Engineering
Geology Study around Malekhu area and
proposed Buddhi
Gandaki Hydroelectric Project area
CHAPTER 1
1.1
Introduction
Geology is the
study of the earth, its history, its exterior as well as interior, and the
processes that act upon it. Geology is an important way of understanding the
world around us, and it enables scientists to predict how our planet will
behave. Scientists and others use geology to understand how geological events
and earth’s geological history affect people.
The two days
geological tour to Malekhu, Dhading and Gorkha district were on 4th
and 5th of Bhadra. On first day we were taken to Gorkha district to
study rock mass and rock mass classification by RMR system, rock slope
stability and underground excavation and support system of test adit of buddhi
gandaki. On the second day we studied about preparing engineering geological
map along Malekhu Bhandra road section, weathering profile and mass movement.
1.2
Objective
ü Study fo rock mass
and rock mass classification by RMR system along Benighat to Arughat road
section.
ü Study of
underground excavation and support system of text adit of Buddhi Gandaki
ü Study fo Rock
slope stability along Benighat to Arughat road section
ü Preparation of
Engineering geological map along Malekhu Bhandra road section
ü Study of
weathering profile
ü Study of mass
movement
1.3
Instruments used
Ø Brunton Comapss : A brunton compass is an instrument used
to determine compass bearing, horizontal angle, and to measure the inclination
of object. It applies the same principle
as a compass, clinometers and level. The brunton’s north compass needle
attracts to magnetic nort. The compass needle points to graduated circle, which
is the compass bearing for that direction. When brunton is on its side, a vertical angle measurement is taken by
clinometers. Another use is to determine the dip of bed.
Ø Geological Hammer:
A geologist's hammer, rock hammer, rock pick or geological pick is a
hammer used for splitting and breaking rocks. In field geology, they are used
to obtain a fresh surface of a rock in order to determine its composition,
nature, mineralogy, and history and field estimate of rock strength. Most
commonly the tool consists of a combination of a flat head, with either a
chisel or a pick head at the other end.
v
A
chisel head (pictured), which is shaped like a chisel, is useful for clearing
covering vegetation from exposures.
v
A
pick head, which terminates in a sharp point to deliver maximum pressure, is
often preferred for harder rocks.
v
A
flat head is used to deliver a blow to a rock with the intention of splitting
it.
CHAPTER 2
Study fo rock mass
and rock mass classification by RMR system along Benighat to Arughat road
section.
2.1 Location
: 2 km from Trishuli Bridge along Benighat to Arughat in Gorkha district
Intact rock : Intact rock is the term applied to rock
containing no discontinuities.
Rock mass : Rock mass is a mass of rock interrupted by
discontinuities with each constituent discrete block having intact rock
properties.
2.2 Rock type
Types
of rocks are:
(a)
Igneous rock: The process of rock
forming by cooling and solidification of molten mobile material magma by
crystallization is called magamtization. In this processs magma lose its heat
gradually on the upward movement, it lose heat and becomes crystals by
crystallization. The rock formed by this process of magamtization is called
igneous rock.
Example
: granite, rhyolit, basalt, gabbro, etc.
(b)
Sedimentary rock: These rocks,
formed from the process of sedimentation are called sedimentary rocks.
Sedimentation process is accumulation, compaction, cementation consolidation of
sediments by weathering of old rocks either igneous, metamorphic and
sedimentary and are then transported by geological agents like water, wind,
ice, etc.
Example:
conglomerate, breccias, sandstone, limestone, etc.
(c)
Metamorphic rock: Those rocks are
formed from the alteration of the pre-existing rocks(sedimentary-igneous) by
the process of metamorphism are called metamorphic rock. Metamophism is the
process by which the existing rocks are altered into new rock under the
influence of pressure, temperature and chemical solution. In metamorphic rock
the minerals are arranged in preferred orientation.
Example:
Gneiss, schist, slate, quartzite, phyllite, etc.
