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Safety in Construction
Prof. Uma Maheswari
Department of Civil Engineering
Indian Institute of Technology, Delhi
Module No # 03
Lecture No # 12
Trench Cav-ins
So in the last week we started with intruding the fatal 4 hazards so we starts from falls, Cav-ins
caught in between or struck by electrocution and so on. So I also said we will be discussing each
one of the hazards along with where generally you know it is seen along with the operation also
we will be discussing. So last week we have seen on falls and concreting work and demolition
primarily in the scaffold and farm work where the falls is predominantly seen.
So this week we will see on Cav-ins and excavation is one of the major operations where Cav-ins
are real killer. So if you talk about excavation then Cav-ins are the major hazard we will also be
discussing what are the other hazards available also. So excavation as such I will be covering in
this particular class.
(Refer Slide Time: 01:16)
Now why should you go for excavation so the top soil as such is not suitable for construction and
it comprises of mineral matter or organic matter and so on. So water, air etc., so that is why it is
not really you know really suitable for construction and hence the soil as to be removed. I am
talking about surface excavation and deep excavation also generally happens to support a
structure and its foundations.
So even the top soil has to be fully leveled and cleared and also the separate you know holes or
openings has to be made earth has to be cut to support this structure and its foundation. So when
you say earth work it is not just one single operation. There are so many steps in the earth work
operation itself. So as such if you start looking at there are some preparatory issues which
implies before you did and then you have think of how do you did for the choice of equipment
and combination and so on.
Then digging out how exactly you have to dig along with protection of sides and the shores until
the foundation is completed preventing from side collapse. Then whenever you encounter with a
water table dewatering or bailing out the water from the pit or the trench then back filing of this
trench pit one side excavation of foundation is completed. And then you dump these surplus
earth into some specified locations are a marked locations.
So these are all the different steps you have in earth work operations and now we are going to
see different hazards available in all these steps. Now let us start with before you dig what are
the major precautions you have to take. And before understanding that let us discuss about
something on IS code and what they talk about types of excavation. So as given in SP 62 there
are several types of soil.
Soft soil or loose soil so what happens here is examples sand, gravel, loam, clay, mud, black
cotton soil and so on. So you can use ordinary digging tools like for examples shovels, spade or
rakes for actually you know cutting of the earth. The next type of soil is hard or dense soil
examples stiff heavy clay hard shale or lime concrete. So you may have to be close in applying
your energy and force.
So you may have to accordingly use repose or something because you may have to closely work
on and loosen the soil so that you are digging of the earth. The next category is ordinary rock
again in rock you have several categories one is rock which does not require blasting and the
other one blasting is prohibited. So in ordinary rock so lime stone hard laterite un-reinforce
cement concrete etc.
So all these are examples and maybe you have split with crowbars or picks hard rock which
requires blasting. So any rock or boulders are the examples and you can go with quartzite,
granite, basalt, RCC are the examples of hard rock you have to blast the ground in order to do the
cutting of the earth. The next is hard rock where in blasting is prohibited due to some reason or
the other. You can use chiseling, edging or any other agreed method by the engineering in
charge.
(Refer Slide Time: 05:05)
So what are the types of excavation now again there are several words which comes in research
in industry practice starting from structural excavation, bulk excavation, confined excavation,
sloped excavation and then stepped excavation and so on. Like these there are, lot of terms
coming in but let us think about based on protective measures which we have to follow in the
construction site.
So accordingly the first category is confined or structural excavation so once your drawings are
all completed you give a good for construction point and based on that drawing you actually
know cut the earth only to that portion. So or it can be trench excavation or only for individual
footings or foundations you cut the earth as the walling. So you may use close cutting of earth
only in specified locations then that is primarily called confined or structural excavation.
The next type is bulk excavation wherein your quantum of excavation is voluminous and you
have to use several, equipment like as shown in the figure here. Several, equipment and which
may be moving on in the site for cutting of the earth. Excavation in rock for relatively a hard
rock you can use rippers or pneumatic breakers for very hard rocks you know you can go for
blasting operations.
For example this is a confined excavation so this is like a trench excavation shown here and this
is actually a bulk excavation. So here you can see it is a very large site and the bulk excavation
lot of equipment’s will be available in the site as seen here for the work. And these are the
excavation work in rocks primarily you have to use a rippers or a breaker or even drilling
machines for cutting of an loosening the earth.
(Refer Slide Time: 06:56)
Now before you dig you have to post warning messages to the public and you have to prevent the
public from the entering into the construction. So what you should do is first provide sufficient
number of notice boards and other danger signals and warning signals to prevent the accidents.
