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Safety in Construction
Prof. Uma Maheswari
Department of Civil Engineering
Indian Institute of Technology, Delhi
Module No # 03
Lecture No # 13
Tunneling Safety
So last class we have discussed about excavation and the major hazard or risk with respect to
excavation in terms of safety is primarily cav-ins and fatality with related to cav-ins. So we have
discussed different forms of excavation like for example in bulk excavation or trenches and we
have discussed all the safety precautions with regard to the same. So in continuation now let us
discuss some of the other hazards. So in a way we call it as a confined space.
Confined space is a major hazard when especially when you are doing very narrow excavation or
may be in terms of tunnels. A, confined spaces are a real big hazard so we will discuss about
confined spaces first and then the different hazards in regard to tunneling excavation. So we will
talk about different types of tunnel excavation and also on micro tunneling also we will discuss.
(Refer Slide Time: 01:26)
So let us discuss what is confined space OSHA defines confined space as an; very enclosed
space which is partially closed or completely enclosed space which is not intended primarily as a
place of work. You cannot work there for 8 hours per day or something like that. So some of the
characteristic features of a confined space it is large enough for entry or exit maybe for 1 or 2
workers. And it is not designed for continuous worker occupancy.
Which is what; I said you cannot stay there for beyond certain duration unless there is a demand
on doing it. Examples for; confined space manhole, borehole or silos then reactors, pits and
trenches, sewers or drains and chambers and ducting. So if a worker has now how do, you say a
worker has entered a confined spaces? If a worker has either put is head or maybe the lower part
of body is inside the space.
Or maybe his upper portion or full body is inside the space then he is considered that he has
entered the confined space. Then he has to follow all the safety protocols with regard to the
confined space. So now is it possible to really list out all the confined spaces generally it is not
possible to list out all the confined spaces. Because some places are termed confined only during
construction or maybe; during fabrication or during modification and so on.
It is not a complete confined space all together so confined spaces should have certain of his
characteristic then it is called confined space. And once it is termed as confined space then all the
safety protocols of the confined spaces as to follow.
(Refer Slide Time: 03:12)
What are the typical hazards in confined space? You may have flammable atmosphere you may
have explosive atmosphere. There may be toxic substances which maybe like dangerous
poisonous or gases in the form of gas, vapor or fumes likely to be present. You may have oxygen
deficiency or you have oxygen excess also maybe if you have chosen a wrong breathing
apparatus that can also be real serious hazard.
And then when you talk about gases different gases can be an; different levels. So suppose if I
have a confined space this maybe the confined space you may have large density gases in the
bottom then this. And then there may be 3 layers based on the depth you have to differentiate the
layers of the confined space. And do the gas detectors for testing the levels of gases and all these
3 layers. Then only you can really detect the presence of the particular gas which is inside the
confined space.
And the worker should be taking a portable gas detector also suppose if the gas levels are
changing periodically they are advised to take portable gas detectors while they are working
inside the confined space. Other hazards in confined spaces are engulfment, excess of heat, lack
of natural light, no ventilation, restricted entry and exit. So suppose if there is an emergency they
may not find a place to know quickly come up on hazard.
So that is also a really big danger in terms of a confined space never go into a confined space
alone so confined space will have so many risks. Now first thing that we should always we are
always talking about is PPE and I have been telling PPE can minimize many of the severe
hazards.
(Refer Slide Time: 05:00)
So what are the required PPE in terms of confined spaces? As usual you have the required PPE
minimal PPE which have required and especially for confined spaces there are some additional
PPE’s recommended. For example respirators are generally 2 forms when you talk about
confined spaces one is supplied air respirators and other one is air purifying respirators. When
your oxygen level is deficient then you may have to carry a oxygen along with your mask which
comes something like this.
And then you may have to enter a tunnel or a confined space sometimes the air maybe toxic or it
is not suitable for breathing then you may have to carry an air purifying apparatus which need
not come with the cylinder is only for purifying the air. So the mask will only purify the air
which you are breathing in. And other thing is always you know recommended to have a full
safety harness and this safety harness along with the life line so as shown in these 2 figures.
So this safety harness is having a full body safety harness along with the lifeline so which is
connected in a tri pod format. So this is a tri pod format with which the worker has to enter and
there should be a standby worker on the outside. Who should be able to see any signal which is
communicated by the worker inside the confined space to the person outside? And this person
also should be capable of pulling out the worker in terms of in case of an emergency.
