Killi zeminlerin şişme davranışına ön yüklemenin etkisi

Abstract

Dünyanın birçok bölgesinde, özellikle de kurak ve yarı-kurak alanlarda şişen zeminlere sıkça rastlanmaktadır. Genellikle aktif kil mineralleri ihtiva eden bu zeminlerin kapiler yoldan su alarak büyük hacim değişikliklerine uğramakta, bilhassa, yol, havaalanı gibi hafif yapılarda büyük hasarlar meydana getirmektedirler. Bu tür zeminlerin şişme davranışlarının ve bunlara etkiyen faktörlerin önceden belirlenmesi ile meydana gelebilecek zararlar azaltılabilmekte ya da tamamen önlenebilmektedir. Bu çalışmada, zeminlerin şişme potansiyelini etkile yen başlıca faktörler arasında yer alan önceden yüklemenin dolayısıyla aşırı konsolidasyon oranının şişme davranışı üzerindeki etkisi araştırılmıştır. Laboratuvarda hazırlanmış aktif kil minerali içeren zemin numuneleri üzerinde ödometre aletiyle, sabit hacim şartları altın da şişme basınçları belirlenmiştir. Daha sonra, aynı zemin numuneleri farklı ön yük değerlerinde konsolide edilmiş ve şişme basınçları tesbit edilmiştir. Deneyler sonucunda elde edilen grafikler de aşırı konsolidasyon oranları ile şişme basıncı arasındaki ilişki tartışılmıştır.

