Rak-50 3149 Soil Test

Tutorial 1 - Simulation of a laboratory tests

1

T01. Simulation of laboratory tests: Soil Testing tool


2

Introduction

The SoilTest tool in PLAXIS has been created with the aim to compare the soil behaviour as defined by constitutive model and a set of input parameter values with the results of laboratory tests. It is possible to work on several models, and by considering the influence of the different constitutive parameters on the material behaviour, the user can check all the values PLAXIS has calculated during of the input. This process surely represents one of the primary aspects involved into a validation process of a FEM-based analysis.

The convenience of SoilTest is due to the possibility to simulate simple tests without the need of the creation of any finite element model. Since the utility is based on a single point algorithm, it totally ignores the weight of the sample: one is asked to input just a few parameters (depending on the selected model) and then run SoilTest.

This tutorial is the first step in becoming familiar with the use of the tool, allowing the performance of both oedometer and triaxial tests on a soil sample. The curves obtained from the tests will be the starting point for further considerations on the selected model; as for this exercise the user is asked to work with Mohr Coulomb model.


3

MOHR - COULOMB MODEL: OEDOMETER

Input

Firstly, the constitutive parameters and the modality of the test to perform have to be defined.

Start PLAXIS Input and select a new project from the Quick select window.

Through the Project properties menu (Figure 1) one can insert the general data of the project. The Project properties window contains the tab sheets Project and Model. The Project tab sheet contains the project name and description, the type of model (Plane strain or Axisymmetry) and acceleration data. The Model tab sheet reports the basic units for length, force and time as well as the dimensions of the draw area.

To carry out this exercise a title is needed, and eventually some arbitrary information in the Comments box; as already pointed out in the introduction, no geometry has to be created so the other General data can be left unmodified.

Figure 1. Project properties: Project tab sheet.


4

Click the OK button to proceed with the input of constitutive parameters.

Save the project selecting File Save as.. from the main menu.

In order to predict the soil behaviour, a suitable constitutive model with appropriate parameters has to be assigned.

Select the Materials button on the toolbar to load a material sets window showing the contents of the project data base (the same window can be reached by choosing Soil & Interface from Materials in the main menu).

In PLAXIS, both soil and material properties of different structures are stored in material data sets. There are basically four types of material sets: Soil & Interfaces, Plates, Geogrids and Anchors. All data sets are stored in a material database. Usually before the mesh generation, from the database, the data sets have to be assigned to the soil clusters or to the corresponding structural objects in the geometry model. The first soil test performed is an oedometer test on a sand specimen, so a Soil & Interfaces data set has to be defined (Figure 2).

Figure 2. Material sets window.


5

Create a new set by clicking the New button.

In the General tabsheet appearing (Figure 3), it is requested to insert a representative name for the soil type (i.e. Sand) as well as the constitutive model (Mohr-Coulomb) and the material type (Drained); a Comments box allows the user to write further information. As stated before, there is no need for any weight value.

Click the Next button or click the Parameters tab to proceed with the input of model parameters (depending on the chosen model).

Insert the values shown in Figure 4.

The values of the shear modulus Gref and the constrained modulus (or oedometer modulus) Eoed are automatically calculated by PLAXIS relating them to Youngs modulus and Poissons ratio according to Hookes law of isotropic elasticity:

( )EG

2 1=

+ (1)

Figure 3. Sand Data set: General.


6

( )( ) ( )oed

1 EE

1 2 1

=

+ (2)

No values have to be entered on flow parameters, interfaces or initial tab.

Click OK to save the material just defined and to go on.

The SoilTest option is available from the Material sets window if a soil data is selected. Alternatively, the SoilTest option can be reached from the Soil and interface material set dialog. In that case the open soil material set must first be saved before the soil test option can be started.

Select the soil test option, a separate window will open (Figure 5).

That window contains a menu, a toolbar and several smaller sections. The menu and toolbar allow for saving and loading of soil test results. The main selection is the Test type window, which shows an overview of the current test settings.

Figure 4. Sand Data set: Parameters.


7

It contains tab sheets for different types of tests, as well as a list of previously run tests that have

been opened from the menu. Below the test type window an overview of the material model and corresponding parameters of the current data set is shown.

Choose Oedometer to proceed.

Calculations

Now it is requested to list the different phases of the oedometer test. Each phase is defined by a

Duration, a vertical Stress increment and a Number of steps in which the stress increment is divided. The initial state is always stress free. In order to perform this exercise only one phase is

needed.

Insert the values shown in Figure 5 in the fields and delete the second default phase by the button Remove.

The duration can be set equal to zero since time is not an issue in this case.

A negative stress increment represents a compression in units of stress (whereas a positive stress increment implies unloading or tension); the number of steps indicates how many sub-parts the stress increment is divided in.

Figure 5. SoilTest main window.


8

Click the Run button to start the calculations.

The Results window will show several diagrams which are typical to display the result of the current test (Figure 6).

Curves

The first diagram (Figure 7) represents the ratio between the vertical stress 1 and the corresponding strain 1 . The test ends when the input value ( 21 500 mkN= ) is reached. The picture shows a linear dependence between the vertical stress and the vertical strain; the slope

representing the oedometer modulus Eoed.

By substituting the input values to the corresponding parameters into Equation (2), one obtains:

( )( ) ( )

2oed

1 0 3 40000E 53846 kN m

1 2 0 3 1 0 3- .

- . .

= =

+

In order to check if that value corresponds to the slope of the curve, the list of Cartesian points has

to be considered.

Figure 6. Results window: Diagrams.


9

Figure 7. Curves: sig_1 eps_1.

Double-clicking one of the graphs to open the selected diagram in a larger window.

The Data tab sheet lists the data points that are used to plot this diagram (each of them is representative of a single loading step, Figure 7).