2.3 Weathering: Weathering is the process of breaking down rocks by mechanical and chemical processes into
smaller pieces. Mechanical weathering may be caused by the expansion and contraction
of rocks from the continuous gain and loss of heat, which results in ultimate
disintegration. Frequently, water seeps into the pores and existing cracks
in rocks. As the temperature drops, the water freezes and expands. The
pressure exerted by ice because of volume expansion is strong enough to
break down even large rocks. Other physical agents that help
disintegrate rocks are glacier ice, wind, the running water of streams
and rivers, and ocean waves.
In
mechanical weathering the rock gets weathered by physical phenomenon such as by
friction, heat etc and the composition of rock remains same but in case of
chemical weathering due to the chemical reaction the composition of rock gets
changed. For example;
Orthoclase to form clay minerals,
silica, and soluble potassium carbonate
Follows:
Most
of the potassium ions released are carried away in solution as potassium carbonate
is taken up by plants. The chemical weathering of plagioclase feldspars is
similar to that of orthoclase in that it produces clay minerals, silica, and
different soluble salts. Ferromagnesian minerals also form the decomposition
products of clay minerals, silica, and soluble salts. Additionally, the iron
and magnesium in ferromagnesian minerals result in other products such as
hematite and limonite. Quartz is highly resistant to weathering and only
slightly soluble in water.
The
weathering process is not limited to igneous rocks. As shown in the rock cycle
Sedimentary
and metamorphic rocks also weather in a similar manner.
2.4 Intact rock strength: Intact
rock strength (IRS) is a major rock property. Intact rock strength determines
the strength of the intact rock block material and as such governs partially
the strength of a rock mass In order to determine the strength of the intact
rock in field generally Schmidt Hammer
Rebound Test is followed. On the basis of the height of rebound the strength of
rock mass is designated. The accuracy of
the work in this method depends upon the experience of the engineer or
geologist.
There
are other various methods to find the strength
in laboratory of the Intact Rock, few of them has been given below:
1.
Unaxial tensile Test: This method can
further be divided into types
a.
Direct tensile strength test
b.
Point load Test
c.
Brazilian test
2.
Unaxial Compressive Test
3.
Triaxial compressive test
Tensile
strength describes the capacity of the rock to resist tensile stress.There are
direct and indirect methods for measurement of tensile strength (ISRM 1985).
The indirect methods have been dominant in determining tensile strength of
rocks in the past due to their ease in sample preparation and testing
procedure. The indirect methods include point load test and Brazilian test.
2.5
Discontinuities in rock : Discontinuities in rocks are:
Ø
Fault: A fault is a
rupture in rocks along which there has been a relative displacement of the two
sides parallel to the fracture plane. Fault is the result of brittle
deformation due to tensional or compressive forces.
Ø
Fold: Folds are the
deformational structure on the rock strata fromed due to compressional forces.
Folds are the ductile deformation i.e. deformation which does not rupure or
fracture.
Ø
Joints: Joints are the
fractures along which there has been no relative displacement along the
fracture plane. Joints are the result of brittle deformation due to tensile or
shearing stesses.
2.6 Properties of discontinuitites
·
Orientation
of discontinuities
·
Spacing
of discontinuities
·
Continuity
of discontinuities
·
Separation
and infilling of discontinuities
·
Groundwater
condition
2.3 Rock Mass
classification
RQD (Rock Mass
Designation):
RQD is defined as the percentage of intact core pieces longer than 100 mm (4
inches) in the total length of core.
RQD =(Summation of length of core > 10cm)/(total length of core)x100
Palmstorm
suggested, when no core is available but discontinuity traces are visible in
surface exposures or exploration adits, the RQD may be estimated from the
number of discontinuities per unit volume. The suggested relationship for
clay-free rock masses is: RQD = 115 - 3.3 Jv (1) where Jv is the sum of the
number of joints per unit length for all joint (discontinuity) sets known as
the volumetric joint count.
RMR(Rock Mass
Rating)
:
Bieniawski (1976) published the details of a rock mass
classification called Geomechanics classification or the Rock mass rating (RMR) system. Over
the year this system has been successively refined as more case have been
examined and reader should have aware
that Bieniawski has made
significant ch anges in the ratings assigned to the different parameters .
