Geomechanics of Large Stone Structures: A Case History from the Washington

National Cathedral

R. Mark

Princeton University, Princeton, New Jersey, USA

R. Richards, Jr.

SUNY, Buffalo, NY, USA

C. Mark

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, USA

ABSTRACT: The Washington National Cathedral is one of the largest masonry structures in the USA, and like

many of its European Gothic counterparts, it required nearly a century to construct. The design was altered

during this period, resulting in greater loadings than were originally anticipated on the soil beneath the

foundation. When signs of continuing differential settlement were observed in the early 1990's, the Cathedral

approached the US Bureau of Mines (now the National Institute for Occupational Safety and Health (NIOSH))

for assistance in monitoring the movements ofthe massive towers and walls. An array of dual-axis tiltmeters

was installed about 40m above ground level to measure wall and tower inclination, with the data sent over a

dedicated phone line to the NIOSH Pittsburgh Research Laboratory for processing. Mechanical gages and

string-potentiometers are also being used to measure motion across cracks and joints, and differential pier

settlement is being monitored with optical-level surveying. Data have now been collected for almost 7 years,

and they show a long-termsettlement trend masked by dramatic diurnal and seasonal thermal movements. The

paper discusses the major findings of the study, implications for the future of the Cathedral, and conclusions

regarding the use of geotechnical monitoring at major national monuments. 

1 ACKNOWLEDGMENTS

The authors are indebted for the vital contributions

made to this project by Joseph Alonso, Chief Stone

Mason, and Jan Dalberto, Construction Archivist ,

Washington National Cathedral; and by Paul Jeran,

Geologist, and TomMarshall,EngineeringTechnician,

of the NIOSH Pittsburgh Research Laboratory.

2 INTRODUCTION: BUILDING HISTORY

Comparable to the campaigns for construction of the

great cathedrals of the Middle Ages, it took some eight

decades for the Washington National Cathedral to be

completed. The period of its construction, spanning

1907 to 1990, witnessed twogreatwars, a world-wide

economic depression, and other intervals of fiscal and

political turmoil. Indeed, just as in the Middle Ages,

the rate of cathedral construction varied greatly with

time according to the availability of funds.

The original design for a Protestant Episcopal

Cathedral inthe DistrictofColumbia (Washington, DC)

was conceived by the English architect George

Frederick Bodley. Henry Vaughan, who had

apprenticed with Bodley before moving to Boston,

became the second Cathedral architect following

Bodley's death in1907. The apse of the Cathedral (Fig.

1) was largely completed during his tenure which

ended with his death in 1917. All remaining questions

of design were subsequently resolved by the a third

group of principal architects, the Boston firm of

Frohman, Robb, and Little, with the firm of Cram and

Ferguson acting as consulting architects. Frohman

himself continuedworking at the Cathedral site until he

retired in 1971. 

The entire foundation, including that for the

projected west front towers, 8 min depth, was in place

by 1924. The structure of the choir was completed in

1930. Work slowed during the depression and WWII,

but the three eastern-most bays of the 31-meter tall

(floor to vault keystone) nave and both transepts were

standing by 1966, by which date the 92 meter high

crossing tower was also complete. Before raising the

western-mostbays ofthe nave, the relatively diminutive

western wings were constructed between 1963 and

1968 on spread footings isolated from the footing for

the west front towers. The nave was completed in

1976, but little further work was carried out at the site

until 1983 when the 72 m-high west towers (Fig. 2)

were raised in an intensive six-year building campaign

(Feller 1989).
Figure 2. View of the west front and west wings of

Washington National Cathedral.

ApseChoir

Crossing

 Tower

North

Transept

South

Transept

Nave

West Wings

West Towers

Narthex

Figure 1. Ground-level plan of the Washington

National Cathedral

Figure 3. High vaults of the nave of the Washington

National Cathedral

Asthe architects ofthistwentieth-centuryCathedral

sought to follow Gothic architectural precedent, they

rejected modern construction employing stone veneer

on a steel or reinforced concrete skeleton. The

Cathedral is not built exclusively of stone block,

however. Instead, the main vessel of load-bearing

masonry, about 150 m in length, was constructed with

exterior Indiana limestone walls. The walls,

constructed without expansion joints, average about

one-third of a meter thick, and encase brick cores

reinforced bysteelrods. The covering vaulting (Fig. 3)

is entirely of stone. 

