Question

The article “Mechanistic-Empirical Design of Bituminous Roads: An Indian Perspective” presents an equation of the form y=a(1/x1)b(1/x2)cy=a\left(1 / x_{1}\right)^{b}\left(1 / x_{2}\right)^{c} for predicting the number of repetitions for laboratory fatigue failure (y) in terms of the tensile strain at the bottom of the bituminous beam (x1)(x_1) and the resilient modulus (x2).(x_2). Transform this equation into a linear model, and express the linear model coefficients in terms of a, b, and c.

Solution

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Given:

y=a(1x1)b(1x2)cy=a\left(\dfrac{1}{x_1}\right)^b \left(\dfrac{1}{x_2}\right)^c

Rewrite the model as the product of powers of xix_i:

y=ax1bx2cy=ax_1^{-b}x_2^{-c}

Take the natural logarithm of each side of the previous equation:

lny=ln(ax1bx2c)\ln y = \ln (a x_1^{-b}x_2^{-c})

The logarithm of a product is the sum of the logarithms:

lny=lna+lnx1b+lnx2c\ln y = \ln a+\ln x_1^{-b}+\ln x_2^{-c}

The power of a value in a logarithm can be moved to right before the logarithm (power property of logarithms).

lny=lnablnx1clnx2\ln y = \ln a-b\ln x_1-c\ln x_2

Finally, we include a possible error ϵ\epsilon which can occur in any predicted model:

lny=lnablnx1clnx2+ϵ\ln y = \ln a-b\ln x_1-c\ln x_2 +\epsilon

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