Experimental study on seepage failure mechanism in an upper-fine-lower-coarse composite sandy strata and its preliminary engineering application#br#
PAN Hong1，2，CAI Lei1，2，LUO Junxing1，3，CAO Hong1，2，LUO Guanyong1，2
(1. School of Civil Engineering and Transportation，South China University of Technology，Guangzhou，Guangdong 510640，China；2. The State Key Laboratory of Subtropical Building Science，South China University of Technology，Guangzhou，Guangdong 510640，China；3. CCCC Fourth Harbor Engineering Institute Co.，Ltd.，Guangzhou，Guangdong 510230，China)
Abstract：Highly water-rich sand strata is prone to causing seepage failure and is the main risk source for underground construction. At present，the mechanism of seepage failure in the composite strata of“upper fine and lower coarse”is not clear. By analyzing and comparing 14 groups of sand erosion model test data，it is found that the seepage damage characteristics of the composite strata composed of the lower coarse sand and the upper fine sand are significantly different from that of the single strata. The lower layer has small energy consumption and mainly plays the role of water supply，while the upper layer of fine sand has small permeability and bears most of the seepage force，thus causing rapid loss of fine sand particles and leading to sand boiling/static liquefaction. Under the same boundary conditions，groundwater disturbs the soil of the composite strata more extensively，and the soil loss is more significant and the loss rate is faster. The sand boiling phenomenon is most significant when the seepage outlet is close to the interface of coarse sand and fine sand and on the side of fine sand. Based on the observations obtained from the test，the causes of the large area collapse in the “2.7” subway accident in Foshan in 2018 were discussed. It is considered that the composite sand layer of “fine top and coarse bottom" and the unique location of the leakage outlet - "the interface of fine sand and medium coarse sand” are the main causes of the large area collapse of the accident. In similar projects，it is necessary to pay attention to the identification of the composite sand layer and make good sealing measures to prevent the emergence of the seepage outlet.
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2023, Vol. 42 
