Instruction Manual
Model 4855
Pile Tip Pressure Cell
No part of this instruction manual may be reproduced, by any means, without the written consent of
Geokon, Inc.
The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for errors, omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 2003, 2004, 2009, 2013 by Geokon, Inc.
(Doc REV D 6/13)
Warranty Statement
Geokon, Inc. warrants its products to be free of defects in materials and workmanship, under normal use and service for a period of 13 months from date of purchase. If the unit should malfunction, it must be returned to the factory for evaluation, freight prepaid. Upon examination by Geokon, if the unit is found to be defective, it will be repaired or replaced at no charge.
However, the WARRANTY is VOID if the unit shows evidence of having been tampered with or shows evidence of being damaged as a result of excessive corrosion or current, heat, moisture or vibration, improper specification, misapplication, misuse or other operating conditions outside of
Geokon's control. Components which wear or which are damaged by misuse are not warranted.
This includes fuses and batteries.
Geokon manufactures scientific instruments whose misuse is potentially dangerous. The instruments are intended to be installed and used only by qualified personnel. There are no warranties except as stated herein. There are no other warranties, expressed or implied, including but not limited to the implied warranties of merchantability and of fitness for a particular purpose.
Geokon, Inc. is not responsible for any damages or losses caused to other equipment, whether direct, indirect, incidental, special or consequential which the purchaser may experience as a result of the installation or use of the product. The buyer's sole remedy for any breach of this agreement by Geokon, Inc. or any breach of any warranty by Geokon, Inc. shall not exceed the purchase price paid by the purchaser to Geokon, Inc. for the unit or units, or equipment directly affected by such breach. Under no circumstances will Geokon reimburse the claimant for loss incurred in removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or software, however, Geokon, Inc. neither assumes responsibility for any omissions or errors that may appear nor assumes liability for any damages or losses that result from the use of the products in accordance with the information contained in the manual or software.
TABLE OF CONTENTS
Page
1. INTRODUCTION ........................................................................................................................................... 1
1.1
T
HEORY OF
O
PERATION
............................................................................................................................... 1
1.2.
P
ILE
-T
IP
P
RESSURE
C
ELL
D
ESIGN AND
C
ONSTRUCTION
............................................................................. 1
2. INSTALLATION ............................................................................................................................................. 2
2.1.
P
RELIMINARY
T
ESTS
................................................................................................................................... 2
2.2.
P
RESSURE
C
ELL
I
NSTALLATION
................................................................................................................... 2
3. TAKING READINGS ..................................................................................................................................... 4
3.1.
O
PERATION OF THE
GK-403 R
EADOUT
B
OX
............................................................................................... 4
3.2
O
PERATION OF THE
GK404 R
EADOUT
B
OX
................................................................................................. 4
3.3.
O
PERATION OF THE
GK-405 R
EADOUT
B
OX
............................................................................................... 5
3.4.
M
EASURING
T
EMPERATURES
...................................................................................................................... 5
4. DATA REDUCTION ...................................................................................................................................... 6
4.1.
P
RESSURE
C
ALCULATION
........................................................................................................................... 6
4.2.
T
EMPERATURE
C
ORRECTION
....................................................................................................................... 6
4.3.
B
AROMETRIC
C
ORRECTION
......................................................................................................................... 6
5. TROUBLESHOOTING .................................................................................................................................. 7
APPENDIX A - SPECIFICATIONS ................................................................................................................. 8
A.1.
S
TRESS
C
ELLS
............................................................................................................................................. 8
A.2
T
HERMISTOR
(
SEE
A
PPENDIX
B
ALSO
) ......................................................................................................... 8
APPENDIX B - THERMISTOR TEMPERATURE DERIVATION .............................................................. 9
APPENDIX C. ATTACHING THE PRESSURE CELL TO THE REBAR CAGE .................................... 10
LIST of FIGURES, TABLES and EQUATIONS
Page
F
IGURE
1.
