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Karpenko_2014_eng (Barcelona)

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Karpenko_2014_eng (Barcelona)

  1. 1. Peculiarities of implementation of primary numerical modeling when determination of the forces of negative friction on pile side surface in condition of its test with static load The State Scientific Research Institute of Building Constructions, Kiev, Ukraine Junior researcher - D. Karpenko Proceedings of the 23rd European Young Geotechnical Engineers Conference, Barcelona 2014
  2. 2. From experience we know that several factors (the type and location of possible sources of soaking the soil; value of settlement thickness and level of soil settlement because of their own weight; physical and mechanical characteristics of the underlying soil; permissible uneven of base deformation; vertical nature of section; depth of laying of ground beam and other) should be taken into account additionally in the design of foundations. The necessity to consider the additional actions on the piles (negative or downdrag friction force on the lateral surface of the pile) arising in case of outrunning settlements of surrounding soil is the main feature of pile foundations designing in settling soil because of its own weight when soaking or additional overload. Prognosis of bearing capacity of piles should be made taking into account the possibilities of decreasing of weak soil resistance during soaking and arising of negative friction forces on the lateral surface.
  3. 3. There are three most typical design cases for taking into account of soaking influence: 1) Soaking source is located directly within the pile foundation when the surrounding soil in contact area of the pile with soil under settlement is in water-saturated state (see Figure 1, a); 2) The source is located at some distance from the pile foundation, therefore a soil has a natural or prescribed humidity in the upper part along all contact and it has humidity close to full water saturation in bottom part (see Figure 1, b); 3) The gradual rise of the groundwater level, which leads to increasing of soil moisture up to full water saturation, therefore the pile interaction with soil at the top is determined by natural or prescribed humidity, and by complete water saturation of the soil at the bottom (see Figure 1, c). Figure 1. Cases of pile foundation soaking: 1 – piles; 2 – soaking source; 3 – curve of distribution of water to the sides of soaking source; 4 – rise of the groundwater level; 5 – impermeable soil layer.
  4. 4. In addition to loads arising under the structure action, the pile may be subject to actions arising under the additional movement of the soil in which it is installed. This phenomenon is known as Europe's term "negative friction on the lateral surface of the pile" when we say about further consolidation of soil around the pile. This consolidation leads to additional forces of friction on pile lateral surface directed down along pile and it reduces the pile bearing capacity in general. The soil moving relative to the pile in other areas (e.g., upward or horizontally) can cause lifting, stretching or lateral displacement of the pile. Standard EN 1997-1 requires using one of the two listed below ways for neutralization of movements: 1) to analyze the interaction of piles with the soil when taking into account the actions of the "soil- upper structure"; 2) bearing capacity of the pile should be considered as the upper limit of its value when taking into account the resistance of the soil as an equivalent of direct action, which is defined separately. The direct methods are absent in these Norms for such detailing.
  5. 5. Everyone knows that the most reliable data on the bearing capacity of drilling piles can be obtained only when they are tested in static compressing load. Test of the piles are generally carrying out for the natural state of the soils. The negative friction is taking into account when possible flood of territory and when the piles are testing for withdraw load. Such tests are difficult (time consuming, long time and expensive) if loess thickness is 25...30 m, and the design results are approximate. Especially since the gotten data are the result of the instantaneous load of piles, and they do not permit to analyze the change of the stress-strain state of a weak base in time or they are characterizing a specific area or engineering- geological section only. It complicates the formulation of the general laws of pile behavior. In practice, it is possible to solve the problem of calculation of pile foundations on base of experimental data if we use well-developed numerical methods.
  6. 6. Many studies were performed according to the arrangement of the early experimental bored piles at the construction site. Piles dimensions were taken on the recommendations of the simulation. The value of the negative forces of friction Pn was simulated as follows. The withdraw load was applied to top of single pile without extension. This pile was installed between a day ground surface and mark corresponding to the calculated depth of action of negative friction of soil (soil in natural moisture and water saturation of settling soil). The withdraw load was gradually increased up to time when software complex PLAXIS gives message "about development of plastic deformation of the soil". This moment will correspond to the maximum axial withdraw load (the amount of negative friction). Modeling of single located pile cluster from 4 piles was carried out (as it was foreseen in design) in order to approximate the initial evaluation of the work of single pile taking into account predefined negative friction forces on the lateral surface of the pile.
