Deformation And Failure Mode Of The Large-scale Geotextile Sand Container Cofferdam On Soft Foundation

In recent 10 years, the offshore engineering develop fast, a new method named “geo-container cofferdam” is applied to many practical projects in China. The sandbag is made of geotextile with large sizes in length and width. Sandbags are filled with pumped sea sand and stacked together layer by layer. Many theoretical obstacles remain unsolved, which has restricted the application of this advanced technology. Combine with numerical analysis and centrifugal model tests, the mechanical behavior, failure mode and mechanism of the geo-container cofferdam were studied. The main contents and conclusions in this paper are as follows:(1) Centrifuge model tests of geo-container cofferdam on soft clay foundation(with and without PVDs) were conducted. The deformation and strain of the geocontainers were obtained. The results shows that the geotextile sand containers have a good integrity and stability. For the case without PVDs, the uneven settlement of soft foundation was remarkable, geocontainers near the slope toe were in tension, and those under the cofferdam centre were pressurized. For the soft clay installed with PVDs, deformation of the foundation was more even, the tension of the geocontainers was higher. The distribution of tension was more localized, and the max tension under the toe of cofferdam reaches around 86% of the tensile strength. The presence of PVDs and sand mattresses increases the strength of the clay foundation.It is difficult to measure accurately the strain of geosynthetics in centrifugal model test with conventional strain gauge. To solve this problem, a kind of flexible glue and a suitable pasting method were put forward. Through a set of self-developed test equipment, two kinds of strain gauges were demarcated.(2) Numerical analysis was also presented to studied the effect of geotextile strength to the deformation and failure mode of the geo-contain cofferdam. When the strength of foundation and geotextile were low, the plastic failure would firstly appear in the foundation, then the deformation of geotextile increase rapidly and pulled cut, lead to the sliding failure of geo-container cofferdam. While the foundation strength is low, and the geotextile strength, the failure mode was that the geo-container cofferdam had a big settlement and the foundation was in shear failure. Based on the failure reason, some advises were put forward for the construction of geo-container cofferdam.(3) The geo-container cofferdam has a large scale, like a boat floating on the soft foundation. When the geotextile has enough strength and stiffness, the sand containers would not be pulled cut, even the foundation has lost its stability. Based on the Terzaghi and Rowe bearing capacity theory, a new calculation method for the geo-container cofferdam ultimate bearing capacity was put forward to the second failure mode.(4) It is important to maintain stability of soft foundation during the cofferdam rapid constructuion. Two deformation variables, the settlement velocity at the cofferdam surface center and the horizontal displacement velocity at the slope toe, are taken as the controlling index. Based on a serious of field data, a critical state line is suggested by Zhou Xiaowen as an essential index of stability control, the stability and instability zoon are separated. It was proven that this critical state line is suitable for the geo-container cofferdam constructed on the soft foundation.