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A specific characteristic of those piles is their very considerable influence on soil properties during the installation, which renders classical bearing capacity calculation methods insufficient. Some methods for calculating the bearing capacity of screw displacement piles have already been presented in literature, for example, by Bustmante and Gianesselli , , Van Impe , , Maertens and Huybrechts , Ne Smith  as well as Basu and Prezzi .
This paper proposes a new method of calculating the bearing capacity of screw displacement piles in non-cohesive soil which is based on CPT results. It has been devised as a result of research project No. N N , carried out in At 6 experimental sites screw displacement pile static loading tests were carried out together with CPTU tests of the subsoil. The results allowed us to establish soil resistances along the shaft ts as well as under the pile base qb and their correlations to the CPT soil cone resistances qc.
Two approaches, both adapted to the general guidelines of Eurocode 7 EC7 , were proposed: a classical approach and the second approach with load transfer functions application. LIST OF SYMBOLS Ab As D MT qb Qb qb;ult qc Qc qcb qcs QN Qs QTn Rc;cal Rb;cal Rs;cal Rc;d Rb;d Rs;d pile base surface, pile shaft surface, pile diameter, torque during the penetration of pile auger into the soil, unit soil resistance under pile base, pile base resistance, ultimate unit soil resistance under pile base, unit soil resistance under CPT cone, total load on pile head, unit CPT cone resistance equivalent for pile base, unit CPT cone resistance equivalent for pile shaft, part of total pile load carried by bearing soil layers, pile shaft resistance in bearing soil layers, pile shaft resistance in upper weak soil layers, total bearing capacity of pile, bearing capacity of pile base, bearing capacity of pile shaft, computational total bearing capacity of pile, computational bearing capacity of pile base, computational bearing capacity of pile shaft,.
KRASISKI characteristic total bearing capacity of pile, characteristic bearing capacity of pile base, characteristic bearing capacity of pile shaft, pile base displacement, pile base displacement at ultimate base resistance, pile shaft displacement, pile shaft displacement at ultimate shaft resistance, unit soil resistance along pile shaft, ultimate unit soil resistance along pile shaft, reference pile base displacement in transform function method, reference pile shaft displacement in transform function method, partial coefficient for pile base bearing capacity, partial coefficient for pile shaft bearing capacity, variation coefficient for pile shaft resistances, variation coefficient for pile base resistances, correlation coefficients.
The soil structure comprised weak organic soils peat and mud in the upper layers and load bearing saturated fine and medium grain sands at the base.
The experimental sites were selected in association with several road building projects. These columns and piles are very similar to one another and can be generally termed Omega system piles.
Before test piles were installed, CPTU subsoil tests were carried out at each of the six sites. Most of the screw piles were equipped with specialist measurement instrumentation to gauge the axial force distribution along the shaft during successive static load tests. The instrumentation comprised a chain of five vibrating wire extensometers to measure the length changes of particular pile shaft sections.
The principles of how this particular instrumentation works have been described by the author in other papers , . A total of 21 piles were used at the experimental sites and the results from 16 pile tests were considered reliable enough to be used in further analysis.
A detailed description of the screw pile tests, the results of these tests and their subsequent interpretation is found in the authors other works . Figures 1 and 2 present the test results of just one of the piles. Figure 1 shows pile shaft axial force distributions during the application of loads, a graph of qc resistances taken before and after pile installation, and a graph of torque readings MT taken during. The axial force distributions presented in Fig. Figure 3 presents graphs of ts and qb soil resistance values based on the displacement results of the 16 screw piles under investigation.
In most cases, resistance ts is between kPa and kPa, while qb ranges from kPa to over kPa. Differences between resistance values result chiefly from the difference in the mechanical properties of the soil at various experimental sites.
The averaged qc resistance values varied from approximately MPa to over 25 MPa. The ultimate values ts;ult were assumed as ts at displace-. Most of the plots in Fig. The points plotted in coordinate system in Fig. Figure 4 also presents the values of variation coefficients s and b.
Bearing capacity of pile For the proposed method the author has adopted the classical formula for calculating the bearing capacity Rc;cal of screw piles, recommended also by EC7. Surfaces Ab and As should be calculated in relation to the nominal diameter of the pile formed. The values of qb;ult and ts;ult are calculated on the basis of the empirical data in Fig.
In the above formulas 2 and 3 values qcs;i and qcb should be given in MPa and moreover they should fulfil the following criteria. Correlation coefficients 3 and 4 EC7 recommends the use of 3 and 4 correlation coefficients, which base bearing capacity of piles calculations to the number n of subsoil profiles. Table 1 shows the recommended coefficients. The values cited are marginally lower than those recommended by EC7. They have been divided by 1. Table 1 Correlation coefficient values 3 and 4 n number of profiles under study.
In the method proposed, which is based on CPT subsoil test results, the number n is understood as the number of CPT carried out in the foundation area of a given building. According to EC7, the characteristic values of pile bearing capacities mean the ultimate load bearing. In the method proposed, coefficient values s and b are regarded to be the settlement criterion. Experience and calculation analysis show that for most constructions the safe settlement of individual piles oscillates in the region of 10 mm.
The way of determining coefficients b and s is adopted in accordance with the diagram in Fig. Verification of calculations with real pile test The proposed screw displacement pile bearing capacity calculation method was verified by comparing it with the real test measurements of one example pile. The calculations were based on CPT readings carried out at the same site as the real pile.
