Analytical study on exit gradient at base aquitard of deep excavations under dynamic artesian water

Based on one - dimensional leaky model for base aquitard of deep excavations, analytical solutions for the excess pore water pressure induced by variation of artesian water head and the corresponding exit gradients in the base aquitard are derived. If the value of permeability coefficient of the base soil is large enough, the calculated results of the exit gradient using the suggested method agree with the traditional ones, thus the validity of the suggested method is verified. According to the analytic solutions for the exit gradient, the variation laws and influence factors of the exit gradient in excavations under dynamic artesian water are analyzed. The results show when the thickness of the excavation base aquitard is certain, the greater the permeability coefficient, the compressibility modulus or the variation period of artesian water head of the soil, the more significantly the exit gradient fluctuated, the less its amplitude attenuated and phase shifted. The influence factor of the exit gradient is described by the dimensionless factor, which is positively correlated with the permeability coefficient and the compressibility modulus of the soil, but negatively correlated with the angular frequency of variation of artesian water head and the squared thickness of the aquitard. Finally, a practical project indicates that the lower permeability coefficient in the base aquitard leads to unapparent amplitude damping and significant phase lagging of the exit gradient induced by the artesian water head variation, which has detrimental effect on the seepage stability of excavation projects.