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Transient strain response of foundation soil under impact load is a critical concern for performance evaluation of geotechnical infrastructures such as airport pavements and railway embankments.In this paper,the development of transient strain in dense sand foundation was investigated based on a series of laboratory falling ball tests.The fiber Bragg grating(FBG)strain sensors have been horizontally embedded in the sand at three depths.When the ball fell from eight different heights,the distribution and variation of strains were automatically recorded at the frequency of 1000 Hz.The FBG monitoring results show that the strain-time curve can be divided into four consecutive phases,i.e.the nonlinear increase phase,the linear increase phase,the nonlinear decrease phase and the residual strain phase.The time corresponding to the peak strain in each test is highly consistent,which is about 100 ms,regardless of the falling ball heights and the FBG buried depths.In all the tests,the depth of influence of the impact load was about 20 cm.Under the same FBG buried depth,the sand strain increases linearly with the increase of the falling ball height;under the same falling ball height,the sand strain decreases exponentially with the FBG buried depth.From the perspective of strain rate,the above rules are more obvious.To evaluate the reliability of the FBG monitoring data,a simply calculation is performed.The results show that there was no relative slip between the fiber and the soil,which means the strain measurements represented the real strain of sand in the tests.