Failure Envelope Of Bearing Capacity For Spudcan Foundation Considering Spatial Variability Of Clay

Study on the bearing capacity a nd fa ilure mechanis m of spudcan foundat ion for the Jack- up plat form is t he key to design a n effic ient, safe and applicab le Jack- up platfor m. Because of comp lex seabed soil profiles and combined loads on spudcan due to storms, waves and self- weight, study on the bearing capacity and failure mecha nism is s ignificant to ensure the safet y of the spudcan foundation. Besides, it is d ifficult to imp leme nt this research w ith site invest igat ions and centrifuge tests. There fore, this study develops random finite eleme nt ana lys is to research the perfor mance of spudcan fo undat ion under multidirectional loads in the spatially variable soil.Comb ining spatia l var iabilit y theories, Monte-C arlo simulat ion w it h finite eleme nt method into random finite e leme nt analys is, this study a ims to invest igate how the spatia l variab ility of soil and mult id irectio nal loads affect the fa ilure enve lope of bearing capacity, fa ilure mechanis ms, fa ilure probabilit ies, factors of safety and mobil ized shear strength, etc.Firstly, this study builds up the finite ele ment model of so il and spudcan foundation based on the software of AB AQ US. The bearing capacit ies fro m finite eleme nt ana lyses are verified by comparing w it h numer ica l simulat ions, theoretical studies and centr ifuge test results from other researches. Secondly, the three- dimens iona l random fields of soil strengt h are mapped onto the finite eleme nt model. Then, the vert ical, horizonta l and rotat ional ult imate bearing capacit ies are obtained fro m the bearing capacit y-disp lacement curves by random finite e leme nt analysis. The bearing capacit ies and failure mechanis ms in random soil are compared subsequent ly w it h results from nor ma lly consolidated cla y. The differences of fa ilure mecha nis ms between t he random soil and unifor m soil are ana lyzed according to soil displacement vectors and contours of equiva lent plastic strain. 400 fa ilure envelopes acquired by the probe tests are sorted into the probabilist ic failure enve lope. Finally, new model of fa ilure envelope is built for the spudcan foundation deeply bur ied in spatia lly var iable soil and under multidirectional loads.This research proposes the formula of mobilized shear strength a ccording to the princ iple of virt ua l work. The effects of var iat ion of coeffic ient(C OV) for soil strength on the bearing capacit y are invest igated. Results show that there is a reduct ion of t he mean a nd an increase of the C OV s of mobilized shear strength wit h higher C OV s of undrained shear strength. Such var iations of t he mobilized shear strength consequent ly w ill result in a reduction of the mean and an increase in the COV of the bearing capacity.The fa ilure probabilit ies and factors of safety are calculated according to the cumulat ive probability d istr ibut ion of bearing capacit y. It is also demonstrated that the simp lified reliab ility- based method in many codes wit h invar iable factors of safety cannot govern a n acceptably unifor m leve l of reliabilit y and even the requirement of targeted failure probability over variable COVs of soil properties.