Investigation of Pore Pressure Migration During Piezocone Tests

压电锥耗散试验是一种用仪器测得的锥通过土柱的方法，在岩土工程、环境工程和其他与地下水有关的领域都有应用。该测试通常用于表征水通过土壤渗透的容易程度（即土壤渗透性），这对于工程分析是必要的，例如预测土壤固结沉降率或预测地下水渗漏引起的污染物运输。然而，由于几个主要的简化假设，解释这些测试的方法是有限的。该测试通过将锥钻穿过土柱，测量锥肩处的孔隙水压力，然后暂停钻进，记录孔隙水压力的变化情况。在饱和土中，穿透锥周围土体的变形引起了超孔隙水压力场。然后，当渗透暂停时，随着时间的推移，当它们在消散曲线中消散到静水条件时，会记录下多余的孔隙水压力。利用耗散曲线解释土壤渗透性的现有方法假设：(i)超孔隙水压力是水平迁移的，（ii）实测孔隙水压力在静水条件下的衰减是单调的。然而，有一个重要的非单调压锥耗散试验数据集，其中测量的超孔隙水压力最初增加，然后降低到静水条件，这可能是由于垂直超孔隙水压力迁移。本项目将研究和表征非单调压电锥耗散试验的原因，从而改进压电锥耗散解释方法，并从试验数据中更准确地表征土壤渗透率。该项目还包括在K-12工程推广活动中进行演示，以增加俄勒冈州波特兰市小学生对土木工程概念的参与。本项目的目的是表征的机制，有助于非单调的压电锥耗散曲线与数值压电锥模型。本研究假设垂直和水平方向的超孔隙水压力迁移有助于形成非单调的压锥耗散曲线。此外，还假设这些贡献受土壤应力历史（即超固结比）、土壤性质、水力导率各向异性和渗透排水条件的影响。这些假设将通过使用直接轴对称渗透模型和MIT-S1本构模型对饱和粘土中压电锥耗散的数值模拟进行验证。模拟将研究：(i)土壤应力历史和土壤性质对渗透锥周围超孔隙水压力分布和后续耗散曲线的作用；（ii）垂直和水平孔隙水压力迁移对耗散曲线的作用；（iii）水力导度各向异性对垂直和水平孔隙水压力迁移的影响；（iv）渗透过程中部分排水条件对上述因素的影响。这些分析结果将综合起来，提出非单调耗散试验的解释方法，并重新解释现有的非单调耗散试验。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.