Date of Award
Open Access Dissertation
Civil and Environmental Engineering
Charles E. Pierce
Gypsum, which is dihydrate calcium sulfate, (CaSO4.2H2O), is widely available in different sizes, from the size of a rock to the size of a few micrometers as a mineral, in different types of soils. It is primarily found mainly in arid and semi-arid areas around the world at different depths. Soils with gypsum and even gypsum rocks are very hard in their dry state. However, these soils and rock will experience remarkable dissolution upon wetting.
The dissolution phenomenon, which takes place in soils that contain gypsum, creates different geotechnical problems within the soil’s profile, along with foundation issues for structures that have been constructed on this type of soil. Noticeable structural damage and collapses, which occurred in heavy and light structures, such as earthen dams, power plants, houses, and even roads, were related to the subsidence of soil with gypsum due to gypsum dissolution. The behavior that was exhibited by this problematic soil has captured the attention of civil engineers and scientists since the first quarter of the 20th century.
Many studies were conducted in different types of soils that contain gypsum within different soil profile depths to investigate the effects of this mineral on the properties of these soils. Those studies concluded that there are many factors that control the solubility of gypsum within the soil profile.
Studies that have been presented in this dissertation focused on the effects of the most important factors on the dissolution of gypsum, which are gypsum content, wetting- drying cycles, static and moving water. As a result, on soil stability, mass loss, porosity, and soil permeability were investigated.
Various tests and treatments were performed on two types of soils to study these effects. These soils are poorly graded sandy soils with different gypsum content that were brought from New Mexico. The first soil consisted of 93% gypsum (high gypsum soil), while the second soil consisted of 31% gypsum (medium gypsum soil).
First, these soils were classified, and then different types of hydration methods were used to measure the gypsum content. An evaluation of one of the dehydration method was conducted to select the best approach to measuring the gypsum content for different soil types, regardless of the size of the gypsum particles. Then, a measurement method was selected to determine the dissolved gypsum in the water and convert this measurement into a gypsum mass.
Three different additives (activated fly ash, asphalt emulsion, and Portland cement) were evaluated to select the appropriate additive that would enhance the mechanical properties of these soils. The selection of these additives was based on two criteria. First, additive must be inexpensive and available. Second, the impact of the additive on the environment and its ability to treat soil with gypsum was also considered.
Based on these criteria, two additives were chosen: fly ash and asphalt. Fly ash is a waste material that is produced after the combustion of coal in power plants.
Asphalt waste is a byproduct or sometimes is found in waste material that is produced from refining crude oil.
However, since asphalt binder requires high temperature in order to be melted and mixed with soil, and, considering safety concern and ease of use, a decision was made to use an anionic asphalt emulsion instead. A third option was to use Portland cement type I/II as a reference additive.
The studies were shown in four different objectives in Chapters 4, 5, 6, and 7.
Chapter 4 discusses the activated fly ash treatment, in which 12 M of KOH was used to activate Class F fly ash to treat both soils. 10%, 20%, and 30% doses of activated fly ash were used in the treatment with curing periods of 7 and 28 days. Then, soil specimens went through 12 cycles of wetting and drying. The results showed that this treatment enhanced the soil stability and reduced the dissolution of gypsum. However, this treatment did not prevent or mitigate mass loss.
The second objective is discussed in Chapter 5. This chapter discusses the effects of static water on the dissolution of gypsum, and its results on soil stability, mass loss, and porosity for specimens that were treated with two different additives: anionic asphalt emulsion (6%, 12%, and 18%), and type I/II Portland Cement (9%). The results showed that the use of asphalt emulsions significantly improved soil stability, reduced gypsum dissolution and porosity, and mitigated mass loss when compared to the use of cement.
Chapter 6 discussed the effect of moving water on gypsum dissolution and soil permeability for both soils in three different states: soil in its natural condition, soil that was treated with 6% asphalt emulsion, and soil that was treated with 18% asphalt emulsion. Two different approaches were used to prepare and mix the asphalt emulsion with the soils. The first approach was conducted by mixing the soils with 6% asphalt emulsion, curing them, and then compacting them in constant head permeability cells.
The second approach was performed by mixing the soils with 18% asphalt emulsion, compacting them in PVC molds, and then curing them before placing them in flexible wall permeability cells.
The results showed that moving water had significant effect on the dissolution of gypsum, particularly for high gypsum soil. Moreover, the second treatment approach had a significant impact on soil permeability by reducing it to a state where water no longer flowed through the high gypsum soil. This approach also led to a reduction in gypsum dissolution for both soils.
The unconfined compressive strength for different specimens treated with activated fly ash, asphalt emulsion, and Portland cement were measured and compared with each other, as described in Chapter 7. Although the results suggested that the highest strength was achieved by treating both soils with 9% Portland cement, this option is not preferable due the sulfate attack that occurred in the soil upon wetting due to the presence of gypsum. The use of asphalt emulsion remained the best and preferred treatment.
Finally, Chapter 8 provides the conclusions and the recommendations, based on the results that were provided. that the results of these studies suggested. The outcome of the studies that have been presented in this dissertation show that gypsum content has a significant impact on soil stability.
Moreover, moving water contributes to the deterioration of these soils by increasing the dissolution of gypsum, which suggest that and preventing direct contact between the water and the gypsum particles will enhance soil stability. The effects of wetting-drying cycles can be mitigated by using asphalt emulsions or asphalt binders. The use of asphalt emulsions enhances different properties for soil with gypsum.
Faris, M. K.(2020). Evaluating the Stabilization of Gypseous and Gypsiferous Sands Using Different Chemical Additives to Mitigate Gypsum Dissolution. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/6140