3 GEOTECHNICAL ISSUES

Fromthe point ofview ofstructural integrity, the design

ofthe Cathedral has beengenerallysuccessful. As with

all monumental buildings, a few problems relating to

local details of construction have came to light. One

example is the damage experienced by a large pinnacle

already in place for 30 years, which was caused by

water infiltration into the brick core behind its thick

outer stone facing (Bork et al. 1995).

One of the more critical problems is differential

settlement of the foundation. Evidence of differential
Ground

A: Fill Surface

B:Sand 

C:Sand 

D : Fat Clay 

C: Sand 

E: Sandy Silt 

D : Lean Clay 

0m

12.5m

14m

16.5m

19m

23m

34m

8m

F

L

Soil

Layer

C:Sand 

Footing

Figure 5. Soil section obtained from test boring near

the west towers.

Figure 4. Prospective of the south flank of the

Washington National Cathedral, by Donald Robb in

1934, indicating grade below the South transept outer

wall.

settlement is mostpronounced inthe vicinity ofthe west

towers, and includes:

• Distortion and cracking of the stone tracery

supporting the glass panels within the large

windows beneath the high vaulting of the nave bay

adjacent to the west towers;

• Transverse cracks across the nave floor and over

the high vaulting in this same region; 

• Shearing distortionofinterior pillars supporting the

west towers;

• Distortion and cracking of tracery in both of the

diminutivewesternwings adjoining the westtowers

(e.g. in intricate, stone- mullioned cabinet doors of

the Rare Books Library), and;

• The lowering of the narthex floor, beneath the west

towers, which was presumably perfectly flat when

it was placed in 1976, and is now six centimeters

below the floor elevation at the outer walls of the

western wings.

Elsewhere, transverse cracks have developed over

the high vaulting adjacent to the crossing tower, and

outward rotation of the south transept end wall is also

indicated by cracking in the floor over the highvaulting

adjacent to this wall. While none of these

manifestations has as yet posed the necessity for

structural intervention, they do indicate a need for

carefully monitoring the building structure.

The differential settlement is largely a consequence

of several crucial, early decisions made by the

Cathedral architects. The first of these, a determination

made around the time of the original design (1907), is

linked to the positioning of the Cathedral on the

building site. The south transept armlies at the summit

of a long, steep hill extending away from the end wall

ofthe transept(Fig. 4)thatwasfilled to furnish a moreor-less level building site. Secondly, no core samples

were taken ofthe subsoil below the foundations during

any stage of construction. Instead, the foundation was

designed using informationobtained fromtest pits. The

Cathedral is sited on one ofthe highestelevations in the

District of Columbia, and thus it might appear that

bedrock would lay close to the surface. However, the

testpitsshowed thatthe upper 10 mof ground consisted

of layers ofrelatively competentsilty sand and clayey

sand. It was assumed that these soils continued down

to bedrock. In fact, test borings taken by Schnabel

Engineering Associates (1994) near the west towers

found thatthe hard shale bedrock is 35 mdeep, and that

the lower 25 m of soil includes layers of “fat” (fineparticle high-plasticity) and “lean” clays. Laboratory

testing indicated thatthe clay soils display primary and

secondary consolidation behavior, and that they are

overconsolidated. Figure 5 is an idealized soil section

based on those borings, and Table 1 shows

representative geotechnical properties.

Table 1. Properties of soil layers beneath the west towers

shown in Figure 5 (Schnabel Engineering Associates, 1994).

STRATUM Soil

Classification1

Penetratio

n

Resistance

2

Plastic

Limit

Liquid

Limit

B (Sand) SC + SP 10 to 30 - -

C (Sand) SM, SC+SP 8 to 100+ 8 to 13 30 to36

D

F

 (Fat Clay) CH 6 to 12 31 to 33 53 to

56

D

L

 (Lean Clay) CL 15 to 42 - -

1From ASTM D-2487-83, Definition of Soil Group Names.

