The In-Situ Test Study On Super-Sized Caisson Force During Sinking

During the super-sized caisson sinking construction period, the key factor which the caisson can suck mud to smoothly sink and stably lift lies in the sidewall friction and end resistance. The current relevant design specifications are often based on research of large diameter piles and medium-sized caisson, but the research of super-sized caisson is scarce. Therefore, through conducting the actual super-sized caisson in-situ force monitoring test during sinking and analyzing the observed data, and comparing with the calculated values of the relevant existing norms. The main contents and conclusions are as follows:(1) During the super-sized caisson implantation period, when the effective contact stress of cutting curb re aches or exceeds 0.4～0.5MPa, or the caisson settlement -time curve finally tends to be stable, which can be the basis of stability of caisson implantation, and then combined with the two methods, it is found that the result is more accurate determination.(2) In the process of the super-sized caisson suction mud sinking, the force relaxation zone was found near the foot blade, differing from sidewall friction resistance distribution supplied by Specification for structural design of reinforced concrete sinking well of water supply and sewerage engineering CECS 137:2015. Thus, it is indicated that sidewall friction resistance distribution recommended by the specification are supposed to be modified when applied in the super-sized caisson.(3) With the depth of caisson increasing, sidewall friction resistance proportion accounting for the total sinking resistance is also gradually increasing, namely, when the depth of the caisson is 10.5m, the proportion is 34.95 percent, and end resistance exceeds sidewall friction, while 53m, the proportion reached 68.19 percent, and sidewall friction exceeds end resistance. At the end time of sinking, reducing the sidewall friction should be the main measures taken to assist sinking when it is difficult for the caisson to sink.(4) In terms of calculating the foundation bearing capacity of this caisson with different depth, Code for design of building foundation GB50007-2011 always exceeds the Code for design on subsoil foundation of railway bridge and culvert TB10002.5-2005; When the depth is 10.5m, the ratio of the former and the later reaches 1.58, besides, with the depth increasing, the ratio is gradually decreased; In contrast to the former, the later seems more conservative, with a more higher safety factor.(5) At the different depth, the in-situ test value of sidewall friction resistance always exceeds the value calculated by Specification for structural design of reinforced concrete sinking well of water supply and sewerage engineering CECS137:2015; When the depth is 10.5m, the maximum ratio of the former and the later reaches 18.52, but minimum reaches 2.49 with a depth of 53m. With the depth of caisson increasing, Rankine earth pressure theory calculation method is closer to measured value than the value of the experience, and the error is smaller.