Koni penetrasyon deneyi ve geoteknik tasarımda kullanılması

Abstract

Bu çalışmada zemin ve temel mühendisliğinde uzun yıllardır yaygın olarak kullanılan arazi deneylerinden koni penetrasyon deneyi (CPT) incelenmiştir. Koni penetrasyon deneyi hakkında genel bilgiler verildik ten sonra bu deneyle ilgili olarak araştırmacılar tarafından yapılan çalışmalar geniş bir şekilde ele alınmıştır. Koni penetrasyon deneyi ile ilgili verilen korelasyonlar Borçelik geoteknik etüdü sırasında yapılan arazi ve laboratuvar deneylerinden elde edilen datanın değerlendirilmesin de kullanılmıştır.

The idea to determine the shear strength by pushing or dropping a cone into the soil was developed very early. Initially the depth of penetration was taken as an indication of shear strength. A pocket penetrometer was developed in Denmark by the Danish State Railroads. This pocket penetrometer was used extensively to determine in the laboratory the shear strength of stiff clay and the allowable bearing pressure for spread footings. The Dutch cone* penetrometer was initially developed by Barentsen in the Netherlands. The aim of the penetrometer was to determine the thickness and bearing capacity of 4m thick hydraulic fill near the town of Vlaardingen in Holland. Several other cone Penetrometers have been developed. Cone penetration tests are performed in order to obtain data on one or more of the following subjects : 1) The stratigraphy of the layers, and their homogeneity over the site. 2) The depth to firm layers; the location of cavities, voids and other discontinuities. 3) Soil identification. 4) Mechanical soil characteristics. 5) Bearing capacity of piles. Relative to other soil engineering methods for exploring site stratigraphy and obtaining data for preliminary design, the CPT has the out standing advantages of often providing better speed and economy, more detailed and precise data, and data better suited to many ordinary soil engineering design problems. It has the disadvantages of not obtaining a soil sample for visual/lab inspection and of sometimes severaly limited depth capability. vi The CPT apparatus consists of a thrust machine and reaction system (rig), and a penetrometer, including measurement and recording equipment. In the CPT, a cone on 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 either the combined resistance to penetration of the cone and outer surface of the rods or the resistance of a surface sleeve. Incontrast to cohesionless soils, cohesive soils form a less important field of use for the CPT, because established alternative methods are available. In N.C and lightly OC clays, parameters can be obtained from in-situ vane tests and labaratory tests on specimens from thin-walled stationary piston samplers. Nevertheless, the CPT has the [advantages of rapid caverage and good identif icaton. of variations in stratification. In OC clays, particularly stiff, fissured clays, vâne tests are unsuitable, and laboratory tests for strength and elastic modulus suffer from the effects of sample disturbance. Unfourtunately, the makrofabric of such clays also makes for difficulty in the interpretation of CPT results. For determination of constrained modulus, laboratory tests are preferable. Clearly, a combination of CPT and alternative methods may be beneficial. The CPT has an important role in the explanation of cohesionless soils, because there is a lack of satisfactory alternative methods. Laboratory testing is generally not feasible, because of difficulty of obtaining undisturbed samples. Alternative in-situ tests include the plate-loading test and the screw-plate loading test. Direct derivations from cone resistance of relative density, angle of shearing resistance and modulus values (either the constrained modulus or Young's modulus) depend on emprical correlations. These have some backing from calibration chamber tests, but it should always be remembered that such correlations are limited in the range of soils to which they apply. There have been a number of emprical correlations between cone tip resistance and SPT blowcount. It has been generally believed that qc/N rations 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. vii 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 standard for electrical CPT soil classification for the last several years has been the soil behaviour chart. The soil classification lines in this chart represent varying soil consistency for specific soil types, but only for a vertical effective stress of 1 tsf. As in all foundation design, it is necessary to consider both safe bearing capacity the ultimate bearing capacity divided by a suitable factor of safety and allowable bearing capacity related to tolerable settlements. Ultimate bearing capacity can be calculated using standard bearing capacity formula and bearing capacity factors. Generally, the direct use of q in settlement calculations for foundations on sand is preferable to methods in which q is first converted to SPT blow count, N. However, there ii a quick check method via SPT which is often useful to indicate the probable extent of a settlement problem. This method uses the data prensented which are from site records where ground conditions and settlements were known. In each case, it is suggested that probable settlement can be taken as equal to half the upper limit value and that maximum settlement does not normally exceed about 1,5 times the probable value. Another method which has considerable merit, although it is indirect, is that of Burland and Burbidge. This method is derived from an extensive review of case histories, mostly based on SPT but including some where the CPT was used. A rapid conservative estimate of settlement of a footing on sand can be directly obtained from q, using relationship proposed by Meyerhof. A method which has been much used, but has now been superseded, is that of DeBeer and Martens. It is based on Terzaghi-Buisman formula. For direct use of CPT values in calculating settlements of footings on sand, the Schmertmann method is probably the best available. On clay, as with footing on sand, it is necessary to consider both safe and allowable bearing capacity. Ultimate bearing capacity can be calculated from undrained shear strength, Cu, using standard formulae. This requires values of Nk for use in the equation. The value of Nk to be adapted is influenced by the factor of safety to be used and other considerations. viii There are two methods to estimate settlement on clay : 1- The method of Skemptonand Bjerrum which combines immediate settlement evaluated by elastic theory on consolidation settlement from a modification of Terzaghi's theory of consolidation. 2- A method using linear elastic theory to calculate total settlement. The cone penetrometer test results are used extensively for pile design, both for estimating soil parameters, for application of the conventional design methods, and as a seperate approach to pile design. The ultimate bearing capacity of a pile is the sum of the ultimate end-bearing capacity and the ultimate shaft resistance-. Safe bearing capacity is then calculated bv applying a factor of safety to ultimate bearing of the pile or separate factors of safety to the components ultimate end-bearing capacity and ultimate shaft resistance. Allowable bearing capacity depends on the settlement which can be tolerated. In a uniform deposit of sand, below a certain depth a parallel-sided displacement pile achieves on ultimate bearing capacity equal to the cone resistance. The depth below which occurs is known as the critical depth» It varies with soil stiffness and it ranges between 4 and 20 pile diameters, the critical depth increasing soil stiffness. A typical value of 8 is often adapted. Sand deposits are seldom uniform, and, in practice, it is necessary to drive a composite q value, q, to take account of the variation of q above and below the pile toe, and a procedure for doing this. In evaluating qC2# trials are made with a number of depths, below pile toe, between 0.7 d and 4.0 d, and the lowest resulting qC2 is adapted. Typical Dutch practice in assessing qT is to limit the value of qc used (normally to 30 MN/m<2), and to limit the ultimate end-bearing capacity to a value not exceeding 15 MN/m2, which depends on OCR. Some further reduction may be required if weaker layers exist between 4 d and 10 d below pile toe level. Shaft resistance directly can be calculated from local side friction or cone resistance. Although methods are available for calculating the bearing capacity of piles in clay in terms of effective stress parameters, it is more usual to use the undrained shear strength, Cu. This can be obtained from CPT results using The methods described in this thesis. At present, there are no commonly adapted procedures for determining pile xi bearing capacity in clay direct from CPT results, and other methods are preferred. At present, there is no direct method of calculating the settlement of a pile from CPT data. However, there are some indirect methods. This thesis provides guidance on the use of the cone penetration test and on the interpretation of test results and their use in design. The result of CPT tests performed in Borçelik project have been given as an example for its application.