Practices in Planning, Design and Construction of Head Race Tunnel of a Hydroelectric Project.

Abstract—A channel/tunnel, which carries the water to the
penstock/pressure shaft is called head race tunnel (HRT). It is
necessary to know the general topography, geology of the area, state
of stress and other mechanical properties of the strata. For this certain
topographical and geological investigations, in-situ and laboratory
tests, and observations are required to be done. These investigations
play an important role in a tunnel design as these help in deciding the
optimum layout, shape and size and support requirements of the
tunnel. Scope of the proposed paper: - 1. Role of Geological
Knowledge:- Pre-knowledge of geological data will provide the
engineer with a better insight of the tunnelling conditions and so
allow improved estimates to be made of anticipated progress. 2.
Geometric Design of HRT: - The size or cross sectional area can be
determined from the amount of water (discharge) that is to be
conveyed through HRT under the given head difference, construction
ease, geology along its alignment etc. 3. Hydraulic Design of HRT: -
The hydraulic studies are carried out for the purpose of alignment of
HRT and design of lining system for HRT. 4. Structural Design of
HRT: The structural design investigates the loads that are expected
on the tunnel opening from the surrounding rockmass and whether a
support is required to hold it in place or a lining is necessary to resist
the pressure of the rock and water pressure from the saturated joints
and cracks of the surrounding rocks. 5. Geological problems during
tunnelling and remedial measures: - These should not be ignored
during the construction as these lead to cost and time over runs and
increase the cycle time drastically.
Keywords—Tunnelling, geology, head race tunnel, rockmass.
INTRODUCTION
LANNING is very important component for the optimum
development of a hydroelectric project. Investigations at
site depend upon the nature of the site. Based on the
investigations optimum dimensions of different hydroelectric
components i.e. dam, reservoir, HRT and powerhouse can be
worked out that could give the most economical scheme [3].
P
Best practices in planning, design and construction of HRT
is taken care of at feasibility report stage, detailed project
report stage, tender preparation stage, construction design
stage and implementation stage. The good practices involved
in planning and design at various stages includes complete
integration of interdisciplinary knowledge. An organization’s
past experience, adequate staffing in terms of specialized
engineering groups and exposure to new construction and
design practices are essential inputs for a good design.
Moreover, in planning, design and construction of HRT,
following considerations are taken into account:

Mohit Shukla is with the Department of PSU, SJVN Ltd., Shimla, India (E-
mail: shuklamohit5@gmail.com).
1. Plan alignment considering the geology to be met and
topographical consideration
2. No. of construction faces, length and position of adits
with a view to optimize the construction schedule
3. Size of the tunnel decided on the basis of the velocity,
head loss and economic consideration
ROLE OF GEOLOGICAL KNOWLEDGE
Geological investigations for tunnel site are conducted in
three stages. In the initial stage, a desk study is undertaken
using available maps and aerial photographs to obtain an
overall impression of the geological conditions and to plan
subsequent investigations. The second stage requires a more
detailed investigation and it is geared to the determination of
the feasibility of a particular location. At this stage,
consideration is given to alternate selected tunnel alignments.
Once a tunnel site is selected, then investigation enters the
third phase when special additional work is conducted to assist
the final design and estimation of tunnel costs. The
investigation should produce a geological map of the area and
a cross section along the centre line of the tunnel. Wherever
possible, the position of the water table should be shown on
the section [2]-[4].
The geology along a tunnel alignment plays an important
role in many of the major decisions that must be made in
planning, designing, and constructing a tunnel. The feasibility,
geology along the tunnel alignment and cost of any tunnel
depends solely upon the geology. An adequate intensity of site
exploration, from which geological and hydrological
mappings and ground profiles are derived, is most important
for choosing the appropriate tunnel design and excavation
method. The rock conditions of the tunnelling location need
thorough exploration to study the response of the immediate
rocks surrounding the tunnel to disturbance caused by
excavation; rock strength, collapse potential and rock pressure
in order to satisfy the chosen design and construction method.
It has long been recognized that detailed and prior knowledge
of the geological character of a tunnelling site is a pre-
requisite to successful design, construction and operation for
the majority of such projects.
