 Next after we are starting that is your breast excavation means where it is required first thing is once we are saying that application of soil mechanics topic is your breast excavation where it is required particularly for underground construction in built up areas generally what happen it has been excavated with side slope side slope if I draw this is the ground surface then how this excavation has been done by going through the side slopes it has been done. So if there is any structure if there is any structure then vertical cut off vertical cut off vertical cut off has been done. So if I draw there are two figures one is with your side slope other is your other figure is your kind of vertical cut off and with this this is sigma v this is your sigma n what is your breast excavation generally breast excavation is a underground construction where this underground construction has been used for example metro rail metro rail where this underground construction generally done and now you can say that metro rail underground construction why this breast excavation is required if you look at that generally underground construction has been done by means of cutting from the ground surface if this is my ground surface from side slope by digging inside by side slope. So that it will be stable you can cut and inside so that construction can be done below the ground but problem arises when this problem arises suppose like a crowded or metro areas metro areas suppose there are already building existing in the ground surface nearby the excavation nearby the excavation what will happen you cannot do this excavation by means of your side slope it is not possible rather you have to go for vertical excavation the moment you go for vertical excavation to make it stable because this vertical excavation it is stable up to certain depth for granular soil you cannot do this vertical excavation but for cohesive soil you have to go for vertical excavation and that too up to a certain depth beyond these it is not going to stable. So to make this stable breast excavation generally breast structure has been provided and so that you can cut or you can excavate vertically that is why this vertical means the breast excavation comes into picture this is your existing structure that means for a area like metro cities metro cities underground or metro rail are the construction you have to do it vertically you can go for below the ground surface and you go for the construction it is not possible side slope excavation you cannot do it. So that is why in that case you need to have your breast excavation or breast support you have now how this construction has been done this is basically a introduction class if you look at here in simple way there is a structure here it is a structure existing here it is structure existing and there is a underground construction may be metro rail is passing somewhere else here it is proposed to have metro rail construction below here like Alcata as well as in Delhi. So in this case in this case you cannot do the movement you go for side slope kind of this side slope excavation by means of side slope what will happen this structure it will topple or may be some settle may to occur you do not want it should be some kind of disturbance should be there in the structures. So in that case what you are supposed to do you have to go for vertical excavation by means of breast structures if I draw it now look at this how the vertical excavation has been done this is called strut and these are walls walls means by means of sheet pile or diaphragm wall sheet pile or diaphragm walls. So the construction technique is like that that means this vertical excavation has been done by means of your breast structures this breast structure this breast structure consist of at the top you go for construction at your breast structures this breast structure this breast structure consist of at the top you go for construction first part is that up to certain depth up to certain depth you excavate then insert your pile sheet pile or diaphragm wall then provide your then then go for another excavation then provide your this struts. So that what will happen you can go vertical excavation whatever the depth you want to do at the same time it will hold the pressure both the sides it is not going to fail. So this is a simple construction technique how this in detail we are going later on. So if I say that what is breast structure breast structure is consist of kind of strut and wall it is struts as well as walls this wall may be a sheet pile or by means of a diaphragm wall. So what will happen before construction the lateral rigidity why the struts you provides struts it is required. So that lateral rigidity closure the strut that means closure the strut lateral rigidity will be kind of it is a better and it has also disadvantage like obstruction to construction that means the struts why we are providing in breast excavation breast structure struts particularly it will improve your lateral rigidity that means lateral deflection or lateral bending will be less and more the closure the struts lateral rigidity will be better. Once you are saying that lateral rigidity is better that means obstruction to your construction that means you should compromise between the spacing of the struts while this construction of particularly breast structures or breast excavation below the ground surface you have to compromise with the number of struts and minimize this spacing of the struts during the construction otherwise what will happen more the struts I am going to provide at regular interval what will happen it is very difficult to have your construction process you cannot do the construction. So these are your techniques how the construction has been done now let us go to the failure modes of breast structures what are the different failure modes this is an introduction where this breast structure or why this breast excavation