Analysis of the vertical load bearing capacity and settlement of a pile group

Transcription

1 Enineerin manual No. 17 Updated: 07/2018 Analysis of the vertical load bearin capacity and settlement of a pile roup Proram: Soubor: Pile Group Demo_manual_17.sp The objective of this enineerin manual is to explain how to utilize the GEO 5 PILE GROUP proram. Introduction All analyses in the Pile Group proram can be divided into two roups: Sprin Method, Analytical solutions. The Sprin Method calculates the deformation of the entire pile foundation and determines the internal forces alon the lenths of the individual piles. The loadin is defined as a eneral active combination of N, M, M, M, H, H. An important result of a sprin method analysis is the x y z x y rotation and the displacement of the riid pile cap and the dimensionin of the reinforcement cae for individual piles. The Sprin Method is further explained in the followin chapter 18. Analysis of deformation and dimensionin of a pile roup. The analytical solution is used to analyse the vertical bearin capacity of a pile roup loaded solely by a vertical normal force. This type of analysis yields the vertical bearin capacity of the pile foundation and the averae settlement of the pile. The analytical solution is further divided accordin to the soil type: for cohesive soils, for cohesionless soils. The vertical bearin capacity of a pile roup in cohesive soil is considered in undrained conditions. It is determined as the bearin capacity of an earth body in the form of a prism drawn around the pile roup accordin to the FHWA. Only the total soil cohesion (undrained shear strenth) c u is specified for the purpose of the analysis. 1

2 The settlement of a pile roup in cohesive soil (in undrained conditions) is based on the calculation of the settlement of a substitute spread foundation (the so-called consolidation settlement of a pile roup, often abbreviated as the 2:1 method). In order to assess the pile roup settlement, the analysis incorporates the influence of the foundation depth and the thickness of the deformation zone accordin to the methodoloy for the calculation of the spread foundation settlement. When doin a pile roup settlement analysis in Czechia or Slovakia, it is possible to use the procedure accordin to CSN Ground under spread foundations. The assessment of a pile roup in cohesionless soil is based on procedures identical to those used for the analysis of a sinle pile in cohesionless soil (as described in chapter 13. Analysis of the vertical load-bearin capacity of a sinle pile). The only addition to the process is the usae of the so-called efficiency of a pile roup, which reduces the total vertical bearin capacity of the pile foundation. The loadin curve of a pile roup in cohesionless soil is constructed the same way as the curve for a sinle pile (shown in chapter 14. Analysis of sinle pile settlement) accordin to prof. H. G. Poulos, with the exception of the total settlement of the pile roup. The total settlement is increased by the so-called roup settlement factor f, which allows for the roup effect of individual piles. The manitude of this parameter depends on the eometrical arranement of the pile roup. Problem specification The eneral specification of the problem was described in the previous chapter (12. Pile foundations Introduction). Carry out all the calculations for the vertical bearin capacity of a pile roup in accordance with EN (DA 2) based on problem 13. Analysis of the vertical load-bearin capacity of a sinle pile. The resultant of the total loadin, which consists of N, M y, H, acts at the upper base of the pile cap, riht at its centre. x Problem specification schema pile roup 2

3 Solution We will use the GEO 5 PILE GROUP proram to analyse this problem. To simplify the problem and quicken the input of the eneral problem parameters (the desin, soil, assinin and the profile) we will import data from problem no. 13. Analysis of the vertical load-bearin capacity of a sinle pile. In this analysis, we will assess the pile roup usin the same analytical methods that we previously used when analysin a sinle pile (NAVFAC DM 7.2, EFFECTIVE STRESS and CSN ). We will focus on other input parameters that affect the overall results. Specification procedure In the Settins frame we will click on the Select settins button and then choose the Standard EN 1997 DA2 calculation settin. We will keep the analysis type option on analytical solution. In our particular case we will consider the type of soil to be cohesionless because we will assess the piles in drained conditions. Dialoue window Settin list Frame Settins We will import the data to avoid inputtin everythin aain. In order to import the data into the Pile roup proram, we first have to open the file from manual no Analysis of vertical bearin 3

4 capacity of a sinle pile in the GEO5 Pile proram. In the upper toolbar, we will click on the Edit button and then select the Copy data option. Proram Pile Subsequently, in the GEO 5 Pile Group proram, we will aain click on the Edit button in the upper toolbar and choose the Paste data option. Throuh this step, the data required for the analysis will be transferred and a sinificant part of the work with insertin input data will be done. Proram Pile Group In the dialo window that appears, we will choose to paste all data except for Settins, Load - LC and GWT + subsoil. Dialoue window Data to paste 4

5 Now we will move to the Structure frame. Here we can specify the round plan dimensions of the base slab (the pile cap), the number of piles in the roup, their diameter and the spacin of their centres (between piles in direction x, or y ). We will set the width of the pile cap as 6.50 m and input the number of piles as 2 in both x and y directions. Frame Structure 5