It is to be noted that inspection and mapping of strata
should continue during tunnel construction. This
information helps to complete the picture of the geological
setting as revealed by the site investigation and help the
geologist predict any changing condition in advance of the
tunnel heading.
DESIGN ASPECTS OF A TUNNEL
A. Tunnel Layout
Practices in Planning, Design and Construction of
Head Race Tunnel of a Hydroelectric Project
M. S. Thakur, Mohit Shukla

Tunnel layout is the route layout of the tunnel in plan [5].
The layout is usually governed by the geology, topography
and the type of hydropower scheme adopted. It must be
noted that the complicated geological conditions and
extraordinary geological occurrences such as intra-thrust
zones, very wide shear zones, geothermal zones of high
temperature, cold/hot water springs, water charged rock
masses, intrusions, fault planes, etc. should preferably be
avoided while deciding the layout and sound,
homogeneous isotropic and solid rock formations should
be selected.
B. Selection of the Alignment
While selecting the alignment the following points should
be considered:
(i) It should be the Shortest Possible: This would
ensure minimum losses and shall be economically
cheapest.
(ii) It should be Straight as far as Possible:
Introduction of bends in the alignment shall involve
losses at all such bends and the cost of tunnelling
would also increase,
(iii) It should be Easily Accessible: An easy access near
the entrance and exit to the tunnel becomes essential
for the construction facility,
(iv) Careful selection of entry and exit locations with
minimum length and depth of approach cuttings and
no weathered, loose fractured layers slope towards
portals.
However, it is not always possible to follow a straight
alignment because of the topographical and geological
obstructions prevailing along the length of the tunnel. This
is discussed below with an example.
The alignment of the tunnel is greatly affected by the
topography and geology.
SJVN Ltd had two alternatives i.e. to keep the HRT on the
left or right bank. The right bank which is comparatively
gentle in nature is dominated by overburden, mainly in the
form of slope debris with scanty rock exposures and lot of
cultivation. The right bank slopes are dissected by 23
nallahs. In contrast the left bank slopes are rocky with less
nallahs. Moreover the left bank consists of massive
quartzite for considerable length which would be suitable
for butting of the dam and for locating the intake of the
HRT. Therefore, the nature of the left bank slopes and
rock type influenced the decision to keep the HRT on the
left bank.
Selection based on geology dominates the accessibility
criteria:
Note: It is not necessary that we have to always choose the
location which is easily accessible. As in the above case
though a PWD road was available on the right bank from
Devraulli to proposed Power house site still right bank
was not chosen, as the geological mapping of the right
bank alignment of HRT has suggested a unfavourable
geological conditions whereas the general steep slopes of
the hills and steep side faces in the first few kilometres of
HRT (from dam site) indicate the overall favourable
topography for placing the HRT on the left bank.
C. Selection of the adit and exit locations
In the choice of the tunnel axis the careful selection of the
adit and exit locations is important. It has occurred more
than once that an incorrectly selected portal has broken
down shortly after the commencement of excavation and
has had to be abandoned for another site. Conditions are
especially favourable where weathered, loose, fractured
layers slope towards the portal. If these are perforated by
an approach cutting before a sufficiently resistant tunnel
portal structure is built, the entire slope may be mobilized
and sliding can no longer be arrested.
Rockfall protection measures comprising of scaling and
cleaning of the rock slope and rock fencing will be prudent
to install as part of the portal work to protect workers and
equipment during portal and tunnel excavation. Moreover,
the correct selection of length of approach cutting is also
of great significance. For economic reasons its depth
should not exceed 20-25 m yet at the same time the adit
section should not be sited in a sliding layer as shown
below.
Fig. 1 Design of a Portal [2]
The adit section shall not be sited in a sliding layer and highly
resistant structures must be designed in uncertain layers and
the coping of the portal (capping or covering of a wall to
manage stressful circumstances) should reach above the
fractured layers covering the slope.
D. Deciding the number of Adits
The alignment of the tunnel is so chosen that adequate vertical
and horizontal cover for the tunnel is ensured and there are
minimum bends on its way commensurate with the maximum
distance between two adjoining adits as 5 Km. The optimum
number of adits is decided based upon the planned
construction schedule. The location of the adits has to be
chosen based on the existing topographical and geological
conditions so that the planned targets are met and the
resources are utilized optimally. The size of the adits should
be wide enough to cater easy movement of traffic both ways
so that the speed of tunnelling is maintained. Keeping in view
the length of the tunnel, the provision of access in the adit
plugs shall be provided. This will facilitate the future
inspection inspection of the HRT after emptying when
essential.
