1 00:00:11,800 --> 00:00:18,090 So today we will evaluate the capacity of the pile plug so that we can determine what 2 00:00:18,090 --> 00:00:23,800 is the N bearing capacity we can limit as proposed by Randolph we can see here he has 3 00:00:23,800 --> 00:00:29,410 integrated the internal you know the vertical pressure and arrived at the limiting capacity 4 00:00:29,410 --> 00:00:36,350 which will be either the frictional capacity plus the rate of the soil or the plug capacity 5 00:00:36,350 --> 00:00:39,989 determined here is depending on the soil at the base 6 00:00:39,989 --> 00:00:45,290 So that is why not only just because of the type of soil but also depends on the soil 7 00:00:45,290 --> 00:00:51,280 within the plug and the frictional resistance within the pile inside also will determine 8 00:00:51,280 --> 00:00:56,739 what amount of N bearing that we can allow So basically that is the starting point from 9 00:00:56,739 --> 00:01:02,269 where they started Now if you look at the simplified approach when the plug the soil 10 00:01:02,269 --> 00:01:08,450 inside is lower than the actual level of soil outside due to several reasons 11 00:01:08,450 --> 00:01:13,850 One of the reason is the course grind material get densify and you get slightly lower but 12 00:01:13,850 --> 00:01:20,290 off course the depth by which it goes down is not substantial sometime you get a plug 13 00:01:20,290 --> 00:01:25,760 ratio of 0.9 0.95 like that it is not going to be completely not there So many times we 14 00:01:25,760 --> 00:01:33,270 get a 90% plug ration means 10% of the depth inside soil in not there So you can calculate 15 00:01:33,270 --> 00:01:38,420 the plug weight by using density and height of layers 16 00:01:38,420 --> 00:01:44,320 So you can see here one important approach taken by Randolph also is to assume slightly 17 00:01:44,320 --> 00:01:51,530 less denser soil a loose soil plug and a densified soil plug so they called it density ratio 18 00:01:51,530 --> 00:01:56,540 of the soil plug within the pile plug itself So you can see here the weight of the lower 19 00:01:56,540 --> 00:01:59,880 wedge and weight of the upper wedge slightly different because of the different type of 20 00:01:59,880 --> 00:02:02,650 density assuming that the soil outside is same 21 00:02:02,650 --> 00:02:08,399 But here he got multilayer soil so you need to proportionate proportional to the depth 22 00:02:08,399 --> 00:02:15,080 of the soil layers outside and correspondingly reduce the height So it is a little bit geometric 23 00:02:15,080 --> 00:02:22,850 exercise plus the inner friction capacity basically taken only on the denser soil the 24 00:02:22,850 --> 00:02:26,490 friction capacity and upper soil is ignored because it is anyway loose and it is not going 25 00:02:26,490 --> 00:02:31,150 to offer that much basically so that is why we can see the frictional capacity is only 26 00:02:31,150 --> 00:02:34,970 taken in the insides and only on the lower soil 27 00:02:34,970 --> 00:02:40,930 So when you add all of them together and that will be the the plug capacity for the bottom 28 00:02:40,930 --> 00:02:46,300 So if you have higher plug capacity because the soil is very good at the bottom does not 29 00:02:46,300 --> 00:02:50,190 mean that you can take it because anyway when you have a N bearing of higher capacity the 30 00:02:50,190 --> 00:02:56,300 soil plug will start move that means the plug will fail before the pile files So this is 31 00:02:56,300 --> 00:03:00,740 the limiting capacity basically when there is no scour scour I think we will discuss 32 00:03:00,740 --> 00:03:06,350 about it in the later part of the course where we will talk about the causes and type of 33 00:03:06,350 --> 00:03:08,550 scour and the nature of scour 34 00:03:08,550 --> 00:03:14,240 Scour is nothing but removal of soil in vicinity of the the structures or the pile that you 35 00:03:14,240 --> 00:03:20,220 constructed on sea floor basically because of the obstruction the increased flow circulation 36 00:03:20,220 --> 00:03:25,090 will remove the particles away and forming a small crater around which is called scour 37 00:03:25,090 --> 00:03:29,860 and is very-very serious when you have a granular type of material it takes away very easily 38 00:03:29,860 --> 00:03:34,750 and except the if the granular material is heavier then it may not be possible 39 00:03:34,750 --> 00:03:40,100 So the scour means the soil is removed in the vicinity in this particular case there 40 00:03:40,100 --> 00:03:46,670 is no scour so is the inner plug is smaller than or height is lower than the outer When 41 00:03:46,670 --> 00:03:53,490 you go to the same thing if you look at this picture the soil original soil floor is somewhere 42 00:03:53,490 --> 00:03:58,160 and the soil have been removed because of the scour so that is the difference otherwise 43 00:03:58,160 --> 00:04:02,500 the calculation is same but the inner soil within the soil plug has not been removed 44 00:04:02,500 --> 00:04:04,290 because it contained within the soil 45 00:04:04,290 --> 00:04:10,880 So that is the a simple geometry so in here we have to understand two things one is the 46 00:04:10,880 --> 00:04:15,099 internal friction the external friction so the internal friction is taken as 80% of the 47 00:04:15,099 --> 00:04:21,000 external friction as you can see here the pile the soil within the pile is confined 48 00:04:21,000 --> 00:04:26,280 so the development of 100% friction is somewhat questionable So generally we assume lower 49 00:04:26,280 --> 00:04:32,319 friction capacity of the internal pile surface to soil to the external surface to soil 50 00:04:32,319 --> 00:04:38,370 So normally we take about 80% and you can see here the length of the pile plug which 51 00:04:38,370 --> 00:04:45,660 is typically about 90% that means penetration is say LP 90% of that is basically the height 52 00:04:45,660 --> 00:04:51,219 of the plug inside so that is the meaning of point nine LP is the plug ratio and you 53 00:04:51,219 --> 00:05:00,190 can see here the ratio of the denser to loose soil is also around 70 80 typically last 20 54 00:05:00,190 --> 00:05:03,719 30% of the soil is going to be slightly loose 55 00:05:03,719 --> 00:05:07,689 Unless otherwise any information is