A reliable and acceptable method readily available to the unexploded ordnance (UXO) community for modeling projectile penetration is needed to help support planning munitions response efforts. Lack of penetration information can result in inaccurate time and cost projections, and the clearance of areas to depths greater than penetration capabilities of a projectile.

The objective of this project was to demonstrate the functional capabilities of the software UXO-PenDepth for estimating ordnance penetration depths.

UXO-PenDepth is a 3-D projectile penetration code that calculates the trajectory of a rigid axisymmetric projectile impacting a soil target. Input parameters include munitions type, impact velocity, impact angle, subsurface soil structure (layers and thicknesses), and a soil penetrability index (SNUM). The soil penetrability index is an empirical value and typically ranges between 5 and 12 for sand-silt-clay soil mixtures having dry to wet moisture states. Both halfspace and two-layer soil scenarios are presented. The soil scenarios include sand, silt, and clay half-spaces with varying soil penetrability indexes that represent dry, moderate, and wet moisture conditions. The two-layer scenarios are combinations of sand, silt, and clay with different first layer thicknesses. Depth penetration range (DPR) plots are generated that provide a summary of projectile penetration depth curves over a range of impact velocities and impact angles.

For the sand, silt, and clay half-spaces with SNUM of  5 to 12 (dry to wet moisture states) and ordnance size 81-mm and smaller plus the 105-mm HEAT, the depth of penetration achieved using minimum impact parameters is < 1 m; penetration depths achieved using maximum impact parameters is < 2.4 m.

Two general observations were noted for the two-layer soil models. First, depth of penetration increases as first-layer thickness increases when the soil resistance of the first layer is less than the soil resistance of the second layer, and penetration depth is less than or equal to the half-space penetration depth. Second, depth of penetration decreases as first-layer thickness increases when soil resistance of the first layer is greater than the second layer soil resistance, and penetration depth is greater than the half-space penetration depth.

The values of the reported UXO recovery depths are generally within the estimates obtained using UXO-PenDepth with the minimum firing parameters. Although the majority of the recovered UXO are within the UXO-PenDepth minimum firing parameter estimate, it is important to note that the majority of the recovery depths for a given UXO category are shallower than the shallowest UXO-PenDepth penetration estimate. Possible discrepancies could be attributed to: 1) the modeled UXO in this study are not representative of the recovery data, 2) differences in subsurface soil structure and soil parameters, 3) variations in firing parameters and methods of firing, and 4) UXO movement after initial penetration and prior to recovery.

Two primary concerns regarding implementation of UXO-PenDepth involve the acquisition of ordnance parameters, physical characteristics and firing information, and the use of an empirical soil resistance parameter. The munitions database in UXO-PenDepth contains ten ordnance input models plus the 1/7 semi-armor piercing (SAP) scale model. Although the input models range in size from 20 mm to a 100-lb bomb, these models do not adequately represent the multitude of ordnance types found on firing ranges. The munitions database should be expanded to represent the most common munitions currently found or suspected to exist on firing ranges, and newer munitions being used. Although the UXO-PenDepth user has the ability to generate an ordnance input file, some of the information needed to generate the input file requires access to databases that typically are not accessible by the general population. The inclusion of additional ordnance input files would enhance the program and be more useful to regulators and those involved in munitions response efforts.

The properties of the soil model used in UXO-PenDepth are represented by the empirical soil resistance parameter SNUM. Although use of an empirical soil resistance parameter is adequate for estimating penetration depth, the development of new forcing functions would allow generation of a site-specific penetration resistance. This would allow the ordnance depth of penetration to be modeled specifically to a site’s soil conditions. In situ soil properties would be required, but could be obtained with hand-held cone penetrometers and moisture meters.