Date of Award

August 2019

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

Advisor(s)

Shobha K. Bhatia

Keywords

centrifuge test, dewatering, empirical model, geotextile, large scale, performance

Subject Categories

Engineering

Abstract

Over the past two decades, geotextile tube dewatering technology has been widely used to handle high water content waste materials in a more sustainable and economical way. The function of a geotextiles tube in a dewatering project is to retain the solid particles inside the tube and release the effluent from the tube through the pores in its surface. Several small-scale (Pressure Filtration Test – PFT, Falling Head Test – FHT) and medium-scale (Geotextile Tube Dewatering Test – GDT, Hanging Bag Test – HBT) performance tests are available to predict the results of the geotextile tube dewatering process in the field. Often, an assessment of dewatering performance is based on four criteria: namely, solid retention, effluent quality, volume reduction, and dewatering time. Existing small scale tests can be used to evaluate the solid retention, volume reduction, and effluent quality of the dewatered slurry based on the axial flow. Compared to small-scale tests, large-scale tests, which are based on three-directional flow, can provide more accurate results on solid retention, effluent quality, and volume reduction. However, a large amount of slurry and time are major drawbacks with these tests. Further, all of these tests provide limited useful information about the dewatering time.

A recently developed Pressurized Two-Directional Dewatering Test (P2DT) and its associated empirical model address the challenge of assessing dewatering performance based on aforementioned four criteria, while also eliminating the large slurry requirement. A previous study verified the applicability of the P2DT and its empirical model for Tully Sand and an unusual industrial waste product. The main objectives of this study are to: 1) modify the pressure application mechanism in existing small-scale laboratory tests (PFT and P2DT) and compare their performance, 2) study the factors affecting the compressibility of slurries using the centrifuge test, 3) evaluate the pertinence of the empirical model for a variety of sediments, and 4) develop a framework to predict the dewatering performance of large-scale geotextile tubes.

To achieve these objectives, 120 centrifuge tests, and nearly 30 PFT and P2DT were performed using five different sediments, including two natural soils and three coal ash sediments. A common, monofilament woven geotextile was used for the dewatering tests. The study evaluated the effect of direct air pressure vs bladder system in the PFT and constant vs variable pressure in the P2DT using Tully Sand slurry. A pressure application mechanism was found to have an influence on the dewatering outcomes like effluent turbidity and final solids concentration in the PFT. Compared to a bladder system, effluent turbidity increased by 40% and the solids concentration of the filter cake increased by about 4% when direct air pressure was applied in PFT. In the P2DT, variable pressure application improved the correlation (R2 = 98.5%) between P2DT results and the empirical model at a lower pressure; however, the pressure application mechanism did not affect the dewatering outcomes like effluent turbidity and final solids concentration of the filter cake.

In spite of the different dewatering and compression mechanisms of slurries, the centrifuge test was able to predict the solids concentration of Tully Sand slurry and two other materials within 5% – 10% of PFT and P2DT results. Centrifuge test results were used to identify the predominant factors affecting the compressibility of different slurries. The initial solids concentration did not affect the final percent of solids in both unconditioned and conditioned slurries of natural soils and coal ash sediments. Based on the centrifuge test results for 21 dredged sediments and soils, the solids concentration of unconditioned slurries was found to increase with the specific gravity within a band of 2.1 – 2.9. For flocculated slurries, in addition to specific gravity, the floc quality affected the maximum solids concentration. Because of its higher resistance to compression, flocculated slurries achieved a solids concentration 5% – 15% lower than that of unflocculated slurries.

Using an empirical model with dewatering parameters predicted from the P2DT, the solids concentrations were estimated within 8% for the Tully Sand slurry, which was dewatered using a geotextile tube sized 1.5 m in diameter and 1.8 m in length.

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Open Access

Available for download on Thursday, September 16, 2021

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Engineering Commons

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