given you can by default you can use the values 56 00:05:07,689 --> 00:05:14,939 of the numbers I have given here and especially this friction though API does not actually 57 00:05:14,939 --> 00:05:20,490 differentiate between the internal friction and the external friction you know the give 58 00:05:20,490 --> 00:05:27,949 the engineers choice to decide a value of internal friction but generally we use 80% 59 00:05:27,949 --> 00:05:33,849 So 80% means when you calculate the frictional capacity of the outer surface to soil is 100 60 00:05:33,849 --> 00:05:41,060 % the inner surface to soil is 80% because is well confined and unable to mobilize that 61 00:05:41,060 --> 00:05:46,240 much shear between the soil and the pile so that is some information you have to keep 62 00:05:46,240 --> 00:05:52,150 in mind which you will not find in the course Course does not necessarily limit 0.8 they 63 00:05:52,150 --> 00:05:55,809 just allow you to use 100 % 64 00:05:55,809 --> 00:06:01,229 So the same thing which we can do a calculation for the scour case where the soil in this 65 00:06:01,229 --> 00:06:08,089 outer vicinity has been removed exactly same except that the plug ratio to the final form 66 00:06:08,089 --> 00:06:13,550 of penetration will be higher because you just have to take into account the scour also 67 00:06:13,550 --> 00:06:18,560 because that is the only difference otherwise calculation method is exactly same there is 68 00:06:18,560 --> 00:06:23,919 no difference The reason why we do this I think hope you have understood is basically 69 00:06:23,919 --> 00:06:28,849 because the pile plug matters in determining the capacity of the N bearing 70 00:06:28,849 --> 00:06:34,360 It is not that only the N bearing is determined only by the soil by which you are actually 71 00:06:34,360 --> 00:06:38,470 resting on it so that is the thing that you need to remember Whenever you have a soil 72 00:06:38,470 --> 00:06:43,059 here based on the soil you calculate the N bearing very similar to the bearing capacity 73 00:06:43,059 --> 00:06:49,039 that we have determined for clay type of soil we have used the lower bound solution for 74 00:06:49,039 --> 00:06:51,869 four CU and upper bound solution of 6.28 75 00:06:51,869 --> 00:06:58,119 So in between you can take a value which will be your bearing value but if you take that 76 00:06:58,119 --> 00:07:05,009 value and find out the capacity limitation based on the plug and whichever is lower you 77 00:07:05,009 --> 00:07:09,610 have to use that So that is why the limiting plug capacity has to be calculated before 78 00:07:09,610 --> 00:07:17,570 you proceed further Next one basically computation of ultimate capacity 79 00:07:17,570 --> 00:07:23,150 I think the first in fact yesterday we were talking about how the summation of capacity 80 00:07:23,150 --> 00:07:29,119 is arising from the skin friction and the N bearing Two cases we have seen plugged case 81 00:07:29,119 --> 00:07:33,710 unplugged case plugged case is basically the full N bearing with external friction and 82 00:07:33,710 --> 00:07:38,770 unplugged case internal friction external friction plus the annular N bearing So that 83 00:07:38,770 --> 00:07:42,930 is the thing that I have written here in a basic idea is compression plugged compression 84 00:07:42,930 --> 00:07:43,930 unplugged 85 00:07:43,930 --> 00:07:50,159 Let us only look at this two so you can see here this is a external friction and the N 86 00:07:50,159 --> 00:07:55,740 bearing and this is internal friction external friction and the N bearing on the wall of 87 00:07:55,740 --> 00:08:02,469 the pile So you have a two cases whichever goes lower automatically you will have to 88 00:08:02,469 --> 00:08:07,700 assign As I mentioned you know basically you don’t need to do this if you don’t need 89 00:08:07,700 --> 00:08:11,330 to really do both of them as long as you are able to determine whether the pile is plugged 90 00:08:11,330 --> 00:08:12,330 or unplugged 91 00:08:12,330 --> 00:08:17,520 If you are able to calculate the plugging is going to form by means of summation of 92 00:08:17,520 --> 00:08:24,129 the weight of the pile plus the internal friction and then compare with the weight of the soil 93 00:08:24,129 --> 00:08:27,099 So you can determine whether the pile is plugged or not then you don’t need to really do 94 00:08:27,099 --> 00:08:31,810 this automatically this will govern So that is where you know you are not able to determine 95 00:08:31,810 --> 00:08:37,070 whether the pile is plugged or not plugged then you can find out the capacities of the 96 00:08:37,070 --> 00:08:45,690 external plus the full N bearing internal external and then annular N bearing and whichever 97 00:08:45,690 --> 00:08:50,990 comes lower you can assign that means the pile has plugged or not plugged accordingly 98 00:08:50,990 --> 00:08:57,100 For tension capacity is only the skin friction can be taken because the load is applied upwards 99 00:08:57,100 --> 00:09:03,459 so you cannot take the N bearing so in this case external friction multiplied by the surface 100 00:09:03,459 --> 00:09:09,790 area of the pile which is A means is the surface area of the pile O means whether outside or 101 00:09:09,790 --> 00:09:12,440 I means is inside 102 00:09:12,440 --> 00:09:17,579 So you can just calculate that wall thickness of detecting the wall thickness For tension 103 00:09:17,579 --> 00:09:25,529 unplugged and the only difference you can see here unplugged means you here the internal 104 00:09:25,529 --> 00:09:30,610 friction plus external friction That means you have a friction inside friction outside 105 00:09:30,610 --> 00:09:36,519 Whereas when the pile is plugged you only have a the skin friction outside So these 106 00:09:36,519 --> 00:09:42,290 four cases basically just to calculate the capacity and compression capacity in tension 107 00:09:42,290 --> 00:09:47,040 when the pile is applied with a downward load its compression load is applied again the 108 00:09:47,040 --> 00:09:50,319 gravity is basically tension 109 00:09:50,319 --> 00:09:58,930 So the parameters involved is the surface area internal external and the end area either 110 00:09:58,930 --> 00:10:04,970 the annular the steel wall area or the total area depending on