Army

Photogrammetric soil density analyzer

Accurate calculations done quickly in the field

Sensors

Determining soil density is critical when designing or building structures, or excavating for construction. If a structure stands on soil with a low or extremely variable density, it can collapse, sink, deform, or otherwise become unsafe. Architects and engineers must either redesign a structure to fit a particular soil density or alter the soil density to ensure stable structural footing. Either scenario requires first testing the soil to determine density by one of several techniques.

  • Nuclear density gauges (NDG) are the fastest density testing systems, requiring less than 5 minutes to conduct an individual test. An NDG uses nuclear sources to determine bulk soil density and water mass within the soil. These radioactive sources present a health risk to users and a danger if stolen for illegal purposes. Constant monitoring of users’ radiation exposure levels is required and the devices must be stored under high security. Transport of an NDG requires significant paperwork and can be difficult in both the US and around the world.
  • Volume replacement systems require the user to excavate a hole and fill it with calibrated materials. The technique is laborious, time intensive, and not size scalable as users must transport calibrated materials to the site. To obtain an accurate volume, the calibrated materials must fill all the gaps in the hole, which may prove difficult for coarser soils. Finally, adding the calibrated materials to the hole may enlarge the hole, making any resulting data inaccurate.
  • Electronic devices require pre-calibration to the soil of interest before returning a useable soil density to the operator. Device calibration requires the use of a second density device such as a volume replacement system or NDG, or laboratory work requiring several days of testing the soil of interest. Calibration increases expense, labor, and time, limiting the usefulness of these devices to continuous testing on a single soil.

Given the above issues with current approaches, there is a need for an accurate, easily used, soil density measurement system that does not require extensive calibration, or significant health, safety, or security measures.

To address these needs, Army scientists and engineers have developed an apparatus for analyzing soil density utilizing a user-selected ground plane. The apparatus is a data processor configured with software to perform photogrammetry (making measurements from photographs, especially for recovering the exact positions of surface points). After a user excavates soil, measures the mass of the excavated soil, and takes multiple images of the excavation site in combination with a calibration object (one of many flat rings of known size and weight), a data processor uses the various values obtained from the collected images to create a point cloud data object. The processor uses this point cloud data object to create a visual representation of the hole and make volumetric calculations. Together with the soil mass, the volume data allows for the calculation of soil density.

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