2Minimum and maximum values in blows per ft, per ASTM D1586.
Figure 6. Comparative west-front facade profiles for

Washington National Cathedral

Some light onthe decisionnot to conduct test borings

is shed by a surviving exchange of letters from 1922

between Scientific American editor J. Bernard Walker

(who held a degree in Civil Engineering) and the thenDean of the Cathedral, Dr. B.C.F. Bratenahl. Walker

(1922) wrote that: "probably more trouble has been

caused by uneven settlement of foundations than by all

other causes combined.... In view of the size and weight

of the Washington Cathedral it would be wise to ...

determine the character of the sub-soil and its absolute

bearing capacity." Walker then suggested that "it would

be highly desirable" to have General William Barclay

Parsons proceed with the work. Apparently Parsons had

already submitted a proposal to the Cathedral to take

what he considered to be an adequate number ofborings

for about $15,000, and Walkerthoughtthe estimate most

reasonable. 

Dean Bratenahl (1922) replied to Walker in a long,

thoughtful letter that he: "would rather leave these

matters to such Gothic architects as [the consultants]

Cramand Fergusson, who have given their whole life to

Gothic construction, than to leave the matter to

engineers. Furthermore, the soil as we go down, is

almost like cement; it is so hard and compact that we

need no shorings for the sides nor do they need to be

sloped.... We should take thought before we pay a sum

almost equal to the entire fee to the architects [for all

their work to date]."

Certainlythe most significant decision influencing the

settlement rate of the west towers was made after the

footing had already been placed. Shortly after the

appointment of the third group of principal architects,

Philip Frohman (1921) argued that the size of the west

towers should be dramatically increased on the basis of

aesthetics: "The whole front has a narrow, constricted

appearance... it seems lacking in that beauty and

grandeur which should be possessed by the facade of a

great cathedral." In a letter to Frohman from one of his

partners written in 1925, Donald Robb supported

Frohman's aesthetic view, butwarned that: "There is the

matter of the foundation mat already in place.... If the

necessary extensions to this mat are poured within a

short time, they will have ample time to come to their

permanentlevel[ofstrength] before the superstructure is

added."

Unfortunately, this advice was not followed.

Ultimately, the upper portion of the great concrete

footing was modified to accept the enlarged towers, but

the lower portion of the footing (its critical 21 x 49 m

"footprint" on the soil 8 m below the surface) remained

unaltered. The west towers, which were finally

completed in 1989, are close to 10 m taller and wider

than the originally proposed facade (Fig. 6). 

While the original design of the mat footings gave a

uniform bearing pressure of 29 T/m2 beneath the crossing

tower, and presumably the same for both west towers,

the bearing pressure of the revised west tower design

(including the footing) is estimated to be 47 T/m2.

Computation of the predicted settlement of the west

towers, using the laboratory soil strength values,

indicatesthata primaryconsolidation ofapproximately

75 mm would have been expected. The primary

consolidation should have been essentially complete

aftersixyears. The long-term secondary consolidation

is predicted to be about 10 mm over next 10 years,

followed by a further 10 mm over the next 100 years.

4 OBSERVATION AND INSTRUMENTATION 

Monitoring at the Cathedral is being carried out using

four overlapping strategies. The first two, primarily

within the province of the Cathedral staff under the

direction of the mason foreman, comprise: 

• Mechanical crack measurements: Quarterly

recording of gap widths at 10 locations bymeans of

(automotive)feeler gagesset between pairs ofrigid

blocks fixed to both sides of existing cracks, and;

• Level elevations: Monthly recording of dumpylevel elevations of metal pins setinto the eight west

tower piers and the two western-most main arcade

piers of the nave relative to datum pins set into the

end walls of the west wings. 

The other two are being carried out under contract

by the Disaster Prevention and Response Branch ofthe
-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

6/15/1994 6/15/1995 6/14/1996 6/14/1997 6/14/1998 6/14/1999 6/13/2000

Date

Deflection (mm/30m)

-50 50 150 250 350 450 550

Temperature

Figure 7. Inclination variation and instrument temperature (uncalibrated) vs. time from southwest tower tiltmeter. 

