Identification and Removal of Unexploded Ordnance at a WWII Bombing Range in an Area Undergoing Rapid Development

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1 Identification and Removal of Unexploded Ordnance at a WWII Bombing Range in an Area Undergoing Rapid Development David G. Nichols, P.G. Kestrel Horizons, LLC, 20-B Creekview Court, Greenville, SC Marie R. Arrant Kestrel Horizons, LLC, 20-B Creekview Court, Greenville, SC Jorgen Bergstrom General Engineering Geophysics, LLC, 2040 Savage Road, Charleston, SC ABSTRACT The Conway Bombing and Gunnery Range was used by the US Army from 1942 to 1947 to allow pilots to practice bombing, strafing and rocket firing. The former range is located between Conway, South Carolina and Myrtle Beach, South Carolina, in an area that is undergoing rapid commercial and residential development. The range was initially investigated by the US Army Corps of Engineers (COE) in the late 1990s. There initial investigations included review of historical information about the facility and gathering geophysical information over a very small percentage of the total range. Area III of the range is two miles in diameter (2010 acres) and was used for a wide variety of armaments but mostly 4-pound incendiary bombs, 2.25-inch rockets and 100- pound practice bombs. The vast majority of these weapons did not carry a full charge but rather a smoke charge, which contained a small amount of black powder allowing the pilot to see the weapon strike. We are performing an investigation and removal action on a parcel containing 325 acres, of which 250 acres are upland and 75 acres are considered jurisdictional wetlands. We used the Geometrics G-858 magnetometer and the Geonics EM-61 time-domain metal detectors to identify subsurface anomalies during the geophysical prove out. The G-858 was superior in routinely identifying UXO like objects to a depth of six feet below ground surface. In addition to meeting the requirements in the COEs Engineering Evaluation and Cost Analysis (EE/CA) report the property imposed an additional requirement of identifying UXO to a depth six feet. In a five acre test area, both instruments were used in combination to determine if the potential to reduce the number of false positives existed. False positives are particularly troublesome because each one requires further investigation costing time and money resulting in no OE/UXO being removed. The cost of managing false positives can be a significant portion of the removal phase of the project and thus reducing the number has the potential to yield cost savings. INTRODUCTION Page 1 of 10

2 The Site was originally privately owned and used as farmland and for timber harvesting. From the Site was used by the U.S. military as a training bombing and gunnery range. The Site was part of the former Conway Bombing and Gunnery Range (BGR) and was heavily used until 1946 when Myrtle Beach Army Air Force (AAF) converted from wartime to peacetime training and use of the Conway BGR significantly diminished. Myrtle Beach AAF was closed in November On February 4, 1948, the Conway BGR, owned by the government in fee, was declared surplus to government needs and transferred to the War Assets Administration. In October 1948, leases on 34,685 acres were terminated and the land was returned to the International Paper Company, the owner 1. The land was then used as forestland for timber production and management. The Site was transferred to a land holding company in two parts in 1999 and Timber was last harvested on the Site in Figure 1: Site Location Map Transmission Line 501 Socastee Swamp Carolina Forest Boulevard Site Location Intracoastal Waterway Approximate Scale 0 10,000 feet Atlantic Ocean The former Conway BGR contained Ranges II, III, IV, VII, XX, a moving target range, two turret ranges, a machine gun range, and a rifle range. The Site is located in former Range III. According to the Engineering Evaluation/Cost Analysis (EE/CA) 2 prepared for the US Army Corps of Engineers, Range III was used for practice bombing, demolition bombing, dive bombing, strafing, incendiary bombing, skip bombing, and rocket firing. The Site contains acreage in both the Target Zone and the Safety Zone, a circle with a one-mile diameter that surrounds the Target Zone. The most common weapons identified in the EE/CA include 4-lb. incendiary bombs, 100-lb. sand filled practice bombs and 2.25-in. rockets. Page 2 of 10

3 Until the 1990s, the Carolina Forest Community remained virtually undeveloped (see Figure 2). Approximately 10 years ago, International Paper Realty began selling blocks of land to developers and builders. In 1998, records indicate that International Paper and Horry County launched a 20-year plan for the 12,000-acre Carolina Forest Community. The community is located approximately 10 miles from Myrtle Beach, and now contains shopping centers, banks, churches, a U.S. Post Office, public golf course, a Horry County Fire Station, restaurants, and many other retail facilities. The community also contains neighborhoods with home values that range from the mid-$120,000 to $300,000 or more. SITE RESTORATION Phase I Environmental Site Assessment A Phase I Environmental Site Assessment was conducted prior to beginning any on-site OE/UXO investigation. The Phase I did not find any recognized environmental concerns beyond the potential for unexploded ordnance. This allowed the Site work plan to be developed with a sole focus on removing the OE/UXO from the property based on COE recommendations. Figure 2: Aerial Photograph of Site at Time of Phase I ESA Site Remediation A work plan was developed based on COE standards including the recommended strategy for remediation listed in the EE/CA with the exception of the acres located in the Safety Zone. The EE/CA recommends surface clearance of OE/UXO and removal to depth under the footprint of construction for the Safety Zone in Area B, whereas the Target Zone is to be cleared to depth over all of the area. To provide the most flexibility Page 3 of 10

