the slope. Localized erosion of joint filling material, or zones
of weathered rock, can effectively decrease interlocking between adjacent
rock blocks. The loss of interlocking can significantly reduce the rock mass
shear strength may allow a previously stable rock mass to move. In addition,
localized erosion may also results in increased permeability and groundwater
flow.
b) Groundwater.
Groundwater occupying the fractures within a rock mass can significantly
reduces the stability of a rock slope. Water pressure acting within a
discontinuity reduces the effective normal stress acting on plane, thus
reducing the shear strength along the plane. Water pressure within
discontinuities that run roughly parallel to a slope face also increase the
driving forces acting on the rock mass.
c) Temperature.
Occasionally, the effects of temperature influence the performance of a rock
slope. Large temperature changes can cause rock to spall due to the
14
accompanying contraction and expansion. Water freezing in discontinuities
causes more significant damage by loosening the rock mass. Repeated
freeze/thaw cycles may result in gradual loss of strength. Except for periodic
maintenance requirements, temperature effects are surface phenomenon and
are most likely of little concern for permanent slopes. However, in a few
cases, surface deterioration could trigger slope instability on a larger scale.
d) State of stress.
In some locations, high in-situ stresses may be present within the rock mass.
High horizontal stresses acting roughly perpendicular to a cut slope may
cause blocks to move outward due to the stress relief provided by the cut.
High horizontal stresses may also cause spelling of the surface of a cur slope.
Stored stresses will most likely be relieved to some degree near the ground
surface or perpendicular to nearby valley walls. For some deep cuts, it may
necessary to determine the state of stress within the rock mass and what
effects these stresses may have on the cut slope.
2.8
Stabilization method
Stabilization methods are essential or for stabilising the structures that need to
be safe and stable for along period of time. Proper design and construction
procedures may help to reduce degree of instability induces into the rock mass and
construction cost. Selecting the suitable stabilization methods depending on the type
of weakness planes, mode of failure of rock mass, mechanisms and techniques of
how a method stabilizes the rock mass, strength and type of rock and type of
structure to be constructed and the degree duration of stability required. Different
modes of failure need different types of stabilization methods. Most cases need
some combination methods either for preliminary of permanent installation. For
tropical country like Malaysia, which gains a rain along the year need a proper
drainage system to control the dissipation of water into the rock mass either in form
of surface water or run off. Presence of water will increase the instability of slope.
15
The stability assessment of rock slopes frequently indicates an impending
failure is possible. In such cases, a number of methods are available for improving
the overall stability. An appreciation of the mechanics associated with rock slope
stability together with an understanding of treatment methods for improving the
stability of potentially unstable slopes permit the detailed planning and
implementation of a slope stability program. The available treatment methods
include alteration of slope geometry, dewatering to increase resisting shear strength,
rock bolts, wire mesh to preventing the rock fall and erosion protection (US Army
Corps of Engineer, 1994).
a) Slope geometry
In the absence of an imposed load, the forces which tend to cause the
instability of a slope are direct function of both slope height and angle of
inclination. A reduction of slope height and/or angle of inclination reduce the
driving forces and, as result, increase stability. In addition, since the majority
of rock slope stability problems are three-dimensional in nature, a few
degrees of rotation in the strike of the slope can in some cases, causes a
potentially unstable slope become kinematically stable.
b) Dewatering
In the presence of ground water within a rock slope can effectively reduce the
normal stress acting on the potential failure plane. A reduction in normal
stress causes a reduction in the normal stress dependent friction component of
shear strength. Groundwater induced uplift can be controlled by two
methods, internal drains and external drains. In this respect, drainage is often
the most economical and beneficial treatment method.
(1) Internal drains.
Properly designed and installed internal drains can effectively reduce
groundwater levels within slopes thereby increasing stability. The
specific design of an effective drain system depends upon the
geohydraulic characteristics of the rock mass (i.e. joint spacing,
condition and orientation, as well as source of groundwater). As a
16
minimum, a effective drain system must be capable of draining the
most critical potential failure surface. In climates where the ground
surface temperature remains below freezing for extended periods of
time, the drain outlet must be protected from becoming plugged with
ice.
