The Failure Envelope Of Caisson Foundations Under Combined V.H.M Loadings

In civil engineering,hydraulic engineering,coastal and offshore engineering,structures are usually subjected to vertical and lateral loads.Laterally loaded structures such as bridges,earth-retaining structures,wind turbines,offshore structures are usually transferred a complex loading combination to the beneath foundations.The deep-water bridges and offshore foundations receive a complex loading combination represented in a vertical load as a result of self-weight of the superstructure,soil fill,surface surcharge,equipment,etc.while horizontal and moment loadings are due to the harshest of environments such as waves,winds,currents,earthquakes,etc.These different vertical,horizontal and moment loadings can be transferred to the underground soil layers through special foundations as caisson foundations,shallow foundations resting on the sea bed,etc.However,the designated foundation system will depend on the type of structures,the depth of the sea bed,environment conditions,geological conditions of sea bed soil,the size and the cost of the foundations.This thesis presents experimental and numerical investigations and techniques which have been developed to find out the locus of the failure envelopes of shallow foundations and caisson foundations subjected to vertical,horizontal and moment loadings.Based on the available literature,experimental and numerical investigations,the main research works and conclusions are as follows:Validation of the effect of the sand density on the shape and the size of the failure envelope method through analyses of experimental results of shallow foundations subjected to eccentrically vertical and inclined loads resting on dense sand and medium dense sand soils.A literature review focused on the available knowledge regarding the influencing factors on the bearing capacity of shallow foundations subjected to combined loadings as load inclination factor and load eccentricity factor,are documented and presented in this work.These load inclination factor and load eccentricity factor are compared with new formulas obtained from laboratory small scale test results and transferred to be as interactions diagrams of a three-dimensional failure envelope.The comparison shows that some inclination factors and load eccentricity factors are adequately captured the test results data.However,the elliptical equations obtained by previous researchers to predict the V-H-M cross-sections are slightly overestimated the laboratory test results.And thus new equations to describe the different cross-sections of the failure envelopes,for both dense sand and medium dense sand soil have been proposed.Investigations of the effect of sand density(dense sand and medium dense sand)on the shape and the size of the different cross-sections of the failure envelopes shows that,the dense sand soil and medium dense sand soil almost gives the same shape of the failure envelopes,but there always exists a little bit difference between the maximum predicted values of the horizontal and moment loadings corresponding to a fixed vertical load ratio.Failure envelops of caisson foundations carrying vertical,horizontal and moment loads.This work presents results of a three-dimensional finite element analysis model used to investigate the shape of the failure envelope of caisson foundations in sand soil.The caisson model has a depth to breadth ratio D/B=1 and the interface condition soil-foundation friction coefficient ?=0.3.After the presentation of the results of vertical-horizontal(V-H),vertical-moment(V-M)and the horizontal-moment(H-M)cross-sections of failure envelope corresponding to different vertical load ratios(V/Vu),three equations to describe the shape of the different cross-sections of the failure envelope are proposed.Moreover,formulas are proposed to describe the effect of load eccentricity and the load inclination factors for caisson foundations under combined loads.These new formulas are applicable where the caisson embedment ratio D/B=1.The effect of the embedment ratio on the shape and the size of the failure envelope of caisson foundations carrying vertical,horizontal and moment loads represent a part of this work.Based on a three-dimensional finite element analysis model for a caisson foundation with the embedment ratios(D/B)varies from one to three,and the interface conditions friction coefficient ?=0.3.Two and three-dimensional normalized failure envelopes for each embedment ratio are compared and presented in this work.The comparison shows a significant effect of the embedment ratio on the shape and the size of the failure envelopes,i.e.the shape of the failure envelopes of vertical-horizontal(V-H)and the vertical-moment(V-M)are reduced with the increase of the embedment ratio,while the shape of the horizontal-moment(H-M)cross-sections are not depending to the embedment ratio.The investigated failure mode mechanisms corresponding to different vertical load ratios in the horizontal-moment(M-H plane)also seen to be similar,regardless of the variation of the embedment ratios(D/B).And thus,new equations are proposed to predict the locus and the shape of different vertical-horizontal,vertical-moment and horizontal-moment cross-sections of the failure envelopes of caisson foundation corresponding to different embedment ratios.Three-dimensional finite element analysis results have been used to investigate the effect of the interface conditions on the shape and the size of the failure envelope of caisson foundations carrying vertical,horizontal and moment loads installed in sand soil.Where the foundation embedment ratio(D/B)varies from one to three,and the interface conditions friction coefficient(?)takes values 0.3,0.5 and 0.7,respectively.The bearing capacities of caisson foundations under uniaxial vertical,horizontal and moment loadings are discussed.Two and three-dimensional normalized and non-normalized cross-sections of the failure envelopes for each embedment ratio are compared and presented in this work.The effect of the interface conditions on the shape of the different cross-sections of the failure envelopes seen to be similar and not affected,while the size of the failure envelopes expanded with increasing the interface friction coefficient(?)and the embedment ratios(D/B),respectively.The investigated failure mode mechanisms corresponding to different vertical load ratios in the horizontal-moment(M-H)plane also seen to be similar,regardless of the variation of the interface condition friction coefficient(?).Formulas to describe the bearing capacity depth factors applicable for caisson foundations with embedment ratios vary from one to three are presented in this work.