E. Tunnel Cross-section
The aspect requires the determination of the size and shape of
the tunnel. HRT is classified on the basis of flow i.e. Pressure

flow and open channel flow system and the shape of the tunnel
i.e Circular, D-Shaped, Horse-Shoe & Egg shaped sections.
Influence of Geological Environment on shape of Cross-
section: A factor of considerable influence on the shape of the
cross-section is the type of geological environment in which
the tunnel is constructed. The magnitude of external loads i.e.
the rock pressure depends on the inherent strength of the rock
material, the quality of which is responsible for the ratio
between vertical and lateral pressures acting on the tunnel. In
loose, soft and weak rock materials large rock pressures and a
relatively considerable lateral thrust may be expected. The
greater the relative magnitude of the lateral thrust the more
advantageous a circular section will be found to be. In solid
rocks showing no tendency to weathering, tunnel sections
excavated with an arched roof may serve without resort to any
lining. In deciding the shape of the cross-section, in addition
to structural consideration, earth pressure theory may also
serve as a guide i.e. cross section is to be selected in
accordance with the relative magnitude of vertical and
horizontal loads (Rankine’s earth pressure theory). Where the
vertical loads are relatively larger, an ellipse with a vertical
major axis is preferable, while for relatively large horizontal
pressures one with a horizontal major axis will prove more
suitable, but in practice a shape composed of circle segments
is used instead of an ellipse [2].
Influence of Construction method and lining material on
the shape of Cross-section: The method of construction must
be chosen in accordance with the prevailing soil condition but
may be influenced to some extent by the availability of
equipment, machinery and materials. Conventional mining
methods are suitable for driving horseshoe and flat arched
sections, and, less economically, for circular sections whereas
shield method is restricted almost exclusively to circular
section. The free face method can be used for the cross-section
of any desired shape. Moreover the material of tunnel lining
also influences the shape of the tunnel. Since materials
capable of resisting compressive stresses only (brick or stone
masonry, concrete) are limited to structures composed of
arches e.g. circular, elliptical and horseshoe sections whereas
materials capable of resisting tensile and bending stresses
(reinforced concrete, steel) can be used for lining sections of
any desired shape.
HYDRAULIC DESIGN OF HRT
While carrying out the hydraulic design of HRT, head losses
and hydraulic gradient line for HRT have to be worked out
both for steady state and transient conditions. The hydraulic
design includes working out the losses occurring during the
flow of discharge through the tunnel and the design of lining
based upon the losses [5].
A.Economic Diameter
Diameter optimization studies for the design discharge are
carried out to determine the diameter of the tunnel. Designers
have to select the velocity of flow of water (4-6 m/s) in a
HRT. A higher velocity means more head loss and more
abrasion of concrete lining. Less velocity, however, leads to a
bigger diameter of HRT, which increases cost and the time of
construction. The optimization analysis is based on computing
the total evaluated cost for the range of tunnel diameters
considered and adopting a tunnel size having least evaluated
cost. Thus an economical diameter of a HRT is found by the
trial-and-error approach and shall be examined for practical
consideration such as space required for excavation
equipment.
B. Transition Shapes
The transition shapes are required at the tunnel entry and exit
inorder to reduce the head losses to minimum and avoid
cavitation. The length and shape of the tunnel depends upon
the velocity and flow conditions prevailing in the tunnel,
economics, construction limitation etc. Hydraulic model
studies are conducted to determine an efficient and
economical transition.
C. Tunnel entrance and exits
The entry into or exit from the tunnel has to be carefully
designed. Since at these points the water enters or leaves the
tunnel, they are prone to hydraulic head loss therefore, a
proper transition shape has to be provided to keep the loss
minimum and to avoid cavitation. To minimize head losses
and to avoid zones where cavitation pressure may develop, the
entrance to a pressure tunnel shall be streamlined to provide
gradual and smooth changes in flow. To obtain best inlet
efficiency the shape of entrance should simulate that of a jet
discharging into the air and should guide and support the jet
with minimum interference until it is contracted to the tunnel
dimensions.
