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Assessment of Bored Pile Verticality Using an Ultrasonic Caliper
Ultrasonic calipering of bored pile excavations allows for assessment of pile excavation shape. In a typical application, the device is lowered into the excavation in incremental depths. At each depth, a 360° sweep of the excavation wall is performed. A circle is fitted to the data points using a non-linear least-squares technique to approximate the cross-sectional profile of the pile for verticality, perimeter area, and volume calculations. To determine pile verticality, one profile ring is selected as the datum ring. The geometric centers of the datum ring and all other profile rings are compared. A center offset is computed for the divergence of each profile ring center point from the datum ring center point. “Encroachment” is also assessed as the portion of the excavation wall which would encroach into the perfectly vertical projection of the datum ring to the depth in question. Verticality is computed as either the maximum encroachment or center offset divided by change in depth, and may be expressed as an angle, a percentage, or as a deviation: depth ratio. Typical practice for specifying required verticality is the simple ratio of deviation:depth, which does not take into account the diameter of the pile. Thus, a specified 1:100 tolerance allows for a deviation of 300 mm at a depth of 30 m, regardless if the pile diameter is 600 mm or 3,000 mm. For the former, this deviation represents 50% (a full radius) of allowable deviation, creating a vertical projection into which a rebar cage cannot be lowered without scraping the side wall, and/or a column support with a large built-in eccentric load. For the latter, it represents a 10% of diameter deviation, which is much less significant in terms of functionality, whether this is bearing capacity (axial pile) or creating a moisture barrier (secant wall). To overcome this, a normalized verticality calculation is presented. Rather than simply taking the ratio of deviation to depth, the compound ratio (deviation/diameter):(depth/length) is proposed.
Assessment of Bored Pile Verticality Using an Ultrasonic Caliper
Ultrasonic calipering of bored pile excavations allows for assessment of pile excavation shape. In a typical application, the device is lowered into the excavation in incremental depths. At each depth, a 360° sweep of the excavation wall is performed. A circle is fitted to the data points using a non-linear least-squares technique to approximate the cross-sectional profile of the pile for verticality, perimeter area, and volume calculations. To determine pile verticality, one profile ring is selected as the datum ring. The geometric centers of the datum ring and all other profile rings are compared. A center offset is computed for the divergence of each profile ring center point from the datum ring center point. “Encroachment” is also assessed as the portion of the excavation wall which would encroach into the perfectly vertical projection of the datum ring to the depth in question. Verticality is computed as either the maximum encroachment or center offset divided by change in depth, and may be expressed as an angle, a percentage, or as a deviation: depth ratio. Typical practice for specifying required verticality is the simple ratio of deviation:depth, which does not take into account the diameter of the pile. Thus, a specified 1:100 tolerance allows for a deviation of 300 mm at a depth of 30 m, regardless if the pile diameter is 600 mm or 3,000 mm. For the former, this deviation represents 50% (a full radius) of allowable deviation, creating a vertical projection into which a rebar cage cannot be lowered without scraping the side wall, and/or a column support with a large built-in eccentric load. For the latter, it represents a 10% of diameter deviation, which is much less significant in terms of functionality, whether this is bearing capacity (axial pile) or creating a moisture barrier (secant wall). To overcome this, a normalized verticality calculation is presented. Rather than simply taking the ratio of deviation to depth, the compound ratio (deviation/diameter):(depth/length) is proposed.
Assessment of Bored Pile Verticality Using an Ultrasonic Caliper
Sinnreich, Jon (author) / Singh, Roberto J. (author) / O’Doherty, Colm M. (author)
IFCEE 2018 ; 2018 ; Orlando, Florida
IFCEE 2018 ; 607-612
2018-06-06
Conference paper
Electronic Resource
English
Assessment of Bored Pile Verticality Using an Ultrasonic Caliper
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