Feature: Movement of the Mall
Professional Surveyor Magazine - September 2012
For more than a century, the NGS has been monitoring settlement and subsidence at the National Mall in D.C. These efforts are now being augmented by the National Park Service as they repair damage to the Washington Monument by a recent earthquake in Virginia.
On August 23, 2011 the mid-Atlantic area was shaken by a magnitude 5.8 earthquake centered near Mineral, Virginia. Earthquakes of such magnitude are highly unusual for the region around the nation’s capital. Fortunately, the effects of the quake were generally mild and caused only modest amounts of structural damage to buildings and other structures.
One structure that did incur significant damage was the Washington Monument. In part due to its height of slightly more than 555 feet, the upper portion of the Monument suffered several significant cracks that have forced the closure of the structure until repairs can be made by the National Park Service (NPS
Since closure, NPS has been engaged in efforts to determine the extent of damage caused by the quake to not only the Washington Monument but also the numerous other monuments and memorials in the area of the Washington Mall. One area of concern was what, if any, settlement or subsidence occurred in the area of the Washington Monument and other structures to the west, including the Jefferson and Lincoln memorials.
Few visitors to the sites today appreciate that all of the structures from the Washington Monument west to the Potomac River are constructed on fill of sand, gravel, and blue clay. A 1950 report by the U.S. Geological Survey shows that the 25-foot-deep foundation of the Monument is approximately 65 feet above bedrock. This type of fill can be especially vulnerable to the effects of soil liquefaction that have been noted during numerous other earthquakes.
Past Mall-leveling Campaigns
As it just so happened, at the time of the event the National Geodetic Survey (NGS) had been engaged in developing an agreement with NPS to perform periodic geodetic leveling observations in the vicinity of the Mall area. NGS and its predecessor agency, the U.S. Coast & Geodetic Survey (USC&GS), had conducted 17 different leveling campaigns in and around the area since 1884. Many of the surveys were performed as small projects, including only certain parts of the area commonly referenced as the Mall.
This area is most often recognized by the public as that portion of the city that extends from just east of the Capitol building to the Potomac River, sandwiched between Constitution Avenue on the north and Independence Avenue on the south. Many of city’s biggest attractions are located in this area, including buildings of the Smithsonian Institution, National Art Galleries on the east side, and numerous memorials on the west such as those honoring Washington, Jefferson, and Lincoln, as well as memorials for World Wars I and II, the Korean War, and the Vietnam conflict. This hugely popular area is administered by NPS National Mall and Memorial Parks (NMMP).
Because the various leveling campaigns did not cover large enough areas to give a consistent picture of any height changes that might be occurring in the area, the agreement being developed between NGS and NPS would call for periodic reobservations of the Mall area, typically every three to five years, that would provide sufficient measurements to bench marks outside of the Mall and in the same epoch so as to provide an accurate assessment of vertical motions that may be occurring.
We already know that the Washington Monument, weighing almost 91,000 tons, has been settling since it was completed in 1884. The previously mentioned USGS report indicates that the Monument had settled 0.05 ft (1.5 cm) from 1900 to 1930 for an average of approximately .0016 ft per year (.0005 m). This compares very closely with the values derived from the different leveling campaigns performed by C&GS/NGS (Table 1,
see end of article).
Measuring Settlement from the Quake
NPS’s new concern was to determine if there may have been any additional settlement caused by “the event” (the earthquake of August 23). To provide that information, NGS was asked to perform a new leveling program as quickly as possible at the beginning of 2012.
Working closely with NMMP staff, NGS drafted a plan for First-Order Class II leveling that would start at existing bench marks to the north and west of the Capitol, run west along the Mall to the Washington Monument, circle the area of the Tidal Basin to include the Jefferson Memorial, follow the Potomac River north to the new Martin Luther King and Lincoln Memorials, then east across the area of the Reflecting Pool, back to the area of the Washington Monument, and then continue north across Constitution Avenue, across the Ellipse, to tie into a deep rod bench mark at the southwest entrance of the White House.
