Effect of Frequency and Vertical Stress on Cyclic Degradation and Pore Water Pressure in Clay in the NGI Simple Shear Device

The effects of the frequency of cyclic shearing and vertical consolidation stress on the cyclic degradation and cyclic pore water pressure in normally-consolidated kaolinite clay having $\text{PI}=28$ were investigated with the help of the cyclic strain-controlled Norwegian Geotechnical Institute (NGI) simple shear test. The testing program encompassed three cyclic shear strain amplitudes, ${\gamma }_c=0.1,0.25,\text{and}\hspace{0.17em}0.5\%$, two vertical effective consolidation stresses, ${\sigma }_{\mathit{vc}}^{\prime }=220\hspace{0.17em}\text{and}\hspace{0.17em}680\hspace{0.17em}\text{kPa}$, and three frequencies, $f=0.001,0.01,\text{and}\hspace{0.17em}0.1\hspace{0.17em}\text{Hz}$. The results reveal that the cyclic degradation parameter, $t$, which measures the rate of cyclic degradation with the number of cycles, increases substantially with $f$ and decreases with ${\sigma }_{\mathit{vc}}^{\prime }$. If $f$ is increased 10 times, $t$ may increase by 20–50%. If ${\sigma }_{\mathit{vc}}^{\prime }$ is increased from 220 to 680 kPa, the $t$ parameter may decrease by 20–38%. The cyclic pore water pressure normalized by ${\sigma }_{\mathit{vc}}^{\prime }$ decreases with $f$ and ${\sigma }_{\mathit{vc}}^{\prime }$.