S
HOWING A
P
ILE
T
IP
P
RESSURE
C
ELL
................................................................................................ 1
F
IGURE
2 C
ONCRETE
C
ONE
.................................................................................................................................. 3
F
IGURE
3 GK405 R
EADOUT
U
NIT
.......................................................................................................................... 5
E
QUATION
1 D
IGITS
C
ALCULATION
..................................................................................................................... 6
E
QUATION
2 C
ONVERT
D
IGITS TO
P
RESSURE
................................................................................................... 6
E
QUATION
3 T
EMPERATURE
C
ORRECTION
......................................................................................................... 6
E
QUATION
B-1 C
ONVERT
T
HERMISTOR
R
ESISTANCE TO
T
EMPERATURE
.......................................................... 9
T
ABLE
B-1 T
HERMISTOR
R
ESISTANCE VERSUS
T
EMPERATURE
......................................................................... 9
1
1. INTRODUCTION
1.1 Theory of Operation
The ability of cast-in-place piles to support a load relies on friction along the pile and on end bearing. The load distribution along the pile can be measured by embedding strain gages at different depths along the pile and by comparing the measured strains at different depths with the strains at the top of the pile, very close to the applied load, which are assumed to be equivalent to 100% of the applied load. This ratio method does not require knowledge of the concrete modulus. Another method, to determine the actual end-bearing load taken by the pile tip is to measure it directly by installing a pressure cell between the pile tip and the ground below. Application of the load to the top of the pile causes pressure to be developed inside the pile tip pressure cell and this pressure, when multiplied by the area of the pile tip pressure cell, is directly equivalent to the end bearing load.
1.2. Pile-Tip Pressure Cell Design and Construction
The basic cell is manufactured to be close to the diameter of the pile. It is comprised of two circular stainless steel plates welded together around their periphery, leaving a thin space between the plates filled with de-aired hydraulic oil. This oil filled space is connected via a pressure tube to two vibrating wire pressure sensors. (Using two sensors provides a measure of redundancy, desirable in this critical situation, where one of the sensors might be damaged by the concrete placement.) End-bearing pressure applied normal to the plate is balanced by a corresponding build-up of internal oil pressure, which is measured by the sensor. The use of de-aired hydraulic oil guaranties that the modulus of the pile tip pressure cell is equal to or greater than the modulus of the surrounding concrete. This ensures that the pressure measured by the cell is characteristic of the pressure across the entire crosssection of the borehole and that there is no error created by a certain amount of the load being transmitted directly through the concrete around the edges of the pressure cell.
Figure 1. Showing a Pile Tip Pressure Cell
2
Lugs are provided on the back of the upper plate for welding to the rebar cage as shown in
Figure 1.
Lugs are also welded to the lower plate for attachment of short lengths of rebar, which are designed for pre-embedment in a concrete cone or cylinder, covering the entire lower plate.
This cone, or cylinder, depending on the shape of the bottom of the excavated shaft, prevents the formation of voids beneath the pressure cell caused by the entrapment of air or water.
During concrete curing, temperatures very often rise and will cause the cell to expand in the still green concrete. On cooling, the cell contracts leaving a space between it and the surrounding concrete which, if allowed to remain, might prevent the transmission of pressures from the concrete to the cell. To overcome this, provision is made so that the cell can be inflated until it comes back into perfect contact with the concrete on both surfaces.
This re-inflation is performed from the surface through a hydraulic line operating through a check valve. (An alternative method, using a pinch tube and remote crimping device, is also available).
The vibrating wire sensors are standard Geokon Model 4500H transducers inside all-welded housings. The sensors are hermetically sealed and are connected via waterproof connectors to an electrical cable leading to the surface. The sensor housings also incorporate a thermistor that permits measurement of temperature at the cell location.
2. INSTALLATION
2.1. Preliminary Tests
It is always wise, before installation commences, to check the cells for proper functioning.
Each cell is supplied with a calibration sheet, which shows the relationship between readout digits and pressure and also shows the initial no load zero reading. The two transducer’s electrical leads (usually the red and black leads) are connected to a readout box (see section 3) and the zero readings given on the sheet is now compared to the current zero readings. The two readings should not differ by more than
≈50 digits, after due regard to corrections made for different temperatures, barometric pressures and height above sea level and actual cell position (whether standing up or laying down).
