PILE LOAD TEST
INTRODUCTION➱
Something relevant for onshore and coastal areas where the possibility of load testing is feasible whereas for offshore conditions full-scale load testing is not feasible because of several constraints In any case to understand what involves such load test will be very useful so that you can see what alternatives can be worked out for offshore conditions.
But in the near shore around less than 20-25 meters we have done pile Load testing but quite cumbersome quite expensive and time-consuming So the pile Load test if you look at the design of foundation you know there has always been a debate because of the uncertainty associated with the soil parameters.
Always it is understood that design is verified by testing to full scale at the side of the final construction If you look at many of the international course obviously they state that in the engineering based design is not hundred percent sure.
So if you go back to British course or European course including Indian course specially for pile foundation, you will see that every pile needs to be tested to its capacity ,
So if you have seen in a construction site several hundred piles testing every one of them becomes you know elaborate as well expensive time-consuming it puts the whole project in a different schedule of construction activities,
So you select critical piles or piles that may represent actual site conditions So among hundred number of piles may select say 2 piles or 3 piles which may represent actually the site conditions ,So normally most of the modern-day projects we take about 3 to 5 percent of the piles,
That means if you have hundred piles you do the testing of 5 piles So you select them in randomly to distribute all around the site in such a way that you can extract information from this pile testing so that you can represent them in the design process,
But by doing that what you are actually going to do is you take this so-called test parameters and then that calculate and go back to the design revisit the design and adjust the design parameters to suit the tested pile information,
So if the tested file is giving the design or the failure load is lower than the design load means your design parameters what you have used in your calculations are on a higher side or vice versa So you can go back and then adjust the parameters until that you get the design capacity versus failure,
Load is almost close by so this adjustment is even now many of the projects we try to do this so that in the future installation become almost reasonably correct So that means this testing has to be done up front before the construction starts so that is the idea behind pile load testing,
That means you get a comfortable level of confidence which is very essential for foundation system,because you cannot have an uncertainty on a foundation for structure which may actually pose a big threat So the pile Load test has been there for several decades for onshore and offshore projects of near coastal areas,Several kinds I would say So since,
What is the purpose of pile load test?
PROCEDURE OF PILE LOAD TEST➱
Now you see the procedure how we want to start the project with low testing we want to do the load tasting up front before the actual construction starts so you can say sacrificial testing that means once you do the testing of this piles the piles will never be able to use it because you already have failed the pile to ultimate load capacity that means you have a different category of testing
Or you can actually test the same pile which you want to use it as a part of the structure that means you cannot fail the pile but you can actually apply the loading until the load that it may get during its service You know so you can see here now planning is required what type of testing is planned for that particular site whether you want to do a full-scale failure to load test or you want to a working load test,
The test that may actually take you to the level of working load and stop then you actually construct the structural system on top of it So this pile Load test is quite useful in a sense so we will look at various test and the procedures available for us So we will be looking at both horizontal load test and vertical load test,
You know we have specially the coastal areas and the berthing structures and offshore structures we have considered amount of horizontal load compared to onshore structures And then we will look at some of the testing some special testing by which you could actually get the similar information,
But that kind of empirical method you do some testing and extract the information and prorate it to obtain the axial failure load which nowadays seems to be very economical and is quick specially the dynamic testing and some of these are commonly used,
So if you select 5 percent of the piles for testing using gravity method the remaining 95 percent you know still uncertainty exist because if you look at a site your construction site say 1 kilometre by half kilo meter wide industrial construction and you select only 5 remaining 95 percent of the pile is distributed all along this site,
Now you may conclude based on 5 piles that these remaining piles are safe we have adjusted the design but what surety you have over the distribution of soil within a 1000 meter length it may be very difficult decision So instead you go for the remainder of the pile with simplified method instead of going and putting a big weight and then measuring the displacement and strain you do a simplified method of pro rating the capacity based on certain small scale testing which is what we used in dynamic testing and which is very useful in many cases,
It proved to be So if you do a full-scale testing and on the same pile you do a dynamic testing compared capacity then you can actually get a very good feel how the dynamic testing is fairing with regards to the full-scale gravity testing and your design your theoretical design you have 3 numbers to compare and then you can come to a conclusion,
And then the remainder of the piles you go and do a So you can see the correlation is very important from gravity testing to a dynamic testing to a engineering design then you go back and do the remainder of the pile you have only engineering design adjustment plus the dynamic testing ,
So this procedure is adopted in most of the large-scale projects but of course if you look at only some projects were only 5 piles 10 piles are there and subject and dimensions of the structure is so small like 50 meter by 50 meter the variability may not be that great so you can do only one pile test and you can leave it,
And in the application of such simplified methods for pile monitoring for offshore structures is very much useful like what we have learned about your medical scheme to calculate the number of blows required to drive a pile to a particular depth of penetration using a particular hammer,
You can back calculate and it is called pile monitoring system that means you will use the information during driving and then calculate what resistance it would have been offered by the the soil during driving,
And then you just adjust it to long-term capacity using several parameters and then you can predict or at the end of driving you can say the pile would have achieved this much capacity,
Now what you did avoiding here you are not going to place a big weight and try to do a measurement of displacement which is quite cumbersome especially when the pilot very large and the load is huge ,
And that is why the pile monitoring system is one of the indirect means of getting pile capacity in offshore condition and that is what we are going to discuss at the last And in the recent times in the last 10 years,
Or so another method of permanent implant of device into the pile system which is adapted for concrete piles at least You know basically before you install the concrete pile you would a load cell at the bottom of the concrete pile itself,
And just cause the pile and afterwards you have a activation system a wire will come to the ground so you can activate the hydraulics and you just push the pile against the soil resistance And this can be done for ultimate test,
Or it can be so this O-cell test is also employed in some of the onshore projects bridge projects in US but very in this part of the world because it is quite expensive you need to embark a complete load cell inside the pile itself,
