Feeling the Pressure – The use of load cells during pile load tests

2 - GK-502 Load Cell Readout with Load Cell

Pile load tests, with the exception of the Osterberg and Statnamic testing methods, use one or more hydraulic jacks to impose loads on the top of the pile. The loads imposed are calculated from the “effective” cross-sectional area of the piston multiplied by the internal oil pressure acting upon it as measured by a Bourdon style pressure gage. The effective cross sectional area is usually less than the theoretical because some of the load is lost due to the friction generated by the O-rings sealing the piston. Additional friction and loss of load can also be the result of tilting of the jack piston inside the jack housing caused by the top surface of the pile not being parallel to the surface of the reaction frame against which the jack piston pushes. Spherical seats on top of the jack piston will take care of this problem.

To ensure accuracy the hydraulic jacks are calibrated frequently along with their attendant pressure gages. This calibration is normally performed in a testing machine of known accuracy. Loads applied to the jack by the testing machine are deduced from the piston area and the pressure indicated by the jack pressure gage at each increment of load.

2 - GK-502 10-pin connectorsPressure gages tend not to have graduations fine enough, and a dead spot caused by hysteresis between increasing and decreasing loads reduce the precision of the pressure gage and its ability to record small changes of load accurately. On account of this it is thought to be prudent to have a second means of measuring loads in the form of a load cell that can also record and store the test data electronically.

Load cells can either be of a solid construction or they may be annular in cross-section where they may also be used with hollow ram jacks to load tie backs and rockbolts. Load cells may be of two kinds – vibrating wire, (VW), or electrical resistance, (ER), depending on the type of strain gages used to measure the compressive strains developed in the body of the load cell when the loads are applied. In general, vibrating wire load cells are preferred where the load cell will be left in place to monitor the loads over long periods of time – perhaps amounting to decades – while electrical resistance type load cells are usually preferred for short term pile tests.

Some mention should be made concerning the agreement between loads indicated by the jack and by the load cell. Very often the agreement is poor with discrepancies of as much as 10 to 20 per cent of the full-scale load. The reason for this lies in the different conditions of loading produced by warped, non-parallel and uneven bearing plates that produce a different calibration factor than that measured in the ideal conditions of testing machine. This suggests that, whenever possible the load cell and jack should be calibrated in tandem using the same type and configuration of bearing plates that will be used in the field.

2 - GK-502_Load_Cell_control_panelTo measure the output from an electrical resistance (ER) type load cell requires an instrument that can accurately read the microvolt output from a Full Wheatstone Bridge network of electrical resistance strain gages: the Geokon Model GK-502 Readout Box has been especially designed for this task.  Care has been taken to make sure that the GK-502 can be used outdoors by making it rainproof and operable in hot or cold weather. Displayed numerals are large in size and can be backlit for easy reading in poor light.  Connections to the load cell are by means of a 10-pin connector so no breaking of finger nails trying to push bare wires into poke-home terminal strips.

ER load cells may or may not use remote sensing techniques where a separate pair of leads is used to measure the voltage input to the Wheatstone Bridge at the load cell itself and not at the GK-502 location. This removes any inaccuracies that may arise due to temperature induced changes in cable resistances or to varying contact resistances. The GK-502 will sense whether or not the load cell has been wired for remote sense and will automatically adjust itself so that it will read either type of load cell.

A further refinement of the GK-502 is its ability to display the load in engineering units: Internal programming allows the input of load cell calibration factors and no-load readings, through a user friendly menu, and then converts the measured microvolt output into the desired units be they mV/V, pounds, kilograms, kips, tonnes, etc…

The GK-502 also permits storage of the pile test data which can be downloaded through a 10-pin USB connector to a spreadsheet for further analysis.

For more information about the Geokon GK-502 Readout Box, please call
1 (603) 448-1562 (USA) or 1 (450) 441-5444 (Canada) and ask to speak to a sales associate or visit www.geokon.com/GK-502