In
order to rate a rock mass following six
parameters are used:
i.
Intact rock strength
ii.
Rock Quality Designation ( RQD)
iii.
Spacing of the discontinuities
iv.
Condition of discontinuitites
v.
Ground water conditions
vi.
Orientation of discontinuities
In applying this classification
system, the rock mass is divided into number of structural regions and is
region is classified separately. The boundaries of the structural regions usually coincide with
a major structural feature such a fault or with a change in rock type .
The Rock Mass Rating System is
presented in following table Rock Mass Rating System( After Bieniawski 1989).
Table of Rock Mass Rating System
(After Bieniawski 1989) has been referred for obtaining data given below.
2.3 FIELD
DATA
SN
|
parameter
|
value
|
rating
|
1
|
strength of intact rock
|
hard
|
15
|
2
|
RQD = 115-3.3jv ; jv
= 22
|
115-3.3x22=42.4
|
8
|
3
|
spacing of discontinuitites
|
16,5,12,10,15m
maximum = 16
|
20
|
4
|
Condititon of discontinuities
a. Discontinuity length
b. Aperture
c. Roughness
d. Infilling material
e.Weathering rating
|
3.5m, 2m 1m, 5m
2.5cm, 1cm, 0.5cm
slightly roughness
soft
Moderately
|
2
0
3
0
3
|
5
|
Groundwater
|
none
|
15
|
TOTAL RATING = 54
ROCK QUALITY = FAIR(41-60)
chapter 3
Study of
underground excavation and support system of text adit of
Buddhi Gandaki
Location 2 : At tunnel, 2.5km form Trishuli Bridge along
Benighat to Arughat road
3.1 Introduction
Tunnels : Tunnels
are underground passages or routes through hills or mountains used for
different purposes. They are made by excavation of rocks below the surface or
through hills or mountains. Tunnels are driven for variety of purposes and are
classified accordingly. Chief classes of tunnel are: Traffic tunnel,
hydro-power tunnel and public utility tunnel. Metros which are symbolic of
great progress achieved by advanced countries are a version of tunneling and in
fact may involve a good length of tunnels as their essential component. Tunnneling
has been one of the most challenging jobs for the engineers. Excavation below
ground for whatsoever purpose need very sound knowledge about the soil and
rocks to be excavated on the one hand and keep the excavations so created
tunnels safe and stable at economically viable costs for the entire life of
these projects on the other hand. Like buildings, roads, railways and many
other construction jobs, tunneling projects are included in the most impotant
developmental activities of the big nations. Geological information is an
integral part of all the processes involved in preparing designs, executing
excavations and construction of all types of tunnels.
3.2 Site selection for Tunnel and Geological considerations
1. Rock Type : Since tunnels pass through underground rock masses,
obviously the nature of rock types which are encountered along tunnel alignment
is very important for the safety and stability of the tunnel. Competent rocks
i.e. those which are strong, hard, massive though difficult to tunnel will be
safe but loose, incompetent rock rock though fast to tunnel is not safe and
require linning. If the tunnel extends for considerably long distance, the kind
of rocks in route may vary from place to place i.e. competent at some places
and incompetent at some other places.
2. Discotinuity : presence
of discontinuitites make rock unsafe. The bearing of structures in tunnels is
very impotant for two reasons:
Ø
they modify the competency and
suitability of rocks for tunneling
Ø
they may create or prevent ground water
problems, which are of critical importance in tunneling.
Joints, faults and tilted characters are the
most common strucrual features associated with rocks.
3. Fold : Anticline
fold is more stable than syncline. circular is more stable type of shape of
tunnel because stress acting on any direction gets deflected but in semi
circular shape at ends stess is concentrated. Horse shoe shaped is preferred
shape. Folded rocks are under strain. When excavations for tunnels are made in
folded rocks, such rocks release the strain energy which may occur in the form
of rock bursts or rock fails or bulging of the sides of the floor or the roof.