is required particularly once again in metros it is not possible to excavate by means of side slope rather you need to have excavate means as like vertically you have to go down below the ground surface for that you need to have a breast structures consist of the moment you say that it is breast structures it consist of struts as well as walls and the struts has been provided to improve the lateral rigidity and walls has been provided to prevent the flow of soil or water inside this breast structure. Now next step is your now we are coming back to this topic now this failure modes what are the different kind of failure modes in diaphragm wall or seat pile wall with your struts or breast structures first part is your consideration of bottom heave as the depth increases this is my depth if you look at here how the bottom heave produce this is your bottom heave that means as depth increases over burden pressure increases over burden pressure increases when over burden pressure is if you see how the bottom heave comes into picture what is the bottom heave you see this is the depth of the excavation look at here up to this depth of the excavation it may not be it may not possible or it is not going to occur that bottom heave first understand what is the bottom heave if this is my depth of the excavation as depth of the excavation increases from here to here what will happen this side this side is a full of soil so what will happen over burden pressure will increase over burden pressure will increase the movement at the bottom at the bottom if over burden pressure is more than if over burden pressure is more than your bearing capacity what will happen by means of this pressure this will go so there will be a some kind of heave soil will come out in the excavation area so heave will be produced so because of this heave what will happen whatever the construction you have made it it may fail so this is called consideration of bottom heave in design so it is not possible for a shallow depth excavation you have to go for a breast excavation so you need to have breast structure for deeper excavation once you go for a deeper excavation that means what will happen your over burden pressure will be more once over burden pressure will be more than your bearing capacity what will happen the soil here will push inside push inside your excavation so there will be a heave kind of things will form that means whatever the structure you have constructed here it may over the period of time it may fail by means of a bottom heave this is one kind of failure mode one of the failure mode now second failure mode is your kind of a clay bursting clay bursting means now look at the structure it is planned to have your excavation and there is a clay layer excavation in the clay layer so then below this clay there is a sand so this is your impermeable blanket now there is a water table somewhere else here so what do you mean by clay bursting that means suppose you imagine suppose you imagine you can go for a vertical cut if there is a clay generally for cohesive soil vertical cut up to height you can go for see by gamma without any support if there is a clay soil you can cut the clay soil or vertical excavation you can do without your breast structures up to height of for see by gamma see your unit cohesion and gamma once you go beyond this if there is a clay layer and below there is a sand below there is a sand it may possible that this water table is here clay bursting that means because of water table and sand layer it will come out it will come out clay bursting clays will come out in your excavation so that is other way of kind of another failure mode that means the condition is if sand near the vicinity there is a clay layer then then your clay bursting formed clay bursting is nothing but it is an impermeable layer impermeable blanket then is your sand because of water table this entire part will go inside your excavation so that is called second failure mode that is called clay bursting now third failure mode is your yielding of support yielding of support that means support has been constructed in breast excavation by means of diaphragm wall or seat diaphragm wall or seat pile wall and struts if you look at the breast structures that means this is consist of diaphragm wall or seat pile and with this along with this these are all your struts now if I construct if this is your ground surface and vertical excavation has been made by means of your breast structures with the help of breast structures the vertical excavation has been made what will happen you should take enough care so that the wall should not fail by means of bending wall should not fail by means of bending that means wall not fail by bending so this is called yielding of support if wall fail by means of bending if wall fail by means of bending means the support structure will be yielded so that means you have to consider this bending this wall should be safe against the bending to stop wall to fail by means of bending appropriate struts should be provided at regular interval so that yielding of the supports or the failure of your wall by means of bending should not be avoided this is your third failure criteria till now we consider there are three one is your by means of bottom if second is your clay bursting third is your yielding of your support yielding of your wall support then fourth is your ground movement look at this construction this excavation has been made with the help of breast structures then what happened if there is a nearby structure there is a nearby structure enough care should be taken once you do the excavation enough care should be taken so that the soil nearby should not move should not move because of this excavation this that means you will take care of all these things the ground movement should be as minimum as possible definitely there will be a ground movement but try to do as minimum as possible otherwise what will