(Refer Slide Time: 06:45)
So now let us talk about before you enter confined spaces first of all you should know what are
the hazards? And how to manage all those hazards confined spaces are really a life threatening
danger and entry is generally is only by permit. So there are 2 types of confined space one is
called permit required confined spaces and other one is a non- permit required confined spaces.
Non permit required confined spaces also require some restrictions and not everybody is allowed
to enter.
But there need not be too much of there is no presence of too many dangerous issues inside in a
permit required confined spaces there may be too many suspected to have too many hazards
levels and risks. So for people to entry and hence you will have restricted entry and exist you
may not be allowed to stay for longer time 2 or more workers not be allowed to enter. So these
are some of the issues with regard to permit required entry spaces.
And before you even go for a permit or a non permit better you should really see whether such
an entry into confined spaces is really required or not. Suppose if you can alter the work and do
the same work outside the confined space then you should go ahead in doing it rather than taking
all the risks and entering the confined space to carry out a specific task. So permit required
confined spaces are the most hazardous and required any qualified person to complete a safety
check list simply call a permit.
Before you enter in the space and trench more than 4 feet depth is generally termed as a permit
required confined spaces because it may contain harmful substances and gases. Entry, permit
requirement what are the minimum thing you need to have in the entry? Identification and
location of the confined space sometimes you may have too many confined spaces you have to
have an identification and location.
And outside the confined space this permit has to be struck on the wall or near to the confined
space. So that people who are close by should be aware that some people have already entered a
confined space and they are actually working inside for this specific task and so on. So purpose
of entering the confined space time at which, the worker is supposed to enter duration of his stay.
What are the ventilation arrangement the worker has taken in? What are the lighting
arrangements a worker has taken in?
What is the air quality of a particular place all these should be monitored first and it should be
listed along with the entry permit? Only everything is done and proper values then only the
worker is allowed to enter the confined space. Safety inspections gas test also has to be done and
this work permit along with the signature of the foreman competent person it actually struck on
the wall near to the confined space.
Confined spaces are dangerous places and never go without a permit most of often it is better to
not enter a confined space without the proper work permit.
(Refer Slide Time: 09:56)
Before entering the confined spaces some of the issues number 1, isolation. So if possible
disconnect and properly lock off the power supply and all other issues and machinery or the part
going to work as to be really isolated. So that actually preventing, all the hazards in the confined
space entering into a work area. So disconnect and properly lock off the power supply and the
machinery and equipment that could cause hazards in a confined space and blank of pipelines,
service lines which can also cause hazards.
So try to isolate the portion of the area which you want to work in simpler way that is what it is?
And try to take effective steps to prevent ingress or in-rush to the confined space of hazardous
gas, vapor, dust, fume etc., Cleaning and cooling before entry ensure that no sludge or other
deposits will give off hazardous gas, vapor, dust or fume during the course of work. If the steam
cleaning is used then sufficient time should be given for cooling of the whole chamber before
you enter into the confined space.
When the size of the entrance is too narrow then you should also have a proper airline breathing
apparatus and that also should be carried along with you. And when you are taking an air
breathing apparatus you should also know that it can restrict the passage or movement of the
workers.
(Refer Slide Time: 11:24)
Provision of ventilation increase a number of openings to improve the ventilation mechanical
ventilation; maybe necessary in many of the places to ensure adequate supply of fresh air. And
do not use oxygen to freshen up the air because this may have a risk of explosion also. Testing of
the air should be free from toxic and flammable gases and if there is no deficiency in oxygen
then the air is fit to breath you should be testing the air quality properly.
And the testing should be carried out by competent person using a suitable gas detector because
the type of gas detectors also is very critical. Because you are detecting the atmospheric
condition based on the detector which; should be properly and accurately calibrated. Further
precaution better to carry an air monitoring device which is a portable decide device as on when
required should be taken along with the workers. And who should be able to know the air quality
in the earth inside the confined space.
Provision of special tools and lighting non sparking tools and specially protected lighting
systems are essential and where flammable or potential explosive atmosphere or likely to be
present. So before entering the confined space again some more additional instructions number 1
is emergency plan you should have a proper emergency evacuation plan. Again these emergency,
evacuation plan also should be properly designed by the competent person and it should be
formally you know workable and so on.
Those all evaluation policies also as to be cross checked before; entering a confined space. And
sufficient supply of approved; breathing apparatus vessels containing oxygen or air.
(Refer Slide Time: 13:14)
Then safety harness and audio visual alarm etc. should all be properly ensured. And the people
outside the confined space should also be able to hear some of the signals which is given from
the workers who have entered the confined space. So everything should be properly you know
planned in advance before entering the confined space. Apart from that emergency services or
procedures like fire services, first aid all should be available at a, close vicinity before a worker
as entered a confined space.
