Research On Rapid Nondestructive Detection, Evaluation And Control Of Subgrade Strength

Subgrade is the support of pavement structure. In practice, the usual damage phenomenon of the pavement are due to the lack of subgrade strength, deteriorated stability and excessive deformation due to external loads in most cases. The strength and stablility of subgrade and the service performance of subgrade and pavement as a whole are based on the quality of construction of subgrade. One key technology to ensure service property of the road and service life of high-grade highway is how to quickly and reliably evaluate the quality of subgrade construction, effectively control the quality of construction process and timely eliminate the quality bugs of subgrade construction in our province and even the country. In this paper, rapid non-destructive testing, evaluation and control techniques of subgrade strength are studied as the following:1. The characteristics of rapid non-destructive testing equipment FWD (falling weight deflectometer) and PFWD(portable falling weight deflectometer) applied in the detection of subgrade strength are introduced in detail. We analysed performance indicators, tested procedures and test results of FWD and PFWD, and presented technical points that worth our notice and technical superiority of FWD instrument in the course, then we proposed two ways to assess the bearing capacity of pavement structure and their respective scope to adapt. Whether PFWD is reliable to test subgrade resilient modulus is analysed, and the result shows that PFWD can be used to test subgrade resilient modulus.2. Adaptive information Genetic Algorithm is proposed. In anti-calculation of the modulus, for the objective function shows as a long, narrow and large flat area when it is in the vicinity of the optimal value, a new kind of anti-calculation on the modulus——adaptive information Genetic Algorithm is put forward. The mechanism that whether we should take self-adaptive segmentation algorithm modulus which is depended upon the size of the information so that the latter part of anti-calculation algorithm in the search space could be reduced is proposed for the first time. In addition, the new algorithm also improves the real crossover operator by using a new exploration strategy. Thereby, it strengthens the local search capabilities in the latter part of the new algorithm. The introduction of this new mechanism and the new strategy makes solving efficiency of anti-calculation on the modulus much faster and reduces the algorithm complexity. The algorithm improves the anti-calculation accuracy and the speed so that it could meet the requirements of anti-calculation on the modulus in the engineering practice.3. The research of the method to evaluate uniformity of subgrade construction quality is conducted. Firstly the mathematical model involved in evaluation is established, and surface fitting method which has been widely used in statistics into the field of evaluation of subgrade is introduced for the first time. With measured data, we can fit out the real modulus surface. Further more, based on the fitted-out surface, uniformity evaluation method——pseudo-variance mean value comprehensive evaluation method is proposed. The subsequent simulation trials showed that the method could effectively evaluate the actual situation of subgrade construction quality.4. According to the basic principles of Reliability-based Design of Highway, the impact on the reliability of pavement that comes from the subgrade resilient modulus coefficient of variation is studied, and the theory and methods to analyse subgrade uniformity based on the reliability of pavement were determined. Also, the method that subgrade resilient modulus coefficient of variation can be taken as an evaluation index of subgrade uniformity was proposed. Based on the specified target reliability of pavement in" Unified Standard for Reliability-based Design of Highway Engineering Structural"(GB/T50283-1999), division evidence of reliability used in evaluation of subgrade uniformity was established, and then classification of subgrade uniformity is divided into superior, middle, and poor. Through the creation of finite element model, structural reliability of asphalt pavement structure and the structure of cement concrete pavement was analysed and calculated under different subgrade resilient modulus, different coefficient of variation respectively, and the critical value of coefficient of variation of subgrade resilient modulus to classify subgrade uniformity was got, and then evaluation criteria of subgrade uniformity based on reliability was established.5. About the difference between subgrade design parameters and detection index of construction quality in China’s current specifications, through on-site investigations and tests of formed subgrade that were conducted in32sections of12highways, the test results of subgrade resilient modulus E b with on-site bearing plate method, Beckman Beam beam deflection value1, subgrade modulus E p measured by PFWD, practical compactness K with sand replacement method, water content w, consistency w c and so on were got. Based on the standard of the subgrade resilient modulus E b with on-site bearing plate method, comprehensive experience relationship between subgradeo resilient modulus and deflection were determined and the relationship between subgrade resilient modulus and construction indicators. The relationship between the design parameters of the subgrade resilient modulus and compactness used as construction control was establisher organically, so reasonable values of resilient modulus which provide reasonable reference for pavement design and subgrade construction quality control could be determined.