Research On Underwater Vacuum Preloading Technology In Intertidal Zone

With the development of China economic strength and the exploration in coastal zones, more and more projects need to be constructed in intertidal zones in neritic area. However, under the influence of geological environments, recently deposited silty clay or silt is distributed in China costal area, posing great difficulties in construction of port engineering. Currently for improvement of underwater soft soil foundation, cement deep mixing method (CDM) or sand compaction pile method (SCP) are utilized, featured with high cost and need of large special construction equipment. Meanwhile, the vacuum preloading technique is the first choice for soft soil foundation improvement, featured with a quick preloading without instability of foundation, no need of surcharge and involved exploration and transportation. Therefore it is significant to research the application of vacuum preloading technique in improvement of underwater soft soil foundation.Due to the limitation of construction technologies, it is difficult to apply all the procedures of the vacuum preloading technique under water. As a transit stage of application of the vacuum preloading technique under water, we have improved the current vacuum preloading technique by considering the characteristics of the tidal environment of the intertidal zone. The improved vacuum preloading technique has been applied successfully in4projects in intertidal zone and produced good result. Based on these application, a full set of underwater soft soil foundation improvement technique by vacuum preloading has been formed with the following innovations:(1) The improvement mechanism of underwater soft soil foundation improvement technique by vacuum preloading is similar to that of conventional vacuum and surcharge combined preloading technique, featured with a variant hydraulic load. In the design, the preload can be assigned to be the sum of vacuum preload beneath the membrane and the average hydraulic load on the membrane, and the latter can be obtained from the tidal level and ground elevation.(2) By summarizing the experiences from4projects improved by underwater vacuum preloading technique, improvements were made on the conventional sand cushion construction, plastic vertical drain, laying-out of sealing membrane. So a full set of construction technique has been formed.(3) A series of automatic data recording and transmission techniques have been developed based on the characteristics of underwater monitoring. Therefore the automatic monitoring can be achieved by automatic measurement, data acquisition, storage and transmission.This research has showed that it is necessary to apply underwater vacuum preloading technique in more projects in intertidal zone in order to shorten construction period and save cost. So it is expected a wide application in engineering construction.