Permanently you cannot extract them afterwards so it is just left over there So this O-cell is basically Oster berg cell name after the person the professor who found this method for one of the bridge project in US,
It is in use for quite very useful and few projects we have used it for offshore or coastal so we will go through each one area by little bit detail So the purpose of pile load test is to find out the install pile capacity and verify and remove the uncertainty of associated with the assessment,
That we have gone through do during boring taking Sample and then to laboratory test and then properties and unknown properties are calibrated using past you know information So all those things will be removed once you have the actual capacity versus the displacement,
And to assess the load displacement basically there is no critical information that easy to relate the capacity versus displacement You might have already got the methodology to evaluate the bearing capacity,
A multi layer site what you saw was basically a single soil layer giving a theoretical relationship between the capacity and the displacement whereas when we have a multi layer soil the behavior itself is going to be complex and the only way to get the load displacement relationship is to carry out complete load test,which will be a prototype in nature you are not doing a scale model like our laboratory,
So the primary purpose is to link and assess the load displacement characteristics of a particular pile at a site So the test can be of in fact 3 categories basically the one that normally carry out is the static load test the load is static in nature and that the dynamic load test which is what we were talking about,
The quicker and the cheaper methods Then we have the bidirectional method using O cell you can actually do this way or the other way In the static load test we have got variety of load application methods,
One is the constant rate of penetration imagine if you have the pile already installed and try to do the loading the displacement is not under control so that means if you go by this method whatever displacement comes you will actually note down whereas if you actually control the displacement by changing the load according to the displacement characteristics,
The rate of petition can be controlled So but the first one is quite obviously very difficult to do because you have to continuously having a monetary system of displacement and adjust the loading according to the rate of penetration,
Not many of the contractors have this facility because you need a feedback loop so that when the displacement increases you have to decrease the loading and it is only feasible by hydraulic means where as normally most of the most load test,
You see in the field they all just put big weights so removal of weight is going to be very difficult So most of the time we do maintained the load test you put one weight weight for several hours according to the procedure and monitor the displacement,
If the displacement become constants that mean for that particular load soil has achieved its maximum possible displacement Then go to the next load displacement and apply the loading wait for several hours and vice versa you remove the loading and look at any elastic rebound happening or is a plastic failure,
So basically this maintained load test is commonly used in most of the projects very rarely we go for this particular method Dynamic load testing we have very similar idea like you know the pile hammer when you take a hammer and drop onto the pile and you see the stress waves travel through the pile and get reflected,
If the absorption is not enough and you measure the statistics of the transmitted stress waves and reflected stress waves and depending on the reflection and the transmission you back calculate using the same principle,
What we were looking at the the dynamic equation and calculate that what would have been the resistance offered by the soil because this much waves has reflected back from the soil itself So that is indirect means but before going and doing the actual testing at the site you may have to actually do a calibration in a known material which is easy to do,
After that you can compare with the known material versus actual pile at the site this is you got 2 types of tests one is the high strain dynamic testing the other one is the low strain dynamic testing just the weight of the hammer and the low strain dynamic testing is normally preferred because for concrete piles,
If you do a high impact the pile itself will actually fail Pile monitoring I think we will talk about it little later it is a simple idea of using the driving information or driving records number of blow counts and you know the duration between the blow,
Sometimes you will have major stresses at the pile tip at the top and calculate back the resistance which is also is dynamic because you are using the pile the hammer impact loading Bidirectional method is quite useful only for the O cell testing which is you can do testing in both directions, because the load cell itself is planted into the the pile foundation,
So you can do vertical load test you can do compression tension I think obviously some of the piles in coastal areas for example berthing structures you will have compression loading and tension loading depending on the magnitude of horizontal load,
So the pile needs to be tested for compression and tension as we calculate the capacity you know pile capacity we calculate using skin friction pleasant bearing that is for compression For tension you will have only skin friction depending on whether the pile is plugged or unplugged,
You will see the internal and external So in this case the tension testing is required for only few number of files for example even if you design a structure the whole structure does not have any tension loading that you do not need to do this type of testing,
But most of the berthing structures will have tension because the gravity loading is very less and it is predominantly going to resist the horizontal loads like ship berthing structures so you will see a huge amount of tension coming at you have to make sure that the pile has sufficient penetration to take the tension loading,
It is not the whole structure will pulled off In many cases what we normally do is if you are unable to penetrate that much longer than you actually do a anchoring of the pile into the ground means you will do a smaller hole we will talk about one of the days,
I think later in one of the sessions about encourage files only for tension loading which is very essential for such type of design Then we have also lateral test static and cyclic and you can see here static test is going to give you certain capacity which will degrade when you apply the same loading,
Several times because the top layer of the soil as we have learned from our P-Y curve near the seabed you can see the soil gets disturbed quite a bit because of the repeated nature of loading and degrade the displacement or increase the displacement degrade the capacity,
So we need to see after how many cycles the capacity gets that means you would see that displacement would be more Then we have loading limit whether to load to the ultimate failure stage that means the pile will go into permanent deformation and the soil or you want to do a routine test by which you will not destroy the pile,
You will only do a extend of maximum working load the structure may get and stop it so that the same pile can be used for permanent construction as part of the structure so either way we can decide Only problem is there is an uncertainty in the second one you know you do not know what behavior,
It will go through after it achieves that so-called working load limit because it can fail straight away plastic deformation can happen or it can have a redundancy after that which will not be revealed when you do the working load test,
Whereas when you do this you will be able to find out at what load the pile is failing then you know what is the factor of sifting because the factor of safety is defined as the ultimate load by working load or ultimate capacity by working load,
