Select Prisms
Function Name:
Use this function to select prism data from a Prism table in the survey database in preparation for graphing or reporting the displacement, velocity and acceleration of the measured attributes. All subsequent functions in the Stability Monitoring module operate on the set of prism data which is created by this function.
This function may be used as often as required while in the Stability Monitoring module to obtain different data from the Prism table for analysis. Each time this function is used, any data which have been obtained from an earlier use of this function are discarded and replaced with data retrieved by the current use of this function.
Each prism has a number of measured attributes recorded in the Prism table for each observation. Any of these attributes may be selected for subsequent graphing and reporting by this function. Some attributes are automatically extracted while others must be selected for extraction.
The attributes which are automatically extracted for graphing are: X, Y, Z, Cumulative Displacement (cum. disp.) and the recorded date of the measurements (prism_date). The Cumulative Displacement is the total change in location from the first observed position of a prism to any new location for that prism. Other attributes which are commonly extracted for graphing are the slope distance to the monitoring prism.
For each attribute which is extracted for graphing, three additional attributes are also calculated. These attributes are derived from the measured attributes and the times of the observations and are:
- delta
The change in the value of a measured attribute from one observation to the next
- velocity
The rate of change of a measured attribute with respect to time. The date/time of the observations are used to calculate the velocities. To minimise environmental effects in the velocities and accelerations, the velocities may be averaged over a number of measurements. An increasing velocity curve is a good indicator of an impending failure.
- acceleration
The rate of change of the velocity of a measured attribute with respect to time. Constant or increasing acceleration is a good indication of impending failure.
Choose Survey > Survey database > Stability monitoringand then Select prismto invoke the function.
Prism table
Enter the name of the Prism table from which the monitoring prism data is to be extracted for graphing and analysis.
The survey database may contain more than one prism table. Whether you choose to have only one Prism table or to have more than one Prism table is simply a matter for how you choose to organise the monitoring prism data. Different measurement techniques, that is, total station recording and level traverses may require different tables with different attribute fields.
Measured attributes
Choose each of the measured attributes which are to be extracted from the Prism table for graphing. The attributes: X, Y and Z are always extracted. The derived attribute of Cumulative Displacement (cum. disp.) is also always extracted. The Cumulative Displacement is calculated by using the X,Y and Z coordinates of successive locations of a monitoring prism to determine the total displacement from the first location to each subsequent location of a monitoring prism.
Other attributes which are commonly extracted for graphing include the slope distance from the setup station to the monitoring prism and the slope distance from the setup station to the backsight station. The slope distance from the setup station to the monitoring prism is a very good determinant of absolute movement of the monitoring prism while the slope distance from the setup station to the backsight station can be important to ensure no appreciable movement of the setup station is occurring. This movement, if it is present, can sometimes mask undesirable movement in the monitoring prisms and lead to failure to predict the collapse of pit walls.
No. of points to average velocity & acceleration
The movement in monitoring prisms is often very small even when measured on a weekly basis. Calculation of velocity and acceleration values with such small movements can sometimes give erratic results when based on successive observations. Environmental factors such as temperature and humidity can sometimes have an undue influence on these derived values. To counter this, you may choose to average the velocity and consequently the acceleration values over a number of observations.
For example, if you choose to average the velocities over four measurements velocities will only be calculated for the fourth and successive measurements and the accelerations will only be calculated for the fifth and successive measurements. The velocity for the fourth measurement will therefore be calculated from the change in value (of each selected attribute) from the first to the fourth measurement while the velocity of the fifth measurement will be calculated from the change in value from the second to the fifth measurement. The acceleration of the fifth measurement will therefore be calculated from the difference in velocity from the fourth to the fifth measurement.
You may choose to eliminate this averaging effect by entering 1 as the number of points over which the velocity and acceleration measurements are to be calculated.
Datum time for graphs, Time units for graphs
The graphs which are presented by this module draw time along the X axis. For convenience the X axis is drawn as a linear scale with time units of either seconds, minutes, hours, days or weeks. You must choose the starting time which represents ZERO on this linear date/time axis. All labels on the X axis are then relative to this datum time. You must also choose which units are required for the X axis to produce the desired results.
The datum time must be entered in the format YYY-MM-DD HH:MM:SS with trailing places being left blank if they are irrelevant. For example, the beginning of 1997 may be entered as "1997" or the beginning of June 1997 may be entered as "1997-06".
Complete the SELECT MONITORING PRISMS form and choose Apply to display the DEFINE QUERY CONSTRAINTS form.
Table Name
This is the name of the table from which the data is to be extracted and cannot be changed on this form.
Function
The function to be performed is displayed.
Field Name
Enter the name of the field upon which you wish to impose field constraints.
Operand
Enter the operation that will define the field constraints. Valid entries for the operand are:
| < | less than |
| <= | less than or equal to |
| > | greater than |
| >= | greater than or equal to |
| <> | not equal to |
| = | equal to |
Constraint Value
Enter the value of the constraint that you wish to apply to the named field. When entering a character constraint value, e.g. station_id, it is important to enter the name exactly as entered earlier when setting up the database.
Complete the DEFINE QUERY CONSTRAINTS form and choose Apply to commence extraction of the prism data from the Prism table in preparation for graphing and reporting.
Result
All monitoring prism data is extracted from the prism table in preparation for use by the graphing and summary functions of the Stability Monitoring module. A message indicating the number of prisms which have been extracted is displayed on completion of the data extraction.