4 for land development, the entire Site is cleared to a depth of six feet below ground surface. Brush Clearing and Surface Ordnance and Explosives Removal Before beginning the OE/UXO detection and removal, the trees and brush were removed from all upland acres with escort by OE/UXO personnel. The removal of the trees and brush allow for faster geophysical surveying and removal of OE/UXO by removing the tree roots and vegetation that interfere with the surveying and removal. A stand of oak trees exist onsite that have historical significance. These trees were saved per the request of the Client. Figure 3: Aerial Photograph after Brush Clearing and Surface OE/UXO Removal Once the site was cleared of brush and vegetation, the OE/UXO contractor conducted surface clearance over all upland acres to create a safe working environment for the geophysical survey crew. Digital Geophysical Mapping of the Subsurface General Engineering Geophysics, LLC (GEG) performed the geophysical investigation at the former Conway Bombing and Gunnery Range in South Carolina. A geophysical prove out was conducted to evaluate methods, equipment, and processes to be used at a digital geophysical mapping (DGM) project at the site. The geophysical equipment evaluated by GEG included an EM-61-MK2 time-domain electromagnetic metal detector manufactured by Geonics, and a G-858G cesium vapor magnetometer manufactured by Geometrics. The positioning instruments evaluated by GEG included a 5800 RTK/GPS Page 4 of 10

5 real-time kinematic global positioning system, and a Geodimeter 600 laser positioning robotic total station, both manufactured by Trimble. The work consisted of conducting a geophysical prove-out to determine, among other things, the range of detection of both the EM-61-MK2 and the G-858G for the site specific conditions existing on the Site. GEG constructed two prove-out grids at the site. GPO site 1 was 12,000 square feet while GPO site 2 was 10,000 square feet. At GPO site 1, GEG buried items recovered from the site during surface clearance by Environmental Hazards Specialists International, Inc. (EHSI). The items included fragments of the most common ammunition expected at the site and non-ordnance items such as railroad spikes. Ordnance items included fragments and warheads of 4-pound incendiary bombs, 2.25-inch sub caliber aircraft rockets, 100- pound practice bombs, and 5-inch high velocity aircraft rockets. At GPO site 2, GEG buried iron pipes of the same dimensions as intact 4-pound incendiary bombs and inch sub caliber aircraft rockets to simulate the expected response from intact UXO, and one 5-inch high velocity aircraft rocket warhead. Analysis of the EM-61-MK2 and the G-858G data show that the EM-61-MK2 data provides more detailed information of buried objects within two feet of the surface, which is the zone that is expected to be most affected by UXO. The G-858G data has poorer horizontal resolution, but can detect most potential UXO objects at a deeper depth. GEG buried iron pipes to simulate intact 4-pound incendiary bombs and 2.25-inch sub caliber aircraft rockets. The 4-pound incendiary bombs and the 2.25-inch sub caliber aircraft rockets are assumed to be the smallest potential UXO targets at the site. These targets were detected to a depth of six feet with the G-858G. Typical size fragments of 4- pound incendiary bombs, which are expected to be the most common UXO-related target at the site, were reliably detected to a depth of two feet with EM-61-MK2 and G-858G. Some of these targets were detected to a depth up to four feet with the G-858G. Based on the results of the GPO, the G-858G was chosen as the primary geophysical instrument for the DGM, using RTK/GPS for positioning support. Furthermore, it was decided a pilot test would be conducted to determine if using the EM-61-MK2 in addition to G-858G in target rich areas had a chance to significantly reduce excavation and removal time by better pinpointing targets. Part of the reasoning included it was likely that some false positives could be avoided by analyzing data from both methods. False positives are amplitudes above the amplitude threshold that are not caused by subsurface metallic objects. Figure 4 shows a comparison between the two instruments in terms of being able to correctly identify a no contact. The G-858 is clearly superior in this regards. Page 5 of 10

