Static and cyclic properties of expansive clays treated with lime and fly ash with special reference to swelling and resilient moduli

Soils with high plasticity and high swell potential have great volume changes due to water content fluctuations in an unstable manner. As a result of this expansion, the pavements and/or lightweight structures, which are settled on that kind of subsoils, are damaged. Expansive soils can be improved by adding chemical stabilizers such as lime, fly ash, micro cement, etc. The resilient modulus value, on the other hand, provides a basic relation between the stress and strain of the material in order to perform the structural analysis of the layered pavement systems under cyclic conditions. Lime and fly ash are the most popular agents used in chemical stabilization method. However, it is not very clear whether these agents are effective enough or not effective at all in very highly or extremely highly plastic clays. Moreover, swell and shrinkage characteristics of the lime and fly ash stabilized soils are not well-known when compared to the strength behavior of the treated soils. Another not well-known aspect of the subject is cyclic loading properties of the native expansive soils and stabilized soil-agent mixtures. It is believed that the resilient modulus properties are needed to be investigated in this context. Finally, the effect of mineralogical composition of the clay soils and chemical components of the agents are believed to be investigated as far as the swelling is concerned. The clays and the lime and fly ash agents used in this research are identified by means of Rietveld, XRD and XRF tests. SEM analyes have also been performed. In this research, static and cyclic loading properties of the two natural clays, which are extremely highly and very highly plastic expansive soils, are investigated by utilizing lime and fly ash agents for stabilization. Lime has been considered as chemically active additive. Fly ash, on the other hand, is considered as filler material and its experimental results can be accepted as reference values to compare them with the results of lime. Static research has been executed with special reference to swelling behavior. Resilient modulus properties have been examined under variable stress states within the scope of dynamic research. Two different soil samples, which are taken from different sites of Ankara, have been investigated. They are namely, Esenboğa and Bilkent Clays. Esenboğa Clay is an extremely highly plastic clay (PI=101%). Bilkent Clay is a very highly plastic clay (PI=76%). Both disturbed and undisturbed soil samples have been taken from both sites. Firstly, reference tests (index etc.) have been carried out on untreated natural soil samples. Then, both clays are mixed with the lime and fly ash agents separately at different percentages of dry weight of the soil (i.e. 1%, 3%, 5%, 7% and 9% for lime, and 5%, 10%, 15%, 20% and 25% for fly ash). Methylene blue tests have been performed on untreated and lime-treated Esenboğa and Bilkent Clays for each percentage. The oedometer tests have been performed to determine the swelling and compressibility characteristics of the soil samples. The unconfined compression tests have been executed for both non-soaked and soaked conditions. These static tests have also been performed for Esenboğa Clay with lime agent at each percentages prepared for 7, 28 and 56-days curing periods. The Esenboğa and Bilkent Clays, which are mixed with the lime agent at different percentages of dry weight of the soil (1%, 3%, 5%, 7% and 9%), have been tested for the determination of resilient modulus values. Resilient moduli have also been found for soil samples prepared for 7, 28, 56 and 90-days curing periods in each lime percentages. SEM analyses have been performed for 7% lime-treated Esenboğa Clay and also for natural Esenboğa and Bilkent Clays. Optimum values have been evaluated for swell potential and compressive strength. The correlations between the agent percentages and index properties, the swell percentage, the swell pressure, time effect, compressive strength and resilient modulus are presented. As a result of mineralogic analyses of the soil samples, it is understood that the Esenboğa Clay is composed of mainly illite (about 87%). However, it is determined that the amount of smectite minerals are not negligible (about 4%). It is interesting to note, on the other hand, that the Bilkent Clay is completely composed of palygorskite (attapulgite) mineral. The optimum lime content is about 3% as far as the swelling is concerned in Esenboğa Clay. In a design of sub-base of surficial structures, this optimum lime content can be taken as a reference. Swell tests should be executed with this reference value. In case of not satisfying the design criteria, this optimum lime content should be increased. Laboratory swell percentage tests show that about 8% lime content for Esenboğa Clay may just be sufficient for design criteria according to Highway Standards in Türkiye. Laboratory tests show, on the other hand, that the fly ash agent may not be sufficient alone for swelling design criteria in such type of extremely and/or very highly plastic clays. Lime agent effect on swelling behavior, therefore, is much more effective than the effect of fly ash agent. Swell pressures are about 6.5 times greater in fly ash improvement with 20 to 25% amounts than the pressures found in lime improvement with 7 to 9% amounts in Esenboğa Clay. When the variation of unconfined compressive strength with lime content for non-soaked and soaked conditions is considered without curing, it is noticed that the unconfined compressive strengths are greater in non-soaked condition up to 5% lime content. Starting from 7% however, the strengths are greater in soaked condition due to self-hardening mechanism which can be called also as pozzolanic reactions. Time effect of lime stabilization in the extremely highly plastic Esenboğa Clay has been investigated in this research. For the curing period of 28 days, 5% lime content is the optimum content at which the swell percentage is decreased to null in Esenboğa Clay. This result is also valid in swell pressure tests. Soaked and non-soaked samples have been tested to get unconfined compressive strengths in Esenboğa Clay with different curing periods. It has been concluded that the unconfined compressive strengths increase dramatically as compared to non-cured strengths for all cure periods especially for non-soaked samples. 28-day curing period can be accepted as optimum. Unconfined compressive strengths of soaked samples are always lower than the ones in non-soaked condition in all lime contents for curing periods of 7, 28 and 56 days. The optimum lime and fly ash contents for Bilkent Clay are about 3% and 10% respectively as far as the swelling is concerned. Since lime is more effective than the fly ash, it is highly probable that the design criteria are to be satisfied at reasonable lime contents. Laboratory swell percentage tests show that 9% to 10% lime content for Bilkent Clay may be sufficient in a transportation sub-structure design as far as the swelling criteria are concerned. Laboratory tests show that the fly ash agent may not be sufficient alone for swelling design criteria in such type of very highly plastic clays. The effectiveness of lime agent on swell behavior can also be observed in swell pressure tests in Bilkent Clay. Swell pressures are about 2.7 times greater in fly ash improvement with 20 to 25% amounts than the pressures found in lime improvement with 7% to 9% amounts in Bilkent Clay. The methylene blue test results obtained are compatible with the results mentioned above. That is to say, the swell pressure and the swell percentages along with the methylene blue (MBV) and activity values decrease with the increase in lime content. When the variation in the mean resilient modulus values are considered for the extremely highly plastic Esenboğa Clay, 3% lime content seems quite effective for both non-cured and cured samples. 56-days curing period gives maximum mean resilient modulus values in this clay. In Bilkent Clay which is very highly plastic, 3% lime content can be accepted effective at early stages (i.e. at non cured and 7 days cured conditions) as far as the mean resilient modulus values are concerned. However, 7% lime content gives generally maximum increases in resilient modulus values at later cure periods of 28, 56 and 90 days. 90-days curing period gives always maximum resilient modulus value in the tests for Bilkent Clay. It should be noticed here that the Bilkent Clay is composed of 100% palygorskite clay mineral which may influence the results. The results achieved in this research show that lime agent alone can be utilized quite effectively in the extremely highly and/or very highly plastic expansive clays obtained from two different construction sites in Ankara, especially on the swelling properties. However, this result is not valid for the agent fly ash which is an industrial waste. It is recommended to use fly ash with a calcium-based agent like lime or cement to improve such type of very highly plastic expansive clays in terms of swelling criteria since utilizing fly ash especially is quite advantageous from an economical and environmental points of view.