U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-99-194
Date: June 2000
Development and Field Testing of Multiple Deployment Model Pile (MDMP)
4.1 Overview of Peripheral Test Accessories
The MDMP requires peripheral systems to conduct the various testing procedures. Two Data Acquisition Systems (DAS) measure the response of the instrumentation during the different stages of testing. A loading frame provides the reaction for both compression and extension static load tests. A hydraulic system applies the loads for these static load tests. A drill rig, equipped with a drop hammer, is used to install the MDMP.
Three basic test procedures are conducted with the model pile during the pile history. During installation, the pile is driven with a standard SPT hammer (0.623 kN (140 lb)) or Casing hammer (1.33 kN (300 lb)). The pile is monitored during driving utilizing the Pile-Driving Analyzer (PDA). After driving, the soil/pile response is measured with time. During the initial period after installation, the response is measured (using a Hewlett Packard (HP) DAS) at a sampling rate of approximately 3 to 4 Hz over a period of about 2 h. Thereafter, the frequency is decreased as the soil/pile system approaches an equilibrium state. Several static load tests, both in compression and tension, are conducted periodically on the model pile to determine the gain of capacity with time. Additional tests such as dynamic restrikes at the end of the test sequence and/or rapid load/unload cycling to determine the ultimate capacity may also be performed.
All of these responses are measured by the various MDMP sensors using an elaborate DAS composed of an HP DAS, PDA, connection box, and cables. Depending on the test procedure, some sensors are monitored while others are not. A schematic of the DAS with the other related components of the model pile is shown in Figure 28. The figure is color-coded to enable easy identification of the various cables and associated connections. Table 14 provides a list of the components as numbered in Figure 28.
Table 14. List of Components as Shown in Figure 28.
4.2 Hewlett Packard Data Acquisition System
The Hewlett Packard Data Acquisition System (HP DAS) is used throughout the testing sequence. The HP DAS is required to trigger and store data from nine channels at 4 Hz. The data are recorded to hard drive and floppy disk periodically to ensure data recovery. The HP DAS consists of two components: the HP 75000 Series B cage VXI Bus DAS and an IBM-compatible 486 PC. The HP 75000 Series is composed of a mainframe HP E1301A with several modular components. Figure 29 is a photograph of the HP DAS system.
Figure 29. Hewlett Packard Data Acquisition System (HP DAS).
The mainframe has a front-panel keyboard and display. Modules are installed in the mainframe that control the different DAS functions to include module-to-module synchronization. The modules installed in the mainframe HP E1301A are: a 5½-Digit Multimeter (HP E1326B), a 16-Channel Relay Multiplexer Module (HP E1345A), and a 4-Channel D/A Converter Module (HP E1328A). The 5½-Digit Multimeter can be used as stand-alone or combined with multiplexers to form a scanning multimeter. The multimeter measurement functions include: DC Voltage, root mean squared (RMS) AC Voltage, 2-Wire Resistance, 4-Wire Resistance, Temperature, and Strain. The 16-Channel Relay Multiplexer switches up to 16 channels, where each channel has High (H), Low (L), and Guard (G) connections. Field wiring is connected to a terminal block that plugs into the Multiplexer. The 4-Channel D/A Converter Module provides four independent 16-bit digital-to-analog converter channels. Two operating modes are available - calibrated or non-calibrated - with typical output voltage ranges of ±10.922 V DC or ±12V DC and typical output current ranges of ±21.84 mA DC or ±24mA DC (HP User's Manuals).
The software program HP VEE was used to trigger the scanning multimeter, set the number of channels to be monitored and the sampling frequency, display real-time data to the screen, and store the data with a time stamp to a hard drive. HP VEE is a Windows-based iconic programming language that was installed on an IBM-compatible 486 PC operating at 33 MHz (Helsel, 1994).
The HP DAS records nine signals during static loading and three signals during dynamic loading. These signals are routed to the HP DAS from the connection box, which supplies the excitation voltage. Table 15 presents all the instruments in the overall MDMP system with respect to the data collection mode (dynamic vs. static). During static loading, the signals from the static surface instruments and all MDMP sensors are recorded, except for the MDMP accelerometers. During driving, the signals from the pore pressure transducer, total lateral pressure transducer, and slip joint LVDT are recorded.
Table 15. MDMP Data Acquisition and Instrumentation Configuration.
