NCDOT S EXPERIENCE USING HYDRAULIC SETTLEMENT GAUGES. David L. Teague,P.E. NCDOT GEOTECHNICAL ENGINEERING UNIT

Transcription

1 NCDOT S EXPERIENCE USING HYDRAULIC SETTLEMENT GAUGES David L. Teague,P.E. NCDOT GEOTECHNICAL ENGINEERING UNIT

2 OUTLINE Introduction Benefits of Hydraulic Settlement Gauges (HSG) over traditional settlement plates Installation procedures Calibration for temperature Case Study MSE Wall in Wayne County, NC (Currently under construction) Results compared to traditional settlement plates (for completed project)

3 INTRODUCTION What are Hydraulic Settlement Gauges?

4 Traditional Settlement Plate

5 BENEFITS OF HSG Once installed, HSG are completely buried, therefore they are not subject to being damaged or destroyed during fill placement and compaction efforts. Traditional settlement plates may be subject to surveyor error. Readings can be taken anytime by in-house personnel. A surveyor is not required. Readings can be taken during construction as well as anytime after construction. Traditional settlement plates must be removed before paving.

6 BENEFITS OF HSG (continued) Possible to remove HSG and re-use on another project (Save $$$) Roughly the same cost as Traditional Settlement plates. Traditional = $700-$800 each HSG = $900-$1000 each

7 INSTALLATION PROCEDURES Dig trench from cell location to reservoir. Ten feet of initial head is ideal. Minimum 2-3 ft. Length of cables can be up to 300 ft. Helpful if contractor is available with excavator to dig trench. Trench needs to be 18 inches deep.

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10 INSTALLATION PROCEDURES Dig hole for settlement cell. Hole should be approximately 2 ft. square and 18 inches deep. Attach settlement cell to plate by either bolting vertically or attaching horizontally. Hand tamp the bottom of the hole so it is flat. Do not want any wobbling of the plate once it is placed in hole Place plate with attached cell into hole

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14 INSTALLATION PROCEDURES Lay tubes flat and straight with minimum hills and valleys. Some, but not a lot of slack in tubes.

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17 INSTALLATION PROCEDURES Backfill trench and cover cell with 2-3 ft. of soil Coil excess tubing and bury it. Connect tubes to reservoir. Must insulate tubes coming from out of ground to readout box from temperature. Can use pipe insulators with slotted PVC Pipe. Add fluid to top of reservoir. If any air is in the system it will affect results.

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21 TAKING READINGS Readings are taken using a Vibrating Wire (VW) Data Recorder. VW Data Recorder records Temperature and Frequency. Temperature is used in the formula to convert frequency to pressure, then converted to elevation.

22 TAKING READINGS Check Dessicant chamber for saturation Fill Reservoir until fluid is up to overflow hole. Wait 10 minutes (if fluid is added). This allows any air bubbles to come to surface. Read Temperature Take Readings of VW Data every 30 seconds for 5 minutes. Frequency should be stable. If reservoir has the potential to settle, survey readings are needed. It is best to position reservoir on a post or other stationary position that is not subject to movement.

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25 EXAMPLE CALIBRATION RECORD OF SETTLEMENT CELL

26 CONVERTING FREQUENCY TO PRESSURE To convert frequency reading to pressure in PSI, use the following formula: P = (A x F^2) + (B x F) + C Where P = pressure in PSI F = Frequency in hertz A, B, and C are determined by manufacturer at time of calibration

27 CONVERTING FREQUENCY TO PRESSURE To correct readings for temperature, use the following formula: Pt = P + ((Tcal Tnow) x m) Where Pt = corrected pressure Tcal = Temperature at calibration Tnow = Current temperature of cell (VW Data recorder). Not ambient temperature. m = sensitivity from manufacturers calibration

28 AIR TEMPERATURE CORRECTION We have noticed that on hot days the sensors seem to read about ¼ inch too low and on cold days the sensors read about ¼ inch too high. This is not satisfactory for our purposes. Therefore we started using an air temperature correction which works like this:

29 AIR TEMPERATURE CORRECTION Corrected temperature = ((Tu x St) + (Ta x AT)) / Tu +TA) Where Tu = Length of tubing underground St = Temperature measured at sensor (from data recorder) Ta = Length of tubing above ground AT = Air temperature at time of reading

30 AIR TEMPERATURE CORRECTION The idea is we want to find the average temperature of the tube, which is the weighted average of the buried tube and the exposed tube. So if the tube is 150 ft. long and 5 ft. is exposed, the ground temperature is 15 C and the air temperature is 30 C the corrected Temperature would be: ((145 x 15) + (5 x 30)) / ( ) = 15.5 C

31 AIR TEMPERATURE CORRECTION The corrected temperature is then used in place of the sensor temperature when calculating settlement. So far, when using the air temperature correction, all the anomalies have disappeared.

32 CASE STUDY R-2554A Wayne County, NC Project consists of a 45 ft. high MSE Wall adjacent to a bridge. Subsurface conditions indicated that significant settlement would be possible. Traditional settlement plates were installed as part of the project contract. HSG were installed by NCDOT in-house forces as a pilot project to compare results.

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45 PREDICTED SETTLEMENT UNDER WALL Maximum predicted settlement = 381 mm = 15 in.

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47 Results from completed project Jacksonville Bypass, Onslow County, NC

48 Another benefit of HSG is that it is possible to retrieve the cells after settlement is complete. The cells can be reused on another project, saving money. Cells are placed inside outer PVC pipe. A cable is attached to the cell and run out to the end of the embankment or outside the wall footprint. When settlement is complete, the cell is pulled through the pipe and retrieved.

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53 INSTALLATION COMPLETE

54 HSG Retrieval on Sunset Beach project

55 THE END Questions? Questions?