Expansive soils are found in many parts of the world, especially in semi -arid areas. An expansive soils is generally unsaturated due to desiccation. Expansive soils also generally contain clay minerals that exhibit high volume change upon wetting. The large volume change upon wetting causes extensive damage to structures, in particular light buildings and pavements. In the United States alone, the damage caused by shrinking and swelling soils amounts to about 9 billion dollars per year, which is greater than the combined damages from natural disasters such as floods, hurricanes, earthquakes and tornadoes. Therefore, the problems associated with swelling soils are of enormous finacial proportion. Foundation heave has often occured as a result of the changes in the water content of the soil. These changes can originate from the environment or from man-made causes Non-uniform changes in water content will result in differential heaves which can cause severe damage to the structure. predicting the future matric suction in the field specific environmental condition. The swelling of soils in general is caused by the presence of expanding clay minerals, hydration of cations on clay surfaces and release on intrinsic stresses caused by over-consolidation or dessication of soils. The swelling soils responsible for structural and road damages are mostly shales which do not contain expandible minerals. Thus factors other than mineralogy play an important role in the formation of expansive soils. The x shales of the region are a very dry and intact condition due to hot and arid climate, having in situ water contents even lower than their shrinkage limits. This result in enormously high water intake potential upon. The infiltration of water as a result of land occupation, destroyed leading to considerable heave movements. Reliable prediction of in situ heave is a prequisite in developing more effective and economical design of structures on expansive soils. The choice of stabilization procedures or soil treatment technique for the purpose of minimizing the effect of soil volume change on the integrity of structure may also be guided by the magnitude of predicted heave. A preliminary step in investigation associated with expansive soils is qualitative characterization or classification of potential volume change. The qualitative characterization serves the purpose of warning of potential problems. In the identification methods, consistency limits and shrinkage properties are taken as the basis for the swell classification. The magnitude of soil suction is also considered as an important factor controlling the swell. However, include parameters that reflect the in situ soil conditions such as natural dry density, soil fabric and stress conditions. Thus, a meaningful estimation of field behaviour can not be obtained by the classification system other than possible existence of problematic swelling soil conditions. Once potentially expansive soil is identified and a qualitative indication of swell is made, quantitative characterization of swelling soil is performed to estimate the amount of anticipated volume change. Techniques available for this purpose include, - Oedometer method - Empiricial relationship - Suction analysis Initial moisture content and initial dry density and other environmental conditions are extremely important in determining the amount of swell. The following factors influence the results obtained in loaded swell tests on soils of any mineralogical composition: - Initial moisture content: Expansive soils are not subject to volume change unless there is an increase in moisture content. A drier soil swells more than a wet soil. For remolded samples, it is obvious that the initial moisture content controls the volume change. Field condition and construction specifications dictate the moisture requirement. xi - Initial dry density: The single most important factor affecting swelling characteristics of swelling soils is density. In remolded tests, the initial compaction condition is critical. Swell tests may decide the degree of compaction required in the placement of fill. Since moisture content and dry density are closely related. - Surcharge pressure: A small surcharge load in the range of 2.5 to 7 kN/nruz has been suggested for seating load in the swell test. Since swell is very sensitive to change in pressure in the lower ranges of pressure, the use of low surcharge pressure may lead to erratic and erroneous test results. - Time allowed for swell: The time required for the soil to reach its maximum swell potential may very considerably depending essentially on the initial density, permeability, and the thickness of the sample. For remolded samples, generally 24 hours is sufficient to obtain 95 percent of the total available swell. At the same time, for undisturbed high density clay shale, it may require several days or-even a week before complete saturation can be achieved. For remolded samples, the initial added water must be evenly distributed. This requires a minimum curing time of six hours for reproducible results. - Size and thickness: Sample thickness affects the time required for total saturation Great thickness may introduce excessive side friction. At the same time, in a cotting from an undisturbed sample, a small thickness may introduce surface disturbance and exclude the possible effect of granular particles, fissures and seams in the soil. A reliable and reproducible test which is to be considered as a basis for the classification of potential expansive soil must be standardized at least for the following environmental conditions: For undisturbed sample: - Surcharge pressure - Size and thickness - Time required for the test - Initial moisture content and density must be qualified For remolded sample: - Initial moisture content - Initial dry density - Method of compaction - Surcharge pressure - Size and thickness - Time required for the test XII - Curing time allowed The amount of total heave and the rate of heave on which a structure is founded are very complex. Unlike settlement prediction, the heave estimate depends on many factors which can not be readily determined the following basic factors should be considered: - Climate, climatic conditions involving precipitation evaporation and transpiration affect the moisture in the soil. The depth and degree of desiccation affect the amount of swell. Climatic conditions partially affect the desiccation. - The thickness of expansive soil stratum. In most cases, the thickness of the expansive soil stratum extends down to a great depth, and the practical thickness is governed by the surface water penetration into the startum. The thickness of the expansive soil stratum is controlled by the depth to water-table. - The depth to water table. Soil below the water table should be in a state of complete saturation; consequently, no swelling of clay should take place for the portion of soil below the water table. - The nature and degree of desiccation of the soil. The predicted amount of heave depends on the initial condition of the soil immediately after construction. If the excavation is exposed for a long period of time, desiccation takes place, and upon subsequent wetting, more swelling may take place. - Permeability and rate of heave. The permeability of the soil determines the rate of ingress of water into the soil either by gravitational flow or diffusion, and this in turn determines the rate of heave. The higher the rate of heave, the more quickly the soil responds to any changes in the environmental conditions, and thus the effect of any local influences is emphasized. At the same time, the higher the permeability, the greater the depth to which any localized the higher the permeability, the greater the depth to which any localized moisture penetrates, thus engendering greater movement and greater differential movement. Therefore, the permeability is an important factor; the higher the permeability, the greater the probability of differential movement. In this investigation, soil samples which show swelling characteristic from Istanbul arround Ambarlı and Maslak have been tested. Overconsolidation ratio which is one the major factor that affect swelling potential is investigated on swelling behaviour. Swell pressure of the high plastic soil samples, prepared under the same conditions in the laboratory, are determined in oedometer under the constant volume conditions. Then the samples were consolidated under the different pre-loads and xiii measured swelling pressures. Graphics obtained from experiments and regregation analysis between overconsolidation ratios and swelling pressure are discussed in respect to purpose of the aspect.