welcome back to the channel guys it's gabriel with your engineering tutorials and tips channel
in this video i'm going to go over some basic points that you should be aware of
when designing a pile again if you have any questions just drop a comment below
or hit me up on instagram if you want to have a chat about engineering in general and of course
don't forget to like the video and subscribe to the channel so surely in your engineering
journey you're going to come across a scenario where piles will be necessary in your foundation
and that usually happens when the site you're working at has a layer of weak soil at shallow
depths on top of a much deeper layer of a stiff or dense material another possible use of piles
would be for the design of retaining systems or any other structures subjected to lateral loads
as2159 classified piles into non-displacement and displacement piles in simple words if you're
drilling a hole taking out soil and replacing with concrete that's a non-displacement pile on the
other hand if the pile is pushing through the soil and displacing it laterally and compacting around
that's a displacement pile an example of a known displacement pile would be a cfa or continuous
flight auger pile or a board pile a displacement pile would be a driven pile or a screw pile for
example every pile has its advantages and disadvantages so first step you need to take
in your design is to choose the most adequate pile to your specific case scenario for example
if noise and vibration are unacceptable for the adjacent property i wouldn't choose a driven pile
a board pile board piles are hard to do if you have a cohesionless soil like sand because the
side walls will keep collapsing into the hole next you need to identify the degree of exposure
that your pile will be subjected to it is a non-aggressive a mild a moderate a severe or very
severe you will need this information to choose the concrete aggregate and the minimum cover to
the reinforcement as per table 6.4.3 so jump into pile actions the structure that you're designing
might be subjected to axial loads lateral loads or both at the same time so when talking about
lacks when talking about axial loads we're gonna have a look at compression and tension and for the
lateral loads we'll have a look at the bending of the pile so let's begin with axial loads which is
a little bit easier than lateral so you can do an analysis based on soil properties for
cohesionless or cohesive soils or rock a second option would be in-situ tests like spt and cpt
or if it's a large project you might choose a more expensive method like load tests which could be
a controlled stress test or controlled strain tests so let's talk about vertical loads now
the total pile bearing capacity is given by the shaft capacity plus the base capacity so the
shaft capacity is given by the skin friction times the surface area of the pile in contact with soil
which is pi times diameter times the length of the pile and the base capacity is given by the base
pressure times the area of the base which is pi pi times d pi times d squared divided by four right
bearing versus friction versus a combination of both if your pile is sitting on rock then the base
is doing most of the work and the sides are doing virtually nothing at all however if the pile is
embedded into rock or or embedded into strong soil the base is definitely doing all the work on the
other hand if your pile is in a weak soil the size will contribute a lot with the capacity so the
friction will do most of the work and finally in short piles the base will do most of the work so
those board piers you see in house footings since they are since they're a couple a couple
of meters long friction will not do much for the calculations the bearing capacity of the
base is pretty straightforward it is the bearing pressure times the area of the base
i won't get into the formulas to find the bearing pressure because as a structural engineer
i will assume the geotechnical engineer will give me this information so i'll just
get these values from the soil report there's one thing that i would like to emphasize and
when you're doing a structural inspection so as a structural engineer you're going to go inside very
often to do structural inspections so you got to make sure the bottom of the hole is nice and clean
because you will not get enough bearing capacity and the pile might settle with time
if you have loose material in the bottom okay so just make sure the contractor
cleaned that out so there are multiple ways to calculate the bearing capacity of the shaft
the one i use is the alpha method for clays and be the method for sense
the power shaft capacity for the alpha method is given by the average skin friction
times pi times the diameter times the length the fms is equal to alpha times the undrained cohesion
and alpha is given by the coefficiency times the vertical effective stress
divided by the undrained cohesion to the power of 0.45 so notice that alpha is dependent on the
vertical effective stress which is gamma times depth therefore
therefore um it makes sense it makes sense that alpha will increase with the depth right so