So you will only be able to ascertain the factor of safety if you are able to find the ultimate load of what you will not be ascertain that factor of safety because in the working load test you only have that level it may actually fail after just going slightly higher than the working load which will not give you the comfort-ability of the factor of safety,
So that is why the working load test is normally not preferred but then cost versus your factor of safety and design requirements You have to decide how many number of piles you want to do ultimate load test and how many you will do a working load test,
But of course if the test result so large scatter then you will increase the number of piles that you do testing If you do a 5 piles and all 5 of them are shows very reasonable matching of results and you may have actually conclude that no further testing is required but each one of them shows different results and different variations in displacement characteristics you may actually decide to do,
Furthermore addition to the pile testing which again is is actually an procedure is decided by the the site representatives I think we have discussed the performance is design validation I think the primary purpose is design validation whatever corruption you have made during your bore holes and to the design stage,
You would like to validate that those assumptions are nullified so that your process of construction can go Quality control for sure in concrete pile is one of the biggest worry in fact steel pile you do not have such issues but the quality control of concrete pile construction,
Because everything goes underwater you have to displace I think we have discussed about the construction of concrete pile you have to displace the slurry which was poured inside by means of good quality concrete but the quality could not be ascertained because nobody can go inside So when you do this kind of low strain dynamic testing you could also assess the concrete the honey combing structure,
If the concrete is not done properly you will see that the reflected waves will differ from the solid concrete that if you have you can actually have uh testing of pile itself whether the pile is sound enough to reflect and transmit the waves or if the pile has got low honey combing inside also can reveal Similarly for steel piles if the file has been broken during driving and if you do this testing and it will reveal that if the stress waves are not coming you could easily find out where is the fault,
So that is the idea behind this the low strain dynamic testing So basically quality control of the construction itself can be verified by means of carrying out cross correlation test Sometimes you do a design development you know you basically you have a large number of some projects of thousands of piles like if you go to some of the large-scale industrial projects,
You may have several hundred piles So you do a representative pile testing and use that information to further develop and then come up with a design procedure In many cases if you if you look at some of the large-scale projects they actually develop new design procedures specific to the site using the design data collected from the pile load test plus the portable information,
And they come up with their own design strategy and the you know you do not need to follow the standard procedure given in either any of the codes or regulations because it is proved by testing at the site so that will be done sometimes And in many cases always you will use it for further refinement of academic interest,
I think these these are secondary but of course insufficient site investigation normally you do not do a construction without the site investigation but then all depends on you know the the owners intention sometimes some government organisations may not be able to do a site investigation,
Before the hour of contract to the contractor because of the nature of limitations they have They may not be able to allocate funds separately for site investigation and separate for construction it is part of the construction itself So what happens is when a contractor comes to the site they have no clue what is the site so they will be doing the site investigation so that is the time when the design becomes difficult the design has to be evolving based on the available information,
But not actual site information Many cases that happens so you do that at that time when the project starts you have assumed certain parameters during design and then you proceed with the construction but then do the testing 1st Once you do the testing your assumed information verified and several other limitations on you know basically cost is one of the primary parameters,
Nobody wants to do testing if they can live an engineered design I think everyone will be happy but that will not be the Now guidance from Handbook of pile load testing this is just give you an idea of how to decide how many piles is good for a particular site,
You know just of course this is only a guidance it will need to be decided by the site engineer and the and the owner of course for sure depending on how much of time and money is located for that purpose,
But typically you can see the complex or very much unknown ground conditions you know it is isolated place no one has constructed in the vicinity so then probably you can go for a large number of So you see here they recommended is 1 preliminary pile test per 250 piles so that means preliminary means this pile will not be allowed to be used in the permanent construction,
So you do a separate file at an isolated location within the periphery of the the boundary where the structure is going to be built It will not be within inside because what you do not want you do not want to fail the soil between close vicinity of a working pile,
For example you have a pile here and just half a meter away or 1 meter away you want to construct a pile and fill it which is not very good because you cannot install another pile in the because the soil already have mobilized its full strength and failed,
So normally you will go little bit away but not very far you cannot do the testing several kilometres away it does not represents the actual site conditions So within the parameters probably representative location you will do a 1 preliminary test for 250 piles,
So you can see here it is only a guess there is no rule book or it is not representing some kind of you know relationship it is only a guess that it may actually give you some information plus 1 working pile test for every hundred piles,
That means among the remainder of the piles every hundred piles you select one and then do a where the the ground conditions are very much unknown probably not very clear No previous pile test data is available in the site new piling technique is used in case in that area every time,
They were using a concrete pile but suddenly you are going to use a test you are going to use a steel pile then it is an unknown experience very limited information available in such cases you do this Where as if it is the other cases you know your risk is reduced with reasonable information then you can increase or decrease the number of piles per testing,
So you see here 1 preliminary test for 500 piles which and then for very low or in fact you got plenty of data then you can also use 1 in 500 1 in 100 or 1 in 200 In fact codes are not suggesting any of these numbers because they leave it to the,
You know the owners or consultants who are representing the design validation So normally we design something very similar like this 1 in 250 but not very projects will have 250 piles you know maybe 10 20 50 so if you do one pile that itself is good enough,
So this is only a guidance which if you are representing your company for pile testing or if you are writing a specifications for pile testing you can put that kind of number Rabbit load test versus dynamic load test you know the kind of capacity that you expect you know 30 mega Newton is $ 3000 tonnes and what kind of hammer,
You will be using just information that you can use it that much is practically possible to do the testing So if you remember we were talking about pile capacity in offshore pile system I think we were talking about 20 to 30 mega Newton for each pile which is something like this,