Communications and workers who have entered the, who are ready for entering the confined
space should know when there is an emergency what is a way to communicate to the workers
stand by workers who are outside the confined space. And how to raise an alarm how to give
audio signal or how to you know communicate through some means are the other which is
preplanned.
And you know well in placed being the people who are outside the confined space and the
workers who have entered the confined space. Shut down in case of emergency which; has
already happen what is the way to shut down and what all as to be shut down? All these should
be planned in advance before entering a confined space.
(Refer Slide Time: 14:34)
Liaison and rescue so when the work is being carried out so as shown here so in a confined space
there should be an another standby workers you can see the standby worker has seen in this
photo should be assigned to station outside the confined space. To maintain the communication
so there is a worker who is entered confined space and there is a worker who is outside. And
both these workers should have all the PPE’s including safety harnesses to rescue the worker
who has entered.
So this rescue personal should be properly trained in rescue physically fit to and readily available
to carry out any rescue operations and rescuing the worker in case of any emergency. And there
are 6 types of all this rescue practices opening less than 24 or opening greater than 24. It can be
side portal it can top portal or it can be bottom portal accordingly you will have rescue devices
set in place.
And the rescue generally called tripod rescue it is off 3 categories number 1 is self-rescue. The
next one is non-entry rescue and third one is entry rescue. Self-rescue is the worker who has
entered the confined space he himself would be able to you know come out of the confined space
in case of any danger that is primarily called self-rescue. Non-entry rescue is the standby worker
will try to help or assist the worker who has entered the confined space.
But he will not enter into the confined space so without entry he will try to know rescue the
worker who is inside the hazard. Third one is entry rescue in this case the standby worker he has
to enter the confined space to rescue the worker who is inside the confined space. Again there are
so many types of rescuing you can have blanket rescue you can have full through legs you can
trap the worker and push him out.
Like this there are so many options of rescuing the worker all these instructions and training
should be given to the worker before the entry to the confined space happens.
(Refer Slide Time: 16:43)
The primary operation where this confined space is predominant we will just look at tunneling
construction. Because these days metro constructions are really popular and let us now discuss a
little, non-hazards with regard to tunneling operation. Tunnel is generally not linked to metro or
a railway project tunnel is primarily underground passage. And it can be anywhere it can be
through a mountain or beneath the sea or under a waterway or it can be any form.
And it may be for a pedestrian cyclist or road traffic motor vehicles railway traffic or a canal and
some tunnel are constructed primarily for carrying water also of a sewers or other hydro-electric
purposes or may be for even telecommunication cables and so on. The choice of tunnel again it
depends on geological condition hydrological conditions cross section and length of the tunnel
all these matters.
Limits of surface disturbance and how you want to put up the equipment and so on accordingly
the choice of tunneling happens.
(Refer Slide Time: 17:51)
Now if you broadly see there are several types of tunneling construction methods number 1 is cut
and cover method. We will discuss about drilling and blasting and also on TBA method of
excavation immerse tube tunnels. These are the 4 types I am going to show here in cut and cover
there are simple I made the process into simpler 4 steps as given in literature. Number 1 first you
have to put the sheet piles or diaphragm walls so these are the sheet piles and diaphragm walls.
And are pipe walls so who are installed to support the excavation step number 2 when the
excavation starts continuing here to the bottom of the tunnel. Temporary road decks as seen here
maybe put up or placed on the put road surface. And this road can be you know opened up for
smooth vehicle or pedestrian flow. So simultaneously what happens construction of station
concourse and platforms will start continuing underneath the temporary road decks.
And in parallel you may have you may also continue with other areas of excavation and the
process starts continuing. Once the roads surface is reinstated and after the construction of
station concourse and platform is fully completed. You can do the back filling and make the
permanent roads so that the normal road is actually made available. This is primarily called
pattern cover method primarily you are cutting and covering and then allowing the movement of
tunneling construction to happen.
The next is drilling and blasting even today this is a very popular method but once the advent of
TBM has been launched drilling and blasting is not done very frequently in almost all tunnels. So
before the launch of TBM drilling and blasting was one of the methods when you want to build
up a tunneling.
(Refer Slide Time: 19:44)
So before the advent of TBM this was the only economical way of excavating long tunnels
where any type of material is inside the soil (()) (19:54) was receivable. Even including hard
rock digging is not possible. So here again there are 4 simpler steps drill the holes in the tunnel
put proper holes and then start installing the explosives. So keep the explosives like this and
detonate the explosives.
