Date of Thesis

Summer 2018

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Description

Soil-bentonite (SB) cutoff walls are commonly employed in the US to control groundwater flow and subsurface contaminant migration. In these applications, both the short-term (as built) integrity of the barrier and the potential for degradation in the integrity of the barrier over time are of critical importance. Although many laboratory studies have been conducted to investigate the performance of SB cutoff walls as hydraulic barriers and the various factors affecting this performance, field investigations are scarce. With support from the National Science Foundation, a 200 m long, 7 m deep, and 0.9 m wide SB cutoff wall has been designed, constructed, and instrumented to serve as a long-term field research site for investigating the in-situ properties and behavior of SB backfill.

This thesis study investigates the properties of this full scale SB cutoff wall, focusing on the hydraulic conductivity (k) of the wall based on backfill samples collected during construction, undisturbed samples collected after construction, and in-situ k measurements performed in the two years following construction. In-situ consolidation and stress measurements are presented along with groundwater monitoring data showing the performance of the wall as a hydraulic barrier for the time period from wall construction up to 20 months after construction.

SB backfill k was measured using flexible-wall tests (70-mm diameter) on remolded specimens prepared from surface grab samples collected during construction; flexible-wall tests on undisturbed specimens collected from the wall after construction; small rigid-wall (oedometer) tests (64-mm diameter) on remolded samples prepared from grab samples and undisturbed samples; larger-scale rigid-wall tests (150-mm diameter) on remolded specimens prepared from grab samples; and slug tests conducted within the wall at various locations and depths. Applied effective stresses in the laboratory tests primarily ranged from 1-35 kPa, encompassing the range of in-situ stresses (7-15 kPa) measured in the wall backfill after load transfer and consolidation occurred.

The results indicate low spatial variability in k for a given test type, consistent with the observed homogeneity of the backfill. Modest variability in k was observed among the different test types, with the slug tests and large rigid-wall tests generally yielding slightly higher k relative to the flexible-wall and small rigid-wall tests at representative stress (i.e., measured field effective stress compared to similar laboratory effective stress). Measuring in-situ k with the single well slug test is shown to be a viable method that is capable of assessing the longevity in performance of the wall. Several methods of interpreting slug test data are compared and analyzed for practical use.

The results also indicate that the range of in-situ k measurements is slightly lower than the range of k for remolded flexible-wall specimens tested at similar effective stresses to those observed in the wall. Variability of the in-situ k was greater than variability in k of the remolded or undisturbed specimens tested in the laboratory. In-situ measurements indicate that k decreases with depth, the unsaturated region in the upper wall has the highest k and variability of k. Adjacent monitoring well readings show high performance of the wall in defect free portions with a solid key, while areas with known defects and a poor key show little difference in the water level across the wall.

Keywords

hydraulic conductivity, soil-bentonite, cutoff wall, slurry, slug test, bentonite

Access Type

Masters Thesis

Degree Type

Master of Science in Civil Engineering

Major

Civil Engineering

First Advisor

Michael A. Malusis

Second Advisor

Jeffrey C. Evans

Third Advisor

Mary Beth Gray

Comments

NSF funded research investigating the properties and behavior of a full-scale soil-bentonite cutoff wall.

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