6 Figure 4: Comparison of EM 61 with a G 858 in Identifing a No Contact Percentage G Data (G/Gmax) E Data (E/Emax) G/GMax or E/EMax Addressing False Positives Prior to conducting the geophysical prove out we considered the merits of determining if the number of false positives (a magnetic anomaly that could not be found in the field) could be reduced by using both the EM-61-MK2 and the G-858G in the same area. Reducing false positives is important because it has a direct impact on total project cost. The results from the prove out indicated that the G-858G did a superior job in determining the presence of magnetic anomalies that were similar in magnetic signature to the smallest anticipated UXO. The combination of the EM-61-MK2 and the G-858G actually increased the number of false positives identified in the field. Of the detected targets, the EM-61-MK2 had a false positive rate of 55% and the G-858G had a false positive rate of 27%. Table 1. False Positives Detected During Field Test Total EM-61-MK2 G-858G Targets Digs Hits False Positives Page 6 of 10

7 During the field test, there were no instances where the EM-61-MK2 identified a potential OE/UXO object that the G-858G did not identify. The G-858G detected 96% of the hits and the EM-61-MK2 detected 64% of the hits. The four percent of anomalies that were not detected by the G-858G consisted of non-oe scrap metal. Subsurface Removal of OE/UXO As the digital geophysical surveying has progressed, the OE/UXO contractors have investigated the anomalies detected during the survey. As to be expected, fewer anomalies exist as the crews move further from the center of the target zone. Based on work conducted on other tracts of land in Carolina Forest, it was anticipated that the removal phase of the project would take approximately one year. As work has progressed on this site specifically, the estimate has shifted to approximately six months. This is attributed to both the efficiency of the OE/UXO contractors and having removed the trees and brush from the site before work began. A grid system of 100 ft x 100 ft grid cells was surveyed over the entire site. The survey and removal work is documented using this grid system. Figure 5 outlines the projected completion date based on work conducted as of February Figure 5: Grid Completion Date No. of Grids /15/ /15/ /14/ /14/2005 1/13/2006 2/12/2006 3/14/2006 4/13/2006 5/13/2006 Grid Completion Date There are approximately 1000 grids that must be surveyed and investigated to six feet below ground surface. Based on the current rate of completion, the subsurface removal should be completed in April Page 7 of 10

8 Figure 6. Cumulative Weight of OE, UXO and Scrap Amount of OE and OE Scrap (lbs) Cumulative Weight Log. (Cumulative Weight) Grids Figure 6 shows is a graph showing the cumulative weight of OE, UXO, and scrap removed from the site. As would be expected the amount of material removed from grids in the Target Zone exceeds the weight of material removed from the Safety Zone. Thus far the Target zone yields about 87 pounds per acre while the Safety Zone yields about 20 pounds per acre. The amount of non-oe scrap is very small generally less than 5 percent of each grid. THE COST OF CHANGING YOUR MIND Initially the owner elected to reserve 19 upland acres for the disposal of trees and topsoil stripped from the site. A prospective buyer wanted that land for development and required the removal of the material as part of the sales agreement. Dealing with this material in the first place would have been less expensive. For example we will now have to double handle 35,000 cubic yards of material at a cost of at least $8 per cubic yard. In addition we will incur incremental costs for remobilizing two contractors and be delayed in achieving the final completion date. The total cost will be approximately $300,000 which is about ten percent of the total project cost. Page 8 of 10

9 CONCLUSIONS The site owner is most interested in being able to assure prospective purchasers that UXO has been removed to a depth of six feet below ground surface. In addition the owner wants to be able to assure the buyer that the requirements of the EE/CA have been met. In both regards the G-858 provided superior data. While both instruments met the owner s criteria the G-858 did so quicker and more accurately. The resulting cost savings amount to at least 25 percent of total project cost. This area of South Carolina is developing rapidly and landowners are anxious to turnover property quickly. As a result of this there is pressure to get the work done as efficiently and effectively as possible. It is important to maintain a high level of quality control so that the work is performed to the highest available standard. Repeating work of this type is costly and time consuming and will cause problems for all the parties. Lastly, it is important to have the site owner establish the site development criteria early in the project and then not change the criteria. A change that causes rework or that causes material to be moved twice gets very expensive very quickly and while it won t affect the ultimate sale of the property is can have a marked effect on the profit. Page 9 of 10

10 REFERENCES 1 Archives Search Report Findings for the former Conway Bombing and Gunnery Range. US Army Corps of Engineers Rock Island District, Project Number I04SC September Engineering Evaluation / Cost Analysis (EE/CA) Conway Bombing and Gunnery Range. Parsons Corporation, Contract Number DACA87-95-D0018. September Page 10 of 10