The Dover Water Works site located on Lowell Avenue has been utilized by the City to provide the residents of Dover clean water back to 1888. Prior to that time, the site was owned by a private water company which supplied water to City residents. One of the initial projects undertaken when the Dover Water Commission was formed in 1888 was the construction of a pump station at the site (shown in the picture to the right). Another major undertaking was the construction of a slow sand filter at the site in 1908. This structure was utilized to provide clean drinking water to the City residents until circa 1954. Since that time, the sandfilter remained hidden beneath fill and trees at the site and long since forgotten to many during the passage of time.
An opening to the underground Sand Filter was discovered during an initial site survey. Construction plans for the structure were not located and the following information is derived from information available in the City of Dover files, site inspections, and from the Water Commissioners reports for 1908 and 1909 available at the Dover Public Library. The filter structure measured approximately 100' by 200' with an estimated height of 15'. It was divided with a concrete partition wall into two filter chambers 95' square. The concrete roof was supported by square columns joined in each direction by lowrise arches, all of which were cast together as an integral plain concrete (unreinforced) unit (See Below).
Six manholes were located in the roof of each chamber. The concrete floor was buried under a 9" layer of gravel followed by a 26" layer of sand (revealed by digging), which served as the water-filtering medium. The floor was understood to be equipped with drainage channels that gather the filtered water and direct it to collection piping but that was not confirmed by the limited excavation. The height from floor to ceiling at the top of the arches was measured at 14'-2" (assuming a 6" roof thickness and 4" floor thickness gives an overall height of 15'). The arched columns were 20" square, spaced 13'-9" on center; the arches rise 32 inches. An arched opening on the east side of the north chamber was uncovered by backhoe excavation. This was a service or "clean-out" door used for maintaining the filter medium. An "operating chamber" sheltered by a narrow brick structure was located midpoint of the east side, serving both chambers but physical remains of this feature were not found. The photo shown below shows the operating chamber building and clean outdoor (now buried). The principal of operation according to historical information on similar structures: the filter was loaded with gravel and sand to a height of perhaps 3 feet; raw water was flooded onto the top of the sand to percolate down to the floor where it was gathered into the water distribution system. The top layer of sand was periodically removed and replaced with clean sand as previously described. More details on the sandfilter are provided in a historic report prepared for the site (see below).
Given the age of the sand filter, a Section 106 process was initiated in accordance with the National Historic Preservation Act (36 CFR U.S.C. 470). Based on the Project Review and Project Area Form completed by Rich Casella of Historic Documentation Company, Inc. the site was deemed eligible for listing in the National Register of Historic Places. The City and the UNHSC worked closely with the New Hampshire Division of Historic Resources to develop meaningful mitigation and execution of a Memorandum of Agreement. The mitigation measures included additional documentation (forthcoming), large-format photography, and an interpretive sign placed at the sign at the completion of the project to document the site's historic significance.
Additional photographs and a "virtual tour" of the sandfilter can be seen here:
Virtual Tour: http://inveridux.com/SRP/Underground%20Bunker/tour.html