So the next one is barriers are primarily you know obstacles or obtrusions so that the public is
not intervening in the excavation work.
And this should be provided for any excavation shaft or pit or opening having a vertical distance
of more than 2 meters. Then fencing so this is primarily a picture of fencing so fencing is
actually erected for deep excavation and then there is lot of likely hood of public you know
frequenting that area.
(Refer Slide Time: 07:46)
So the next precaution so first is you have posted a message you have allowanced to the public
you have given enough warning signals on excavation is going on. The second precautions, that
you have to do is identify all the buried underground utilities. There are so many underground
utilities which are buried inside the ground and you may have to identify all of them so that do
not mess up with those utilities when you are actually putting your know excavator equipment
there.
So all the utilities which are all you know passing through the site has to be identified before you
start the digging and so and there are, lot of means of identifying also. So utilities much be
exposed before digging the site engineer should identify the underground utilities using detectors
you can also use ground penetrating radar and there are other devices also available. So you may
have to use an appropriate device and an appropriate choice of GPR based on what you wanted
to scan.
You are actually scanning the ground and you should know what exactly you have to scan? And
what is the depth of scan? What is the intensity or the accuracy with which you want to scan?
Based on that you may have to choose equipment sometimes even your scanning measurement
may go wrong. You may not be able to know get a right data or sometimes there may be some
mystery abstraction present inside the soil.
And you would not know what exactly it is? So you should not be leading that it is a doubtful so
you can also do trial pits. So trail pits can be done manually at several locations in order to
identify those underground utilities. So you can do at trial pit at very close to those utility and
watch for what is the utility? And once you have you know confirmed as what is the utility?
Then you can identify all the others and then go.
So always even if the utilities are all found or even if you think and confident and all utilities are
identified still you have to proceed with proper cautions when you are doing the digging
operation. And the work permit as to be obtained for either emptying the utility provided it is a
sewerage line or a waterline or something. Or diverting the route or maybe de-energizing if it is
an electric cable or something.
So you have to take adequate measures from the concerned authority and you have to know
divert or safe guard the site first of all the buried utilities and then you may have to work. And all
the workmen who are working in the site wear and proper shoes, gloves and other PPE’s as
recommended. Suppose if you have identified or you have a suspicion on some underground
electrical cables then the cables should be de-energized.
Otherwise also the equipment which you are using for example tools like crowbar or spades etc.,
they should be well insulated. So that you do not the workers do not get an electric shock while
they are digging those places. And the employees also should determine all the estimated
locations of these installations so that it is easy for them to divert and turn. Once you identified
all the utilities then the next step is you have to mark those utilities on to the ground.
There are some color codes and color symbols for the marking all those utilities on the ground
you can use a flag as shown in the figure here.
(Refer Slide Time: 11:28)
You can either you know put color flags and then say that these are the lines where utilities are
available or you can also apply spray paints to show that utilities are available. And American
public works association APWA as recommended uniform color codes which is followed in
several countries. White color is generally recommended for the proposed excavation pink color
for temporary survey markings.
Yellow color for gas, oils, steam petroleum or gaseous materials red generally is used to show
electric power lines cables, condults and lighting cables. Blue for portable water orange for
communications, alarms, signals, cables or other issues. Purple for reclined water, irrigation and
slurry lines and green for sewers and drain lines. So according to these color codes can mark and
show the presence of the utilities which you have identified and raise out before you start with
the excavation work.
And now the first step I told before you dig is first you have to post adequate warning, messages
signs and then you may have to proper barriers, fencing and so on. Second is identifying all the
undergoing utilities the next step is there maybe adjacent structures close to your side. Your site
may not be like too spacious that you do not have any neighboring structure close by will have so
many neighboring structures close by.
So you have to analyze what are the neighboring structures close by; and you have to see
whether those foundation. The foundation of those neighboring structures, are interfering with
your excavation or not. And sometimes what happens your site maybe you may be excavating
some portion it may be closed to one foundation of other existing building. And suppose if you
are taking out soil from a close base structure.
That structure may start weakening and it may lose its strength. So there are some ways of
preventing that then only you have to start with our excavation work.
(Refer Slide Time: 13:46)
So no excavation works shall happen below the level of any foundation unless underpinning. So
underpinning is actually providing an additional support like this below the structures so that the
structure is not falling off. Or these are the pictures of shoring or shoring etc., to prevent collapse
of this structure or excessive settlement. So the foundations adjacent to or below which the
excavation is to be made shall be supported by either adequately designed sheet pilling shoring
or any other aggrieve method.
