Impacts that change the spatial relationships of objects or sediment in archaeological deposits can reduce the integrity of the archaeological resource. These disturbances can include soil compaction, soil mixing, and erosion. Depositional integrity can be a critical determinant of an archaeological site’s significance and therefore protective status. If a site is eligible for listing on the National Register of Historic Places (NRHP or National Register), an impact on data or information that makes that site significant is considered an adverse effect on the site. Typically, both a determination of National Register eligibility and a determination of adverse effect use excavation-based techniques in the assessment to determine depositional integrity, whether there is any loss of integrity, and the loss of archaeological data that resulted from the loss of depositional integrity. Excavation can be time consuming and expensive. If there is loss of depositional integrity and associated archaeological data, the loss must be mitigated. Mitigation is often accomplished through data recovery in the form of controlled excavation.
A large number of archeological resources are located on Department of Defense (DoD) sites where training includes vehicular activities. When vehicles create ruts, the depth of the rut is the visible disturbance. The subsurface depth of impact and its influence on the depositional integrity is an unknown. The objective of this project was to develop and use methods to measure and assess vehicle impacts on buried archaeological deposits.
Multiple experiments were conducted, with each experiment building on the results of the previous ones. The first experiment was a core compaction test that verified the relationship between bulk density and magnetic susceptibility. Then a geotechnical model was developed, which provided a tool for estimating the compaction profile under a rut based on stress curves under footings with static loading. The accuracy and shortcomings of the geotechnical model were demonstrated in later tests. The first series of tests provided a detailed investigation of compaction of uniform soil within a large wooden box. These experiments were used to refine the measurement techniques, to verify the geotechnical model, and to develop a better understanding of the depth and distance that a surface impact could propagate into the subsurface. The same rut formation methodology and measurement techniques were applied in the field at the Yakima Training Center, Washington. Magnetic modeling was used to interpret the results.
The results from the box tests demonstrated that cone penetrometer and down-hole volumetric magnetic susceptibility measurements could be used to accurately determine the magnitude of compaction and that the geotechnical model accurately predicted compaction in the homogeneous soil. The field experiments also indicated that cone penetrometer and down-hole volumetric magnetic susceptibility measurement techniques were capable of identifying the depth and width of compaction under a rut. Unfortunately, the field experiments also indicated that the geotechnical model was less accurate in the heterogeneous sediment. The model did not have the functionality to account for the change in soil type limiting the depth of soil compaction. The magnetometer measurements made as part of the field compaction experiments indicated that the change in the magnetic signature created by the ruts was easily identifiable. Although magnetometer surveys with subsequent modeling did not appear to be a viable option for identifying depth of sediment compaction at existing vehicle impact sites, analysis of the anisotropy of magnetic susceptibility did.
This project validated tools and methods that could be used to conduct archaeological site assessments more quickly and cost-effectively, enabling site managers to make decisions about when it would or would not be acceptable to allow military vehicles to drive over areas with known or suspected archaeological materials in the subsurface and what type of archaeological materials would be affected if a drive-over occurred.