通过对甘肃省定西市兰渝铁路(夏广段)LYS-1标胡麻岭隧道的红层软岩进行不同高径比试样单轴抗压强度对比试验，研究红层软岩单轴抗压强度的尺寸效应。加工试件直径50 mm，高20~100 mm。定义无量纲量尺寸效应系数为非标准样与标准样所测单轴抗压强度之比。试验数据拟合得出红层软岩尺寸效应系数与高径比、平均弹性模量之间线性相关，拟合效果良好;高径比＜2.0，尺寸效应系数随着高径比的增大呈先减小后增加的趋势。联立拟合直线求解，得出尺寸效应系数取极小值时的高径比。对试样进行受力分析，通过研究不同破坏形态，得出红层软岩试样尺寸效应系数变化的原因，且发现红层软岩尺寸效应明显。

The size effect of red bed soft rock's uniaxial compressive strength was researched in this paper.The testing samples are taken from Humaling Tunnel of the Xiaguan-Guangyuan Section of Lanzhou-Chongqing Railway,Dingxi city,Gansu Province,China.The samples are of cylindrical shape with different ratios of height to diameter(diameter 50mm,height 20-100 mm).Results show that the red bed soft rock has obvious size effect.The dimensionless size effect coefficient is defined as the ratio of uniaxial compressive strength of nonstandard sample to that of standard sample.Fitting of test data shows that size effect coefficient is linearly related with height to diameter ratio and average elastic modulus.A mathematical model is established to illustrate this linear function.When the height to diameter ratio is smaller than 2.0,the size effect coefficient decreases and then increases with the increase of the ratio.The height to diameter ratio corresponding to minimum coefficient is obtained.Moreover,stress analysis is conducted to research the destruction patterns of red bed soft rock,and the causes of size effect coefficient variation are given.