The accuracy and credibility issue of interpretation of measurements carried out during load tests on instrumented piles is still difficult and complex. It is evidenced by numerous publications that describe and analyse the problem, for example, Fellenius et al. (2000), Fellenius (1989, 2001, 2002c), Hayes and Simmonds (2002), Krasiński (2012) and Maertens and Huybrechts (2003). The interpretation results are largely influenced by factors such as:
geometric heterogeneity of the pile shaft along its length (thickening, narrowing); material heterogeneity of the pile core (variability of the concrete quality and stiffness modulus with the depth and stratification of the soil; non-linear characteristics of this stiffness); pile reinforcement heterogeneity (variable number of longitudinal bars along the pile); inaccuracy and unreliability of the measuring system (type of equipment used – vibrating wire extensometers or fibre optic systems), important due to small values of the measured deformation and occurrence of an initial axial force of unknown value in the pile shaft (so-called residual force) before the start of the pile load test.
geometric heterogeneity of the pile shaft along its length (thickening, narrowing);
material heterogeneity of the pile core (variability of the concrete quality and stiffness modulus with the depth and stratification of the soil; non-linear characteristics of this stiffness);
pile reinforcement heterogeneity (variable number of longitudinal bars along the pile);
inaccuracy and unreliability of the measuring system (type of equipment used – vibrating wire extensometers or fibre optic systems), important due to small values of the measured deformation and
occurrence of an initial axial force of unknown value in the pile shaft (so-called residual force) before the start of the pile load test.
This article deals with the last mentioned factor concerning the residual force, considered in case of screw displacement piles. During interpretation and analysis of numerous load test results (performed on screw displacement piles), the authors have observed symptoms indicating the presence of residual forces. Based on the analyses and established regularities, a proposal of how to identify the residual force value and then how to apply it in the results interpretation was developed. The article describes the procedure of such identification and presents its verification based on two selected load tests performed on screw displacement piles equipped with retrievable vibrating wire extensometers.
The purpose of performing instrumented pile load tests is to determine the value and distribution of the axial force along the pile shaft, which then allows to determine the contribution of individual soil layers along the shaft and under the pile base in transferring the external load applied to the pile head. Tests rely on measurements of the pile shaft deformation, currently carried out using vibrating wire extensometers or the fibre optic technique. Due to relatively low deformation values, high accuracy of the measuring device is required. More favourable measurement results are obtained with the fibre optic technique, mainly due to higher resolution (Kania and Sørensen, 2018; Sieńko et al., 2018). The disadvantage of which, however, is higher cost, more complicated technology that requires specialised and qualified service and problems with temperature compensation.
The value of axial force (
Examples of the
The discussed interpretative approach is subjected to errors resulting from possible heterogeneity of the concrete stiffness modulus (
Presence of residual forces in the pile shafts of various technologies has been the subject of many publications, including Cooke (1979), Fellenius et al. (2000), Fellenius (2002c), Maertens and Huybrechts (2003), Kim et al. (2004), Siegel and McGillivray (2010), Krasiński (2012) and Sahajda (2015). In the case of screw displacement piles, initial compressive forces might be generated mainly as a result of negative friction occurring in the upper parts of the subsoil (Fellenius, 2002a; Van Impe et al., 2013). This friction is caused by the secondary settlement of the soil, which was raised during pile formation by a displacement auger. In that process, poorly permeable soils deform with practically no volumetric changes and excess pore water pressure is generated. While this excess is slowly dissipated, layers of poorly permeable soil are consolidated and aforementioned secondary settlement occurs. Soil consolidation takes a long time and may also continue after the concrete has hardened and causes the activation of compressive force in the pile shaft. The discussed mechanism is presented graphically in Fig. 3. In critical cases, due to the residual force, the pile shafts may even get damaged in the early stages of concrete maturation (with still low strength). Movement of a heavy construction equipment (e.g. pile machine) on the ground level might be an additional factor causing negative friction. The mentioned compressive force in the pile shaft is generally referred to as residual force, although this name is more appropriate for the case of driven piles.
It is not always easy and possible to determine the presence of initial compressive force in the pile shaft. We may find out about its existence during the intermediate or final unloading of the pile. If in all measuring sections, after unloading remain permanent deformations (permanent shortenings), it can be assumed that there was little or no initial compressive force in the pile before the load test was started. However, if in some measuring sections, the deformations after unloading are of the opposite sign (elongation), this is a clear evidence of the initial compressive force. An example of such a situation is shown in Fig. 4. The described phenomenon has been observed in the vast majority of tests on instrumented screw displacement piles, of which the authors have so far carried out over 50.