And till the excavation of this particular site is open and these excavated materials also should
not be piled against the walls of all these neighboring structures because it can collapse. Those
buildings can also collapse so that you have to take care off.
(Refer Slide Time: 14:34)
Then the next step is site clearance. So as I told you may have to follow clear and level the
ground before you start with your excavation. So site clearance what all you should think off so
before site clearance itself you should have some usable structure are there some existing
structures, some trees you want to safeguard some archeology monuments, structures etc., If they
are there then you should proper permission for safeguarding that so that your excavator or your
excavation work is not known bombarding with that.
So the site should be cleared of vegetation, trees, saplings any rubbish which is remind up to a
distance of 50 meters outside the clearance area. And roots and the trees shall be removed to the
depth of least 60 centimeters below the ground level. So that should happen and the, hallows of
the, you know removed roots and so on. Should be filled up earth leveled ramped and you should
must ensure it has attain the normal compactor level.
And the trees measured at a height of 1 meter above the ground level should be cut only after
you get a proper permission from the engineer in charge.
(Refer Slide Time: 15:47)
In the existing structures if they are all available it should be safeguarded properly and then only
you should start with your excavation. So all many of the safety precautions starts from you
know from even before you start your excavation number 1 public safety should ensure that you
have seen earlier. The hazard should be removed or supported before the excavation starts.
Excavation sites should be protected or supported and to safeguard the employees and take care
when you are walking neat excavation sites.
Because with the movement of vehicle may with the flow of water or maybe with a dumb of soil
flows by. The land maybe slippery and you may be tending to slip or trip and fall. So better you
should be very careful when you are walking near excavation sites and always try to barricade or
cover any well pits or shafts. And the employees must be trained to operate any equipment and
so general precaution is when any lifting or digging equipment is happening.
So do not go near that so stay away from the loads and stay away from the vehicles which are
loading and unloading. When mobile equipment’s are operated close to the edge of the
excavations site you should be you know stopping those processes. Because vibration of that
equipment can naturally disturb the soil which you have cut and use appropriate PPE. These are
the steps which are like generic steps which you keep in your mind before you start with your
excavation.
Now let us talk about the actual excavation and there Cav-ins, are more. So Cav-ins are generally
happens one is trenches and other one is excavation but generally trenches excavation trenches
are considered to be number 1 killer in Cav-ins. So let us now discuss what is a trench? What is
an excavation?
(Refer Slide Time: 17:50)
So when any width of your cut earth is less than 15 feet then it is primary called as trench. And if
it is more than 15 feet it is generally called as an excavation whether it is a trench or an
excavation they are all man made cut it is not a natural hole on the earth. So primarily it is a man
made cut formed by removing some portion of the earth and this excavation can be done may be
for a building basement or for a road bed and so on.
So what is a trench? It is a narrow underground excavation so primarily your depth of the trench
is more compared to your width. As I told the width is generally less than 15 feet and depth is
really you know steeper and steeper. That is primarily called as a trench so possibilities of Cav￾ins are more inner trench compared to your regular excavation. So that is deeper than it is not
wider than 15 feet. So trench Cav-ins fatalities are major killer in construction. So let us see what
is this cav-ins, fatalities?
(Refer Slide Time: 18:53)
And some statistics on what is this cav-ins all about? So some dirt facts soil is considered to be
dirty let us talk about some dirt facts. Most of the soil; are at least like very heavy soil is not light
weight. So statistics says so one cubic yard of soil weighs around 2000 to 3000 pounds on any
phase so 1 cubic feet of soil weighs almost more than 100 pounds. So again the weight of soil
depends on type of material amount of water and air contained in it.
And all the compassion levels and so with which it has been disturbed earlier so all these are
inclusive of weight of the soil. How is this soil or falling on worker is linked with this 1 cubic
foot of soil is equal to a weight of a car. So imagine a car a normal medium sized car and just
falling on you so that is the weight of the soil which will fall on the workers. And what happens
is it takes very less second for the worker to react on to the soil which falls on to the worker.
And if you see the statistics on death or fatality with regard to trenching or cavi-ins incidents
37% of most of these incidents have occurred at death trench death less than 5 feet. It is not too
steeper and depth but still many fatalities have happened. So ironically if you see the more,
deeper you go there are not too many fatalities or adequate safety precautions are already in
place.
