Bearing Behavior Of Subsoil Under Gravity Foundation With Anchors For Offshore Wind Turbines

The offshore wind turbine foundations are generally subject to complexloading due to their special working conditions. In the worldwide, the existing typesof offshore wind turbine foundations have some problems, e.g. high cost andconstruction difficulty etc. Therefore, it is urgent to develop new types of offshorewind turbine foundations which are inexpensive and easy to construct.Based on conventional gravity foundations, a novel type of offshore windturbine foundation was proposed by using pre-stressed anchors. Such type offoundation is named as gravity foundation with anchors for offshore wind turbinesin this study. In this new type of foundation, the anchors are arranged uniformlyalong the perimeter of inner and outer rings, which are called anchor rings, at thebottom of foundation. Aiming at the turbine with a capacity of3MW, the size ofconventional gravity foundation, which had been summarized by predecessors, hasbeen reduced. The subsoil within calculated area has been arranged one layer ofsandy soil. In case of single or compound loading conditions, the bearing behaviorof subsoil and the mechanism of its failure have been mainly analyzed. The effectsof key parameters of the anchors and their main functions have been analyzed aswell. In addition, the bearing capacity of the novel foundation has been comparedwith that of traditional gravity foundation (without anchors). The conclusions areshown as follows.The anchors can improve the maximum bearing capacity of the subsoil. Withthe single vertical loading condition, as the anchors can improve the stiffness oflocal subsoil, the settlement reduces by33%when vertical load reaches normalloading condition; when extreme loading condition is achieved, the settlementreduces by37%. Thus, its ultimate bearing capacity is2.7times of that of thefoundation without anchors. With the single horizontal loading condition,pre-tension of the anchors increases the pressure between the foundation and thesubsoil, whose increment can be80%. In this way, skid resistance has beenimproved so that the horizontal displacement decreases by25%when it reachesnormal loading condition and decreases by28%when extreme loading condition isachieved. At the same time, the ultimate bearing capacity increases by12%. Furthermore, the anchors effectively resist the moment load. With the singlebending moment, when normal loading condition is achieved, the rotation angle ofthe foundation reduces by68%; when extreme loading condition is achieved, thefoundation without anchors has lost stability, while the rotation angle of the novelfoundation is still within the permitted range and the ultimate bearing capacity of itsbending moment is2.1times as that of the foundation without anchors.Meanwhile, the presence of anchors is found to change the failure mode of thesubsoil under single load component. When subject to vertical load component, theanchors transfer loads to deep soil stratum and then change the general shear failureof the subsoil into punching shear failure. Under horizontal load component, theanchors produce an increase in contact pressure between foundation and subsoil,and then enhance the integrity of the subsoil, thereby increasing the depth of thesubsoil mass that fails. Additionally, under the moment load component, as theanchors transfer load downward, the local shear failure of the pressed subsoilchanges into punching shear failure.Moreover, increasing the number of anchors not only improves the bearingcapacity of foundation but also reduce the axial force of each anchor. However,when the number of anchors is increased to certain extent, the spacing between twoanchors is too small and then the load-carrying zones in the subsoil are overlapped,thereby causing the superposition of stress and reducing the bearing efficiency ofanchors. Therefore, the number of anchors should be optimized. Regarding theeffect of diameter of anchors, it is found that the bearing capacity of foundationincreases almost linearly with the diameter of anchors. In other words, the larger thediameter of anchors the greater contribution for the stability of foundation providedby the anchors. Therefore, the diameter of anchors should be as large as possible.The increase in the diameter of anchor rings is equivalent to the increase in the armof anti-moment provided by the anchors. However, since the anchors can changeequivalent stiffness of the subsoil, bearing capacity for vertical loading increasesfirstly but decreases subsequently as the diameter of anchor ring increases.Consequently, it is essential to evaluate bearing capacity from various aspects andchoose appropriate diameter of anchor rings for design. Within the scope of thisstudy, it is appropriate to use the average value of the diameter of inner and outerrings as55%of the basal diameter of the foundation. In addition, the bearing capacity of gravity foundation with anchors undercombined loads was analyzed. It is found that bending moment plays a major roleunder the combined action of horizontal load and bending moment. Failureenvelope curves of subsoil in the V-H load plane were constructed by both Swipemethod and fixed displacement ratio method. The curves are hyperbola-like. Underdifferent values of bending moment, the failure envelope curves in the V-H loadplane have similar shapes. As bending moment increases, the size of failureenvelope curves are reduced gradually.