  7. 7. Í àñèï í èé ãðóí ò (ñóãëèí êè í åî äí î äí î ð³äí ³ â ñóì ³ø êó ç áóä³âåëüí èì ñì ³òòÿì äî 10%) Ñóãëèí êè ëåñî â³ , ï èëóâàò³ æî âòî -áóð³, òâåðä³, ï ðî ñ³äàþ ÷³ Ñóï ³ñêè ëåñî â³ ï èëóâàò³, êàðáî í àòí ³, òâåðä³, ï ðî ñ³äàþ ÷³ Ñóãëèí êè ëåñî â³ ëåãê³, ï èëóâàò³, æî âòî -áóð³ ç ÷åðâî í èì â³äò³í êî ì , êàðáî í àòí ³, òâåðä³, ï ðî ñ³äàþ ÷³ Ñóï ³ñêè ëåñî â³ ï èëóâàò³ ï àëåâî -æî âò³, ç ï ðî æèëêàì è êàðáî í àò³â, ï ðî ñ³äàþ ÷³ Ñóãëèí êè ëåñî â³ ï èëóâàò³ , ç âêëþ ÷åí í ÿì êàðáî í àò³â (IL= -0.5; E=23Ì Ï à), ï ðî ñ³äàþ ÷³ Ñóï ³ñêè ëåñî â³ ï èëóâàò³, êàðáî í àòí ³, â³ä òâåðäèõ äî í àï ³âòâåðäèõ (I L= -0.25; E=21Ì Ï à) ï ðî ñ³äàþ ÷³ Ñóï ³ñêè ëåñî â³ òÿæê³ , ï èëóâàò³, ç âêëþ ÷åí í ÿì êàðáî í àò³â (IL= -0.14; E=21.5Ì Ï à) Ãëèí è ï èëóâàò³ ÷åðâî í î -áóð³, òâåðä³, ç âêëþ ÷åí í ÿì êàðáî í àòí èõ ñòÿæî ê (IL= -0.27; E=19Ì Ï à) Ãëèí è ï èëóâàò³ áóðî âàòî -ñ³ð³, òâåðä³, ç âêëþ ÷åí í ÿì êàðáî í àò³â (IL= -0.27; E=19Ì Ï à) Ï ³ñêè ñåðåäí ³ ç ë³í çàì è äð³áí èõ, æî âòî -ñ³ð³, ç ï ðî ø àðêàì è ñóï ³ñê³â, ñóãëèí ê³â, ì àëî âî ëî ã³ ³ âî ëî ã³, ù ³ëüí ³ (E=41Ì Ï à) Ãëèí è ñ³ð³, çåëåí î âàòî -ñ³ð³, òâåðä³, ç ï ðî ø àðêàì è ï ³ñê³â ³ ñóï ³ñê³â (E=18Ì Ï à) Óì î âí ³ ï î çí à÷åí í ÿ ˳òî ëî ã³÷í ³ âèäè ãðóí ò³â 1-1 Filled soil - Loessial loams, silty, firm, subsiding - Loessial loamy sands silty, firm, subsiding - Light loessial loams, silty, firm, subsiding - Loessial loamy sands silty, subsiding - Loessial loams silty, subsiding - Loessial loamy sands silty, firm to half-firm, subsiding - Loessial loams heavy, silty, subsiding- Clays silty, firm - Sands of middle size, containing low humidity, compact - Clays firm, with sand and loamy sand layers - Legends Lytological types of soil Figure 2. Embarking of a pile foundation on geologic section. Modeling of single located pile cluster from 4 piles was carried out (as it was foreseen in design) in order to approximate the initial evaluation of the work of single pile taking into account predefined negative friction forces on the lateral surface of the pile.
  8. 8. Figure 3. Finite-element 3D model "pile foundations-settling base".
  9. 9. Figure 4. Generation of water pressure, kN/m2 .
  10. 10. Figure 5. Iso-fields of soil displacement in vertical direction Uy, m (section 1-1).
  11. 11. Figure 6. Iso-fields of soil displacement in vertical direction Uy, m (section 2-2).
  12. 12. Figure 7. Iso-fields of soil displacement in vertical direction Uy, m (section 3-3).
  13. 13. Table. Comparison of field tests of piles with expansion according to numerical studies.
  14. 14. According to the calculations of numerical simulations, the bearing capacity of the pile cluster, taking into account the forces of negative friction on the lateral surface of the pile is 4468 kN, provided that the working design is 4000 kN under prescribed conditions. In the numerical simulation there was also found that the process of gradual infiltration of water into the soil and its watering operates on a pile in two ways. From the first side, the water saturation has place for increased soil volume and most part of the pile cluster is under overhanging soil action as the flow of water is action on loess soil. From the second side, the cohesion forces on contact of pile with soil are decreased when the water content is reduced. I.e., the specific cohesion and angle of internal friction is reduced by more than 10...15%. when soaking the loose (loess) soil The value of the bearing capacity of piles is under influence of changes in soil under expansion due to soil compaction. Thus the values of strength and deformability can be increased by 10% or more.
  15. 15. 2) The negative friction force depends on the case of base soaking, depth of soaked layer, the properties of settling soil of base and other factors. The soaking by parallel layers of settling loose base below is more dangerous for the drilling piles (including the piles with extension) then top soaking; 3) The proposed simplified method allows to research preliminary the drilling piles work (including piles with extension) in structure of clusters) in loose settling soil due to its own weight in view of the negative forces of friction for different cases of soaking with necessary accuracy to select the structural measures for design. This method has a strict definition, i.e. it provides the reliable estimates in determining the allowable load on pile foundations of the specified type under construction architectural designs of the project; 4) The results of these studies were used in the construction of the shopping center wholesale and retail trade in Zaporozhe, which confirmed the reliability of the calculations. GENERAL CONCLUSIONS The following conclusions may be done on the basis of detailed analysis of the interaction of the pile with surrounding settlement of loess massif: 1) Comparison of the calculated values of the obtained negative friction forces on the lateral surface of the pile with the experimental data shows that the difference of these values is 8 ... 13% ( in terms of the construction site);
  16. 16. THANK YOU FOR YOUR ATTENTION Peculiarities of implementation of primary numerical modeling when determination of the forces of negative friction on pile side surface in condition of its test with static load

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