基于服役需求的机场粉土道基临界动应力研究

Determination of critical dynamic stress for airport silt subgrade based on serviceable requirement

在循环荷载作用下土样应变发展特性的研究中,安定理论被广泛应用。通过该理论将塑性应变发展状态进行划分,可确定临界动应力。但该传统方法忽略了工程中的服役需求,其取值未考虑在临界动应力范围内,道基土体变形的持续发展,最终超过允许变形,引起道面板错缝、跑道断裂等问题。针对以上问题开展大型动三轴试验,获得道基粉土在不同压实度、围压、制样方法和动应力下变形发展规律,结合服役需求中的破坏应变,提出考虑道基服役需求的临界动应力模型,主要结论有:1)压实度和围压对最终塑性应变均有较大影响,且与应变有较好的指数关系。考虑压实度、围压和动应力幅值建立的最终应变预测模型能较好地反映试验结果;2)制样方法对塑性应变的发展有明显的影响,在动应力小于180 kPa时应变差在0.2%左右,而在动应力较大时,不同制样方法之间应变差异显著增大,最大可超过2%,远大于规范中最大允许差异沉降值0.1%~0.2%。实际工程中多压实工法处理道基,可能导致道基在长期荷载作用下产生较大的差异沉降;3)基于服役需求提出的临界动应力在道基沉降值和差异沉降值上均符合规范要求,且与传统方法临界动应力计算结果相比,相同条件下动应力值约为80%,对应塑性变形增长速率仅为40%,充分考虑机场服役需求,取值更加严谨,对实际工程更具指导意义。

In the study of strain development characteristics of soil specimens under cyclic loading,the shakedown theory has been widely used.The critical dynamic stress is derived,based on the development status of plastic strain.However,this traditional method overlooks the actual serviceable conditions of the subgrade.The deformation of the subgrade continues to develop within the range of the aforementioned critical dynamic stress.In this case,the subgrade deformation may exceed the allowable values,resulting in the mismatch and crack of pavement slab.Based on the case,a series of large-scale dynamic triaxial tests were carried out,which obtained the deformation development of subgrade silt under different conditions(degree of compaction,confining pressure,sample preparation method and dynamic stress).The main conclusions are drawn as follows.(1) Both degree of compaction and confining pressure have a large influence on the final plastic deformation,and both have a good exponential relationship with the dynamic stress.(2) The sample preparation method has a remarkable influence on the increase of plastic strain,which may result in a large differential settlement when the subgrade subject to the long-term loading.The accumulated plastic strain of the static compression samples is higher than that of the compacted samples by 0.2%,before the dynamic stress is less than 180 kPa,and then the difference increases significantly,the maximum can exceed 2%,which is much more than the maximum allowable differential settlement value in the specification.(3) Compared the critical dynamic stress proposed by the serviceable performance with the traditional result,the critical stress controlled by failure strain is about 80%,which corresponds to a plastic deformation growth rate of only 40%.That indicates the critical dynamic stress in this study is more accuracy in field application.