If tunnel alignment is parallel to
the axis of a fold, then the condition is desirable because similar formation
or formations with similar physical conditions i.e. stress-stain conditions are
encountered along the course of the tunnel. Tunneling along limbs only is the
desirable and not along crests and trough.
Tunnel along the crest: Along the
bends of folds, along crests, the rock masses may be in a highly fractured
condition due to the development of tensional joints. As a consequences of
this, if tunnels are driven in such places, there may be frequent fall of rocks
from the roof.
Tunnel alignment along the trough: Tunnels
along troughs also encounter unfavourale conditions, because rock masses there
will be harder and more resistant. This means excavation through them will be
difficult process. Further, the inclination of bedding planes may guide the
percolated water towards the trough and creates undesirable ground water
problems.
Tunnel alignment perpendicular to
the axis of fold is undesirable because different rocks formations are
encountered from place to place along the length of tunnel and also tunnel has
to pass through a series of anticlines and synclines. In anticlines limbs will
be under great strain and crest under low strain causing physical heterogeneity
while in syncline core regions will be greatly stained.
In case of horizontal or gently
inclined bed conditions will be favourable for tunneling. This is preferable
because thicker formations are more competent and hence tunnels through them
will be safe and stable.
4. Fault : Fault
are harmful and undesirable. The active fault zones are places where there is
scope for further recurrence of faulting, which will be accompanied by the
physical displacement fo litho units. Hence, such faults lead to dislocation
and discontinuity in the tunnel alignment. Therefore, irrespective of the
relation of the attitude of the fault with the tunnel coarse, the occurrence of
any active fault in tunnel is very undesirable.The fault zones, even if
inactive, are places of intense fracturing which means that the are zones of
great physical weakness. Such a remedial mesure of lining with concrete also
becomes necessary because fault zones, being highly porous, permeable and
decomposed, are the potential zones to create ground water problems,
5. Groundwater : Groundwater
above tunnel is danger as large quantities of groundwater may gush out(to suddenly let
out large amounts of a liquid) and inundate (to cover an area of land with
large amount of water). Groundwater makes easier the movement of rocks upon
each other causing slips along joints and bedding planes.
Ground
water conditions affect soft-ground tunneling more than any other single
factor. Many tunneling hazards like failure of roofs, swelling or squeezing of
ground etc. are all intimately related to ground water conditions. Thus hard,
dry and compact clays might be quite safe and easy for excavation along tunnel
line but when the same formations happen to be overlain by saturated sands and
gravels, these might become the most plastic and difficult formations to
tunnel.
6. No. of joints : Joints are plane of complete separation in rock masses and
represents weakness. Closely spaced joints in all types of rocks are harmful. Joints
which strike is parallel to the tunnel axis naturally persist for long
distances and hence are undesirable and joints with strike oblique or
perpendicular to the tunnel axis will have a limited effect on them.
7. Overbreak : Overbreak
indicates the quantity of rock broken and removed in excess of what is required by the perimeter of the proposed
tunnel. An excavation through hard rocks necessarily involves the removal of
some of the rocks outside the proposed perimeter of the tunnel. The quantity of
the rock broken and removed, in excess is required by the perimeter of the
purposed tunnel, is known as overbeak. The geological factors which govern the
amount of overbreak are:
·
the nature of rock
·
the orientation and spacing of
joints or weak zones in them
·
in case of sedimentary rock, the
orientation of the bedding plane and thickness of beds with the alignment of
the tunnel.
Overbreak adds to the cost of
tunneling, particularly if lining is required so overbreak should be as minimum
as possible.
Requirement for the site selection for tunnel/Dam
1.
Lithology
a.
Hard rock is most favourable.
b.
Soft rock gives the problem of
squeezing and swelling
2.
Geological Structures
a.
Horizontal bedding is preferred
b.
Tunnel is driven parallel to the
strike Creates problem.
c.
Syncline fold is better for the
stability of tunnel
d.
There should not exist the fault
zone
SOME TERMS
Rock bolting : Rock bolt is a long anchor
bolt for stabilizing rock
excavations, which may be used in tunnels or rock cuts. It transfers load from
the unstable exterior, to the confined (and much stronger) interior of the rock
mass. It prevents rock falling.
