| Along with the development of our country's pavement, nondestructive testing and evaluation is becoming more and more important. There are many achievements in backcalculating the pavement layer moduli and evaluating the pavement bearing capacity based on nondestructive testing data have been obtained in the last 30 years almost. Genetic algorithms(GA), as a rising biological modeling optimization method, have many good properties. Some preparative study about the application of GA in pavement backcalculation was made in this paper.GA is a selfadapting global optimization method, which is simulating the evolution procedure of biology in the environment. It can prompt good structures through simulating the Darwin's theory of "survival of the fittest, elimination of the poor". It can keep the good model and find the better model through simulating the procedure of the heredity and mutation of Mendelism, and finds the best one at the end of the population evolution.We can get the global optimal solution through single GA or SA without question in theory, but prematurity and local optimal solution is always met in operation of single GA or single SA. The fundamental theory of GA is introduced systematically in this paper. Some improvement measures are offered to eliminate the prematurity phenomenon and local optimal solution results. According to the compensatory property in pavement backcalculation, some improved methods are offered too. The main contents of this paper are as follows:1. Some improved methods are offered to the simple GA and to the application of GA in pavement backcalculation.(1 population generating uniformly instead of Population generating stochastically.Initializing population stochastically is always adopted in traditional GA, which will maybe omit some good models or generate some similar chromosomes, and then the unnecessary search will arise. Initializing population uniformly is used in this paper. With this method, the initial population is scattered in whole solution space, and the searching time can be reduced remarkably.(2) 'Metropolis' accepting rule of simulated annealing algorithm, backfire and annealing strategy are applied here.The overall frame of the algorithm in this paper is GA's frame, the mutation results should be judged according to the 'Metropolis' accepting policy. When the temperature is high, the deteriorative chromosomes are accepted at a high rate and will not be accepted in zero degree. If the temperature approximates to zero but the global optimum solution has not been gotten, then we should rise the temperature, and make the searching jump off the local optimum solution, and then redo the procedure of crossover, mutation, annealing, etc. until the global optimum solution is gotten. This procedure is named as backfire and annealing. Because new stochastic factor was introduced, the algorithm can avoid plunging into local optimum solution and prematurity phenomena.(3) Keep the fittest one in mutation operator.The best chromosome of the population is kept in mutation procedure to avoid population degeneration.(4) Adjust the searching area adaptive automatically.After some generation, when the results of evolution are good in a sense, according to the information of current population and the specific condition of GA in pavement backcalculation, the searching area should be automatically adapted. This measure can improve local searching ability.2. According to the fundamental of system identification and the improved GA, a backcalculation program has been developed with Fortran90.System identification is to identify system character parameter according to the input and output. The basic principle is to set up a reasonable model to simulate the unknown system at first, and then to modify the model parameter through iteration procedure to minimize the output data error between theoretical model and real system. For the pavement backcalculation, the mathematical model can be defined as follows:n- the number of control poi... |
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