Search

A recent study on active trapdoor stability has been completed by the authors using Terzaghi's three stability factors approach. It was concluded that the superposition approach is an effective way to evaluate the stability of cohesive-frictional soils. This technical note aims to extend the previous active trapdoor study to perform a stability assessment of a passive planar trapdoor (i.e., a blowout condition) in cohesive-frictional soil. Note that this passive trapdoor problem represents the blowout stability of soils due to defective pipelines under high water main pressures, in spite of the frequent media news about the water main bursts which enlightens the relevance of the problem. Numerical solutions of upper and lower bound finite element limit analyses are presented in form of the three stability factors (Fc, Fs, and Fγ), which consider the effect of cohesion, surcharge, and soil unit weight respectively. In the event of passive trapdoor stability, this technique can be used to determine a critical blowout pressure due to a water mains leak. The study continues with a series of sensitivity analyses with a widely selected range of parameters including the cover-depth ratio (H/B) and the drained frictional angle (φ). The influence of these parameters on the three stability factors is discussed, and a practical example of adapting these approaches is also introduced. All numerical results are provided in the forms of design charts and tables that can be efficiently used with confidence in design practice. [ABSTRACT FROM AUTHOR]

Pit-in-pit (PIP) excavations in an aquifer-aquitard system likely undergo catastrophic failures under the hydraulic uplift, the associated undrained stability problem, however, has not been well analyzed in the past. To this end, a hypothetical model of PIP braced excavation in typical soil layers of Shanghai, China is developed using the finite element limit analysis (FELA) tool. The FELA solutions of safety factors (FSs) against hydraulic uplift are verified with the results from the finite element analysis with strength reduction technique (SRFEA) and existing design approaches. Subsequently, FELA is employed to identify the triggering and failure mechanisms of PIP braced excavations subjected to hydraulic uplift. A series of parametric studies considering the various geometric configurations of the PIP excavation, undrained shear strengths of aquitard, and artesian pressures are carried out. The sensitivities of relevant design parameters are further assessed using a multivariate adaptive regression splines (MARS) model that is capable of accurately capturing the nonlinear relationships between a set of input variables and output variables in multi-dimensions. A MARS-based design equation used for predicting FS is finally presented using the artificial dataset from FELA for practical design uses. [ABSTRACT FROM AUTHOR]