Take Initial No-Load Reading of pressure and temperature with the cell laying flat on the ground. These are important readings and will be used in future calculations of pile-tip load.
By standing on the cell it should be possible to change the readout digits, causing them to fall as the pressure is increased.
Checks of electrical continuity can also be made using an ohmmeter. Resistance between the gage leads should be approximately 180 ohms, ±10 ohms. Remember to add cable resistance when checking (22 AWG stranded copper leads are approximately 14.7
Ω/1000' or 48.5
Ω/km, multiply by 2 for both directions). Between the green and white should be approximately 3000 ohms at 25° (see Table B-1), and between any conductor and the shield should exceed 20 megohm.
2.2. Pressure Cell Installation
The short pieces of threaded re-bar are screwed into the lugs welded to the bottom plate of the pressures cell. A depression is excavated in the ground and lined with sand. The shape of the depression should match the profile of the bottom of the pile shaft, whether conical or
3 cylindrical. This depression is now filled with concrete of the same type as the rest of the pile and is heaped in the middle so that when the pile tip pressure cell is lowered into it the concrete extrudes out to the side carrying any voids with it. The concrete is left to harden.
The lugs protruding from the upper plate of the pressure cell are welded to the rebar of the rebar cage as shown in figure 1. (More details of this operation will be found in Appendix C)
The cables from the sensors and the hydraulic pressure line to the check valve are fixed firmly to the rebar using nylon Tyraps every one meter or so. The cables and hydraulic line should be positioned so that they are protected from the concrete tremie pipe and from being scraped as the rebar cage is lowered into the hole.
Preparation of the bottom of the hole should ensure that the surface below the pressure cell is clean of debris and is pre-filled with wet concrete to a depth of 30cm. It is important that there be no voids below the cell.
One technique to ensure that there are no voids below the pile tip load cell is shown in
figure 2
Figure 2 Concrete Cone
A concrete cone is constructed and attached to the under-surface of the load cell in the following manner: The load cell is manufactured at Geokon so that it has threaded lugs welded to the underside. Pieces of rebar, supplied with the load cell, are threaded at one end and are threaded into the lugs so that they form two circles of rebars; short rebars on the outside circle and longer ones nearer the center. A hole is excavated in the ground in the shape of the desired cone and then it is completely filled with concrete. The pile tip load cell is then placed over the top of the hole so that the rebars poke down into the concrete.
After the concrete has set up then the load cell with the concrete cone attached is welded to the bottom of the rebar cage.
Take readings of pressure and temperature at regular intervals throughout the installation process. It should be possible to monitor the pressure of the wet concrete as it is poured.
4
Allow the concrete set up and cool to a temperature close to ambient. It may be observed that there is a drop in cell pressure from the value observed with the wet cement when the hole was filled. Connect the cell re-inflation line to a hydraulic hand pump filled with de-aired oil and pump oil into the cell while observing the pressure reading. The cracking pressure of the check valve is set so that it will open at a higher pressure than the static head of oil in the re-pressurization tube plus any suction that will be generated by the weight of the concrete attached to the lower plate. As soon as the pressure begins to rises sharply above this value and reaches the wet cement value then stop pumping. Disconnect the pump from the hydraulic line and cap the end of the re-pressuring tube.
3. TAKING READINGS
3.1. Operation of the GK-403 Readout Box
The GK-403 can store gage readings and also apply calibration factors to convert readings to engineering units. Consult the GK-403 Instruction Manual for additional information on
Mode "G" of the Readout. The following instructions will explain taking gage measurements using Mode "B".
Connect the Readout using the flying leads or in the case of a terminal station, with a connector. The red and black clips are for the vibrating wire gage, the white and green clips are for the thermistor and the blue for the shield drain wire.
1. Turn the display selector to position "B". Readout is in digits (Equation 1).
2. Turn the unit on and a reading will appear in the front display window. The last digit may change one or two digits while reading. Press the "Store" button to record the value displayed. If the no reading displays or the reading is unstable see section 5 for troubleshooting suggestions. The thermistor will be read and output directly in degrees centigrade.
