[Disarikan dari makalah yang dipresentasikan di International Conference on Advanced Characterisation of Pavement and Soil Engineering Materials dengan judul asli: Development of Vs-CBR-DCP Empirical Correlation Model for Determining Dynamic Stiffness of Pavement Base Layer using SASW. Makalah asli bisa merujuk pada Proceeding of Advanced Characterisation of Pavement and Soil Engineering Materials (copyright) 2007 Taylor & Francis Group, London, ISBN 978-0-415-44882-6]
ABSTRACT: The stiffness of the base layer is an important parameter for designing the pavement thickness needed to support traffic loadings. It is normally related to the California bearing ratio (CBR). Currently, the CBR could be obtained from laboratory and field testing using Dynamic Cone Penetrometer (DCP). These methods are time consuming, destructive and costly. The spectral analysis of surface waves (SASW) method is hereby introduced as an in situ non destructive seismic technique to obtain the CBR and DCP values from the measurement and correlation of the dynamic properties of the pavement system. The relationship between the shear wave velocity and dynamic stiffness of the SASW were found to correlate well with DCP and CBR. The empirical correlations of CBR to dynamic stiffness in terms of elastic modulus were found to be similar to the correlation suggested by Shell (1978). Preliminary analysis also indicated that the empirical model was useful for predicting pavement base layer modulus.
The performance of pavement structures is affected by the stiffness of the base and subgrade layer. In order to effectively measure and evaluate the stiffness of those layers, a non-destructive test (NDT) which is economic and fast is needed. The spectral analysis of surface wave (SASW) is an NDT method based on the dispersion of Rayleigh waves (R waves) to determine the shear wave velocity, modulus and depth of each layer of the pavement profile. The SASW method has been utilized in different applications over the past decade after the advancement and improvement of the well-known steady-state (Jones 1958) technique. Much of the basis of the theoretical and analytical work of this method for pavement investigation has been developed by Heisey et al. (1982), Nazarian & Stokoe (1984), Röesset et al. (1990, 1991). For practical purposes, an empirical correlation between the seismic parameter (i.e. shear wave velocity) produced by SASW and the conventional pavement assessment (i.e. dynamic cone penetrometer test) is required to enhance assessment of pavement conditions. Experimental investigation of the SASW test on pavement base layer is presented in this paper. An empirical correlation between shear wave velocity obtained from SASW with the dynamic cone penetrometer (DCP) and the corresponding to the California Bearing Ratio (CBR) will be obtained. The use of DCP test is considered because the method is commonly employed for predicting the bearing capacity of base layer as it is fast and provides the easy understanding of the material parameters.
A set of impact sources of various frequencies were used to generate R waves on the pavement surface. The propagation of the waves were detected using two receiving accelerometers of piezoelectric DJB A/123/E model and the analog signals were then transmitted to a Harmonie 01 dB (IEC 651-804 Type-I) acquisition box and transferred digitally a notebook computer. Several configurations of the receiver and the source spacings were required in order to sample different depths. The measurement configuration of the SASW test used in this study is the mid point receiver spacings. In addition, the short receiver spacings of 5 and 10 cm with a high frequency source (steel ball bearings of 10 and 20 g in weight) were used to sample the asphaltic layers. Longer receiver spacings of 20, 40 cm and 80, 160 cm with a set of low frequency sources (a set of hammers of 1, 2 and 5 kg in weight) were employed to sample the deeper base and subgrade layers.
The actual shear wave velocity of the pavement profile was then produced from the inversion of the composite experimental dispersion curve. In the inversion process, each layer of pavement profile was assumed as a homogeneous layer extending to infinity in the horizontal direction. The last layer was usually taken as a homogeneous half-space. In this study, a theoretical dispersion curve was then calculated based on the initial profile using an automated forward modeling analysis of the 3-D dynamic stiffness matrix (Kausel & Röesset 1981). The theoretical dispersion curve was ultimately matched to the experimental dispersion curve of the lowest root-mean-square (RMS) error with an optimization technique of the maximum likelihood (Joh 1996). Finally, the profile with the best-fitted (representing the lowest value of RMS) of the theoretical dispersion curve to the experimental dispersion curve was used to represents the most likely pavement profile of the site.
The shear wave velocities from the SASW are then correlated to the DCP and the CBR values for evaluation of the bearing capacities of the base materials. The increase in the shear wave velocities correlates well with the increase in the CBR values, while the shear wave velocities decrease with the DCP values. The empirical equation derived between the shear wave velocities have significant correlations with the CBR and DCP value. The correlation coefficient, R2 of 0.94 are obtained for the base layer. The empirical correlation between the CBR and DCP values to the dynamic elastic modulus from SASW for the base layer have been found. The results show a good agreement between the dynamic elastic modulus from the SASW test and the CBR value with a deviation range of ± 20 %.