whether is plugging unplugging 111 00:10:04,970 --> 00:10:11,940 and F knot and F phi which is the skin friction capacity in terms of say unit values per square 112 00:10:11,940 --> 00:10:17,380 meter per square feet or per square inch which we need to find out this are geometric values 113 00:10:17,380 --> 00:10:21,370 which is not a problem because is as long as the pile diameter is given we can calculate 114 00:10:21,370 --> 00:10:27,060 Whereas the F knot F phi and then the unit N bearing which is very similar to the bearing 115 00:10:27,060 --> 00:10:31,829 capacity of shallow foundation which we saw the other day so Q So these are the three 116 00:10:31,829 --> 00:10:36,149 parameters which we require to calculate depending on the type of soil that is what we are going 117 00:10:36,149 --> 00:10:41,379 to see today So if you know three parameters then you can find out what is the capacity 118 00:10:41,379 --> 00:10:47,329 then accordingly you can decide what is the total capacity available 119 00:10:47,329 --> 00:10:52,250 So the first one lets introduce one method simple method for you know cohesive type of 120 00:10:52,250 --> 00:10:58,199 soils where the strength is given in terms of undrain shear strength for clay type of 121 00:10:58,199 --> 00:11:08,690 soil and it is a fraction of that will be taken as the frictional resistance or frictional 122 00:11:08,690 --> 00:11:13,740 capacity between the pile and the soil So alpha is basically a dimensionless parameter 123 00:11:13,740 --> 00:11:19,880 this alpha and that parameter needs to be calculated depending on what type of soil 124 00:11:19,880 --> 00:11:21,720 and where it is located 125 00:11:21,720 --> 00:11:27,949 So can see here numerical formula of this type is given here is alpha is proportional 126 00:11:27,949 --> 00:11:33,980 to the overburden pressure so basic idea is soil value is nothing but CU divided by P 127 00:11:33,980 --> 00:11:40,490 knot P knot is nothing but the height of this the layer in which you are calculating from 128 00:11:40,490 --> 00:11:46,209 the surface of the seabed so basically multiplied by the density P knot is nothing but height 129 00:11:46,209 --> 00:11:47,389 times density 130 00:11:47,389 --> 00:11:53,680 So only thing is because undrain condition in underwater we have to take the effective 131 00:11:53,680 --> 00:12:00,160 unit weight rather than the total so basic this is just gamma H so as you can see P knot 132 00:12:00,160 --> 00:12:04,170 will keep on increasing as you go down so if it you if it is under surface of the soil 133 00:12:04,170 --> 00:12:09,350 its zero because the height is zero as you go down it will increase as p knot increases 134 00:12:09,350 --> 00:12:10,850 the value differs 135 00:12:10,850 --> 00:12:15,300 So there are two criteria given here if the soil value is greater than one used this 0.5 136 00:12:15,300 --> 00:12:21,170 and soil value is less than one use this limiting value of to the power minus 0.5 So this is 137 00:12:21,170 --> 00:12:27,449 the difference that we have to adopt so basically whatever you do the calculation alpha value 138 00:12:27,449 --> 00:12:34,700 can up cannot be greater than 1 So that means the frictional resistance can only be as much 139 00:12:34,700 --> 00:12:38,110 as the undrain shear strength and not more than that 140 00:12:38,110 --> 00:12:43,310 The soil cannot actually give a friction resistance more than the undrain shear strength that 141 00:12:43,310 --> 00:12:47,319 is the thing that you need to make sure that remember to limit as soon as you calculate 142 00:12:47,319 --> 00:12:51,889 the alpha value you check whether it is less than one or greater than one if it is greater 143 00:12:51,889 --> 00:12:59,351 than one limit the value to one and value depends on the strength plus your location 144 00:12:59,351 --> 00:13:03,839 of the layer from the seabed 145 00:13:03,839 --> 00:13:08,730 The more that you go down you can easily understand now for example the layer near the surface 146 00:13:08,730 --> 00:13:13,970 and clay layer say 100 meters down you could easily see that the 100 meters down the layer 147 00:13:13,970 --> 00:13:19,000 is having so much of overburden effect which will allow will not allow the soil to move 148 00:13:19,000 --> 00:13:24,870 away from the pile so that means that you have to take into account the effect of overburden 149 00:13:24,870 --> 00:13:28,829 which is going to cost the of soil to the pile 150 00:13:28,829 --> 00:13:32,660 So that is why in think that the few days back we were talking about overburden pressure 151 00:13:32,660 --> 00:13:38,209 is going to be very important parameter in the deciding what is the capacity of pile 152 00:13:38,209 --> 00:13:42,050 foundation because is going there we are going deeper the deeper you go the soil in this 153 00:13:42,050 --> 00:13:48,079 vicinity of the pile is going to help in keeping the soil together with the pile and frictional 154 00:13:48,079 --> 00:13:50,009 resistance will definitely increase 155 00:13:50,009 --> 00:13:55,180 But there is the limit as you can see here for example if you don’t have this limit 156 00:13:55,180 --> 00:13:59,779 alpha is less than or equal to one this P knot is going to keep increasing and I you 157 00:13:59,779 --> 00:14:04,339 can see here the alpha value is as you go down maybe more than one which is not going 158 00:14:04,339 --> 00:14:10,730 to be the case Why did they do this is basically to limit and based on experiments you know 159 00:14:10,730 --> 00:14:16,519 several times experiments on frictional resistance have been carried out and basically in cannot 160 00:14:16,519 --> 00:14:19,329 be more than the strength of the soil itself 161 00:14:19,329 --> 00:14:24,589 Even though the depth of the soil or the depth of the pile is very large like few 100 meters 162 00:14:24,589 --> 00:14:29,940 like you know offshore foundation if you go as much as 150 meters so that is where this 163 00:14:29,940 --> 00:14:34,630 limitation is there So you have to remember to limit the alpha value to less than 1 What 164 00:14:34,630 --> 00:14:38,759 it means is undrain shear strength this is the limit by which the friction capacity can 165 00:14:38,759 --> 00:14:46,760 be mobilized So this is typically called