Figure 7. Data list: sig_1 eps_1.

1

E oed


10

By choosing a generic point, for example the one representative of the eighth step ( 21 55 mkN=; 31 1002143,1

= ), it is possible to obtain the same Eoed value as before:

23 538461002143,1

55mkNEoed =

=

The second curve (Figure 8) is a 31 diagram. The slope of the graph is the inverse value of the earth pressure coefficient at rest 0K :

30

1

K1-

= =

(3)

0

1 1 0 3 2 33K 0 3

- ,

,

,

= =

By selecting the Cartesian points associated to ninth step ( 21 40 mkN= ; 23 14,17 mkN= , Figure 9) the slope can be confirmed:

0

1 40 2 33K 17 14

,

,

= =

Figure 8. Curves: sig_1 - sig_3.

1/Ka

1/Ko

1/Kp

-325

-30


11

The dotted lines represent the two boundaries for allowable values of ( 1 , 3 ) points on the graph. The steepest line is related to the failure in active thrust conditions, so it is characterized by a slope

which is the reciprocal of the active thrust coefficient aK ; on the contrary the lower line is related to a failure in passive thrust conditions (slope p1 K ):

( )( )a

1 sin 321 1 sin 3 25K 1 sin 1 sin 32

,

-

+ + = = =

( )( )p

1 sin 321 1 sin 0 31K 1 sin 1 sin 32

-

,

= = =

+ +

One can easily verify that by choosing two points laying on those lines, for example (2

1 325 mkN= ; 2

3 100 mkN= ) and ( 21 30 mkN= ; 23 100 mkN= ). The diagram in Figure 10 is represented in a deviator stress against mean stress plane ( 1 3q -= ;

1 32p3

+ = ); the broken line shows the failure condition (it is the projection of the Mohr-

Coulomb surface on the 'pq plane).

Figure 9. Data list: sig_1 sig_3.


12

MC6 sin

3 sin = (4)

( )( )MC

6sen 321 29

3 sen 32,

= =

The same steepness value is confirmed by looking at the point ( 2129 mkNq = ; 2100 mkNp = ).

The last graph (Figure 11) is a plane with the Mohrs circle relatively to the final stress condition ( 21 500 mkN= ; 23 214 mkN= ).

The Mohr-Coulomb criterion tan'+= c is there represented by a dotted line; looking at the

point ( 2480 mkN= ; 2300 mkN= ) one immediately obtains:

( )c 0 62 32'arctan arctan . = = = (5)

Figure 10. Curves: q p.

MC

129


13

which is the friction angle and the slope of the line as well.

Note

The last diagram (Figure 11) in the plane shows how in an oedometer test, by choosing a radial stress path less steep than the MC -line in the 'pq plane, it is impossible to reach failure;

the broken line wont ever be tangential to Mohr circle. That is the reason why the oedometer test is performed in order to collect information about the material stiffness; no data are provided about its

resistance. In estimating the material strength a shear test is needed, such as a triaxial test like the one shown in details in the next section.

Figure 11. Curves: tau - sig.

'


14

MOHR COULOMB MODEL: TRIAXIAL TEST (D)

Input

Keeping the results just achieved in the oedometer test section, it is possible to simulate a triaxial test on the same material specimen (Figure 12). So click on the Triaxial tab sheet in order to proceed.

Calculations

The triaxial tab sheet contains facilities to define an isotropically consolidated standard drained or

undrained triaxial loading test.

The parameters needed here are obviously different from those required by an oedometer test: a cell

pressure value which is kept constant during the performing of the test ( 23 100 mkN= ), the maximum vertical strain value 1 (the test is in deformation control, insert of 5% or 0,05-depending on PLAXIS version- for that parameter), which means a total deformation the number of steps the maximum strain is divided in (100 steps), finally the selection between drained or consolidated undrained test (go for Drained). Click on the Run button to start the test and in order to make the corresponding diagrams appear on the video (Figure 13).

Figure 12. Triaxial test.


15

Curves

The diagram in Figure 14 shows the relationship between the deviator stress q and the vertical strain

a. The curve is composed by two parts related to two different types of behaviour: a linear elastic

branch, followed by the sample failure with perfectly plastic behaviour.

Note

In the Lecture dealing with hardening models it will be shown that for a proper modelling of soils,

the transition between elastic and plastic state should be smooth. This is the main improvement introduced by the more advanced models which are explained and

used in next Lectures and Exercises.

The second curve (Figure 15) relates the main principal stresses each other. The dotted lines represent respectively the compression and extension failure conditions; being constant the minor stress during the test performance, the stress path results with a vertical up to failure due to high

compression load. Just checking on the Data list, the user can easily find out that the specimen fails

when the maximum principal stress reaches the value of 2329 mkN (same value as the one shown in Figure 15, for a cell pressure of 2100 mkN ).

Figure 13. Triaxial test curves.


16

The third plot in Figure 16 is a 'pq representation: starting from the isotropic consolidation

condition ( 2100 mkNp = on the x-axis), the sample moves towards the Mohr-Coulomb line (

MC ) with a constant slope equal to 1 to 3.

Figure 14. Curves: q eps_1.

Figure 15. Curves: sig_1 sig_3.

E ref

1


17

This can be proved as usual by choosing a couple of points from the Data list, or simply by

substituting the constant cell pressure condition ( 03 = ) in the stress path slope expression:

Figure 16. Curves: q p.

3

1 1 MC

Figure 17. Curves: tau sig.


18

32 31

31

+

=

=

pq

(6)

The last diagram (Figure 17) shows a plane with the Mohrs circle touching the failure line. In that condition the principal stress values can be pointed out on the x-axis.

Rak-50 3149 Soil Test

Documents

Transcript of Rak-50 3149 Soil Test