So it is not that we cannot do it can be done but at a very expensive system design because water depth is more and you want to design a reaction pile for such type of testing will become costlier than your jacket so that is why we avoid,
I think we just discussed about static maintained load of test versus static penetration test or the so-called constant penetration test you can read some of the information about the past history of what was done the maximum load of what was achieved in the previous several years it can be done to that much of load 3000 times,
So you will see if you go around some of the the pile way construction I think MRT construction is going on I think 6 months back so many places they were doing pile testing for the pillars the big pillars you will see that a use platform is built and concrete blocks are stacked upon each was about 600 tonnes,
So you can see that the bulkiness of that platform to load this 600 tonnes because you calculate a 600 tonnes by either by sandbags or by concrete blocks you will get 3 to 4 layers of concrete blocks will come So safe working load ultimate load and design verification load sometimes differ from we discussed about safe working load is a working load of the pile as per the design requirement Ultimate load includes the factor of safety that means either 2 or 2 and half or whatever the design factor of safety,
As per the design code If it is a IS code you normally use 2 or 2 and a half whereas API code we use 1.51 through and so this is the test load that you will apply if you do ultimate load test Sometimes design verification load is in between these 2 we go 50 percent overload instead of failing the pile,
We do a pile load test taking between the working load and the ultimate load sometimes 25 percent higher than the working load Still the pile can be reused because you are not failing it but it is slightly overloaded just to see that how the behavior goes,
Because if the pile fails immediately after reaching the working load after say 5 percent the the factor of safety is not there anymore So design verification load it can be any one of them or it can be in between also A typical onshore pile load test we will just read the methodology how it is carried out,
So you can here this is a good ground condition means the ground at the surface is reasonably good So we have a gravity based support system something like this you construct pillars sometimes we actually use concrete blocks,
You know simply put on 4 Corners make a steel beam something like this and just you have your pile previously installed which is going to be tested and then with a prepared pile head it could be a concrete piece or steel piece placed on top and then you start stacking up your weight because while you are stacking up because people have to go up and load,
It or you your Crane has to bring material and then just keep stacking up And then you will place a hydraulic jack or Jacks 1 or 4 of them normally 4 of them is used and just jack it up So what happens is previously when it was loading is done at that time no load is going to the pile and safe because for a longer duration,
You do not want to keep this condition because it will topple or slides down to sideways you will be putting big danger on the pile itself as well as the system So normally you have this is called Cantalage weight distribution system this is feasible only when the ground conditions are good number 1 and also the the distance from the pile should be sufficient enough that the load does not actually alter the soil conditions in the vicinity of the pile,
Imagine if you do this and if the soil gets actually squeezed and get compressed the pile capacity that you are actually going to measure is not going to be represented in the actual site condition because already the soil has been compressed and consolidated,
So that is why you have to keep this support system quite away from the pile itself so you have to keep it 3 diameter to 4 diameter away so this cantalage becomes actually very large depth girders normally you will see 2 meter-3 meter girders spanning between the supports something like this you have to keep 10 meters away if it is a 2 meter-3 meter pile,
So they will be away so that that influence of the reaction is not coming back to the soil So this is a typical onshore pile load test that using you know spread footing type reaction system so that is the idea put behind So this test can be repeated if it is a working load test you just load-unload load-unload,
But load will not be applied in one single stage normally because then you you will only get one load one displacement relationship So you actually divide the total load into say several steps if it is a 300 times divided into say 5 or 6 steps and put 300 into say 6 steps means 50 tonnes and just look at the displacement then go to hundred tonnes wait for some time and till the displacement becomes constant,
Then you take the displacement reading go and put the further loading so it is just done in steps And then similarly you can do a unloading so from 300 you come to 250 and 200 and so on so you can see whether the loading path and unloading path is same or different,
If there is a plastic deformation then you will see that there is a different load path from loading to unloading so that also can be verified If it is a soil condition that the top surface soil is not good or it is going to distribute the load very close to the the pile that is being tested then you may actually look at installing additional piles and this is the case most of the cases it happens like this So install additional number of piles in and around the test pile,
Normally 4 number of piles are installed and you prepare a cantalage which is nothing but a just a grid of beams which will support these weights to re stacking during unloading process and then when you want to do the testing you simply use the hydraulic jacks to activate,
And then lift it up So you can see the difference between soft bound you know soil condition at the surface to good soil condition the expense increases several folds You can see here here just only a simple pedestal support quick to make easy to remove where as when you go to this one you can see here for one pile load test,
You are installing 4 number of piles so you can see the the multiplication of the time and the cost and the removal you have to remove these piles you cannot leave them there itself unless it is isolated place,
So basically the procedure is same only the difference is depending on pile condition you will do the support reaction and cantalage according to the requirement And if you are going to do the same testing in coastal waters 5 meter 10 meter 15 meter 100 meter water depth can we do this,
So that is where we bifurcated into a different idea if it is a 20 meter water depth yeh probably you can design a pile which will be directed around the other load testing pile and make a platform something like this and then construct this frame load and unload can be done up to say 10 meter 15 meter 20 meter,
But when you go beyond 25 meters will easily see that the diameter of the pile tested and diameter of the support frame the diameter 0 becomes too large If you actually have to do a pile test in say jacket location 100 meter water depth because jacket is still not there because jacket will come only after you complete all your design and everything,
You cannot do testing of a pile through a leg for sure it is impossible So when you want to do this 100 meter water depth imaging you may have to actually specially fabricate a jacket vertical in nature is not it something like this and keep the frame there and put the pile inside and do the testing,
That means, it becomes another project so that is why this pile testing is completely not feasible so maximum pile testing I think for coastal areas people have done up to 20-25 meters maximum beyond which you will see that the design of the system itself becomes a problem,
Specially when you are doing so much loading coastal areas 300 400 500 tonnes But for jacket type of structure if you want to mobilize weight of 3000 tonnes which is becoming a bigger problem so how to avoid even in coastal areas even in land based structures can we do testing without this weight,