alpha method of for only clay type of soil it cannot 166 00:14:46,760 --> 00:14:51,029 be used for sandy material so will just quickly look at the alternative 167 00:14:51,029 --> 00:14:58,300 Or in fact before that we can just quickly spend some time on typical values of CU I 168 00:14:58,300 --> 00:15:05,870 think we have already seen this table from the interpretation from SPT results is similar 169 00:15:05,870 --> 00:15:12,339 You could see here very low SPT is of something around less than 10 and the values increasing 170 00:15:12,339 --> 00:15:21,670 So as long as you can find out alpha typically you know 0.5 0.6 0.7 very rare cases you will 171 00:15:21,670 --> 00:15:28,959 go into 0.8 and 1 so you will see that there frictional resistance is in the order of 50 172 00:15:28,959 --> 00:15:35,029 to 60 % of the CU values you will get reasonably and as long as you can get into SPT value 173 00:15:35,029 --> 00:15:40,569 of greater than 50 you will see that the soil is going to get a good amount of frictional 174 00:15:40,569 --> 00:15:43,449 resistance 175 00:15:43,449 --> 00:15:47,420 Just to show you that variation of alpha you know we were this is what we were looking 176 00:15:47,420 --> 00:15:54,309 at you know the alpha value is going to limited by one even though when you go deeper so you 177 00:15:54,309 --> 00:16:01,339 can see here three different graphs are given one for 5KPA to 50 KPA 100KPA 100KPA is basically 178 00:16:01,339 --> 00:16:08,899 the green one you know it took almost 58 meters before it becomes one Because it is basically 179 00:16:08,899 --> 00:16:13,839 the density value is 8 kilo-newton per is submerged density 180 00:16:13,839 --> 00:16:22,519 Whereas for at slightly increased density you can see here it is alpha value becomes 181 00:16:22,519 --> 00:16:27,380 one slightly earlier and it depends on the strength because that equation if you look 182 00:16:27,380 --> 00:16:33,370 at numerical formula you know the Si is the ratio of the Cu divided by the overburden 183 00:16:33,370 --> 00:16:42,769 pressure So you can see here for lower strength soil like 5 kpa it becomes 1 as early as near 184 00:16:42,769 --> 00:16:48,069 the surface So that is the so depending on the type of soil where it becomes one the 185 00:16:48,069 --> 00:16:55,579 limiting plug is achieved can be calculated and then accordingly applied Another chart 186 00:16:55,579 --> 00:17:08,980 showing different in fact is the same or different is different That is the variation of alpha 187 00:17:08,980 --> 00:17:16,900 I think is the same or its quadratic This is linearly varying CU 188 00:17:16,900 --> 00:17:22,390 This is basically the constant CU CU is constant throughout the layer whereas we were seeing 189 00:17:22,390 --> 00:17:29,950 that the previous bearing capacity calculation with CU is varying from surface to some depth 190 00:17:29,950 --> 00:17:36,840 below So you can see here is is just basically a curvilinear versus linear just because the 191 00:17:36,840 --> 00:17:41,540 CU value is constant here and the CU value is changing all the way from the surface to 192 00:17:41,540 --> 00:17:45,340 a down 193 00:17:45,340 --> 00:17:50,840 The second method which we called it beta method is applicable for you know the sandy 194 00:17:50,840 --> 00:17:55,870 material or cohesionless soils and it’s a similar idea only thing is the parameters 195 00:17:55,870 --> 00:18:03,350 involved is different You can see here the tan delta is the parameter which we discussed 196 00:18:03,350 --> 00:18:10,390 the other day with respect to angle of internal friction is internal failure angle of the 197 00:18:10,390 --> 00:18:11,390 material 198 00:18:11,390 --> 00:18:17,950 Whereas the delta is the angle that actually forms between the foundation and the soil 199 00:18:17,950 --> 00:18:21,990 which h we called it the friction angle and that needs to be determined it is not exactly 200 00:18:21,990 --> 00:18:28,920 same as phi and in the past in several literature you can find some relationship between phi 201 00:18:28,920 --> 00:18:32,770 and delta which I have given you a table 202 00:18:32,770 --> 00:18:40,341 So if have a phi value of 20 degrees delta is approximately by about 15 degrees Many 203 00:18:40,341 --> 00:18:44,870 times we take two third or sometimes some literature you will see delta will be phi 204 00:18:44,870 --> 00:18:52,030 minus you like API code they recommend something like this but if you look at some of the literature 205 00:18:52,030 --> 00:18:57,520 you will find you can take two third of phi value as delta but in many case when you are 206 00:18:57,520 --> 00:19:02,540 using API code they have given you this table nicely so you don’t need to really do an 207 00:19:02,540 --> 00:19:07,520 approximation You can remember is 5 degrees less than the phi value so that is something 208 00:19:07,520 --> 00:19:08,910 that you can remember 209 00:19:08,910 --> 00:19:13,030 So you see here the frictional resistant depends on three parameters one is the frictional 210 00:19:13,030 --> 00:19:18,260 angle between the pile and the soil and the overburden pressure which is nothing but as 211 00:19:18,260 --> 00:19:27,630 you go deeper and deeper this overburden pressure is going to increase and K is the k is the 212 00:19:27,630 --> 00:19:31,430 earth pressure coefficient which we I think we calculated active passive earth pressure 213 00:19:31,430 --> 00:19:36,010 So when you drive the pile inside a granular soil the soil is getting squeezed away so 214 00:19:36,010 --> 00:19:41,950 there will be a soil pressure coming towards so basically this coefficient of lateral earth 215 00:19:41,950 --> 00:19:47,400 pressure can be calculated as long as you know where is the depth it is the ratio of 216 00:19:47,400 --> 00:19:54,810 the horizontal to normal and this because it is a 100 % displacement pile it will be 217 00:19:54,810 --> 00:19:59,970 more but in this case is open ended steel pipe pile when you drive amount of soil displaced 218 00:19:59,970 --> 00:20:01,640 is very limited 219 00:20:01,640 --> 00:20:07,540 So that is why we will see lower value will be used delta I think not a problem you can 220 00:20:07,540 --> 00:20:16,430 calculate as per the relationship between phi