East/west inclination (west is positive) is indicated by the solid trace above the graph axis. Temperature variation is indicated

by the broken trace.

National Institute for Occupational Safety and Health -

Pittsburgh Research Laboratory (formerly, the US

Bureau of Mines). These comprise: 

• String potentiometers: Hourly recording of crack

movement at four locations using Technovision

model PT-10-A string potentiometers that

incorporate a spring-loaded cable rotating a shaft

coupled to a precision potentiometer, and;

• Tiltmeters: Hourly recording of tower and wall

inclination from four Applied Geomechanics model

716 dual-axis tiltmeters thatincorporate electrolytic

sensors, one for each axis, together with temperature

sensors. 

The tiltmeters, fixed to interior walls of the western

towers and the southern transept, are located about 40 m

above ground level; the crossing tower tiltmeter is

mountedat about 50 melevation. A central 12-volt D.C.

power supply, with battery backup, powers the

instruments, and their signals, sent over a dedicated

telephone line, are recorded in Pittsburgh (only midnightreadings are used indata presentation). Monitoring

of the Cathedral was initiated at the beginning of the

summerof1994. Results to date, therefore, are based on

close to seven years of data. 

5 RESULTS

One ofthe significant findings ofthe studywasthat daily

and seasonal temperature fluctuations give rise to

appreciable motion throughout the building structure,

most notably cyclic opening (and closing) ofjoints and

cracks in the walls, floors, and vaults adjacent to the

central and west towers. The normal, yearly

temperature range for the District of Columbia,

according to the US National Weather Service, is 56

degrees C; and the maximum recorded yearly range is

67 degrees C. The large amplitude of the seasonal

cyclic component of movement, combined with a small

number of measurement points, makes it difficult to

discern the relatively slight long-term movement from

these data. Movements caused by annual seasonal

variation can be an order of magnitude greater than

those caused by the underlying settlement. The cyclic,

seasonal, opening and closing of transverse cracks

above the high vaults adjacent to the towers

necessitates their periodic, careful examination,

particularly to ensure that even small masonry

fragments that may be produced by this type of

movement do not fall to the floor below. 

The tiltmeter data display the highestseasonal (and

daily) variations of any of the instruments. The

maximum yearly range of east/west inclination of the

southwest tower is 4.2(10)-4 radians, which

corresponds to 12 mm of lateral movement at an

elevation of 30 m (Fig. 7). However, regression of the

data obtained overthe entire studyperiod indicates that

the net change in tilt has been negligible. The trace of

north/south south-west tower inclination indicates less

seasonal variation than along the east/west axis. 

Level elevation measurements in the west towers

also show non-linear seasonal effects, but they are
 

0

2

4

6

8

1 0

1 2

1 4

1 6

1 8

5/31/1995 5/21/1996 5/12/1997 5/3/1998 4/24/1999 4/14/2000 4/5/2001

Date of Measurement

Settlement, mm

Outside Piers

Inside Piers

Figure 8. Level elevation measurements from the piers supporting the west towers. The “inside piers” are located closer

to the longitudinal centerline of the Cathedral, the “outside piers” are located closer to the wings.

-2

-1

0

1

2

3

4

5

6

6/15/1994 6/15/1995 6/14/1996 6/14/1997 6/14/1998 6/14/1999 6/13/2000

Date

Movement (mm)

Figure 9. Gap movement (in mm) vs. time measured by a string potentiometer installed over the nave high vault

adjacent to the south-west tower. Annual averages and the regression line through them are also shown.

much less pronounced. However, ongoing slow

settlement is clearly evident in the data. Regression

lines through the measurements on each of the eight

supporting piers indicates that the average rate of

differential settlement during the last six full years is

approximately 1.4 mm/year (Fig. 8). The r-squared for

these regressions averaged 0.8. The measured rate is

slightly greater than the calculated value of 0.9 mm/year

for secondary settlement during this period. While there

is no evidence yet thatthe rate ofsecondarysettlementis

diminishing, the current rate is appreciably less than it

was during the period of primary settlement (when it

averaged about 8 mm/year).