This is one of the very good idea so that you can avoid for example if I go and connect this beam with this pipe so what will happen I do not want to put the weights here so instead of using this weight as a reaction what we want to do is we have simply weld this this beam with this pile So what happens,
When you are actually putting up the hydraulic jack the reaction will come from soil itself so that is basically the idea in the recent times no one wants to do a this this is called Dead weight method of testing,
And you can just look and eliminate the so much of weight bringing the weight stacking upon and removal can be removed provided these piles which were installed as a reaction pile or a support pile has sufficient pullout capacity otherwise that pile will come,
You do not know whether you are measuring the this displacement of this pile or whether you are measuring the pullout of the other pile which will become a very complicated situation,
PILE LOAD TEST PART -2
Let us continue with the pile testing so by this I think you could understand the 3 concepts concept 1 foundation by means of simple spread footing for the cantalage itself And basically pile foundation where the ground condition does not permit Load distribution around the test pile the primary concern is we should not put undue load to the pile soil interface which is being tested,
So that is why we have to space it larger enough so that the pressure from the thus upload pile cantalage support pile does not affect or influence the result of the test pile itself that is one concern The 2nd concern you know when you are doing measurement of a displacement of the pile being tested you need to have a firm ground or a darter line by which you will be able to measure the the settlement,
For example in this case the ground condition is good and also you have accessible ground so you just put the darter line which is not going to change over period of time during the testing So if there is a easy way of putting a dial gauge with respect to the ground and then measure the movement of the pile,
Now if you go to the offshore condition what happens if you have no firm condition or firm ground you have to use only the the pile which is supporting the cantalage but at the time of testing it is not loaded anymore,
So you can use that as a reference and use a beam or some welded connection so that you can use the dial gauge So what we need is the the firm place it is not moving during the testing so that is one of the 2nd concern And then when you are doing the testing,
In any case it is not influencing the soil-pile interface or the capacity of the test pile which is being done the now-that is one of the idea so you need to keep it faraway 3 to 4 diameters so your cantalage anyway will require a larger space to stack up the waste and things like this Very rarely in coastal areas we do this because,
It is very expensive one of the tests could take months to set up and do it and bring back A typical cantalage test which is just on land so you can see here even in this particular case it was done in one location in Kakinada where last year before you can see piles were driven so this ground condition was so bad that even you cannot spread the load which coming from so much of sandbags,
So we need to have piles going down to a firm ground because the total top 15 meter of soil is play soft clay and if you try to put some spread footing it will definitely think under this kind of support reaction load during stacking and then you cannot retrieve and every time when you just load and unload the foundation becomes no use,
So that is the picture that you are seeing the pile is being tested which is at the middle and basically the pile will be provided with a pile cap steel plate and these are the jacks which are just placed between the cantalage and pile head just to stack of you know And the dial gauge can see here is placed at this place and this is the reference to the pile head itself,
And this is the bottom line which is not moving So when the pile is moving up the dial gauge will show the displacement every time when your how much pressure you are applying The alternate ways also because instead of instead of going for cantalage with self weight or dead weight,
We can actually use this method by using the pile itself as a reaction frame So you do need to stack up weights only thing is the load is produced onto the test pile by means of the compression load because you are just holding this cantalage frame on the sides using the pile so this is being a welded connection,
So always this anchor pile will be under tension so you need to have sufficient capacity of the anchor pile so that so that means if you are testing a 3000 tonnes here each one of them will have to be divided by 4 and recently have designed with the adequate penetration because it is only tension At least this pile have the N bearing whereas the anchor pile will not have N bearing so you need to have sufficient penetration sometime it may become very large,
And basically bigger bigger than the test pile because you need bigger capacity in tension than the test pile is being a compression capacity so you have to be little bit So that is where this method becomes little bit difficult because it might become very expensive,
When you compare with this maybe better because you do not need to mobilize so much of material movement every time because it is not one-time you will see that later on the sandbags have gone back and forth several times because you have to do loading sequence and unloading sequence and you have to repeat the test at least 2 times,
And that is where you find this is a time-consuming process and also very expensive Anchor pile seems to be very good but then caution needs to be taken in terms of design make sure that when the pile is being tested this is the anchor that you have designed it is not going too much,
Otherwise you will be measuring actually relative movement of the pile anchor pile and the pile being tested Typical reaction pile setup so you can see here this is the pile being tested pile requires to be tested otherwise you have the jack being placed at the and these are the pile heads the left side and right side you see the pile heads which the piles are being driven with the pile head,
And when you just push this hydraulic jack upwards the reaction being applied to the test pile and these the reaction piles on the left side and right side should have sufficient capacity and basically that should be designed And just a closer view of how the hydraulic jack is placed in the girder,
So that it reacts against the test pile as well as the reaction pile Another load test using concrete blocks instead of sandbags many times people use this because it is easy to handle and can be used in multiple places only require storage,
But in any case in construction sites you will find many of these concrete blocks uhh spare capacities available there but many times people use these sandbags because easy to dispose because they can be used for construction work no extra storage space is required and each one of them will be weight before placing it on the top so that you know the weight what it is only the stability of them,
Sometimes bit of a worry because even a small eccentricity caused by either placement or by you know stability the whole thing can be turned In many places if you notice pile load test failed because of the cantalage tilted or cantalage failed which will cause quite a bit of problem,
Whereas using concrete blocks is quite useful because it is very stable sometimes these concrete blocks will come with a sheer case so you can have an opening at the bottom and you can when you place it it will not go anywhere and horizontal stability is well-established,
We can have precast blocks specially for testing not just for any other purpose So the ultimate load test is basically done to the load which is taken to the maximum failure capacity of the pile which will be 2 times or 3 times depending on what the factor of safety used in the particular project of particular core of design,
It is most of the time for IS codes we design for factor of safety for 3 which is for concrete piles which actually says 2 to 3 but then mostly we use factor of safety of 3 that means you find out what is the load that is designed for for working conditions take it to factor of safety of 3 and then find out So typically,
