Numerical And Experimental Research On The "Pot-cover" Effect In Airport Runways Caused By Seasonal Temperature Change

The water-gas migration equilibrium between subgrade and atmosphere changes with the construction of airport runways or other ground structures, inducing water accumulation in soil surface. The phenomenon is named "pot-cover" effect. The rising of water content, combined with traffic loads, will cause differential settlement to subgrade or even damages to pavement structures, seriously affecting the airworthiness of runways.In order to reveal the formation mechanism of "pot-cover" effect, numerical simulation, field test, model test were carried out. Based on coupled theory of heat transfer and unsaturated porous media moisture migration, the finite element analysis software OpenGeoSys is used to study moisture movement process in different kinds of soils under the influence of pavement structure’s blocking effect and environmental temperature change. The influence of seasonal temperature change, initial temperature and saturation of soil is also analyzed. In addition, as a comparison to the simulation results, in-situ water content test was carried out in Changzhi Airport in Shanxi province. Based on the work mentioned above, a soil-column model apparatus was developed, in which temperature control is in circumferential direction. By this new apparatus we could maintain a constant temperature gradient. Moisture migration test of loess was performed under temperature gradient. More attention was paid on the significance of temperature gradient and overlying structure in the formation of "pot-cover" effect. The experiment process was also simulated by non-isothermal multiphase flow method. The results of simulation analysis, field test and experiment are as follows:(1) Moisture migration process in subgrade soils is obviously influenced by seasonal temperature change. Temperature decrease in surface of subgrade will cause upward water transportation. Therefore, water content in surface will increase, described as water enrichment. The main influence depth is about 1m. Seasonal temperature change impact silty clay the most, maximum saturation change of which could reach nearly 6%, the second for clay, while water content in sand is relatively less affected.(2) Water accumulation amount in shallow subgrade is closely related with initial condition of backfill soil. Water enrichment phenomenon will become more obvious with the rising of initial water content and temperature. Analysis indicates the effect of seasonal temperature change on water content in surface soil is greatly weakened as saturation decrease.(3) Pavement’s blocking effect has significant influence on water content. Within 1m below the pavement, soil water content is obviously higher than that of the same depth in subgrade far away from airport runways. Furthermore, water content in 50cm is higher than deep soil, appearing as "pot-cover" effect. The analysis results and in-situ test are consistent.(4) During the test period (11 weeks),3 models were carried out with different temperature control and surface boundary. Water distribution pattern of the 3 models have great distinction. As the permeability coefficient of compacted loess is relatively low, the variation of water content is less than 2%. Temperature gradient caused by low-temperature boundary in surface, combing with covering structure, contribute to the water enrichment, and water content in 5cm increase by 7.4%.(5) The existence of covering structures has great effect on moisture transport process. Under the same temperature boundaries, water content significantly reduces in subgrade far away from airport runways,8.6% lower than that of the same depth in subgrade with covering structure.(6) The simulation of experiment process could better reflect the law of soil moisture variation in different depth, indicating that non-isothermal multiphase flow method is an appropriate approach in "pot-cover" effect study.