D. Velocity
Average permissible velocity in a concrete lined tunnel may
lie between 4 – 5 m/s. For a steel lined tunnel, higher velocity
can be adopted and velocity as dictated by economic studies
shall be chosen. Permissible velocities in tunnels of different
surfaces i.e unlined, steel lined and concrete lined also depend
upon the sediment load carried by the water. Depending upon
the nature of sediment load the velocity may be varied
accordingly.
CHOICE OF TUNNELLING METHODS
The choice of tunnelling method is influenced by several
factors which include the following:
1. Ground conditions: This is the main factor as it may not
only influence the choice of method, but may also present
major limitations on some particular methods. The
following ground conditions are encountered:
(a) Soft ground: The soft ground composition consists of
clays, gravels, sands, weathered rocks in various states of
decomposition and presents ease of excavation
particularly by hand methods.
(b) Rock conditions: Low rock strength may prove
advantageous to machine excavation but require special
support considerations while high rock strengths may
preclude machine excavation but require minimal
temporary support.
(c) Mixed face conditions: Tunnelling at bedrock (Solid
unweathered rock lying beneath surface deposits of soil)
horizon results in the upper part of the face being in the
soil or heavily weathered rock whilst the lower part of the
face is in the rock; this condition presents problems with

machine tunnelling and in connection with the provision
of effective temporary support.
2. Tunnel Size: Microtunnels with diameters of less than 0.9
m to full face TBM tunnels of upto or greater than 12 m
diameters all require special consideration to be given to a
comprehensive assessment of ground conditions;
increasing tunnel size introduces changes in significance
of particular tunnelling problems.
3. Environmental Aspects: Drill and blast operations can
be restricted, if not precluded, in connection with tunnel
drivage in urban areas; water table and ground water
drainage pattern changes can result from tunnelling
activities and affect the surface; other factors include
problems encountered with the intermittent or continuous
occurrence of water and gases from the rocks in close
proximity or even remote to the tunnel.
.
STRUCTURAL DESIGN OF A HRT
The basic philosophy adopted in design of an underground
excavation (tunnelling, surge tanks, power houses etc.) is to
utilise the rock mass itself as the principal structural material,
creating as little disturbance as possible during the excavation
process. The amount of reinforcement required is worked out
depending upon the behavior of the rock mass when the
opening is made in the rock. The structural design is an
important aspect which has to be dealt carefully one the tunnel
is designed geometrically and hydraulically [1], [6], [8].
A. Rock load and its estimation
The most important potential loads acting on underground
structures are earth pressures, i.e. rock pressure and water
pressure. It is general rule that every stress produces a strain
and leads to displacement of individual rock particles. For
movement a rock particle needs space. In confined state the
movement of rock particles is prevented hence the stresses
accumulate and reach very high values which are in excess of
their yield point. By excavating the cavity, opportunity is
given for deformation towards its interior. In order to maintain
the cavity, the intrusion of rock masses must be prevented by
supporting structures and the loads acting thereon is referred
as rock pressure. The magnitude of rock pressure is greatly
affected by the magnitude of deformation produced. The
modern tunnel designers shall predict the (primary) stress
conditions prevailing in the interior of the non-uniform rock
mass (depending on the previous tectonic history) and design
the tunnel for the same as well as for supporting the rock load
which is likely to be developed on account of the opening
made in the rock (secondary stresses). The magnitude of
secondary pressures developing around the cavity is governed
by a variety of factors, such as the size of the cavity, the
method of its excavation, support method and bridge action
period. There are numerous methods of rock load estimation
based on the assumption of the formation of a natural ground
arch above the cavity.
The true art in tunnelling lies in the anticipation of the
development of large rock pressures, which is far more
effective than to find the means of resisting rock pressures
which have already developed [2].