If you look at 300 tonnes is a working load most of the 1 meter-1 and half meter diameter piles then you multiply by 3 you will take to nearly thousand tonnes so imagine thousand tonnes what could be the requirement for dead weights Working load test is typically one and a half times safe working load so we want to go for slight overloading,
But not to the ultimate stage As long as you want to see passing of the working load and also have little bit of budget get depends on how much design margin is available at sometimes be reduced to 25 percent we normally call it overload test Even mechanical devices nowadays everyone of them the working load test is not acceptable,
You have to go for slightly overload because if the load exceeds the working load even by slight amount it should not fail so that is why overload test is about 25 to 50 percent So the 2 methods of testing that we were discussing the other day is constant rate of penetration or maintained load constant rate of penetration is just you can see here 0.75 mm per minute,
So it is a quite slow process and that eventually required by the British code 800 is for design of foundations And maintained load test typically used in most part of this construction site in India is very easy to carry out you do need a sophisticated loop back system simply put the weight and get the whereas the constant rate of penetration,
You need a good amount of instrumentation to read the the penetration backward and adjust the pressure accordingly and it can only be done by hydraulic system not by dead weight and cantalage method it is just not feasible,
Whereas the maintained load test even in a very simple method which you need a dead weight and a jack And the load shall be maintained constant until settlement rate is decreased to 0.25 mm per hour so what we normally do is use you set up your cantalage put the weights safer example 900 tonnes divided into say 5 steps or 6 steps,
So the step 1 of the load and this activates hydraulic jack to transfer the load to the test pile relieving the Anchorage piles or support system and then monitor the displacement Initially displacement of the pile may be larger slowly reducing indicating that the soil has achieved its failure state at that load and then it will go constant or may slightly increase in a very low implement,
So once you see that it becomes flat the displacement graph is flat that means you are not expecting any more images settlements you may actually expect long-term settlement which may be several years which is we are not interested in that So basically then how do you decide when to stop the or when to restart,
The unloading process because every time you are going to unload it bring to 0 then stack up to the next increment So 1st time say you arriving 200 tonnes and bring to 0 that means you have to take out the load and load it again to 400 tonnes so that process when you want to start as soon as you see the displacement relationship is going flat,
That will be better but if the rate is still within this then you can go that means when the displacement is happening you cannot do unloading process So maintained load test is quite simple we will see the procedure later on something like this so you see here you put the 200 tonnes and then that the displacement is increasing,
So what you will see is when you are doing unloading process if there is elastic part it will come back There is permanent deformation of the soil that will not come back that will be residual so it will keep on stacking up so that is what we are interested in Similarly we go to the 2nd 2nd step so when the 200 ton weight is placed on the cantalage,
When you relieved the hydraulic jack the load will go back to the support stock then put another 200-300 tonnes on top of it It will become 400 ton take the hydraulic jack and load it on to the pile and just repeat the procedure until you reach the total test load it could be ultimate test load or it could be working test load,
And every time you do this you will see a loading-unloading curve you can see so basically you will be taking this as a displacement 9 mm 8.5 mm for 200 tonnes so you can go and plot here load displacement in a different relationship,
And typically we will take several hours because get this flat curve sometimes you may have to wait longer time depending on type of soil if it is a sandy material you will get the displacement quickly after that it will just become flat If it is a clay it might take little bit longer time but then instead of setting this criteria IS codes actually gives the time to wait for one hour depending depending on type of soil,
You may only vary little bit So if you look at the procedure given in IS codes it will give you the timing every time step how much time you need to wait and then start the process of loading-unloading In fact this this particular procedure is taken from pile testing manual which I was talking about day 1,
So the design specifications load all it is basically the test load if it is ultimate test design verification load will be your total load multiplied by the fact of safety 25 percent 50 percent 75 percent so this procedure is just segmented into 4 subdivisions 30 minutes so once you load 25 percent load wait for 30 minutes and 100 percent load wait for little longer time,
And then during the unloading process only because relieving of elastic compression will take just like this it is not going to take time so unloading time they are just allowing you to only little time 10 minutes And then again we start the process 100 percent and you are going to do a overloading test,
There you are going to wait for little longer time beyond the design verification load if it is a working load test and then repeat the procedure So basically this is just typical time versus increment of load,
I have just taken from the you know the manual of pile load testing from the Association of civil engineering contractors every everywhere they used this but if you look at the IS codes they may actually gives slightly different numbers and different timings,
You know so depending on which jurisdiction you want to use So if you look at this graph load displacement curve for ultimate load test up to failure So you can see depending on type of foundation material you will see 3 types of so if you look at the blue one N bearing and V crock basically it increases and then suddenly fails,
So you will see a certain capacity reaching the rock is trying to fail so almost like elastic and perfectly plastic so something similar like this And if you look at the black one friction pile in soft clay or sand you will see that highly nonlinear it keeps increasing and then the failure rate will be slow down achieving certain capacity,
Or else if you have a fiction pile in stiff clay also will be very similar but then there will be a downgrade effect because the stiff clay broke down when it is achieving the so typically you will see something similar when you plot the when you extract the displacement from such plots,
Nowadays you can automate these kinds of things so automatically we will get a time bound graph then you can later take the displacement come up with the displacement versus the applied load graph is something like this and basically you need to decide,
Now what capacity is to be taken as ultimate or working load So depending on what the allowable all ultimate reflection that you want to permit remember when we were doing the TZ curve for clay and sand we were talking about 0.1 inch for sand and then 1 percent for clay something like this,
So that corresponds to the ultimate failure whether you want to consider here for steel piles has to be decided by what is your ultimate failure to state you want to define So once you define that limiting load or limiting displacement then you can go onto the graph you can take this is my ultimate failure load of the pile beyond which we will not be able to take it Saturday is the idea behind construction of this diagram is just to make sure that your TZ graph you have made is going to be reflected here,
And if you get a representative from here and that will be realistic because this is based on test where as the other one is based on historical data and empirical methods So you see here in this particular one the ultimate capacity at 25 mm settlement is taken as 425 tonnes,
