MIDAS Geotechnical Training Series

Transcription

1 MIDAS Geotechnical Training Series Fully Coupled vs Consolidation Analysis Seongwan Bae Angel F. Martinez MIDAS Information Technology Integrated Solver Optimized for the next generation 6-bit platform Finite Element Solutions for Geotechnical Engineering

2 Project Overview (Fully Coupled vs Consolidation Analysis) Integrated Solver Optimized for the next generation 6-bit platform Finite Element Solutions for Geotechnical Engineering

3 Overview STEP 0 Geometry Import STEP 0 Geometry Works STEP 0 Mesh generation STEP 0 Boundary Conditions STEP 0 Load Conditions STEP 06 Define Water Level STEP 07 Define Construction Sequence STEP 08 Analysis Case STEP 09 Perform analysis and check result

4 0 Consolidation_by several methods Overview In order to accelerate consolidation without additional embankment and potential shear failure of soil, a uniform atmospheric pressure can be applied to each depth of the soft layer. This example is to compare the results between Suction drain method and Embankment with free drainage by fully coupled and consolidation methods Main objectives for this analysis are as follows. Initial and long term settlements during and after construction Compressibility Lateral ground flow Slope failure / reduction of bearing capacity Differential settlement on adjacent structures, services (road) Improvement methods drainage, solidification, compaction -

5 0 Material for Soil and Structures Name Embankment AS Ac AS Material Isotropic Isotropic Isotropic Isotropic Model Type Elastic Elastic General Modified Cam Clay Elastic Elastic Modulus (E) Poisson s Ratio(v) Unit Weight(r) Ko Porous Unit Weight(Saturated) Initial Void Ratio(eo) Permeability (kx,ky,kz) (m/day) Non-Linear OCR Slope of Consol line (λ) Slope of Over Consol Line (k) Slope of critical State Line (M)

6 0 Soil Property (D Plain Strain Element) Name Embankment AS Ac AS Property D D D D Model Type Plain Strain Plain Strain Plain Strain Plain Strain Material Embankment AS Ac AS -6

7 0 Construction Sequence ( sets) * Analysis Case (Fully Coupled Suction Drain). Initial state of ground. Apply suction to each depth at an interval of m (00 days). Leave time (000 days) * Analysis Case (Fully Coupled Embankment). Initial state of ground. Embankment (00 days). Leave time (000 days) * Analysis Case (Consolidation Embankment ). Initial state of ground. Embankment (00 days). Leave time (000 days) -7

8 Fully vs Semi Coupled Analysis Semi coupled analysis Consider P.W.P for stress analysis Time-Independent nonlinear static analysis Drained: Long-term stability Undrained: Short-term stability Seepage Stress [One way coupled] Fully coupled analysis Time-integrating method / Time-Dependent analysis Changes in stress/stain/pore pressure with time Governed by permeability of ground Seepage Stress [Two way coupled interaction] Semi Coupled or Sequential analysis is the simplest way to consider the pore water pressure distribution by conducting seepage analysis beforehand, and reflecting it in the total stress/effective stress relationship equation of the stress analysis conducted in the following step. This method can be used to understand the static stress state of the given steady groundwater flow. However, since deformation due to stress analysis does not influence the seepage phenomenon inversely, there is no two-way coupling. Fully-coupled Stress-Seepage analysis is the two-way coupled analysis between seepage analysis and stress analysis. Both analyses are used to solve the coupled equation. It can display the pore water pressure, stress or deformation changes with time.

9 Consolidation vs Fully Coupled <Common> Two way coupled interaction Time dependent behavior based on quasi-static analysis <Difference> Consolidation analysis Fully coupled analysis Fully coupled DOF for displacement and excessive pore stress Constant hydrostatic pressure Simulate dissipation of excessive p.w.p with time DOF for displacement and total pore stress Simulate changes in hydrostatic pressure with time. All seepage boundaries (head, flux, review) are available. Consolidation [Relation b/w two analyses] The consolidation analysis begins with the assumption that steady state pore water pressure can be maintained, and is used to see the changes in excess pore water pressure. In other words, this analysis is used to simulate the phenomenon of how excess pore water pressure changes with the changes in load/boundary conditions. Fully-coupled Stress-Seepage analysis does not follow assumption that steady state pore water pressure is maintained. Hence, it is suitable for simulating the transient seepage phenomenon, stress analysis and stability in abnormal condition in a fully coupled form. Unlike the consolidation analysis, it is possible to define the changes in seepage boundary conditions with time, boundary flow rate etc. In other words, for Fully-coupled Stress-Seepage analysis, it is possible to use all the transient seepage boundary conditions, structural load and boundary conditions.

