Installation Of Stiffened Caisson In Clays

The successful installation of caissons(bucket foundations)is the premise of their work.The stiffeners are usually applied to increase the property of structure,avoiding buckling during installation.However,the introduce of stiffeners brings a lot of uncertainties in mechanical behavior during caisson installing in clay,including soil-flow mechanism,inner soil heave and penetration resistance.Aiming at this problem,the international standards,including API and DNV,are still lack of the theoretical system and calculation method.Through the modified RITSS method,the researches on the installation of stiffened caisson in different soil profiles(single-layer uniform clay,soft-over-stiff clay and stiff-over-soft clay)have been conducted in this dissertation.The research results contribute to understand the surrounding soil-flow mechanism during the stiffened caisson installation process,and reveal more difficult properties of penetration resistance.The main work and conclusions are including:(i)The large deformation finite element program(RITSS)has been modified for tracking the geometric configuration change of boundary line of multiple-layer soil during contacting with the structure concave.The numerical results from modified RITSS program have been compared with the data of centrifuge tests,which can validate the reliability and stability of modified RITSS program.(ii)The continuous installation of stiffened caisson in single-layer uniform clay has been simulated using the modified RITSS program,revealing the stiffeners influences on the surrounding soil-flow mechanism.Both ‘soil-soil sliding failure’ and ‘soil-skirt sliding failure’have been observed.Furthermore,it can be found that soil failure mechanism around the stiffeners is conducted by the ratio of stiffener spacing and stiffener width(s/b): ‘soil-soil sliding failure’ is conducted when s/b≤5;‘soil-skirt sliding failure’ is conducted when s/b>5.(iii)Based on the parametric analyses results of the effect of caisson geometry and soil property on the soil-flow mechanism and soil heaving mechanism,the prediction method of the soil heave inside caisson was proposed.Aiming at ‘soil-soil sliding failure’ and ‘soil-skirt sliding failure’ in single-layer uniform clay,the corresponding formulars of penetration resistance were proposed.(iv)The modified large deformation finite element method(RITSS)has been used to simulate the continuous installation of stiffened caisson in soft-over-stiff clay.The results show that in the initial penetration depth the soil failure mechanism of top-layer clay is same with installation in single-layer uniform clay.With deeper penetration,partial top-layer soft clay flows into the gap between stiffeners and taken into bottom-layer stiff clay,so shearing failure is formed between the soft clay trapped between stiffeners and the second-layer stiff clay.Considering the different soil-flow mechanism in different stage,calculation methods of penetration resistance for differebt penetration stage were proposed.Based on the inner soil heaving properties,the estimating formular for inner soil heave has been proposed.(v)The penetration process of stiffened caisson in stiff-over-soft clay has been analyzed through modified finite element approach,and three kinds of soil-flow mechanisms were proposed.The first mechanism is that top-layer stiff clay has not back flowed,and bottom-layer soft clay has back-flowed into gaps.At this time,the top-layer stiff clay is taken as ‘float’beyond the bottom-layer soft clay,so the penetration in stiff-over soft clay can be equivalent to single-layer clay.The second mechanism is that top-layer stiff clay backflows into gap and taken into bottom-layer soft clay,forming the stiff-soft clays shearing failure.The third mechanism is that the soil inside caisson has a same penetration rate with caisson,which is similar with the ‘total plugging’ of inland open-end pile.Based on the numerical analyses results,the distinguishing method for three mechanisms was proposed,and the corresponding formulars for estimating inner soil heave and penetration resistance for different mechanisms were established.