The number of drops you will see in the grid depends on your current settings in File Settings.
Each line in the grid presents the test results for the drop. You can use the command buttons and Next point and Previous point to scroll through the points.
Click Analysis >File Settings to open the window. It can also be accessed from the Options menu item in the Plot feature for some graph types.
These settings should be checked every time before doing analysis, as it besides settings for drop selections, also contains important values used for calculation of Elastic moduli and overlay design thicknesses.
In Drop Selection you have a range of options for selecting the drops to be used with the analysis. Usually it is not recommended to use all drops, because a number of the drops can be regarded as seating drops, where the structures changes due to the load impacts. Which options you want to use depends on the structure, the drop sequences and the purpose of the analysis. Use the facilities available in the Plots - Rawdata functions, to evaluate which drops that are suited for the analysis. This will give information about seating, abnormalities etc.
It may often be appropriate to try out different drop selections, and compare the results. Notice that you have the possibility to only use drops from tests with a specific plate radius. To evaluate the thickness of the top layer in a multi-layer system, it is necessary to have measurements done with different plate sizes, and for that reason they should also be included in the analysis. However, in some situations you may have problems with the stability of the equipment, doing tests with the small plate size, and for that reason you may want only to include drops made with the big plate.
At the bottom you have an option for selectingLast drop in each sequence. A sequence of drops is defined by same load range, plate size and geophone distances. Selecting this option will skip all drops where the next drop has the same configuration.
When you click Save the lines in the data grid will change reflecting the new setup, so you can easily see if you got the selection that you wanted.
Stress distribution and Poisson’s ratio is used during calculation of elastic moduli. The effect on the calculated surface deflection modulus is seen from the equation:
In LWDmod a factor of 2 is used for uniform distributions and pi / 2 is used for the rigid case, which may be more correct for cohesive materials.
For Poisson’s ratio 0.35 is regarded as a reasonable value for granular materials. Some literature suggests 0.50, but it is probably more reflecting older standards more than a correct value.
The Design frame contains values used with the estimation of overlay thickness. The surface modulus must be calculated at a specific Radius and Contact Stress. When specifying a design surface modulus, it must be connected to the plate radius. The contact stress only matters in case of the presence of nonlinear elastic materials, where this will influence the calculated surface deflection.
Modulus of design material should be set to a value that reflects the properties of the material that is supposed to be used for overlay.
When saving these material values, they are saved with the current file. They will not change the default values that are set in Settings from the Main menu.
Use only centre deflection can be checked, in case tests have been made with more than one geophone, but only the deflections from the centre geophone should be used in the analysis. In some cases the use of multiple geophone output could be more a disadvantage rather than a useful contribution to the analysis. This could be the case when testing on granular materials with low compaction, where movements in the material could cause influence on the deflection readings away from the centre. Looking at the surface moduli plot can give an indication of whether it is reasonable to use the additional geophone readings, like having increasing values with geophone distance.
Use centre deflection – offset should be checked when permanent deformation seems to occur during the tests (see: Offset). Use the Plots - Time History to study this in details.
8.4Structure and Seed Values
Before any analysis can take place, structural information must be entered in terms of definitions of layers and seed values.
By default the frame for entering this information look like this:
The subgrade is predefined and count as one layer. If the construction consists of more layers than the subgrade, then the number of layers should be entered in the field No. of layers. By changing this value to "2" the input frame will change: It is also possible to enter 3 layers, but back-calculation can only be performed on the top layer and the subgrade. In this case the entered seed E-value will be regarded as a fixed value:
For a 3-layer structure the Fix E1/E2 can be checked, in which case the ratio between E1 and E2 will remain the same as indicated by the entered seed values.
All thickness fields and E-value fields must be filled in with values. For the check boxes left of the fields, checked means LWDmod will calculate a best fit value based on the seed value entered, otherwise it will be regarded at a fixed value, and not being changed during backcalculation.
The modulus of subgrade is entered as a stress dependant equation:
If a linear-elastic subgrade is desired, then n should be entered as 0, and the checkbox must be unchecked. Following is an example of input data for a 2-layer system:
Notice that the checkbox for calculating thickness of the top layer is disabled. This means the thickness is fixed to the entered value. Only if drops for the test point is made with various plate sizes, it is possible to let the program try to estimate the thickness during back-calculation. In any case it is recommended to get the best possible information about the structure, so the best input values can be selected.
Depth to bedrock is an option to allow for influence from bedrock. Again it is possible to fix the value by leaving the checkbox empty, or to calculate the value by marking the checkbox. To calculate the values demands tests with more than 1 geophone, so in this case the checkbox is disabled because only one geophone was present.
In the input frame for structures there is a Deflections button. This will calculate the surface deflection based on the input values and display them in the grid:
The measured and calculated deflections can then be compared. The RMS value is calculated as either the percentage or absolute difference between the deflections. The option can be set in the checkbox beneath the grid.
Em is calculated as the subgrade modulus at the top of the subgrade, and is the value normally used with strain/ stress related design criteria for subgrade materials. The surface modulus of the subgrade can be found from this values by multiplying with (1 – 2 * n), where n is the non-linearity.
Eo is the surface deflection modulus on the top of the construction. In case of a 1-layer construction (subgrade only) and linear elastic subgrade (n = 0) then Em and Eo will be the same.
To do back-calculation for a single point click the Calculate point button. See more in the topic Calculation of Elastic Moduli.