Research On Dynamic Characteristics Of Road Subgrade Based On Impact Load

At present,the detection methods of subgrade compaction quality can be basically divided into three categories,namely,compaction represented by the sand cone method,mechanics represented by the bearing plate method,and experience represented by the compaction meter value(CMV)test.The first two categories are commonly used in the evaluation of road engineering compaction quality,and the third category is also used in the current intelligent compaction research and engineering construction.However,all three types of methods do not involve the parameter of the natural frequency with the mechanical properties of the subgrade system itself.Considering that the subgrade is subjected to periodic vibration of rollers or intermittent dynamic effect of traffic load during construction and operation period,it is reasonable and necessary to research the dynamic properties of the subgrade based on the vibration mechanical.In view of this,this paper focuses on the natural frequency testing and identification of the indoor compaction specimen system and the compacted subgrade system by means of theoretical analysis,numerical simulation and indoor and outdoor tests,which provides a basis for judging and optimizing the rationality of the compaction technology.Meanwhile,the reliability of the identification result of the natural frequency of the compacted subgrade at the construction site is demonstrated by the resilient modulus measured by the bearing plate method.Firstly,according to the vibration theory,the identification method of dynamic parameters of indoor compaction specimens based on damping single-degree-of-freedom and multi-degree-of-freedom theoretical models was established.The research on the dynamic response of the soil specimens with the compaction degree of 91%and 93%under sinusoidal steady-state excitation showed that the natural frequencies of the two specimens were around 24.20 Hz and 26.14 Hz respectively.The experimental modal analysis under impact excitation showed that the two compacted soil specimens mainly showed the 3 modal response and the 1st-3rd natural frequencies in the range of 4-5 Hz,16-18 Hz and 26-28 Hz respectively.Using the self-developed preliminary hammering tester instrument for subgrade,the on-site hammer modal tests were carried out on lime-treated soil subgrade(rolled 5 times in total,and tested each time),compacted gravel soil subgrade and cement-treated gravel soil subgrade.Based on the collected signals,the dynamic properties of the subgrade were studied from the frequency domain perspective,and a method for testing and identifying the modal parameters of the subgrade was established.The analysis results showed that the lime-treated soil subgrade of each rolling pass generally showed the modal response of no less than 5 modes under impact load,which was combined with the vibration frequency range of commonly used rollers.The average lower natural frequencies(less than 100 Hz)of the subgrade of interest to the engineering were 17.22 Hz,49.60 Hz and 80.62 Hz for the first 3 modes of the soil subgrade,36.71 Hz and 78.07 Hz for the first 2 modes of the gravel subgrade.and 29.78 Hz,60.71 Hz and 83.19 Hz for the first 3 modes of cement-treated gravel subgrade.Meanwhile,the response of the subgrade model under simple harmonic loads was calculated by using finite element software Abaqus and combining with the relevant parameters of the hammering tester instrument.The results showed that the finite element model also exhibits multi-mode characteristics consistent with the compacted subgrade,and the natural frequency of each mode increases approximately linearly with the increase of stiffness,and the growth rate of higher modal frequencies was greater than that of lower modal frequencies.In order to verify the validity of the natural frequency reflecting the compaction quality,the stiffness characteristics of the three material subgrades were tested by using the bearing plate method,portable falling weight deflectometer(PFWD)and Geogauge test device,which was carried out at the same measurement point as the hammering test.Statistical analysis showed an ideal positive linear relevance between the stiffness characteristics parameters,indicating the reliability of the test results.Significant positive linear relevance was also found between each of the lower natural frequencies of the subgrade and the stiffness parameters such as the resilient modulus.respectively,in agreement with the Abaqus simulations,indicating the reliability of the identified lower natural frequency results of the compacted subgrade.Therefore,it was considered that each lower natural frequency can be used as an index of compaction quality,and can be used as the basis for selecting or optimizing the vibration frequency of the roller.The lower natural frequency of the soil subgrade did not increase with the increase of compaction times.The stiffness characteristics test also showed that the resilient modulus and layer stiffness of the lime-treated soil subgrade showed a serrated gentle change with the increase of compaction times,which was far from the generally accepted law of gradual increase,indicating that the compaction technology used for this material was unreasonable and it was necessary to improve it.