So it starts blasting and all the debris and the soil spoil you actually take it up through dumb
trucks and it is transported away. Number fourth step is install, and support the proper tunnel
with tunnel lining and you are ready for the next blast to happen keep moving with the tunnel
construction progress.
(Refer Slide Time: 20:32)
The next is TBM is nothing but tunnel boring machine so it works underground with less noise,
dust, vibration. And TBM is generally preoccupied with all the equipment’s and setup. So you
need not have an additional setup for transporting and doing anything because it can do the hole,
operation with the machine on its own. And disruption and minimize the impact on the
community and traffic.
So it is highly efficient compared to other methods due to several work procedures it can occur
simultaneously. It itself can cut and take off all the mud outside and deposit in an, other place.
And you can also have concrete pre-case segments with which it is forms a tunnel lining also is
generally placed with the TBM itself. Now what are the 4 steps here also? So primarily you have
to put a launching shaft under retrieval shaft.
So shaft is nothing but the first initial excavation you do vertically down so that the TBM can be
placed. So you have to have 2 locations launching and retrieval shaft. Then second step is
assemble the TBM in the launching shaft so like this the TBM setup is done you bring parts from
different parts of the world and assemble and keep it ready in that place. The third step start
cutting and excavating the tunnel as it is seen here and it starts moving ahead.
So this excavation of tunnel is happening without any disturbance to the any of the buildings or
any adjoining structure which is above the tunneling. And the TBM starts moving arrives at the
deceiving shaft and it then dismantle for transportation. If it has to continue then it keeps on
continuing like this so recently this is a very famous method of tunneling construction. The next
one is immersed tube tunnels wherein you have to do a tunnel placing of a tunnel inside or under
the sea bed or inside a, water body that is a main issue here
(Refer Slide Time: 22:39)
So typically a immersed tunnel is made up of sinking precast concrete boxes so these are the
precast concrete boxes generally you use lot of ship to guide the setup you bring it to the place
where it has to be dropped and it is own self (()) (22:57) it just gets dropped down and with the
help of lining agents it just becomes water tight and seals of the water so that there is no ingress
on water inside.
So first step you have to do trench dredging and first you clear of the soil below you can use any
equipment’s like for example your clamp shells or drag line anything is used. This is primarily a
clamp shell and you start immersing a, tube units with the help of lay barge first fix the direction
and set up this. And then you place all the precast box units together along with the seal and it
starts getting water types seal. And then you can start back filling the earth over its so that it is a steady asset tunnel. Now safety
precautions especially in terms of tunneling now let us talk about safety personnel.
(Refer Slide Time: 23:53)
There are primarily 2 safety personnel one is competent person and other one is certified
workers. Competent person we call it as a competent foreman so this foreman should be able to
you know execute all the operation inside the tunnel or a shaft very safely. Shafts are the vertical
excavation which we do and tunnel is primarily the horizontal excavation which we do. So
whether it is inside a shaft or inside the tunnel the competent foreman should be able to execute
the job very safely.
In larger jobs responsibilities and functions in the respect of safety arrangements maybe
delegated to an independent qualified and the competent supervisor. Safety committee shall also
be appointed when there are 100 or more workers are employed in tunneling operation. And this
committee has to meet once in 15 days to discuss all the safety problems in the site work.
Certified workers and these workers are specially trained for to take of themselves in terms of
even self-rescue and so on.
And when there is any accident or something they should be also trained to report same accident
to the site supervisor and any incident what all as to be informed. And how to take of themselves
in case of any hazard approaching the tunnel all these workers are generally trained well in
advanced. And they are primarily called certified workers. The next issue when you are entering
a tunnel is you should have a proper first aid arrangement.
A first aid arrangement has to be kept for any injured worker so it should be maintained in every
work site. And it should be available for emergency at a quick reach it has to be reaching to the
injured worker.
(Refer Slide Time: 25:43)
Depending on magnitude of the work availability of ambulance also should be available at a very
short notice shall be there. And at least one experience first aid attendant should be there and
who can take of these injured workers. It is recommended generally the workers, foreman;
supervisors also should know how to deal with first aid methods to avoid any serious impact on
to any of the accidents which happen inside.
Stretches other equipment necessary to remove the injured workers also should be available. And
when there are more than 50 person working in a shift artificial respiratory arrangement also
should be provided. And PPE all the person who are entering tunnel should wear a proper PPE.