Rock
obtained is fair rock so tunnel support at this site are :
Rock mass class
|
Excavation
|
Rock bolts(20mm
diameter fully grouted.
|
Shotcrete
|
Steel sets
|
III-Fair rock
RMR : 41-60
|
Top heading and
bench
1.5-3m advance
in top heading
Commence support
after each blast
|
Systematic bolts
4m long, spaced 1.5-2m in crown and wells with wire mesh
|
50-100mm in
crown and walls with wire mesh
|
None
|
CHAPTER 4
Study fo Rock
slope stability along Benighat to Arughat road section
4.1 Introduction
Kinematic analysis is a method used to analyze the potential
for the various modes of rock slope failures (plane, wedge, toppling failures),
that occur due to the presence of unfavorably oriented discontinuities. Slope stability
analysis is performed to assess the safe design of human-made or
natural slopes (e.g.
embankment, road
cuts, open-pit
mining, excavations, landfills etc.) and the equilibrium conditions. Slope stability is the resistance of inclined surface to failure by sliding or collapsing. The
main objectives of slope stability analysis are finding endangered areas,
investigation of potential failure mechanisms, determination of the slope
sensitivity to different triggering mechanisms, designing of optimal slopes
with regard to safety, reliability and economic designing possible remedial measures, e.g.
barriers and stabilization.
4.2 Types of failure
In rock slope, therer are three types of failures which occur
on the basis of orientatin of discontinuities with respect to the orientation
of hill slope or cut slope. The three type of failures are:
(1) Plane failure: Planar failure of rock slope occurs when the mass of rock
in a slope slides down along a weak plane. Conditions of plane failure are:
ü The joint plane and the hill slope should dip in same direction.
ü The dipping of the joint should be less than the dip of the hill slope.
ü The strike difference should be between 20°.
ü The dip of the joint should be more than internal friction angle.
(2) Toppling
failure:
The toppling failure is possible when the planar features dip opposite to hill
slope or cut slope and the hill slope is steep enough than the planer features.
In general, the hill slope or cut slope is at least 55o. Conditions
of toppling faiure are:
ü The joint plane and the hill slope should dip in opposite direction.
ü The strike difference should be between 20°.
ü The dip of the joint should be
more than internal friction angle.
(3)
Wedge failure: The wedge failure is possible, when two
planes intersects obliquely across the slope face and their line of
intersection plunges at the same direction as the dip direction of hill slope
or cut slope. The line of intersection must be on daylight zone of slope face
i.e. inclination of this line is less than that of hill slope or cut slope
face.
According to the relation of dip direction of wedge
and natural slope, the wedge are of three types. When a wedge shows an
intersection parallel or close to the direction of the slope(up to 32o
on either side of the direction of slope), it is defined as a central wedge,
and from 32o to 65o known as lateral wedge. Behind 65o,
it is described as a very lateral wedge.
Conditions for the Wedge failure:
ü The wedge and the hill slope should dip in same direction.
ü The dipping of the wedge should be less than the dip of the hill slope.
ü The strike difference should be between 20°.
ü The dip of the wedge should be more than internal friction angle.
4.2
Stereonet
Slope stability is
done by using stereonet. A stereonet is a lower hemisphere graph on to which a
variety of geological data can be plotted.
Stereonets are used in many different branches of geology. Stereographic
projection involves plotting 3D data (planar or linear) on to a 2D surface
(stereonet) where it can be manipulated and interpreted.
FIELD
DATA
data
1
|
data
2
|
data
3
|
data
4
|
data
5
|
data
6
|
|
strike
|
N84oW
|
N31oE
|
N45oE
|
N85oW
|
N45oE
|
N80oW
|
dip
amount
|
84o
|
76o
|
87o
|
86o
|
82o
|
74o
|
dip
direction
|
N2oE
|
N60oW
|
N39oW
|
N7oE
|
N37oW
|
N9oE
|
Hill slope:
dip direction :
N46oE
dip amount : 56o
strike : N44oW
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