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Potential Serious Threat
To The Washington Monument
As documented in 1934, 1973, and 1984
Building in the highly saturated soils at the Rainbow Pool, which is situated in what was until the late 19th century the confluence of the Tiber and Potomac rivers, will require pumping of groundwater with the potentially serious and unintended consequences of destabilizing the Washington Monument. Historically the extensive gardens designed by the McMillan Plan of 1901-1902 for the Washington Monument Grounds were not built as a result of engineering studies in 1930 that concluded that the Monument's stability might be seriously affected by shifting the dirt load. The report states that "The stability of the sand and gravel stratum, which supports the masonry of the Monument, evidently depends to a considerable extent upon the carefully worked-out balance of the mound of earth from which the Monument rises and the resulting equilibrium of pressures on the wet and mobile substratum of mud."(1)
A 1973 engineering feasibility report for a visitor's center on the Washington Monument grounds found the Monument had settled due to drops in groundwater levels caused by recent construction: "In fact an approximate 12 foot of drawdown between 1962 and 1973 increased the effective stress in the subsoils by almost 0.4 tons per sq. ft. This may have been responsible for about 1/4 inch of settlement."(2) A 1984 study prior to sidewalk improvement reiterated the recurring observation of settlement: "The increased rate of settlement that has occurred since the late 1960's is very probably due to an effective increase in overburden stress as a result of lowering of the ground water level in the general area by construction activity."(3)
It is unknown what further lowering of ground water may have occurred as a result of construction of the subterranean Smithsonian Museum of African Art and Sakler Gallery of Art at 10th St. in the 1980s, and the recently completed Ronald Reagan Federal Building at 14th and Constitution Ave.
Construction of the WWII Memorial will occur closer to the Washington Monument than the activity documented in 1973 and 1984. It will occupy the 25-year flood plain and will require extensive excavations below the water table. Pumping of ground water could create a cone of depression and subsistance as the soil contracts, effects that could cause further settling and potentially destabilize the foundations of the Washington Monument.
Additional unforeseen problems of depressing the Memorial into the soil of a floodplain include: build up of salts in the soil with adverse effect on the historic Olmsted elms surrounding the site, and potentially catastrophic flooding of what will undoubtedly be a heavily visited Memorial.
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1 "Improvement of the Washington Monument Grounds. Communication from the President of the United States transmitting A report on the improvement of the Washington Monument Grounds authorized by the independent offices act of 1931, approved April 19, 1930, together with several plans and estimates thereof (Washington: 1934), pp. 1-2.
2 "Visitor Facility for the Washington Monument Washington, D.C." prepared for the National Park Service by Hartman-Cox Architects, September 20, 1973, p. A-15.
3Settlement Analysis for Washington Monument Grounds Improvement NPS: D18 (NCR-NACC) (XF54)" February 13, 1984, p. 3: "The magnitude of settlement that has occurred between 1891 and the late 1960's is consistent with an appropriate theoretical rate of secondary compression of the underlying soil strata. The increased rate of settlement that has occurred since the late 1960's is very probably due to an effective increase in overburden stress as a result of lowering of the ground water level in the general area by construction activity." (i.e., basement excavation and depressed roadways) "The Mall area between 12th Street and the Monument has been dewatered to an appreciable extent by the construction along 12th Street."(p.3) "We suggest that before earthwork commences the Coast and Geodetic survey, or other organizations which have been maintaining precise measurements on the Monument, be requested to go through a cycle of observations. These should be repeated after the filling operation is complete for the purpose of demonstrating, for future planning purposes, the response of the Monument to this modest filling operation."
(Note that the 1984 study was done for the Park Service to evaluate the effects of new sidewalks, involving relatively minor changes down to 17th Street. No additional study has been undertaken since then even though major renovations to the Monument are ongoing and a visitor's center is yet to be built.)
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