Analysis of vertical load-bearing capacity of a single pile

Transcription

1 Engineering manual No. 13 Udated: 06/2018 Analysis of vertical load-bearing caacity of a single ile Program: File: Pile Demo_manual_13.gi The objective of this engineering manual is to exlain how to use the GEO 5 PILES rogram for the analysis of vertical load-bearing caacity of a single ile in a secified ractical roblem. Problem secification A general secification of the roblem was given in the revious chater (12. Pile foundations Introduction). All analyses of the vertical load-bearing caacity of a single ile shall be carried out in comliance with EN (Design aroach 2). The resultant of the loading comonents N M y H acts at the ile head. 1,,1, x,1 Problem secification schema single ile Solution We will use the GEO 5 PILES rogram to analyse the roblem. In the text below, we will describe the solution to this roblem ste by ste. In this analysis we will assess a single ile using various analytical calculation methods (NAVFAC DM 7.2, EFFECTIVE STRESS and CSN ) and focus on the inut arameters, which influence the overall results. 1

2 Secification inut First of all, click on the select settings button (on the bottom of the screen) in the Settings frame and then select otion no. 4 - Standard EN 1997 DA2 analysis setting. Further, we set the method of the analysis of a vertical load-bearing caacity of a ile using the analytical solution. In our case we will assess the ile in drained conditions. Dialog window Setting list For the initial assessment of the ile, we will use the NAVFAC DM 7.2 method, which is the default one for this analysis setting (see the figure below). We will not analyse horizontal bearing caacity in this task, so we check the Do not calculate horizontal bearing caacity otion. Frame Settings Next, go to the frame Profile, where we ll add a new interface at 6,0 m. 2

3 Frame Profile add a new interface Then, we will go to the frame Soils, where we define the arameters of soils required for the analysis and assign them to the rofile. The NAVFAC DM 7.2 method requires that the soil tye is defined first, i.e. whether it is a cohesive or cohesionless soil layer. All the arameters listed below influence the magnitude of skin friction R s kn. Soil (Soil classification) Unit weight 3 kn m Angle of internal friction ef c Cohesion of soil ef / c u kpa Adhesion factor Bearing caacity coefficient CS Sandy clay, firm consistency S-F Sand with trace of fines, medium dense soil 18,5 24,5 - / 50 0,60 0,30 17,5 29,5 0 / - - 0,45 Table with the soil arameters Vertical bearing caacity (analytical solution) For the 1 st layer, which is considered as an undrained cohesive soil (class F4, firm consistency), we must in addition secify the total soil cohesion (undrained shear strength) c u kpa and the so-called adhesion factor. This factor is determined relative to the soil consistency, ile material and total soil cohesion (for more details visit the rogram hel F1). 3

4 Dialog window Add new soils soil CS For the 2 nd layer, which is considered a cohesionless soil (class S3, medium dense), we must in addition secify the angle of skin friction, which deends on the ile material. Furthermore, we must define the coefficient of lateral stress K, which is affected by the tye of loading (tension ressure) and by the ile installation technology (for more details visit the rogram hel F1). To simlify the roblem, we will select the otion calculate for both variants. 4

5 Dialog window Add new soils soil S-F Then, assign the soils to the rofile in the frame Assign. Frame Assign assigning soils to rofile 5

6 Next, we will define the load acting on the ile in the frame Load. The design (calculation) loading is considered in the calculation of the vertical load-bearing caacity of the ile, while the service load is considered in the calculation of settlement. Therefore, we will add a new design load as shown in the figure below. Dialogue window New load In the Geometry frame we will secify the circular cross-section of the ile and further determine its basic dimensions, i.e. its diameter and length. Then, we will define the tye of the ile installation technology. Geometry frame In the Material frame, we will secify the material characteristics of the ile the unit weight of 3 the structure = 23.0 kn m. 6

7 Material frame We will not change anything in the GWT + subsoil frame. In the Stage settings frame we will leave the ermanent design situation set and then continue to the assessment of the ile using the Vertical caacity frame. Analysis of vertical load-bearing caacity of a single ile NAVFAC DM 7.2 analysis method In the Vertical caacity frame, we must firstly secify the calculation arameters affecting the k magnitude of the ile base bearing caacity kn R b. First, we will define the critical deth analysis coefficient, which is derived from the so-called critical deth deending on the soil density (for more details visit the rogram hel F1). We will consider this coefficient as k = 1, 0. Another imortant arameter is the coefficient of bearing caacity N q dc dc, which is determined by the soil internal friction angle relative to the ile installation technology (for more details visit the rogram hel F1). In this case we will consider N = ef q 7