4.3 Pile-Driving Analyzer
The Pile-Driving Analyzer (PDA) is a signal conditioning and data acquisition system developed by Pile Dynamics, Inc. The PDA monitors pile driving in order to estimate pile capacity and determine pile stresses during installation. The PDA used in this research consists of a 486 SLC 25-MHz processor with 8 Mb RAM and a 240-Mb hard disk. The PDA has eight channels, thereby having the capability of monitor eight sensors and four strain gauges, 2 piezoelectric accelerometers, and 2 piezoresistive accelerometers. The system has a built-in automatic balancing of all signal conditioning. The maximum sample rate is 20,000 Hz and records 1029 data points on each channel. For model pile testing, this high sample rate is needed to accurately record the sharp rise of the hammer impact, similar to the one developed during standard penetration testing (SPT). The recorded force and velocity data can be further analyzed with software such as the case method and CAPWAP to predict soil behavior and estimate static pile capacity.
Figure 30 is a photograph of the PDA system. The PDA collects the dynamic instrumentation signals from the three MDMP load cells and accelerometers and the surface strain gauge and accelerometer during driving only. These eight signals are routed to the PDA from the connection box when the three load cell switches on the connection box are set to the dynamic position (see Table 15).
Figure 30. Pile-Driving Analyzer (PDA) Data Acquisition System.
An alternative data collection procedure can be employed during driving if another PDA is used to collect the surface measurements. With a second PDA, two strain gauge and two accelerometer signals can be attached to the drill rods at the surface and monitored without the use of the connection box. The first PDA can be used as previously described with only the three strain gauge and accelerometer signals within the MDMP routed through the connection box and recorded by the PDA. This alternative provides a more reliable measurement of force and velocity readings (and, therefore, energy) at the pile top (drill rods).
The connection box serves as the nerve center for the entire MDMP DAS. The connection box was designed and fabricated by Gary Howe, Civil Engineering Laboratory Director at the UMass-Lowell. All the cables from the MDMP and the surface instruments are routed through this box before being connected to the respective DAS (either HP or PDA), as shown in Figure 28. This connection box accepts the input signals from the various sensors, supplies the excitation voltages, and routes output signals to the correct DAS. The fundamental design requirements of the connection box were: (1) to allow instantaneous switching from dynamic to static readings (e.g., at the end of driving) and (2) to simplify the data acquisition process by centralizing all of the cables and connections in one place so that repairs could be made relatively easily in the field if problems were encountered. Wire diagrams of the connection box are presented in Appendix F.
4.4.2 Power Supply Requirement
The connection box supplies a total of four different direct current (DC) excitation voltages (+5, +15, -15, and +18 V) via a dual DC external power supply. The pressure transducer for the pore pressure measurements requires a constant current. A circuit board provided by the manufacturer supplies a constant 4-mA current and is capable of amplifying the output signal. This circuit board requires a constant voltage supply of +18 V. The DC-LVDT displacement transducer requires an excitation voltage of ±15 V DC. The lateral pressure cell requires an excitation voltage of 5 V DC. The connection box internal circuitry supplies all the required excitation voltages through a connection to an external power supply.
A schematic of the back faceplate of the connection box is shown in Figure 31. The connection box accepts three different input cables. These three cables are connected to the left side of the back faceplate. One cable contains all of the MDMP instrumentation wires (down hole input), while the other two cables carry the dynamic and static ground-surface instrumentation wires. Refer to Section 4.5 for details on the various cables. Included in the connection box requirements was the incorporation of the available PDA cables. In order to do so, socket receptacle connections were fabricated. These receptacles are push-on connections that allow the cables to be pulled out if a sudden jerking motion occurs, rather than having the wires severed.
Three different output cables route the input signals to the appropriate DAS, depending on the data collection mode (either static or dynamic). The three different output connections are shown on the right side of the back faceplate in Figure 31. The static output connection carries the signals measured during static testing to the HP DAS. The dynamic output is split into two connections, depending on the type of accelerometers. The output from the two piezoelectric accelerometers and their associated strain gauges in the MDMP load cells (top and middle) is routed to the piezoelectric receptacles of the PDA. The output from the piezoresistive accelerometer and associated strain gauge in the tip load cell of the MDMP and the piezoresistive accelerometer and strain gauge at the surface is routed to the piezoresistive receptacle of the PDA.
The output signals are wired directly from the input cable to the HP DAS. The accelerometers from the model pile are also wired in the connection box for direct attachment to the PDA. Using the connection box, other instrumentation, such as thermometer and an inclinometer can easily be added in the future.