The data also indicate that the eight piers are

settling at slightly different rates. The average for the

outer four piers ranged from 0.118 to 0.123 mm/year,

while the inner four rates range from 0.153 to 0.156

mm/year. It appears that the concrete mat foundation,

whichis believed to be unreinforced, is bowing slightly

in response to the soil pressure. 

The automatic, effectively-continuous data

recording from the string potentiometers has been

effective in showing long-term crack opening trends.

For example, regression of data from the string

potentiometerlocated overthe highvaultadjacent to the

south-west tower indicates a rate of 0.43 mm/year (rsquared = 0.95 based on yearly averages). The
maximum yearly range, due to seasonal effects, is as

much as 4.5 mm/yr (Fig. 9). The less frequent

mechanical measurements broadly agree with the string

potentiometers. 

Instrumentation mounted on the crossing tower and

the south transept wall display diurnal and seasonal

movements that are similar in both form and magnitude

to those measured on the west towers. There is no

evidence of long-term trends at any of these locations,

however.

6 CONCLUSIONS

Although the program of monitoring the Washington

National Cathedral is ongoing, preliminary inferences

may be drawn concerning both the state of the building

fabric and the strategies for monitoring. Most

importantly, the rate of west tower (differential)

settlement, more than a decade after final completion of

the towers, appears to have slowed, although itremains

appreciable. While the ongoing settlement imposes the

need for a programof more-or-less continuous mending

of affected stonework, the towersthemselves appear to

be stable. Certainly, a catastrophic failure due to

excessive settlement and rotationis highlyunlikely. The

same salutary prognosis for long-term stability of the

crossing tower is implied by the tiltmeter data received

from it. 

The combination of mechanical and electronic

devices, especially where there is an overlap of

measurement points, has provided rubustness and

credibility to the data. Furthermore, automatic,

continuous data recording hasfacilitated discrimination

of long-term trends. Even so, given the appreciable

thermal movement, data from a great many cycles is

required. Without several years of measurement, a

single, unusual season (a particularly hot summer, for

example) canskew the data enoughto significantlyeffect

regression projections. In fact, the lively thermal

activity of the Cathedral has important implications for

the long-termbehavior ofotherlarge masonrystructures.

The direct involvement of the Cathedral staff in the

monitoring process has had a number of valuable

benefits. For one, it has served to heighten everyone's

understanding ofpossible structural problems within the

building. Historical studies have shown that animportant

factor contributing to the success of pre-scientific

construction was the detailed observation by the

designer/builder of the building fabric. This ensured

thatsteps were taken to eliminate tensile cracking in the

masonry and any other undesirable behavior (Mark

1990). Successful conservation of historic buildings can

only be enhanced by similar perception onthe partofthe

building staff. 

The strategy described herein can be readily applied

in any large masonry building to help determine sources

of structural distress and to ascertain rates of crack

motion. Indeed, National Parks Service engineers

recently visited the Cathedral to observe the

application of the remote sensing instrumentation. The

result of the visit was a decision to employ similar

instrumentation to monitor the tallest masonry structure

ever constructed, the Washington Monument.

7 REFERENCES

Bork, R., Livingston, R.A., & Mark, R. 1995.

Restoration of a crossing pinnacle at Washington

National Cathedral. A.P.T. Bulletin, 26(2-3): 5-9.

Bratenahl, B.C.F. to J.B. Walker, letter dated

July 10, 1922. 

Feller, R.T. 1989. Completing Washington Cathedral

for Thy Great Glory, Washington Cathedral,

Washington

Frohman, P.H. 1921. Report to the Cathedral Chapter,

October 21.

Mark, R. 1990. Light, Wind, and structure. The MIT

Press, Cambridge:170.

Robb, D. to P.H. Frohman, letter dated October 21,

1925. 

Schnabel Engineering Associates. 1994. Geotechnical

Engineering Report, Settlement Investigation, The

Washington National Cathedral (Our 942036).

Walker,J.B. to DeanB.C.F. Bratenahl, letter dated July

5, 1922.