Only for shallow depths of trenches where in people take it very easily their lots of fatalities have
happened. So fatality many of the fatality has happened only when the depth of the trench is less
than 9 feet itself not more than that. And most often in during these fatalities protective systems
where; not in place that also you should understand. So what happens is when this weight of the
soil is on you it can actually suffocate the victim and it can also create bone injuries.
So compared to the death there are many permanent fatalities permanent disability also has
happened to the workers. So the trenches trend to collapse very quickly leaving no time to react.
But there are some warning signals which are given before a collapse happens; ground settlement
can be seen. Side walls you can see lot of cracks and minute movement or dislocation of some
pieces of the earth.
Pebbles or small pieces of soil can start falling that it shows a warning sign that this soil may
collapse at any point of time. There can be changes on to the side walls from where you have cut
so the cav-ins are possible when you are doing the cut of the earth why because? See when a soil
is undisturbed you have actually the pressure coming on the top. And also the capillary action
and other pressures coming on the bottom and also the soil pressure which; comes on all the
sides.
So all these pressures tenses to be equalized and soil is actually very stable the minute you start
cutting the earth what happens is some forces on these equal movement is disturbed. And as a
result it starts collapsing and lot of accidents starts happening. But if you are very watchful in the
construction site for all these small signs of warning then you can easily prevent accidents in the
construction site.
So cav-ins can happens but accidents can be prevented provided you keep proper safety
precautions. Now what is the statistics on fatality in trenching accidents this is primarily from
Bureau of Labor statistics which says 2011 to 2016 if you see this is the trend of the accident,
you can see here. So there are quite of few, fatality which has happened and for almost these 2
digit number at least 3 digit number of permanent disability also is reported in several document.
So there are quite a few fatalities and quite a few major injuries or permanent disabilities have
also happened as a result of collapse of soil. Now what can be reasons for soil in stability? One
reason as I said is primarily you are disturbing the mass and wherein whereas you have all the
forces intact. And some forces are getting disturbed because you are just cutting the earth and
taking it away that is other scientific reason.
But there are other reasons also which can be manmade which can create a soil instable and it
can start collapsing.
(Refer Slide time: 24:10)
The trench wall collapse when they stand unsupported for a long period of time. You are cutting
the earth and the side portions are standing firmly it would not be standing firmly for you know
longer hours of duration maybe for some time it can stand and before that you should put proper
safety you know precautions so that it is all standing firm. If you leave the earth as such you
know without putting a safety barrier or barricade in terms of shoring or shielding.
Then automatically it starts collapsing and it closes the trench opening which you have done.
And before we discuss on what are the reasons for instability we will also discuss when you are
not you need to be bothered about side wall protection. No protection is required in a trench
when the material is solid or stable rock and trench is also less than 5 feet deep. So the trench can
go till 5 feet deep you need not have a protection.
If the soil is investigated as a solid rock or a stable rock then you need not worry about stable or
any safety protection. It can stand on its own suppose for everywhere you should know what can
happen if something goes inside anything can go wrong especially the soil anything is possible.
If the soil type is not determine or it is difficult to identify what type of soil then better to classify
it has type C soil.
We will discuss about type A, B, C soil after few slides and when you have less than 5 feet when
you gone know when you are just below 5 feet and so on. A competent person should actually
start inspecting the excavation and determine whether there is no indication of a potential cav￾ins. Then you start putting in all your safety shield and barriers and then you can go in ahead in
containing with your excavation.
Now let us come back to factors which actually you know which are primarily behind the
instability of a soil. Number 1 is your soil which is excavated from the cut portion which you are
dumping in very close by. Suppose you use an equipment or excavator and start cutting the earth.
You actually no turn the boom and put the dump the soil in a little close by location. So there are
some guidelines as to where how much distance away from the cut the earth you can dump those
soils.
So always follow that, guidelines so otherwise if the soil pile is very close by to the cut earth. So
that can be imbalance load which can happen and easily cav-ins, are possible. The next is vehicle
which are passing by it can be suppose if you are close to a busy street or something. There may
be movement of vehicle and you excavation can be a disturbed with the movement of that
vehicle which can create vibrations and that can let you to cav-ins accidents.
Other one is vibration because you wanted to know use some vibrators just to loosen the soil that
can also know, create lot of vibrations and the soil is loosened and obviously there may be a cav￾in accidents. The next is your types of soil suppose if you have not identified a proper soil type
and you have chosen a wrong protective measure for safeguarding the side collapse. The
obviously it is not going to be a preventive measure at all.