and you know the delta Beta is replaced 221 00:20:16,430 --> 00:20:22,060 by the earlier we were having this type of relationship K times P knot into 10 delta 222 00:20:22,060 --> 00:20:27,200 in the later part of the code they have replaced this value P knot is kept here and K into 223 00:20:27,200 --> 00:20:30,580 10 delta is replaced by a direct parameter called beta 224 00:20:30,580 --> 00:20:37,320 Which if you go to this table if you look at this if you calculate k times Tan delta 225 00:20:37,320 --> 00:20:43,340 is this and beta is this value In fact originally the previous version of the code they used 226 00:20:43,340 --> 00:20:49,630 to give this delta value and k value and we calculated ourselves Instead of that the revised 227 00:20:49,630 --> 00:20:54,760 version of code directly give because one of the reason is people were trying to use 228 00:20:54,760 --> 00:21:02,150 or many of the projects got into issues because even though when the delta value is say 20 229 00:21:02,150 --> 00:21:07,590 degrees its very loose to loose a basically the soil is so loose that the material is 230 00:21:07,590 --> 00:21:09,560 not going to offer any frictional resistance 231 00:21:09,560 --> 00:21:15,940 But because we have a phi value and delta value people used to use this for capacity 232 00:21:15,940 --> 00:21:21,600 equation So new code says if it the material is identified as a very loose sandy granular 233 00:21:21,600 --> 00:21:27,470 material you can’t use it for any a strength inclusion so you have to exclude them from 234 00:21:27,470 --> 00:21:32,500 calculation that is why specifically for attempting to make sure that the people are misusing 235 00:21:32,500 --> 00:21:38,710 the code is the values of beta is given but not the K and delta 236 00:21:38,710 --> 00:21:45,410 So if you if we just look at the table two table the previous table and the new table 237 00:21:45,410 --> 00:21:50,440 that you can see here specifically the value of beta is given rather than the earlier code 238 00:21:50,440 --> 00:21:54,760 we used to have a delta angle and you can calculate yourself and then misuse it The 239 00:21:54,760 --> 00:22:01,830 idea is to avoid the use of loose material into foundation capacity calculation So that 240 00:22:01,830 --> 00:22:10,510 makes slightly comfortable So you can see here in this table as much as angle of friction 241 00:22:10,510 --> 00:22:20,730 is 40 degrees you could achieve reasonable frictional ratio 56 % of the 10 overburden 242 00:22:20,730 --> 00:22:21,730 pressure 243 00:22:21,730 --> 00:22:27,520 So overburden pressure more you will be having the frictional resistance higher isn’t it 244 00:22:27,520 --> 00:22:32,550 but here also we need to have a similar limitation to what we are doing here in here we limited 245 00:22:32,550 --> 00:22:39,300 the alpha value to 1 similarly API recommends to limit the total value that you get from 246 00:22:39,300 --> 00:22:46,150 beta times P knot to a limiting value so if you go the table you will see that the new 247 00:22:46,150 --> 00:22:53,170 table limiting soft friction value though you can calculate using beta times p knot 248 00:22:53,170 --> 00:22:58,190 If the value exceeds whatever is specified here you have to limit to the limiting value 249 00:22:58,190 --> 00:23:04,330 you need to remember this For example there are two units given one is in kips feet square 250 00:23:04,330 --> 00:23:09,700 the second one is in kilo Pascal so you have to use if you are using metric units take 251 00:23:09,700 --> 00:23:14,330 the values given in the brackets So that the limiting value that you have to 252 00:23:14,330 --> 00:23:23,770 So after calculating beta times p knot and find out whether the value is exceeds because 253 00:23:23,770 --> 00:23:29,790 you see here beta times P knot means is basically a linear increase with the depth The more 254 00:23:29,790 --> 00:23:34,150 that you go knot is going to be substantially higher if it is 100 meters K knot will be 255 00:23:34,150 --> 00:23:38,700 very large value multiplied by this beta you will see that the frictional resistance is 256 00:23:38,700 --> 00:23:44,100 going to be so you can ask a question why we are limiting again the similar idea the 257 00:23:44,100 --> 00:23:48,520 larger depth does not mean that all the soil going to offer the overburden effect to squeeze 258 00:23:48,520 --> 00:23:51,070 the pile soil interface intact 259 00:23:51,070 --> 00:23:56,980 Because after certain depth what happens is soil is going to be self self sustaining and 260 00:23:56,980 --> 00:24:03,370 it is not going to give you the so much of effect on the frictional resistance that is 261 00:24:03,370 --> 00:24:07,730 why you have to limit this after a certain depth you will not be taking into account 262 00:24:07,730 --> 00:24:13,420 this limiting value So basic idea is two methods one is alpha method the other one is beta 263 00:24:13,420 --> 00:24:19,380 method Alpha method you have alpha times CU beta method is P beta times p knot 264 00:24:19,380 --> 00:24:25,090 Both of them involve overburden pressure calculation overburden pressure is nothing but height 265 00:24:25,090 --> 00:24:30,260 times the density of the soil above Suppose you have three layers how do you calculate 266 00:24:30,260 --> 00:24:35,580 basically layer one times density plus layer two times the density plus the layer three 267 00:24:35,580 --> 00:24:39,890 times the density will be the overburden pressure at the place where your so you should know 268 00:24:39,890 --> 00:24:43,340 how to calculate and all that 269 00:24:43,340 --> 00:24:50,780 The next thing is the N bearing so think skin frictional is very simple after all the simple 270 00:24:50,780 --> 00:24:55,410 algebraic calculations and easy to find out Only thing is you need to remember to limit 271 00:24:55,410 --> 00:25:01,340 the values if it is clay you limit the value to alpha equal to 1 if it is sand you have 272 00:25:01,340 --> 00:25:07,130 to limit the multiplied value beta times p knot to the code specified limits and that 273 00:25:07,130 --> 00:25:13,410 limits are given here for different types of material Medium dense is 67kpa very dense 274 00:25:13,410 --> 00:25:19,120 115kpa so if you if you calculate and find that