Constant rate of strain (CRS) and incremental loading (IL)consolidation [3]. Constant rate of strain consolidation test reduces the required time for consolidation test using standard oedometer test (IL) from almost two weeks time to few hours. During CRS test the load is applied continuously; continuous record of stress and deformation are made, which improves the accuracy of consolidation properties determination in particular the preconsolidation pressure P c .Because of its many advantages, the CRS test is in many countries, a standard test for the determination of consolidation properties of clayey soils. It is adopted by the Swedish geotechnical institute, the Norwegian geotechnical institute, the French Laboratoire Central des Ponts et Chaussées (LCPC), and the American Society of Testing and Materials (ASTM) [4]. However, prior selection of a suitable strain rate for a given soil specimen, is still a problem to an extensive use of the CRS test in practice [4,5], and many researches were conducted to examine strain rate effect on measured CRS consolidation parameters [6][7][8][9].Comparison of CRS test results with those of conventional oedometer test has permitted to develop some criteria for the CRS test results acceptance. These criteria are essentially the relative pressure criterion R u [2,3,10], the liquid limit LL criterion [11], and the standardized strain rate parameter β based on the large strain theory [4,12]. The relative pressure criterion R u defined as the ratio of excess pore water pressure at the base of specimen ∆u H to applied vertical stress ∆σv is generally the most used criterion. However, literature survey shows important differences in the acceptable range of relative pressure values. Table 01 summarizes some relative pressure values recommended by authors for some types of soils, with relative pressure range varying from 5 to 50%. ASTM standard (ASTM 4186-06) recommends values of relative pressure to be between 3 and 15% during steady state stage of CRS test [13]. Similar range of relative pressure is also recommended by the French Laboratory (LCPC) [14].
Accurate prediction of the rate of consolidation settlement is dependent upon a reliable method of determining the coefficient of consolidation (cv) and the time to end of primary consolidation of the natural soil. For this purpose, and to circumvent errors associated with graphical procedures in evaluating cv, a FORTRAN program was developed to determine the best fit of laboratory consolidation test data using a combination of a probabilistic method and a minimization of the sum of squared residual (SSR) method. The proposed computer procedure does not use any particular point on the time-settlement curve. The program uses ranges of probable values of d0, d100, and cv. For each set of values d0, d100, and cv selected in a random manner, the SSR is computed. The set of values with the smallest SSR is considered as the best fit. This procedure leads to suitable reference values of the coefficient of consolidation and the end of primary consolidation, and permits an assessment of the accuracy of classical graphical log(t) and t1/2 procedures.
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