温度对CPTU在海底泥中测试结果的影响及修正方法

Effect of Temperature on CPTU Measurements in Seabed Sediments and Correction Method

温度改变会影响孔压静力触探(CPTU)的测量结果.利用CPTU调查了海底泥的特性,由于海底泥的锥尖阻力(qc)和侧壁摩阻力(fs)较小而且调查时海底泥和海水中的温差较大,fs大多数是负值且随深度逐渐减小.据此提出了在没有安装温度传感器的情况下考虑温度对CPTU在海底泥中测试结果影响的修正方法:首先通过室内温度校正试验获得qc和fs的温度校正系数;然后假设浅层海底泥的fs随深度线性增加,根据相关测量结果或经验估计“实际”的fs剖面;再根据测量和“实际”的fs剖面的区别估计地基中的温度剖面;最后根据温度剖面和温度校正系数对CPTU的测量结果进行温度修正.对本文CPTU测试数据的温度修正结果表明,如果不考虑温度的影响,CPTU在海底泥中的fs会被低估,其值会随深度减小甚至变为负值;在约3.5m深度处,海底泥的qt(考虑孔压修正的锥尖阻力)可能会被高估50%以上;海底泥的不排水抗剪强度(su)将会被高估,温度修正后CPTU所得su更接近于室内试验所得的su.在本文所建议的修正方法中,唯一不确定的参数是正常固结海底沉积物的fs随深度的增加率(α).本文通过反分析得出的α=0.2kPa/m,黏土矿物类型和沉积环境等因素可能会影响α的大小.实际中应该根据相关的测量数据确定α的大小,如果没有相关的测量数据,可以采用本文建议的α值.

The measurement results of the piezocone penetration test(CPTU)are influenced by temperature.In this study,we used the CPTU to investigate the properties of seabed sediments.We found that the values of sleeve friction(fs)were generally negative and decreased with increasing depth owing to the lower values of the cone resistance(qc)and fs in seabed sediments and the greater temperature difference between the seabed sediments and seawater.We propose a correction method for considering the temperature effect on CPTU measurements in seabed sediments for cases in which there is no temperature sensor in the CPTU probe.First,we obtain the temperature correction factors of fs and qc through laboratory temperature calibration testing. Then,we estimate the “actual” fs profile according to the local experience or measured data based on the assumption that the fs value of shallow seabed sediments linearly increases with depth.Then,we estimate the ground temperature profile based on the difference between the measured and “actual” fs profiles. Finally,we correct the CPTU measurements based on the ground temperature profile and temperature correction factors.The temperature correction results for the CPTU measurements in this study show that if temperature correction had not been performed,the fs values of seabed sediments might have been underestimated.Further,we found that the values of fs generally decreased with increasing depth and became negative.In addition,the qt values might have been overestimated by more than 50% at a depth of approximately 3.5m,the undrained shear strength(su)values might have been overestimated,and the corrected CPTU-measured su values were comparable with those measured in the laboratory.In the proposed method,the only uncertain key parameter is the rate of increase of fs with depth(α)of normally consolidated young seabed deposits.For this case study,we back-calculated α=0.2kPa/m.The value of α may change with the deposit environment and type of clay mineral.The value of α should be determined based on the available measured data,but if there are no directly measured data for determining the value of α,the proposed value of α in this study can be adopted.