your values are higher you should limit 275 00:25:19,120 --> 00:25:22,840 that so that is the idea behind it 276 00:25:22,840 --> 00:25:29,730 The next thing is the bearing at the bottom Now you can see and recollect what we derived 277 00:25:29,730 --> 00:25:34,770 over the last few classes about the bearing capacity of shallow foundation in clay type 278 00:25:34,770 --> 00:25:40,870 of soil when phi is equal to 0 we derived a lower bound as four times CU and we derived 279 00:25:40,870 --> 00:25:46,870 6.28 CU when the foundation is at the surface of the seabed isn’t it not below 280 00:25:46,870 --> 00:25:53,720 Now you can imagine this pile is going to be installed 100 meters 50 meters 60 meters 281 00:25:53,720 --> 00:25:58,600 down into the earth that means is overburden effect will come If you go back and recollect 282 00:25:58,600 --> 00:26:06,220 the formula for the footing bearing capacity is four times CU plus Q knot You know the 283 00:26:06,220 --> 00:26:10,140 effect of overburden will come the deeper you go the bearing capacity is going to substantial 284 00:26:10,140 --> 00:26:12,880 increase and that exactly the idea here 285 00:26:12,880 --> 00:26:18,080 You can see here instead of four times CU or six times CU we have 9 times Cu because 286 00:26:18,080 --> 00:26:23,799 the pile foundations are going to be definitely installed on the top of the soil soil surface 287 00:26:23,799 --> 00:26:28,320 No point of installing a pile which is going to touch down the surface only So it is going 288 00:26:28,320 --> 00:26:33,360 to be several meters deep and overburden effect will be taken into account So that because 289 00:26:33,360 --> 00:26:38,270 of that you can see here the increased bearing capacity because we are going to install the 290 00:26:38,270 --> 00:26:40,090 pile and the effect of overburden 291 00:26:40,090 --> 00:26:46,610 Instead of directly calculating they taken as 9 times CU base on several test So that 292 00:26:46,610 --> 00:26:55,620 is why the bearing capacity of pile foundation terminated in a clay layer is 9 times CU as 293 00:26:55,620 --> 00:27:01,920 a numerical number we sometime called it this nine replaced by NC Very similar to our Terzaghi 294 00:27:01,920 --> 00:27:02,920 bearing capacity equation 295 00:27:02,920 --> 00:27:10,270 You have the C times NC so here also similar and cohesionless soils similar idea P knot 296 00:27:10,270 --> 00:27:16,290 times NQ again NQ is the bearing capacity factor similar to the bearing capacity factor 297 00:27:16,290 --> 00:27:22,110 we were having in shallow foundations only thing is this number will be slightly different 298 00:27:22,110 --> 00:27:29,530 different from the one that you chop a chart indicating the various values of NQ N gamma 299 00:27:29,530 --> 00:27:36,120 NC so N bearing capacity is very easy to calculate this is 9 times CU no limit 300 00:27:36,120 --> 00:27:42,140 As long as C value is larger you can use it but as we know very well clay means N bearing 301 00:27:42,140 --> 00:27:49,190 is going to be very-very limited We saw the values of strength you see here C values is 302 00:27:49,190 --> 00:27:54,630 going to somewhere around even if you hit a very strong clay you are going to hit 400kpa 303 00:27:54,630 --> 00:28:00,670 400kpa is nothing compared to if you look at a granite or granular material you can 304 00:28:00,670 --> 00:28:03,120 get as much as ten thousand 305 00:28:03,120 --> 00:28:07,730 So that is where the difference clay is not going to offer you a bigger capacity but clay 306 00:28:07,730 --> 00:28:12,890 can offer a bigger frictional capacity because the length is more whereas here if you multiply 307 00:28:12,890 --> 00:28:20,270 this 400 with whatever the pile surface area at the bottom you will not get any capacity 308 00:28:20,270 --> 00:28:25,850 So that is why there is no limiting value for bearing capacity of the pile foundation 309 00:28:25,850 --> 00:28:28,910 at the terminated at this clay type of layer 310 00:28:28,910 --> 00:28:35,060 So you can use as much value doesn’t matter Whereas when you look at cohesionless soil 311 00:28:35,060 --> 00:28:39,610 P knot is going to be substantially larger if you drive the pile to one hundred meters 312 00:28:39,610 --> 00:28:44,490 and p knot will be very large Even if you take a typical value of say for example 100 313 00:28:44,490 --> 00:28:50,950 meters penetration density of soil is say 2 ton per cubic meter I think typical sandy 314 00:28:50,950 --> 00:28:56,420 material will be something like this and because is underwater you get minus one 315 00:28:56,420 --> 00:29:03,500 So you get 1 ton per cubic meter if you take one hundred meters times one you get nearly 316 00:29:03,500 --> 00:29:11,230 one hundred ton per square meter is the overburden pressure which is applied surrounding the 317 00:29:11,230 --> 00:29:16,830 pile material the pile itself Multiplied by a numerical coefficient depending on you know 318 00:29:16,830 --> 00:29:22,700 type of soil you can calculate or find out from this table see basically you can see 319 00:29:22,700 --> 00:29:26,200 here N bearing factor given by API 320 00:29:26,200 --> 00:29:32,340 Very similar to the numbers that we look for shallow footing and you can pick up say for 321 00:29:32,340 --> 00:29:41,200 example if it is a very dense sand is 50 so 50 times hundred ton So you can see here the 322 00:29:41,200 --> 00:29:48,750 numbers becomes very large 500 500 in fact 5000 ton per square meter So it is a large 323 00:29:48,750 --> 00:29:54,170 value capacity even if you have a pile of 1 square meter you are going to get a huge 324 00:29:54,170 --> 00:29:59,150 capacity coming from there and that is why you want to terminate the pile in a sandy 325 00:29:59,150 --> 00:30:04,400 material rather than clay material even if you have the best form of clay you are not 326 00:30:04,400 --> 00:30:06,530 going to achieve any N bearing capacity 327 00:30:06,530 --> 00:30:11,730 So that is why many times when you are designing a pile we want to look for where the good 328 00:30:11,730 --> 00:30:18,170 material is whether its rock or sand of good capacity you terminate the pile there you 329 00:30:18,170 --> 00:30:22,660 don’t even need to worry about the skin friction because