Zeminlerin indeks özellikleri ve SPT-CPT deneylerinin karşılaştırılması

Abstract

Bu çalışmada, arazi deneylerinden, Standart Penetrasyon deneyi (SPT) ile Koni Penetrasyon Deneyleri (CPT) arasın daki ilişkiler incelenmiştir. Standart Penetrasyon Deneyi ve Koni Penetrasyon Deneyi ile ilgili mevcut korelasyonlar, Kmalı-Sakarya otoyolu güzergahı üzerinde bulunan Karasu, Sakarya ve Beylikçayır viyadüklerinin ve İzmir Denizbostanlı'da yapımı tasarlanan Atakent projesi Geoteknik etüdleri sırasında yapılan, arazi ve laboratuvar deneylerinden elde edilen dataların değerlendirilmesinde kullanılmıştır. Standart Penetrasyon Deneyi ile Koni Penetrasyon Deneyi hakkında genel bilgiler verildikten sonra bu deneyler ile ilgili bağıntılar ele alınmıştır. Standart Penetrasyon deneyi ile Koni Penetrasyon Deneyi kıvam limitleri ve granülometri açısından incelenmiştir. SPT darbe sayısı (N) ile drenaj sız kohezyon (Cu), plasti- site indisi (ip), gibi zemin parametreleri arasındaki ilişkiler araştırılmıştır. Ayrıca plastisite indisi ve likit limit arasındaki ilişkiyi gösteren plastisite kartı üzerinde eldeki datalar işlenerek, gerekli sınıflandırma lar yapılmıştır. Koni Penetrasyon deneyinden elde edilen uç mukavemeti (qc), çevre sürtünmesi (fs), sürtünme oranı (FR=fs/qc) ve zemin parametreleri arasındaki korelasyonlar incelenmiştir. Koni Penetrasyon deneyinin kazık tasarımında kullanımına ait bilgiler verilmiş, Koni Penetras yon deneyinden elde edilen, uç mukavemeti ve kazık birim uç mukavemeti arasındaki ilişkiler incelenmiştir.

In-situ tests are an important part of ground investigations to provide necessary information for a safe and economical design of civil engineering projects. The Standard Penetration Test - SPT - and Static Cone Penetration test- CPT - are well established in-situ tests for soil investigations. The SPT and CPT test results are frequently used in foundation designs by employing empirical relations. Thus to assess empirical correlations between these tests are important in engineering applications. Significant improvements have been made recently in the interpretation of SPT and CPT test results. In those cases where only SPT results are available engineers who are more familiar with CPT interpretations may translate the SPT-N values into CPT-qc-values, and the reverse is also possible. So there is a need for a reliable SPT-CPT correlation, for various types of soils. It has been emphasised in this thesis that different interpretation methods can lead to large differences in the soil parameters obtained by in-situ testing. In this study samples were taken from the second Istanbul Bosphorus Highway Crossing which is a part of the Kmalx-Sakarya Motorway and izmir-Atakent project located in Denizbostan- li-tzmir. The Karasu Viaduct, Sakarya Viaduct and Beylik- çayır Viaducts located in Kxnali-Sakarya Motorway have been considered. This thesis gives guidance on the suitability of Cone Penetration Test (CPT), Standard Penetration Test (SPT) for obtaining design parameters for clays, silty clays and sands. Penetration tests are also very valuable during the detailed exploration phase especially in silt, sand and gravel so that the compressibility of the soil and the settlements in the different stratas can be estimated. It is also possible to get an indication of the shear strength from the measured penetration resistance, so that the ultimate bearing capacity of footings and piles can be evaluated. In most cases the settlements will govern the design rather than the ultimate bearing capacity of the soil. Importance of in-situ tests can be explained as follows; The very soft or sensitive soils, stony soils, sands and gravels, soft fissured or shattered rocks and fractured rocks, on which foundations are to be constructed, v do not readily lend themselves to good sampling, or may require the use of expensive samplers outside the scope of routine investigation. When these are encountered in-situ tests may provide the best means of obtaining engineering design parameters. Many forms of in-situ penetration tests are in use worldwide. Designs and applications procedures by penetration methods can be found, for example in Sanglerat (1972), the "Proceedings of the European Symposium on Penetration Testing" (Stockholm, 1974), and the International Society for Soil Mechanics and Foundation Engineering's (I.S.S.M.F.E) "Report of the Sub-Committee on the penetration Test for use in Europe" (1977). Penetrometers can be divided into two broad groups. The simplest, dynamic penetrometers consist of tubes or points driven by repeated blows of a drop weight. Static penetrometers are normally pushed hydroulically into the soil. Although many forms of penetrometer test are used in different countries around the world, only two tests are widely accepted. The so-called "Standard Penetration Test" and the "Static Cone Penetration Test" are accepted by A.S.T.M. and the I.S.S.M.F.E. in Europe. The standard penetration test (SPT) is a relatively old method. This method was developed in the