(2) External drains.
External or surface drains are designed to collect surface runoff water
and divert it away from the slope before it can seep into the rock
mass. Surface drains usually consist of drainage ditches or surface
berms. Unlined ditches should be steeply graded and well
maintained.
c) Rock bolts.
Rock bolts is a tensioned reinforcement element consisting of a rod, a
mechanical or grouted anchorage and a plate and nut for tensioning or for
retaining tension applied by direct pull or by torquing. Rock bolts with
sufficient length will increase the inherent strength and holding the unstable
discontinuities.
d) Wire mesh
The most common type of surface treatment is wire mesh or fabric attached
directly to the reinforcement elements. Such surface treatment can and
should be used as a routine part of construction. The benefits include
stabilization of deeper rock by holding loosened rock in place and control of
unstable areas by containing loose rock rather than allowing it to fall. The
safety benefits are obvious. The fabric should be galvanized if it is
permanently exposed but may be ungalvanized if it is to be eventually
covered by shotcrete or concrete. Examples of surface treatment are shown
in Figure 2.9 and 2.10.
17
e) Erosion protection.
Shotcrete, frequently with the addition of wire mesh and/or fibers, is an
effective surface treatment used to control slaking and reveling of certain
argillaceous rock types that can lead to erosion problems. The treatment also
prevents loosening of rock mass due to weathering processes and provides
surface restraint between rock bolts.
Figure 2.9 : General surface treatment-wire mesh and shotcrete (US Army
Corps of Engineer, 1980).
Figure 2.10 : Typical local structural treatment of wide seams and fractured
zones (US Army Corps of Engineer, 1980).
In combination with rock bolt system, the use of shotcrete as surface
treatment is very effective in preventing surface raveling. As with rock bolts,
shotcrete is generally most effective (except in swelling ground) if it is applied
shortly after excavation. When permanently exposed, the shotcrete should always be
placed in combination with welded wire fabric or chain link fabric. Drain holes
drilled through the shotcrete into the rock should be constructed to relieve
hydrostatic pressure.
18
2.9
Ground water flow
Tropical countries like Malaysia receive more than 3000mm precipitation
every year. Therefore some remedial work is need to control the water dissipation
into rock mass especially there are presence of large scale discontinuities like joints
which can effected to instability of slope. This water dissipation from water surface
or run off almost defined as ground water level, which depends on rock mass
properties like porosity, permeability and etcetera.
The presence of ground water in the rock mass surrounding an open pit has a
detrimental effect upon the mining program. Water pressure reduces the stability of
the slope by reducing the shear strength of the potential failure surfaced. Water
pressure in tension crack or similar near vertical fissures reduces stability by
increasing the forces tending to induce sliding. High moisture content results in an
increased unit weight of the rock and hence give rise to increased transport cost.
Changes in moisture content of some rocks, particularly shales, can cause accelerated
weathering with resulting decrease in stability.
Erosion of both surface soils and fissure infilling occurs as a result of the
velocity of flow of groundwater. This erosion can give rise to a reduction in stability
and also to silting up of drainage system. Discharge of groundwater into an open pit
gives rise to increased operating cost because of the requirement to pump this water
out and also because of the difficulties of operating heavy equipment on very wet
ground. Blasting problems and blasting cost are increased by wet blast holes.
Liquefaction of overburden soils or waste tips can occur when water pressure within
the material rises to the point where the uplift forces exceed the weight of the soil.
This can occur if drainage channel are blocked or if the soil structure undergoes a
sudden volume change as can happen under earthquake conditions.
The most important effect of the presence of groundwater in rock mass is the
reduction in stability resulting from water pressure within the discontinuities in the
rock. Therefore, any work that related to the slope either for soils or rock slope
19
needs proper drainage system to control the presence of ground water level or surface
water and run off.
