Reactions between lime, alumina released from clay during pozzolanic reactions, and sulfates present in some soils (causing the formation of the highly expansive crystalline mineral ettringite) have been responsible for the deterioration and ultimate failure, by expansion, of several lime stabilization projects. The mechanisms for these reactions were hypothesized, and a laboratory research program using lime-treated artificial soil samples of compacted kaolinite-sand and montmorillonite-sand mixtures, incorporating known amounts of sulfates, was designed and undertaken. The strength, swelling, compositional, and micromorphological characteristics of the treated samples were determined following different curing times and soaking conditions. Ettringite formation was evidenced in all lime-treated samples, whenever sulfates were present. A non-expansive monosulfate calcium-aluminum-hydrate forms first in the high alumina content lime-treated kaolinite-sand mixes. The monosulfate converts to an expansive trisulfate form (ettringite), after a period of a few months. Conversely, ettringite starts to form at the early curing stages (after a few days) in the low alumina lime-treated montmorillonite-sand mixes. It was found that the amount of heave following ettringite hydration and growth is a function of the amount and rate of release of alumina into solution. The amount and type of sulfates present is also a factor influencing the quantity and crystal morphology of ettringite formed. Moreover, temperature and humidity fluctuations were also found to play an important role in the overall ettringite-related heave mechanism, as they affect reaction rates, solubilities of species, and the overall stability fields of a soil system's components. Continuing research will extend the results reported here to a more quantitative specification of conditions leading to deleterious reactions in lime stabilization applications. Concurrently, treatment methods that may prevent such adverse reactions are being explored.

Experimental studies on poorly draining soil-reinforcement interactions were reviewed in a companion paper by Zornberg and Mitchell in 1994, leading to the conclusion that permeable geosynthetic inclusions are useful for reinforcing marginal backfills. This conclusion is strengthened by lessons learned from the case histories described in this paper. There are no design guidelines for reinforced soil structures using poorly draining backfills. Nevertheless, several of these structures have already been constructed, and the performance of some of them has been reported. Good structure performance is strongly dependent on maintaining a low water content in the poorly draining fill. Large movements occurred in reinforced structures when pore water pressures were generated, and failures were reported in marginal backfills reinforced with impermeable inclusions that became saturated after rainfalls. Benefits and applications of reinforcing poorly draining backfills are addressed, and research needs aimed at formulating a consistent design methodology for these structures are presented.

In this and a companion paper (Mitchell and Zornberg 1994), the use and performance of reinforced soil structures constructed with poorly draining and/or cohesive backfills is evaluated. This evaluation shows that proper design and construction can result in stable, durable, and economical earth structures. Permeable reinforcements may be especially useful for soil structures with poorly draining backfills because the drainage capabilities of the geosynthetic can dissipate excess pore water pressures, thus enhancing stability. Consequently, the design of a safe and economical structure should address two aspects specific to poorly draining backfills: the cohesive soil-reinforcement interaction and the reinforcement drainage characteristics. The present paper focuses on experimental and analytical studies undertaken to evaluate these issues. Tensile strength, durability, and creep response of geosynthetics embedded in marginal soils are also addressed. There is strong experimental evidence that permeable inclusions can effectively reinforce clay structures.