united States around 1925 by A.H. Mohr (Terzaghi, 1953). It is by far the most commonly used penetration testing method today. In Japan, SPT is used during the exploratory phase in more than 90% of all borings. The most controversal aspect of the Standard Penetration Test is the operation of the hammer. In North and South America the procedure described in ASTM (1586-84) using a rope and a cathead to lift the hammer or a free falling hammer is generally followed. In Europe, Asia and Africa, the test is generally carried out in accordance with the European Reference Standard (ISSMFE, 1977). The SPT is most commonly used in Turkey. It is known that many conventional foundations are designed in Turkey by employing the SPT-N value. Despite continuous efforts to standardize the SPT procedure there are still continuous problems associated with its repeatability and reliability. The standard penetration test has the advantage of being used in almost all part of the world. The test equipment is simple, inexpensive and rugged, representative but disturbed samples are obtained. It can be used for classification of the different soil strata and that the test can be carried out in almost all soil types and also in weak rock. VI The standard penetration test is mainly used to estimate the relative density, the strength and deformation characteristics of mainly granular soils, the ultimate bearing capacity potential of sand and silt. It is usually possible to get some indications from SPT of the shear strength: of cohesive soils. In a standard penetration test a 51 mm diameter thick walled split spoon sampler is used which is driven down into the bottom of a borehole using a 63.5 kg (140 lb.) hammer from a height of fall of 0.76 m (30 in.). In Brazil, a 41 mm diameter sampler is often used. The diameter of the borehole should be between 60 and 200 mm. In gravelly or stony soils the sampler is often provided with a solid driving shoe to prevent damage to the sampler during the driving. The number of blows (N,J required to drive the sampler 0.3 m is counted. This N30 value (blows/0. 3m) is the so-called standard penetration resistance of the soil. The testing is usually terminated when the penetration resistance exceeds 50 blows/0.3 m. Terzaghi and Peck (1948) suggested that for very fine or silty submerged sand with a standard penetration value N1 greater than 15, the relative density would be nearly equal to that of a dry sand with a standard penetration value N where : N = 15 + 1/2 (N'-15) If this correction were not made, Terzaghi and Peck suggested that the relative density of even moderately- dense very fine or silty, submerged sand might be over estimated by the results of the standard penetration tests. A correlation study between SPT-N and the relative density of sand was expressed by Terzaghi and Peck (1948). According to Terzaghi and Peck, if sand is subject to liquefaction, it is possible that after the initial few hammer blows the soil starts to flow and partly fills the boring. The data obtained from the SPT is a direct evaluation of the compactness of the sand. The values of N obtained from the SPT may then be correlated to the values of 0, N and N. q Y Meyerhof (1965) presented comparative data between the standard penetration test and the classical static penetration tests. Meyerhof estimates that the, penetration resistances of the static, dynamic and standard penetration tests exhibit the same change as the depth increases. For fine or silty medium dense to loose sands, it was shown by Meyerhof that the correlation may be expressed as follows : q « 4 N ^c vii 2 2 Where, q = cone static resistance in kg/cm (100 kN/m ) and N= nSmber of blows per 0.30 m of penetration of the SPT where N may have to be corrected in case of fine sand. Results from standard penetration tests in medium to firm clays and soft rocks may be used to estimate the shear strength of the soil, and hence to provide estimates of end-bearing pressure and of skin friction. Stroud (1974) has presented a correlation of N value with undrained shear strength, and shown that the ratio Cu/N is about 4-5 kN/m (0.04-0.05 kg/cm2) for clays of medium plasticity. As the plasticity index reduces to below 20* the ratio Cu/N starts to increase rapidly to 6-7 kN/m (0.06-0.07 kg/ cm 2) or more. The most common static penetration testing methods is the cone penetration test (CPT). The quasi-static Dutch cone penetration test has a development history of ever 40 years. Many engineers throughout the world have found this test a useful and economical tool for site investigation and geotechnical design. The modern advances of using trucks with up to 20 ton thrust capacity, and the inventions of the friction-cone and electrical cone tips have made this test even more attractive technically without sacrifice of economy. Dr. Schmertmann re- introduced this test into the U.S.A. in 1965. The CPT has not so far been used as widely (and as often) as it might be in Turkey and it is hoped that this thesis will encourage its