So though it is highly nonlinear some amount of plastic deformation have happened compared to when you go to the blue if you look at it this is completely elastic and after that only becomes the plateau so you can see the difference So that is the way we want to determine,
When you do a testing I want to determine what is the ultimate capacity The thing is you have to sum up what is the displacement at which you would like to take for example when you are designing a structure you will always have a limiting deflection in both horizontal and vertical working load conditions,
So then if you are allowable capacity you want to find from this then suppose your fix rate 10mm is the allowable displacement during service life of the structure then 10 you just go to 10mm the pile has achieved a 350 tonnes,
Now if you take the ultimate capacity apply a factor of safety of 3 or 2 you will only get something like 120 tons so that is what will be revealed here If you do not know about this graph is going what you are trying to do is if you take 420 tons join from here straight away to 0 make a straight line that is what you are doing by means of factor of safety 420 divided by factor of safety of 3 on load,
Whereas if you have this this graph you know very well that you can allow maximum of 10 mm during its service life with the factor or on factor load then you can decide how much margin you have almost large margin is available,
When you take 10 mm and restrict to 10 MM is the displacement 420 decided by factor of safety of 3 you will get around 130 tons that is what you are going to permit But actually the pile has got deserved capacity up to 350 at the same 10 MM displacement,
So that will be the difference when you have nonlinear load deflection curve especially after testing So this will give you a picture how much extra margin you have because of the soil behavior and if you take this one you will not get anything better is not it Because you are grabbing almost linear relationship,
If you take that will be the capacity of say 550 tonnes divided by straight away factor of safety of 3 you will get somewhere around 180 tonnes Whether it is 10 MM or 5 MM you are going to just linearly prorate so does not matter So that is the idea of constructing or establishing load displacement characteristics up to failure,
If we have only established up to for example some lower displacement because you are not going to do a failure test you are going to do a working load test just up to the linear portion then you may not actually reveal how the behavior after this this is why at least one or 2 piles at the site you have to do up to failure to see how much margin,
You have It can behave this way which we do not know unless you do the testing Then we also move onto horizontal load test basically similarly you can do horizontal load test One of the biggest problems is doing reaction she just now we have learned about vertical load test with support arrangement,
Somehow we can make it because it is similar piles can be driven but when you do horizontal load test you need to have some ground or firm support condition just to measure the horizontal displacement of the pile and also to transfer the reaction from the jacks which is going to be a big challenge So doing it onshore itself is a challenge,
Then when you are doing it in coastal or offshore conditions it is even a challenge because you have to design a system stronger than the test pile itself and install it And also you can carry out cycling load test in cases of coastal structures where cyclic behavior is required for degradation effects So typically you see here some working platform is required,
And I would have just straightaway gone to the coastal areas where some of the berthing structures we still do this lateral load test to establish the capacity in horizontal direction but most of the structure on land normally we do not do you know horizontal load test not really required,
Because predominantly it is not so much problem so far for buildings and bridges predominantly gravity load So you see here we have a test pile which is in yellow colour and we have a reaction pile and just the jack,
Now you see here the jack is placed in between attached to both the pile heads and the jack is trying to expand by giving horizontal load through the test pile the reaction is transferred to the reaction pile Noise they reaction pile stable the reaction pile displacement is smaller or ignoble then what you are measuring is the pile head displacement otherwise he will be measuring the relative displacement,
Because this pile is also moving the other pile also moving and how much we have to find out So you need a chili credit system to find out what will be the displacement which will be quite tough,
Because you can theoretically calculate but then it may not be correct A reference will be used another few numbers of piles is required or system is required to support all this as well as access to these places,
So you will see that horizontal load test becomes reasonably expensive because you have reaction frame and you also have a reference frame the reference frame the green colour what you see is a reference frame against which you will do the measurement of displacements,
And they are need to be placed sufficiently you know wider spacing otherwise what happens when you are doing when you are doing reaction here when the pile is trying to bend it puts undue pressure on the soil and that soil gets affected by this particular location and behavior of the lateral capacity is going to be affected to some extent and that is why you have to have minimum of 3 diameter or 4 diameter or 5 diameter,
Once it become wider spacing mounting of the jack becomes a problem you need to design a frame and that is going to put you onto support system together it has to be mounted onto the 2 piles which needs to be free to move so that is one of the challenge A typical cyclic load test results or plot how you carry out loading and unloading process basically horizontal load versus lateral load displacement,
You take it one cycle at particular load level bring it to 0 repeat it few times just to get the repeatability and again increase the higher load repeat it again load and unload since there is no dead weights are involved it is only hydraulic jack doing such load test is very easy then you repeat it again and again for various loads and stop,
When you actually reach the expected displacement for example ultimate load test you are trying to do for 100 mm maximum If you are doing a ultimate horizontal failure load test then you can go up to failure of the pile and stop otherwise you have a predetermined load by which it will not be a larger capacity,
Most of the pile if you look at 50 tonnes or maybe maximum 70-80 tons unlike the vertical capacity where you have thousands of tons or sometimes very large number Here it all be you know restricted by the pile horizontal shear because if the soil does not fail what will happen the pile will fail,
So if you look at the Shell capacity of the pile that will give you the maximum magnitude by which you can test in fact will apply factor of safety on it What you do not want you do not want the system to fail before the soil fails system means the pile itself or the anchor pile itself,
So you will find out what is the bending capacity of the pile or structural capacity and shear capacity Make sure that you have got sufficient factor of safety available on the structural system prior to go to ultimate load test,
Because you do not want the system to fail when there will be a catastrophe And in some cases in the coastal structures like berthing structures we may require pullout test because some of the piles are actually going to have tension capacity requirement especially when you build a berthing structure the front pile will be in tension the back pile will be in compression,
When you are having shift or berthing against the berthing structure So such you may require a pullout test instead of a compression test it is exactly opposite of it only the cantalage placement and the hydraulic jack placement is in the reverse condition,