10 Key Boundary Conditions General Water Level [Line(D), Surface(D)] Water Level for Mesh Set Create a changing groundwater level by selecting a geometry shape on the work screen. Seepage / Fully Coupled Analysis Nodal Head Nodal Flux Surface Flux Review Input the head of the model. Both the constant head value for steady state analysis and the changing head value for Transient analysis can be entered by applying the Seepage boundary condition function. The Nodal head is used as a boundary condition for Seepage/Consolidation analysis (Fully-Coupled). Consolidation Analysis Draining Condition Used to simulate the domain where the excess pore pressure is 0 (drain). The Drainage condition is used as a boundary condition for Consolidation analysis. Non Consolidation The elements have an additional pore pressure degree of freedom, as well as displacement degree of freedom, at the nodes. Used to model non-consolidation layers to turn off PP degree of freedom. -0

11 Concepts of Coupled Analysis Fully Drained < Outline of coupled analysis> Semi-Coupled Consolidation Fully-Coupled Nodal head(total head) O X O Nodal head(pressure head) O X O Boundary review O X O Drainage condition X O O (total head=0) Non-consolidated condition X O O Node/surface flow O X O <List of applicable load/boundary conditions for each coupled analysis type>

12 Modeling (Fully Coupled vs Consolidation Analysis) Integrated Solver Optimized for the next generation 6-bit platform Finite Element Solutions for Geotechnical Engineering

13 0 Create New Project Main Menu > New Analysis Setting > Model Type > D Unit System > kn-m-day -

14 0 Geometry works (Create or Import from AutoCAD) Main Menu > Import > DXF D (Wireframe) Select AutoCAD DXF File > Embankment Consolidation vs Fully Coupled CAD.dxf Open Select OK -

15 0 Define Material / Property for Soil & Structures * Can Import from the existing model file (Refer to Project Overview > Material / Property) Mesh > Prop. > Property > Import Select Embankment Consolidation vs Fully Coupled Final.gts file Select All Select OK -

16 0 Generate Mesh (D Element) Mesh > Generate > D > Auto - Area Select Edge(s) > Select edges for Embankment as highlighted in the figure. Input element Size : (m between two nodes) Select Property : Embankment Input Mesh Set Name : Embankment 6 Select Apply (Follow the same procedure for below soil layers) 6 It is possible to input different size of element for each area. All nodes at each boundary line will be connected automatically. This is to control Mesh Quality and the number of elements. -6

17 0 Generate Mesh (D Element) Mesh > Generate > D > Auto - Area Select Edge(s) > Select edges for AS as highlighted in the figure. Input element Size : (m between two nodes) Select Property : AS Input Mesh Set Name : AS 6 Select Apply (Follow the same procedure for below soil layers) 6 After creating mesh for all area at once, it is possible to change Property. Go to Mesh > Element > Parameters > D -7

18 0 Define Boundary Condition (Water Level) * Water Level Function will be defined for initial water level. Static / Slope Analysis > Boundary > Water Level > Edge Select Target Edge(s) > Select edges to define water level as highlighted in the figure. Interval : (m) Initial Water Level Input Name : Water Level () Select OK In the works tree, there are water level functions created automatically. -8

19 0 Define Boundary Condition (Suction Drain) Seepage/Consolidation Analysis > Boundary > Nodal Head Select Object(s) > Select 9 edges for Suction drain as highlighted in the figure. Value : - Select Type : Pressure Boundary Set Name : Suction Practical suction pressure is within the range of 0~00kPa, depending on the field condition. - = - x 9.8kN/m = 9kPa -9

20 0 Define Boundary Condition (Drain by Total Head) Seepage/Consolidation Analysis > Boundary > Total Head Select Object(s) > Select 0 edges as shown Value m Pressure Boundary Set Name Draining Total Head Repeat for bottom layer with m Pressure Pore pressure set to given values -0

21 0 Define Boundary Condition (Consolidation) Seepage/Consolidation Analysis > Boundary > Draining Condition Type Edge Select Object(s) > Select 0 edges as shown Boundary Set Name Draining Condition Pore pressure goes to 0 on selected nodes. -

22 0 Define Boundary Condition (Consolidation) Seepage/Consolidation Analysis > Boundary > Non Consolidation Type Element Select Embankment Boundary Set Non Consolidation Pore pressure degree of freedom turned off. -

23 0 Define Boundary Condition (Ground) Seepage/Consolidation Analysis > Boundary > Constraint > Auto Boundary Set Name : Ground Boundary -

24 06 Load Condition (Self Weight) Seepage/Consolidation Analysis > Load > Self Weight Load Set Name : S/W -

25 07 Define Construction Stage (Create Stage Set) * Construction stage will be defined for each stage set. Seepage/Consolidation Analysis > Construction Stage > Stage Set Stage Type > Consolidation Set Name : Consolidation Embankment Select Add Stage Type > Fully Coupled Stress Seepage 6 Set Name : Suction Drain Select Add 8 Repeat for Fully Coupled Embankment Set Select Suction Set and Click Define CS -