use. Correlations of CPT results with engineering parameters and performance presented herein should be viewed with the appropriate degree of caution. This varies from one correlation to another, and comments on this aspect are included in this study. The practice described in this thesis is that adapted in U.K., which closely follows European practice. Although cones for penetration testing are available in various sizes, the only cones considered are, with minor exceptions, those with a 60° point angle and a diameter of 35.7 mm (cross sectional area 1000 mm^). In the CPT, a cone at the end of a series of rods is pushed into the ground at a constant rate, and continuous or intermittent measurements are made of resistance to penetration of the cone. If required, measurements are also made of the combined resistance to penetration of the cone and outer surface of the rods on the resistance of a surface sleeve. viii The CPT apparatus consists of a thrust machine and reaction system (rig), and a penetrometer, including measurement and recording equipment. CPT delineates the soil profile. This can be done with greater accuracy than can be achieved from conventional boring and sampling. However, the accuracy is not greater than can be obtained in certain soils with a sampler such as the Delft Continuous sampler, or by continuous sampling with a stationary piston sampler. Moreover, such sampling has the advantage that it permits visual inspection of the soil. It is advisable to check soil identification by the CPT by direct comparison against one or more boreholes, preferably with continuous sampling. It should also be emphasised that local experience may well indicate differences from the data presented above. Every opportunity should be taken to derive correlations based on local conditions. The cone penetration test consist in pushing into the soil, at a sufficiently slow rate, a series of cylindrical rods with a cone at the base, and measuring continuously or at selected depth intervals the penetration resistance of the cone, and if required the total penetration resistance and/or the friction resistance on a friction sleeve. Cone penetration tests are performed in order to obtain data on one of the following subjects : 1) The stratigraphy of the layers, and their homogenity over the site. : 2) The depths to the firm layers; the location of cavities, voids and other discontinuities. 3) Soil identification 4) Mechanical soil characteristics 5) Bearing capacity of piles The CPT has an important role in the exploration of cohesionless soils, because there is a lack of satisfactory alternative methods. Laboratory testing is generally not feasible, because of the difficulty of obtaining undisturbed samples. CPT results can also be correlated into equivalent SPT-N values in order to use correlations between N and engineering parameters and performance. The cone penetration test has produced excellent continuous profiles of undrained shear strength. The major problem is the evaluation of the in-situ undrained shear strengths (Cu) appropriate to the particular design problem, since the Cu depends on the stress path followed during shear. The recommended general empirical correlation using an N value of 15 is related to the field vane strengths. IX There have been a number of empirical correlations between cone tip resistance and SPT blowcount. It has been generally believed that qc/N ratios vary depending on soil type and test apparatus and procedure. It can be seen that the ratio of cone tip resistance to SPT blowcount decreases with decreasing fines content or increasing mean grain size. Cone penetration test data may be analyzed to provide information on indices of soil strength and soil classification. Initially/ CPT soil classification for the Dutch Cone Penetrometer was based only on the friction ratio. Soils with friction ratios of less than 2,5 percent classify as sand, greater than 3,5 percent as clays, and between 2 and 4 percent as mixtures and/or silts. The soil behaviour chart has been developed by using electronical CPT. The soil classification lines in this chart represent varying soil consistency for specific soil types, but only for a vertical effective stress of 1 kg/cm (100 kN/m2). In this thesis, general description of SPT and CPT with their historical background are presented. The cone penetration test (CPT) and the standard penetration test (SPT) carried out by General Directorate of Highways "KGM" at a number of sites are used to obtain empirical correlations between the parameters of these two tests and the correlation between SPT results (N values) and cohesive soil parameters (Cu, qu) also between liquid limit (Wl) - plasticity index (Ip) have been examined. Finally, the results have been compared with the previous studies. Moreover, datas obtained from îzmir-Atakent project which is located in Denizbostanlı-îzmir are also examined and it has been observed that present study is in general agreement with the previous studies made by various researches.