The magnitude of earth pressures is in general independent of
the strength and the time of installation of the supporting
structure and it is only its distribution which is affected by the
deformation of the latter however, the magnitude of rock
pressure is influenced decisively by the strength and the time
of installation of props. This is because deformation
following the excavation of the cavity in rock mass
surrounding the tunnel is of plastic nature and exceeds over a
period of time. The period required for final deformations and
thus for pressures to develop, generally increases with the
plasticity of the rock and with the depth and dimension of its
cross-section. The magnitude of deformation and
consequently that of stresses can be limited by installing an
efficient designed rock support system at the proper time.
B. Importance of Bridge Action period while deciding the
Tunnel Support
The necessity of tunnel support arises from the fact that the
excavated rock has a tendency to drop out of the roof of the
tunnel. The time available to us for installation of the supports
depends upon the bridge action period (the time between
blasting and the beginning of collapse of the unsupported
roof.) which may range from a few hours to few weeks. The
bridge action period for cohesionless sand or completely
crushed rock is almost zero. Hence, if a tunnel passes abruptly
from fairly sound rock into such materials (cohesionless sand
or completely crushed rock), excessive over breaks at the
point of transition are inevitable.
Example: An accident took place during the excavation of
Chukha HRT of Chukha hydropower plant at Bhutan where a
seam filled with sand and water was encountered. As soon as
the rock partition between the rock and the seam was blasted,
sand and water flow of the order of 30 to 50 cusecs (cubic feet
per sec) was observed. If any such geological problem is
encountered, proper investigation of the medium ahead of
face by probe holes and thereafter its treatment through a pre-
drainage and pre-grouting becomes mandatory. Therefore it
becomes necessary that geological investigations should be
continued during construction. The importance of bridge
action is explained below.
Fig. 2 Importance of Bridge Action Period [2]
However, if tunnel bridge action period happens to be quite
large, one should not misunderstand that the tunnel needs no
supports because of the fact that the rock loads go on
developing for weeks and months together after the tunnelling
operations are over.
Example: An accident took place in the Chukha Power House,
Bhutan where the cavity (25.5 m span) was left unsupported

for 35 days and the accident occurred on the 36th day
bringing down about 800 cu m of rock and resulting in the
formation of a dome with a maximum height of about 6 m near
the crown. This is due to the fact that if the cavity is left
unsupported, load at the crown goes on increasing to such an
extent that it exceeds the crushing strength of the rock and at
that stage the crown caves in resulting in dome formation.
C. Excavation and Rock Support System
1. ROCK REINFORCEMENT
The concept of rock reinforcement or active support in rock is
aimed at limiting rock mass deformation by reinforcing the
worn out rock mass in strong and intact rock mass. Such
action is intended to improve the mechanical properties of the
rock mass by interlocking blocks of ground or applying
additional confining stresses, which in both cases aim to limit
the magnitude of the rock mass deformations. This is in direct
contrast to the concept of using free standings or passive
supports such as steel sets where in general the support only
applies an opposing stress to ground movement as a result of
its own deformation by the movement it is designed to
prevent. Such movement can lead to significant deterioration
of the rock mass by extensive rock failure or the opening up of
rock joints which can result in creating inherently unstable
tunnel conditions.
D. Mechanisms of support
There are four basic and relatively independent mechanisms
by which rock reinforcement can improve the stability of a
rock mass.
1. In relatively competent and well-jointed rocks, instability may
be the result of individual blocks detaching from the roof
when the gravitational loading exceeds their retaining load
provided by shear resistance and cohesion along joints. Such
bolts can be stabilised by rock bolts with an anchorage force
capacity greater than the weight of the block in question.
2. In weaker blocky rock conditions, tunnel closure can occur
either through blocks sliding or rotating or through rock
yielding and these can be opposed by tensioned or untensioned
bolts designed to maintain the shear stress along the
discontinuities.
3. In laminated or stratified rocks, instability may be as a result
of slip along bedding planes in addition to bed separations.
Fully grouted, untensioned rock bolts can be used to preserve
the inner strata shear strength and thus prevent such actions
and give rise to forming a significant load sustaining
reinforced beam.
4. Rock failure in overstressed rocks can be minimised by
increasing the degree of confinement or the minor principal
stress. In tunnelling, the minor principal stress vector acts in
general perpendicular to the wall. Thus tensioned rock bolts,
installed relatively quickly after excavation, can provide
additional confinement and limit the extent of rock failure by
increasing the allowable major principal stress.