3. The unit will automatically turn itself off after approximately 2 minutes to conserve power.
3.2 Operation of the GK404 Readout Box
The GK-404 is a palm sized readout box which displays the Vibrating wire value and the temperature in degrees centigrade.
The GK-404 Vibrating Wire Readout arrives with a patch cord for connecting to the vibrating wire gages. One end will consist of a 5-pin plug for connecting to the respective socket on the bottom of the GK-404 enclosure. The other end will consist of 5 leads terminated with alligator clips. Note the colors of the alligator clips are red, black, green, white and blue.
The colors represent the positive vibrating wire gage lead (red), negative vibrating wire gage lead (black), positive thermistor lead (green), negative thermistor lead (white) and transducer cable drain wire (blue). The clips should be connected to their respectively colored leads from the vibrating wire gage cable.
Use the POS (Position) button to select position B and the MODE button to select Dg
(digits).
Other functions can be selected as described in the GK404 Manual.
The GK-404 will continue to take measurements and display the readings until the OFF button is pushed, or if enabled, when the automatic Power-Off timer shuts the GK-404 off.
5
The GK-404 continuously monitors the status of the (2) 1.5V AA cells, and when their combined voltage drops to 2V, the message Batteries Low is displayed on the screen. A fresh set of 1.5V AA batteries should be installed at this point
3.3. Operation of the GK-405 Readout Box
The GK-405 Vibrating Wire Readout is made up of two components:
• the Readout Unit, consisting of a Windows Mobile handheld PC running the GK-405
Vibrating Wire Readout Application
• the GK-405 Remote Module which is housed in a weather-proof enclosure and connects to the vibrating wire sensor by means of:
1) Flying leads with alligator type clips when the sensor cable terminates in bare wires or,
2) by means of a 10 pin connector..
The two components communicate wirelessly using Bluetooth
®
, a reliable digital communications protocol. The Readout Unit can operate from the cradle of the Remote
Module (see Figure 3) or, if more convenient, can be removed and operated up to 20 meters from the Remote Module
Figure 3 GK405 Readout Unit
For further details consult the GK405 Instruction Manual
3.4. Measuring Temperatures
Each Vibrating Wire Concrete Stress Cell is equipped with a thermistor for reading temperature. The thermistor gives a varying resistance output as the temperature changes.
Usually the white and green leads are connected to the internal thermistor. The GK 401
Readout Box does not read temperatures – a digital ohmmeter is required.
1. Connect the ohmmeter to the two thermistor leads coming from the stress cell. (Since the resistance changes with temperature are so large, the effect of cable resistance is usually insignificant.)
2. Look up the temperature for the measured resistance in Table B-1. Alternately the temperature could be calculated using Equation B-1.
Note: All readout boxes will read the thermistor and display temperature in
°C automatically.
6
4. DATA REDUCTION
4.1. Pressure Calculation
The basic units utilized by Geokon, and displayed on the readout box Channel B, for measurement and reduction of data from a Pile Tip Pressure Cell are "digits". Calculation of digits is based on the following equation;
Digits
=
1
Period
2
×
10
−
3
or
Digits
=
Hz
2
1000
Equation 1 - Digits Calculation
To convert digits to pressure the following equation applies;
Pressure
= (Initial Reading - Current Reading) × Calibration Factor or
P = (R0 - R1)×C
Equation 2 - Convert Digits to Pressure
The Initial Reading is obtained as described in section 2.1). The Calibration Factor (usually in terms of PSI or MPa per digit) comes from the supplied Calibration Sheet.
4.2. Temperature Correction
The vibrating wire sensor is relatively insensitive to temperature fluctuations and usually the effect of temperature is insignificant and can be ignored. But, if desired, correction for temperature effects on the sensor can be made using the factors supplied on the calibration sheet. See Equation 3. Also, there are spurious temperature effects caused by the mismatch between temperature coefficients of the cell and surrounding concrete. This effect is not quantifiable in the laboratory and, hence, no correction factor for this effect can be supplied.