Fellenius (2002a) has proposed an identification method based on the concept that residual force value in the upper pile sections is equal to a fully mobilised negative skin friction of the pile. Therefore, the soil resistance obtained from uncorrected load test in these layers consists of two directions of friction (negative and positive); that means, its value is doubled and for proper interpretation, it should be divided by 2. In the lower pile sections, ‘true’ soil resistance value is determined from the correlations based on CPTu sounding. Then, the residual force can be assessed by extracting the estimated uncorrected soil resistance value (based on pile load test results) from the one based on CPTu sounding (for more details of the Fellenius method, see Fellenius (2002a, 2002b, 2015)). The method has been tested and verified to be legitimate (Kania et al., 2020); however, it must be underlined that it is only an estimation. Authors indicate that the value of initial compressive force (residual force) in the pile shaft can be estimated directly from deformations of the measuring sections recorded during the pile unloading phase. For this purpose, the concrete stress–strain characteristic in the first measuring section (G1), corresponding to the pile unloading phase, should be determined. The characteristic should be defined for changes (decreases) in stress and changes in concrete deformation, calculated according to the equations (1, 2) given as follows:
In the final stage (
It was observed that the higher concrete class and reinforcement in the pile shaft are, the lower permanent deformation and the more linear the Δ
The values of forces
If in the final (
Such a case can be seen, for example, in Fig. 8a. It should be taken into account that some sections cannot fully relax due to the soil resistance along the pile shaft. This phenomenon can be neglected in case of weak soil layers along the upper part of pile shaft. Additionally, it may be advantageous to perform an additional one or more load–unload loops, resulting in a further gradual degradation of the residual soil resistance along the pile shaft. If during such loops, an additional increase in the pile sections’ length is observed, then estimation of the residual force values (
The proposed method of estimating the residual force value was used to interpret the test results of two exemplary instrumented screw displacement piles. Piles differ significantly in length (12.6 and 7.5 m) and in the ground conditions they were embedded in (two different experimental fields). Pile no. 6293 was installed in the substrate dominated by organic and non-cohesive soils, and pile no. 9 in the substrate composed mainly of cohesive soils. Ground conditions were identified by CPTu soundings. During the static load tests of both piles, vibrating wire extensometers measuring system was used (six and seven retrievable Geokon 1300 Model A-9 extensometers). In both cases, elongation of the pile shafts after unloading was observed, which allowed to qualify these cases for further analysis.
Pile no. 6293, with a diameter of
The interpretation results of extensometric measurements have already been presented in Fig. 1 and 2. Fig. 4 shows measured deformations of the individual pile sections, where an evident elongation of sections G3–G6 can be observed after the pile was completely unloaded.
In the first stage, test and measurement results were interpreted without any residual force effect. The result of such an interpretation in the form of force distribution along the pile in successive load steps is shown in Fig. 2. After further analyses and calculations, charts of unit soil resistance mobilisation along the shaft
In the second stage, test results were interpreted according to the influence of residual force
For comparative purposes, the graphs of unit resistances
Pile no. 9, with a diameter of
When analysing the interpretation results of the considered example no. 2, it should be noted that the value of the
The comparison presented in Fig. 18 shows regularity similar to that observed in example no. 1. Taking the residual force
The example cases and analyses presented in the article show that the presence of the residual force
The proposed method (procedure) of identifying and estimating the value of residual force in instrumented piles is, according to the authors, very clear and easy to apply. It is based on shaft deformation changes measured during pile unloading. The observed length increase in some of the pile measuring sections is a clear evidence of the residual force presence in the pile before the loading test. The advantage of this method is also the fact that it takes into account non-linear stiffness characteristics and plastic (permanent) deformations of the concrete pile.
The discussed method has been applied to screw displacement piles, in which the presence of residual force is a common phenomenon and where a measurement system built of retrievable vibrating wire extensometers performing section readings was used. Nevertheless, it can also be applied to piles of other technologies, if only relaxation (elongation) of their shafts after unloading is observed in the measurements. Residual forces may also occur in bored piles, especially with injection under the base and in driven piles. The method is also suitable for other measurement systems, for example, optical fibres.
In order to increase the accuracy of residual force identification, it is advisable to perform one or more additional load–unload loops besides the standard load and unload cycle.
In the case of no elongation of the measuring sections after unloading the pile or obtaining relatively low values of the residual force, its influence may be ignored in the interpretation of the pile test results.
The dissemination of the residual force identification method in combination with good solutions to other measurement problems (listed in the first paragraph) will undoubtedly increase the quality and credibility of the results of static load tests on instrumented piles and restore their attractiveness and popularity.
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