2.10
Weathering
Rock surfaces that have been exposed to the elements will show evidence of
disintegration of a surficial layer (Legget and Hatheway, 1988). It is including the
changes, which occur essentially in place when rocks are contact with the
atmosphere, surface water and organisms. The engineering projects like rock sloping
have come to grief because of failure to appreciate the influence of weathering on
construction problems. In other instances, failure to anticipate deep or irregular
weathering has led to greatly increased costs (Schultz and Cleaves, 1956).
The two types weathering agencies are physical and chemical. The former
includes such item as temperature changes, crystals growth, pressure and the action
of plant roots and burrowing animals. The second involves numerous and often
complex reactions. Physical weathering results in disintegration and chemical
weathering in decomposition of parent material (Schultz and Cleaves, 1956).
Products of weathering or the degree of rock is weathered is controlled
primarily by weathering resistance of individual rock forming minerals. These
minerals respond differently to weathering processes, meaning that each instance of
rock weathering is a case of individual mineral breakdown; some minerals respond
faster than others. Degree of weathering can be classified into classes; Zone 1(Grade
I) to Zone 6 (Grade VI). Where is the degree of weathering increased from Zone I to
Zone III. Table 2.1 shown descriptions of zone and weathering grade of rock.
20
TABLE 2.1 : Description of zone and weathering grade of rock (Attewell, 1993).
Weathering
Zone (material
grade)
Zone 6 (Grade
VI)
Descriptive
terms
Material description and likely engineering
characteristics
Residual soil.
Completely degraded to a soil; original rock
fabric is completely absent; exhibit large volume
change; the soil has not been significantly
transported.
Stability on slopes relies upon vegetation rooting
and substantial erosion & local failures if
preventive measures are not taken.
Rock is substantially discoloured and has broken
down to a soil but with original fabric (mineral
arrangement & relict joints) still intact; the soil
properties depend on the composition of the
parent rock.
Can be excavated by hand or ripped relatively
easily. Not suitable as foundation for large
structures. May be unstable in steep cuttings and
exposes surfaces will require erosion protection.
Rock is substantially discoloured and more than
50% of the material is in degraded soil condition;
the original fabric near to the discontinuity
surfaces have been altered to a greater depth; a
deeply weathered, originally strong rock, may
show evidence of fresh rock as a discontinuous
framework or as corestone; an originally weak
rock will have been substantially altered, with
perhaps small relict blocks but little evidence of
the original structure.
Likely engineering characteristics are as in Zone
5.
Rock is significantly discoloured; discontinuities
will tend to be opened by weathering process and
discolouration have penetrated inwards from the
discontinuity surfaces; less than 50% of the rock
material is decomposed or disintegrated to a soil;
rock samples containing discolouration are
noticeably weaker than the fresh undiscoloured
rock; an originally weak rock will comprise relict
blocks of substantially weathered material.
Occasionally may be excavated without blasting
or cutting (i.e. by block leverage at the
discontinuities); will be relatively easily crushed
by construction plant moving over it in situ, may
be suitable as rock foundation (with some
reinforcements); joints may exhibit lower
strength characteristics, so rendering side slopes
unstable.
Zone 5 (Grade
V)
Completely
weathered
Zone 4 (Grade
IV)
Highly
weathered
Zone 3 (Grade
III)
Moderately
weathered
21
Table 2.1 (Continued)
Zone 2 (Grade
Slightly
II)
weathered
Zone 1 (Grade
I)
Fresh
Some disclouration on and adjacent to
discontinuity surfaces; discoloured rock is not
significantly weaker than undiscoloured fresh
rock; weak (soft) parent rock may show
penetration of discolouration.
Normally requires blasting or cutting for
excavation; suitable as a foundation rock but with
open jointing will tend to be very permeable.
No visible sign of rock material weathering; no
internal discolouration or disintegration.
Normally requires blasting or cutting for
excavation; may require minimal reinforcement
in cut slope unless rock mass is closely jointed.