Centrifuge Modelling And Analysis Of Rainfall-induced Shallow Failure In Unsaturated Soil Slope

As southeastern coastal area in China has a humid climate,lots of landslides in soil slopes are induced by rainfall during the rainy season and typhoons every year.With the continuous development of China's economy,more and more infrastructure construction,such as civil architecture,high-speed rail,highway and so on,are being built.Landslide disaster has become an environmental problems cannot be ignored.Additionally,extreme events will be more frequent,more intense under global warming.It is of great importance to investigate the failure mechanism,forecasting and warning method of rainfall-induced slope instability.Unit cell and soil column test under lg and centrifuge modelling test under Ng were carried out.The results from the above tests were analyzed by numerical simulations and analytical methods.The main research works and conclusions are as follow.(1)Laboratory loess soil column tests subjected to a constant water head at the top or a high temperature at the bottom were respectively performed.The response of the soil column was monitored and a numerical model was proposed to simulate the coupled thermo-hydraulic process.The unsaturated permeability coefficient curve of soil both in-situ and in laboratory were measured using the instantaneous profile method.The results show that the method of constant water head is more convenient and time-saving to measure the unsaturated hydraulic conductivity of silt.The unsaturated hydraulic conductivities measured in-situ were abou 1 orders of magnitude higher than those measured in laboratory.The heating of the bottom accelerated the drying process of the soil column by almost 22 d compared with the conditions without heating.The water vapor diffusion was significantly enhanced by the bottom heating.(2)A rainfall simulator and a miniature TDR probe used in the centrifuge modelling test were developed.The results show that the working performance of the rainfall simulator under Ng was stable and robust.The uniformity of simulated rainfall could reach 85%.The measured TDR waveforms of the soil with same water content were almost identical with each other under different value of g,which indicated that the super gravity environment has no effect on the measurement of the TDR probe.The absolute error of the measured water content by the TDR probe could be controlled within 2%.During the acceleration of centrifuge,the tensiometer could maintain higher suction and water in it had not cavitated.After the g reached a stable value,the measured suction approached the initial value(the maximum difference was 1.1 kPa)under 1g during a relatively short period(about l0min).(3)Centrifuge model tests of rainfall infiltration into horizontal foundation in unsaturated loess soil under three different values of g were conducted.The TDR probes,tensiometers and bender elements were used to monitor the response of the loess soil.The results show that the time correspond to the abrupt turning point in the duration curve measured by TDR probes,tensiometers and bender elements were all consistent with each other.This accordance indicated that the reliability of the measurement and effectiveness of the cooperation between TDR probes,tensiometers and bender elements.The migration rate and time of wetting front during rainfall infiltration both conformed to the theoretical scaling laws under different value of g.(4)A series of unconfined compression tests of sand and 17 centrifuge model tests of unsaturated sand slope instability were carried out.The test results were interpreted by mumerical modeling and back analysis.The results show that the total effective internal friction angle((?)'cril+?)calculated by the equation of Zhan&Ng was higher than that(i.e.41.40)determinated by the equation of F&M about 10°.(?)b based on the equation of Zhan&Ng was lower than that on the equation of F&M.Especially when suction was lower,the difference was greater.The safety factors Fs and theoretical slip surfaces based on the equation of Zhan&Ng are more consistent with the observations in the tests than those based on the equation of F&M.The equation of Zhan&Ng is able to reasonably account for analysis and calculation of the surficial failure happened in unsaturated soil slopes.(5)Centrifuge modelling of rainfall-induced slope failure in silt soil subjected to three different rainfall intensities were performed.The test results were interpreted by numerical modeling and back analysis.The results show that the observed failure mode started from local failure at the slope toe,extended upward and finished with a global shallow landslide.The depth of the slip surface was between 1 to 3 m.The safety factors Fs based on the equation of Zhan&Ng was more consistent with the observations in the tests than that based on the equation of F&M.It was found that the data points of relationship between rainfall intensity and rainfall duration from the three centrifuge modelling tests were close to the Intensity-Duration curve proposed by Li,Zhan et al.(2012)for the early warning of landslides in residual soil slope in southeast China.(6)The numerical modeling was conducted to analyze the effectiveness of re-compaction and vegetation cover to the superficial layer of unsaturated soil slope.For clayey and sandy silt,the final depths of wetting front within the re-compacted slope were all lower than those of loose slope.Because of re-compaction,the water permeability of soil significantly decreased.The re-compacted layer acted as an impeding layer,which hinders rainfall infiltration and preserves the initial soil suction underneath.For the stability of superficial layer,FS predicted for the re-compacted slope was significantly larger than that for the loose slope during the rainfall,which was attributed to the larger effective friction angle((?)').Fs of re-compacted slope considering the effect of vegetation was larger than that for the loose slope,too,which was due to the larger initial negative pore water pressure distributions.