E. Concrete Lining
Lining in tunnels is a very important component and makes up
for 30 to 40 percent of the total cost of tunnel [1]. Tunnels
forming part of water conductor system have to be invariably
lined with cement concrete-plain or reinforced or steel lined,
However, in cases where a tunnel is meant for operation for
short periods and where it has to be abandoned after it has
served the purpose e.g., diversion tunnels the lining could be
avoided.
Structural design of tunnel lining requires a thorough study of
the geology of rock mass, the effective rock cover, results of
in situ tests and other mechanical characteristics of the rock.
The designer should assess the rock load acting on the lining
and the internal pressure which is to be transmitted to the rock
mass based on the result of these investigations.
The function of concrete lining is one or more of the
following:
• To reduce head losses in the system;
• To protect steel ribs from deteriorating;
• To prevent leakage of water;
• To protect the turbines by preventing loose rock particles
falling into the water and being carried to the turbines;
• To take that part of the internal pressure not taken up by the
rock.
GEOLOGICAL PROBLEMS DURING TUNNELLING AND
REMEDIAL MEASURES
Tunnelling may be associated with many problems like
problems of stress, ground movement, squeezing and swelling
ground conditions, heavy ingress of water, high geothermal
conditions and sometimes gases apart from varied lithological
and structural geological features.
A. Tunnelling Problems
Various problems are encountered during tunnelling. The
various problems quoting examples from Nathpa Jhakri
Hydroelectric Project (H.P.) and practically adopted solutions
will be discussed [7], [9].
1. Occurrence of Geothermic (Hot Water) Zones in the
HRT
During excavation of the 27.4 km long HRT of Nathpa Jhakri
HE project many geological problems associated with
underground excavation were encountered. One of the most
difficult problems was the occurrence of geothermic zone of
about 3.5 km length. A number of hot water springs occurred
at places where deep acquifers have been tapped by tectonic
dislocations like faults, shears and joints. Hot water with a
temperature ranging from 350
C to 550
C was seeping and
gradually increasing through the rocks and the rock
temperatures were also in the vicinity of 550
C, this started
from RD 16950 onwards. The dewatering arrangements
provided at that time could not tackle the quantity resulting in
accumulation of water. The total discharge of water was
around 100 lit/sec.
Consequences and preventive measure:
a. Excavation Problem:
There was high temperature and humidity inside the tunnel
due to heavy seepage of hot water. It was extremely difficult
to work under such conditions. The problem was further
compounded by dust generated due to blasting and exhaust of
construction machinery.
Preventive measures taken to reduce the effect of hot water:

1. Improving the ventilation by installing additional fan of
200KW, 180000 cum/hour capacity at the adit portal thereby
increasing the amount of fresh air through the booster fans.
2. Spraying of cold water on rock inside the tunnel.
3. Providing cold water bath arrangements for workers.
4. Very short duration of work shifts i.e. 2 to 4 hours.
Moreover before concrete lining was started, the hot water
inflows were channelized by providing half PVC pipes with
mortar to the rock surface and diverting it outside through the
shutter with the help of a pipe, in order to achieve a rock
surface as dry as possible. On completion of concreting these
pipes were grouted with cement mortar.
b. Durability of tunnel concrete lining:
The entire HRT was to be concrete lined using RCC/PCC. It
was suspected that the concrete may face long term durability
problems under such high temperature of seepage water. The
CSMRS (Central Soil and Material Research Station) were
entrusted to access the problem and to suggest some suitable
measures against it.
CSMRS during their studies found that the seepage water
was of chemically aggressive nature. Besides this some of the
aggregates proposed to be used were suspected against alkali
silica reaction. Taking all these factors into account,
CSMRS/CWC suggested the use of blended cements with low
water cement ratio for concreting of entire HRT lining. Both
Portland puzzolana cement with the ash content of 25% and
Portland slag cement with slag content of 50% have been used
for tunnel lining.
c. Precautions taken during initial filling
Initial filling of water conductor system of the project at a
rate of 1.0 m/hour ensure slow and uniform saturation of rock
mass, thereby avoiding overstressing of rock mass due to high
gradient of seepage flow. Further, the filling was carried out
with suitable pressure steps with pause of 24 hours between
each pressure step. As soon as the water in the HRT reached
the geothermal zone, the filling rate was reduced to 0.3
m/hour. This slow rate of filling in hot water zone prevented
cooling shock of the lining and also ensured that favourable
cracking of the lining as anticipated by the provision of
reinforcement really takes place.