Additionally it was decided that a spur line should be observed from the vicinity of the Jefferson Memorial south to the single, long-term tide gauge in the District of Columbia maintained by the National Ocean Service
, Center for Operational Oceanographic Products and Services as part of the National Water Level Observation Network. Ultimately this network included 57 existing and 5 new marks for a total of 62 bench marks covering approximately 10.9 miles (17.50 km) double run leveling (Figure 1
One of the most immediate considerations faced by the NPS and the NGS level team was timing. Not wanting to run levels in the winter meant that early spring was the best opportunity. Because these operations were to be conducted from the NGS headquarters office in Silver Spring, Maryland, the logistics of travel were not an big concern; however, putting the team together so as to have the least impact to their ongoing programs and to tourist traffic during leveling was a major issue.
The weather in early spring was very cooperative, and the team, consisting of an observer, two rodmen, and a pacer, began observations on March 7, 2012. All observations were conducted with a Trimble DiNi 12 geodetic level (Figure 2
) with 3-meter calibrated rods (Figure 3
). After some discussion with NMMP staff, the decision was made to begin the actual observations at the Washington Monument to complete the loop consisting of the Tidal Basin and the Jefferson, MLK, and Lincoln memorials as soon as possible because these areas are most visited by tourists and the cherry blossoms were coming out a bit early.
While the team was working on this part of the project, significant efforts were being made by NMMP staff to get their colleagues at NPS President’s Park, which covers the Ellipse and the White House, to allow the level crew access to the Ellipse and the two significant bench marks there (the Meridian Stone and the Zero Milestone), as well as access to a deep rod bench mark (RS) located near the southwest entrance to the White House grounds. All of the NPS staff involved were extremely supportive, which eventually ensured that the level team could connect to these extremely important marks.
The first geodetic leveling to the Washington Monument was conducted by USC&GS in 1884 as part of the initial transcontinental leveling program begun in 1877; it formed the foundation of a consistent national vertical reference frame that we enjoy today as the North American Vertical Datum of 1988
(NAVD 88). At that time, a single connection was made to a bench mark at the Washington Monument that was shortly covered by construction and not seen again until 1999. This was followed in 1901 by another level program that established four bench marks at or near the Washington Monument that were subsequently releveled in 1907 and 1912.
When C&GS revisited the Monument in 1921, the marks from 1901 had all been destroyed, so they were required to establish a new mark. This mark, designated C 1 (HV1842), was a standard C&GS bench mark disk embedded in the sill at the entrance to the Monument on the east side—a very conspicuous location. This mark was releveled as part of five separate C&GS/NGS (the agency was renamed in 1970) projects during 1926, 1942, 1972, and 1974, providing a high degree of integrity to estimates of settlement at the Monument. During 53 years, from when the mark was set to the final leveling, the five surveys indicated a total settlement of 1.6 cm (.05 ft) for an annual rate of .3 mm/year.
Since the 1980s
The next survey conducted by NGS was in 1984, and by that time bench mark C 1 had been removed as a result of some remodeling of the entranceway. In an effort to ensure that there would be good-quality bench marks available for future surveys, NGS decided to install special marks of a style similar to those used in previous surveys of the White House (see sidebar).
Two such marks, designated A8 (HV8076) (Figure 5
) and B8 (HV8077) (Figure 6
), were placed in the Monument on the north- and south-facing walls. During 2009 similar style marks were set in both the Jefferson and Lincoln memorials. The location of all of these marks is such that they can accurately reflect any height changes to the monuments as well as being inconspicuous so as to have minimal impact on the appearance of the monuments.
White House-style marks, made of made of stainless steel, consist of a cylindrical plug 9 cm long, 2.5 cm in diameter with a treaded cap that can only be removed with a special tool (Figure 4
). To use the mark, you must remove the cap and insert into the plug a stainless steel pin 23 cm long and specially machined to fit into the opening.
Place the rod on the pin during observations, and then remove the pin replace the cap to make sure the plug doesn’t fill with debris.
Difference in Settlement Rates
Since 1984 these marks have been releveled seven times: during 1992 (twice), 1998, 1999, 2000, 2009, and 2012. Over that time both marks appear to have settled 4.3 mm, or approximately .2 mm per year (Table 2
and Table 3
). Based on the multiple leveling campaigns conducted by USC&GS/NGS since 1901, the Washington Monument appears to have settled 5.8 cm (0.19 ft.) or an average rate of .5 mm per year.
