To get a correct and useful test results, it is very important to make sure, that there is a good contact between the plate and the surface, to obtain an even stress distribution under the plate. Remove stones and other objects that could be disturbing. If the surface is uneven, it is recommended to put a thin layer of well graduated sand to make the surface even, and so the plate will be positioned as horizontal as possible.
Before the drop is released, use the geophone repositioning device to re-centre the sensor. Check also that the input values on the computer are set correctly, especially for the plate radius.
Do the number of drops as required, use different drop heights and as many drops as necessary to get stable readings for the deflections.
During testing check regularly that the essential parts of the testing device is free of dirt.
Tests on very cohesive or moistened soft subgrades can cause difficulties. For this purpose the plug to put in the centre of the plate for locking the centre geophone can be used.
3.3Raw Data Files
LWDmod can import two types of raw data files. The normal type to use is the *.kdb file generated on the PDA during testing. Alternatively raw data can be imported from Excel files, which have the format of the Excel export from the previous Keros Toolbox program. This option is mainly to secure backwards compatibility, although older versions of the KDB files also can be imported.
Regarding the import from Excel files you must be very careful not to change anything in the format or structure of the file. Please follow these main rules:
Changes in column headings or name of the sheets are not allowed.
Do not delete points or drops in the Excel file. This must be done from the LWDmod program.
Fields with time or date must not be edited.
Inserting or deleting columns or rows is not allowed.
3.4Preparing Data for Analysis
Before doing any analysis it is important to ensure that the data are of the best possible quality, and structured in an appropriate manner.
5 steps are involved to secure the quality:
Prepare the field testing accurately and preferable follow a standard procedure.
See: Field Test Procedures
Correct mistakes from the field testing resulting in poor or wrong data.
See: Edit Data
Name points (locations) and files logically, to avoid identification problems.
Use the facilities in LWDmod to eliminate points or drops with test results that are obviously not suited for analysis. See: Deleting Drops and Points
Use the facilities in LWDmod to select the drops for analysis that are suited for the purpose of the analysis. See: File Settings and Analysis Techniques
The deflections varies according to physical laws with load size and plate size, but other effects could also influence the deflection reading like compaction of material during load pulse, contact area between surface and plate, transversal movements of the geophone sensor during the impact and cut-offs due to maximum values exceeded for the deflection.
An important step in the process of the results is therefore to eliminate drop which are not suited for analysis. LWDmod provides facilities for detecting these drops and to remove them.
From the Plots - Deflections facility it is possible to delete drops from the graphical interface. Deflections that are unusual high will be seen from graphs. Remember first to normalize the deflections.
From the Plots - Surface Deflection Moduli facility drops can be deleted as well. From these graphs it is possible to see unusual high surface moduli, reflecting unusual low deflections.
From it is possible Edit Data it is possible both to delete single drops, and to delete a complete test point, including all drops.
Use also the Plots - Time History graphs to find abnormalities. Drops with negative Offset (the deflection curve settles below zero), and drops with significantly odd shapes should be removed.
Following are examples of drops, and drop series which should be considered to be deleted:
This topic summarizes the tools and possibilities you have for analyzing the data to get the most useful results. Please read first the Preparing Data for Analysis chapter.
Be careful about which drops from a test point that are selected for the analysis. The selection can be controlled from File Settings and by disabling single drops from the grid in the analysis window. In general be aware of not using seating drops and drops with abnormal deflection readings. If possible, include drops made with various load levels and plate sizes as this information allows the analysis to calculate non-linearity and layer thickness.
It is easy and quick to change the drop selection, so use the facilities to experiment with various settings to compare and select the most beneficial setup.
For multi-layer systems the layer thickness should preferable be known from measurements or construction data. In many cases when testing on older constructions the information is not available, and the input may be based on the best guess. It may be useful to run the analysis with various input for the layer thickness to evaluate what gives the best fit and most reliable E-values.
If drops are made with various plate sizes, the analysis will allow calculation of the top layer thickness; however in some situations the iterations can go in the "wrong" direction and produce unrealistic results.
Seed values are the start values in the iteration procedures, and the results are often depending on the choice of seed values. It is therefore important to enter these values as realistic as possible. In some cases you may want to fix the value, by unchecking the box left to the input field. If tests are made on sites, where bedrock close to the surface is a possibility, you should include this option in the input values.
You have also the option to run new analysis, where the previous results are used as seed values.
3.6.4Tests with multiple geophones
Up to 3 geophones can be attached to the equipment. How valuable the input from the additional geophones are will vary with the construction, the materials, the surface, the contact and probably other factors. A first hint can be seen from the consistency of the deflections from multiple drops. Remember to normalize, and to compare only drops with the same plate size.
If results from the analysis using all geophone look suspicious, then try to analyze using only the centre deflection. This option is available from File Settings.
If drops are made with multiple geophones, and various plate sizes, then it may be useful to only use centre deflection for estimating top layer thickness, and following enable all geophones and use the calculated top layer thickness as fixed input for a new analysis.
Study the results both in the grid and in the result frame to evaluate the quality. In the grid notice if any drops come out with especially high RMS values. It may then be appropriate to run the analysis again without these drops included. Compare the results with any other information you have about the tested structure to validate if they are within a reasonable range.
You can try if results can be improved by increasing the input values to the iterations in terms of Width, Steps and Alternate.
When running analysis for all points, remember to check the results for all points, and if some come out with doubtful values, then try to redo analysis for these specific points, changing some of the input parameters.
When evaluating the results for all points, then use the facilities for statistics and sectioning in the PLOT feature.