2. Highly sheared rock with heavy seepage
Very poor ground conditions were encountered in the reach of
tunnel between Rattanpur downstream downstream and 6th
face adit. The rocks, in the shear zone encountered, were
quartz mica schist, sericite schist, biotite and amphibolite
band. It is mostly rich in sericite and biotite schists with
multiple small shears filled with clay and squeezing in nature.
The heavy ingress and seepage of water encountered have
made the shear zone material in flowing conditions. Further
advancing of excavation was not possible with the available
equipment. After detailed study of the ground conditions and
various other solutions, the excavation of tunnel was done
with DRESS (Drainage-Reinforcement-Excavation-Support-
Solution). DRESS methodology is completely flexible and
effective in responding to extremely heterogeneous rock mass
with large rock loads and squeezing behaviour under high
water head with risk of huge inflow in unstable excavation
profile and reliable of time and cost estimates. But this method
is a costly alternative and all other possibilities for excavation
like multi-drift method, etc. should be explored before
adopting this methodology.
DRESS involved pre-drainage of ground ahead of face with
24m long drainage holes and stabilization of the crown ahead
of tunnel face by steel pipe umbrella arch, up to a
predetermined length, followed by excavation in small steps
by mechanical means and support thereof.
Sequence of activities involved:
The details of various steps carried out with dress
methodology are described as under:
(a) Draining of the rock ahead of the face: Before opening
of the face, advance drainage was done all around and ahead
of the face to eliminate the detrimental influence of water
pressure on face stability. Six to eight drainage holes of 77mm
diameter up to 24m length depending on the site strata, in an
upward inclination of 15 degree were drilled with a hydraulic
drilling rig using DTH hammer. M.S. Pipe of 50mm diameter
12m grouted and 12m perforated protected with geotextile
(impervious felt) were provided in the drilled drainage holes to
avoid the blockade of drainage system. These drainage holes
have been provided in alternate forepoling blocks.
(b) Face improvement: After providing the drainage system,
stability of the face and ahead was improved by cement
grouting with W.C. ratio 1:1. Sometimes when grouting was
not possible due to encountering of gougy sheared material
mixed with clay the face was stabilised by shotcreting and
grouted anchor bars of 25mm diameter 8m long.
(c) Umbrella fore poling arch: Fore poling (casing) of steel
pipes was provided ahead of the face before excavation of the
face using the hydraulic drilling rig. In this the crown of the
tunnel above springing level was supported with 12m long
steel pipe forepoles (casing) of 114.3mm outer diameter with
6mm thick wall and in an upward direction of 6 degree over
rib R1 of the block. The forepoles were spaced @ 400mm C/C
spacing. After drilling and installing of the forepoles, cement
grout in W.C ratio of 0.75 to 0.45 was placed at a maximum
pressure of 5 kg/cm2
.
(d) Face advance in heading excavation: After stabilising
the crown and the zone ahead of the tunnel face by forepoles,
drainage holes, shotcreting and grouting, the tunnel advance in
one forepoling block of 12m length was carried out up to 8.75
m length in a variable diameter of excavation from 11.65 m to
13.45 before the next block of forepoling. In this 8.75 m
length of the tunnel advance, a total number of 12 sets of ribs
of ISMB 300 × 140 @ 750mm c/c spacing were provided in a
sequential advance of 0.75m to 1.50m depending upon the
stand up time of rock strata. Rib R1 was installed at the
minimum excavation line and subsequent ribs R2 to R12 were
provided in a variable minimum excavation line radius to
facilitate the forepoling machine working in the next block of
forepoling. After protecting the crown with
shotcrete/wiremesh, the excavated section was supported with
ribs and the space between the rock surface and the rib
intradose have been filled with shotcrete. In the second round

of excavation, the central portion was first excavated up to the
previous round advance and again the excavation for this
round was carried out in an advance of 1.5m in the same way
as explained above. After excavating and supporting the
forepoling block up to rib R12 (last rib of the block), the
excavation of Rib R1 of next block was done and the rib is
installed and supported with wiremesh and shotcrete. The ribs
were anchored at springing level with 25mm dia 6m long
cement grouted anchor bolts with ISMC 150 × 75 runners
joining three to four sets of ribs. The face was then sealed off
with shotcrete for improvement of face and ahead before start
of excavation of next forepoling. The excavated reach was
further supported with radial rock reinforcement in the form of
32mm dia 6m long hollow core self drilling cement grouted
rock bolts. In such type of bolting the rock bolts are installed
simultaneously with drilling having the steel bit attachment to
the rock bolts at the drilling end. Grout was then pumped
through the bolts itself forcing out water, debris etc. and filling
of all fissures voids and complete grouting of the bolts was
ensured. A temporary invert arch of 350mm thick shotcrete
was also provided to prevent heave of an unsupported invert
and punching of steel ribs from the arch support into soft rock.
