Kil zeminlerin karakteristiklerinin ASTM yöntemleri ile belirlenmesi
Evaluating the swell characteristics of clay soils according to astm test methods
- Tez No: 66460
- Danışmanlar: PROF. DR. ERGÜN TOĞROL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Geoteknik Bilim Dalı
- Sayfa Sayısı: 101
Özet
ÖZET Kil zeminlerin mühendislik bakış açısından en dikkate değer özelliklerinden biri de, üzerindeki yükden bağımsız olarak, su emmesi sonucu hacim değiştirebilmesidir. Bu hacim değişikliklerinin zaman zaman yüksek boyutlara vararak büyük maddi hasarlara yol açması sorunu özellikle önemli kılmıştır. Şişme basıncının önceden doğru olarak tahmin edilmesiyle uygun tasarımlar yapılabilmekte ve yapıda oluşabilecek hasarlar en aza indirilebilmektedir. Şişme basıncının tahmini için çeşitli laboratuar yöntemleri ve ampirik bağıntılar geliştirilmiştir. Bunlardan en yaygın olarak kullanılanları ise laboratuar ödometre yöntemleridir. ASTM deney yöntemleri, nisbeten örselenmiş veya sıkıştırılmış kohezyonlu zeminlerin kabarma veya oturma miktarlarının belirlenmesi için üç alternatif laboratuar metodunu kapsar. Bu yöntemler, (a) belli bir eksenel (düşey) basınç altında meydana gelecek kabarma veya oturmanın büyüklüğünün, (b) yanal deformasyonuna izin verilmeyen deney numunesinin hacim değişmesini engelleyecek düşey basıncın büyüklüğünün belirlenmesinde kullanılır. Bu tez çalışmasında, ASTM' de önerilen üç alternatif şişme deneyi ele alınmış ve bu yöntemlerle, mümkün olduğunca standarda bağlı kalınarak, şişme deneyleri gerçekleştirilmiştir. Yapılan çalışmalar ve elde edilen veriler ışığında bu yöntemlerin bir karşılaştırması yapılmış ve varılan sonuçlar açıklanmıştır. VII
Özet (Çeviri)
SUMMARY One of the most notable characteristics of clays from the engineering point of view is their susceptibility to slow volume changes which can occur independently of loading due to swelling. These volume changes may result in many structural damages which causes great financial losses in many part of the world. Differences in the period of and the magnitude of precipitation and evaporation are the major factors influencing the swell response of an active clay beneath a structure. Expansive clay minerals absorb moisture into their latttice structure before volume changes occurs. In densely packed soil having small void space, the soil mass has to swell to accommodate the volume change of expansive clay particles. Reliable prediction of swell is essential for the development more effective and economical design of structures on expansive soil. Many procedures are proposed for this purpose. There are three different methods of classifying potentially expansive soils. The first, mineralogical identification, can be useful in the evaluation of the material but is not sufficient in itself when dealing with natural soils. From the mineralogical standpoint, the magnitude of expansion depends on the kind and amount of clay minerals present, their exchangeable ions and the internal structure. Another group includes the indirect methods, such as the index property, PVC method, activity method and empirical relationships which are valuable tools in evaluating the swelling property. Most of empirical relationships are derived from data for specific climatic and environmental conditions and hence, should be used with caution. The third method, direct measurement, gives the most useful data in evaluating the swell parameters. This method includes many test procedures which are simple to perform and not require any costly laboratory equipment. Swell potential and swell pressure are two swell parameters for defining the magnitude of swell of expansive soils. Swell potential is defined as the vertical volume change from an oedometer-type test as a percentage of original height of an undisturbed specimen from its natural moisture content and density to a state of saturation under an applied load equivalent to the in situ overburden pressure. VIIISwell pressure is defined as the pressure which prevents the specimen from swelling or that pressure which is required to return the specimen back to its original state (void ratio, height) after swelling. The most satisfactory and convenient method of determining the swelling potential and the swelling pressure of an expansive clay is by laboratory testing methods. Laboratory testing procedures generally involve the use of the one-dimensional consolidation apparatus (i.e., oedometer). The soil sample is enclosed between two porous stones and confined in a metal ring. The diameter of ring ranges from 2 to 4 inches. The thickness of the specimen ranges from one-half to 1 inch. The soil sample can be allowed to access water both from top and bottom. Oedometer test apparatus enables an easy and accurate measurement of the swelling parameters of an expansive soil under various conditions. Laboratory-prepared test specimens should duplicate the in situ soil or field-compacted soil conditions as closely as possible because relatively small variations in unit weight and water content can significantly alter the measured heave and swell pressure. Constant volume“ and ”free swell" test procedures are commonly used in the analysis of swell by oedometer techniques. In constant volume test procedure, the sample is subjected to a token load and submerged in water. As the sample attempts to swell, the applied load is increased to maintain the sample at a constant volume. This procedure continued until there is no further tendency for swelling. The applied load at this point is referred to as the swell pressure. The sample is then further loaded and unloaded in the conventional manner. Free swell oedometer test can also be used to measure the swelling pressure and swelling properties of a soil. The sample is initially allowed to swell freely with a token load applied. The load required to bring the sample back to its original void ratio is termed the swelling pressure. Environmental conditions are extremely important in determining the amount of swell. So, the swelling characteristics vary greatly with the variation of one or more of the environmental or placement conditions. The initial water content, dry density, surcharge pressure, compaction method for remoulded specimens etc. influences the results obtained in loaded swell tests on soils of any mineralogical composition. The initial moisture content of the expansive soils controls the amount swelling. If the moisture content of the clay remains unchanged, there will be no volume change. When the moisture content of clay is changed, volume expansion will take place. Slight increases of moisture content are sufficient to cause the swelling. Directly related to initial moisture content, the dry density