is N bearing capacity is 330 00:30:22,660 --> 00:30:28,080 going to be surely a very high So because of that we need to definitely find out the 331 00:30:28,080 --> 00:30:35,450 limiting value so the API gives you limiting N bearing value not simply because you cannot 332 00:30:35,450 --> 00:30:37,300 infinitely increase N bearing 333 00:30:37,300 --> 00:30:43,970 So if you go for very dense granular sand even after you calculate the N bearing by 334 00:30:43,970 --> 00:30:50,490 NQ times P knot you have to limit the value to the values given here in think you will 335 00:30:50,490 --> 00:30:55,470 you have to be little bit careful here also The values given here kips feet square and 336 00:30:55,470 --> 00:31:02,980 in bracket is mega Pascal So you can see here 12 mega Pascal which is nothing but you know 337 00:31:02,980 --> 00:31:07,620 12 mega Pascal concrete you know if you look at the M30 concrete what is the capacity Or 338 00:31:07,620 --> 00:31:09,770 what is the unconfined compressive strength 339 00:31:09,770 --> 00:31:16,730 Is 30 mega Pascal so you can see here 12 mega Pascal means as good as lean concrete of somewhere 340 00:31:16,730 --> 00:31:22,410 around 10 to 15 mega Pascal So it is going to be a good material so that is the limit 341 00:31:22,410 --> 00:31:28,370 you have to use for different material like dense medium or slightly loose medium so can 342 00:31:28,370 --> 00:31:33,790 go as much as 29 mega Pascal So that is the limiting value so can see this table is very 343 00:31:33,790 --> 00:31:38,480 useful you are going to infact you have to use this new table 344 00:31:38,480 --> 00:31:43,790 You have to use is a similar value so basically this giving you limiting N bearing value this 345 00:31:43,790 --> 00:31:49,220 gives the limiting soft friction value it also gives you the bearing capacity factor 346 00:31:49,220 --> 00:31:55,000 which we need to take and multiply and the friction factor The only thing this what you 347 00:31:55,000 --> 00:32:00,090 have is the density or so called relative density Now you remember when we were doing 348 00:32:00,090 --> 00:32:03,460 the first few classes when you are doing the testing 349 00:32:03,460 --> 00:32:08,320 The first thing you want do is the classification once you know the right classification like 350 00:32:08,320 --> 00:32:13,700 this you will have a several literatures to take out the strength values so once the classification 351 00:32:13,700 --> 00:32:20,760 is correct and you can go into whether the pure sand or sand with silt or silty sand 352 00:32:20,760 --> 00:32:26,010 you can use this table if you are not falling within this range you will have to find out 353 00:32:26,010 --> 00:32:29,510 reasonable relationship with other parameters 354 00:32:29,510 --> 00:32:33,680 That is when you will actually go into finding out the delta value from literature and come 355 00:32:33,680 --> 00:32:39,900 back here Many times you will you will not find direct classification lie this if you 356 00:32:39,900 --> 00:32:47,040 fall outside this for example you have a instead of silt you may have a clay fifty-fifty You 357 00:32:47,040 --> 00:32:51,200 may not be able to come here because is not classified under this group so have to go 358 00:32:51,200 --> 00:32:56,830 somewhere in the literature find out the equivalent strength form and then come back here either 359 00:32:56,830 --> 00:33:02,730 with this value comparison or you can use the old value of delta and calculate 360 00:33:02,730 --> 00:33:13,120 So you will have to do according to the situation So basically the four parameters we have now 361 00:33:13,120 --> 00:33:19,800 calculated the skin friction for sand clay I think very simple clay and sand you should 362 00:33:19,800 --> 00:33:25,270 be able to determine based on which strength parameter is given Even if it is not specifically 363 00:33:25,270 --> 00:33:30,790 given to you if a phi value is given its going to be a sandy material if it’s a C value 364 00:33:30,790 --> 00:33:34,520 is given it is a clay type of material and you can 365 00:33:34,520 --> 00:33:41,220 One of the disadvantage of this API method is when you have a sandy clay C phi soil you 366 00:33:41,220 --> 00:33:47,870 will not be able to do this so you will you will find that difficulty you know in assigning 367 00:33:47,870 --> 00:33:53,820 frictional resistance or N bearing value So you have to see which is predominant you know 368 00:33:53,820 --> 00:34:00,000 if you are finding some places in you will find sandy clay you know you will you will 369 00:34:00,000 --> 00:34:05,270 have a mixture you will have a C value of some lower amount phi value 370 00:34:05,270 --> 00:34:12,040 In such cases you can ignore the minor component and used it That is one of the weakness of 371 00:34:12,040 --> 00:34:18,280 this API method which makes this simplified but there is little bit complication but if 372 00:34:18,280 --> 00:34:22,750 you into the literature you will find there are several other forms of equations where 373 00:34:22,750 --> 00:34:30,280 C phi soil cab be represented so the N bearing is also very simple 9 times CU and NQ times 374 00:34:30,280 --> 00:34:34,889 p knot and this is limited by the limiting value this is not limited by limiting value 375 00:34:34,889 --> 00:34:41,270 Whereas the skin friction both of them are limited and we should know how to approach 376 00:34:41,270 --> 00:34:47,960 This we have discussed just now this is also discussed basically clay no limitation sand 377 00:34:47,960 --> 00:34:55,879 is limited by 12 mega Pascal old API table which is no more in existence so will not 378 00:34:55,879 --> 00:35:02,090 be will not be giving in any examination or testing we will use this table so this numbers 379 00:35:02,090 --> 00:35:06,730 you don’t need to really memorize in case if it is required for examination point I 380 00:35:06,730 --> 00:35:12,780 will print it and give it to you so that you can use it 381 00:35:12,780 --> 00:35:21,260 The next method which is also useful in terms of estimating the capacity is the CPT so you 382 00:35:21,260 --> 00:35:29,310 can see I think CPT gives you a continuous form of resistance as you drive a cone I think 383 00:35:29,310 --> 00:35:35,580 this was introduced