happen because of your vertical excavation nearby structure will fail by means of your ground movement so this is another condition or another failure criteria has to be considered for your design point of view now how the construction procedure has been made earlier I said the construction techniques how the construction has been particularly done that will see here suppose this is a construction procedure let us see these are all your walls along with the walls these are all your struts once excavation has been made this part has to be made available so that some underground tunnel or may be underground metro rail has to be provided how the construction is there techniques if you can I can write step by step bentonite after y bentonite after excavation then d wall or sheet pile wall it should be minimum 3 meter wide then your reinforcement has been provided if you look at this construction how the construction has been started how the construction procedure if this is my ground surface this is the ground surface initially you start this construction excavate up to certain depth once your excavation has been done then what happened side by side you provide bentonite bentonite is a mixture of soil plus cement bentonite is a mixture of soil plus cement plus water with mixture of cement soil and water the movement you do the excavation and add bentonite it will make side soil hard it will make side soil hard so that by that time you insert your diaphragm wall or sheet pile wall so soil movement will be restricted so that means if I say this is my ground surface try to understand this construction procedure start first step of excavating initially you excavate here you excavate first you excavate after excavation it is not like one by one it will be done immediately after excavation providing the diaphragm wall it takes certain time so what will happen this soil nearby the soil there is a chance the ground movement will occur because already this this part of the soil you have taken out this part of the soil below the ground surface has been taken out to prevent this what happen the bentonite mix has been provided at the both the ends of the excavation as it is a mixture of soil and cement and water so it will harden the nearby the part of the soil so it will it will stop the ground movement nearby then what happen you insert your diaphragm wall then once you insert what happen then start construction then again start construction then again provide your bentonite again provide your bentonite okay then construction then provide bentonite you go up to till you want to go for your required depth then once the required depth you achieve then provide your the wall what will happen the start should be provided at regular interval because this bentonite mixture is a short term phenomena the strength it harden over the period of time it may lose so what will happen then you provide your start at regular interval so that it prevent it prevent your one of this technique that is called lateral yielding it may possible because of soil because of soil pressure because of soil pressure if you do not provide the struts this diaphragm wall may fail or sheet pile wall may fail by means of bending to prevent this bending immediately after providing this wall or sheet pile walls you have to provide your struts at regular interval so that means you are in other way you are you are stopping this you are you are making this or preventing the wall to bend at the end what happen at the end also at the bottom at the bottom you have to also again provide one strut why at the bottom you have to provide a strut if you remember here there is a chance as the over burden pressure increases there will be a bottom heap so to stop that this bottom heap you have to provide strut along with bentonite bentonite then what will happen use create a passage you create a passage in between so that underground construction can be taken care once the wall is stable the movement along the two sides it has been stopped so there will not be any movement there will not be any movement parallely at the side of the wall and at the same time what will happen this diaphragm wall or the sheet pile wall it won't bend it will be stable so the construction space has been created so you go there then you can do the construction for your material and other part of these things so this is our this is your construction technique how you proceed step by step this bentonite slurry has to be provided means bentonite slurry is basic aim it is for temporarily provided so that immediately before providing before providing your d wall or the sheet pile walls the ground should not lateral movement that means lateral movement of ground should be prevented it is natural the movement you cut a ground vertical cut off then what happened from the lateral side of the soil try to move once it will try to move the structure in nearby particularly in metro city it will get distorted means there will be a failure or maybe a differential settlement may occur so to prevent short term before applying before inserting your d wall then you go for bentonite so step by step bentonite then d wall then again construction then go for bentonite then provide your d wall like this you reach your required depth once you reach your required depth immediately to prevent lateral yielding of your walls you have to provide struts at regular interval and one strut along with your bentonite you have to do bentonite slurry by means of bentonite this bottom part will be hardened so that even if the pressure will be more even if the pressure will be more this should not fail by means of heave and also some strut has to be provided at the bottom so that it prevent heave as well as lateral displacement of your wall once it is over then your space has been created for your underground construction so I will stop it here then next class will start the design part of your breast exhibitions