of the clay is another index of expansion. When dry density decreases, swelling pressure rapidly increases. For undisturbed soils, dry density is the in situ characteristic. For remoulded soil, the swelling pressure varies with the degree of compaction. rxA reliable and reproducible test which is to be considered as a basis for the classification of potential expansive soil must be standardised environmental conditions. ASTM D-4546 defined three standard test methods for one dimensional swell parameters of cohesive soils. ASTM test methods cover three alternative laboratory methods for determining the magnitude of swell or settlement of relatively undisturbed or compacted cohesive soils. The methods can be used to determine (a) the magnitude of swell or settlement under known vertical (axial) pressure, or (b) the magnitude of vertical pressure needed to maintain no volume change of laterally constrained, axially loaded specimens. Method A- The specimen is inundated and allowed to swell vertically at the seating pressure(pressure at least 1 kPa applied by the weight of the top porous stone and load plate) until primary swell is complete. The specimen is loaded after primary swell has occurred until its initial void ratio/height is obtained. Method A measures, (a) the free swell, (b) percent heave for vertical confining pressures up to the swell pressure, (c) the swell pressure Method B- A vertical pressure exceeding the seating pressure is applied to the specimen before placement of free water into the consolidometer. The magnitude of vertical pressure is usually equivalent to the in situ vertical overburden pressure or structural loading, or both, but may vary depending on application of the test results. The specimen is given access to free water. This may result in swell, swell then contraction then swell. The amount of swell or settlement is measured at the applied pressure after movement is negligible. Method B measures, (a) the percent heave or settlement for vertical pressure usually equivalent to the estimated in situ vertical overburden and other vertical pressure up to the swell pressure, and (b) the swell pressure Method C- The specimen is maintained at constant height by adjustments in vertical pressure after the specimen is inundated in free water to obtain swell pressure. A consolidation test is subsequently performed. Rebound data is used to estimate potential heave. The swell pressure obtained by this method shall be corrected upward by a suitable construction procedure. Soil disturbance and the process of adjusting vertical pressures may allow some volume expansion to occur, which reduces the maximum observed swell pressure. XMethod C measures, (a) the swell pressure, (b) preconsolidation pressure, and (c) percent heave or settlement within the range of applied vertical pressures. The relative swell/settlement potential of soil determined from these test methods can be used to develop estimates of heave or settlement for given final moisture and loading conditions. The initial water content and void ratio should be representative of the in situ soil immediately prior to construction. Selection of test method, loading, and inundation sequences should, as closely as possible, simulate any construction and post-construction wetting and drying effects and changes in loading conditions. ASTM states that Methods A and C have produced estimates of heave consistent with observed heave. Method B may lead to estimates of heave less than observed heave. Method A has not been recommended for evaluation of swell pressure and consolidation parameters for settlement estimates because sorption of water under practically no restraint may disturb the soil structure. Swell pressures by Method C corrected for specimen disturbance may be similar similar to or slightly greater than those by Method A. Estimates of the swell and settlement of soil determined by these test methods are often of key importance in design of floor slabs on grade and evaluation of then performance. However, when using these estimates it is recognised that swell parameters determined from these test methods for the purpose of estimating in situ heave of foundations and compacted soils may not be representative of many field conditions because: (a) Lateral swell and lateral confining pressure are not simulated. (b) Swell in the field usually occurs under constant overburden pressure, depending on the availability of water. Swell in the laboratory is evaluated by observing changes in volume due to changes in applied pressure while the specimen is inundated with water. Method B is designed to avoid this limitation. (c) Rates of swell indicated by swell tests are not always reliable indicators of field rates of heave due to fissures in the in situ soil mass and inadequate simulation of the actual availability to the soil. (d) Secondary or long-term swell may be significant for some soils and should be added to primary swell. (e) Chemical content of the inundating water affect the volume changes and swell pressure. (f) Disturbance of naturally occurring greatly diminishes the meaningfulness of the results. In this thesis, ASTM standard test procedures were investigated in the quantitative analysis of swell. Laboratory oedometer tests were conducted on undisturbed and remoulded specimens according to ASTM testing procedures. Remoulded specimen compacted at nearly optimum water content to maximum density in the Standard XIAASHO compaction test. Test results were summarised in Chapter 3, permitting comparison of procedures A, B and C. It is observed that Method A gives the maximum swell pressure values. Method B gives higher magnitudes for the swell parameters than Method C where water entry is restricted by relatively high magnitude of vertical stress which also restores the influence of sampling defects. The different values in the swell parameters are due to differences in loading and wetting conditions It is also observed that, remoulded expansive clays tend to swell more than their undisturbed equivalents. Expansive clays expand very little when compacted at low densities and high moisture but expand greatly when compacted at high densities and low moisture. It is observed that the soil specimens may exhibit different amounts of swell after compaction if they are compacted by different compaction method. In the light of the data evaluated from the tests it is suggested that Method B provides reasonably accurate estimates for swelling parameters of compacted clays when prepared wet of optimum water content. But Method C may be proposed for undisturbed soils because this procedure accounts for sampling disturbance. XII
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