Temperature Correction
= (Current Temperature - Initial Temperature) × Thermal
Factor
or
PT = (T1-T0) x K
Equation 3 - Temperature Correction
4.3. Barometric Correction
Barometric pressure fluctuations will be sensed by the cells. However, the magnitudes (±0.5 psi) are usually insignificant.
7
5. TROUBLESHOOTING
Maintenance and trouble shooting of Pile Tip Pressure Cells is confined to periodic checks of cable connections. Once installed, the cells are usually inaccessible and remedial action is limited.
Consult the following list of problems and possible solutions should difficulties arise.
Consult the factory for additional troubleshooting help.
Symptom: Stress Cell Readings are Unstable
Is the readout box position set correctly? If using a datalogger to record readings automatically are the swept frequency excitation settings correct? Channel A of the GK-
403< GK-404 and GK-405 can be used to read the stress cells. To convert the Channel
A period display to digits use Equation 1.
Is there a source of electrical noise nearby? Most probable sources of electrical noise are motors, generators and antennas. Make sure the shield drain wire is connected to ground whether using a portable readout or datalogger. For all Readouts connect the clip with the blue boot to the bare shield drain wire of the stress cell cable.
Does the readout work with another pressure cell? If not, the readout may have a low battery or be malfunctioning.
Symptom: Pressure Cell Fails to Read
Is the cable cut or crushed? This can be checked with an ohmmeter. Nominal resistance between the two gage leads (usually red and black leads) is 180
Ω, ±10Ω.
Remember to add cable resistance when checking (22 AWG stranded copper leads are approximately 14.7
Ω/1000' or 48.5Ω/km, multiply by 2 for both directions). If the resistance reads infinite, or very high (megohms), a cut wire must be suspected. If the resistance reads very low (
<100Ω) a short in the cable is likely.
Does the readout or datalogger work with another stress cell? If not, the readout or datalogger may be malfunctioning.
8
APPENDIX A - SPECIFICATIONS
A.1. Stress Cells
Model:
Ranges:
Sensitivity:
Accuracy:
Linearity:
4855
2 MPa (300 psi), 3. MPa (450 psi), 5 MPa (750 psi),
7.5MPa (1100 psi), 10Mpa (1500psi), 20 MPa (3000 psi),
0.025% FSR
0.10% FSR
0.25% FSR (standard)
0.1% FSR (optional)
Operating
Temperature:
Frequency range
Dimensions:
Material:
Electrical Cable:
-30 to +70° C
1400-3500Hz
Diameter to suit the pile, Thickness approx 50mm
303 & 304 Stainless Steel
2 twisted pair (4 conductor) 22 AWG
Foil shield, PVC jacket, nominal OD=6.3 mm (0.250")
Consult the factory for other sizes or options available.
A.2 Thermistor (see Appendix B also)
Range: -80 to +150° C
Accuracy: ±0.5° C
APPENDIX B - THERMISTOR TEMPERATURE DERIVATION
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
T
=
1
A
+
( )
+
( )
3
−
273 2
Equation B-1 Convert Thermistor Resistance to Temperature
Where; T
= Temperature in °C.
