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By: Its  05/12/2011
Keywords: Readings, Tubes, inclinometer

Foundation Assessment

  • Pile Integrity Testing
  • Pile Driving Analysis
  • Dynamic Load Testing
  • Cross-Hole Sonic Logging
  • Inclinometer Monitoring

Pile Integrity Testing

The sonic echo technique is based on the one-dimensional stress-wave theory.  The test is performed by striking the pile head with a hand held, lightweight, plastic-tipped hammer. An accelerometer transducer, held against the pile head, measures the vibration caused by the initial impulse, and reflections of the shock wave from within the pile.

Each pile is tested at several points to ensure that a consistent signal is being obtained. These signals are stored electronically to enable subsequent interpretation and analysis of the pile’s acoustic response.

Pile Driving Analysis & Dynamic Load Testing

When a hammer or drop weight strikes the top of a foundation, a compressive stress wave travels down its shaft at a speed c, which is a function of the elastic modulus E and mass density.

The impact induces a force F and a particle velocity v at the top of the foundation. The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modules.

The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile.

Strain transducers and accelerometers are connected to a Pile Driving Analyzer (PDA) or PAL-R, for signal processing and results


Cross Hole Sonic Logging

The Cross Hole Analyzer (CHA) uses Cross Hole Sonic Logging (CSL) technology to determine the quality of concrete between pairs of PVC or steel tubes pre-installed in drilled shafts.

A transmitter lowered down one tube sends a high frequency signal to a receiver in another in another tube. The sensors (transmitter and receiver) move down the tubes, scanning the entire length of the shaft. Repeating the procedure for each pair of tubes allows for the investigation of defects both along the length and by quadrant.

The transmitter and receiver have PVC cables that come in various lengths, and may each be positioned at different depth levels for maximum testing flexibility.

Inclinometer Monitoring

An inclinometer system includes inclinometer casing, an inclinometer probe, a control cable and an inclinometer readout unit.

 Inclinometer casing is typically installed in a near-vertical borehole that passes through a zone of suspected movement, the bottom of which is anchored, and presumed fixed.

The inclinometer tubes have four grooves at 90 degree spacing’s around the inside surface of the tube which run the length of the tubing. The inclinometer probe has wheels that fit into the grooves on the opposite sides of the tube. Taking readings with an inclinometer involves carefully positioning the probe into the grooves, lowering it down to the bottom of the casing and pulling it up twice, once to obtain readings in the A0 direction (direction of expected movement) and once to obtain readings in the A180 direction. Measurements are typically at 0.5m depth intervals.

It should be noted that due to the construction of the probe, readings in the A direction are slightly more accurate than readings taken in the B direction.

The rate, depth, and magnitude of movement is calculated by comparing data from the initial survey to data from subsequent surveys.

 The inclinometer readings are presented as both cumulative and incremental displacement plots. Cumulative plots illustrate the actual casing profile. Incremental plots illustrate displacements that occur at each single depth. These plots make it easy to find the zone where maximum movement is occurring.

Keywords: inclinometer, Readings, Tubes