The small difference between these two settlement rates can easily be reconciled by the fact that there were periods of approximately 21 years since 1901 during which no leveling was performed, and that prior to 1984 there were several different bench marks were used in the various leveling efforts.
That fact that there have been considerable advances in leveling-instrument technologies over that time is likely of only modest consequence because the leveling standards used by USC&GS/NGS have remained virtually unchanged for almost 80 years and the distance over which the leveling has been conducted from the Washington Monument’s primary bench mark designated A (HV1841) is less than 100 meters.
All of the leveling conducted by USC&GS/NGS is considered to be at least First-Order Class II and have an estimated accuracy between adjacent marks of not more than .7 mm as described in the “Federal Geodetic Control Committee Standards and Specification for Geodetic Control Networks” of 1984. Because the distance from the primary bench mark and the Washington Monument is so short, this standard has been used as the threshold for describing the accuracy of the height differences.
Additional local leveling from monument A to bench marks at the Washington Monument was conducted by USC&GS in 1907 and 1912. During that time the leveling indicated settlement of the Washington Monument of approximately 1.8 cm.
Station A was subsequently tied into the growing national vertical network being developed by USC&GS in 1914. The station was reobserved as part of six different regional leveling projects during 1926, 1942, 1972, 1974, 1992, and 2012. Those campaigns also included the Capitol bench mark (HV2017) that’s one of the oldest geodetic bench marks in the District of Columbia, having been set in 1884.
A comparison of height differences from the Capitol bench mark to A over that time indicates station A appears to have uplifted approximately 1 cm sometime during 1926 to 1942. Additional observations from 1942 to 2012 seem to indicate that station A has continued to uplift but only in the range of 3 to 4 mm (Table 4
). Because the leveling distance from the Capitol to the Washington Monument is approximately 3 km, this difference is in excess of the expected leveling error. This analysis does not suggest any reason for the difference.
While visual inspection reveals the extent of structural damage due to the earthquake, how much permanent physical displacement or subsidence of the Washington Monument was there? The NGS reports nothing that was inconsistent with the legacy—subtle patterns of subsidence evidenced by more than a century of various leveling campaigns—although the July 2012 NGS report is also frank in offering that “the lack of significant, repeat, re-observations precludes any determination of settlement.”The immediate needs during the repair are being addressed in the interim. NPS has indicated to the NGS that scaffolding for the Washington Monument will likely be required for repair and that the NGS will continue to monitor during that phase and beyond, although details have not completely been worked out. Suggestions for ongoing monitoring from the NGS include leveling campaigns every two to three years to be augmented by periodic GNSS or even continuously operating GNSS sensors.
The latter speaks to another serious consideration in structural monitoring: Do localized level runs alone capture regional subsidence and crustal motions? Augmentation through live ties to the NGS CORS network provides a potentially powerful resource with which to monitor both regional and localized motion.
Research into past sporadic campaigns underscores the need to develop empirical trend data from which to evaluate the effects of acute events such as this recent earthquake. It is important to initiate an ongoing and comprehensive program monitoring these national treasures, and it’s good news that the NGS and NPS are in the midst of planning a program, not only for the Washington Monument but for many other Mall features including the White House. We hope that such plans are put in place before seemingly inevitable “disaster amnesia” sets in.
View the related video here
Bench mark A, sometimes called the “Mini-Monument,” was originally set by the Office of Public Buildings and Grounds (OPG, later called the Office of Public Buildings and Public Parks and then absorbed into the U.S. Army Corps of Engineers). The bench mark, set during the reconstruction of the foundation of the Monument sometime before 1889, consists of a concrete obelisk 12.5 feet long and 3 feet wide at its base, tapering to 1.5 feet at the top and resting on a concrete platform that is 12 square feet. The top of the obelisk is flat to support a level rod, is recessed a little over a foot below ground level, and is protected by a cast iron utility access cover. The mark is rarely seen by the public, and even the occasional news story or YouTube video only partly describes this mark’s significance (www.youtube.com/watch?v=ioZIwoFT_LA
Bench mark A was first used by USC&GS in a leveling program in April, 1901 when USC&GS was requested by the OPG superintendent to validate the height difference between this mark and several others located at the corners of the Washington Monument that had previously been determined by OPG in 1899. The work conducted by USC&GS showed no difference at that time.
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