it's easy to understand that the deeper you go the more the soil will compress the pile but there's a
limit for that so the limit for alpha is one okay so you can't go above one and the coefficiency
varies from 0.4 to 0.5 for board piles and equal or greater than 0.5 for driven piles
now if you have cohesion-less soil like sand you can use the beta formula which is again
the skin friction times the surface area which is pi times d times l um
and fms is is given by ks times sigma times the tangent tangent of delta so the k value is the
lateral earth pressure coefficient it is the same k value that you use to calculate retaining walls
and it is given by 1 minus sine theta so for example for for an angle of friction of
30 degrees k is equal to 1 minus sine theta which is 0.5 and that means that half of the pressure
of that soil is pushing sideways okay so 0.5 so that means half of the pressure is horizontal
so sigma again is the vertical effective stress and delta is two-thirds of the angle of friction
a couple of things to take into consideration i usually apply a factor of safety of 3 for the
shaft capacity so the result you get in the end just divide by 3 by 3 that's um that's your answer
also as 2159 clause 4.4.1 says that we should ignore the capacity of the first 1.5
times the diameter of the pile so for example if the pile has a diameter of 450 so 1.5 times 450
675 so you start the calculation from 675 downwards okay so the first 675
gives you no capacity and this accounts for a weak soil at the surface as well as
cracks in the soil okay also remember there's a limit where the friction stops increasing okay
from 20 times the diameter downwards the pressure becomes constant so this triangular shape here
will become constant at a certain depth so this triangle becomes becomes a rectangle
and this you know it's not going to increase forever so uplift on piles
let's suppose you have a transmission tower a transmission tower like this one to design
when the wind blows in this direction you will get uplift force in this pile
so you need to quantify this force and design the power to take take these loads also um
reactive soils may cause uplift loading on piles so the soil swells and lift the pile up like here
the uplift capacity of piles is given by skin friction times the area the area surface area of
the pile in contact with soil plus the self weight of the pile as 2159 clause 4.4.2 states that
in uplift the load displacement behavior is typically more brittle than in compression
the corn pull out mode of failure may be critical for shallow piles socketed into rock or fizzard
clays so you've got a a cone failure here so it goes diagonally diagonally from the bottom of the
pile here and f fm st which is the skin friction intention is often less than fmse which is the
skiing friction for compression because of poisson and order and other effects now usually seven fmst
is usually 75 of fms different literatures might get different values that's that's usually um
the value that i use so lateral loads wind is the main lateral load on most structures soil will
also exert lateral loads on retaining walls and all these force will cause bending moment on the
piles another load that can cause bending moment on pile is eccentric vertical loads as join 159
states that any pile needs to be designed for a bending moment of force times 0.05 times the
diameter as a minimum right and if you go to clause um 7.2.1 the minimum 75 millimeters okay
but if you have an eccentricity that's greater than both of these numbers you obviously will use
this eccentricity okay the greatest one you have several methods to work out the lateral capacity
of a pile the ones i believe are most used are bronze method branch handsome method py curves
winkler springs and numerical analysis like the plexus software just a couple just a couple of
comments to finalize this subject if you have a board pier connected to a cross-bracing system
the pier will have to be designed to take lateral loads and uplift okay so this wind
load will cause the diagonal bracing to go in tension and then you're gonna have a reaction
horizontal reaction you're gonna have an uplift reaction bending moments will have to be um
to be calculated this force this reaction will have to be resolved okay a common misconception
i see young padawans doing out there is using a pain support for the columns and thinking that the
pile will not be subject to bending moment however the horizontal reaction will still cause bending
moment on the pile okay so don't forget about that just be aware of this horizontal force that
will cause bending moment okay to wrap things up this is an extensive subject there are a multitude
of topics that that we can cover within pile foundations i haven't discussed buckling of piles
um sediment sediment of piles deflection power groups piles and rock screw piles
so if you find yourself in a position where you have to design a pile make sure you understand
every possible scenario and all the potential modes of failure of the pile okay so you design
your pile accordingly all right i hope you enjoyed the video and i'll see you next time