Using shear wave velocity for general airfield pavement design

Abstract

Listen

Translate article

Rapid and efficient assessment of the airfield subgrade stability is critical to new airfield construction in unprepared terrain, and to repair of existing airfields as well. Shear strength is a measure of stability and describes the load carrying capacity of the subgrade soil. Inadequate soil subgrade strength can result in failure of the soil support system beneath airfield pavements, causing catastrophic damages to aircraft and ground equipment. For military and many civilian pavement applications, the California Bearing Ratio (CBR) value is an index used to evaluate subgrade soil strength. The CBR is the ratio of the resistance to penetration developed by a subgrade soil to that developed by a specimen of standard crushed stone base material. Currently, the Dynamic Cone Penetrometer (DCP) is one of the most widely used methods to estimate field subgrade CBR values. The results obtained by CBR tests are traditionally used with empirical curves to determine the required thickness of pavement and its component layers. However, current Mechanistic-Empirical (M-E) pavement design guidelines used to design civilian roadways and airport pavements recognize that elastic pavement design is not governed solely by the strength of the unbound pavement materials, but by the stiffness of these materials under small deflections. Stiffness describes the tendency of a soil to deform under various loading conditions. When stiffness is evaluated in terms of stress and strain, over small strain levels and under specific cyclic loading conditions, it is given as a resilient modulus, MR. The resilient modulus values are typically obtained from CBR values by using various empirical relationships. However, it must be remembered that the CBR values are estimated using DCP values. Consequently, the error from the individual empirical equations are compounded when combined into a final equation. Geophysical measurements, such as shear wave velocity, provide a means to obtain a modulus value directly. Thus, eliminating the need for a multistep process. The geophysical properties of a subgrade soil system are directly related to parameters such as soil type, pore structure, degree of saturation, stress history and stress state. These parameters are also directly related to the strength and deformation (i.e. stiffness) behavior of the soil system. Thus, there is a high likelihood that geophysical measurements in soils will provide a reliable means to evaluate and predict engineering behavior in subgrade soils. In addition, to providing a more intrinsically natural (as opposed to a purely empirical) relationship for modulus, geophysical techniques are readily adapted to remote sensing platforms. This paper presents the results of a preliminary effort to use geophysical measurements such as shear wave velocity to calculate pavement design parameters. The shear modulus and the modulus of elasticity are determined directly from the measured shear wave velocity using fundamental principles of elastic theory and the theory of wave propagation through an elastic material. The moduli can then be used to calculate pavement design parameters such as pavement thickness, allowable load, and the number of passes. These pavement design parameters will be directly related to shear wave velocity and will be obtained through a much simpler process than CBR.