26 08- Define Construction Stage (Suction Drain) Stage : Initial state 7 Time Step : Initial value () Activated Data : Mesh Sets for initial state of ground (D elements only except Embankment ), Ground Boundary, Self Weight Check Define Water Level for Global Input m, Select Water Level () 6 6 Check Clear Displacement Select Save Drag & Drop 7 Select New -6

27 08- Define Construction Stage (Suction Drain) Stage : Suction Time Step : 00 days / 0 steps Activated Data : Suction Select Save Select New Drag & Drop -7

28 08- Define Construction Stage (Suction Drain) Stage : Leave Time Time Step : 000 days / 0 steps Deactivated Data : Suction Select Save Select Close Drag & Drop *Create other sets by same procedure or by making copies of sets and modifying boundaries applied. -8

29 08- Define Construction Stage (Embankment) Stage : Initial state 7 Time Step : Initial value () Activated Data : Mesh Sets for initial state of ground (D elements only except Embankment ), Ground Boundary, Self Weight Check Define Water Level for Global Input m, Select Water Level () 6 6 Check Clear Displacement Select Save Drag & Drop 7 Select New -9

30 08- Define Construction Stage (Embankment) Stage : Embankment Time Step : 00 days / 0 steps Activated Data : Embankment And Draining Total Head Select Save Select New Drag & Drop -0

31 08- Define Construction Stage (Embankment) Stage : Leave Time Time Step : 000 days / 0 steps Select Save Select Close -

32 08- Define Construction Stage (Consolidation) Stage : Initial state 7 Time Step : Initial value () Activated Data : Mesh Sets for initial state of ground (D elements only except Embankment ), Ground Boundary, Self Weight Check Define Water Level for Global Input m, Select Water Level () 6 6 Check Clear Displacement Select Save Drag & Drop 7 Select New -

33 08- Define Construction Stage (Consolidation) Stage : Embankment Time Step : 00 days / 0 steps Activated Data : Embankment, Drainage Condition and Non- Consolidation Select Save Select New Drag & Drop -

34 08- Define Construction Stage (Consolidation) Stage : Leave Time Time Step : 000 days / 0 steps Select Save Select Close -

35 09 Analysis Case Analysis > Analysis Case > General Title : Case Name Solution Type : Construction Stage Select Construction Stage Set : Suction Drain Analysis Control : Check each option in Red Box 6 Select OK 7 Select OK 7 (Create nd and rd Analysis case for Embankment stage by fully coupled method and consolidation method set with the same options. 6 -

36 09 Review of Analysis Control (Geometric Nonlinearity) Analysis > Analysis Case > General > Analysis Control Consider geometric nonlinear effects in stress, fully coupled and slope stability analysis. Analysis can take into account load nonlinearity. Reflecting the effects of follower loads, where the load direction changes with the deformation. In case of large deformation analysis, the user can check more reasonable behavior. In case of embankment, Initial Configuration option in General tab is applicable to check more realistic behavior. [Staged Consolidation analysis for embankment] [Geometric nonlinearity + Initial configuration option] Normal pressure Pressure in specified direction [Directional change of pressure load due to the large deformation] [Result comparison] [Linear geometry without option]

37 0 Perform Analysis and Check Results Analysis > Analysis > Perform Select OK -7

38 Post Processing (Pore Pressure) Consolidation method. From the RESULTS WORK TREE Check Pore Pressure Head for the last step in LEAVE TIME for each of the analysis cases. Fully Coupled Embankment Fully Coupled Suction -8

39 Post Processing (Settlement with time) Consolidation method. From the RESULTS WORK TREE Check Total Displacements for the last step in LEAVE TIME for each of the analysis cases. Fully Coupled Embankment Fully Coupled Suction -9

40 Post Processing (Settlement with time) Result > Advanced > Extract Select Analysis Set : Suction Drain Select Result Type : 6 Displacements Select Results : TY TRANSLATION(V) (vertical displacement) Select Nodal Results Extraction Select Table Select Two Column Export to Excel 6 Repeat process for other cases and graph settlement comparison 6-0

41 Results Comparison <Fully Coupled Suction drain> - Suction drain: -0 kpa (60days) Residual displacement Max displacement Suction drain period Suction drain removed [Maximum displacement vs. time] [Pore pressure distribution with suction drain] Stage (Embankment step ) <Consolidation> [Excess Pore Pressure distribution with suction drain] Stage (Embankment step ) [Excess Pore Pressure vs Time -

42 Fully vs Semi Coupled Analysis Initial water level <Excavation - dewatering> - Comparison of sequential and fully-coupled analysis layer layer [Semi Coupled Vertical displacement after days] [Full Coupled Vertical displacement after days] -

43 Pressure Head Fully vs Semi Coupled Analysis <Excavation - dewatering> - Comparison of sequential and fully-coupled analysis Pore Pressure vs Time [Semi Coupled Pore Pressure Head after days] Time (day) [Full Coupled Pore Pressure Head after days] -

44 END Integrated Solver Optimized for the next generation 6-bit platform Finite Element Solutions for Geotechnical Engineering