Investigation of Pore Pressure Migration During Piezocone Tests

Portland State University, Portland OR

Investigators

Abstract

The piezocone dissipation test, where an instrumented cone is advanced through the soil column, is used throughout geotechnical engineering, environmental engineering, and other groundwater-related fields. The test is routinely used to characterize how easily water seeps through soil (i.e. soil permeability) which is necessary for engineering analyses such as predicting rate of soil consolidation settlements or predicting contaminant transport by groundwater seepage. However, methods for interpreting these tests are limited due to several major simplifying assumptions. The test is performed by advancing the cone through the soil column and measuring porewater pressures at the cone shoulder, then pausing penetration and recording how porewater pressures change. Soil deformations around the penetrating cone induce an excess porewater pressure field in saturated soil. Then when penetration is paused, excess porewater pressures are recorded over time as they dissipate to hydrostatic conditions in a dissipation curve. Available methods to interpret soil permeability from dissipation curves assume that: (i) excess porewater pressure migration is horizontal, and (ii) measured porewater pressure decay to hydrostatic conditions is monotonic. However, there is a significant dataset of non-monotonic piezocone dissipation tests, where measured excess porewater pressure initially increases then decreases to hydrostatic conditions, which may be due to vertical excess porewater pressure migration. This project will investigate and characterize the causes of non-monotonic piezocone dissipation tests, which will lead to improved piezocone dissipation interpretation methods and more accurate characterization of soil permeability from the test data. The project also includes demonstrations at K-12 engineering outreach events to increase the engagement of Portland, Oregon grade school students with civil engineering concepts. The objective of this project is to characterize the mechanisms that contribute to non-monotonic piezocone dissipation curves with a numerical piezocone model. This project hypothesizes that vertical and horizontal excess porewater pressure migration contribute to non-monotonic piezocone dissipation curves. It is also hypothesized that those contributions are affected by soil stress history (i.e. over-consolidation ratio), soil properties, hydraulic conductivity anisotropy, and penetration drainage conditions. These hypotheses will be tested with numerical simulations of piezocone dissipation in saturated clay using a direct axisymmetric penetration model and the MIT-S1 constitutive model. The simulations will investigate: (i) the role of soil stress history and soil properties on excess porewater pressure distribution around the penetrating cone and subsequent dissipation curves, (ii) the role of vertical and horizontal porewater pressure migration on dissipation curves, (iii) the impact of hydraulic conductivity anisotropy on vertical and horizontal porewater pressure migration, and (iv) how the above factors are affected by partial drainage conditions during penetration. The results of these analyses will be synthesized to suggest interpretation methods for non-monotonic dissipation tests and to re-interpret existing non-monotonic dissipation tests. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.