Importance of logistics in long tunnel: The logistic
supports in the tunnel such as, proper ventilation, dewatering
and lighting, etc. are the lifeline for the speedy tunneling [10].
These supports become much more important when the tunnel
is excavated through water charged mediums and long
tunnelling from one face. As the human body cannot function
properly in the absence of any organ, similarly speedy
tunnelling cannot be executed in the absence of any logistic.
Example: Tunnelling (10.6 km HRT) in Dulhasti H.E. project,
on river Chenab, in J&K, remained a bottleneck due to poor
logistics like ineffective ventilation and managing huge
discharge of water inside the tunnel and frequent occurrence
of geological problems like encounter with the shear zones
gradually slowed down the progress of the tunnel which could
have been easily tackled with proper planning of the logistics.
Seeing such problems following recommendations are made:
1. Long tunnelling (more than 2.5 km) from one face is to be
avoided. If unavoidable, then another alternative such as
ventilation through intermediate shaft, at a suitable location,
should be planned.
2. Systematic monitoring of pollutants level in the air, air flow
(m3
/sec) and air speed (m/s) are mandatory to improve the
system.
3. In water charged medium, sufficient gradient of tunnel should
be provided for under gravity drainage. Simultaneously, a side
drain is necessary for effective drainage and to save the invert.
4. If any geological problem is encountered, proper investigation
of the medium ahead of the face by probe holes and thereafter
its treatment through pre-drainage and pre-grouting becomes
mandatory.
5. As far as possible minimum pumping stations are to be made
and rehandling of water is to be avoided.
CONCLUSION
It has been observed that tunnels bring the project on critical
path. Designers have to give proper consideration to the
design standards and come to a final design only after detailed
study of the topographical, geological conditions prevailing on
the site. Further, geological exploration prior to the planning,
detailed geological investigation parallel to planning should be
done and to be continued during the construction in order to
ascertain whether the driving method adopted is correct or
needs to be modified based on the prevailing conditions.
REFERENCES
[1] Tunnelling- Design, Stability and Construction by B.N Whittsker and
R.C. Frith
[2] The Art of Tunnelling, Second English Edition (1973) – Karoly Szechy.
[3] Manual on The Planning and Design of Hydraulic Tunnels – Central
Board of Irrigation and Power (2012).
[4] Hydropower Engineering (Module 5) – NPTEL.
[5] IS:4880 – 1987 Design of Tunnels conveying water.
[6] Underground Infrastructures: Planning, Design and Construction - R.K.
Goel, Bhawani Singh, Jian Zhao.
[7] Water and Energy International: Special Issue on 1500 MW NJHPS,
CBIP, New Delhi (April-June 2008)
[8] Civil Excavations and tunnelling: A Practical Guide BY RATAN TATIYA
[9] Tunnelling problems encountered at Nathpa Jhakri Hydroelectric
Project (H.P.) by S.P.S. Chauhan, R.K. Gupta, G.L. Bansal.
[10] Importance of Logistics in Long Tunnel (A case Study of Dulhasti H.E
Project, NHPC) – S.C. Gupta and P.K. Gupta.

Practices in Planning, Design and Construction of Head Race Tunnel of a Hydroelectric Project.

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