Experimental Study Of Spudcan Punch-through Behavior In Double-layered Ground

Applications of jack-up offshore platform are increasingly common with the exploitation of offshore oil and gas resources. The support system of jack-up offshore platform is three independent legs located at the bottom of the platform. Each leg stands on the seabed supporting by independent spudcan. Safety of offshore platforms is directly linked with spudcan bearing capacity. Therefore, research on spudcan bearing characteristics is concerned by more and more scholars. Seabed usually composed by sand overlying clay. In such ground, spudcan penetrate into the lower clay layer quickly when the maximum load exceeds bearing capacity of upper sand layer, punch-through failure occurred. Punch-through failure occurs in a short time, may lead overturning of offshore platform, causing huge economic losses. Therefore, it is crucial to research spudcan penetration mechanism in this foundation form. This paper carried out multiple sets of centrifugal penetration tests on the basis of summarizing previous scholars’achievements, spudcan penetration characteristics in sand ground and sand overlying clay are investigated. The main works carried out in this paper are as follows:(1) Preparation of sand ground model:Sand pouring method is the most frequently used method in model test. Conventional sand pouring method can produce sand in middle and dense state. In order to obtain sand with wider range of relative density, independently developed sand pourer device was used. And on base of the study of conventional factors such as drop height, flow rate, etc, the influence factors on relative density like sand head moving rate and moving path, sieves, etc were further studied. By changing these factors, sand samples with loose state were produced out. The uniformity of produced sand samples was tested by small boxes in different sizes and CPT method. The results showed that the uniformity was good. By comparing these two test methods, testing uniformity with small boxes was simple and easy to operate with relatively larger error, and CPT method turned out to be credible and reliable.(2) Centrifugal penetration test in sand ground:Multiple sets of penetration test of 1g indoor and centrifugal state were carried out before penetration test in sand overlying clay. Factors that affect penetration characteristics like relative density of the sand, spudcan diameter, base form were studied. The results showed that penetration resistance increases with relative density of the sand and spudcan diameter. Soil surface displacement was observed during penetration process in 1 g test, and the influence area was given out.(3) Spudcan foundation is prone to suffer punch-through hazard in sand overlying clay. Multiple sets of centrifugal model test in sand overlying clay were carried out on the base of penetration test in sand. Factors that affect spudcan penetration characteristics, such as spudcan diameter, relative density of sand layer, undrained shear-strength of clay layer, relative thickness of sand layer, loading mode were studied. On the base of these studies, punch-through speed under stress controlled loading mode was discussed. The results showed that the relative thickness of the sand layer, spudcan diameter and undrained shear-strength of clay layer were key factors that affect peak penetration stress. Under stress controlled loading conditions, punch-through occurs in a short period of time with relative higher speed. And the penetration speed is faster when the undrained shear-strength of clay is low. There was no significant difference in penetration curve under different loading mode.