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Soil Characterization

The centrifuge laboratory is equipped with a variety of tools that may be used to identify and characterize soil properties.

Full-flow Penetrometer (T-Bar)
Cone Penetrometer (CPT)
Bender Elements

In-flight Robot Tools

The centrifuge in-flight robots (2DOF and 4DOF) are capable of using a variety of tools. Several of these may be used for in-situ soil characterization of both sands and clays. The full-flow penetrometer, which is often fitted with the T attachment, is used to measure the undrained shear strength profile of clays. The cone penetrometers are capable of generating the tip resistance for granular soils.

Full-flow Penetrometer (T-Bar)

T-Bar testing is used to determine the in-situ undrained shear strength for various cohesive soils in the field and in the centrifuge. It is an example of a full flow penetrometer test, which can utilize a bar, sphere, or plate. The centrifuge-based T-bar was developed at the University of Western Australia (Stewart and Randolph, 1991). Numerical analyses regarding their mode of failure (Randolph and Andersen, 2006) have been performed. The tool was eventually produced at a prototype scale and tested in field. Typical field T-Bars are 50 mm in diameter and 200 mm in length. The bar and attachments available at the RPI Centrifuge laboratory are shown in the following figures. They were manufactured by the University of Western Australia’s Centre for Offshore Foundation Systems. The T-Bar penetrometer is preferential to the CPT in soft cohesive soils. While the cone penetration test also produces a continuous profile of the soil, it relies on empirically derived correction factors that are dependent on advanced knowledge of the soil loading history (Almeida and Parry, 1984). The T-bar instead makes use of the plasticity solution for a cylinder moving laterally through a cohesive soil (Randolph and Houlsby, 1984). The methodology is dependent on a local flow-around failure method. Therefore, shallow depths produce data which must be corrected for a different failure mechanism.

The figure above depicts data generated by T-Bar testing at 50g plotted with handheld penetrometer probe points collected at 1g.

Cone Penetrometer (CPT)

CPT testing is used to determine the bearing capacity for granular soils both in the field and at the centrifuge facility. The tool interfaces with the robotic in-flight loading system and is capable of accepting multiple diameters in order to account for differences in g-level and grain size. The facility currently possesses 3mm, 6mm, 8mm, and 12mm diameter cone penetrometers. The following video illustrates the 12mm CPT in use while the centrifuge is spinning.

In-situ Non-destructive Tools

Centrifuge models may be constructed with instrumentation that provides soil characteristics via non-destructive methods. These include using bender elements and accelerometers to determine mechanical wave velocities as they propagate through the model. The bender elements are specifically designed in order to generate shear waves well below the small-strain threshold for typical soils. Accelerometers may be used in conjunction with small seismic inputs to produce shear wave velocities.

Bender Elements

Bender elements are piezoelectric transducers that are capable of generating and detecting mechanical waves in soil. They may be used to determine the small-strain shear modulus, shear wave velocity, and compression wave velocity of a soil media. They are composed of a ceramic (Navy Type II Lead Zirconate Titanate) that exhibits piezoelectric properties. Under electrical excitation, the material will either physically expand or contract, depending on the sign of the input. Conversely, it will produce an electrical response when physically strained. The facility uses custom hardware and software in order to facilitate rapid collection of dense instrumentation arrays. Bender Elements Data Acquisition (BEDAQ) is a software suite that allows researchers to rapidly collect, verify, and analyze data from piezoelectric transducers. en in LabVIEW and interfaces with several National Instruments modules in addition to custom-made components. It includes several features to facilitate data collection and analysis. These include:

  • High channel capacity: software supports up to 16 receivers and 16 transmitters for high-density tomography applications.
  • Auto-Save: when enabled, data is automatically saved based on the experimental parameters and time of test.
  • Save/Load Settings: settings can be saved and recalled in future testing.
  • Signal Stacking: improves signal/noise ratio using a smart stacking algorithm that amplifies the real signal while reducing random and cyclical noise.
  • Digital Filtering: feature allows for digital filtering of data when signal stacking is not sufficient (rare).
  • Signal Generator: user-selectable signal generator for the transmitters produces a single pulse of a sine or square wave.
  • Data Viewer: recall and view data quickly and easily while utilizing powerful graph manipulation tools.

The following video showcases the software’s stacking algorithm. The sensors used in the video are setup to produce noisy signals via electromagnetic interference. The software is able to reduce the ambient noise and produce a true visible signal.

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