8 Vertical caacity frame assessment according to NAVFAC DM 7.2 The design vertical bearing caacity of a centrally loaded ile R c kn consists of the sum of the skin friction R s and the resistance on ile base R b. To meet the condition for reliability, its value must be higher than the magnitude of the design load V d kn acting on the ile head. NAVFAC DM 7.2: R = kn V = kn SATISFACTORY c d 0 Analysis of vertical load-bearing caacity of a single ile EFFECTIVE STRESS analysis method Now we will get back to the inut settings and carry out the analysis of the vertical bearing caacity of a single ile using other analysis methods (Effective stress and CSN ). In the Settings frame, click on the Edit button. Then, in the Pile tab, select the Effective stress otion. The other arameters will remain unchanged. 8

9 Dialog window Edit current settings Then, we will roceed to the Soils frame. This analysis method requires that we additionally define the coefficient of ile bearing caacity which affects the magnitude of skin friction R s kn. This arameter is determined by the soil internal friction angle more details visit the rogram hel F1). and the soil tye (for ef Dialog window Edit soil arameters soil CS 9

10 Dialog window Edit soil arameters soil S-F The other frames remain unchanged. Now we will get back to the Vertical caacity frame. For the Effective Stress method, we must first secify the value of the coefficient of bearing caacity N, which significantly affects the ile base bearing caacity R b kn the soil internal friction angle ef. This arameter is determined by and the soil tye (for more details visit the rogram hel F1). The significant influence of this arameter on the result is demonstrated by the following table: for N = 10 (ile base in clayey soil): R b = kn, for N = 30 (ile base in sandy soil): R b = kn, for N = 60 (ile base in gravelly soil): R b = kn. In our roblem, we consider the coefficient of bearing caacity N = 30 (the ile base in sandy soil). The guidance values of N can be found in the rogram hel for more details visit F1. 10

11 Vertical caacity frame assessment according to the Effective Stress method EFFECTIVE STRESS: R = kn V = kn SATISFACTORY c d 0 Analysis of vertical load-bearing caacity of a single ile CSN analysis method Now we will get back to the Settings frame, where we will change the analysis method for drained conditions by clicking the Edit button and changing the analysis method to CSN All the other inut arameters will remain unchanged. 11

12 Dialogue window Edit current settings Note: The analysis rocedure is resented in the ublication Pile foundations Comments on CSN (Chater 3: Designing, art B General solution according to grou 1 of the limit states theory, age 15). All rogram rocedures are based on the relationshis contained in this text, with the excetion of calculation coefficients, which deend on the assessment methodology adoted (for more details visit the rogram hel - F1). 12

13 Now we will go back to the frame Soils, where it is necessary to define effective soil arameters for each soil. Dialog window Edit soil arameters soil CS Dialog window Edit soil arameters soil S-F 13

14 Subsequently, we will re-assess the ile in the Vertical caacity frame. We will leave the coefficient of technological influence equal to 1.0 (the analysis of vertical load-bearing caacity of a ile without the reduction due to installation technology). Vertical caacity assessment according to CSN frame CSN : R = kn V = kn SATISFACTORY c d 0 Vertical load-bearing caacity of a single ile analysis results The values of the total vertical bearing caacity R of a ile differ deending on the analysis methods used and the inut arameters assumed by these methods: c NAVFAC DM 7.2: adhesion factor, ile skin friction angle, coefficient of lateral soil stress K, critical deth analysis coefficient dc k, coefficient of bearing caacity q N. EFFECTIVE STRESS: coefficient of ile bearing caacity 14,

15 coefficient of bearing caacity N. CSN : soil cohesion c ef kpa, soil internal friction angle. ef The results of the analysis of the vertical bearing caacity of a single ile in drained conditions relative to the analysis method used are resented in the following table: EN , DA2 (drained conditions) Analysis method Pile skin bearing caacity R s kn Pile base bearing caacity R b kn Vertical bearing caacity R c kn NAVFAC DM EFECTIVE STRESS CSN Summary of results Vertical bearing caacity of a ile in drained conditions The total vertical bearing caacity of a centrally loaded single ile of the design load R c is higher than the value V d acting on it. The fundamental reliability condition for the ultimate limit state is met; the ile design is therefore satisfactory. Conclusion It follows from the analysis results that the total vertical bearing caacity of a ile differs in each calculation. This fact is caused both by the different inut arameters and by the chosen analysis method. The assessment of iles mostly deends on the chosen analysis method and the inut arameters describing the soil. Designers should always use calculation rocedures for which they have the required soil arameters available, for examle values resulting from the results of geological surveys or values that reflect local ractices. It is certainly imroer to assess a ile using all analysis methods contained in the rogram and choose the best or the worst results. 15

16 For the Czech and Slovak Reublic, the GEO 5 software authors recommend calculating the vertical load-bearing caacity of a single ile using the following two methods: An analysis taking into consideration the value of the allowable settlement s = lim 25 mm (the rocedure according to Masoust, which is based on the solution of regression curves equations). An analysis according to CSN The ile analysis rocedure remains identical with that contained in CSN, but the loading and calculation coefficients reducing the soil arameters or the ile resistance are secified according to EN This analysis therefore fully comlies with EN