Now next comes, sanitation and drinking water. So sanitation facilities shall be provided close by
now it depends on the length of the tunnel.
(Refer Slide Time: 26:46)
At what interval you want to keep the closets. So dry closets or water closets or closet cars can
be provided in the tunnel at the scale of one unit for every 15 men in the shift unless it is
practically impossible. Closets shall be regularly cleaned and they should be effectively cleaned
disinfectant also should be provided in any case the entire tunnel should also be treated with the
disinfectant periodically to have any unhygienic situations removed.
Next drinking water so at least 5 liters per person employed in a shift drinking water facility
should be given in the tunnel. And especially when you are inside a very tunnels and it should be
stored in a clean container with the tight fitting lid and material handling.
(Refer Slide Time: 27:42)
So those, material required for the work in progress alone should be kept inside the tunnel and all
other material shall be removed and kept outside. Even if there is any sufficient space still all the
other material which are, not required for the work progress should be all kept outside. And the
workers should be able to have clear space free of any obstacles to escape in case of any mishap
which happens inside the tunnel.
Especially when you are having a flammable liquids or something which is required should not
be stored inside a tunnel and it should be under the supervision of a qualified person. All the
combustible material including rubbish should be continuously removed from such areas. And
spills of flammable liquid all shall be cleared up immediately. Containers of all the flammable
liquid shall be tightly capped and closed.
So all sources of ignition shall be prohibited in areas where flammable liquids are stored,
handled and processed. Fire extinguishers and fire buckets also should be provided in case of all
these hazardous materials are present. Next is communication facilities communication can be
different formats.
(Refer Slide Time: 28:55)
Number 1 is telephone system and the warning signals telephone it shall be provided to ensure a
positive and quick method of communication. So between all control locations and portal of the
tunnels and when you are longer than 500 meters and for shafts when it is longer than 50 meters
in depth. Warning signals so irrespective of length and the bends in the tunnel should have a
proper arrangement for transmitting and communicating any message you want to give to the
people outside.
It can be electrically operated bells or it can be a walky-talky so whatever form you can still you
should be able to communicate the people outside. So up to 100 meters length of the tunnel only
one of the system mentioned above shall be provided. And whereas when you have a tunnel
length which is more than 100 meters at least 2 or more communication systems and warning
systems should be in place.
Red and green lights of adequate sites brightness and provide a suitable intervals and straight
lengths curves, cross over points etc. to regulate the construction traffic. Red is primarily for stop
or green is to for slowing down. So like this communication is primarily given for the all the
locomotives and other traffic which comes inside the tunnel. Communication systems shall be
subjected to daily check whether it is properly serviced or whether it is properly you know
maintained.
Otherwise the service is required and it should be in place every day before the tunneling
operation starts the communication systems should be properly checked. Now what about
electrical installation and lighting?
(Refer Slide Time: 30:33)
So all parts of electrical installations shall be constructed installed and maintained to prevent of
any electric shock fire hazard and so on. And it should also be having adequate strengths and that
it is not you know getting damaged with the underground conditions and other issues like a
water, dust and thermal reaction or chemical action and so on. Wherever possible which should
be properly insulated and shall have all life parts enclosed.
So primarily in terms of switch boards there should be a passage of not less than 60 centimeters
wide should be maintained in front of the switch boards. And rubber mats we place in front of
the switch boards and no person should be using the switch board as like the clock room or
change room or a locker room or a storage room. And it there should be totally prohibited and
non-electrical equipment shall bare the essential details of voltage, amperes and circuit diagrams
and so on.
As far as possible the combustible material should not be used in any room where there is
electrical apparatus kept there. No flammable materials are also shall be stored in rooms,
recesses or compartments, which contains electrical apparatus. So try to you know segregate the
hazards which are put together. Adequate first extinguisher equipment suitable for using on all
live parts should also be readily kept and that should be kept at least once in a month. When fire
breaks out the supply of electricity should be cut off immediately.
(Refer Slide Time: 32:13)
And the fire should be attacked as soon as possible and reported to the nearest available
supervisor. All places where electrical apparatus is installed shall be adequately ventilated in
order to ensure proper cooling of the apparatus and dilution of the flammable gases are ensured.
Minimum of 50 lux is maintained inside the tunnel and shaft headings especially while drilling,
mucking or other scaling operations. The lighting in general inside the tunnel or outside an
approach road shall not be less than 10 lux.
This is how the lighting efficiency is calculated. Emergency lights there shall be installed at the
working faces and at intervals along the tunnel to help escape of workmen in case of accidents.