6 Then, in the Geometry frame, we will define the depth from the round surface, the pile head offset, the pile cap thickness and the lenths of all piles in the roup. All the individual piles in the roup have equal diameters and lenths. Frame Geometry 3 In the Material frame we will specify the unit weiht of the structure = 23.0 kn m. Frame Material After that, we will define the loadin. The vertical bearin capacity of a pile roup is analysed usin desin loads; the service load is used in a settlement analysis. Click on the button Add and add one new desin load and one new service load as shown in the fiures below. 6

7 Dialoue window New load Desin (calculation) load Dialoue window New load service (imposed) load We will carry out the pile roup assessment in the Vertical capacity frame. To meet the condition of reliability, the value of R must be bier than the manitude of the actin desin load V d (for more details visit the proram help F1). Usin the NAVFAC DM 7.2 analysis method and pile roup efficiency La Barré (CSN ) accordin to the initial analysis settins, the results of the vertical bearin capacity of the pile roup are as follows: 7

8 La Barré (CSN ): = R = kn Vd = kn SATISFACTORY Frame Vertical cap. Note: The calculated vertical bearin capacity of a pile roup in cohesionless soil must be reduced because individual piles statically affect each other. The assessment in the proram contains several methods of determinin the pile roup efficiency. This dimensionless fiure (usually within the rane from 0.5 to 1.0) reduces the total vertical bearin capacity of the pile roup the number of piles in the roup n x, n y ; the spacin of piles in the roup on centres s x, s y ; the diameter of piles in the roup d. R with respect to: The pile roup efficiency depends solely on the set eometry of the pile roup, not on the analysis method. 8

9 We can check how the vertical bearin capacity chanes when we use other methods of determination of pile roup efficiency. First, we will o back to the Settins frame. There, we will click on the Edit button in the bottom centre part of the screen and select the remainin possibilities UFC A, and Seiler-Keeney step by step in the Pile roup tab. Dialoue window Edit current settins Usin other analysis methods, the calculation in the proram is analoous to the problem solution from manual no. 13. Analysis of vertical bearin capacity of a sinle pile. In the case of the effective stress method, we will set the coefficient of bearin capacity N p to 30. The results of the analysis of the vertical bearin capacity of a pile roup in cohesionless soil (i.e. in drained conditions) relative to the method used and the pile roup efficiency are presented in the followin table: La Barré (CSN ): = 0. 84, UFC A: = 0. 80, Seiler-Keeney: =

10 EN , DA2 (cohesionless soil) Analysis method Efficiency of pile roup Bearin capacity of sinle pile kn R c Bearin capacity of pile roup kn NAVFAC DM EFECTIVE STRESS CSN Summary of results Vertical bearin capacity of pile roup in drained conditions R Conclusion (vertical capacity of a pile roup) The calculated vertical bearin capacity of a pile roup R in cohesionless soil must be reduced (usin the so-called pile roup efficiency ) because individual piles statically affect each other. In eneral, individual piles in a roup affect each other more when the spacin on centres is decreased. The desiner should always carefully consider, whether to use the calculation in drained or undrained conditions for an analytical solution of vertical bearin capacity of a pile roup. The two calculation types are sinificantly different. Analysis of pile roup settlement The analysis of a pile roup settlement is completely identical with that applied to a sinle pile, the only difference is that the calculated settlement is in addition multiplied by the roup settlement factor f. Note: The manitude of the roup settlement factor f depends on the eometrical arranement of the pile roup, i.e. the diameter of piles in the roup and the width of the pile cap. We analyse the settlement of a pile roup accordin to the Poulos theory. We will use values of modulus E s from manual no. 14 Analysis of sinle pile settlement (17 MPa for the 1 st layer, 24 MPa for the 2 nd layer). The maximum settlement is considered as 50 mm. 10

11 Frame Settlement NAVFAC DM 7.2 method The analysis results are presented in the followin table: Analysis method of vertical bearin capacity of pile roup Load at the onset of mobilization of skin kn friction R yu Settlement of pile roup s mm usin force V = 4000 kn NAVFAC DM EFECTIVE STRESS CSN Summary of results Settlement of pile roup accordin to Poulos Conclusion (pile roup settlement): It follows from the analysis results that the vertical bearin capacity of a pile roup is different as far as the total settlement is concerned. The analysis of the pile roup settlement in cohesionless soil (drained conditions) is based on the linear settlement theory, which needs the values of skin friction R s and the resistance on pile base R b as input. In contrast, the settlement of a pile roup in cohesive soil (undrained conditions) is based on the calculation for a substitute spread foundation. This calculation method is often titled the consolidation settlement of a pile roup or commonly abbreviated as the 2:1 method. For this pile roup settlement assessment, the effect of the depth from round surface and the depth of the deformation zone in 11

12 accordance with the methodoloy for assessin the settlement of spread foundations is introduced into the calculation. The two calculation methods sinificantly vary and provide completely different results. The GEO5 developers recommend the vertical bearin capacity and settlement of a pile roup to be calculated accordin to local customs. 12