LnR
= Natural Log of Thermistor Resistance
A
= 1.4051 × 10-3 (coefficients calculated over the −50 to +150° C. span)
B
= 2.369 × 10-4
C
= 1.019 × 10-7
Ohms
201.1K
64.09K
60.17K
56.51K
53.10K
49.91K
46.94K
44.16K
41.56K
39.13K
36.86K
34.73K
32.74K
30.87K
29.13K
27.49K
25.95K
24.51K
23.16K
21.89K
20.70K
19.58K
18.52K
17.53K
187.3K
174.5K
162.7K
151.7K
141.6K
132.2K
123.5K
115.4K
107.9K
101.0K
94.48K
88.46K
82.87K
77.66K
72.81K
68.30K
Temp
-50
-33
-32
-31
-30
-29
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-42
-41
-40
-39
-38
-37
-36
-35
-34
-49
-48
-47
-46
-45
-44
-43
Ohms
16.60K
6905
6576
6265
5971
5692
5427
5177
4939
4714
4500
4297
4105
3922
3748
3583
3426
3277
3135
3000
2872
2750
2633
2523
15.72K
14.90K
14.12K
13.39K
12.70K
12.05K
11.44K
10.86K
10.31K
9796
9310
8851
8417
8006
7618
7252
53
54
55
56
47
48
49
50
51
52
57
58
59
60
61
62
63
64
65
66
67
68
69
35
36
37
38
39
40
41
42
43
44
45
46
Temp
+30
31
32
33
34
Temp
-10
13
14
15
16
7
8
9
10
11
12
17
18
19
20
21
22
23
24
25
26
27
28
29
3
4
5
6
-2
-1
0
+1
2
-9
-8
-7
-6
-5
-4
-3
Ohms
2417
1212
1167
1123
1081
1040
1002
965.0
929.6
895.8
863.3
832.2
802.3
773.7
746.3
719.9
694.7
670.4
647.1
624.7
603.3
582.6
562.8
543.7
1733
1664
1598
1535
1475
1418
1363
1310
1260
2317
2221
2130
2042
1959
1880
1805
93
94
95
96
87
88
89
90
91
92
97
98
99
100
101
102
103
104
105
106
107
108
109
75
76
77
78
79
80
81
82
83
84
85
86
Temp
+70
71
72
73
74
Ohms
525.4
301.7
292.4
283.5
274.9
266.6
258.6
250.9
243.4
236.2
229.3
222.6
216.1
209.8
203.8
197.9
192.2
186.8
181.5
176.4
171.4
166.7
162.0
157.6
507.8
490.9
474.7
459.0
444.0
429.5
415.6
402.2
389.3
376.9
364.9
353.4
342.2
331.5
321.2
311.3
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
115
116
117
118
119
120
121
Temp
+110
111
112
113
114
122
123
124
125
126
150
Ohms
153.2
97.3
94.9
92.5
90.2
87.9
85.7
83.6
81.6
79.6
77.6
75.8
73.9
72.2
70.4
68.8
67.1
65.5
64.0
62.5
61.1
59.6
58.3
56.8
149.0
145.0
141.1
137.2
133.6
130.0
126.5
123.2
119.9
116.8
113.8
110.8
107.9
105.2
102.5
99.9
55.6
Table B-1 Thermistor Resistance versus Temperature
9
10
Appendix C. Attaching the Pressure Cell to the Rebar Cage
The eyebolts are threaded into the three lugs welded to the top plate. The three hooks and chains supplied with the cell are then hooked to the eyebolts and used to lift the cell and position it close to the bottom of the rebar cage, (within 2m). The cables and hydraulic lines are then routed along the rebars leaving an extra 2 meters of slack between the cell and the bottom of the cage. A steel rope of approximately 1 meter length is tied to one of the eyebolts and to the bottom of the cage. The purpose of the steel rope is to make sure that the cables and hydraulic lines cannot be ripped off when the rebar cage is lifted from the horizontal to the vertical.
The bottom of the rebar cage should have some standard form of support for the
pressure cell. These standard designs include a steel cross at the bottom and a steel belt around the periphery, adjacent to the bottom. These particular features require that tapped lugs be welded on the upper plate of the cell at specific locations to
match corresponding holes drilled in the cross-piece.
When the cage has been lifted to a vertical position it is lowered onto the cell. The bottom of the cage should be guide by at least two people to prevent it from twisting and swinging. The holes in the crosspiece are lined up with the threaded lugs on the pressures cell and then the bolts supplied with the cell are used to bolt the cell to the crosspiece. The chains can now be removed from the eyebolts.
Three pieces of rebar, approx. 1.5 m long are bent into a hook shape at one end. The hooked ends are hooked into the three eyebolts and the other ends are welded to the rebar cage. The assembly is now ready to be lowered into the shaft. While lowering the cage into the shaft the cables and the hydraulic line are tensioned step by step and fastened to the longitudinal rebars of the cage with cable ties at intervals of 1 m or so.