soil investigation time CPT is nothing but a cone fitted with instruments 384 00:35:35,580 --> 00:35:41,890 like strain cages and load cells so when you drive this cone into the soil the strain cages 385 00:35:41,890 --> 00:35:47,460 gives you lot of information skin friction resistance as well as the N bearing and the 386 00:35:47,460 --> 00:35:51,610 total load taken to push the cone into the soil 387 00:35:51,610 --> 00:35:58,460 So if you can get gather this information and a its one of the advantages it is continuously 388 00:35:58,460 --> 00:36:04,620 available unlike other forms of testing where you have to excavate the boring stop and do 389 00:36:04,620 --> 00:36:08,920 the testing at that particular location and then further bore and then do the testing 390 00:36:08,920 --> 00:36:14,450 Here the cone is continuous so you will get something like this like an analog signal 391 00:36:14,450 --> 00:36:19,440 and if you are able to do a numerical integration of this frictional resistance you will be 392 00:36:19,440 --> 00:36:21,390 able to get the capacity very fast 393 00:36:21,390 --> 00:36:29,320 That is why nowadays you know from the field you get the report within few days like if 394 00:36:29,320 --> 00:36:35,020 you talk about 20 years back you know they take a sample and then bring it to the laboratory 395 00:36:35,020 --> 00:36:39,660 Do all the testing they take about few months before you can see any test parameters Nowadays 396 00:36:39,660 --> 00:36:50,530 most of the modern soil you know investigation companies have this vessels fitted with CPT 397 00:36:50,530 --> 00:36:56,080 instruments as soon as they complete that boring or testing within next days you get 398 00:36:56,080 --> 00:36:58,680 the report saying that this the capacity you can achieve 399 00:36:58,680 --> 00:37:03,890 So that is one of the greatest advantage of this CPT and especially electronic CPT with 400 00:37:03,890 --> 00:37:08,730 all this strain gauges you can get the results as soon as just you come out you can get the 401 00:37:08,730 --> 00:37:13,770 result because everything is based on computer program as soon as you start driving you will 402 00:37:13,770 --> 00:37:22,370 get the information so this CPT methods were not recognized because of a complexities involved 403 00:37:22,370 --> 00:37:24,170 not everybody is able to do it 404 00:37:24,170 --> 00:37:30,310 Previous versions of the code they only say we leave it to the geo technical experts to 405 00:37:30,310 --> 00:37:36,490 decide depending on the site But now what they have done in the recent revision of API 406 00:37:36,490 --> 00:37:43,170 or other codes infact ISO codes or British codes they have included this methods as part 407 00:37:43,170 --> 00:37:49,690 of their evaluation procedure and that is why we will go into details one by one because 408 00:37:49,690 --> 00:37:56,170 is very important because many of thegeo technical reports will provide you with this information 409 00:37:56,170 --> 00:38:01,640 You should know how to integrate and which method to use There were several ideas in 410 00:38:01,640 --> 00:38:07,780 fact that is why I have given you the phi as mentioned by API API says you can use any 411 00:38:07,780 --> 00:38:14,650 one of the method simplified numerical integration or university method or fugro method this 412 00:38:14,650 --> 00:38:20,030 are some private companies So they have given slightly modified procedures based on two 413 00:38:20,030 --> 00:38:24,950 things one is the and at the same side you drive the pile until to the twisting and compare 414 00:38:24,950 --> 00:38:26,000 the capacities 415 00:38:26,000 --> 00:38:33,640 So they have calibrated this methods basically and several locations and API has given you 416 00:38:33,640 --> 00:38:38,850 the complete procedure for four methods One of the important issue with this when you 417 00:38:38,850 --> 00:38:45,400 are doing this integration each one method has used adjusting parameter I think I mentioned 418 00:38:45,400 --> 00:38:51,880 about this earlier on You know whenever you do the soil testing and you do the piling 419 00:38:51,880 --> 00:38:57,320 and do the load cell and test it and compare the results by theoretical calculation using 420 00:38:57,320 --> 00:39:03,030 the results obtained from laboratory test or from the cone and from the pile testing 421 00:39:03,030 --> 00:39:08,060 If the difference is too large you try do a correction on your method until your results 422 00:39:08,060 --> 00:39:14,660 comes closer so that is what you see here a lot of numerical parameters for adjustment 423 00:39:14,660 --> 00:39:19,180 because they have done their testing they have also done their laboratory investigation 424 00:39:19,180 --> 00:39:23,510 and finally they come up with different-different numbers so each methods 425 00:39:23,510 --> 00:39:31,230 Methods 123 you can see here this methods the last one they don’t have this data in 426 00:39:31,230 --> 00:39:39,000 API So you should use whichever the method and ultimately should get a similar result 427 00:39:39,000 --> 00:39:46,400 So this first one is they simplified ICP is a Imperial college as early as 2002 they started 428 00:39:46,400 --> 00:39:53,240 working under this procedure2005 they published one paper on OMIE and from then a lot of people 429 00:39:53,240 --> 00:39:57,720 were using but then was not recognized by API but in the recent revision they have included 430 00:39:57,720 --> 00:40:08,360 And then UWA also was early as 2005 the method was implanted but not much useful 431 00:40:08,360 --> 00:40:13,820 But now we can recognize and can use it All the methods are based on the integration of 432 00:40:13,820 --> 00:40:19,700 the resistance measured by the cone in the form of strain cage readings continuously 433 00:40:19,700 --> 00:40:24,810 So you can the signal is coming in this fashion and you what you need to do is integrate from 434 00:40:24,810 --> 00:40:31,080 whatever the depth So if you have analytical analog form of reading then you can do a numerical 435 00:40:31,080 --> 00:40:37,320 integrator and then get the values or if you can digitize those values depth wise and do 436 00:40:37,320 --> 00:40:39,200 a simple numerical integration also can be done