You identify the type as C I identify soil type as A. And you have chosen a measure as different
one then, what happens is? It is not going to safeguard so obviously you are going to be you are
having an accident then there. Depths of the cut there are guidelines as to when then depth,
which you have to go you have to bring in adequate measures also. And after some level down in
also have to think off falling accidents.
So from the edge of the ground level of the pit or opening person or vehicle anything can fall
down that also you may have to anticipate. The next is saturation it is through rain sometimes
raining can happen and it may also start collapsing soil. And as a result there can be a cav-in
accident other one is natural capillary action as a result of which the order table can be seen
when you are starting to begin that is one of the reason for cav-ins accidents to happen. In the
last one is buried utilities which are available.
(Refer Slide Time: 29:00)
Safety precautions so as I told you the soil, spoil safety should be you know at least kept 2 feet
away from the edge. And on either, sides and the material storage should be also at least 2 feet
away from your edge. And there is something on 1 feet, 2 feet rule and so on let us talk about
that this safety precautions it is easy for you to remember in terms of numbers. So excavated
materials should be kept away from the edge always maintain that 2 feet distance on either side
of the cut edges.
And in special cases if you are not able to dump it away from 2 feet because you do not have a
proper place to dumb in. Then you can have little closer one but it should be not less than 1
meter and you should also be checking whether that particular land is able to support all your
additional load just because of the cut earth. Now the next is when a person or when a worker is
inside the pit inside the excavator pit he should be able to reach to a nearby access for
evacuation.
And the nearby access should be not more than 25 feet so whatever direction he is not more than
25 feet he should be able to see an access. The access can be in terms of ladder, ramp or stapes or
stairs. And beyond 4 feet or deep then the worker should not be entering unless there is a safe
entry exit point. And if your trench pits are more than 5 feet or deeper you should be looking for
water accumulation hazards also.
The next is types of soil collapse let us talk about you know the pattern of side wall collapse that
is what we have seen has one of the major danger. When you start cutting out the earth the side
walls you know tend to collapse in due to several reasons.
(Refer Slide Time: 31:09)
Number 1 is sliding, slowly the soil particle can start coming down sliding and it can bury the
worker or toppling there can be a cut along the sides. And one portion of the earth can actually
topple on to the worker and there can be accident. Tension cracks there can be lot or lot of cracks
which can happen all along the excavated sides and as a result it can start falling off on to the
worker. The next one is bulging is primarily you know bulging of the bottom portion of the soil.
And the top portion of the soil is more bulged than the bottom portion and as a result you have
the earth to be fallen off. The next is heaving or squeezing it is nothing but with the weight of the
soil on the other ends it starts coming down and actually you know you actually find not a proper
pit which is excavated but actually you know a little bounded one. So that is primarily a called as
heaving or squeezing.
Now what are the different measures of protective systems so whenever you are cutting out the
earth. Then you should be having lot of protection systems are available.
(Refer Slide Time: 32:30)
And number 1 is shields then shoring and sloping so shielding, shoring and sloping are the 3
methods of protecting any open pits or trenches and so on. So this picture is primarily to show
that you know actually the excavator material is lying too much away from your cut portion. And
you can see this pipe laying operation and the excavated earth is you know it is not piled up all
along the walls and little away from the walls.
So the employer shall select and construct the slopes and configurations for sloping and benching
systems we will discuss about what is the sloping benching system for which you should know
what are; the type of soil? And accordingly the sloping and benching system we have to
understand. And sloping and benching are supposed to be called natural ways of safeguarding
the earth.
And the artificial way of safeguarding the earth is either through shores or through shields.
Shores are nothing but some panels are support you keep on the sides all along the cut earth to
prevent the collapse. Shielding’s are like trench boxes they are primarily called trench boxes and
they are like boxes they are just fixed all along their and inside the box the workers can start
working in.
So even if the collapse of the side walls, happen since you are inside the trench box there is no
fatality or accidents happening are really minimized. Now let us talk about types of soil and also
the sloping or as a precaution for the collapse or prevent the collapse of soil.
(Refer Slide Time: 34:09)
So type of soil so A, B, C are the 3 types of the cohesive soil we are only going to talk about the
cohesive soil. For any stable rock you need not think of a sloping sloping is only to give a
slanting nature of the soil cut by nature itself what happens is the soil cav-in will not happen. So
instead of cutting the earth like this your actually know having a slope so that your cut earth
naturally will not yield and cav-in is not happening.
Now there are for stable rock which can stand on its vertical line itself you need not think of a protective pressure at all you need to think of an angle also.