The front faceplate of the connection box is shown in Figure 32. Three switches in the front faceplate enable the data collection from the three MDMP load cells to toggle between dynamic and static modes. When the three switches are placed in the dynamic position during driving, the PDA supplies the excitation voltage and records the output strain and acceleration signals from the three MDMP load cells and the surface strain gauge/accelerometer pair. The HP DAS records the displacement of the slip joint LVDT, pore pressure, and total lateral pressure during driving.
During static loading, the three switches are placed in the static position. The connection box supplies the excitation voltage, while the strain signals from the strain gauges within the three MDMP load cells are recorded by the HP DAS. In addition, the connection box supplies the excitation voltages for the three additional MDMP instruments (pore pressure transducer, lateral pressure transducer, and slip joint LVDT) and the three surface instruments (load cell and two DCDTs), while the HP DAS records the signals. The accelerometer signals are meaningless during static loading and are not recorded.
Six cables are used in the overall MDMP DAS. Three cables collect the data from the surface instruments and MDMP instruments. Three additional cables are used for output to the PDA or HP DAS. The pin connections for the different cables are presented in Appendix F. The following sections present a description of the cables and connections.
4.5.2 Input Cables
A 61-m (200-ft) main cable is used for all the wiring from the various MDMP measuring devices. A total of nine instrument signals from the three load cells, three internal accelerometers, slip joint LVDT, pore pressure transducer, and lateral pressure transducer are transferred via this cable. This cable is the brown line in Figure 28 and is referred to as the "down hole measurements" cable. The MDMP end of the cable is sealed with a watertight connection (MINO-44#20-CCP connector). The other end of the cable has an Amphenol 50-pin connector (#12 in Figure 28). This 50-pin connection connects to the #11 slot in the connection box.
Surface measurements are recorded by two separate cables. The "surface static measurement" cable is a 21-m (70-ft) PDA cable that combines the surface load cell (Lebow load cell) and the two surface DCDT wires at connection #13. This combined cable is the blue line in Figure 28 and connects to slot #10 in the connection box. The other surface cable is referred to as the "surface dynamicmeasurements" cable (red line in Figure 28). This is another 21-m (70-ft) PDA cable that combines one strain transducer and one piezoresistive accelerometer to the connection box in slot #9.
4.5.3 Output Cables
There are three output cables that route the various input signals to the HP DAS and PDA. Two output cables convey the dynamic signals to the PDA. These two cables are designated either as piezoresistive (yellow line) or piezoelectric (green line). Refer to Section 3.4.2 for details on the difference between the two accelerometer types. These cables are specially manufactured for the PDA. The piezoresistive signal connection (#5) is routed from the connection box (slot #6) to the piezoresistive receptacle on the PDA (#2). This cable carries the signals from the tip load cell (strain gauge and accelerometer pair) in the MDMP and the surface strain gauge and accelerometer pair to the PDA. The piezoelectric signal connection (#3) is routed from the connection box (slot #4) to the piezoelectric receptacle on the PDA (#1). This cable carries the signals from the upper and middle load cells (strain gauge and accelerometer pair) in the MDMP to the PDA. All cables are compatible with the PDA, utilizing connector part numbers MS3101A22-14P and MS3106A22-14S.
A 3-m (10-ft) long output signal cable from the connection box to the HP DAS (purple line) was custom-made at UMass Lowell to record all signals other than the dynamic strain and acceleration signals. A 48-pin contact connection (part no. MS3106A36-10S) (#7) connects to slot #8 on the connection box. The other end of the cable is a terminal block connection manufactured by HP (#14) that connects to the multiplexer in the HP DAS. During static loading, this cable routes signals from the surface static measurements (Lebow load cell and two DCDTs at the surface) and down hole measurements (three strain-gauged load cells, slip joint LVDT, pore pressure transducer, and total lateral pressure transducer from the MDMP) to the HP. During driving, this cable carries signals from the LVDT slip joint, pore pressure, and total lateral pressure.
The loading system provides tension and compression loads (and their reaction) for the MDMP static load tests. The static load tests are performed in order to measure the soil/pile interaction. When assessing gain of capacity with time, multiple load tests are conducted with the following requirements: (1) in order to assess the initial capacity, the first load test needs to be conducted as soon as possible after the MDMP installation is completed; and (2) as the gain of capacity of small piles is achieved during a short period (e.g., about 7 days for the MDMP), the load test needs to be of the "fast" load test type, not allowing creep or changes in stress to take place during the load test period. These requirements are accomplished with a pre-assembled portable load frame and hydraulic piston. The reaction is supplied from pre-installed ground anchors as detailed below.