If we just go back to our the this test somewhere here I think something like this so this this one is anchor pile method you are doing a compression test just reverse the process and placement of the hydraulic jack you will get the So basically the cantalage arrangement is attached to the reaction pile,
And there will be stress blocks and you have hydraulic jacks and then you attach your brackets welded to the test pile itself So what you are trying to do is you are compressing the reaction pile and pulling up the test pile so basically you are just reversing the reaction just to get what tension capacity is available Many times you will require this for coastal structures but magnitude will be not very big,
Few hundred tons So what are the things that we need to note down when you are doing a compression load test the support frame needs to be designed appropriately because you you have such a large weight to be placed Support piles needs to be placed away by 3 diameter that makes the so if you have 2 meter diameter pile you need to go for either side 3 meters 3 times 2 meters 6 meters so the frame becomes 12 by 12 or 15 by 15 Structural capacities of test pile,
As well as the support piles leads to be made sure that it has good adequate capacity Special care must be given to the test pile because it is a cantilever whereas actually the same pile when it is working in the actual structural system it may not be a cantilever it may be combined pile heads with the other structures,
Whereas when you are doing testing it is a pure cantilever you must remember what is the difference between pile in a temporary condition which is a cantilever to a supported pile in the permanent condition because of high level buckling The load capacity will come down drastically,
So you cannot think about always you have pile 1 meter diameter in a permanent system which is connected to a bigger structure which may not behave as a pure cantilever so the effective length and the conditions are different you are 1 meter diameter pile may work there whereas the same 1 meter diameter pile,
When you bring to the test category it becomes a pure cantilever number 1 high-level buckling load will be definitely half or less than the actual buckling capacity When you are doing the same ultimate load test here you do not want the pile to fail So what we really need to do is keep the diameter same because you need you do not want to change the pile-soil interface is not it Because we want to test the 1 meter diameter,
But what you can do is you can increase the wall thickness to increase the bending stiffness buckling capacity shear capacity in such a way that structure does not fail but the soil pile interface is being tested,
You understand the idea know so you need to make sure this otherwise if you bring the same pile and testing before the soil fails the pile collapses and then it puts the whole system into problem that is what you have to make sure So what IS codes are suggesting instead of what we saw was 25, 50, 75 they have divided into 5 sub-segments of loading 20, 40,60.80 to 100 so it is just a little difference,
And basically loading-unloading cycles almost similar I am not very sure whether I have copied the time cycle You can look at the code the time is also given 30 minutes up to 6 hours so you can refer to the code but what the criteria,
I wanted to look at it is basically when do we decide it has achieved its ultimate capacity and it is applicable only for concrete piles you cannot apply this one to steel tubular piles uhh it is not under that particular code So if you look at this graph,
I have just plotted one of the recent last year test two thirds of final load at which the total displacement attains value of 12 MM So you just look at the graph and just go around apply the criterion number 1 the safe load on a single pile for initial test should be least of the following,
So the criteria is given Do the testing comeback here plot the graph and look at these criteria whichever is lower that is your safe working load so that is the criteria that you are going to apply And of course this is written on the basis of onshore structures you know 2911 is a code for both the concrete piles for onshore applications not even for coastal applications that is why you cannot blindly apply this principle to a coastal structure on or offshore structure where displacement get area are different from dawn show structures,
You will see that they are very strict in terms of displacement because these are applicable to buildings and bridges or to some extent industrial structures on land where they are controlled by deflection Whereas if you go to coastal structures and offshore structures the displacement are definitely going to be larger,
You cannot even think of the criteria here because here they are talking about 25 MM When you think about coastal or offshore structure you are going to have several hundred millimetres of displacement because of the the magnitude of horizontal load you have to be a bit cautious and apply the principle differently 50 percent of final load at which the total displacement equal to 10 percent of the pile diameter,
So these are the 2 criteria so if you see there the criteria 1 and criteria 2 you can find out whichever is giving you the capacity I think even the lateral load also similar crater is given 50 percent of the final load at which the total displacement increases to 12 MM final load at which the total displacement corresponds to 5 MM and then any load corresponds to any specified displacement this is some at least some option is given for horizontal load,
So according to your project specific requirement so you specify I can go out to 50mm then I can look at so that if that is the thing that we need to have a decision-making process just doing load test alone will not be good enough you do not know what to do with that what you need is the demarcation what will be the acceptable displacement for that particular type of structure and applicable code in force,
What the API says we do not have a procedure to restrict the displacement unfortunately there is no requirement because we do not need because offshore structures are subjected to so much higher loading that if you restrict 25 mm displacement for example your structure will be so much bigger and may not be practical,
So that is why the the adapted method by API is to design by the factor of safety which gives you an adequate safety against pile failure or premature failure and at the same time you have a ductile material which is steel,
And you are not worried about failure by other means of fracture because the worry in concrete structure is larger displacement either in vertical direction or in horizontal direction can cause superstructure failure by fracture because the concrete cannot fail by the tail,
So that is one of the reasons why most of these concrete cores they restrict the displacement to smaller where as the offshore structures we never build using it except the concrete gravity platforms all the fixture types of structures are built by steel material has higher tensile capacity and also have a ductile characteristics In that reason we go by the method of engineering-based design based on soil properties with adequate factor of safety and we leave that we do not want to apply the restricting displacement,
The reason why we developed TZ all that is to make sure that pile is actually subjected to the actual load load displacement behavior and the bending stresses are calculated So if we have a larger displacement what happens is the pile is subjected to larger bending and larger bending means the requirement of section and the diameter and the thicknesses are going to increase,
So you design for it rather than trying to limit the displacement of the pile to lower value and increase the larger diameter so that is exactly the the design method adapted by API so so far I do not think any problem with that idea One of the issues with concrete pile supporting concrete structure is the architecture and finishers,
And serviceability requirement that is why it restricts the displacement to a very small value in fact the reason why we restrict to 1 inch is because of that You know if you have the structural system capable of taking the architectural finishes and service functions will be seriously disturbed that is why buildings are designed for smaller displacement,
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