The bearing capacity values of the pile base Rb;d and shaft Rs;d as well as the total load bearing capacity Rc;d were defined for the lower subsoil layers, ignoring the weak upper and surface layers. Total bearing capacity Rc;d therefore needed to be compared with force QN obtained from the test.
For the comparative analysis the calculated Rb;d, Rs;d and Rc;d loads were applied to graphs Q-s obtained from the real test, as is shown in Fig. Calculated pile bearing capacity compared with real pile test results SDP-b3 pile, Pruszcz site. Figure 6 shows that the proposed calculation method produces pile bearing capacity values that generally correlate well with real test results.
In the case of the analysed SDP-b3 pile the piles calculated total bearing capacity Rc;d corresponds to force QN from the test, where pile settlement is at approximately 6 mm. More examples were analysed in paper . Of these, in the decisive majority the calculated pile loads corresponded to the settlements and did not exceed the value of 10 mm.
Bearing in mind that the standard calculation procedure also includes correlation coefficients 3 and 4 with values higher than 1. The method presented below uses transform functions t-z and q-z, proposed by Gwizdaa , . In the computational scheme the pile is treated as an elastic rod with a constant stiffness along shaft EA which is divided into a series of short elements, while the soil is modelled using a set of elasto-plastic bonds distributed along the shaft and as single elasto-plastic bond under the base.
The characteristics of the bonds are nonlinear. Transform functions for screw displacement piles In the calculation method proposed, transform functions are determined on the basis of soil resistances ts and qb in relation to soil resistances qcs and qcb under the CPT cone.
As in formulas 2 and 3 reference stress qref should be given the value of 1. One may note that there are certain differences between formula 2 and formula 7 in the constant and in the index of the first parenthesis. In the transformation function method one obtains from the CPT graph the pile settlement curve Q-s, which is divided into shaft soil resistance Qs and base soil. From the thus obtained curves one can next define the piles computational total bearing capacity Rc;d together with component values Rb;d and Rs;d, by applying for this purpose either the settlement criterion or one of the piles Q-s curve analysis methods, such as the one recommended in Polish standard .
By applying the settlement criterion the pile designer may use individual permissible settlement values. As in the classical approach, loads Rb;d and Rs;d should be reduced using coefficients 3 and 4 with the values set out in Table 1. In this case, coefficients b and s should be ignored, since the Q-s curve prediction as well as loads Rb;d and Rs;d are already defined to a satisfactory level of safety.
Verification of method with sample test results The results of sample verification test are presented in Fig. Results of pile load calculation based on transform function method compared with real test results SDP-b3 pile. Settlements corresponding to forces Rb;d, Rs;d and Rc;d are reduced by the shortening of the pile shaft. In comparison with the classical method results Fig. Therefore, on the basis of this example the transform function method has turned out to be safer or more conservative.
Similar results were obtained in the other comparative analyses, most of which are included in paper . The transform function method of predicting the bearing capacities of piles may be considered an alternative to conventional methods. Although it is more conservative and laborious, its advantage lies in the fact that it gives the designer greater choice in determining pile load bearing capacity. This might depend on the type of construction and especially on its sensitivity to settlements.
Moreover, the transform function method provides a full nonlinear Q-s pile characteristic which may next be used in static analyses of the construction to be founded on piles. For this reason these methods are reliable, as has been confirmed by comparing calculated pile load bearing results with real test results. Furthermore, these methods comply with the general Eurocode 7 recommendations and are therefore also up-to-date from the official point of view.
Application of the above methods should, however, be restricted to piles using SDP, CMC, FDP or Omega augers with diameters D ranging from to mm and inserted in load bearing layers comprising fine and medium coarse sands.
Extending the use of these methods to other piles e. The methods presented above provide net results regarding the load bearing capacity of piles, i. This should be taken into account at a later stage of pile design. One should note that the presented methods have been developed relatively recently and the versions in this paper are certainly not the final ones.
In the future they are bound to be modified and corrected on the basis of analyses concerning practical use as well as subsequent pile test verifications. The design of screw displacement piles should also be examined with regard to resistances to the auger of a pile forming in the soil. An appropriate diameter and pile length should be selected for a given piling machine torque.
The issue of resistances to pile augers being screwed into the soil has also been studied in research project  and will be the subject of a separate publication by this author.
Balkema, Rotterdam, , Tom 1: Technologie i obliczenia. Eurokod 7. Projektowanie geotechniczne; cz. Fundamenty budowlane. Nono pali i fundamentw palowych. Learn more about Scribd Membership Home.
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Embed Size px x x x x Abstract: The article is a review of the current problems of the foundation pile capacity calculations. The article considers the mainprinciples of pile capacity calculations presented in Eurocode 7 and other methods with adequate explanations. Two main methodsare presented: method used to calculate the short-term load capacity of piles in cohesive soils and method used to calculatethe long- term load capacity of piles in both cohesive and cohesionless soils. Moreover, methods based on cone CPTu result are pre-sented as well as the pile capacity problem based on static tests.
Proposal for Calculating the Bearing Capacity
The paper presents the trial of the theory of elasticity application to estimate pile stiffness in a combined piled-raft foundation. Two methods were applied: a simplified numerical model and closed-form analytical solution. Results were compared with the field tests. Keywords: combined piled-raft foundation. Beata Kutera, Ph.
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