4.6.2 Loading Frame
Figures 33a and b are a schematic and photograph of the static loading system, including the load frame, load application system, reaction system, and surface measurements. There are two steel plates made of type 4130 plate steel, each 25.4 mm (1 in) thick. The lower plate has five through holes, one in the center for the model pile to pass through and four in the corners for threaded support rods to be attached to the top plate. The top plate has several through holes as well. Like the bottom plate, it has a center hole for the pile to pass through and four holes for the threaded support rods. In addition, six holes are used to attach the double-acting ram to the top plate and four holes in the corners are used to attach the load frame to the anchor system. The threaded support rods used to connect the bottom and top plates are 91.44-cm- (36-in-) long, 38.1-mm- (1.5-in-) all-thread rod with hex nuts to secure both ends. The four threaded support rods slide into four steel tubes (sleeves), 41.275-mm (1.625-in) I.D. x 6.35-mm (0.25-in) wall thickness x 76.2 cm (30 in) long, that separate the top and bottom plates. A threaded disk screws onto the top of the hydraulic ram. The disk has six threaded holes that match up with the six holes on the top plate of the loading plate. Six 25.4-mm- (1.0-in-) threaded rods connect the disk to the top plate to secure the hydraulic ram. The machine drawing of load frame components is presented in Appendix C with the shop drawing of the MDMP.
Figure 33b. Photograph of the MDMP Static Load Frame.
The frame is designed to resist both upward loading (tension) and downward loading (compression). A loading rod is used to transfer loads from the hydraulic ram to the drill rods. The hydraulic ram is bolted to the loading frame. The loading rod bolts to the top of the ram, passes through the ram, and screws into the load cell and/or drill rods below. For tension loading, the ram pushes up on the bolted loading rod. The reaction load to this upward ram movement is transferred downward to the loading frame and ultimately the ground anchors provide the reaction load. For compression loading, the ram is extended prior to loading. The ram pulls downward on the bolted plate and transfers the load to the drill rods. The reaction load to this downward ram movement is transferred from the ram to the frame that is attached to ground anchors with turnbuckles. Four ground anchors, type 816 Chance 20.32-cm (8-in) No-Wrench Anchor, resist the upward load. The maximum load for the 25.4-mm (1-in) diameter rod of each anchor is 160 kN (36,000 lb). The soil at the Newbury Site appears to be type 1 or 2, which correlates to an anchor capacity of 142 kN (32,000 lb) (Chance, 1992).
4.6.3 Hydraulic Loading System
A double-acting hollow-plunger hydraulic cylinder (Enerpac RRH-10010) is used to apply the load to the model pile. The ram has a capacity of 890 kN (100 tons) when advanced, 602 kN (67.7 tons) when retracted, and 254 mm (10 in) of travel. A two-speed electric high-pressure hydraulics pump (Power Team PE214S) supplies the hydraulics pressure for the hydraulic cylinder. The hydraulic pump has three functions: advance, hold, and return. The hydraulic pump does not have a control to regulate the speed of the hydraulic cylinder. To control the speed of the cylinder, a flow control valve (Parker F600S) is placed in line with a maximum operating pressure of 34.5 MPa (5000 psi). The valve controls the flow of the hydraulic fluid in one direction and allows free flow in the opposite direction. The valve has different color bands that are used as a reference scale for quick adjustment. For fine adjustment, the first three full turns control at low flow and the next three full turns open the needle valve to full flow. Two of these valves are used to control the hydraulic cylinder in both directions so compression and extension static load tests are possible at a controlled displacement rate.
4.7 Driving System
A typical drop hammer and cathead is being used to drive the MDMP. The rated energy of the driving system is approximately 475 J (350 ft·lb) (based on a ram weight of 0.623 kN (140 lb) and an average stroke of 0.762 m (2.5 ft)). Figure 21 is a schematic showing a typical drill rig drop hammer used in SPT exploration. The drill hole is advanced by conventional methods (e.g., standard wash and drive drilling). A 10.16-cm- (4-in-) diameter casing is then driven to the top of the testing zone. The hole is cleaned out and the MDMP is then attached to the drill rods and inserted to the top of the test zone. The MDMP is driven approximately 3.05 m (10 ft) (MDMP length) below the top of the testing zone.